GB2098945A - Steadying a platform in a vessel on heaving seas - Google Patents

Abstract

A moonpool structure is located within a ship 10 or comprises or forms part of a leg of a semi-submersible. The upper end of the moonpool is enclosed and a pontoon 15 floats on the sea surface 12 within the moonpool. Means is provided to vary the air pressure of the air space 14 within the moonpool to keep the sea surface and so the pontoon at a steady vertical distance from the sea bed 36; this means comprises an air inlet/outlet 30 fed by a blower or compressor 32 via a valve 33 controlled by an electronic control 31. The control 31 is fed data from an air pressure sensor in the air space 14, and a wave gauge 35 which senses wave pressure in the ambient sea close to the ship's hull at the bottom of the moonpool. Alternatively, rams or springs act on the pontoon to retain it against vertical movement. The pontoon 15 supports a work platform 20 above the moonpool or houses work areas, or forms a land platform or a runway or couples with work apparatus so by retaining the pontoon vertically stationary a stable work area is provided on a heaving vessel. The platform 20 may be supported by more than one moonpool structure which cooperate to retain the platform steady. <IMAGE>

Description

SPECIFICATION Marine vessels This invention relates to marine vessels, which term includes ships, semi-submersibles, tethered platforms and other floating offshore structures such as wave-energy structures, and more especially, to marine vessels which have a 'moonpool', i.e. a vertical passage within the vessel, normally open at its upper end, and open at its lower end to the sea to provide a column of sea water within the vessel.

A vessel operating in a seaway is subjected to forces of wave current and wind and its responses depend, to a large extent on the design characteristics of the vessel. Thus the vessel may heave, roll or pitch and for effective working when the vessel is stationary, it is essential for the heaving motion to be as small as possible.

Proposals have been made for stabilizing a vessel i.e. heave compensating, by transferring water ballast in a controlled manner from one side of the vessel to the other, (UK 1 603224/US 4261277); also known from US 4176614 is a means of stabilizing a semi-submerisible by air pressure control within the legs of the vessel. US 4176614 also discloses air pressure control within a moonpool to damp oscillations of the water column within the moonpool.

The above prior art requires that the entire vessel be stabilized and the reference to the pressurised moonpool is an independent embodiment unconnected with stabilizing the vessel.

It is an object of this invention to provide a vessel in which a selected area can be stabilized, i.e. retained vertically stationary to the sea bed independently of the vessel's motion.

According to one aspect of the present invention, there is provided a moonpool structure for the marine vessel, comprising a moonpool which is enclosed at its upper end and has means for controlling air pressure within the air space above the sea surface therein, a pontoon which floats on the sea surface within the moonpool and supports a work area so that, by controlling the air pressure in the air space in the moonpool the pontoon and consequently the work area is retained at a steady vertical distance from the sea bed independently of the vessel's motion.

Preferably, the pontoon supports a work area above the enclosed moonpool structure, said work area being mounted on the pontoon by one or more pillars which pass through substantially air tight aperture means in the moonpool enclosure.

Preferably also, the means for controlling the air pressure in the air space of the moonpool comprises air pressure sensing means within the moonpool to monitor the air pressure in said air space, wave pressure sensing means adjacent to the vessel to monitor wave pressure, electronic control means for assessing the pressure readings, and actuating means controlled by said control means to compensate for fluctuations of pressure within said air space.

According to another aspect of the present invention there is provided a method of retaining vertically stationary relative to the sea bed a platform floating in the sea surface within an enclosed moonpool, the method comprising the steps of sensing the air pressure in the air space of the moonpool and simultaneously sensing the wave pressure of the ambient sea, electronically calculating the difference, namely the pressure due to the vessel's heaving motion and actuating means to compensate for said difference whereby the pontoon is retained steady relative to the sea bed.

According to another aspect of the present invention there is provided a marine vessel which has a moonpool structure comprising; a moonpool the upper end of which is enclosed and substantially air-tight; an air inlet and an outlet communicating with the air space above the sea surface within the moonpool structure and means for supplying air to and withdrawing air from said air space; a pontoon floating on the sea surface within the moonpool and supporting a platform structure above the moonpool by support means extending slidably through the enclosing upper end of the mqonpool structure, sensor means to determine the pressure of the air in the air space and the wave pressure of the ambient sea, and control means linked to said sensor means to cause injection or withdrawal or air into or from the air space in the moonpool to compensate for fluctuations of pressure in said air space, whereby the platform structure is maintained vertically stationary relative to the sea bed independently of the vessel's motion.

According to another aspect of the present invention, there is provided a method of retaining the platform structure of a vessel as defined in the preceding paragraph, vertically stationary relative to the sea bed, comprising the steps of sensing the air pressure in the air space of the moonpool, and simultaneously sensing the wave pressure of the ambient sea, electronically calculating the difference, namely the pressure due to the vessel's heaving motion, and injecting or withdrawing air into or from the air space to compensate for said difference so that the air pressure in the air space equates with the wave pressure whereby the sea surface with the moonpool, the pontoon thereon and the platform structure supported by the pontoon are retained vertically stationary relative to the sea bed independently of the vessel's motion.

The platform structure may be a deck area, an accommodation area, a crane mounting, a drilling platform or a hold.

The marine vessel may be a ship within which a moonpool structure is located at any suitable location, e.g. amidships or at the stern; two or more moonpool structures may be provided, for use separately or in conjunction. The marine vessel may be a semi-submersible wherein one or more legs incorporate or comprise a moonpool structure where more than one moonpool structure is provided, they may be used separately or in conjunction. The marine vessel may be dynamically tethered drilling platform one or more legs of which incorporate' or comprise a moonpool structure with the platform structure supported by the pontoons in the or each moonpool structure.

Embodiments of the present invention will now be described, by way of examples, with reference to the accompanying drawings in which: Fig. 1 illustrates a marine research vessel according to the invention from which a submersible can be launched and recovered; Fig. lA illustrates a modified vessel; Fig. 2 illustrates a drill ship according to the invention; Fig. 3 illustrates a tethered rig according to the invention; Fig. 4 illustrates a rig supply ship according to the invention; Fig. 5 illustrates a moored vessel connected by a flow line to a subsea production unit; and Fig. 6 illustrates a moored vessel connected by a flow line to a supply vessel.

Referring firstly to Fig. 1, there is shown a marine support vessel 10 which incorporates a moonpool structure 11, i.e. a well which opens at its lower end into the sea. Thus, sea water is contained in the moonpool the surface 1 2 being at a mean level with the ambient sea 13. The upper end of the moonpool structure is closed to provide an air space 14 above the sea surface in the moonpool. A pontoon 1 5 floats on said sea surface.

A platform structure 20 is located above the moonpool and is supported by one or more pillars 21 which extend from the pontoon 15 through the top 1 6 of the moonpool. The pillar 21 is slidable through an aperture in said cover, the aperture being as air-tight as possible but flexible enough to allow the vessel's angular motion to take place without causing excessive stress on the support pillar 21. The aperture may have, for example, a collar of bristles which allow for angular movement of the pillar, or an apertured plate may be mounted in a universal joint. The platform structure 20, in this embodiment, carries a crane 22 by which a submersible 23 can be launched and recovered.

If the vessel is subjected to heaving, due to wave motion as a vessel often is when stopped or at anchor, the crane would normally also be subject to vertical motion because the sea surface in the moonpool would rise and fall with the vessel (though this movement would be considerably more if the moonpool was open to atmosphere). In some circumstances the crane movement may be so severe that launch or recovery of the submersible would become difficult or dangerous.

In accordance with the invention, however, the crane 22 will not be subjected to the vessel's heaving motion, because means is provided to retain the pontoon stationary. Said means includes an air inlet/outlet 30 by which air can be injected into or withdrawn from the air space 14, and an electronic control 31 for causing air to be injected or withdrawn. The air inlet/outlet 30 communicates with a chamber 32 which may be a blower or compressor via a valve 33 which is actuated by the control 31. The control 31 is supplied with data from a pressure sensor 34 which senses the air pressure in the air space 14 and a wave gauge 35 which senses the wave pressure in the ambient sea, close to the ship's hull and preferably at the same depth as the bottom of the moonpool and preferably at the entrance thereto.

The information needed to enable the correct amount of air to be put into or drawn from the air space by the blower or compressor is the pressure change due to heaving of the vessel, wave pressure at the underwater entrance to the moonpool and pressure inside the air space. The calculations are done with the aid of computers.

The pressure change is based on the equation P 1Va=P2V2.

When P1 ar,d V1 are the initial pressure and volume P2 and V2 are the new pressure and volume.

When the vessel heaves with an amplitude the relation becomes Heave upwards,

Heave downwards

Where I is the distance of the sea surface at P,V, from the top of the moonpool.

The pressure change is therefor P,6 P=P2-P 1= (heave upwards) (I-s) --P,6 P= (heave downwards) (I+s) From the data given, the control 31 can calculate the difference between the wave pressure and the air pressure in the air space 14, and thus deduce the air pressure in the air space which is due to the vessel's heave. Air can be injected into or withdrawn from the air space to compensate for that pressure, which will fluctuate with the heave motion of the vessel. Thus, while the control means is operative, there is a continuous variation of pressure in the air space and continuous compensatory injection or withdrawal of air so that the sea surface in the moonpool is kept constant relative to the sea bed 36 or other selected reference point.

Consequently, so also is the pontoon and the crane platform. A submersible tethered to the crane will also avoid heaving and this will be beneficial to the crew of the submersible when it is under water; also there will be less strain on the support cable.

Another application of the arrangement of this embodiment is the provision of a steady underwater work platform supported by the ship's crane whereby, for example, underwater repair or maintenance work can be done on an offshore rig or other fixed structure.

When the pressure control means is inactive the platform structure will rise and fall with the heave of the vessel; it can be made fast to the vessel. It will be appreciated that the pressure control means need only be active when the vessel is heaving and it is desirable to retain the work area steady relative to the sea bed.

The air inletioutlet for each moonpool structure may include a plurality of ducts opening into the air space 14 for even distribution of the air injected or withdrawn.

In a modified version of this embodiment, the crane 22' is mounted on a platform 20' supported in a column 21' carried by the pontoon 15, and a vertical shaft 1 7 passes through the platform, column and pontoon. A cable 1 8 passes through this vertical shaft for light-duty underwater work; for example the lower end of the cable can carry a camera or sensor 19, or a hook or jaws for lifting or operating valves.

The vertical shaft 17 can be capped when not in use to prevent or reduce surging of the column of water therein. During use, the shaft can be capped by a ring of resilient material such as a collar of bristles, through the centre of which the cable can move.

Tests have indicated that the vertical shaft 1 7 does not adversely affect the efficiency of the apparatus because the enclosed air space 14' above the pontoon still provides the stabilizing effect thereon via the monitored air pressure.

In a second embodiment, Fig. 2, the vessel 40 is a drill ship having a drilling platform 41 on which a drilling unit 42 is located. During drilling operations, a riser 43 depends from the rig through a well or open moonpool 44 in the vessel, and extends down to the sea bed 36.

Normally, the drilling equipment in such a a vessel would include heave compensators so that, when the vessel was subjected to heave, the riser, which would otherwise be subjected to the same vertical motion as the vessel, would have a reduced amount of vertical motion. In such conditions drilling could continue, but in severe adverse wave conditions, the heave of the vessel may be so great that the heave compensators could not function. In such conditions drilling would have to be halted because otherwise damage to the drill riser would result.

In accordance with the invention, drill platform 41, during drilling, can remain vertically stationary relative to the sea bed so that the drill riser is not subject to adverse vertical motion. Thus, heave compensators are not required, and drilling can continue even when the heave of the vessel would for known vessels lead to a shutdown of the drilling operation.

The platform 41 is supported by pillars 21A each of which is carried by a pontoon 1 5A in a closed moonpool 11 A. There may be two or more (e.g. four) moonpools 1 1A and associated parts 1 5A, 21 A, and each moonpool is equipped as hereinbefore described in the first embodiment with means 30A-35Afor controlling the pressure in the air space of the moonpool. The control means for each moonpool operates independently of the other or others and as a result the platform structure supported by the pontoons remains steady at a fixed vertical height from the sea bed 36.

In a third embodiment, Fig. 3, the vessel is a tethered drilling platform such as a tension leg platform 50 which is anchored to the sea bed 36 by vertical mooring lines 51 connected to the legs 52 of the tethered platform 50. As in the second embodiment, a drill platform 53 is provided which is supported by two or more (e.g. four) pontoons 15B each mounted in a sealed moonpool 11 B formed by legs 52 of the tethered platform 50, which are open at their lower ends.

Each moonpool has its associated pressure control means 3OB-33B.

As in the second embodiment, the control means 3OB-35B for each moonpool, when in use, will retain the drilling platform 53 at a s.tationary vertical distance from the sea bed when the tethered platform is subjected to heave.

The apparatus described in the third embodiment in connection with a tethered drilling platform is also applicable to dynamically positioned drilling platforms and semisubmersibles generally.

In a fourth embodiment, Fig. 4, a supply vessel 60 is provided with a closed moonpool 11 C situated aft. A platform structure 20C is located above the moonpool and carried by a pontoon 15C within the moonpool. The associated pressure control means 30C--35C as described more fully in the first embodiment is also provided.

Thus, when cargo 61 is being handled between the vessel 60 and a fixed structure such as an offshore rig 62, and the vessel is subjected to heave, the pressure control means can be operated to retain the platform 20C steady, i.e.

vertically stationary relative to the sea bed 36.

The platform structure 20C may be part of an upper, open, deck or it may comprise a hold.

The apparatus of the invention can also be used for connecting a flow line to a vessel, the connection being made on the steady work area so that the flow line is not subjected to constant movement and consequently stress on the line is reduced. Two examples of this are shown in Figs.

5 and 6.

In the embodiment described with reference to Fig. 5, the pontoon 70 is a hollow structure supporting an internal work area containing pumping machinery (not shown). The flow line 71 is connected between a coupling on the pontoon and a subsea production unit 72, and the pontoon 70 is coupled by a flexible connector 73 to a processing plant 74 on board the vessel, which is moored over the subsea unit 72. The pontoon is retained steady when required by control means 30D to 35D.

In the embodiment described with reference to Fig. 6, a flow line 75 is coupled between two vessels, a supply vessel 76 and a moored processing plant vessel 77. Each vessel has apparatus according to the invention so that the connecting points for the flow line 75 can be held steady by actuation of the control means 30E35E. Flexible connectors 80 are provided between each vessel's platform structure 20E and its supply unit 82 or plant 83.

The above described embodiments are only a few of the many applications to which the apparatus of the invention can be put. For example, the platform structure supported by the pontoon may be a helicopter landing pad; or it may support a crew accommodation area.

One or more legs of a semi-submersible or tethered platform may provide the moonpool for such purposes or for supporting a crane platform.

A floating runway for short take-off aircraft may also benefit from the present invention: by providing a series of moonpools along each side of the runway, it can be retained steady when the supporting structure is subjected to heave. The supporting structure may be a converted tanker hull, or an elongate semi-submersible structure.

The apparatus of the invention may also be incorporated in conventional aircraft carrying vessels for the purpose of providing a steady flight deck.

The term marine vessel includes wave energy structures which may also incorporate apparatus according to the invention.

Where a hollow pontoon is provided as illustrated in Fig. 5, watertight hatches are provided therein so that the work area within the pontoon can be manned when appropriate.

In a modification of the apparatus of the invention, not shown, the air inlet/outlet means 30 may be replaced by other means designed to retain the platform/pontoon steady relative to the sea bed. For example, air controlled rams or springs may be mounted vertically with the air space of the moonpool to engage the pontoon.

The pressure sensing means and electronic control will then adjust the air pressure acting on the rams or spring in accordance with pressure fluctuations in the moonpool air space and so hold the pontoon steady. Alternatively, the rams or springs may be outside the moonpool, and connected to the platform structure, so as to control the position thereof relative to the heaving vessel in accordance with pressure fluctuations within the moonpool.

Claims (14)

Claims

1. A moonpool structure for a marine vessel, comprising a moonpool which is enclosed at its upper end and has means for controlling air pressure within the air space above the sea surface therein, a pontoon which floats on the sea surface within the moonpool and supports a work area so that, by controlling the air pressure in the air space in the moonpool, the pontoon and consequently the work area is retained at a steady vertical distance from the sea bed independently of the vessel's motion.

2. A structure as claimed in claim 1 in which the pontoon supports a work area above the enclosed moonpool structure, said work area being mounted on the pontoon by one or more pillars which pass through substantially airtight aperture means in the moonpool enclosure.

3. A structure as claimed in claim 1 in which the pontoon is a hollow structure forming an internal work area therein.

4. A structure as claimed in claim 1, 2 or 3, in which the means for controlling air pressure in the air space on the moonpool comprises air pressure sensing meahs within the moonpool to monitor the air pressure in said air space, wave pressure sensing means adjacent to the vessel to monitor wave pressure, electronic control means for assessing the pressure readings, and actuating means controlled by said control means to compensate for fluctuations of pressure within said air space.

5. A structure as claimed in claim 4 in which the electronic control means actuates an air supply line to insert or withdraw air into or from said air space.

6. A structure as claimed in claim 4 in which the electronic control means actuates holding means engaging the pontoon and restraining the pontoon against vertical motion.

7. A structure as claimed in claim 3 in which the pillar has a vertical shaft therethrough.

8. A method of retaining vertically stationary relative to the sea bed a platform floating in the sea surface within an enclosed moonpool, the method comprising the steps of sensing the air pressure in the air space of the moonpool and simultaneously sensing the wave pressure of the ambient sea, electronically calculating the difference, namely the pressure due to the vessel's heaving motion and actuating means to compensate for said difference whereby the pontoon is retained steady relative to the sea bed independently of the vessel's motion.

9. A marine vessel which has; a moonpool structure the upper end of which is enclosed and substantially air-tight; an air inlet and an outlet communicating with the air space above the sea surface within the moonpool structure and means for supplying air to and withdrawing air from said air space; a pontoon which floats on the sea surface within the moonpool and supports a platform structure above the moonpool by support means extending slidably through the enclosing upper end of the moonpool structure, sensor means to determine the pressure of the air in the air space and the wave press'ure of the ambient sea, and control means linked to said sensor means to cause injection or withdrawal of air into or from the air space in the moonpool to compensate for fluctuations of pressure in said air space, whereby the platform structure is maintained vertically stationary relative to the sea bed independently of the vessel's motion.

1 0. A vessel as claimed in claim 9 comprising a ship within which at least one moonpool structure is provided.

11. A vessel as claimed in claim 9 comprising a semi-submersible wherein at least one leg thereof incorporates or comprises a moonpool structure.

12. A vessel as claimed in claim 9 comprising a dynamically tethered drilling platform at least one leg of which incorporates or comprises a moor\poo\ structure.

13. A vessel as claimed in claim 10,11 or 12 in which a plurality of moonpool structures are provided.

14. A vessel as claimed in claim 13 in which the moonpool structures are independent of each other.

1 5. A vessel as claimed in claim 13 in which at least two moonpool structures are interrelated for use in conjunction with each other to stabilize a platform supported by said interrelated structures.

1 6. A method of retaining the platform structure of a vessel as defined in the preceding paragraph, vertically stationary relative to the sea bed, the method comprising the steps of sensing the air pressure in the air space of the moonpool, and simultaneously sensing the wave pressure of the ambient sea, electronically calculating the difference, namely the pressure due to the vessel's heaving motion, and injecting or withdrawing air into or from the air space to compensate for said difference so that the air pressure in the air space equates with the wave pressure whereby the sea surface with the moonpool, the pontoon thereon and the platform structure supported by the pontoon are retained vertically stationary relative to the sea bed.

1 7. A marine vessel substantially as hereinbefore described with reference to the accompanying drawings.