This invention relates to methods and apparatus for use in marine pipelaying. More specifically, but not exclusively, the invention relates to methods and apparatus whereby a conventional marine drilling vessel may be adapted to perform pipelaying operations.

In prior-art marine pipelaying systems as employed in laying offshore subsea pipelines (for such uses as the gathering of oil and/or gas from offshore subsea wells, as, for example, in the Gulf of Mexico or the North Sea) , it has been conventional to use one of two main methods to lay the pipe. In the first, or "stovepiping" method, a pipeline is fabricated on the deck of a lay barge by welding together individual lengths of pipe as the pipe is paid out from the barge. Each length of pipe is about 40 feet or 80 feet long. Thus, the pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string.

The stovepiping method requires that skilled welders and their relatively bulky equipment accompany the

pipelaying barge crew during the entire laying operation; all welding must be carried out on site and often under adverse weather conditions. Further, the stovepiping method is relatively slow, with experienced crews being able to lay only one or two miles of pipe a day. This makes the entire operation subject to weather conditions which can cause substantial delays and make working conditions quite harsh.

The other principal conventional method is the reel pipelaying technique. In this method, a pipeline is wound on the hub of a reel mounted on the deck of a lay barge. Pipe is generally spooled onto the reel at a shore base. There, short lengths of pipe can be welded under protected and controlled conditions to form a continuous pipeline which is spooled onto the reel. The lay barge is then towed to an offshore pipelaying location and the pipeline spooled off the reel between completion points. The pipe, which normally is plastically deformed during spooling onto the reel, has to be straightened as it is unspooled.

This method has a number of advantages over the stovepiping method, among them: speed (one to two miles per hour); lower operating costs (eg smaller welding crews and less welding equipment must be carried on the lay barge); and less weather dependency.

Commercial reel pipelaying techniques require the use of certain pipe handling equipment in addition to the reel, including a straightener mechanism. This may take the form of a series of rollers or tracks, or any other arrangement which imparts sufficient reverse bending force to the pipe to remove residual curvature such that after unspooling, the pipe will lay

substantially straight on the sea bottom.

Early reel pipelaying systems employed barges carrying reels mounted for rotation about a vertical axis (referred to herein as "horizontal reels"), the pipe being unspooled and launched from the stern of the barge at relatively shallow angle. Subsequently, a purpose built, dynamically positioned reel pipelay vessel was constructed, known as the "Stena Apache", in which a horizontal-axis reel (a "vertical reel") was incorporated into the hull of the vessel. The Stena Apache was also equipped with variable-angle pipe straightening and launching equipment, allowing a wide variety of pipelaying operations to be undertaken without significant re-fitting of the vessel. Various aspects of the "Stena Apache" are described in the following US Patents, to which reference is also made for a fuller discussion of pipelaying techniques in general, and reel pipelaying in particular:- Springett, et al - US Patent No. 4,230,421 Uyeda, et al - US Patent No. 4,269,540 Yenzer, et al - US Patent No. 4,297,054 Springett, et al - US Patent No. 4,340,322 Uyeda, et al - US Patent No. 4,345,855

All existing pipelaying systems require high capital investment, whether for purpose built vessels or for the extensive re-fitting of existing general purpose vessels. This increases the costs of pipelaying operations, often to the extent of making exploitation of marginal hydrocarbon deposits uneconomical.

It is an object of the present invention to provide methods and apparatus for marine pipelaying which entail lower capital costs than prior pipelaying

systems. It is a particular object of the invention to provide methods and apparatus allowing an existing drilling vessel to be adapted for pipelaying operations with minimal additional equipment. This provides the possibility of marginal deposits being exploited using the same vessel to perform both drilling and pipelaying operations.

In accordance with a first aspect of the present invention there is provided a method of marine pipelaying, of the type in which the pipeline to be laid is wound upon a reel carried by a vessel and is unspooled from said reel as the pipeline is laid from said vessel, comprising the steps of: feeding said pipeline from said reel over a bending shoe vertically spaced above the deck of said vessel and downwardly therefrom into a straightening and clamping assembly disposed below said bending shoe; said straightening and clamping assembly being adapted for vertical movement along said pipeline between an uppermost position adjacent said bending shoe and a lowermost position adjacent said deck, being further adapted to straighten said pipe by movement of the assembly along the length of the pipeline, and being further adapted to releasably clamp said pipeline; clamping said straightening assembly to the leading end of said pipeline, whilst said straightening assembly is in its uppermost position;

lowering said straightening assembly together with the leading end of said pipeline to its lowermost position;

clamping said pipeline by means of second clamp means disposed below the lowermost position of said straightening assembly and unclamping said straightening assembly from said pipeline; returning said straightening assembly to its uppermost position along said pipeline, thereby straightening that length of pipeline extending between said uppermost and lowermost positions of said straightening assembly; and repeating the foregoing steps until the required length of pipeline has been laid.

Preferably, said bending shoe is mounted on a derrick structure extending above said deck.

Preferably also, said straightening assembly is suspended from said derrick structure.

Preferably also, said derrick structure is disposed above a moonpool formed in said vessel and said pipeline enters the water via said moonpool.

Preferably, said reel is a vertical reel, mounted for rotation about a horizontal axis, and said pipeline is unspooled from the bottom of said spool such that the feeding of said pipeline about said bending shoe causes reverse bending of said pipeline.

Most preferably, said vessel is a drilling vessel, and said derrick assembly is the existing derrick of the drilling vessel. Preferably also, said reel, said bending shoe and said straightening assembly are removably mounted on the vessel. Preferably also, said straightening assembly is suspended from an existing crane hook of said derrick and is moved between said

upper and lower positions by operation of said crane hook.

In accordance with a second aspect of the invention, there is provided pipelaying apparatus for use in pipelaying operations of the type in which the pipeline to be laid is wound upon a reel carried by a vessel and is unspooled from said reel as the pipeline is laid from said vessel, comprising: a reel adapted for mounting on said vessel for storing the pipeline to be laid; a pipeline bending shoe adapted to be mounted at a vertical distance above the deck of said vessel; and pipeline straightening means adapted to be mounted below said bending shoe for vertical movement between an uppermost position adjacent said bending shoe and a lowermost position adjacent said deck, being further adapted to straighten said pipe by movement of the assembly along the length of the pipeline, and being further adapted to releasably clamp said pipeline.

Preferably, said vessel is a drilling vessel including a derrick structure located above a moonpool, said bending shoe being adapted to be mounted on said derrick structure.

Preferably also, said derrick structure includes vertically travelling crane means, and said straightening means is adapted to be suspended from said crane means for vertical movement with said crane means.

Preferably also, the apparatus further includes a pipeline clamp adapted to be located below the lowermost position of said straightening means, a pipeline abandonment and recovery winch, and a tugger

winch for use in initially feeding the end of the pipeline from the reel, over the bending shoe and through the straightening means.

Preferably also, said reel is adapted for rotation about a horizontal axis.

In accordance with a third aspect of the invention, there is provided a marine pipelaying vessel comprising a vessel having a derrick structure located above a moonpool, said vessel being adapted for pipelaying operations by the provision thereon of a pipeline storage reel and of pipe diverting and straightening means mounted on said derrick structure, whereby pipe from said reel may be laid via said moonpool.

In accordance with a fourth aspect of the invention, there is provided a method of marine pipelaying comprising adapting a vessel having a derrick structure located above a moonpool for pipelaying operations by the provision thereon of a pipeline storage reel and of pipe diverting and straightening means mounted on said derrick structure, and feeding the pipeline from said storage reel via said diverting means through said straightening means, said pipeline exiting said vessel via said moonpool.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Fig.l is a partial side view of a drilling vessel equipped with pipelaying apparatus in accordance with the present invention; Fig.2 is a partial plan view of the vessel of Fig.l;

Figs.3(a) to 3(f) are a sequence of side views of the vessel of Fig.l illustrating the method of use of the pipelaying apparatus in accordance with the invention; Fig.4 is a further partial side view of the vessel of Fig.l indicating different pipeline departure angles for a given bend radius; Fig.5 is a plan view showing the base and crown of a derrick structure forming part of the vessel of Figs.l to 4; and Figs.6(a) to (c) are side views of the derrick structure of Fig.5 in the directions A-A, B-B and C-C thereof respectively.

Referring firstly to Figs. 1 and 2, there is shown a conventional drilling vessel 10 equipped with a derrick structure 12 located above a moonpool 14. The derrick 12 is itself equipped with a vertically travelling crane hook assembly 16. The derrick 12, moonpool 14 and crane 16 would normally be used in conventional drilling operations undertaken by the vessel 10.

In accordance with the invention, the vessel 10 is adapted to perform pipelaying operations by the addition of a pipe storage reel 18, upon which a pipeline to be laid is spooled, mounted on the deck of the vessel astern of the derrick 14; a bending shoe assembly 20 mounted on the derrick 12 at a distance above the deck of the vessel; and a pipeline straightening assembly 22 suspended from the crane hook 16 beneath the bending shoe 20. Operation of the crane allows the straightening assembly to travel vertically between an uppermost position adjacent the bending shoe 20 and a lowermost position adjacent the deck of the vessel.

The reel 18 is preferably of the "portable" type, which can be loaded with pipe onshore before being lifted onto the vessel. Various portable reels of this type are known for use in other pipelaying systems, for example from US Patents Nos. 3,855,835, 4,157,023, 4,243,345 and 4,260,287. As illustrated the vessel may carry one or more additional reels 18a., according to the available deck space and load capacity, allowing a greater length of pipeline to be carried and laid.

The bending shoe 20 performs a substantially similar function to the "radius controller" of the above referenced US Patents Nos. 4,230,421, 4,269,540, 4,297,054, 4,340,322 and 4,345,855; i.e. diverting the pipe as it is unspooled from the reel 18 for presentation to the straightening assembly 22, and at the same time reverse bending the pipeline to a relatively uniform radius of curvature so as to facilitate subsequent straightening. The bending shoe provides an arcuate path about which the pipe is bent in being fed to the straightening assembly. The pipe bearing surface of the shoe is normally provided with rollers or a conveyor-belt type track arrangement, which may be formed with a transverse sectional profile for centring the pipeline thereon as is well known in the art.

The pipeline is preferably fed from the bottom of the reel as shown, such that it is bent in the reverse direction to the curvature imparted to it by being spooled on the reel 18.

In the present embodiment the bending shoe 22 preferably provides an arcuate surface of approximately 240 degrees. For ease of handling and installation, the

shoe preferably comprises a plurality of arcuate segments which together make up the required arc length, each segment being fitted with its own pipe transporting conveyor or rollers (not shown) . The radius of the arc of the shoe will depend upon the characteristics of the maximum size of pipe which is to be laid. For 6 inch diameter pipe, a radius of about 3.5 metres will be required.

The straightening assembly 22 comprises a three-point straightener, preferably a roller assembly of a type which is known in the art. Three-point straightening of pipelines is discussed in US Patents Nos.3,237,438 and 3,372,461. Examples of roller-type straighteners of the generally preferred type, and their operation, are described in the above referenced US Patents Nos. 3,855,835, 4,157,023, 4,243,345 and 4,260,287. Alternative "roller-track" type assemblies are also described in above referenced US Patents Nos. 4,230,421, 4,269,540, 4,297,054, 4,340,322 and 4,345,855, and further in US Patents Nos. 3,641,778, 3,680,342, Re 30,846 and 4,723,874.

The rollers of the assembly 22 are movable between an open position in which they are disengaged from the pipe and a closed position in which they are in contact with the pipe, and are disposed relative to one another and to the pipe so as to straighten the pipe by reverse bending through movement of the straightener along the length of the pipe, in accordance with principles which are well known in the art.

In conventional reel pipelay systems straightening takes place by holding the straightener stationary while the pipe is pulled therethrough. In the present

case the straightener is moved vertically along a length of pipeline whilst the pipeline is held stationary. The straightening assembly 22 is also adapted to be clamped to the pipeline either by engagement of the rollers thereof or by means of a separate clamp incorporated in the assembly. Clamping arrangements of this type are known in the art and will not be described in further detail herein.

A work platform 23 may also be provided on the derrick 12, affording access to the straightening assembly 22. The platform may be enclosed if required.

The pipelaying equipment fitted to the vessel also includes a pipeline clamp 24, an abandonment and recovery (A&R) winch 26, and a tugger winch 28.

Referring now to Figs. 3(a) to (f) of the drawings, the operation of the pipelaying apparatus is as follows.

Firstly, with the straightening assembly 22 open and at its uppermost position, a tugger cable 30 is fed from the tugger winch 28, through the clamp 24 and straightening assembly 22, and around the bending shoe 20 for attachment to the free end of the pipeline 32 emerging from the reel 18 (Fig. 3(a)). The tugger winch 28 is then operated to pull the cable 30 and pipeline over the bending shoe 20, and through the straightener 22 until the pipeline 32 protrudes approximately 5 metres below the straightener 22, at which point the tugger cable 30 is disconnected (Fig. 3(b)).

The straightener 22 is clamped to the pipeline 32 and is lowered to its lowermost position, where it is clamped by the pipeline clamp 24, and an initiation

cable 34 is attached (Fig. 3(c)). The straightener 22 is then undamped from the pipeline 32, is closed and is returned to its uppermost position, thereby straightening the length of pipe extending between the uppermost and lowermost positions (typically a length of about 24 metres) (Fig. 3(d)).

The clamp 24 is released, the straightener 22 is again clamped to the pipeline 32 and lowered therewith to its lowermost position, the clamp 24 re-engages the pipeline 32, the straightener 22 is undamped, closed and returned to its uppermost position to straighten the next length of pipe.

This cycle of operations is repeated until the free end of the pipe achieves vertical stab-in of the initiation head at a stab-in assembly 36 located on the sea bed (Fig. 3(f)), guided by the initiation cable 34 which extends from the end of the pipe 32, through the stab- in assembly 36 and back to the A&R winch 26. Once stab- in is achieved the initiation cable 26 is detached and the vessel proceeds with the pipelay, straightening the pipeline 32 as it is unspooled from the reel 18 in the same manner as described above.

As seen in Fig. 1, the pipeline 32 exits the straightening assembly 22 substantially vertically. Operational considerations, including the water depth, may require a pipe launch angle of less than 90 degrees. In this case a stinger assembly 40 may be fitted to the bottom of the vessel so as to divert the pipeline to the required angle as it exits the bottom of the moonpool 14. The use of stingers in controlling the launch angle in pipelaying operations is well known in the art and will not be discussed further herein.

Fig. 4 illustrates a range of departure angles for a given bending radius, showing how the length of the required stinger 40 would increase as the launch angle becomes smaller. An angle of 70 degrees is likely to be suitable for a typical range of operations, requiring a stinger approximately 20 metres in length, extending downwards from the interior of the moonpool 14 immediately below the pipeline clamp 24.

The method of pipeline straightening described herein straightens the pipe in a piecewise manner, a given length of pipe being straightened in each cycle, rather than in a continuous manner as in systems where the straightener remains stationary and the pipeline is fed continuously therethrough. In shallow water depths, this piecewise mode of straightening may require the forward speed of the vessel to be varied in synchronism with the cycling of the straightener. However, where the water depth is greater, the length of pipe between the vessel and the sea bed will sufficient to accommodate the slack produced by the piecewise unspooling of the pipe from the reel, so that the vessel may proceed along the pipelay path at substantially constant speed.

The invention thus provides methods and apparatus allowing pipelaying operations to be conducted at relatively low cost, particularly where a drilling vessel is employed and the existing derrick, crane etc. of the vessel are exploited.

Modifications and improvements may be incorporated without departing from the scope of the invention.