WO2004011837A1

WO2004011837A1 - Method and apparatus for laying/recovery of lines on a seabed

Info

Publication number: WO2004011837A1
Application number: PCT/GB2003/003280
Authority: WO
Inventors: Craig Lang
Original assignee: NATURALAY TECHNOLOGIES Ltd
Current assignee: NATURALAY TECHNOLOGIES Ltd
Priority date: 2002-07-30
Filing date: 2003-07-29
Publication date: 2004-02-05
Anticipated expiration: 2005-01-30
Also published as: GB2405680B; AU2003255731A1; GB2405680A; GB0217511D0; GB0500731D0

Abstract

There is disclosed a method and apparatus for laying a line on a seabed, or retrieving a line from a seabed, via a surface vessel on which the apparatus is mounted, the apparatus comprising: a drum which is rotatable about an axis which is substantially parallel to the general plane of the deck of the vessel, and which is capable of reeling-in a length of line, or unreeling a length of line; means for guiding the line to and or from the drum; bearings for supporting the drum and any line stored thereon; a tank in which the drum is located, and which is capable of being charged with liquid in order to exert an upthrust on the drum and thereby relieve at least part of the load of the drum and line which is borne by the bearings; and control means for varying the volume of liquid in the tank according to the instantaneous amount of line on the drum.

Description

METHOD AND APPARATUS FOR LAYING / RECOVERY OF LINES ON A SEABED

This invention relates to a method and apparatus for laying lines on a seabed and / or for recovery of a line from a seabed.

The invention has been developed primarily in connection with the laying / recovery of pipelines for conveying liquid and / or gaseous hydrocarbons, and which vary between relatively flexible types of lines or "hoses" of plastics material of relatively small diameter, up to relatively rigid types of lines in the form of rigid pipes of steel and which are of substantial diameter, typically 12 to 24 inch in diameter.

Despite the more rigid nature of a steel pipe of substantial diameter, as compared with a flexible hose type of pipeline, pipelines of both types can be wound onto a storage drum which is mounted on the deck of a pipe-laying vessel or barge, and which can then be unreeled from the drum in order to be laid on the seabed.

However, while the invention is primarily concerned with the laying / recovery of flowlines i.e. lines which are intended to convey liquid and / or gaseous fluid, it should be understood that the method and apparatus of the invention may be used to handle other types of line, such as particularly telecommunication cables.

Reference will now be made to Figures 1 to 3 of the accompanying drawings, which are diagrammatic illustrations of existing methods of laying pipelines on the seabed.

Thus, current technology for laying pipes on the seabed, both of the relatively rigid steel type as well as composite flexible pipes, is derived from a method used for laying steel pipes wherein the pipe is assembled on the deck of a ship, held in a tracked tensioner, supported by a support member referred to as a "stinger" (floating or on the deck) , and lowered in a catenary to the seabed. This is illustrated in Figure 1 of the drawings, which shows diagrammatically in side elevation the method in operation. The seabed is denoted by reference 1, the surface of the sea by reference 2 , the deck of a ship by reference 3 , and reference 4 denotes a steel pipe being laid on the seabed 1. A tracked tensioner 5 holds the pipe 4, and a support member 6 (which is semi-buoyant) - the "stinger" supports the pipe 4 as it is being lowered to the seabed.

Various developments of this have occurred in order to speed-up the process and to increase its depth laying capability.

Briefly, these developments have included the storage of products on reels, carousels, baskets and longer tensioners to compensate for the increased weight in deep water. Examples of these are the Apache and the NOS DYNLAY system. The known Apache method is illustrated in Figure 2 of the drawings, and which will now be described.

Figure 2 shows in side elevation and diagrammatically the method in operation, and reference 2 denotes the surface of the sea, 3 denotes the deck of a ship, 4 denotes a pipe to be laid from a reel 9 to the seabed, 5 denotes a tracked tensioner holding the pipe 4, 6 denotes the stinger and reference 7 denotes the tip of the stinger 6. The launch angle is shown by reference 8.

The Apache method has recently been modified to avoid buckling the pipe 4 at the tip 7 of the stinger 6 (where full plastic deformation occurs during laying of the pipe 4) and is currently being modified further to accommodate deep water lays by raising the launch angle from about 55° (as shown) , to an angle of about 85°.

The known NOS DYNLAY system is illustrated in Figure 3 of the drawings, which shows in plan view the method in operation. Pipes 4 are to be laid from carousels 10 on the seabed, and tensioners 5 and stingers 6 (also known as connectors) are provided. The NOS system, using rotation about a vertical axis and top loaded baskets is believed to be undergoing development, but is understood to be very expensive and presenting complex design problems, and furthermore cannot handle relatively rigid i.e. steel pipes. In addition to the machinery used for handling steel and composite pipes, there is an extensive, mature technology for handling subsea telephone lines. This is being further developed to handle fibre optics cables which present problems due to their small diameter, the presence of connectors and the shear strength of the coating which is more easily ripped-off than when handling telephone cables.

The existing technology, described above, and using wheel and track tensioners, has been applied to hose laying, but the tensioners (limited by the amount of radial force which can be applied to a hose, and by the coefficient of friction between hose cover and pad) , are becoming impracticably long, difficult to control and expensive to buy and maintain.

With development of oil / gas fields in deep water, all of the foregoing equipment is proving to be uneconomic, and a simple practical and economical approach is required by field operators .

The present invention therefore seeks to provide a novel method and apparatus to meet these requirements, and employs a buoyant drum on which the line can be wound, prior to unreeling to lay the line on a seabed, or onto which a line can be wound during recovery from the seabed.

According to one aspect of the invention there is provided apparatus for laying a line on a seabed from a surface vessel on which the apparatus is to be mounted, said apparatus comprising: a drum which is rotatable about an axis which is substantially parallel to the general plane of the deck of the vessel, said drum being capable of storing a reeled-up length of line to be laid; means for guiding the line from the drum and downwardly to the seabed; bearings for supporting the drum and the line stored thereon; a tank in which the drum is located, said tank being capable of being charged with liquid in order to exert an upthrust on the drum and thereby relieve at least part of the load of the drum and line which is borne by the bearings; and, control means for varying the volume of liquid in the tank as the line is unwound from the drum.

Preferably, the control means is responsive to the load applied to the drum during unwinding of the line.

According to a second aspect of the invention there is provided apparatus for reeling-in a length of line which is to be laid subsequently on a seabed from a surface vessel on which the apparatus is to be mounted, said apparatus comprising: a drum which is rotatable about an axis which is substantially parallel to the general plane of the deck of the vessel, said drum being capable of storing a reeled-up length of line; means for feeding the line to the drum so as to form reeled-up windings of line on the drum as the drum is driven to rotate; bearings for supporting the drum and the line wound thereon; a tank in which the drum is located, said tank being capable of being charged with liquid in order to exert an upthrust on the drum and thereby relieve at least part of the load of the drum and line which is borne by the bearing; and, control means for varying the volume of liquid in the tank as the line is wound onto the drum.

Preferably, in this second aspect of the invention, said control means is responsive to the load applied the drum by the line during reeling-in.

Preferably, the apparatus is adapted to be mounted on the deck of a surface vessel with the axis of the drum extending generally parallel to the fore and aft axis of the vessel e.g. so that line can be reeled-in, or unreeled with respect to the drum, via the stern end of the vessel .

Alternatively, the apparatus may be adapted to be mounted on the deck of a surface vessel with the axis of the drum extending generally parallel to the fore and aft axis of the vessel, whereby line can be reeled-in or unreeled from the drum via the side of the vessel. The control means may be operative to charge the tank with liquid when increased upthrust on the drum is required, and to vent liquid from the tank when reduced or no upthrust is required.

The control means may vent liquid from the tank by admission of gas to the tank.

In a preferred embodiment, the tank may be formed by a moonpool of a ship .

The invention also includes a method of laying a line on a seabed, or retrieving a line from the seabed, utilising apparatus as defined above.

The line may be a rigid pipeline of the type used to convey liquid and / or gaseous hydrocarbons. However, the invention may also be applied to the handling of a flexible line or hose of the type used to conveyor hydrocarbons.

It is already known to use a buoyant drum on which pipeline can be wound, and indeed as long ago as 1944 the PLUTO project used a buoyant drum towed behind a destroyer to lay petrol supply lines along the English Channel subsequent to the D-Day landing . Being a very early use of a buoyant drum for this purpose, it was relatively unsophisticated mechanically, and was not found to be a complete success.

It is also known from US-4,117,692 to use a floating drum to transport a wound-up length of pipeline from a shore location to a pipe laying vessel at sea. The pipeline is wound onto the drum at the shore location, and the inherent buoyancy of the drum is such that it can float despite the load of the pipeline, and therefore can be towed out to sea to meet with the pipe laying vessel at the required pipe laying location. The floating drum is then coupled to the vessel, and the pipeline is unreeled from the drum and fed to the vessel to undergo pipe straightening and tensioning by equipment provided on the vessel, prior to controlled descent to the seabed along a catenary in known manner.

The drum is known as a "pancake" drum in the sense that it floats with its axis perpendicular to the water surface (vertical when the sea is calm) , and the pipeline is reeled onto the drum while it maintains this attitude and is rigidly moored to the shoreline, and is then towed in this position to meet with the pipe laying vessel. The drum must then be coupled with the vessel so as to float independently of the vessel, and the pipeline is then fed from the drum to the pipe handling equipment on the deck of the vessel. Relative up and down movement will take place between the drum and the vessel with wave movements, which is not conducive to controlled unreeling of the pipeline from the drum. Furthermore, there are other drawbacks to use of pancake drums, in regard to subsequent handling by pipe tensioning / straightening equipment, so that horizontal axis drums are generally regarded by most operators as being more preferable.

It is also known from WO94/06678 to provide a method and apparatus for laying a line on a seabed and / or retrieving a line from a seabed and employing a marine vessel and a winch based system including a buoyant drum, in which the drum is installed in or attached to the marine vessel with the axis of rotation of the drum maintained generally parallel to the surface of the sea in which the vessel is floating, and with the drum being at least partly immersed in a body of water to provide upthrust to support at least part of the weight of the drum, and in which the line is wound on the drum for storage and subsequent unreeling in order to lay the line on the seabed, and is wound onto the drum during retrieval of an already laid line from the seabed.

In this known arrangement, the buoyant drum is mounted within the body of the vessel in such a way as to be at least partially submerged in the sea. This may be achieved, for example, by loading the drum, with pipeline wound on it, into a vessel which is provided with a "moonpool" . The static load of the drum and the pipeline wound on it is then borne mainly by the sea, and therefore the load on the bearing or other support for the drum provided by the vessel is very substantially reduced, which facilitates the reeling and unreeling operation.

There are many ways in which the buoyant drum can be mounted in the vessel, and including:

(a) towing the drum (loaded with pipeline) to the vessel, either by the drum floating in the water, or being borne by a transport barge, pontoon or the like, and then by ballasting of the drum and / or the towing support, the drum can be lowered to a position below the hull of the vessel, and then upon de-ballasting, the drum can rise by its own buoyancy into the moonpool of the vessel when the latter is provided with a moonpool;

(b) the drum may be supplied as a replaceable cartridge floating in its own tank of water, and the entire cartridge can then be loaded onto the vessel. In this case, the tank will transfer the load of the drum to the vessel, but the drum itself will float, and therefore any rotary support for the drum upon installation in the vessel again is required to bear a very small proportion of the load, which facilitates reeling and unreeling;

(c) the drum can be installed in the vessel with its axis of rotation extending at any desired angle relative to the general fore and aft axis i.e. longitudinal axis of the vessel, namely:

(i) with the axis of the drum extending perpendicular to the axis of the vessel i.e. across the beam of the vessel, in which case preferably the line is reeled and unreeled through the moonpool opening in the hull of the vessel; or

(ii) the drum is arranged with its axis of rotation extending parallel to the longitudinal axis of the vessel, which is particularly advantageous, in that the length of the drum (axially) can be increased to take up space made available throughout the length of the vessel, and therefore a greater length of line can be wound on the drum, whereas when the drum is mounted transversely of the vessel, necessarily its axial length is restricted by the beam of the vessel.

When the drum is mounted in the vessel with its axis of rotation extending parallel to the longitudinal axis of the vessel, preferably the line is reeled and unreeled over the side of the vessel, rather than downwardly through the moonpool .

A line straightening device is mounted on the deck of the vessel so as to co-operate with the drum, during reeling and unreeling, and in the latter case it acts to unbend the deformation necessarily applied to the line during initial winding onto the drum, and usually and desirably imparts a small degree of reverse bending, prior to descent to the seabed along a required catenary path. A tensioning device may also be provided on the deck of the vessel, to co-operate with the straightening device, and which is used initially to draw the pipeline from the drum, although with requirement for progressively smaller tension force as the line commences its descent to the seabed, when gravity pull of the pipeline will assist the unreeling of the drum. Braking mechanisms may be provided known per se to control the unreeling of the drum.

The formation of a required catenary path will depend upon the depth of water, and will be controlled by forward speed of the drum in the pipe laying direction.

In the case of a transversely mounted drum, the pipe laying direction will be the normal forward propulsion direction of the vessel. However, in the case of the drum being mounted with its axis parallel to the longitudinal axis of the vessel, the pipe laying direction will be in a direction perpendicular to the axis of the drum, and therefore also to the longitudinal axis of the vessel i.e. the vessel is driven e.g. preferably by usual side thrusters in a transverse i.e. crab-wise direction during line laying.

The pipe-laying method and apparatus which will be described below, with reference to Figures 4 to 12 of the accompanying drawings, are also disclosed in publication No WO94/06678, and which serves as a background to subsequent description of preferred embodiments of the present invention.

Figures 4a to 41 are diagrammatic side view illustrations of a succession of 12 steps in performing a line laying method;

Figure 5 shows a practical embodiment of pipe laying vessel ;

Figure 6 is a perspective illustration of an alternative arrangement ;

Figure 7 is a detail view, to an enlarged scale, of part of a pipe bending and guidance system of the arrangement of Figure 6;

Figures 8a and 8b are end views of the buoyant drum and bending and guidance system in more detail;

Figure 9 is a side view of the bending and guidance system and a variable tension pipe withdrawal device;

Figure 10 is an end view, in more detail, of a buoyant drum assembly shown schematically in Figure 6;

Figure 11 is a side view corresponding to Figure 10; and,

Figure 12 is a plan view corresponding to Figure 10.

Referring now to Figures 4a to 41, these illustrate diagrammatically a sequence of 12 steps involved in performing a pipe-laying method, and which in the illustrated example comprises the laying and retrieving of pipelines on the seabed (to convey liquid and / or gaseous hydrocarbons) and in a range encompassing shallow through to extreme depths.

The method employs a winch based system, comprising a buoyant or semi-buoyant drum, which is installed in or attached to a suitable marine pipe laying / recovery vessel. The drum is loaded with a long length of pipeline at an inshore location by any well known technique, and then the drum is either loaded on to the vessel and the vessel sails to the pipe laying site; or else the loaded drum may be towed out to meet a vessel already on site, in which case the drum can either be loaded on a barge or other vessel and towed to site, or if the drum is rendered sufficiently buoyant so that it can float in the water (despite both the dead load of the drum and the live load of the length of pipeline reeled thereon) , it can be towed out to site .

Despite the fact that typically a pipe laying drum may weigh of the order of, for example, 800 tonnes, and the live load of pipeline may weigh about 5,000 tonnes, the typical size of a pipe laying drum may be of the order of 20 meters diameter, and by making it of hollow construction, it can have sufficient buoyancy that the upthrust from the water in which it is at least partially immersed can be equal to the dead and live loads of the drum.

Alternatively, an unloaded winch drum can remain mounted in position on the vessel, and can receive a load of pipeline direct from an inshore location with the vessel located closely adjacent thereto.

Figure 4a shows step 1 of one method of operation, in which a length of pipeline 105 is being loaded from a winch 102 mounted on a support structure 103 at an inshore location, and with the pipeline 105 passing over the stern 106 of marine vessel 101 to a semi -buoyant drum 107 mounted on the vessel. The surface of the sea is denoted by reference 104, and curved arrow 109 denotes clockwise rotation of the drum 107 during the winding of the pipeline 105 on to the drum 107.

Pipeline 105 may be any suitable type of pipeline used for seabed conveyance of liquid and / or gaseous hydrocarbons, and may range from a relatively flexible type of pipeline of synthetic plastics material known as a "hose", up to relatively rigid steel pipes of substantial diameter, ranging from, say, 4 inches up to 36 inches diameter.

Figure 4b shows step 2 of the operation, in which a length of pipeline 105 has been loaded on the drum 107. A hydrostatic test unit 110 is mounted on the support structure 103 and the pipeline 105 is pressurised to the required working pressure and tested against leakage with the test unit 110. Preferably, pipeline 105 is laid under high pressure, as the pressurisation enables more product to be loaded on the drum 107 (assuming the pipeline is capable of being inflated) , because an inflated pipeline 105 provides more resistance to crushing. The pressurisation also facilitates leak detection during loading and laying procedures.

Figure 4c shows step 3, in which the test unit 110 has been removed from the support structure 103. The drum 107 continues to be loaded until all the required lengths of pipeline 105 have been loaded following the procedures of steps 1 and 2. A pig (not shown) is inserted and it is blown through with compressed air until all water has been expelled at free end 108. At another free end (not shown) of pipeline 105, the pig is removed and the pipeline is blanked-off. This other free end is secured to the flange of the drum 107 with a pipe clamp (not shown) .

Figure 4d shows step 4, in which a buoy 112a provided with a bullnose 113 is lying on the seabed 111 and awaiting retrieval to be laid on the deck of vessel 101. Pipeline 114 is attached to drum 107, and 2,000 meters of pipeline from drum 107 are paid out, moving away in the direction of lay. Thrust is applied, so that the pipeline is tensioned and can then be raised from the seabed 111 so that the pipeline is reeled-in, while thrust is maintained until the bullnose 113 approaches the water surface 104.

Figure 4e shows step 5, in which thrust is continued to be applied in the direction of arrow 116, and the bullnose 113 is pulled-up and set into a keyhole (not shown) using a deck winch 112. To cross-haul, the bullnose 113 is removed and haul-in cable 114 is secured on the flange of the drum 107.

Figure 4f shows step 6, in which the drum 107 and the pull-in cable (not shown) on the deck winch 112 are rotated until the pipe flange lifts out and clears the keyhole (not shown) completely. In order to attach the pipeline 105 already wound on the drum 107 to the length of steel pipe 115 already laid on the seabed, the flange of pipeline 105 is attached to the flange on steel pipe 115 to form a joint 125 (see Figure 10), and loaded on to the pipeline 105. The thrust in the direction of arrow 116 is maintained.

Figure 4g shows step 7, in which the pipeline 105 is- lowered whilst rotating the drum 107 anti-clockwise as shown by curved arrow 117, while at the same time maintaining forward thrust in the direction of arrow 116 (the force of which will be determined by the weight of steel pipe 115 and pipeline 105) , sufficient to maintain a catenary with a minimum radius which will not buckle under hydrostatic pressure prevailing.

Figure 4h shows step 8, in which the steel pipe 115 is set on the seabed 111 and the thrust in direction 116 is reduced to a level calculated according to water depth and size of pipeline 105 until the position for attaching a buoyancy device has been reached.

Figure 4i shows step 9, in which a buoyancy device 118 (also known as a "mid-water arch") is attached to the pipeline 105, and the volume and depth of the device 118 will be determined according to prevailing circumstances. The pipeline 105 is lowered onto the seabed 111 until suspended from a cable 119 (see Figure 13) provided on the drum 107.

Figure 4j shows step 10, in which a messenger cable 120 is passed down through a moonpool (not shown) provided in the hull of the vessel, which as is well known comprises an open shaft in the centre of the hull of a ship engaged in deep sea drilling through which drilling takes place. The cable 120 is passed under the vessel and across to a platform or buoy (moorings 123) to connect to the pull-in cable 121 for the pipeline 105. Reference 122 denotes a pull-in winch for the cable 121.

Figure 4k shows step 11, in which the messenger cable 120 is lowered together with the pipeline 105 into the water and a hydraulic hook 124 is released.

Figure 41 shows step 12, in which installation by the vessel 101 is complete. The vessel 101 stands by while the pipeline 105 is hydrotested, in case recovery should be necessary in view of any faults being found.

The description above with reference to Figure 4a to 41 is in relation to the laying of pipeline on a seabed by unreeling of a semi-buoyant pipe laying drum mounted on the vessel, and recovery of an already laid pipeline merely comprises a reverse of these procedures.

To conclude, with regard to the pipe laying procedure, this comprises the following steps in sequence:

Step 1 - load first length of pipeline onto pipe laying drum;

Step 2 - connect hydrostatic test unit to end of first length of pipe, fill hose (venting at elbow on drum adjacent to joint) , pressurise hose to 1.5 times working pressure and check joint for leaks; if successful continue to step 3; if leaks are detected, remake joint and retest;

Step 3 - continue to load drum until all lengths are loaded, and all joints have been tested; insert pig and blow it through with compressed air until all water is expelled at end A; at end B remove pig and blank off; secure end to drum flange with a pipe clamp;

Step 4 - retrieve recovery buoy and lay on deck; maintain position on DP, attach line to drum; pay out 2,000 meters of line from drum, moving away from the line in the direction of lay; apply thrust, tensioning line and raising it from the seabed; reel-in while continue to apply thrust until the bullnose approaches the surface;

Step 5 - continue to apply thrust and pull-up bullnose and set into keyhole using deck winch; remove bullnose and secure haul-in wire on drum flange;

Step 6 - rotate drum and pull-in on deck winch until pipe flange lifts out and clears the keyhole completely; attach hose flange to pipe flange and ease off the deck winch and lower the load onto the hose; maintain thrust and DP control;

Step 7 - lower the hose while maintaining forward thrust (determined by weight of steel pipe and hose) , sufficient to make a catenary with a minimum radius which will not buckle under prevailing hydrostatic pressure;

Step 8 - set steel pipe on seabed and reduce thrust to level calculated according to water depth and hose size until position for attaching buoyancy has been reached;

Step 9 - attach buoyancy, with volume and depth to be defined by operator, and lower hose until suspended from cable on drum;

Step 10 - pass the messenger cable down through the vessel moonpool, under the ship and across to the platform / buoy to connect the hose pull-in cable;

Step 11 - lower messenger and hose cables into the water and release the hydraulic hook; and,

Step 12 - installation by vessel now complete; vessel stands by while line is hydrotested in case recovery is necessary. Recovery of an already laid pipeline is the reverse of the above described laying procedure.

Figures 4a to 41 are diagrammatic illustrations of the successive steps involved in carrying out a pipeline laying / retrieving method, in which a buoyant or semi-buoyant drum 107 is mounted in the pipe laying vessel in such a way that at least part of the dead and live load of the drum is borne by a supporting body of water, and which in the illustrated arrangement comprises mounting of the drum in a moonpool provided in the hull of the vessel . The upthrust applied to the drum, when at least partly immersed, substantially reduces the loads applied to bearing supports for the drum, which maintain the axis of the drum generally parallel to the deck of the vessel and perpendicular to the general fore and aft (longitudinal) axis of the vessel. The bearing supports locate the drum axially, and also guide rotation of the drum during reeling and unreeling. The bearing supports also include thrust bearings (not shown) to locate the drum axially despite wave-induced or other oscillating movement of the vessel.

The bearing supports are not shown in Figures 4a to 41, but preferably comprise bearing assemblies arranged radially outwardly of the side flanges of the drum, and concentric therewith, and which guide the rotation of the drum via the mounting of the drum flanges within the bearing assemblies.

The bearing assemblies may be built-up from a series of circumferentially spaced arcuate bearing pads made of suitable wear resistant plastics material, capable of bearing applied radial loads and circumferential friction loads. Bearing in mind that the drum flanges rotate through a bath of liquid (sea water) present in the moonpool, this water lubricates the circumferential sliding movement which takes place between the flanges and the bearing pads. Further, the lower set of bearing pads will normally be permanently immersed in water, which will lubricate the rotation of the drum, and further it will be the lower bearing pads which have to receive the greater part of the net gravity load of the drum and pipeline. The upper bearing pads i.e. those not immersed in the moonpool will be lubricated by water carried over by the rotating flanges .

To further improve the lubrication effect, some or all of the bearing pads may have apertures in their inner sides facing the drum flanges, and to which any suitable liquid lubricant, preferably water, can be pumped in order to improve lubrication and also to dissipate any heat which may be generated.

The advantages of mounting a buoyant drum in a moonpool, as compared with existing techniques, will be evident from the following description of existing techniques.

The existing pipe laying systems which utilise the reel principle generally comprise a reel and a tensioning cum straightening device which is separate from the drum and is difficult or impossible to use for extreme depths. In one such configuration the use of the machinery for deep water laying involved bending the pipe through an acute angle approaching 80°. This has the major disadvantage of plastically deforming the pipe during deep laying. This applies extremely high crushing forces to the bending rolls, introduces an unacceptably high risk of buckling the pipe and requires extremely high pulling forces to move the pipe around the curve .

Solutions to this problem have been suggested in the past which involve the erection of large bridge cranes between the drum and the launching ramp. These bridge cranes would be used to maintain the pipe in a curve reducing the shear forces at the lift off from the drum and at entry to the ramp avoiding the problems above. However, the capital and operating costs of this system, combined with the impracticality of using men at levels up to 140ft above the deck, make such an arrangement unfeasible.

However, this known arrangement makes provision for launching directly from the drum downwards into the water. There are several difficulties which are overcome and which are provided for in the system;

1. the distance between the point of departure from the drum and the water's maximum surge height permits access by men to effect a butt weld;

2. the tension applied to the pipe at change of wrap can be controlled to avoid buckling or the pipe can be supported by the creation of ramps to prevent buckling;

3. the system is capable of being used for loading the pipe at the dock; (during the loading a back tension must be applied to the pipe to bend it during drum rotation) . This back tension must be maintained on the drum at all times to avoid uncoiling (springing) , and ideally the tension should be maintained by the same tensioning device as is used for loading;

4. access is permitted between the tensioning or straightening elements for the manual butt welding and the attachment of anodes .

General description of a moonpool system.

The system for laying steel pipes (and flexible pipes) offshore comprises a large buoyant drum secured in an open moonpool in the middle of a ship. (Other locations for the drum e.g. the stern or alongside are possible and are intended to be within the scope of the present invention. To lay steel pipes the drum is equipped with rails around the perimeters of each flange on which a bridge crane is mounted. This bridge crane is provided with a pair of rails for a levelwind carriage, LC, which in turn supports a tensioner / straightener device. The deck of the ship is provided with a superstructure to which the bridge crane is attached when loading pipe. The wingwalls in the moonpool are also equipped with structures to which the bridge is attached when laying.

The operation is as follows:

* the bridge is locked to the superstructure and the end of a length of pipe about one km in length is pulled onto the ship over the stern, through the rollers on the tensioner cum straightener (TS) to be welded to the anchor point on the drum.

* the drum is rotated being levelwound by adjusting the position of the levelwind carriage on the bridge.

* at change of wrap a ramp is created using blocks to minimise the risk of buckling. * as each length of pipe is loaded another length is welded into position and inspected by QC .

* loading continues until a continuous length of pipe, several kilometres long is tightly coiled on the drum.

* the TS rolls are locked to maintain tension and security lines installed from the pipe to the carriage structure .

* the bridge is locked to the drum and unlocked from the superstructure and rotated down into the launch position where it is locked to the wingwalls and unlocked from the drum.

* upon arrival at site a line is passed from the anchor point on the seabed and attached to the bullnose.

* the ship thrust is increased and pipe is pulled from the drum, being assisted by the powered rolls on the tensioner, (at this time they are putting power into the laying and drawing power from the ship's electrical system) . The drum is being retarded by its hydraulic motors and heat is being dissipated from the circuit.

* as the pipe descends to the seabed the tension in the catenary increases and the driving force in the tensioner is progressively reduced until it reaches zero and changes direction becoming a braking force to act in the same direction as the drum restraining force .

* during the laying the tensioning / straightening operation is as follows: the pipe leaving the drum is under tension which pulls it straight (zero curvature but with residual stress remaining) . In this condition it enters the straightener where tension is applied. The angle to the vertical of the straightener / tensioner carriage can be adjusted by altering a hydraulic cylinder. This adjustment places the centre line of the tensioner / straightener on the axis of the pipe leaving the drum.

* residual bending is removed from the pipe as it passes through the straightener downstream of the tensioner. (This location for the straightener has the advantage of increasing the clearance of the work area above the surge. It is also possible to position the straightener between the tensioner and the drum) .

* abandonment of the end of the pipe is achieved by stopping the pipelay operation, locking the rolls against rotation and forcing them together, thus firmly gripping the pipe between multiple pairs of rolls. Additional constraint is provided against accidentally dropping the pipe by closing the clamp between the first reactive roll and the active roll in the straightener. A second clamp is closed on the pipe above the first tensioner roll on the pipe between the drum and the tensioner. The pipe is now cut and the upper sets of rollers rotated to provide space for a bullnose to be welded to the lower pipe. An abandonment wire is attached to this to lower it to the seabed. After the pipe is on the seabed the upper end of the wire is passed out of the moonpool and buoyed off.

Advantages

* unlimited water depth capabilities

* less damage to the pipe by minimising the number of plastic bend cycles

* safer operation since the pipe takes the shortest possible path to the water avoiding overhead routing and minimising the number of components which are highly stressed

* the same principle can be used to install much larger pipe sizes

* the change in tension through the pipe laying machinery is reduced

* the machine is simplified and more easily controlled

* the launching ramp is eliminated

* easily and quickly loaded

* easy corrections to launch angle

* bridge can be set so that the ship can lay in very shallow water by going astern

* drum can be used to lay flexible pipes (hoses)

* drum can be used as a heavy lift winch

* drum flanges can be outfitted with wire

* drum can be outfitted with auxiliary drums with separate drives or braked to the main drum * drum can be ballasted down for removal and depositing on the seabed leaving the moonpool free

* drum can be used for laying umbilicals, fibre optics, telephone cable and fibre optics

* drum axis can be used for slip rings or pressure and flow swivels permitting continuity monitoring and pressurising of flexible pipes in extreme depths, (needed to protect against collapse of inner liner)

* drum is manufactured from readily available steel sections using widely applied welding procedures

* drum does not need post weld heat treatment and repairs, modifications can be conducted at sea or inshore by normally qualified welders and fitters

* interior of drum is accessible through standard bolted manhole covers

* corrosion protection is simple being effected by paint and sacrificial anodes bolted to the plates

* drive system and bearings use proven technology

The general description above of a moonpool mounted floating drum sets out the advantages which can be achieved, and a more detailed example will now be described with reference to Figure 5 of the drawings .

Referring therefore now to Figure 5, this shows a pipe laying vessel 201 in more detail, having a moonpool 202 formed in the hull, and in which a buoyant or semi -buoyant pipe laying / recovery drum 203 is mounted so as to be at least partly immersed by sea water present in the moonpool in order to reduce the effective weight of the drum (and pipeline wound on it) which is transferred to the vessel via rotary mountings which support the drum for rotation about an axis which will be generally horizontal in a calm sea, and which extends generally parallel to the deck of the ship and perpendicular to the longitudinal or fore and aft axis.

Figure 5a shows how the drum 203 projects both downwardly and upwardly relative to the hull 204 of the ship.

Figure 5 shows a 12 inch pipe being laid by being unwound from drum 203 after passing through pipe straightener / tensioner device shown schematically only by reference 205. Device 205 acts on the unreeling pipeline in order to at least partly remove the deformation present in the pipeline as a consequence of having been wound on the drum 203, and may, according to requirements, apply a small amount of reverse bend to the pipeline, prior to downward discharge along a controlled catenary path in order to lay the pipe. The pipe 206, which for illustration purposes is assumed to be a 12 inch diameter pipe, passes through the straightener device 205 and then down to the seabed, being guided along a 3 inch diameter guide cable, and the required catenary shape is maintained by application of suitable forward propeller thrust.

The pipeline 206 is readily unreeled to form the required catenary path, by passing downwardly through the opening in the hull defining the entrance to the moonpool, and this provides considerable improvement over known pipe laying barges, where the pipeline is normally withdrawn rearwardly from the drum over the stern of the vessel, after passing upwardly over a bridge structure of substantial height (typically up to 140 feet), before passing through pipe straightener devices. The arrangement shown in Figure 5 is particularly advantageous for laying pipeline in deep water, or shallow water as necessary, in view of the downward passage of the pipeline through the hull of the vessel, and it is only necessary to alter the launch angle of the straightener device 205 to suit different seabed depths .

Drum 203 may be loaded with pipeline in the way described above with reference to Figures 4a to 41. Alternatively, drum 203 with a load of pipeline wound on it may be transported from a shore installation independently to meet the vessel 201 on site, either by being towed in a floating mode through the water by a tug or the like, or by being transported on a submersible barge. Once the drum has been brought alongside the vessel, the drum (or its supporting barge) may be ballasted to lower it in the water to a position below the bottom of the hull, and then with sideways maneouvering , upon deballasting the drum can rise-up within the moonpool and then be mounted on suitable bearing supports provided alongside the moonpool . Further, other means may be adopted in order to load the drum onto the vessel, including use of cranes.

Another way of supplying a loaded drum to a vessel may involve arranging a reusable cartridge assembly, in which the drum is assembled with a supporting tank which can be filled with water to support the weight of the drum, and the entire cartridge can be mounted in the pipe laying vessel to unreel pipe for a pipe laying operation, and when the drum is completely unreeled, the cartridge can be removed and replaced by a fresh one .

The figures of drawings described thus far all show a drum mounted with its axis extending across the beam of the vessel, and within the lateral confines of the hull, whereby the axial length of the drum is necessarily limited by the dimensions of the hull of the vessel in which it is mounted. However, the drum may be mounted on or in the pipe laying vessel with its axis of rotation still extending generally parallel to the deck of the vessel i.e. horizontal on a calm sea, but with the axis of the drum extending other than perpendicular to the general longitudinal axis of the vessel . In an arrangement which will be described below with reference to Figure 6 of the drawings, the axis of the drum extends parallel to the longitudinal axis of the drum, and pipe reeling and unreeling takes place over the side of the vessel. This is a radical departure from existing techniques, and provides technical advantage as will be described below, both in regard to a buoyant or semi-buoyant drum, which is a preferred arrangement, but also in the case of a drum mounted within the hull of the vessel by conventional bearing supports.

Referring now to Figure 6 of the drawings, this shows schematically the mounting of a buoyant pipe handling drum in a pipe laying vessel 301, in which the drum is mounted with its axis of rotation extending generally parallel to the fore and aft or longitudinal axis of the vessel 301. A liquid containment space 302 is formed in the hull of the vessel 301, to provide a body of water in which drum 303 can float at least partly immersed. This may be formed by a "moonpool", or by a tank defined within the hull and which can be filled with water, and in which the drum 303 can be partly immersed, whereby the bearing supports (not shown) for the drum 303 and mounted on the structure of the vessel are required to bear only a small proportion of the dead and live load of the drum 303. The components shown by reference 302 and 303 may comprise a re-usable "cartridge" assembly which can be mounted in the vessel 301 when an existing drum has fully unreeled its load.

An arch structure 304 extends over the drum 303, and on which is mounted necessary pipe straightener / tensioner devices which unbend the unreeling pipeline 305, and control its descent along its path to the seabed after passing over the side of the vessel .

Pipe laying vessels 301 are usually provided with thrusters to control lateral movement of the vessel for adjustment purposes, and these side thrusters can be used to move the vessel in a direction perpendicular to its longitudinal axis and along the intended direction of pipe lay.

The pipe straightener / tensioner device is shown diagrammatically only in Figure 6, and is designated by reference 306, and will now be described in more detail below with reference to Figures 7, 8a and 8b, and Figure 9 of the drawings .

Figure 7 is a perspective illustration of a detail of the deck 307 of vessel 301, and shows drum 303 mounted horizontally with its axis of rotation extending parallel to the longitudinal axis 308 of the vessel. A length of pipeline 305 is shown being unreeled from drum 303 after being subjected to pipe straightening and controlled tensioning by device 306. Although not shown in detail, a suitable travelling arrangement will be provided to facilitate winding or reeling of pipeline onto the drum with successive windings closely adjacent to each other, and also to assist unreeling.

Figures 8a and 8b are end views of the drum 303 and a pipe straightener portion of device 306, which comprises a series of guide rollers 309 which unbend the unreeling pipeline 305 to remove the deformation inevitably present as a consequence of winding onto the drum 303, and downstream of the pipe straightening portion of the device 306, there is a variable tensioner device which will be described below with reference to Figure 9.

The guide rollers 309 are carried by a straightener carriage 310 which is mounted on a bridge 311 which can be adjusted between different angular positions relative to the longitudinal centre axis 312 of drum 303, to vary the launch angle. Figures 8a and 8b show different angular positions which can be taken-up by the pipe straightener portion of the device 306.

Turning now to the tensioning device portion of device 306, evidently during initial unwinding of pipeline from drum 303, the gravity pull of the pipeline is relatively small, and therefore a tensioning device is required to apply a pulling force to pipeline 305 to withdraw it from the drum 303. However, as more pipeline becomes withdrawn, the gravity pull increases, and therefore the tensioning force applied to the pipeline can reduce, and this takes place automatically using monitoring devices known per se for such use.

Figure 9 shows pipeline 305 passing over a lower set of the guide rollers 309 and then through an upper clamp 313 and a lower clamp 314 of a hydraulically operated tensioning device. Four hydraulic cylinders 315 control reciprocating movement of lower clamp 314 relative to upper clamp 313, and which operate in sequence so as to withdraw successive lengths of pipeline from the drum. Evidently, upper clamp 313 will be clamped to the pipeline 305 while the lower clamp 314 is undamped and moving upwardly prior to again being clamped with the pipeline, and then moved downwardly (after release by the upper clamp 313) to draw a further length of pipeline from the drum.

The rotation of the drum 303 can be controlled by braking devices and clamping devices known per se .

Figures 6 to 9 are schematic, or illustrative drawings of the unreeling of pipeline, and which would be applicable to relatively flexible pipeline which can be unwound from the drum along the paths indicated, but for more rigid pipeline it will usually be necessary to define an upwardly curved path which the unwinding pipeline has to follow as it leaves the drum, before curving down to undergo pipe bending reversal by the straightener device.

Figures 10 to 12 generally correspond with the schematic illustration of Figure 6, and shown in more detail the path of travel along which an unreeling pipeline is constrained to follow in practice. Parts corresponding with those already described and illustrated in Figure 6 are given the same reference numerals.

As can be seen particularly in Figure 10, the pipeline being withdrawn from the drum 303, during unreeling, is constrained to form an upwardly curved path over a set of guide rollers 316 before undergoing bending reversal in device 306, followed by downward passage along the catenary path to the seabed. This upwardly curved path is necessary for more rigid pipelines, as is well known to those skilled in the art, in order to assist the unbending process. However, as can be seen from Figure 10, this takes place within the confines of the arch frame structure 304, and within an overall height of the arch structure, which is of the order of the flange diameter of the buoyant drum. This compares favourably with some existing designs of pipe unreeling systems, in which the pipe withdrawn from the drum is required to follow an upward path over supporting deck structure as high as 140 feet, as referred to earlier in the specification.

As described earlier, drum 303 rotates in a body of liquid which supports the dead and live load of the drum, and which is provided in the illustrated arrangement by a tank 302 of water, or by a moonpool. The level of the main deck is shown by reference 317, and the waterline by reference 318.

The above description with reference to Figures 4 to 12 of the drawings is intended to serve as background to description of a significant development thereof, and which comprises the preferred embodiment of pipe-laying method and apparatus according to the present invention.

In the preferred embodiment, the drum is arranged to have a buoyancy which varies according to the load applied to the drum. Evidently, during unreeling of a fully wound drum, the weight of pipe-line wound onto the drum will decrease, and therefore the requirement for (buoyancy) upthrust on the drum will reduce also progressively. However, the downward pulling force of the unreeled pipeline will exert a downward component of force on the drum (and which will be borne partly by the buoyancy of the drum and partly by the bearings which support the drum) .

Therefore, although the weight of pipeline on the drum will decrease during unreeling, the instantaneous (reduced) requirement for buoyancy will not reduce in direct proportion to the amount of pipeline unreeled because of this weight effect of the unreeled pipeline. However, the buoyancy requirement of the drum will tend to reduce as pipeline is withdrawn, if it is required to maintain substantially constant radial support of the drum via its bearings.

Conversely, as the drum winds pipeline around it (during initial loading of the drum prior to launching of a new pipeline, or during recovery of an existing line from the seabed) , there is a requirement to increase the buoyancy of the drum.

The present invention therefore provides a means of controlling the buoyancy of the drum in response to the change in pipe-line load applied to the drum. This may be achieved in any convenient manner, which will include controlling the filling of the drum with ballast, preferably liquid, and the venting of the ballast with gas, depending upon whether the drum load is decreasing or increasing.

A control arrangement may be provided which monitors the instantaneous amount of pipeline on the drum, and the rate of change (increase or decrease) , and which then initiates inflow / outflow of ballast liquid or venting gas accordingly.

Therefore, a drum buoyancy control arrangement is provided which varies the buoyancy of the drum to balance the load applied by the pipeline, so that bearing loads are minimised, and also capex and opex costs for bearings can be minimised (which is a major cost in existing reelships) .

Preferred additional and / or alternative aspects of the development provided by the invention are as follows:

(a) provide a sub-divided drum, so that eccentric loads applied by pipe-lines can be compensated for;

(b) provide a drum with axial seals on both sides, through which water can be introduced / evacuated by compressed air, through a radial slide valve of suitable design;

(c) provide a drum fitted with a deck-mounted gantry crane carrying a bridge fitted with straightening and tensioning equipment designed for laying steel pipe e.g. as shown in Figure 6 ;

(d) provide a drum of a design such that it can be manufactured from steel plates, suitable for construction on shipyard machinery, using existing ship building techniques, so that assembly cost is minimised;

(e) arrange the gantry so that it can pass lengthwise along the deck of the ship, and out over the stern to pick-up loads from the sea over the stern;

(f) fit the drum with shells (daughters) that rotate about the drum axis and carry steel pipes, flexible pipes, or cables e.g. products of different diameters, which can be laid independently of the pipe-line borne by the main drum; and,

(g) provide a drum which can be installed in a tank or "moonpool", so that its buoyancy can be controlled irrespective of the behaviour of the ship in a seaway.

Still further, and alternative aspects of the invention, and preferred aspects thereof, will be described below. These further developments, having independent inventive signi icance, are based on Figures 6, 10, 11 and 12 in

The preferred further developments, provided by the present invention, based on the known arrangements of Figure 6, 10, 11 and 12, comprise: 1. Provide a system in which the flanges of the drum are housed in tanks in the side walls of the moonpool, and are provided with seals between the flange and the side wall so that the buoyancy can be manipulated. This will be a useful feature for ships with a narrow beam, since it maximises the length of pipe which can be carried. It also reduces the size and cost of the bearings required. The depth of water in the side wall tanks can be varied to manipulate buoyancy on the drum and reduce bearing loads .

2. Provide hydrostatic bearings that are pressurised with water, to resist the end loads on the drum caused by rolling.

3. Provide a bridge supported from the gantry spanning the drum, the bridge supporting the pipe-laying machinery.

4. Provide an articulated bridge with one or more horizontal hinges so that it can be manoeuvred to match the radius of the pipe, and provide a suitable launch angle for the pipe .

5. Provide a telescopic section that can be extended over the side to facilitate laying in deep water.

This further development provided by the invention may comprise any one of features 1 to 5 recited above, taken singly. The invention further contemplates providing a pipe laying method and apparatus utilising any one or more of the above five set of features.

Claims

1. Apparatus for laying a line on a seabed from a surface vessel on which the apparatus is to be mounted, said apparatus comprising: a drum which is rotatable about an axis which is substantially parallel the general plane of the deck of the vessel, said drum being capable of storing a reeled-up length of line to be laid; means for guiding the line from the drum and downwardly to the seabed; bearings for supporting the drum and the line stored thereon; a tank in which the drum is located, said drum being capable of being charged with liquid in order to exert an upthrust on the drum and thereby relieve at least part of the load of the drum and line which is borne by the bearings; and, control means for varying the volume of liquid in the tank as the line is unwound from the drum.

2. Apparatus according to claim 1, in which said control means is responsive to the load applied to the drum during unwinding of the line.

3. Apparatus for reeling-in a length of line which is to be laid subsequently on a seabed from a surface vessel on which the apparatus is to be mounted, said apparatus comprising: a drum which is rotatable about an axis which is substantially parallel to the general plane of the deck of the vessel, said drum being capable of storing a reeled-up length of line; means for feeding the line to the drum so as to form reeled-up windings of line on the drum as the drum is driven to rotate; bearings for supporting the drum and the line wound thereon; a tank in which the drum is located, said tank being capable of being charged with liquid in order to exert an upthrust on the drum and thereby relieve at least part of the load of the drum and the line which is borne by the bearings; and, control means for varying the volume of liquid in the tank as the line is wound onto the drum.

4. Apparatus according to claim 3, in which said control means is responsive to the load applied to the drum by the line during reeling-in.

5. Apparatus according to any one of the preceding claims, in which the apparatus is adapted to be mounted on the deck of a surface vessel with the axis of the drum extending generally perpendicular to the fore and aft axis of the vessel.

6. Apparatus according to any one of claims 1 to 4, in which the apparatus is adapted to be mounted on the deck of the a surface vessel with the axis of the drum extending generally parallel to the fore and aft axis of the vessel.

7. Apparatus according to any one of the preceding claims, in which said control means is operative to charge the tank with liquid when increased upthrust on the drum is required, and to vent liquid from the tank when reduced or no upthrust is required.

8. Apparatus according to claim 7, in which the control means is operative to vent liquid from the tank by admission of gas to the tank.

9. Apparatus according to any one of the preceding claims, in which the tank is formed by a moonpool of a ship.

10. A method of laying a line on a seabed utilising apparatus according to any one of the preceding claims .

11. A method of retrieving a line from a seabed utilising apparatus according to any one of claims 1 to 9.

12. A method according to claim 10 or 11, in which the line is a pipeline for conveying liquid and/or gaseous hydrocarbons .

13. A method according to claim 10 or 11, in which the line is a flexible line or hose.