GB2070724A

GB2070724A - Laying underwater pipeline

Info

Publication number: GB2070724A
Application number: GB7938613A
Authority: GB
Original assignee: Santa Fe International Corp; Sante Fe International Corp
Current assignee: Santa Fe International Corp; Sante Fe International Corp
Priority date: 1979-11-11
Filing date: 1979-11-11
Publication date: 1981-09-09
Also published as: BR8007313A; CA1158872A; GB2070724B

Abstract

In the laying and recovery of underwater pipelines from and to a surface vessel having a reel from and onto which relatively inflexible pipe is unspooled and spooled, the thrust imparted to the vessel by the vessel motive force and the dynamic braking force imparted to the reel by a reel drive mechanism are co-ordinated to maintain sufficient tension in the pipeline to hold the pipeline in a desired profile, this coordination for the aforesaid purpose embracing both the stoppage of the forward motion of the vessel to maintain a station keeping mode to enable any necessary operations to be carried out on the pipe, and the subsequent resumption of the forward motion of the vessel.

Description

SPECIFICATION Reel pipe laying and respooling methods background of the invention This invention relates to techniques and meth ods utilized in laying underwater pipelines.

More particularly, the invention relates to lay ing pipelines wherein continuous lengths of pipe are first spooled onto a reel carried by a vessel and are thereafter unspooled into the water as the vessel proceeds along the pipel ine route. Still more particularly, the invention relates to techniques associated with the lay ing of such pipelines, including attachment of sacrificial anodes, effecting repairs to the coating and/or pipe during the laying proc ess, and with respooling the pipeline, either as part of the primary operation or as part of a pipeline salvage operation.

The methods and techniques described herein are applicable to several types of reel pipelaying vessels. Suitable vessel, w ich would be expected to use the methods and techniques described herein include towed barges, converted drill ships and/or ore carri ers, and specially designed and constructed self-propelled reel type pipelaying ships.

Towed barges capable of utilizing the methods and techniques described here have included barges owned and operated by Santa Fe Inter national Corporation (hereafter " Santa Fe") and/or Santa Fe's predecessors-in-interest.

The first known commercial reel type pipelay ing barge, called the U-303, was operated by Aquatic Contractors and Engineers, inc. a pre'decessor-in-interest to Santa Fe. The U-303 utilized a large vertical axis reel, per manently mounted on a barge and having horizontally oriented flanges (generally referred to in the trade as a "horizontal reel").

A combined straightener-level winder was em ployed for spooling pipe onto the reel and for straightening pipe as it was unspooled. The U-303 first laid pipe commercially in Septem ber, 1961, in the Gulf of Meixco off the coast of Louisiana and was used successfully during ,the 196O's to lay several million linear feet of pipe of up to 6" diameter. The U-303 reel pipelaying barge is described in British Patent No. 1,077,331 and in corresponding U.S.

Patent Nos. 3,237,438 issued March 1, 1966 to Prosper A. Tesson and 3,372,461 issued March 12, 1968 to Prosper A. Tesson.

The succesor to the U-303, currently being operated in the Gulf of Mexico by Santa Fe and known in the trade as the "Chickasaw", was built and operated by Fluor Ocean Services, Inc., a successor-in-interest to Aquatic Contractors and Engineers, inc., and a predecessor-in-interest to Santa Fe. The "Chickasaw" also utilizes a large horizontal reel, per manently mounted to the barge such that it is not readily movable from one carrier vessel to another. Various aspects of the "Chickasaw" are described in the following patents: British Patent No. 1,312,592 and corresponding U.S. Patent Nos.: 3,630,461, issued December28, 1971 to Daniel E. Sugasti, Larry R. Russel, and Fred W. Schaejbe; 3,680,342, issued August 1, 1972 to James D. Mott and Richard B.Feazle; and 3,712,100 issued January 23, 1973 to Joe W. Key and Larry R. Russell; and British Patent No. 1,312,593 and corresponding U.S. Patent No. 3,641,778, issued February 15, 1972 to Robert G. Gibson.

A portable pipelaying system designed and built by Santa Fe for use on small supply boat type vessels for laying small diameter pipelines (up to 4" nominal diameter) Has been in commercial use off the coast of Australia since about July, 1978; this portable pipelaying system is described in British Patent Application No. 22079/78 and corresponding U.S.

Patent U.S. Patent Application No. 909,260, each filed May 24, 1978 in the names of Stanley T, Uyeda and John H Cha, as inventors.

Santa Fe has developed and built a selfpropelled reel ship known in the industry as "Apache" which began commercial pipelaying operations in August, 1 979. Features of Apache-type pipelaying ships are described in the following British and U.S. patent and applications: British Patent No. 1,507,959 and corresponding U.S. Patent No. 3,982,402 issued September 28, 1 976, in the names of Alexander Craig Lang and Peter Alan Lunde, as inventors; British Patent Application No. 79/15904 filed May 8, 1979, and corresponding U.S.

Application No. 903,180, filed May 5, 1978 in the names of Charles N. Springett, Dan Abramovich, Stanley T. Uyeda and E. John Radu, as inventors; British Patent Application No. 7915914, filed May 8, 1979, and corresponding U.S. Application Nos.

903,181, filed May 5, 1978 and 35,216, filed May 2, 1 979 as a Continuation-in-Part of said Serial No. 903,181 each in the names of Stanley T. Uyeda, E. John Radu, William J. Talbot, Jr. and Norman Feldman, as inventors.

The present disclosure (and inventive subject matter described and claimed herein) and the above-listed British and U.S. patents and applications are all owned by Santa Fe.

The disclosures of the above listed British and U.S. patents and applications are incorporated herein in their entireties by reference as though fully set forth below.

SUMMARY OF THE INVENTION The present invention was developed to satisfy the requirements of commercial pipelaying operations. More particularly, the invention was developed to meet the requirements of pipeline owners and various govern mental and industry-established regulatory bodies or agencies while at the same time making the reel pipelaying technique commercially practical, economical and competitive with other pipelaying methods, such as the laying technique known in the trade as "stove-piping." This invention also has particularly advantageous utility in such cases where it is desired to remove a pipeline from one location and relay it elsewhere. Reuse of the same pipeline can be environmentally advantageous; e.g., natural resources are conserved, pipe is not left on the sea bottom as "trash", etc.Such reuse can also have economic advantages. The cost of constructing a pipeline of a desired length is generally substantially greater than the cost of respooling a previously laid pipeline of similar length.

Therefore the effective cost of the "new pipeline is substantially reduced in the case where a pipeline is reused.

In one aspect, the invention is directed to a method of intermittently stopping and restarting the laying of pipe offshore from a reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for imparting at least a dynamic braking force to the reel in an unspooling mode, said method comprising the steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying by (1) gradually reducing the forward thrust imparted to the vessel by the vessel motive source, and (2) concurrently gradually increasing the dynamic braking force imparted to the reel by the reel drive mechanism (3) to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain sufficient tension in the pipeline to hold the pipeline in a desired profile; b. locking the reel against further rotation in the direction of unspooling when the vessel has stopped its forward motion; c. thereafter controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under said sufficient tension to hold the desired pipe profile while the vessel is in said station keeping mode;; d. performing any necessary operations on the pipe and/or coating while the vessel is in said station keeping mode; e. thereafter resuming forward motion of the vessel in the pipelaying direction by (1 ) disengaging the means locking the reel aginst the rotation in the unspooling direction, (2) concurrently substantiaily increasing the dynamic braking force imparted to the reel by the reel drive mechanism such that the dynamic braking force exceeds any increases in the forward thrust of the vessel's motive source at least up to the pipelaying thrust level required to move the vessel in the forward direction at a desired laying speed, (3) gradually increasing the forward thrust imparted to the vessel by the vessel motive source up to said pipelaying thrust level, and (4) thereafter gradually reducing the dynamic braking force of the reel drive mechanism at least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

In a further aspect, the invention concerns a method of intermittently stopping the laying of pipe offshore from a reel pipelaying vessel, thereafter respooling a portion of the laid pipeline, and thereafter resuming the laying of pipe, said reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said method comprising steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying by (1) gradually reducing the forward thrust imparted to the vessel by the vessel motive source, and (2) concurrently gradually increasing the dynamic braking force imparted to the reel by the reel drive mechanism, (3) to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain sufficient tension in the pipeline to hold the pipeline in a desired profile; b. locking the reel against further rotation in the direction of unspooling when the vessel has stopped its forward motion; c. thereafter controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under said sufficient tension to hold the desired pipe profile while the vessel is in said station keeping mode; ; d. thereafter respooling at least a portion of the laid pipeline by (1) switching the reel drive mechanism from unspooling mode (2) adjusting the straightening means for a noload condition whereby the straightening force to the pipe passing therethrough in the respooling direction, (3) gradually increasing drive torque imparted to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotating the reel in the spool-up direction, (5) thereafter respooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero, (7) therafter again locking the reel against further rotation in the unspooling direction, and (8) thereafter switching the reel drive mechanism from spooling-up mode to unspooling mode; e. thereafter resuming forward motion of the vessel in the pipelaying direction by (1) disengaging the means locking the reel against the rotation in the unspooling direc tion, (2) concurrently substantially increasing the dynamic braking force imparted to the reel by the reel drive mechanism such that the dynamic braking force exceeds any increases in the forward thrust of the vessel's motive source at least up to the pipelaying thrust level required to move the vessel in the for ward direction at a desired laying speed, (3) gradually increasing the forward thrust im parted to the vessel by the vessel motive source up to said pipelaying thrust level, and (4) thereafter gradually reducing the dynamic braking force of the reel drive mechanism at least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

In a still further aspect, the invention com prises a method of spooling at least a portion of a previously laid pipeline onto a reel pipe laying vessel, said pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the' reel, vessel motive source means for im parting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said method comprising steps of:: a. controlling the forward thrust of the "vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under said sufficient tension to hold ;the desired pipe profile while the vessel is in said station keeping mode; b. locking the reel against rotation in the direction of unspooling when the vessel is in said station keeping mode with the one end of the pipeline secured to the reel; and c. spooling at least a portion of the pipeline onto the reel by (1) placing the reel drive mechanism into a spooling mode, (2) adjust ing the straightening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the spooling direction, (3) gradually increasing drive torque imparted to the reel by the reel drive mecha nism to overcome the forward thrust imparted by the vessel motive source and begin rotat ing the reel in the spool-up direction, (4) thereafter spooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero when the desired amount of pipeline has been spooled onto the reel, and (7) thereafter again locking the reel against further rotation.

BRIEF DESCRIPTION OF THE DRAWING Figures lA-B show plan and side views of a U-303 type reel pipelaying barge, as described in British Patent No. 1,077,331.

Figures 2A-B show plan and side views of a 'Chiskasaw" type reel pipelaying barge, as described in British Patent No. 1,312,592.

Figure 3 shows a perspective view of an Apache-type adjustable pipe ramp assembly on which is located various pipe handling equipment, as described in British Application No. 79/15904.

Figure 4 shows a diagrammatic side view of a reel type pipelaying vessel and the profile of the pipe between the vessel and the sea bottom.

DESCRIPTION OF PREFERRED EMBODI MENTS In the following description of the several types of reel pipelaying vessels shown in the drawing fugures similar elements will be designated by the same reference numerals. All reel type pipelaying vessels have certain features in common, including a hull 10, on the deck of which is mounted a pipe carrying reel 12. One or more pipelines of lengths sufficient to satisfy customer job requirements may be spooled onto reel 12 (up to the design capacity of the reel). Although only a "horizontal" type reel is shown in detail, Apachetype vessels employ a "vertical" reel (i.e., the reel having vertically oriented flanges and mounted for rotation about a generally horizontal axis) and operate (for purposes of the herein described invention) in substantially the same way as horizontal reel vessels.

Pipe handling equipment is located along the pipe path downstream of the reel in the unspooling direction. Such pipe handling equipment may include a level winder/ straightening means 16 and separate tensioning means 18 or a combined level wind/ straightening/tensioning apparatus 20 as on Apache-type vessels.

Additional features of the vessels shown in Figs. 1-3 are described in greater detail in one or more of the above-listed Santa Fe owned British and U.S. patents and applications.

Commercially laid underwater pipelines for carrying oil or gas must meet certain requirements and limits set by the customer (pipeline owner) and/or governmental or other regulatory bodies. For example, the pipe, as it is being laid and as it lays on the sea bottom, should be subjected to minmal residual stress, strain, tension, etc. This means that the pipe as it lays on the sea bottom should be straight and have substantially no residual curvature due to the spooling or laying processes. The "as laid" restrictions are a function of a number of parameters developed by the pipeline designer, including the type of sea bed on which the pipe rests, the size and grade of pipe to be used, the type, amounts, and flow rates of fluid to be carried by the pipeline, and predicted life span of the pipeline.Other parameters relating to, or based on, the geometry (shape) of the pipeline during the pipe laying operation (and taking owner supplied parameters into consideration) are developed by the pipelaying engineers.

The pipe laying engineers must also take into consideration the type of coating on the pipe to be laid in a given operation when developing the pipe laying guideline parameters for use by the operating personnel on board the pipelaying vessel.

In addition, many commercial pipeline owners require that sacrificial anodes be attached to pipe laid offshore to inhibit the corrosive effect of the salt water on the pipe. The pipeline owner may require that anodes be attached at specified intervals, ranging generally between 300 feet and 1,000 feet.

In the "stovepiping" method of laying pipe, anodes are pre-attached to certain pipe sections that are then welded in place at the onboard end of the already-laid pipeline portion.

In the reel pipelaying method, on the other hand, the thin film coated pipe travels through various pipe handling equipment which must make smooth contact with the moving coated outer surface of the pipe to avoid tearing the coating and to adhere as closely as possible to the customer developed "as-laid" requirements. Therefore the anodes must be attached at required intervals aft of the pipe handling equipment, including the straightener/tensioner mechanism, as the pipeline is being unspooled and before it leaves the vessel. It normally takes about 3 to 5 minutes to attach a sacrificial anode to the pipe, requiring that the vessel be stopped.

Thus, when laying a 15,000 foot line with a requirement that anodes be attached every 500 feet, the vessel may have to be stopped 30 times during the lay operation to permit application of anodes.

The novel techniques discribed herein of bringing the pipelayinvg vessel to a stop during a pipelaying operation and thereafter restarting the vessel to continue laying pipe are applicable as well to situations in which the laying operation must be temporarily halted to allow repairs to be made to the pipe and/or coating. In many cases, minor repairs to the coating can be effected without stopping the laying operation; such minor coating repairs are made "on the fly" with the pipeline continuously passing through one or more repair stations before it enters the water.

Sometimes, more extensive repairs must be made, which require that the laying operation be stopped e.g., to permit a damaged pipe section to be cut out and the cut ends rejoined, welded and recoated. The nature and time required for repairs of this type preclude their being done "on the fly".

The anode locations are marked on the pipe and the anodes are applied at a specific anode application station on the along the pipe path.

Coating repair locations may also be premarked (e.g., when noted during the original spooling-on operation) or be visually or elegy tronically observable the operational personnel watching the pipe as it is unspooled.

In order to meet the above-noted pipe laying requirements, a balance must be maintained between the tension on the pipeline and the vessel's motive source, e.g., main propulsion units on a self-propelled shop or the tug(s) powering a towed barge. (Hereinafter, references to the vessel's "motive source" will be taken to mean generally the means employed to provide a forward thrust component to move the vessel in the forward direction along the right-of-way). The person in charge of the pipe laying operation (generally and hereafter in this disclosure called the superintendent") must effectively control the thrust of the motive source to (1) maintain adequate tension on the pipe at all significant times during the laying operation and (2) avoid excessive tension on the pipeline when the reel is braked to a stop, as during an anode application operation.

As noted above, an important spect of stopping the pipelaying vessel during a laying operation is the requirement that, in general, adequate tension must be maintained on the pipe at all significant times. This is necessary to prevent the "sag bend" from exceeding certain predetermined tolerance limits. As shown in Fig. 4, the "sag bend" region of the pipeline occurs at or near the sea bottom where the pipe curves back to the horizontal plane as it comes to rest on the sea bottom. The approximate point at which the pipe touches the bottom is called the touchdown point (TDP). The radius of the sag bend should normally be greater than the minimum permissible radius to which the pipe may be bent without exceeding elasticity limits in accordance with customer requirements. The pipeline should normally be kept under sufficient tension during the laying operation to maintain the proper desired pipe profile between the pipe departure point from the vessel and the sea bottom on which the pipe rests, and, in particular, to prevent the sag bend radius from decreasing below its allowable minimum.

For convenience, hereafter, a "desired pipe profile" is one in which, at all significant times, the maximum allowable working stress, due to the pipelaying operation, in the unsup ported length of pipe between the vessel and the sea bottom will not normally be greater than about 85% of the minimum yield strength of the pipe.

Also as noted above, another important requirement associated the stooping the pipe laying vessel during a laying operation con cerns the avoidance of imparting excess ten sion to the pipe. Controlled tension is im parted to the pipe by one or more of (1 ) the reel through the reel drive mechanism opera tion as a dynamic brake, (2) the main vessel drive thrust, generated by the towing tug or on board main drive source (the vessel's "motive source ), and (3) pipe handling equip ment, including a straightener or straighten er/tensioner assembly, such as described, for example, in one or more of the above-refer enced prior related Santa Fe patents/applica tions.

In order to satisfy the requirement that adequate tension be maintained on the pipe while excess tension be avoided at all signifi cant times during the pipelaying operation, certain procedures have been developed by Santa Fe and/or Santa Fe's precedecessors in-interest while operating the U-303 and/or "Chickasaw". These operating steps are unique to the technique of laying pipe by the reeel method, stopping the reel laying vessel during the laying operation, and subsequently restarting the vessel. Additional techniques described herein pertain to retrieving pipe from the sea bottom, e.g., during a salvage operation.

A. Stopping the pipelaying operation.

When it is necessary to stop a pipeline laying operation in midstream, as, for exam ple, to apply a sacrificial anode to the pipel ine, the pipelaying vessel must be brought to a gradual stop to avoid a rapid sharp increase in tension which could result in damage to the pipe and/or coating. When it is observed that the premarked anode application location (or pipeline section requiring repair) is about to be unspooled, the superintendent orders a gradual reduction in the forward thrust of the vessel to thereby reduce the forward momen tum of the pipelaying vessel. On a towed barge, the superintendent orders the towing tug to reduce its forward thrust; on a self propelled vessel, the superintendent orders a reduction in the forward thrust of the main drive engines. Reducing the forward thrust produces a reduction in the tension holding the pipe in its desired profile.In order to maintain the desired pipe profile as the forward motion of the vessel is reduced to zero, the superintendent orders a corresponding in crease in the dynamic braking force exerted by the reel drive mechanism. The combination of reduced forward thrust and increased braking force serves to maintain the pipe under adequate tension and thus hold the proper profile. As the forward thrust of the vessel is reduced and the braking force of the reel increased, the vessel continues to lay pipe at a gradually decreasing lay rate. The superintendent must therefore begin the stopping sequence well in advance so that when the vessel comes to a complete stop, the portion of the pipe to be worked on will be located at the appropriate work station.

As soon as the vessel comes to a stop and the lay rate has been reduced to zero, the reel is "dogged" or locked in a known manner to prevent further unspooling of pipe. The superintendent then orders any necessary adjustments to the vessel's motive source to modify the forward thrust as required to hold the vessel in a station-keeping position, while maintaining a desired pipe profile. Surges due to sudden changes in the vessel thrust or reel braking force can produce increases in pipeline tension which exceed the maximum allowable stress/residual tension limit requirements imposed by the pipeline owner; an abrupt decrease in the vessel thrust or reel braking force can produce decreases in pipeline tension and cause the sag bend radius (see Fig.

4) to decrease below the minimum permissible radius for the particular pipeline, possibly even resulting in severe damage to the pipe and/or coating due to buckling. It is therefore desirable and preferable to avoid such surges.

Adjustments to the vessel's forward thrust and/or reel braking forces should therefore be made gradually to avoid surging actions as much as possible.

Once the reel has been dogged or locked to prevent further rotation in the direction of unspooling, anode application or repair operations can be performed on the stopped pipe.

B. Respooling pipe.

If the pipeline is to be respooled, the reel drive mechanism is changed over from a dynamic braking mode to a driving mode. It is important that the reel be dogged before this switch over to maintain the pipe under adequate tension at all significant times. The vessel motive source thrust is held at the minimum level necessary to maintain the pipelaying vessel in a station-keeping attitude.

Before beginning to respool pipe, the straightener mechanism is preferably retracted to a no-load (or spool up) position so that it exerts no reverse bending force on the pipe.

Preferably, therefore, no straightening of the pipe will take place during the respooling mode, since the pipe which is being respooled had previously been straightened.

The tensioner mechanism may or may not be retracted to a no-load position, depending upon whether it is necessary or desirable to impart tension to the pipe in addition to or alternatively to reel imparted tension. If a power driven tensioner mechanism is em pioyed, and such mechanism is to be used to impart tension to the pipe in a respooling mode, the tensioner drive mechanism will generally be reversed to drive the tensioner mechanism in the direction of respooling pipe movement at a rate which produces the desired respooling tension or part thereof.

As the next step in a respooling operation, while maintaining station-keeping thrust on the vessel drive source, and while the reel remains dogged, the superintendent orders a gradual increase in power to the reel drive mechanism. The pipeline tension will thus be reacted directly to the reel drive mechanism.

When the reel drive power becomes greater than the forward thrust of the vessel motive source so that the reel just begins to move in the spool up direction, the dog(s) holding the reel against rotation in the unspooling direction are disengaged. Disengagement of the reel dog(s) should not take place until after the reel has begun to move in order that there will be no sudden loss of tension and/or surging in the pipeline.

Respooling tension may in fact be greater than laying tension and is considerably greater than the initial spooling tension, that is the tension on the pipe as it is spooled onto the reel at the pipeline base. To avoid damaging the pipe due to the additional tension imparted during the respooling operation a special technique, called "crossover" spooling, is advantageously and preferably used. One such crossover spooling method is described in detail in British Patent Application No.

22,080/78, filed May 24, 1978, titled "METHOD AND APPARATUS FOR SPOOL ING PIPE ON A REEL"; the disclosure of said British application No. 22,080/78 is incorporated herein in its entirety by reference as though fully set forth below.

Pipe is respooled by generating sufficient torque in the reel to overcome the forwardly directed station-keeping thrust of the vessel motive source and thereby pull the pipelaying vessel back down the right-of-way. The forward thrust imparted to the pipelaying vessel by the vessel motive source should preferably not be so great as to impose a torque on the reel that exceeds the capacity of the reel drive mechanism. Preferably and advantageously the forward thrust of the vessel motive source is set to be sufficiently less than the maximum permitted reel torque so that the respooling rate may be controlled by adjustments to the reel torque through the reel drive mechanism.

Under certain conditions, the force required to respool the pipe may be greater than the maximum torque which can be generated in the reel by the reel drive mechanism while holding the pipe under sufficient tension to maintain the desired pipe profile. In order to maintain such adequate tension on the pipe during the respooling operation, even though the capacity of the reel drive mechanism is exceeded, the tensioner mechanism may be used to provide the additional force required to respool the pipe.

One such circumstance under which the total required respooling force is greater than that which can be generated by the reel alone while maintaining the desired pipe profile occurs when relatively large diameter pipe (e.g.

12" l.D.-1 8" I.D.) is to be respooled from relatively deep water (e.g. 1,000 foot water depths or greater). At such times, a power driven tensioner apparatus can be used to impart an additive force component to the respooling force generated by the reel drive mechanism to produce the total force necessary to respool the pipe.

Another circumstance which may require the use of a power driven tensioner during the respooling process occurs when the total force needed to respool the pipe can create an overtension in the pipe which could result in damage to the pipe and/or pipe coating as the pipe is being respooled onto the reel.

Overtensioning may be avoided by using the powered tensioner to impart a portion af the force required to raise the pipe, with the balance of the required force being taken up by the reel driven mechanism. In this way, the portion of the pipe between the reel and tensioner mechanism will be subject to only the force imparted by'the reel; the total force needed to raise the pipe is only applied downstream of the tensioner.For example, if the combination of water depth and pipe diameter is such as to require a respooling tension of about 200,000 pounds, but the pipe can only safely withstand a maximum spooling tension at the reel on the order of 150,000 pounds (per customer/pipeline owner specification based on such factors as pipe size, grade, coating, etc.), the tensioner mechanism can be set to impart 100,000 pounds of force and the remaining 100,000 pounds can abe imparted by the reel drive mechanism. Thus, the portion of the pipeline downstream of the tensioner will be subject to a total respooling tension of about 200,000 pounds; however, between the tensioner and the reel, the pipe will only "see" a tension on the order of 100,000 pounds, well within the safe operating limits of the pipe and reel.

When the desired amount of pipe has been respooled, the drive torque imparted to the reel by the reel drive mechanism is reduced to balance the forward thrust imparted by the vessel motive source. To avoid tension surges when terminating the respooling operation, power to the reel drive mechanism should be gradually reduced to gradually decrease the reel torque and thereby gradually bring the reel imparted tension into balance with the forward thrust imparted by the vessel motive source. If the reel imparted tension had been supplemented by a powered tensioner during the respooling mode, it will generally also be necessary to reduce the tensioner force in order to reduce the pipeline tension to a value balancing that of the forward thrust component of the vessel motive source.The superintendent must anticipate termination and order the gradual reduction in power well before the respooling termination point.

When the pipeline tension is reduced to a value which just balances the forward thrust on the vessel so that the reel has stopped respooling pipe, the reel is redogged to lock it against rotation in the unspooling direction.

The reel drive mechanism is then changed over from the driving mode to a dynamic braking mode. At this time, the straightener mechanism is reset to straighten the pipe as it is unspooled. If a tensioner mechanism is used, it too, is reset to impart the desired amount of laying tension to the pipe.

C. Restarting the pipelaying operation The pipelaying operation is restarted by gradually increasing the forward thrust imparted by the vessel's motive source. The reel dog is disengaged before increasing the forward thrust of the vessel's motive source but after the superintendent has been assured that the vessel is in the desired station-keeping mode and pipeline tension is properly balanced against the vessel's forward thrust of the vessel motive source up to the thrust level required to move the vessel along the pipeline right-of-way at the desired laying speed.

The dynamic braking force of the reel drive mechanism is established greater than the forward thrust of the vessel motive source; the vessel will therefore remain on station as the motive source thrust is gradually increased up to pipelaying thrust, that is, the thrust necessary to maintain the vessel underway at a desired pipelaying speed with the pipe under a predetermined tension as established by the dynamic braking force of the reel drive mechanism. The superintendent then orders a very gradual reduction in the dynamic braking force of the reel drive mechanism until the vessel's forward thrust begins to exceed the back tension imparted by the reel drive mechanism so that the vessel begins to move forward and the reel begins to turn to pay out pipe.The dynamic braking forcre of the reel drive mechanism can thereafter be reduced at a more rapid rate commensurate with the vessel's ability to accelerate to bring the vessel up to normal pipelaying speed while holding the pipe under sufficient tension to maintain the desired pipe profile.

If anodes are to be applied to the pipeline, the same procedures are followed as described above except that the steps associated with respooling the pipe (section "B" above) may be omitted. Since the vessel must be stopped for 3 to 5 minutes while anodes are applied, all of the above described steps associated with stopping and restarting the pipelaying operation (sections A and C above) are followed.

Minor repairs to the pipe coating may be made "on the fly" at a reduced lay rate.

When, during the unspooling procedure, it becomes apparent before or just as the pipe comes off the reel, that a minor coating repair will be necessary, the superintendent orders an increase in the dynamic braking force of the reel drive mechanism, thereby increasing the back tension on the pipe and reducing the net forward motion of the vessel as long as the forward thrust of the vessel motive source is not increased (e.g., vessel forward thrust is hele constant). The lay rate is therefore reduced to enable such minor repairs to be made. For more extensive repairs, the pipelaying operation must be stopped as per the procedures described above.

When the pipelaying operation is temporarily stopped, e.g., for anode application, major pipe repair, etc., the tension on the pipeline is preferably and advantageously maintained between upper and lower operating limits. The lower limit constitutes the minimum acceptable tension which will maintain the desired pipe profile; the maximum tension limit constitutes the maximum tension under which the pipeline may be held for a given type, size and tgrade of pipe. The reel drive mechanism, in its dynamic braking mode, constitutes the primary control mechanism for maintaining the pipeline tension within the above-noted upper and lower limits. The vessel's motive source is used in conjunction with the reel drive mechanism control to provide a nominal thrust/tension balance about which the reel drive mechanism may be varied for maximum tension control within these limits.Thus the combination of the reel drive mechanism and vessel motive source must provide the capability for (a) station-keeping for anode application and (b) adjustable net forward motion for making coating repairs "on the fly" while paying out pipe and not exceeding the abovestated mechanical limitations of the pipe, while permitting control of pipeline variables through the reel drive mechanism.

It is further noted that the respooling procedures (section "B" above) employed as part of a pipe salvage operation after the pipeline end has been picked up and secured to the reel or the stub end of a pipeline remaining on the reel from a prior operation.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiment described above is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the hereafter appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

CLAIMS 1. A method of intermittently stopping and restarting the laying of pipe offshore from a reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening theh pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for imparting at least a dynamic braking force to the reel in an unspooling mode, said method comprising the steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying (1) gradually reducing the forward thrust imparted to the vessel by the vessel motive source, and (2) concurrently gradually increasing the dynamic braking force imparted to the reel by the reel drive mechanism, (3) to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain sufficient tension in the pipeline to hold the pipeline in a desired profile; b. locking the reel against further rotation in the direction of unspooling when the vessel has stopped it forward motion; c. thereafter controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under said sufficient tension to hold the desired pipe profile while the vessel is in said station keeping mode;; d. performing any necessary operations on the pipe and/or coating while the vessel is in said station keeping mode; e. thereafter resuming forward motion of the vessel in the pipelaying direction by (1) disengaging the means locking the reel against the rotation in the unspooling direction, (2) concurrently substantially increasing the dynamic braking force imparted to the reel by the reel drive mechanism such that the dynamic braking force exceeds any increases in the forward thrust of the vessel's motive source at least up to the pipelaying thrust level required to move the vessel in the forward direction at a desired laying speed, (3) gradually increasing the forward thrust imparted to the vessel by the vessel motive source up to said pipelaying thrust level, and (4) thereafter gradually reducing the dynamic braking force of the reel drive mechanism at least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

2. A method according to claim 1, wherein the step of preforming any necessary operations on the pipe and/or coating comprises the step of applying one or more sacrifical anodes to the pipe at a designated work station located along the pipe path downstream of the straightening means in the unspooling direction.

3. A method of intermittently stopping the laying of pipe offshore from a reel pipelaying vessel, thereafter respooling a portion of the laid pipeline, and thereafter resuming the laying of pipe, said reel pipelaying vessel having a teel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening theh pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said method comprising steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying by (1) gradually reducing the foward thrust imparted to the vessel by the vessel motive source, and (2) concurrently gradually increasing the dynamic braking force imparted to the reel by the reel drive mechanism, (3) to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain sufficient tension in the pipeline to hold the pipeline in a desired profile; b. locking the reel against further rotation in the direction of unspooling when the vessel has stopped its forward motion; c. thereafter controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under said sufficient tension to hold the desired pipe profile while the vessel is in said station keeping mode;; d. thereafter respooling at least a portion of the laid pipeline by (1) switching the reel drive mechanism from unspooling mode to spooling mode, (2) adjusting the straigthening means for a no-load condition whereby the straigtening means imparts substantially no~ straightening force to the pipe passing therethrough in the respooling direction, (3) gradually increasing drive torque imparted to they reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotating the reel in the spool-up direction, (4) thereafter disengaging the means locking the reel against further rotation in the unspooling direction, (5) thereafter respooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero, (7) thereafter again locking the reel against further rotation in the unspooling direction, and (8) thereafter switching the reel drive mechanism from spooling-up mode to unspooling mode;; e. thereafter resuming forward motion of the vessel in the pipelaying direction by (1 ) disengaging the means locking the reel against the rotation in the unspooling direc tion, (2) concurrently substantially increasing the dynamic braking force imparted to the reel by the reel drive mechanism such that the dynamic braking force exceeds any increases in the forward thrust of the vessel's motive source at least up to the pipelaying thrust level required to move the vessel in the for ward direction at a desired laying speed, (3) gradually increasing the forward thrust im parted to the vessel by the vessel motive source up to said pipelaying thrust level, and (4) thereafter gradually reducing the dynamic braking force of the reel drive mechanism as least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

4. A method according to claim 1 or 3, further comprising maintaining a balance be tween the rate of reduction of the forward thrust imparted to the vessel and the rate of increase of dynamic braking force imparted to the reel in step (a) to maintain the sag-bend radius in the portion of the pipeline neara the sea bed above a minimum permissible value to thereby avoid damage to the pipeline due to buckling.

5. A method of spooling at least a portion of a previously laid pipeline onto a reel pipe laying vessel, said pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for im parting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternatively driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said 'method comprising of:: a. controlling the forward thrust of the vessel motive source to maintain the vessel in ea station keeping mode and to maintain the pipeline under sufficient tension to hold the pipeline in a desired profile while the vessel is in said station keeping mode; b. locking the reel againsta rotation in the direction of unspooling when the vessel is in said station keeping mode with the one end of the pipeline secured to the reel; and c. spooling at least a portion of the pipeline onto the reel while maintaining the pipeline under sufficient tension to hold the desired pipeline profile, by (1 ) placing the reel drive mechanism into a spooling mode, (2) adjust ing the straightening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the spooling direction, (3) gradually increasing drive torque imparted to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotation the reel in the spool-up direction, (4) thereafter disengaging the means locking the reel against rotation in the unspooling direction, (5) thereafter spooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero when the desired amount of pipeline has been spooled onto the reel, and (7) thereafter again locking the reel against further rotation.

6. A method according to claim 5, wherein said pipe handling and working means includes powered tensioner means for tensioning the pipeline independently of reel imparted tension, said method comprising the further step of: imparting additional tension to the pipeline through said tensioner means to supplement tension imparted to the pipeline by the reel during said step (c).

7. A method according to claim 6, further comprising the step of: adjusting the tensioner drive force in proportion to adjustments in drive torque imparted to the reel to maintain a substantially constant tension in the pipeline while spooling up said pipeline according to said step (c).

8. A method according to claim 6 or 7, further comprising: imparting a substantially greater amount of tension to the pipeline by the tensioner than by the reel during said step (c), wherein the portion of the pipeline between the reel and tensioner means is subjected to substantially less tension than is subjected to substantially less tension than the pipeline between the tensioner and the sea bottom.

9. A method substantially as hereinbefore described of intermittently stopping and restarting the laying of pipe offshore from a reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for imparting at least a dynamic braking force to the reel in an unspooling mode.

CLAIMS (8 Dec 1980) 2. A method according to claim 1, wherein the step of preforming any necessary operations on the pipe and/or coating comprises the step of applying one or more sacrificial anodes to the pipe at a designated work station located along the pipe path downstream of the straightening means in the unspooling direction.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

e. thereafter resuming forward motion of the vessel in the pipelaying direction by (1 ) disengaging the means locking the reel against the rotation in the unspooling direc tion, (2) concurrently substantially increasing the dynamic braking force imparted to the reel by the reel drive mechanism such that the dynamic braking force exceeds any increases in the forward thrust of the vessel's motive source at least up to the pipelaying thrust level required to move the vessel in the for ward direction at a desired laying speed, (3) gradually increasing the forward thrust im parted to the vessel by the vessel motive source up to said pipelaying thrust level, and (4) thereafter gradually reducing the dynamic braking force of the reel drive mechanism as least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

4. A method according to claim 1 or 3, further comprising maintaining a balance be tween the rate of reduction of the forward thrust imparted to the vessel and the rate of increase of dynamic braking force imparted to the reel in step (a) to maintain the sag-bend radius in the portion of the pipeline neara the sea bed above a minimum permissible value to thereby avoid damage to the pipeline due to buckling.

5. A method of spooling at least a portion of a previously laid pipeline onto a reel pipe laying vessel, said pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for im parting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternatively driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said 'method comprising of:: a. controlling the forward thrust of the vessel motive source to maintain the vessel in ea station keeping mode and to maintain the pipeline under sufficient tension to hold the pipeline in a desired profile while the vessel is in said station keeping mode; b. locking the reel againsta rotation in the direction of unspooling when the vessel is in said station keeping mode with the one end of the pipeline secured to the reel; and c. spooling at least a portion of the pipeline onto the reel while maintaining the pipeline under sufficient tension to hold the desired pipeline profile, by (1 ) placing the reel drive mechanism into a spooling mode, (2) adjust ing the straightening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the spooling direction, (3) gradually increasing drive torque imparted to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotation the reel in the spool-up direction, (4) thereafter disengaging the means locking the reel against rotation in the unspooling direction, (5) thereafter spooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero when the desired amount of pipeline has been spooled onto the reel, and (7) thereafter again locking the reel against further rotation.

6. A method according to claim 5, wherein said pipe handling and working means includes powered tensioner means for tensioning the pipeline independently of reel imparted tension, said method comprising the further step of: imparting additional tension to the pipeline through said tensioner means to supplement tension imparted to the pipeline by the reel during said step (c).

7. A method according to claim 6, further comprising the step of: adjusting the tensioner drive force in proportion to adjustments in drive torque imparted to the reel to maintain a substantially constant tension in the pipeline while spooling up said pipeline according to said step (c).

8. A method according to claim 6 or 7, further comprising: imparting a substantially greater amount of tension to the pipeline by the tensioner than by the reel during said step (c), wherein the portion of the pipeline between the reel and tensioner means is subjected to substantially less tension than is subjected to substantially less tension than the pipeline between the tensioner and the sea bottom.

9. A method substantially as hereinbefore described of intermittently stopping and restarting the laying of pipe offshore from a reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for imparting at least a dynamic braking force to the reel in an unspooling mode.

CLAIMS (8 Dec 1980)

2. A method according to claim 1, wherein the step of preforming any necessary operations on the pipe and/or coating comprises the step of applying one or more sacrificial anodes to the pipe at a designated work station located along the pipe path downstream of the straightening means in the unspooling direction.

3. A method of intermittently stopping the

laying of pipe offshore from a reel pipelaying vessel, thereafter respooling a portion of the laid pipeline, and thereafter resuming the lay ing of pipe, said reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for im parting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode andj impartaing a dynamic braking force to the reel in an unspooling mode, said method comprising the steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying by (1 ) gradually reducing the forward thrust imparted to the vessel by the vessel motive source, and (2) concurrently gradually increasing the dynamic braking force imparted to the reel by the reel drive mechanism, (3) to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain sufficient tension in the pipeline to hold the pipeline in a desired profile; b. locking the reel against further rotation in the direction of unspooling when the vessel has stopped its forward motion;; c. thereafter controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under said sufficient tension to hold the desired pipe profile while the vessel is in said station keeping mode; d. thereafter respooling at least a portion of the laid pipeline by (1) switching the reel drive mechanism from unspooling mode to spooling mode, (2) adjusting the straigthening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the respooling direction, (3) gradually increasing drive torque impartedj to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotating the reel in the spool-up direction, (4) thereafter disengaging the means locking the reel against further rotation in the unspooling direction, (5) therafter respooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero, (7) thereafter again locking the reel against further rotation in the unspooling direction, and (8) thereafter switching the reel drive mechanism from spooling-up mode to unspooling mode;; e. thereafter resuming forward motion of the vessel in the pipelaying direction by (1) disengaging the means locking the reel against the rotation in the unspooling direction, (2) concurrently substantially increasing the dynamic braking force imparted to the reel by the reel drive mechanism such that the dynamic braking force exceeds any increases in the forward thrust of the vessel's motive source at least up to the pipelaying thrust level required to move the vessel in the forward direction at a desired laying speed, (3) gradually increasing the forward thrust imparted to the vessel by the vessel motive source up to said pipelaying thrust level, and (4) thereafter gradually reducing the dynamic braking force of the reel drive mechanism at least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive.

mechanism and the vessel begins to move forward.

5. A method of spooling at least a portion of previously laid pipeline onto a reel pipelaying vessel, said pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source meansj for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said method comprising the steps of:: a. controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and the maintain the pipeline under sufficient tension to hold the pipeline in a desired profile while the vessel is in said station keeping mode; b. locking the reel against rotation in the direction of unspooling when the vessel is in said station keeping mode with the one end of the pipeline secured to the reel; and c. spooling at least a portion of the pipeline onto the reel while maintaining the pipeline under sufficient tension to hold the desired pipeline profile, by (1) placing the reel drive mechanism into a spooling mode, (2) adjusting the straightening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the spooling direction, (3) gradually increasing drive torque imparted to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotating the reel in the spool-up direction, (4) thereafter disengaging the means locking the reel against rotation in the unspooling direction, (5) thereafter spooling at least a portion of the laid pipeline, (6) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero when the desired about of pipeline has been spooled onto the reel, and (7) thereafter again locking the reel against further rotation.

9. A method of intermittently stopping and restarting the laying of pipe offshore from a reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for im parting at least a dynamic braking force to the 'reel in an unspooling mode, said method comprising the steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying by gradually increasing the dynamic braking force imparted to the reel by the reel drive mechanism while maintaining a substantially constant forward thrust imparted to the vessel by the vessel motive source to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain suffici ent tension in the pipeline to hold the pipeline in a desired profile; b. thereafter controlling at least one of the forward thrust of the vessel motive source and dynamic braking force imparted to the reel by the reel drive mechanism to maintain the pipeline under said sufficient tension to hold the desired pipe profile while the vessel is in said station keeping mode;; c. performing any necessary operations on the pipe and/or coating while the vessel is in said station keeping mode; and d. thereafter resuming forward motion of the' vessel in the pipelaying direction by grad ually reducing the dynamic braking force of the reel drive mechanism at least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

10. A method according to claim 9, 'wherein the step of performing any necessary operations on the pipe and/or coating com prises the step of applying one or more sacrifi cial anodes to the pipe at a designated work station located along the pipe path down stream of the straightening means in the unspooling direction.

11. A method of intermittently stopping the laying of pipe offshore from a reel pipelay ing vessel, thereafter respooling a portion of the laid pipeline, and thereafter resuming the layering of pipe, said reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said method comprising the steps of:: a. stopping the forward motion of the vessel in the direction of pipelaying by gradually increasing the dynamic bramking force imparted to the reel by the reel drive mechanism while maintaining a substantially constant forward thrust imparted to the vessel by the vessel motive source to cause the reel to decrease its rotational rate toward zero while continuing to unspool pipe and to thereby maintain sufficient tension in the pipeline to hold the pipeline in a desired profile;; b. thereafter respooling at least a portion of the laid pipeline by (1) adjusting the straightening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the respooling direction, (2) thereafter continuing to gradually increase the drive torque imparted to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotating the reel in the spool-up direction to respool at least a portion fo the laid pipeline, and (3) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zero; and c. thereafter resuming forward motion of the vessel in the pipelaying direction by gradually reducing the dynamic braking force of the reel drive mechanism at least until the forward thrust imparted to the vessel by the vessel motive source exceeds the dynamic braking force of the reel drive mechanism and the vessel begins to move forward.

12. A method according to claim 9 or 11, further comprising maintaining a balance between the forward thrust imparted to the vessel and the rate of increase of dynamic braking force imparted to the reel in step (a) to maintain the sag-bend radius in the portion of the pipeline near the sea bed above a minimum permissible value to thereby avoid damage to the pipeline due to buckling.

13. A method of spooling at least a portion of previously laid pipeline onto a reel pipelaying vessel, said pipelaying vessel having a reel for spooling relativelyin flexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for alternately driving the reel in a spooling mode and imparting a dynamic braking force to the reel in an unspooling mode, said method comprising the steps of: a. controlling the forward thrust of the vessel motive source to maintain the vessel in a station keeping mode and to maintain the pipeline under sufficient tension to hold the pipeline in a desired profile while the vessel is in said station keeping mode; and b. spooling at least a portion of the pipeline onto the reel while maintaining the pipeline under sufficient tension to hold the desired pipeline profile, by (1) adjusting the straightening means for a no-load condition whereby the straightening means imparts substantially no straightening force to the pipe passing therethrough in the spooling direction, (2) gradually increasing drive torque imparted to the reel by the reel drive mechanism to overcome the forward thrust imparted by the vessel motive source and begin rotating the reel in the spool-up direction to thereby spool up at least a portion of the laid pipeline, and (3) thereafter gradually reducing the reel drive torque to a value which substantially balances the forward thrust imparted by the vessel motive source to thereby decrease the rate of spooling rotation of the reel toward zerol when the desired amount of pipeline has been spooled onto the reel.

14. A method according to claim 13, wherein said pipe handling and working means includes powdered tensioner means for tensioning the pipeline independently of reel imparted tension, said method comprising the further step of: imparting additional tension to the pipeline through said tensioner means to supplement tension imparted to the pipeline by the reel during said step (c).

15. A method according to claim 14, further comprising the step of: adjusting the tensioner drive force in proportion to adjustments in drive torque imparted to the reel to maintain a substantially constant tension in the pipeline while spooling up said pipeline according to said step (b).

16. A method according to claim 14 or 15, further comprising: imparting a substantially greater amount of tension to the pipeline by the tensioner than the reel during said step (b), wherein the portion of the pipeline between the reel and tensioner means is subjected to substantially less tension that the portion of the pipeline between the tensioner and the sea bottom.

17. A method substantially as hereinbefore described with reference to the accompanying drawings of intermittently stopping and restarting the laying of pipe offshore from a reel pipelaying vessel having a reel for spooling relatively inflexible pipe thereon, pipe working and handling means for straightening the pipe as it is unspooled from the reel, vessel motive source means for imparting a forward thrust to the vessel, and a reel drive mechanism associated with the reel for imparting at least a dynamic braking force to the reel in an unspooling mode.