GB2634781A

GB2634781A - Subsea system, methodology for determining parameters of optical systems and optical signal transmission

Info

Publication number: GB2634781A
Application number: GB2316076.5A
Authority: GB
Inventors: Murdoch Mckenzie Eaen; Roy Windsor Kristian; Salcedo Thiago; Sen Rajarshi
Original assignee: Baker Hughes Energy Technology UK Ltd
Current assignee: Baker Hughes Energy Technology UK Ltd
Priority date: 2023-10-20
Filing date: 2023-10-20
Publication date: 2025-04-23
Also published as: WO2025082625A3; WO2025082625A2; GB202316076D0

Abstract

A method and apparatus of determining a parameter of an optical signal in a subsea communication network using a variable optical attenuator 192, and a subsea system for transmitting communication signals to subsea hydrocarbon production equipment. A variable optical attenuator receives an input signal at a first subsea module 140, and passes a corresponding signal through the optical attenuator. A predetermined or specified parameter of a received optical signal is thus determined at a further subsea module 120 that is located at a further subsea location that is spaced apart from the first subsea location.

Description

SUBSEA SYSTEM, METHODOLOGY FOR DETERMINING PARAMETERS OF OPTICAL

SYSTEMS AND OPTICAL SIGNAL TRANSMISSION

The present invention relates to a method of determining one or more parameters of an optical signal in a subsea communication network. In particular, but not exclusively, the present invention relates to a subsea network and method of determining parameters of optical systems whereby an unattenuated optical signal is transmitted through a subsea variable optical attenuator disposed in a subsea module. The attenuator helps determine one or more parameters associated with a received optical signal.

In the oil and gas industry, from time to time subsea wells are used to extract petroleum and/or natural gas from a site such as an oil well or the like. Control and monitoring of one or more subsea wells is conventionally achieved through communication between a topside location such as a surface platform, a floating production storage and offloading (FPSO) or the like, and a subsea well, via an umbilical. The topside location usually includes a master control/station which provides monitoring and control of subsea wells topside. The master control is usually controlled by an operator. Conventionally an umbilical carries lines for electrical power, hydraulic supply and data communications between the topside location and a subsea distribution point. From the distribution point, flying leads are conventionally utilised to connect the distribution point and thus the master control located topside to control modules associated with each of the subsea wells in a subsea field. Sometimes there may be more than one umbilical from the master control to more than one of various distribution points and thereby to one or more control modules associated with a subsea well. Nevertheless, data transmitted to and from a given control module to the master control passes through an umbilical.

At a subsea well such as an oil well, gas well, water injection well, or the like, a so-called Christmas tree (or 'subsea tree' in particular) can regulate the flow of fluid out of the subsea well. The subsea tree is usually an assembly of sub-components including a control module among other things. The subsea control module (commonly referred to as a 'control pod') provides an interface between control lines which usually supply hydraulic power, electric power, and signals from a host facility such as the distribution point and the subsea tree to be controlled. That is to say the control module acts as a control system for the subsea -2 -tree/manifold valves. The control module usually contains at least one component called an electronics module. The electronics module is a component of the control module which manages electrical systems on the control module, receives sensor information, processes sensor and other information, stores information and issues instructions to other components of the control module.

Among other things, the electronics module is provided with data values captured by sensors associated with the subsea tree such as temperature, pressure, and the like. Data values are regularly captured by a given sensor based on sensor readings of real-world variables associated with the subsea tree and then sent as transmitted data to the electronics module which monitors the data values and may log the data values or send all or some data values to the master control via the umbilical. Data values received by the topside master control are usually presented to and interpreted by the operator to diagnose issues. The operator may then choose to take positive action if required by issuing commands to the electronics module to affect the real-world variables associated with the subsea tree.

Conventionally the umbilical has a limited bandwidth for transferring data between a subsea well associated with an electronics module and the master control. Therefore, the provision for sending data values captured by a sensor is somewhat limited. Similarly, the amount of information that can be transmitted from topside to components of a subsea installation is limited. This can limit the number of subsea components that can be powered and controlled via a single umbilical. There can thus be a limit to the number of subsea components that can be installed at a particular region of a subsea field. Additionally, expanding a brownfield site or an existing subsea site to include more apparatus possibly at further wellbore locations can be challenging as the existing equipment located subsea often cannot sustain expansion to include additional apparatus and communications.

In current control systems, data values monitored by an electronics module in a subsea tree are sometimes transmitted, via an umbilical, to master control which is located topside. At a master control station, instructions are determined to optimise operation of the subsea tree and then the instructions are transmitted back via the umbilical to the electronics module in the subsea tree to be executed. Many of the current control system solutions are often reliant on getting enough accurate data from subsea to topside to then analyse and make changes. The current systems are limited by the amount of data that can be sent topside: sometimes -3 -the bandwidth limitations from subsea to topside can limit those optimisations and stop the well from performing efficiently, for example at an efficient operating point.

Sometimes, the subsea distribution point may include or may be connected to a communication distribution unit which can sometimes include a power and communication distribution module for helping direct power and communications to desired subsea installations. Often, power, communications and/or data are transmitted from the power and communication distribution module to other subsea installations via copper conductors in subsea cables, sometimes over relatively large distances such as 10 km or more. These cables can sometimes be inefficient and can sometimes transmit only a limited bandwidth of power and communications. Thus, the number of subsea installations that can be connected to a respective power and communications distribution module (PCDM), and the distance between a subsea installation and a power and communications distribution module can be limited.

Given the limited capacity of the umbilical and other conventional subsea cables for transmitting power and communications throughout a subsea system or network, it can be difficult to expand an established site or brownfield site or the like without significant investment in additional cables and/or subsea modules and the like. Furthermore, it can be difficult to account for changes in the distance over which data must be transmitted when new equipment is installed subsea at new locations. Furthermore, the efficiency of some transmission equipment can degrade over time, and it can be hard to account for such degradation in some existing subsea installations.

It is an aim of certain embodiments of the present invention to at least partly mitigate one or more of the above-mentioned problems.

It is an aim of certain embodiments of the present invention to provide a subsea module adapted for optical communications in a subsea hydrocarbon extraction field.

It is an aim of certain embodiments of the present invention to provide a subsea module or unit that is adapted to include or receive a small form-factor pluggable variable optical attenuator. This may be a power and communications distribution unit which may be included in a subsea distribution unit. This may instead be the subsea distribution unit as a whole. -4 -

It is an aim of certain embodiments of the present invention to provide a subsea module (or unit) that can convert an electrical signal into an optical signal and modulate a parameter of the optical signal such that the parameter of the optical signal received at a further subsea module (or unit) is within a desired range or at a desired level.

It is an aim of certain embodiments of the present invention to provide a subsea module or unit that can receive optical signals and can modulate a parameter of said optical signals such that the parameter of said optical signals received at a further subsea module or unit is within a desired range or at a desired level.

It is an aim of certain embodiments of the present invention to module, at a subsea module or unit, an optical signal to account for a distance between the subsea module and a further subsea module tow which the signal is to be transmitted.

is It is an aim of certain embodiments of the present invention to provide a method of determining one or more parameters of an optical signal in a subsea communication network.

It is an aim of certain embodiments of the present invention to provide a method and apparatus for transmitting communication signals to subsea hydrocarbon production equipment.

It is an aim of certain embodiments of the present invention to module an optical signal to take into account a reduction in transmission efficiency of transmission apparatus such as fibreoptic cables over the service life of said transmission apparatus.

It is an aim of certain embodiments of the present invention to provide increased bandwidth in a subsea network such that a subsea network can be expanded to include additional apparatus such as additional subsea distribution units, manifolds, productions trees, injections trees and the like.

According to a first aspect of the present invention there is provided a method of determining a predetermined parameter of an optical signal in a subsea communication network, comprising the steps of: receiving an input signal at a first subsea module that is located at a first subsea location; and transmitting an unattenuated optical signal that is responsive to the input signal through a variable optical attenuator that is disposed at the first subsea module, thereby determining a predetermined parameter of a received optical signal at a further subsea -5 -module that is located at a further subsea location that is spaced apart from the first subsea location.

Aptly the method further comprises modulating the predetermined parameter via the variable optical attenuator responsive to a distance between the first subsea module and the further subsea module.

Aptly the method further comprises modulating the predetermined parameter via the variable optical attenuator responsive to the transmission efficiency of an elongate fibreoptic element connected between the first subsea module and the further subsea module.

Aptly the method further comprises varying at least one characteristic of the variable optical attenuator thereby increasing the predetermined parameter over the service lifetime of the elongate fibreoptic element to account for a decrease in efficiency of the elongate fibreoptic element.

Aptly the method further comprises modulating the predetermined parameter so that the predetermined parameter is maintained within a predetermined range that is an acceptable parameter range associated with the further subsea module.

Aptly the method further comprises transmitting an optical output signal responsive to the unattenuated optical signal from the first subsea module to a further subsea module via an elongate fibreoptic element thereby providing the received optical signal at the further subsea module.

Aptly the method further comprises transmitting an optical output signal responsive to the unattenuated optical signal from the first subsea module to a further subsea module via the elongate fibreoptic element thereby providing the received optical signal at the further subsea module.

Aptly determining the predetermined parameter comprises determining the power and/or magnitude of the received optical signal.

Aptly the method further comprises providing the unattenuated optical signal from the input signal that is an optical signal. -6 -

Aptly the method further comprises providing the unattenuated optical signal via converting the input signal into an optical signal.

Aptly the method further comprises converting the input signal into an optical signal via an optical modem that is disposed at the first subsea module.

Aptly the input signal is an electrical communication signal.

Aptly the method further comprises receiving the input signal from a still further subsea module located at a still further subsea location or from a master control station located at sea-level.

Aptly the method further comprises receiving the input signal via an umbilical.

Aptly the method further comprises transmitting the optical output signal from an established region of a brownfield site at the first subsea location to an expansion region at the further subsea location.

Aptly, wherein the input signal is an optical signal, the method further comprises performing optical signal analysis based on the input signal that is an optical signal.

According to a second aspect of the present invention there is provided apparatus for determining a predetermined parameter of an optical signal in a subsea communication network, comprising: a first communication interface disposed at a housing of a first subsea module, for receiving an input signal; a variable optical attenuator for modulating an unattenuated optical signal disposed within the housing thereby providing a modulated optical signal; and a further communication interface disposed at the housing for transmitting an output optical signal responsive to the modulated optical signal.

Aptly the first subsea module is a power and communication distribution module (PCDM) usable in in a communication network of a subsea hydrocarbon production system.

Aptly the PCDM is locatable in a communication distribution unit (CDU) or a subsea distribution unit (SDU) or a PCDM foundation of a subsea hydrocarbon production system. -7 -

Aptly the apparatus further comprises an optical modem in communication between the first communication interface and the variable optical attenuator for converting the input signal into a pre-transmission optical signal upon which the output signal is responsive.

Aptly the variable optical attenuator is a small form-factor pluggable (SFP) variable optical attenuator.

Aptly the variable optical attenuator is for modulating a predetermined characteristic of the output optical signal to account for a distance between the first subsea module and a further subsea module to which the output optical signal is transmitted via an elongate fibreoptic element and/or the variable optical attenuator is for modulating a predetermined characteristic of the output optical signal to account for a reduction in efficiency of an elongate fibreoptic element, into which the output optical signal is transmitted, over the service life of the elongate fibreoptic element.

Aptly the variable optical attenuator is for determining a predetermined parameter of a received optical signal at the further subsea module.

Aptly a master control station disposed at sea level is connected to and/or in communication with the first communication interface.

Aptly the apparatus further comprises an adaptor device in communication between the first communication interface and the further communication interface for transcribing an electrical and/or hydraulic based communication protocol into an optical based communication protocol. 25 According to a third aspect of the present invention there is provided a subsea system for transmitting communication signals to subsea hydrocarbon production equipment, comprising: a master control station (MCS) located at a topside location; at least one communication distribution unit (CDU) or subsea distribution unit (SDU) connected to the MCS via at least one umbilical; at least one power and communication distribution module (PCDM) disposed at the SDU or CDU or at a PCDM foundation; a variable optical attenuator disposed at the PCDM; and a further subsea module for receiving the output optical signal that is spaced apart from the PCDM and is communicatively connected to the PCDM via at least one elongate fibreoptic element. -8 -

Aptly the further subsea module is a subsea control module (SCM) of a subsea tree for operating subsea hydrocarbon production equipment.

Aptly the subsea system further comprises an optical modem disposed in the PCDM and communicatively connected between the first communication interface and the variable optical attenuator for converting the input signal, that is an electrical and/or a hydraulic signal, into an optical signal.

Aptly the PCDM is disposed in a brownfield region of a subsea hydrocarbon production system and the further subsea module is disposed in a more recent expansion region of the subsea hydrocarbon production system.

Aptly the subsea system further comprises a network communication device that is disposed subsea to send or gather data from various subsea equipment including sensors and instruments.

Certain embodiments of the present invention provide a subsea module or unit (for example a power and communications distribution module and/or a subsea distribution unit) that receives electrical signals, converts said electrical signals to optical signals, attenuates the optical signals via a small form-factor pluggable variable optical attenuator and transmits the signals to a further subsea module or unit such that the received signals at the further subsea module or unit are at a desired magnitude or amplitude (or are in a desired range).

Certain embodiments of the present invention provide an expandable subsea optical communication network with increased bandwidth to support further installations and communications.

Certain embodiments of the present invention provide a subsea optical network in which a signal output from a first module or unit can be modulated to account for a distance between the first module or unit and a second module or unit to which the signal is to be transmitted.

Certain embodiments of the present invention provide for installation of a small form-factor pluggable variable optical attenuator in a subsea module/unit such as a SDU and/or a PCDM installed in a subsea hydrocarbon extraction field to modulate a parameter of an optical signal transmitted from the subsea module/unit to a further subsea module/unit so that the parameter -9 -of the signal received at the further subsea module/unit is at a desired level or within a desired range. The variable optical attenuator allows for a signal with a desired amplitude or magnitude to be delivered at the further subsea module irrespective of the separation distance between the first and further subsea module or the efficiency of the transmission equipment (such as optical fibres and the like) which may vary over time. Furthermore, modulating a parameter of optical signals in a subsea environment can increase the available bandwidth and can allow for expansion of a subsea field or subsea installation.

Certain embodiments of the present invention provide a subsea optical network in which a signal output from a first module or unit can be modulated to account for efficiency losses in transmission equipment such as a fibreoptic cable disposed between the first module or unit and a further module or unit to which the signal is to be transmitted.

Certain embodiments of the present invention provide a subsea module or unit adapter to include or receive a small form-factor pluggable variable optical attenuator. The subsea is module or unit may be a power and communications distribution module that may be located in a subsea distribution unit. The subsea module or unit may be a subsea distribution unit.

Certain embodiments of the present invention provide a method of determining a predetermined parameter of an optical signal in a subsea communication network. An unattenuated optical signal is transmitted through a subsea variable optical attenuator which determines a predetermined parameter of a received optical signal.

Certain embodiments of the present invention utilise a variable optical attenuator disposed of a subsea location whereby the attenuator modulates one or more predetermined parameters of an optical signal in response to a distance between a first and a further subsea module.

Certain embodiments of the present invention provide apparatus for determining a predetermined parameter of an optical signal in a subsea communication network. Determining the predetermined parameter helps optimise operation of a subsea communication network.

Certain embodiments of the present invention provide a subsea system for transmitting communication signals to subsea hydrocarbon production equipment.

-10 -Certain embodiments of the present invention utilise a variable optical attenuator disposed at a PCDM of a communication network to control one or more parameters of a received optical system which then creates a optical signal for onward transmission having favourable parameter characteristics.

Certain embodiments of the present invention provide a subsea module or unit that is adapted to include a lucent connector.

Embodiments of the present invention will now be described hereinafter, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates a subsea field;

Figure 2 illustrates a schematic view of a communication network in the subsea field of Figure 1; Figure 3 illustrates a schematic view of a subsea optical communication network included in the subsea field of Figure 1; Figure 4 illustrates a power and communication distribution module adapted for optical communications; Figure 5 illustrates a further subsea optical communication network and indicates how a small form-factor pluggable variable optical attenuator can be integrated or installed in a subsea 25 module; Figure 6 illustrates a still further subsea optical communication network; and Figure 7 illustrates a schematic representation of a PCDM can receive and transmit signals.

In the drawings like reference numerals refer to like parts.

Figure 1 illustrates a subsea field 100 below a sea surface 110 where a first subsea site 115, a second subsea site 116 and a third subsea site 117 are located. It will be appreciated that there may alternatively be one, two, four or more subsea sites 115, 116, 117 in the subsea field 100. At the first subsea site 115 there are multiple wells 1201,2, (two shown). The second and third subsea sites 116, 117 also have multiple subsea wells 120 (not shown). It will be appreciated that the subsea site 115, 116, 117 may alternatively have one, two, three, four or more subsea wells 120. The subsea site 115, 116, 117 therefore includes at least one wellhead and its respective Christmas (subsea) Tree. Figure 1 thus illustrates a multiple well complex about a seabed 125.

A floating production storage and offloading (FPSO) vessel 130 illustrated as being located above the field 100. It will be appreciated that alternatively, a floating platform, topside location or topside node may be provided instead of the FPSO 130. The FPSO 130 includes a topside controller 132 and an electrical power unit. The topside controller shown in Figure 1 is a master control station (MCS) 132. It will be appreciated that the MCS 132 is an example of a master controller. The MCS is an example of a topside control device. It will be appreciated that other master controllers or other topside control devices could be provided a network master node.

The MCS 132 is used to generate and receive control communications to instruct operation of subsea components and to receive data indicative of the state of various components and sensor readings etc. It will be appreciated that whilst a floating structure is illustrated in Figure 1 the MCS 132 may be a shore-based control centre or a platform-based node or the like. The FPSO is an example of a topside node.

The FPSO 130 is connected via a first umbilical 1351 to a distribution unit 140 in the first subsea site 115. It will be appreciated that the distribution unit 140 is an example of a subsea distribution unit (SDU) and is located at a subsea distribution point in the field. It will be appreciated that alternatively, a Remotely Operated Vehicle (ROV) may instead provide a connection between the FPSO 130 and the distribution unit 140. A topside node is a node of a communication network that is physically located on/at the surface of a body of water. A subsea node is a node that is physically located under the surface of a body of water, e.g., at a depth of lm, 10m, 500m, 10km or more below the surface of a sea, lake, or the like. The distribution unit 140 of Figure 1 is a subsea distribution unit (SDU) however it will be appreciated that the distribution unit may instead be a manifold or the like. The first umbilical 1351 shown in Figure 1 is terminated with a wet mating connector 145 which mates with a corresponding wet mating connector interface 150 of the distribution unit 140. It will be appreciated that alternative connectors may be used. It will be appreciated that the first umbilical 1351 may be terminated at an umbilical termination head (UTA) located on the distribution unit 140. It will also be appreciated that the first umbilical 1351 may be connected -12 -to an umbilical termination assembly (UTA) at an umbilical termination head (UTH) located on the UTA. The UTA may be connected to the distribution unit via one or more flying leads and/or further umbilical's or the like. It will be appreciated that a UTA may instead be integrated with (or disposed within) the distribution unit 140.

As shown in Figure 1, a second umbilical 1352 connects the FPSO 130 and associated MCS to the second subsea site 116. The second subsea site 116 contains a distribution unit and subsea wells (not shown). A third umbilical 1353 connects the FPSO 130 and associated MCS to the third subsea site 117. The third subsea site 117 contains a distribution unit and subsea wells (not shown). The second subsea site 116 shown in Figure 1 is connected to the first subsea site 115 by a first cable 1531. Optionally the second subsea site 116 may not be connected to the first subsea site 115. Optionally the second subsea 116 site may be connected to the third subsea site 117. The third subsea site 117 of Figure 1 is connected to the first subsea site 115 by another first cable 1532. Optionally the second subsea 116 site may not be connected to the first subsea site 115. It will be appreciated that the distribution units in the first subsea site 115, second subsea site 116 and third subsea site 117 shown in Figure 1 are interconnected by the first cables 1531,2. It will be appreciated that whilst three subsea sites 115, 116, 117 are illustrated in Figure 1, there may alternatively be one, two, four, five, six or more subsea sites 115, 116, 117 located in the subsea field 100, operated by the single FPSO 130.

A second cable 160 connects the distribution unit 140 to a first of the subsea wells 1201. A fibreoptic cable 180 connects the distribution unit to a second of the subsea wells 1202. It will be appreciated that the fibreoptic cable is an example of an elongate fibreoptic element. The distribution unit 140 and subsea wells 1201,2 are arranged in a so-called star formation, where the distribution unit 140 is connected to each subsea well 120 individually. It will be appreciated that alternatively the distribution unit 140 may be connected to the subsea wells 1201,2 in a mesh or chain formation, wherein the distribution unit 140 is connected to one subsea well 1201 and each subsea well 1201 is connected to a consecutive subsea well 1202.

In such an instance, it will be appreciated that one subsea well may be connected to another subsea well via a fibreoptic line/cable. It will be appreciated that alternatively a multi-drop formation or any other formation may be used. It will be appreciated that there may alternatively be more than one distribution unit 140. It will be appreciated that the distribution unit 140 may alternatively not be a discrete element but may instead merely be a distribution -13 -point provided by a manifold or Christmas (subsea) Tree or the like. In some embodiments the distribution unit 140 may be referred to as a distribution point or a distribution node.

Each subsea well 120 illustrated in Figure 1 is associated with a respective subsea control module (SCM) 1701,2. It will be appreciated that the SCM 170 may be referred to as a control module. The SCM determines operation of hydraulic driven valves which can be opened and closed using electrical signals communicated from the FPSO 130 or other topside control centre. It will be appreciated that each SCM may additionally control other facets of a respective subsea tree and may additionally relay communications and/or other information from a respective tree back to the distribution point/unit 140 (and optionally topside) which may include status reports of subsea well devices and the like. Each SCM 1701,2 is associated with two subsea electronics modules (SEMs) (not shown). It will be appreciated that an SCM 170 may alternatively be associated with one, three, four or more than four SEMs. It will be appreciated that the SEM may be referred to as an electronics module. It will be appreciated that alternatively, a Remotely Operated Vehicle (ROV) may instead provide a connection between the FPSO 130 and the SCM 170.

As shown schematically in Figure 1, the distribution unit 140 includes a power and communications distribution module (PCDM) 184. The PCDM 184 receives and relays different communications to the respective subsea wells 1201, 1202. The PCDM 184 may also process signals and communications and the like. The PCDM 184 also directs power to the subsea wells 1201, 1202 and components located therein (for example the respective SCMs 1701, 1702). It will be understood that the power is transmitted from topside via a respective umbilical 1351,2. It will be appreciated that, for the system illustrated in Figure 1, the communications are transmitted from topside via a respective umbilical 1351,2. Optionally, it will be appreciated that the power and/or communications may be transmitted to the PCDM 184 from further subsea modules or units, for example from further distribution units or the like that may be located in the same subsea site or may be located in further subsea sites. It will be appreciated that the PCDM may additionally relay communications from downstream devices such as one or more SCMs 170 or other devices located at subsea wells 120 (or located at any other part of the subsea field) to the MCU 132 located topside. This may be for monitoring the devices located in the subsea field or may be data recorded by sensors located at subsea locations or the like.

-14 -It will be appreciated that the PCDM 184, while shown to be located in the distribution unit 140 in Figure 1, may instead be located outside of the distribution unit 140. For example, the PCDM 184 may be located in a foundation separate to the distribution unit 140 or may be located in other subsea units such as in an electronic distribution unit (EDU) or the like. It will also be appreciated that, while only one PCDM 184 is illustrated in the distribution unit 140 of Figure 1, there may be multiple PCDMs located in the distribution unit 140 (for example for redundancy and/or for dual channel communications and/or the like) or located outside of the distribution unit 140. As shown in Figure 1, the PCDM illustrated includes an optical modem 188. The optical modem converts an electrical signal provided to the distribution unit 140 (and thus to the PCDM 184) via the respective umbilical 1351 that connects the distribution unit 140 to the topside (and the FPSO in the system of Figure 1). It will be understood that the optical modem is an example of a converter element that converts a first signal into an optical signal. It will be understood that conventional umbilical's sometimes carry hydraulic lines and electrical power and communications over copper wired connections and thus may not include optical channels to carry optical communications. This limits the available bandwidth for transmitting signals from topside to the distribution unit 140. The optical modem 188 converts electrical signals carried to the distribution unit 140 via electrical conductors in a respective umbilical (that optionally are copper conducing elements or the like) to an optical signal. It will be understood how optical signals (carried via fibreoptic elements and the like) can be more efficient than electrical based signals (carried via copper conductors and the like) and thus the bandwidth available downstream can be increased by utilising optical signals in a subsea communication architecture.

Figure 1 additionally helps show how the PCDM illustrated in Figure 1 includes a variable optical attenuator 192. The variable optical attenuator 192 shown in Figure 1 is a small form-factor pluggable (SFP) variable optical attenuator 192, however it will be appreciated that any other suitable variable optical attenuator may instead be utilised. It will be understood how use of a SFP variable optical attenuator 192 helps reduce the space necessary in a PCDM 184 for including a variable optical attenuator (as the space available in existing subsea modules can be limited) and can allow the SFP variable optical attenuator to be retrofitted to existing PCDMs or other subsea modules/apparatus. It will be understood that the variable optical attenuator reduces one or more parameters of an optical signal to a desired level. This can be achieved via modulating a parameter of the optical attenuator for example modulating a reflectivity of a inner wall of an optical passageway or via increasing a gap/cavity space region in an optical passageway or modulating the thickness of an optical passageway or the -15 -like. It will be appreciated how the variable optical attenuator 192 receives an optical signal (that was generated at the optical modem and that may or may not pass through other intermediate equipment of the PCDM 184) and reduces the magnitude (that is the amplitude or power or the like) of the optical signal to be at a desired level for output into the elongate fibreoptic element 180. The magnitude is an example of a signal parameter. Thus, it will be understood that one or more optical signals can be transmitted from the distribution unit 140 to the second subsea well 1202. It will be appreciated that conversely, electrical signals are transmitted from the distribution unit 140 to the first subsea well 1201. It will be understood that electric signals may also be transmitted to the second subsea well 1202 via further cables not shown in Figure 1, for example via flying leads and the like. It will be appreciated that the SCM 1702 disposed at the second subsea well will be adapted to receive and process optical signals. It will additionally be appreciated how this SCM 1702 may also transmit optical signals to the distribution unit 140 (which may be relayed back to the FPSO 130 optionally by converting the optical signals to electrical signals and transmitting these electrical signals to the FPSO via the respective umbilical) and may include information relating to subsea well operation.

It will be appreciated how the subsea fields 115, 116, 117 may be an existing subsea field or a brownfield site. It will be understood that the second subsea well 1202 may be an expansion region (that is to a say a region of the subsea site that is deployed more recently than the

remainder of the subsea field).

It will be appreciated that two PCDMs may be arranged in the distribution unit 140 to allow for redundancy and/or dual channel communications (despite only one PCDM 184 being illustrated in Figure 1). Optionally any other suitable number of PCDMs may be included in the SDU of Figure 1.

Aptly SPF variable optical attenuator 192 may be a M6200-SFPVOA, SFP variable optical attenuator module or the like. Aptly the variable optical attenuator 192 may have a dark or bright attenuation type, a variable attenuation of from around 0-20dB.

Figure 2 illustrates a schematic 200 of a subsea control system of the subsea sites 115, 116, 117 and FPSO 130. Whilst Figure 1 illustrates a physical system of topside and subsea apparatus, Figure 2 illustrates only a communication network associated with the physical -16 -system including a plurality of nodes. It will be appreciated that alternative subsea control system arrangements may be used instead.

The communication network is an interconnected group of nodes that can communicate with each other directly or indirectly via one or more connections, whereby the one or more connections may be wired connections, wireless connections, or the like. A node is a uniquely addressable location in the communication network. The node may include one or more processing elements. A topside node is a node that is associated with a physical location on/at the surface of a body of water. A subsea node is a node that is associated with a physical location under the surface of a body of water, at a depth of lm, 10m, 500m, 10km or more below the surface of a sea, lake, or the like. Alternatively, the topside node is a node that is physically located on/at the surface of a body of water. Alternatively, the subsea node is a node that is physically located under the surface of a body of water, e.g., at a depth of lm, 10m, 500m, 10km or more below the surface of a sea, lake, or the like. Alternatively, the topside node is an FPSO. Alternatively, the subsea node is an SCM, SEM, distribution unit, manifold, PCDM, UTA, distribution node, subsea tree, or the like.

It will be appreciated that the variable optical attenuator 192 attenuates an output optical signal from the PCDM 184 signal so that a desired magnitude of the optical signal received at the SCM 1702 at the second subsea well 1202 is at a desired level (or within a desired range). As signal loss can occur through a fibreoptic cable along its length, it will be understood how the variable optical attenuator can apply a degree of attenuation to an optical signal (or to a parameter of an optical signal) to be transmitted to the SCM 1702 to account for a distance of separation between the SCM 1702 and the PCDM 184 (thereby accounting for the length of the fibreoptic cable 180) so that the received signal at the SCM 1702 is received at the desired magnitude. Thus, the variable optical attenuator 192 helps accounts for distances between subsea modules and attenuation can be varied accordingly when installing new subsea modules in a subsea field for example.

It will also be appreciated that the efficiency of an optical transmission system can reduce over time. For example, the efficiency of a fibreoptic cable 180 may reduce over its service life in a subsea environment. Thus, it will be understood how, by utilising a variable optical attenuator 192 in a PCDM 184 (or another suitable subsea module or apparatus), an attenuation provided to an optical signal to be sent from the PCDM 180 can be reduced so that the magnitude of the signal received at the SCM 1702 is within the desired range or at the -17 -desired level. Thus, providing a variable optical attenuator in a subsea system can help account for degradation of efficiencies in optical apparatus over time.

In Figure 2 there is illustrated use of a network master 210. The illustrated network master 210 is located topside. The network master 210 is associated with the FPSO 130. It will be appreciated that the topside node may alternatively be a floating platform, a land platform, or the like. It will be appreciated that the network master 210 is the node running on the Master Control Station 132 (MCS). In other words, the network master 210 is a node of the MCS 132. As noted above the MCS 132 illustrated is a topside controller, so it will be appreciated that the network master node 210 may be associated with any topside controller. The network master node 210 communicates with three sub-networks 2201.3. A first sub-network 2201 corresponds to the first subsea site 115. A second sub-network 2202 corresponds to the second subsea site 116. A third sub-network 2203 corresponds to the third subsea site 117. It will be appreciated that the network master node 210 could alternatively communicate with and/or manage one, two, four, or more sub-networks 220. Communication of data between the network master node 210 and any sub-network 220 is facilitated by the umbilical 1351_3. It will be understood that the umbilical 135 is a communication link. The umbilical 1351_3 may include electrical conducting elements or the like for transferring information. It will be appreciated that the umbilical 1351_3 may include a fast copper connection, a direct subscriber line (DSL) connection, 100BaseT connection, other wired connection, wireless connection or the like. The first umbilical 1351 connects the network master node 210 to a first sub-network master 2401. The second umbilical 1352 connects the network master node 210 to a second sub-network master 2402. The third umbilical 1353 connects the network master node 210 to a third sub-network master 2403. The first cable 1531.2 connects one sub-network master 240,- 3 to another sub-network master 2401_3. The first cable 1531,2 may include a fibre optic connection and/or may include a fast copper connection, a direct subscriber line (DSL) connection, 100BaseT connection, other wired connection, wireless connection or the like. It will be appreciated that the first cable is communication link and optionally may additionally transmit power and the like.

The first sub-network master 2401 is a node of the sub-network 2201. It will be appreciated that the first sub-network master 2401 is the node running on the distribution unit 140 shown in Figure 1. The second sub-network master 2402 is a node of the sub-network 2202. It will be appreciated that the second sub-network master 2402 is the node running on a second distribution unit. The third sub-network master 2403 is a node of the sub-network 2203. It will -18 -be appreciated that the third sub-network master 2403 is the node running on a third distribution unit. It will be appreciated that alternatively, the sub-network masters 240 may be running on a manifold or a distribution unit or the like.

Figure 2 also shows a first sub-network node 2501. The first sub-network node 2501 is a node associated with the first subsea well 1201 shown in Figure 1. Similarly, a second sub-network node 2502 is a node associated with the second subsea well 1202. The remaining subsea subnetwork nodes 2503_9 are associated with subsea wells 120. The sub-networks 2201_3 are thus associated with numerous subsea wells 120. In other words, the subsea control system diagram 200 extends over nine subsea wells 120. The arrangement of the network master node 210, sub-network masters 2401_3 and sub-network nodes 2501_9 shown in Figure 2 may be described as a 'point-to-point hybrid'. That is to say, there is a mesh network between the network master node 210 and sub-network masters 2401_3. Aptly there is a treelstar network between the network master node 210 and sub-network masters 2401_3. There is a tree/star sub-network between a given sub-network master 2401_3 and its sub-network nodes 2501-9-Aptly there is a mesh network between a given sub-network master 2401_3 and its sub-network nodes 2501_9. In the tree/star network the given sub-network master 2401_3 is a central hub that each sub-network node 240 is connected. As shown in Figure 2, a number of the sub-network nodes are connected to a respective sub-network master via respective second cables 160 (eight shown). The second cables 160 are of a second cable type and is a copper connection.

The second cable 160 is an example communication link and may be a flying lead or the like.

Figure 2 shows how the sub-network note 1 2501 is connected to the sub-network 1 master 2401 via a fibreoptic cable 280. It will be understood that optical signals are transmitted between the sub-network note 1 2501 and the sub-network 1 master 2401 via the fibreoptic cable. It will be understood that a variable optical attenuator disposed at sub-network 1 master 2401 attenuates the optical signal sent from the sub-network 1 master 2401 so that an optical signal of desired strength is received at the sub-network note 1 2501. It will also be appreciated that the sub-network 1 master 2401 optionally includes a subsea modem (or other converter device) to convert an electrical signal to an optical signal to be attenuated and provided to the sub-network 1 master 2401.

It will be appreciated that the sub-network master 2401_3 is also called a master subsea node 2401_3. Aptly the master subsea node is a hierarchical position in the communication network.

Aptly the master subsea node is defined in the communication network depending on the task -19 -that is being performed. In some embodiments, the master subsea node may be a single subnetwork master 2401. In some embodiments the master subsea node may be a sub-network node 2501_9 running on a subsea tree. It will be appreciated that the sub-network node 2501-9 is also called a slave subsea node 2501_9. Aptly the slave subsea node is a hierarchical position in the communication network. Aptly the slave subsea node is defined in the communication network depending on the task that is being performed. In some embodiments, the slave subsea node may be one or more sub-network masters 2402,3. In some embodiments the slave subsea node may be only one or more of the sub-network nodes 2501_9. It will be appreciated that the first and second sub-network nodes 2501.2 are subordinate slave subsea nodes associated with the first master subsea node 2401. Similarly, any subordinate slave subsea nodes are slave subsea nodes 2501_9 associated with a respective master subsea node 2401,2,3. Communication links are provided between master subsea nodes 240 and slave subsea nodes 250.

Figure 3 illustrates a schematic representation of how optical communication signals may be generated and transmitted from the distribution unit 140 shown in Figure 1. It will be understood that the distribution unit 140 is a subsea distribution unit (SDU). It will be appreciated that the schematic shown in Figure 3 includes only a portion of the subsea field arrangement illustrated in Figure 1. As shown in Figure 3, an umbilical 135 extends from a topside location (that may be a FPSO or the like) to the subsea distribution unit (SDU) 140 disposed at a first subsea location. The umbilical 135 is terminated at an umbilical termination head (UTH) 312 that is disposed on an outer surface of the SDU 140. Alternatively, the UTH 312 may be located on a separate subsea device such as an umbilical termination assembly (UTA), for example, which may be connected to the SDU 140 via one or more further cables (which may be umbilical's or flying leads or the like). It will be understood that the umbilical provides signals such as communications and/or power to the SDU 140. The umbilical 135 shown in Figure 4 is a conventional umbilical and thus provides electrical signals (and optically other types of signals) to the SDU 140. It will be appreciated that the umbilical is an example of a subsea cable which provides communication signals to the SDU 140 in an electronic format.

Figure 3 shows how the SDU 140 includes two PCDMs, a first PCDM 1841 and a further PCDM 1842. Optionally any other number of PCDMs may be included in the SDU 140, for example one, three four or more PCDMs. The two PCDMs 1841, 1842 help provide redundancy and/or dual channel communications in the subsea network of Figure 3. The first PCDM 184 includes -20 -an optical modem 188 for converting the electrical communication signal provided through the umbilical 135 into optical signals. Optionally the second PCDM 1842 also includes an optical modem, receives electrical signals from the umbilical and converts these signals into optical signals. It will be appreciated that the PCDMs 1841, 1842 may additionally receive optical signals or electrical signals from subsea installations or apparatus such as subsea trees and manifolds and the like and may convert these signals to and from optical communication signals. The PCDMs 1841, 1842 may also relay electrical communications to the topside from the subsea network via the umbilical 135.

The first and further PCDMs 1841, 1842 also each include a variable optical attenuator 192 that optionally is a SFP variable optical attenuator. The variable optical attenuator 192 helps attenuate a parameter (such as the magnitude or the amplitude or the like) of optical signals that are output from the PCDMs 1841, 1842 of the SDU 140 to be transmitted further subsea elements, units, installations, or modules at a so that these optical signals are received by the further subsea units (or modules or the like) at a desired level, magnitude or amplitude. It will be understood that the level, magnitude or amplitude of the received optical signal at the further and downstream subsea units are examples of parameters of the optical signal. In the arrangement shown in Figure 3, it will be appreciated that both of the PCDMs 1841, 1842 output attenuated optical signals to downstream subsea field components in order to provide dual channel communications to and from subsea field components. However, it will be appreciated that optical signals may be transmitted out of the SDU 140 from the first PCDM 1841 only, or from the further PCDM 1842 only. Aptly, the first PCDM 1841 may output optical signals that are received by the further PCDM 1842. These signals may be attenuated via a variable optical attenuator located in the first PCDM 1841. Optionally no variable optical attenuator is located in either the first PCDM 1841 or the second PCDM 1842 (so that only one of the PCDMs 1841, 1842 shown includes a variable optical attenuator). In the system shown in Figure 3, both of the PCDMs 1841, 1842 each include a SPF variable optical attenuator 192 and thus attenuate respective unattenuated optical signals (within the respective PCDMs) at such that the optical signals that are transmitted or output from the respective PCDMs are a desired level. As illustrated, one or more optical communication lines 324 (that includes at least one fibreoptic line) is located in the SDU 140 to connect the first and further PCDMs 1841, 1842. Thus, the first and further PCDMs 1841, 1842can send optical signals to, and receive optical signals from, the other respective PCDM located in the SDU 140. The first and further PCDMs 1841, 1842can thus communicate via optical signals.

-21 -Figure 3 helps illustrate how the SDU 140 is connected to further subsea modules, units or components of a subsea field via a further optical communication line including at least one fibreoptic cable 180 that is an external optical communication line. This optical communication line is an external communication line and may be around 5 km or 10 km or 20 km or 25 km or more in length. It will be appreciated that the further subsea modules may be spaced apart from the SDU 140 by around 5 km or 10 km or 20 km or 25 km or more. As shown in Figure 3, the further subsea modules may be subsea manifolds 340 or production trees 342, 344, 346 or injection trees or the like. It will be understood that the subsea modules may be disposed at wellbores 120 shown in Figure 1. For example, individual trees 342 may be located at respective wellbores and a manifold 346 may be located to be in communication with each of the trees 342. Each of the trees and the manifold of Figure 3 may include a SCM 170, such as those shown in Figure 1 for example, for receiving and processing signals transmitted from the SDU 140. It will be understood that the SDU 140 may be connected to one or more further subsea module via respective fibreoptic lines. It will be appreciated that the fibreoptic lines may feed optical signals into a SCM 170 of a further subsea module. It will additionally be understood the SDU 140 may transmit power to the further subsea modules via further cables that may be separate to, or bundled with, the fibreoptic line 180.

It will be appreciated that the further subsea modules are disposed at one or more further subsea locations relative to the first location in which the SDU 140 is disposed. That is to say, the further subsea modules or units are spaced apart from the SDU 140 on the seabed.

It will be appreciated that alternatively, the umbilical 135 of Figure 3 may instead include a fibreoptic line. Thus, the umbilical may transmit optical communication signals from a topside location (for example from a MCU of a FPSO or the like) to the SDU 140. In this instance, the respective PCDMs 1841, 1842 receive optical signals from topside and thus may not include optical modems. Optionally PCDMs 1841, 1842 located in the SDU 140 may still include optical modems for modulating optical signals to be transmitted and demodulating received optical signals, for example from further subsea equipment (for example production trees 342 or manifolds 340). It will thus be appreciated that the respective variable optical attenuators 192 located in one or each of the first and further PCDMs 1841, 1842 thus attenuate optical signals to be transmitted to the further subsea modules 340, 342, 344, 346 that are based on, responsive to or comprise at least some of the optical signals transmitted to the SDU 140 from topside.

-22 -As a still further alternative, it will be appreciated that the umbilical 135 shown in Figure 3 may receive an optical signal from a topside location or from a further subsea module or device. The umbilical 135 in this instance thus includes at least one optical communication line that is optically a fibreoptic line. The umbilical thus then transmits optical signals to and from the SDU 140. The PCDMs 1841, 1842 located in the SDU 140 in this instance may instead include a modem or other conversion apparatus to convert the optical signal to a digital or electrical signal or the like to thereby distribute an electrical signal to the further subsea modules 340, 342, 344, 346. Optionally the optical signal received at the SDU 140 may be attenuated at an initial transmission location (that for example may be topside or a further subsea device) by a variable optical attenuator wherein the attenuation level is selected to account for a transmission length through the umbilical (based on the distance between the initial transmission location and the SDU 140) and/or the current efficiency of the optical system (for example the efficiency of the fibreoptic lines in the umbilical 135) and the like.

It will be appreciated that the signals (either electrical or optical) that are received by the PCDMs 1841, 1842 shown in Figure 3 are examples of input signals. It will be appreciated that the signals transmitted from the PCDMs 1841, 1842 shown in Figure 3 are examples of output signals.

Figure 4 illustrates the PCDM 184 of Figures 1 and 3 in more detail. It will be appreciated that the PCDM 184 includes an external housing that is not shown in in Figure 4. Figure 4 illustrates how the PCDM 184 includes a number of generally cylindrical modules 404, 408, 412, 416. One of the cylindrical modules 404 illustrated in the PCDM 184 a power distribution and control module 404 which is for managing power distribution to various components of a subsea field/system. A further one of the cylindrical modules 416 of the PCDM 184 shown is a central electronics module (CEM) 416 that is for managing communications to and from various components of a subsea field/system. The CEM 416 of Figure 4 includes a SFP variable optical attenuator 192 which allows the PCDM 184 to modulate parameters (for example a magnitude or amplitude or the like) of optical signals provided from the PCDM to downstream components of a subsea optical network, for example SCMs at subsea wells and the like, or to topside locations such as a master control station or MCU. It will be appreciated that optionally the CEM 416 of Figure 3 also Includes an optical modem 188 for converting electrical signals into optical signals or vice versa. Optionally the CEM 416 does not include an optical modem.

-23 -Figure 4 further illustrates how the SFP variable optical attenuator 192 shown in Figure 4 is connected to the CEM 416 via a lucent connector (LC) 440 of the attenuator 192. It will be understood that the CEM includes a further LC 444 that cooperates with the attenuator LC 440. For example the attenuator may include a plug and the CEM, may include a socket or vice versa or the like.

Figure 5 illustrates an alternative subsea optical network including two SDUs 508, 580 connected in series (or in a daisy-chain arrangement). Figure 5 additionally illustrates how variable optical attenuators can be arranged in a subsea module, for example a PCDM, included in a subsea network.

Figure 5 helps illustrate how topside equipment 504 which may include a FPSO and/or a MCS (optionally located at the FPSO) or the like can be connected to a first SDU 508 located in a subsea location via a first umbilical 512. Optionally the first SDU of Figure 5 may be similar to the SDU 140 illustrated in Figures 1 and 3. As is illustrated in Figure 5, the first SDU 504 is connected to the first umbilical 512 via respective wet mate connectors 516 disposed on the outer surface of the first SDU and the end of the first umbilical 512. It will be understood that the wet mate connectors 516 interact in a cooperative manner to connect the umbilical 508 to the first SDU 508. For example, the first SDU 508 may include a female wet mate connector that connects to a male wet mate connector at the terminal end of the first umbilical 508. It will be understood that the first SDU 508 is located at a first subsea location for example at a region of the seabed.

Figure 5 shows how the first SDU 508 includes a PCDM 520. It will be appreciated that the first SDU 508 may include more than one PCDM. It will be understood that the PCDM 520 may include various internal modules such as one or more central electronics module (CEM) 524. The PCDM illustrated in Figure 5 may be substantially similar to the PCDM 184 discussed with respect to Figures 1, 3 and 4. The CEM 524 or alternatively the whole PCDM 520 may be flooded with a fluid, such as a dielectric oil, to help withstand pressure at the subsea location and also to reduce prevent contamination/noise in signals transmitted therein.

Located within a CEM 524 of the PCDM 520 is a SFP variable optical attenuator 528. It will be appreciated that the SFP variable optical attenuator may be similar to the SFP variable optical attenuator discussed with respect to Figures 1, 3 and 4. The SFP variable optical attenuator 528 includes a lucent connector (LC) 532. It will be appreciated that the CEM 524 includes a corresponding LC (such as a LC socket or the like) for connecting to the SFP -24 -variable optical attenuator 528 such that the SFP variable optical attenuator 528 can be plugged into the CEM 524 of the PCDM 520.

It will be appreciated that the SFP 192 variable optical attenuator discussed with respect to Figures 1, 3 and 4 may also include a LC connector that cooperates with a corresponding LC connector disposed in the PCDM 184 CEM 414 shown in Figure 4.

Figure 5 shows how the SFP variable optical attenuator is in optical communication with a dry mate connector 536 of the PCDM 520 which in turn is in optical communication with a dry mate connector 540 of the first SDU 508. The dry mate connector 536 of the first SDU 508 is an internal dry mate connector 508 for connecting the PCDM 520 to the first SDU 508. That is to say that the dry mate connector is located within a housing of the SDU. It will be appreciated that the first SDU and PCDM optionally will include many more connectors and components than those shown schematically in Figure 5. The dry mate connector 540 of the first SDU 508 shown in Figure 5 is in optical communication with a further wet mate connector 544 located on an outer surface of the first SDU 508. It will be appreciated that an optical signal that is either received by the PCDM or generated inside the PCDM responsive to an input signal (for example by converting an electrical signal into an optical signal) is passed into and through the SFP variable optical attenuator 528 to attenuate the optical signal to a desired output level that is to be transmitted from the PCDM (and optionally from the SDU).

The optical signal is transmitted out of the first SDU 508 via the further wet mate connector 544. It will be appreciated that the optical signal passed to the SFP variable optical attenuator 528 (and therefore also the output signal from the first SDU 508) is responsive to at least one signal provided to the first SDU 508 by the umbilical 512 from topside 504. It will be understood that the umbilical may transmit communications and data to the first SDU 508 in electrical or optical form (or both). If communications are transmitted to the first SDU 508 (via the umbilical 512) the first SDU 508 may also include a converter device such as an optical modem for converting an electrical signal into an optical signal to provide to the SFP variable optical attenuator 528. It will be understood that communications may also be converted from an optical to electronic format for transmission through the umbilical to topside.

As shown in Figure 5, the further wet mate connector 544 of the first SDU 508 is connected to an external cable 548 that includes at least one optical/fibreoptic line. Optionally the external cable may be an umbilical type of cable or may be a flying lead or the like. Thus, it will be -25 -appreciated that the attenuated optical signal output from the first SDU 508 is transmitted into the external cable 548. As is illustrated in Figure 5, the external cable 548 is connected to a subsea umbilical termination assembly (SUTA) 552 via a wet mate connector arrangement 556 (or rather via a pair of cooperating wet mate connectors one of which is located at the SUTA and one of which is located at a terminal end of the external cable 548). The UTA includes a fusion splice to optically connect the fibreoptic line located in the external cable 548 to a fibreoptic line of a further umbilical 564 that is a subsea or infield umbilical. As shown in Figure 5, the first umbilical extends from the SUTA 552 to a further SUTA 568 that is located at a position in the subsea field spaced apart from the SUTA 552. The further SUTA 568 includes a further fusion splice 572 for connecting the fibreoptic line of the further umbilical 564 to a fibreoptic line of further external cable 576. The further external cable 576 is connected between the further SUTA 568 and a further SDU via respective wet mate connector pairs. As shown in Figure 5, the further SDU 580 is substantially the same as the first SDU 508. It will be appreciated that the further SDU 580 may be connected to a still further SDU via more optical cables in a daisy chain arrangement.

It will be understood that the optical signal (or a parameter of the optical signal such as amplitude or magnitude) is attenuated by the SFP variable optical attenuator 520 at the first SDU 508 such that the received signal at the further SDU 580 (and at a PCDM located in the further SDU) is at a decide level or within a desire range. It will be appreciated that use of the variable optical attenuator allows a controller to modulate the attenuation of the optical signal in response to transmission distance and system efficiency. For example, is a new subsea device, that is located between the first and further SDUs, was to be connected to the first SDU, and the further SDU (thereby being connected between the first and further SDUs), the optical attenuation of the signal from the first SDU can be attenuated further to account for a reduced transmission distance. This may occur for example in a subsea site expansion where a still further SDU (or other suitable apparatus) may be located between the first and further SDUs.

Although Figure 5 illustrates the variable optical attenuators being located in PCDMs. It was be appreciated that such device may instead be located in any other suitable subsea optical communication apparatus or units or the like.

It will be appreciated that optionally the further umbilical is connected between the first and further SDUs and no SUTAs are required.

-26 -Figure 6 illustrates a schematic representation of a still further arrangement of a subsea communication network 600. The arrangement shown in Figure 6 includes a number of SDUs connected in series (or in a daisy-chain arrangement). Figure 6 shows how an umbilical 602 connects a first SDU 604 to a topside location (for example a FPSO or a MCU that may be located in a FPSO or the like). It will be appreciated that the first SDU may be substantially similar to the SDUs 140, 508, 580 described with respect to Figures 1, 3 and 5. The umbilical 602 carries signals from the topside location to the first SDU 404 and optionally also carries signals from the first SDU 604 to the topside location. The umbilical of Figure 6 is a conventional umbilical and thus carries signals that include electrical signals (power and the like) to the first SDU 604 optionally via one or more electrical conducting elements such as copper wires/cables. Optionally any other suitable umbilical may instead be utilised. The umbilical 602 may additionally carry other types of signals and/or data information and the like. Figure 6 illustrates how the umbilical 602 is terminated at a UTH 606 located on the first SDU 604. The UTH may optically include one or more wet mate connectors and/or the like.

Optionally it will be appreciated that the umbilical 602 may be connected/terminated at a further subsea unit such as a UTA that is connected to the first SDU 604 via further cables and the like. It will be appreciated that the first SDU 604 is arranged at a fist subsea location that optionally is at a first location on the seabed.

Figure 6 illustrates how the first SDU 604 includes two PCDMs 608, 610. It will be appreciated that these PCDMs may be substantially similar to the PCDMs discussed with respect to Figures 1, 3, 4 or 5. The first PCDM 608 illustrated in Figure 5 includes an optical modem (or another converter element/device) for converting one or more electrical signals received at the first SDU 604 from the umbilical 602 into one or more optical signals. The first PCDM 608 thus is able to output an optical signal that is responsive to an input signal (that in this instance is an electrical communication signal) received at the first SDU 604 from the umbilical 602. Optionally the input signal may be a different type of signal. The second PCDM 610 of Figure 6 also includes an optical modem. It will be understood that the second PCDM 610 may not include an optical modem. The second PCDM 610 of Figure 6 additionally receives electrical communication signals (provided to the first SDU 604 from topside via the umbilical 602) and converts these to optical signals. It will be appreciated that the first and second PCDMs illustrated in Figure 6 are in optical communication via one or more optical communication lines (that are fibreoptic lines) located in the first SDU 604. Thus, it will be appreciated that the first PCDM 608 may output optical signals and transmit these to the second PCDM 610 and/or -27 -vice versa. The second PCDM 610 can thus receive optical signals from the first PCDM 608 and vice versa. It will be appreciated however that the second PCDM 610 may instead operate separately from the first PCDM 608 and may thus not receive optical signals from the first PCDM 608 or vice versa. Both the first and further PCDM5 608 and 610 can transmit an output signal that is an optical communication signal to topside and/or further subsea equipment responsive to signals that are input to each respective PCDM. This enables dual channel communications within the subsea system illustrated.

Figure 6 shows how the second PCDM 610 outputs optical signals, via a UTH 6062 into an elongate optical communication element 612 that in Figure 6 is a fibreoptic cable 612. It will be appreciated that the first PCDM is also able to transmit output optical signals to this UTH. The second PCDM includes a variable optical attenuator is a SFP variable optical attenuator. Optionally any other suitable variable optical attenuator may instead be utilised. An unattenuated optical signal within the second PCDM 610 (that is responsive to the input optical is signal provided by the first PCDM 608 to the second PCDM 610or that is received from topside via the umbilical 602) is thus passed through the variable optical attenuator in the second PCDM 410 to attenuate the optical signal (or more specifically to attenuate a parameter of the optical signal such as its magnitude or amplitude or the like) before the signal is passed into the fibreoptic cable 612. Additionally an unattenuated optical signal within the first PCDM 608 (that is responsive to the input optical signal provided by the second PCDM 610 to the first PCDM 608 or that is received from topside via the umbilical 602) can thus be passed through the variable optical attenuator in the first PCDM 608 to attenuate the optical signal (or more specifically to attenuate a parameter of the optical signal such as its magnitude or amplitude or the like) before the signal is passed into the fibreoptic cable 612 The fibreoptic cable 612 thus receives an attenuated optical signal. Figure 6 illustrates how the fibreoptic cable 612 is connected between respective UTA5 6062, 6063 at the first SDU 604 and a second SDU 616. It will be appreciated that the first SDU 604 is an example of a first subsea structure or unit or module and the second SDU 616 is an example of a second subsea unit or structure or module. It will also be appreciated that each PCDM illustrated in Figure 6 is an example of a subsea unit or module.

As discussed, the first PCDM 608 of Figure 6 includes a variable optical attenuator for attenuating the optical signal output or transmitted from the first PCDM 608. Thus output signal may be provided to the second PCDM 610. The output signal transmitted from the first PCDM 608 may not provide an optical signal to the second PCDM 610. The output signal -28 -may instead be provided, as an optical signal, to a further subsea module via the UTH 6062 and fibreoptic cable 612 or optionally via a further UTH and a further fibreoptic cable or the like that are connected to (or integrated with) the first SDU 604. The first PCDM 608, like the second PCDM 610, may thus attenuate an output optical signal to be transmitted to a further subsea module via the variable attenuator that is optionally located therein. Thus, first and second PCDMs 608, 610 may both transmit optical signals out of the SDU and to downstream components of the subsea field or only one of the PCDMs may be responsible for such transmission.

The second SDU 616 is located at a further subsea location that is spaced apart from the first subsea location at which the first SDU 604 resides. The first and further subsea locations may be spaced apart by the order of tens of km or more. As is shown in Figure 6, the second SDU 616 includes two PCDMs 620, 622 for example for redundancy and/or to enable dual channel communications and the like. Optionally one, three, four or more PCDMs may be located in the second SDU 616. The respective PCDMs 620, 622 in the second SDU 616 are third 620 and fourth 622 PCDMs. The third and fourth PCDMs receive optical signals from the first SDU 604. For dual channel communications, it will be appreciated that the third PCDM 620 of Figure 6 received optical signals from the first PCDM 608 and the fourth PCDM 622 received optical signals the second PCDM 610. Optionally the third and fourth PCDMs may each receive communication signals from any or both of the first and second PCDMs (and also from further PCDMs located downstream of the second SDU in the subsea field). Figure 6 also shows how a fifth 624 and sixth 626 PCDM are located in a first electronic distribution module (EDU) 628. The first EDU 628 shown in Figure 6 is a separate unit to the second SDU 616. It will be appreciated that the first EDU 628 may instead be integrated on or in the second SDU 616. Aptly the first EDU 628 may be spaced apart from the second SDU 616. Figure 6 illustrates how one or more flying leads 630 (that are optical flying leads) are connected between respective UTA5 6064, 6065 located on the second SDU 616. The flying leads 630 are additionally connected to the first EDU 628 and are in optical communication which the third 624 and fourth 626 PCDMs. The third and fourth PCDMs are additionally connected via optical communication elements (for example fibreoptic lines) within the first EDU 628. Thus, the third 620, fourth 622, fifth 624 and sixth 626 PCDMs are communicatively connected and can thus send optical signals to and receive optical signals from each other. It will be appreciated that the additional PDCMs 624, 626 located in the first EDU 628 enable the second SDU 616 to direct communications to more downstream subsea units/devices that could be managed by the second SDU 616 alone. Thus, the first EDU 628 may be a retrofitted -29 -unit to an existing subsea system (that is the subsea system shown in Figure 6) to allow the second SDU 616 to manage more downstream subsea apparatus than initially installed in the subsea system.

Figure 6 shows how the second SDU 616 is connected to downstream subsea apparatus via respective cables. For example, the second SDU is connected to a production manifold 632 and to a number of production tree assemblies 634 at various well heads and the like for extraction of hydrocarbons. Figure 6 shows how the second SDU 616 can be connected to such downstream apparatus via cables 635. Additionally, an infield umbilical 636 is connected to a further UTH 6066 located at the second SDU 616 and a remaining end of this umbilical 636 is terminated at a UTA 6067 associated with one or more injection tree 638 (or injection tree arrangement/ assembly). It will be appreciated that the first 620, second 622, third 624 and fourth 626 PCDMs may transmit optical signals to the downstream subsea apparatus. The infield umbilical 636 may thus include one or more fibreoptic lines. Additionally, the one is or more cable(s) 635 located between the second SSU 616 and the manifold 632 and/or production trees 634 may comprise one or more fibreoptic lines. It will be appreciated that the first 620, second 622, third 624 and/or fourth 626 PCDMs may include respective variable optical attenuators for attenuating optical signals to the downstream apparatus.

Alternatively, it will be appreciated that the first 620, second 622, third 624 and fourth 626 PCDMs may instead convert an input optical signal back to electrical/electronic based communication signals. Thus, the second SDU 616 may provide electrical signals to the downstream apparatus shown in Figure 6. It will be appreciated that in this instance, the first 620, second 622, third 624 and/or fourth 626 PCDMs include(s) a modem or other converter device for converting the input optical signal/signals transmitted to a respective PCDM into electrical signal(s). Thus, the downstream cable or cables 635 and infield umbilical may be electrical cables and may include for example one or more copper conducting elements. It will be appreciated that such an arrangement is advantages when providing optical communications to a brownfield site or to an established subsea site wherein existing downstream apparatus (such as injection tree's 638, productions tree's 643 and one or more manifold 632) is located which is configured receive electrical signals. Such a situation may occur when replacing communication protocols between SDUs in a subsea field with optical communication protocols to increase the available bandwidth. For example, this may be in order to introduce further SDUs into a subsea field, for example to manage communications -30 -in new subsea well regions when expanding a subsea field while retaining existing wellbore equipment that is configured to operate via electrical/electronic based communication signals.

Figure 6 also helps illustrate how the second SDU 616 is connected to a third SDU 640 via a further elongate fibreoptic communication element 642 that, in Figure 6, is a fibreoptic cable via respective UTHs 6065, 6067. The third SDU 640 includes a seventh 644 and eighth 648 PCDM. Figure 6 also illustrates how a second EDU 650 is included in the network shown in Figure 6. The second EDU 650 includes a nineth 652 and tenth 654 PCDM. It will be appreciated that the third SDU 640 and the second EDU are connected via flying leads (that are optical flying leads) in much the same way as described with respect to the second SDU 616 and the first EDU 628. It will be understood that the third SDU 640 and the second EDU 650 are substantially similar to the second SDU 616 and first EDU 650 respectively. It will additionally be appreciated that the third SDU transmits communications to downstream apparatus such as manifolds 660 production trees 662 and injections trees 664 and the like.

As shown in Figure 6, a fibreoptic cable extends between the second SDU 616 and the third SDU 640. It will be appreciated that the second SDU 616 can thus transmit optical signals to the third SDU 440 and optionally vice versa. It will be appreciated that optical signals transmitted from the PCDMs 620, 622 in the second SDU 616 and optionally those 624, 626 in the first EDU could be transmitted to the third SDU 640. Optionally some optical signals provided by the PCDMs in the first SDU 604 could pass through the second SDU 616 and be transmitted to the third SDU 640. It will be appreciated that the optical signals transmitted to the third SDU may be attenuated via one or more variable optical attenuators prior to transmission to the third SDU 640. It will be appreciated that one or more such variable optical attenuators may be located in the PCDMs located at the first SDU 604, or in the respective PCDMs located in the second SDU 616 and/or in the first EDU 626 dependent on where the optical signal (to be received at the third SDU 640) is to be transmitted. It will be understood that the variable optical attenuator(s) can be utilised to account for the distance over which optical transmission to the third SDU 640 occurs and/or to factor in efficiency losses in the communication network. It will be appreciated that relevant optical signals received at the third SDU are transmitted to respective PCDMs 644, 648, 652, 654 in the third SDU 640 and in the second EDU 650 for onward transmission to downstream subsea apparatus (that may be via optical or electrical transmission as previously discussed).

-31 -Figure 7 illustrates a schematic representation of how the first PCDM 608 of Figure 6 can receive and transmit signals. It will be appreciated that the PCDM is an example of a subsea module or subsea unit. The PCDM 608 is located in or at a SDU. Alternatively, the PCDM may not be located in/at a SDU and optionally may reside on/at its own foundation (that is a PCDM foundation or the like). It will be understood that alternatively any other suitable subsea module or unit may be utilised, for example a SCM or the like. Optionally the SDU may be considered as a subsea module or unit itself. It will be appreciated that the any of the other PCDMs illustrated in Figure 6, such as the second PCDM 610 of Figure 6, may operate in a similar manner as is shown in Figure 7. It will also be understood that the PCDMs discussed with respect to Figures 1, 3, 4 or 5 may also operate as described with respect to Figure 7.

As illustrated in Figure 7, the PCDM 608 receives an input signal 704 at a first communication interface 708 that is an input communication interface. The first communication interface receives signals and optionally also transmit signals. Optionally the first communication interface 708 is a wet mate connecter or a dry mate connector. It will be appreciated that the input signal 704 received at the PCDM 608 may be transmitted from topside via an umbilical. Optionally the input signal 704 received by the PCDM 608 is transmitted from subsea equipment such as a further PCDM or an SCM or the like and optically may be for transmission to topside. The input signal shown in Figure 7 is an electronic or electrical communication signal. It will however be appreciated that the input signal could be any other suitable type of signal, such as an optical signal or the like.

Figure 7 shows how an intermediate signal that is responsive to (or based on) the input signal 712 is transmitted to an optical modem 716. The intermediate signal may optionally be the input signal or may be a resulting signal derived from the processing or analysis or the like of the input signal by further PCDM components. The optical modem 716 receives the intermediate signal 712 and converts the intermediate signal 712, that is an electronic or electrical communication signal, into an optical communication signal. The optical modem 716 thus outputs an intermediate optical signal 719 that is responsive to (or based on) the intermediate signal 712. It will be appreciated that should the input signal be an optical signal, an optical modem may not be included in the PCDM 608 as the signal would not need to be converted into an optical signal (as it is already an optical signal). Aptly, an optical modem still may be included in order to modulate and demodulate respective optical signals.

-32 -An unattenuated optical signal 720 that is responsive to (or based on) the intermediate optical signal 719 is transmitted from the modem 716 is received by a SFP variable optical attenuator 724. This may be similar to the variable optical attenuator discussed with respect to Figure 1, 3, 4 or 5. It will be appreciated that the unattenuated optical signal 724 may be the intermediate optical signal or alternatively may be a signal resulting from processing of the intermediate optical signal via further PCDM 608 components (not shown). The variable optical attenuator 724 receives the unattenuated optical signal 712 and modules the signal to a desired level (so that at least one parameter of the attenuated signal is modulated to be at a desired level). Figure 7 illustrates how an output signal 732, that is an attenuated optical signal responsive to the unattenuated optical signal 720 is transmitted out of the PCDM via a further communication interface 732 (that is a wet mate or dry mate connector or the like). The output signal 728 is transmitted into a fibreoptic cable 736 (that may be located in an infield umbilical or a flying lead or the like) to be received by a further subsea module or unit. Figure 7 additionally illustrates how the further subsea module or unit 740 receives a received signal 744 that is an optical signal. It will be appreciated that the variable optical attenuator 724 modulates the parameter of the unattenuated optical signal 720 so that a corresponding parameter of the received optical signal 744 is at a predetermined value or level, or in a predetermined range, at the further subsea module or unit 740. It will be appreciated that the further subsea module/unit in Figure 7 is a PCDM disposed in a further SDU. Optionally the further subsea unit is a SCM or an SDU as a whole or the like. It will be appreciated that the PCDM 608 and the further subsea module/unit 740 are spaced apart in a subsea field.

A subsea communication system may optionally include one or more Subsea Control Module electrical flying leads (SCM EFLs).

It will be appreciated that the electrical signals referred to with respect to Figures 1 to 6 may refer to electronic communication signals and they may optionally be bidirectional signals.

It will be appreciated that the SFP variable optical attenuators discussed with regard to Figures 1 to 7 may act to modulate (that optically includes reducing) an amplitude and/or a magnitude and/or another predetermined parameter of an optical signal. This may be achieved by varying a characteristic of the attenuator itself. For example, this may include varying a physical property/characteristic or mechanical property/characteristic, or optical property/characteristic or arrangement of components within the optical attenuator. For example, the SFP variable optical attenuators may include a variable neutral density filter or -33 -the like. The filter may be controlled from a remote locator and may, for instance, include actuator or a motor or similar unit within the SFP variable optical attenuator. Optionally the filter may be automatically adjusted responsive to optical signals passing into and through the attenuator. A variable neutral density filter may, for example, include two or more polarising filters that are arranged in series. One or more of the polarising filters may be rotatable and such rotation of the one or more filters may vary the amount of light that can pass through the rotatable polarising filter. Thus, rotation of one or both of the at least two polarising filters can vary the magnitude of the optical signal provided through the polarising filters. It will be appreciated that the polarising filters may attenuate an optical signal by at least partly polarising light passing through the filters thereby effectively removing certain orientations of light wave propagation (that is to say limiting propagation of light waves oscillating in various orientations). It will thus be appreciated that rotating one or more polarising filters can alter the degree of polarisation of an optical signal passing through said filters.

Alternatively, the SFP variable optical attenuators described with respect to Figures 1 to 7 may include other variable characteristics. For example, the optical attenuators may include gaps in a transmission pathway of an attenuator through which light can escape from the transmission pathway. It will be appreciated that varying the size of the gap can vary the degree of loss of the optical signal (and thus vary the attenuation provided by the attenuator).

As an alternative example, the variable optical attenuator may include variably tapered inner surface of a transmission pathway that helps restrict passage of light through the pathway and/or reflects light out of the transmission pathway. It will be appreciated that by varying the degree of taper of the inner surface, more light can be removed from the transmission pathway (and thus attenuation of the optical signal could be increased). As a still further example, a reflectivity of an inner surface of a transmission pathway within the attenuator could be varied to thereby vary the attenuation provided by the attenuator. Another alternative may be inclusion of a refractive or reflective element in a variable optical attenuator whereby alteration of the orientation of the reflective or refractive element (via rotation of the element for example) in the attenuator may vary the attenuation of an optical signal provided by the SFP variable optical attenuator. Any other suitable method of varying attenuation of an optical signal could instead by utilised in the SFP variable optical attenuator. Of course, a combination of any attenuation methods described herein, or any other suitable attenuation methods, could also be employed in a SFP variable optical attenuator.

-34 -It will be appreciated that any of the SFP variable optical attenuators described with respect to Figures 1 to 7 may be configured to be managed from a topside location. That is to say the degree of attenuation provided by the variable optical attenuators may be managed by an operator at a MCS or the like at a topside location. That is to say attenuation of optical signals provided by the variable optical attenuators can be optimised at topside optionally at a MCS and the like. Optionally the variable optical attenuators may be automatically managed by software, for example at a MCS or the like.

It will be appreciated that the variable optical attenuators described with respect to Figures 1 to 7 may be managed by software from topside that optionally may be at least one proprietary software management interface in the MCS developed for network optimization based on data streaming performance (TX/RX data flow) and optical power input or output and optionally for creating a dynamic network to adapt required topology. It will be appreciated that the subsea modules such as PCDMs and the like (and components located therein such as SFP variable optical attenuators, SFP optical modems and the like) may be individually managed and/or controlled from topside. That is to independent electronic control may be embedded in each module and components thereof. Optionally a hardware assembly resides on the communications electronic module (CEM) inside the PCDM (Power & Communication Distribution Module) for remote control from the topside master control station (MCS). The hardware assembly may utilise a controlled SFP (small form factor pluggable) variable attenuator for optical power output management and developing software functional blocks to control, operate and help optimise the optical links or nodes for subsea network optimization and help ensure communications performance and quality over time throughout the life of field as well as to be able to adapt for future field development expansions without the need to swap modems or develop custom solutions. Optionally, an active optical subsystem is included in subsea modules including a SFP optical modem, attenuator. The system may include a mechanism to control the optical power output link between optical network nodes. Thus, the subsystem can tap into brownfields and greenfield projects combined with an all-electric program. It will be appreciated that the subsystem may help in reducing the associated costs to new or swap subsea communication modules allows the standardization of products for a wide range of applications and improves communication system performance.

Optionally the systems illustrated in Figures 1 to 7 include independent electronic control embedded on standard PCDM SFP optical modems with a software management interface (that optionally is a proprietary software management interface) in the master control station -35 - (MCS) for network optimization based on data streaming performance (TX/RX data flow) and optical power input/output for creating a dynamic network to adapt on any required topology.

Optionally the systems illustrated in Figures 1 to 7 include utilisation of a controlled SFP Variable Optical Attenuator, optical amplifier, and power metering sensor embedded on CEM/PCDM for power output management and network optimization to enable subsea field development to adapt to communications scheme changes or fluctuations throughout the life of the field.

It will be appreciated that the utilisation of optical communication systems or equipment as illustrated in Figures 1 to 7 can help increase bandwidth in a subsea optical communication network and can help enable or improve multiplexing (optionally via wavelength division multiplexing or the like) of optical signal to be transmitted to and/or from subsea modules from/to topside or between subsea modules.

It will be appreciated that any of the SFP variable optical attenuators described with respect to Figures 1 to 7 may optionally include a plug-like interface or a socket-like interface for interacting with a cooperating/mating socket/plug-like interface in a subsea module, sub-module or unit. Optionally, the SFP variable optical attenuator may include software for integrating the attenuator with a unit, module or sub-module into/to which the attenuator is provided so that the attenuator can receive and transmit signals from/to the unit module or sub-module. The SFP variable optical attenuator may optionally include internal circuitry for operation. For example, responsive to instructions provided to the attenuator, the attenuator may include requisite electronics and/or circuitry for operating a motor to vary/modulate attenuation of an optical signal provided to the attenuator.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to" and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

-36 -Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of the features and/or steps are mutually exclusive. The invention is not restricted to any details of any foregoing embodiments. The invention extends to any novel one, or novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

-37 -CLAIMS: 1. A method of determining a predetermined parameter of an optical signal in a subsea communication network, comprising the steps of: receiving an input signal at a first subsea module that is located at a first subsea location; and transmitting an unattenuated optical signal that is responsive to the input signal through a variable optical attenuator that is disposed at the first subsea module, thereby determining a predetermined parameter of a received optical signal at a further subsea module that is located at a further subsea location that is spaced apart from the first subsea location.
2. The method as claimed in claim 1, further comprising: modulating the predetermined parameter via the variable optical attenuator responsive to a distance between the first subsea module and the further subsea module.
3. The method as claimed in claim 1 or claim 2, further comprising: modulating the predetermined parameter via the variable optical attenuator responsive to the transmission efficiency of an elongate fibreoptic element connected between the first subsea module and the further subsea module.
4. The method as claimed in claim 3, further comprising: varying at least one characteristic of the variable optical attenuator thereby increasing the predetermined parameter over the service lifetime of the elongate fibreoptic element to account for a decrease in efficiency of the elongate fibreoptic element.
5. The method as claimed in any preceding claim, further comprising: transmitting an optical output signal responsive to the unattenuated optical signal from the first subsea module to a further subsea module via an elongate fibreoptic element thereby providing the received optical signal at the further subsea module.
6. The method as claimed in any preceding claim, further comprising: -38 -providing the unattenuated optical signal from the input signal that is an optical signal.
7. The method as claimed in any preceding claim, further comprising: providing the unattenuated optical signal via converting the input signal into an optical signal.
8. The method as claimed in any preceding claim, further comprising: transmitting the optical output signal from an established region of a brownfield site at the first subsea location to an expansion region at the further subsea location.
9. The method as claimed in any preceding claim wherein the input signal is an optical signal, further comprising: performing optical signal analysis based on the input signal that is an optical signal.
10. Apparatus for determining a predetermined parameter of an optical signal in a subsea communication network, comprising: a first communication interface disposed at a housing of a first subsea module, for receiving an input signal; a variable optical attenuator for modulating an unattenuated optical signal disposed within the housing thereby providing a modulated optical signal; and a further communication interface disposed at the housing for transmitting an output optical signal responsive to the modulated optical signal.
11. The apparatus as claimed in claim 10, further comprising: the first subsea module is a power and communication distribution module (PCDM) usable in in a communication network of a subsea hydrocarbon production system.
12. The apparatus as claimed in claim 10 or claim 11, further comprising: an optical modem in communication between the first communication interface and the variable optical attenuator for converting the input signal into a pre-transmission optical signal upon which the output signal is responsive.
-39 - 13. The apparatus as claimed in any one of claims 10 to 12, further comprising: the variable optical attenuator is a small form-factor pluggable (SFP) variable optical attenuator.
14. The apparatus as claimed in any one of claims 10 to 13, further comprising: the variable optical attenuator is for modulating a predetermined characteristic of the output optical signal to account for a distance between the first subsea module and a further subsea module to which the output optical signal is transmitted via an elongate fibreoptic element and/or the variable optical attenuator is for modulating a predetermined characteristic of the output optical signal to account for a reduction in efficiency of an elongate fibreoptic element, into which the output optical signal is transmitted, over the service life of the elongate fibreoptic element.
15. The apparatus as claimed in claim 14, further comprising: the variable optical attenuator is for determining a predetermined parameter of a received optical signal at the further subsea module.
16. A subsea system for transmitting communication signals to subsea hydrocarbon production equipment, comprising: a master control station (MCS) located at a topside location; at least one communication distribution unit (CDU) or subsea distribution unit (SDU) connected to the MCS via at least one umbilical; at least one power and communication distribution module (PCDM) disposed at the SDU or CDU or at a PCDM foundation; a variable optical attenuator disposed at the PCDM; and a further subsea module for receiving the output optical signal that is spaced apart from the PCDM and is communicatively connected to the PCDM via at least one elongate fibreoptic element.
17. The subsea system as claimed in claim 16, further comprising: the further subsea module is a subsea control module (SCM) of a subsea tree for operating subsea hydrocarbon production equipment.
18. The subsea system as claimed in claim 16 or claim 17, further comprising: -40 -an optical modem disposed in the PCDM and communicatively connected between the first communication interface and the variable optical attenuator for converting the input signal, that is an electrical and/or a hydraulic signal, into an optical signal.
19. The subsea system as claimed in claim any one of claims 16 to 18 further comprising: the PCDM is disposed in a brownfield region of a subsea hydrocarbon production system and the further subsea module is disposed in a more recent expansion region of the subsea hydrocarbon production system.
20. The subsea system as claimed in claim any one of claims 16 to 19 further comprising: a network communication device that is disposed subsea to send or gather data from various subsea equipment including sensors and instruments.