CA1147566A - Method of laying offshore pipeline from a reel carrying vessel - Google Patents
Method of laying offshore pipeline from a reel carrying vesselInfo
- Publication number
- CA1147566A CA1147566A CA000358052A CA358052A CA1147566A CA 1147566 A CA1147566 A CA 1147566A CA 000358052 A CA000358052 A CA 000358052A CA 358052 A CA358052 A CA 358052A CA 1147566 A CA1147566 A CA 1147566A
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- Canada
- Prior art keywords
- pipe
- vessel
- pipeline
- tension
- reel
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
METHOD OF LAYING OFFSHORE PIPELINE
FROM A REEL CARRYING VESSEL
Abstract of the Disclosure Disclosed are methods and techniques related to the control of pipelaying operations from a self-propelled reel pipelaying vessel. The methods are concerned with 1) controlling pipeline geometry as a function of pipe entry angle into the water and tension on the pipeline; 2) monitoring the excursion of the pipeline outside certain defined limits and controlling the pipeline geometry based on such measured excursions; and 3) compensating for pipeline induced turning moments which would otherwise tend to draw the pipelaying vessel off course and off the predetermined pipeline right of way.
FROM A REEL CARRYING VESSEL
Abstract of the Disclosure Disclosed are methods and techniques related to the control of pipelaying operations from a self-propelled reel pipelaying vessel. The methods are concerned with 1) controlling pipeline geometry as a function of pipe entry angle into the water and tension on the pipeline; 2) monitoring the excursion of the pipeline outside certain defined limits and controlling the pipeline geometry based on such measured excursions; and 3) compensating for pipeline induced turning moments which would otherwise tend to draw the pipelaying vessel off course and off the predetermined pipeline right of way.
Description
Background of the Invention This invention relates to techniques and methods utilized in laying underwater pipelines. More particularly, the invention relates to laying 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 pipeline route.
The methods and techniques described herein are particu-larly suited for self-propelled types of reel pipelaying vessels.
Suitable vessels which would be expected to use the methods and techniques described herein include drill ships and ore carriers converted to carry pipe spooling reels and related reel pipelaying equipment. One such self-propelled vessel constructed specifically as a reel-type pipelaying ship is described in U.S. Patent 4,230,421, issued to Charles N. Springett, Dan Abramovich, Stanley T. Uyeda and E. John Radu; U.S. Patent 4,269,540 iscued to Stanley T. Uyeda, E. John Radu, Willlam J. Talbot, Jr. and Norman Feldman.
The present appliction (and the inventive subject matter described and claimed herein) and the above-listed U.S. Patents are all owned by Santa Fe International 1~7566 Corporation; hereafter the above-listed commonly owned applica-tions will be referred to as -prior related Santa Fe Inventions--.
Prior to the development by Santa Fe of the self-propel-led reel ship known in the industry as -Apache-- (the construction of which is substantially described in the above-listed prior related Santa Fe application) and which was scheduled to begin commercial pipelaying operations in late summer of 1979, most known commercial reel type pipelaying vessels consisted of non-self-propelled barges towed by a tug. One 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-- I.D.) has been in commercial use off the coast of Australia since about July, 1978; this portable pipelaying system is described in U.S. Patent 4,260,287 issued to Stanley T. Uyeda and John H. Cha, and assigned to Santa Fe.
Other patents owned by Santa Fe directed to and describ-ing one or more features of reel pipelaying vessels include:
U.S. Patent No. 3,237,438, issued March 1, 1966 to Prosper A. Tesson;
U.S. Patent No. 3,372,461, issued March 12, 1968 to Prosper A. Tesson;
U.S. Patent No. 3,630,461, issued December 28, 1971 to Daniel E. Sugasti, Larry R. Russell, and Fxed W. Schaejbe, U.S. Patent No. 3,641,778, issued February 15, 1972 to Robert G. Gibson;
U.S. Patent No. 3,680,342, issued August 1, 1972 to James D. Mott and Richard B. Feazle;
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1~47566 ~ U.S. Patent No. 3,712,100 issued January 23, 1973 to Joe ~. Key and Larry R. Russell; and , U.S. Patent 3,982,402, issued September 28, 1976 to ~lexander Craig Lang and Peter Alan Lunde.
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1~147566 ~ A~ t~e ~ n The present invcntion is concerned with methods and tec~niqués related to the control of pipelaying operations from a self-propelled reel ipelaying vessel. The methods are co~cerned with 1) controlling pipeline geometry as a function of pipe entry angle into the water and tension on the pipeline;
The methods and techniques described herein are particu-larly suited for self-propelled types of reel pipelaying vessels.
Suitable vessels which would be expected to use the methods and techniques described herein include drill ships and ore carriers converted to carry pipe spooling reels and related reel pipelaying equipment. One such self-propelled vessel constructed specifically as a reel-type pipelaying ship is described in U.S. Patent 4,230,421, issued to Charles N. Springett, Dan Abramovich, Stanley T. Uyeda and E. John Radu; U.S. Patent 4,269,540 iscued to Stanley T. Uyeda, E. John Radu, Willlam J. Talbot, Jr. and Norman Feldman.
The present appliction (and the inventive subject matter described and claimed herein) and the above-listed U.S. Patents are all owned by Santa Fe International 1~7566 Corporation; hereafter the above-listed commonly owned applica-tions will be referred to as -prior related Santa Fe Inventions--.
Prior to the development by Santa Fe of the self-propel-led reel ship known in the industry as -Apache-- (the construction of which is substantially described in the above-listed prior related Santa Fe application) and which was scheduled to begin commercial pipelaying operations in late summer of 1979, most known commercial reel type pipelaying vessels consisted of non-self-propelled barges towed by a tug. One 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-- I.D.) has been in commercial use off the coast of Australia since about July, 1978; this portable pipelaying system is described in U.S. Patent 4,260,287 issued to Stanley T. Uyeda and John H. Cha, and assigned to Santa Fe.
Other patents owned by Santa Fe directed to and describ-ing one or more features of reel pipelaying vessels include:
U.S. Patent No. 3,237,438, issued March 1, 1966 to Prosper A. Tesson;
U.S. Patent No. 3,372,461, issued March 12, 1968 to Prosper A. Tesson;
U.S. Patent No. 3,630,461, issued December 28, 1971 to Daniel E. Sugasti, Larry R. Russell, and Fxed W. Schaejbe, U.S. Patent No. 3,641,778, issued February 15, 1972 to Robert G. Gibson;
U.S. Patent No. 3,680,342, issued August 1, 1972 to James D. Mott and Richard B. Feazle;
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1~47566 ~ U.S. Patent No. 3,712,100 issued January 23, 1973 to Joe ~. Key and Larry R. Russell; and , U.S. Patent 3,982,402, issued September 28, 1976 to ~lexander Craig Lang and Peter Alan Lunde.
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1~147566 ~ A~ t~e ~ n The present invcntion is concerned with methods and tec~niqués related to the control of pipelaying operations from a self-propelled reel ipelaying vessel. The methods are co~cerned with 1) controlling pipeline geometry as a function of pipe entry angle into the water and tension on the pipeline;
2) monitoring the excursion of the pipeline outside certain definea limits and controlling the pipeline geometry based on such measured excursions; and 3) compensating for pipeline induced turning moments which would othen~ise tend to draw the pipelaying vessel off course and off the predetermined pieline right of ~7ay.
~ The present invention is primarily applicable to a self-propelled reel pipe laying vessel, having a reel for spooling relatively inflexible pipe thereon, pipe working and ha~dling means for straightening the pipe as it is unspooled, pipe guide ~eans for guiding the straightened pipe into the water at a presettable, adjustable exit angle, means for maintaining the pipe under a predetermined adiustable tension, main vessel drive means, preferably including twin screws ?ocated on opposite sides of the vessel longitudinal centerline, and forward and aft thruster means located orward and aft, respectively, of the longitudinal center of the vessel.
During a pielaying operation, the pipe handling equipment and pipe guide means translates across the beam of the vessel as it follows ~or leads) the pipe wrap being unspooled.
Inthe process of translating the pipe guide means across the beam of the vessel, turning moments (in the horizontal plane) are imparted .
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119~7566 to the vessel by the tension in the pipeline. In one aspect, there~ore, the invention comprises a method of compensating for these pipeline tension induced turning moments by generating a reactive force in opposition to the pi~eline tension induced turning moment to thereby correct for deviations in the vessel's course and to maintain the vessel on course along the desired right of way.
A further aspect of ~he method of this invention com~rises monitoring the angle of entry of the pipe into the water relative to a nominal horizontal plane representing the water surace; monitoring the angle of excursion ~7hich the pipe makes relative to a nominal pipe centerline substantially parallel to the nominal preset angle of entry into the water;
and adjusting the nominal pipeline tension if the ~onitored excursion angle remains outside a predetermined pérmissible excursion range for at least a significant time period, for example, greater than the pitching period of the vessel~
A still further aspect of the method of this invention comprises setting the pipe guide means to establish a desired pipe exit angle at which the pipeline substantially enters its catenary configuration before exiting the vessel and pipe guide means; and setting the tensioning means to hold the pipe under a predetermined nominal ten.sion in conjunction with the pipe exit angle, to establish a minimum radius of curvature of the pipe in the sag bend region which is greater than the minimum radius to which that pipe may be bent without exceeding its elasticity llmits as it is unspooled and paid out from the vessel.
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' Brief Descri~tion of the Drawing ' Flgure 1 is a diagra~matic sketch of a sel~-pro~elled reel pipe laying vessel showing the ap~roximate pipe profile bet~7een the vessel and the sea bottom.
Fi~ures 2~-C are diagrammatic sketches of the vessel deck, ramp assembly and pipe, in several conditions of pitching due to sea conditions.
Figure 3 is a aiagram~atic plan view of the vessel showing course-correcting force relationships.
', Description of Preferred Embodiments ' ~ndenJater pipelines for carrying oil or gas must meet certain requirements and limits set by the customer tpipeline owner) and/or governmental or other regulatory bodies.
It is of prir.lary importance that the pipe, as it is being laid and as it lays on the sea bottom, be subjected to minimal residual stress, strain, tension, etc. In terms of pipe laid by the reel me~hod, this means that the pipe as it lays on the sea bottom must be straight and have substantially no residual curvature due to spooling or laying. It is also im~ortant that the pipeline'be laid close to the nominal right of way. The "as laid"' restrictions are developed as a function of a nu~ber 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 parametexs relatin~ to, or based on, the geometry (shape) of the pipeline during the pipe laying operation are developed by the pipe ~ayin~ engineers.
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' ' 7 .. .... , ~C''---AdditionallY, a reel pipelaying vessel and the pipe being laid are subjected to a number of hydrostatic and hydrodynamic forces during a pipelaying operation which must be taken into account and compensated for in order to properly lay pipe so that it meets the customer and regulatory body requirements. Such forces include the effects of wind, waves, and current on the vessel due to its heave, pitch, and roll characteristics.
Self-propelled reel pipelaying ships, including for example, Apache-type vessles described in the aforesaid prior related Santa Fe inventions , have certain distinct advantages over non-self-propelled pipelaying vessels, either of the reel pipelaying type or of the -stove piping-- type; the latter technique involved joining 40 or 80 foot lengths of pipe end to end and mov-ing the vessel ahead an equivalent distance after each such turn-ing to thereby effectively pay out pipe from the vessel. Known commercial vessels employing the -stove pipe-- technique have generally been vessels which maintain their operational position by setting out anchors. Auxiliary support vessels set out the barge anchors in specified patterns and the barge moves along the pipeline right of way by hauling in on some anchors and paying out line on other anchors. In relatively shallow water (up to about 200 feet deep), sufficient anchor line can be pald out to allow the barge to move along the right of way 1,500 to 2,500 feet before the anchors must be raised and a new pattern set. The distance which a stove piping barge can move along the right of way on a single anchor set pattern decre~ses as water depth increases. It is apparent that the limited for~ard movement permitted by this anchor setting technique is not at all suitable for economical reel pipe laying operations.
~ Althou~h towed leel pipelaying bar~es have been found to be quite adequate for the relatively calm waters of th2 Gulf of ~lexico offshore of the United States coastline, they Itlave certain inherent limitations wbich make then unsuitable for use in relatively rough waters, such as are found in the Morth Sea or off the coast of South ~erica or Australi~. One of the principal built-in limitations of a towed bar~e syste~. resides in the towing connection itself. Unlike a self-propelled shipt in which the motive source is effectively connected directly and rigidly to the pipeline (through the reel), the connection between the towing vessel (motive source) and the towed barge (effectively including the pipeline end) is a flexible one which introduces an additional unpredictable and uncontrollable factor into the overall system. In rough water, the barge may be subjected to irregular pulling action as the tow line . ~ .
tightens or sags with relative movement between the tug and barge. ~his may cause the pipeline tension to exhibit sudden increases and/or decreases in ~agnitude which can neither be predicted norcontrolled ef~ectively by the barge operator(s).
A self-propelled reel type pipelaying ship requires neither anchors nor tugs as the motive source. Therefore, compared to stove-piping type barges as described above, a self-propelled reel pipelaying ship is able to move continuously down the risht of way, stopping only ~hen necessary, for example, to install anodes as re~uired by the customer and/or to perform other opera,ions on the pipe, such as coating repair, etc.
Comparecl to towed reel barges, the self-propelled reel ship has a signi~icant advantage in that the motive source of the reel ship can, for practical purposes, be considerea to be i~ed with the reel and pipeline end, thereby eliminating relative movcments ~herebetween due to weather related fac~ors, as no~ed above.
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Co~mercial and practical limitations effectively restrict the operatin~ capability of a towed reel barge. One of the principal requirements in l~ying pipelines offshore from a surface vessel is tha~ in general,aaequate tension must be maintained on the pipe at all significanttimes. This is necessary to prevent the "sag bend" from exceeding certain predetermined toler-ance limits. 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 point at which the pipe touches the bottom is called t~e Touchdown Point (TDPj.
It is important that the radius of the sag bend curvè be kept above the mini~um permissible radius to which the pipe may be bent without exceeding elasticity limits in accord with customer requirements. The pipeline should be kept under sufficient tension at all significant times during the laying operation to maintain the proper profile in the pipe 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 to below its allowable minimum.-It has been found that the relationship between thedeparture or exit angle (also sometimes called pipe entry angle into the water~ and the required tension can be expressed as an essentially linear logarithmic relation where the pipe pro~ile is catenary-shaped in its unsupported length between the vessel and the sea bottom, substantially as represented in Fig. l; i.e., for a given size and grade of pipe and a given lay depth along the right of ~ay, the tension required to hold the sag bend radius above the allowable minimum decreases as the departure angle of the pipe into the water increases. For example, it is necessary to hold about 250,000 lbs. of tension (250 ~ips, where "Kips" equals thousands of pounds) on a ~ipe havin~ an ' ' .. , '' .
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~14~5~6 ~) outside diam~ter of 10 3/4" and 3/~" wall thickness laid in a water depth of 500 feet, if the pipe exit angle is set at aboùt 26, in order to maintain the sag bend radius above the allowable minimum, at an exit angle of 58, the same conditions require a tension of about 60 Kips. (These examplary pipe size and water depth conditions are typical for North Sea operations.) All known commercial reel type pipelaying barges to date have been designated to operate at a relatively fixed departure angle of between about 6 and 12 (relative to a nominal horizontal plane representing the water surface). At this shallow exit angle, the tension required to maintain a catenary shaped pipe profile for deep water (deeper than about 1,000 feet) is typically greater than can be generated by the barge and tug. The pipe therefore assumes an "r" shape (with two inflection points) in its unsupported length between the barge and the sea bottom. The first point of inflection, or "overbend", occurs near the surface as the weight of the pipe imparts a down-ward force vector to the pipe, forcing it to curve downwardly; the second point of inflection occurs at the sag bend.
Referring to Figure 1, a ~eature of"Apache-type" special reel pipelaying ships isthe adjustable pipe carrying ramp assembly 4~ pivotably mounted (generally at the stern) to the deck of the vessel 10, aft of thereel 20. The vessel also comprises main propul-sion propellers 12, one or more forward lateral thrusters 126 and one or more stern lateral thrusters 122. ~Throughout this dis--closure, reference is made to the maln propellers as providing the requisite forward thrust; it is apparent,however,that other suitable drive means could be provided to generate the neces-,0~
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sary for~ard thrust ~nd the re~erence to "propellers" throughoutthis disclosure is intended to encompass other such suitable drive means, except where otherwise specifically noted.) Special pipe handling equipment, which may include, for example, the adjustable radius control member, adjustable straightener tracks, tensioner tracks, pipe clamping assemblies, guide roller assemblies, and pipe angle measuring assembly, is advantageously ~ounted to the ramp assembly 40.
~ n adjustable ramp assembly of this type has not heretofore been incorporated into any ~kno~n commercial offshore reel pipelaying vessel, specifically including the supply boat portable reel system used off the coast of Australia, the t~o r~el pipelaying towed barges owned and used by Santa Fe and/or Santa Fe's predecessors-in-interest since about 1961 and two competitive reel pipelaying barges, one used for a short time in 1972 or 1973 and the other currently in use in the Gulf of ~exico off the United States coast.
The Apache-type reel pipelaying vessel di~fers from prior commercial reel pipelaying barges in its ability to discharge pipe into the water at any desired angle within its operating range of between about 15 and 65, preferably between about 18 and 60. The adjustable ramp assembly of an Apache-type reel ship permits the angle ofentry of the pipe into thewater tobe preset and maintained during a pipe lay operation; the ramp assembly guides - the pipe as it enters the water at the preset exit angle. As noted aboye, all prior known commercial xeel pipelaying barges haYe operated at a fixed, non-variable exit angle of between about 6 and 12. The adjustable exit angle fea~ure of the Apache-type - ~C ..
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vessel enables it to handle a ~ider ranye of pipe sizes in a greater range of water depths than was heretofore possible with fixed lo~ exit angle reel pipelaying barges.
One of the advantages of an ~pache-t~pe adjustable ra~p assembly for setting the pipe exit angle is the virtual elimination of the overbend region (i e~, the bend region occurring as the pipe translates do~mwardly fromtherelatively hori-zontal plane of the barge toward the seabed in therelatively vertical planeof the catenar~. Advantageously andpreferably, the ramp angle and tension are set so that downstream of the straightener/tensioner , apparatus, the pipe will be unsupported; thus, pipe exiting the straightener mechanism and traveling along the ramp assembly will already be in its nominal catenary configuration before and as it en~ers the water. Preferably, as the pipë moves through the straightener mechanism toward the water, all or substantially all of the curvature imparted to the pipe by the reel and other pipe handling elements is removed so that pipe exiting from the straightener mechanism has substantially zero residual stress and zero residual bending moments.
By initially setting the ramp angle and nomlnal pipeline tension to virtually eliminate the overbend as a factor in determining and controlling the final residual pipeline characteristics, the sag bend (i.e., the bend occurring in the translation of the pipe from the vertical to the hori-zontal plane on the sea bottom) becomes a critical factor in the control o~ the pipe as it is laid. The sag bend is con-trolled, at least in par~, as a function of the tension main-tained on the pipe by the functional elements of the pipelaying vessel, including the xeel, straightener /tensioner elements vessel drive assembly, etc. Controlled tension is imparted ~-C
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to the pipe by ~1) the reel throuyhthereeldrive mec~lanismoperating as a dynamic brake, (2) the main vessel drive thrust acting through the vessel main propellers and/or the lateral thruster assemblies, and (3) the tensioner assembly, which may or may not be used, throuyh a rec3ulated tensionin~ force established at the beginnlng of a lay operation and generally maintained throughout the lay operation.
The desired pipelaying tension and the desired entry angle of the pipe into the water are preferably determined on the basis o~ information supplied by th~ pipeline designer.
Such information from the pipeline designer (or customer - pipeline owner) includes (1) the size of the pipe, including internal pipe diameter and ~all thic~ness, (2) the type or grade of pipe, including such information as the pi~e material and minimu~ yield strength, (3) maximum allowable strèss, strain and residual tension, and (4) water depth along the pipeline right of way. An optimum nominal tension and lay anc31e can be determined from these parameters.
One of the criteria which has been developed for laying pip~withan Apache-type vessel is that the maximum allowable working stress, due to pipelayingoperation,in the unsupported length ofpipe between the vessel and thesea bottom should not be greater than about 85% of the minimum yield strength o~ the pipe.
It is also desirable and preferable to minimize the tension imparted to the pipe by the vessel while maintaining operating conditions such that the maximum allowable stress limit and the maximum allowable residual tension in the ~ipeline are not exceeded. This may be accomplished by setting the ramp assembly angle (and thus the pipe entry angle into the water) in conjunction with nominal pipe tension such that the tightest sag bend radius will bP achieved without exceedinc3 the a~ove-noted stress ~nd residual tension li~it.
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` J ~147S66 The ramp assembly angle (and thus the pipe entry angle lnto the water) is set at the b~ginning of the pipelaying opera-tion and is normally not changed during the entire lay operation.
It is possible to alter the ramp angle during a-pipelaying opera-tion, for example, to account for (appreciable) changes in water depth. During the pipe-laying operation, control of the pipe as it ïs being laid is maintained by controlling the tension in the pipe. Such control is normally achieved through ad~ustments in the reel tor~ue andjor tensioner setting and/or in the vessel forward and/or lateral thrust.
Prior to the start of the pipe~aying operation, the ramp angle and nominal pipe tension level are established on the basis of input from the pipeline designer. Also, in the case of an Apache-type vessel wherein the straightener tracks and the radius controller section are independently adjustable relative to each other, the radius controller and the straighteners are set at predetermined positions relative to each other and to the ramp assembly aft of the straighteners so that the tpreferably unsupported length of) pipe between the straightener assembly and aft end of the ramp assembly (at the stern guide roller assembly) will have little or no residual strain between the straightener assembly exit point and the aft end of the ramp assembly.
Under certain operating conditions, the "flexible"
towing connection between a reel barge and its tug will not be adequateito maintain the necessary continuous tension on the pipeline as it is being laid. The tug moves independently of the barge due to wave action. This means that the motive source which provides the forward thrust necessary to maintain tension on the pipeline is susceptible to uncontrolled variations relative to the barge and thus to the pipe. Limited excursions '- ' ' ~
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of tllis type may be acceptable for some sizes of pipe and some sea conditions. Ho~Jever, the ranse of permitted excursions is relatively small and decreases, particularly with increasing pipe size and increasingly rough sea conditions.
- ~ A self-propelled reel ship has the advantage that the forward thrust producing motive force can be considerea to be coupled directly to the pipe end on board the ship so that relative movement be`tween the motive source and the pipe end connected to the vessel is reduced essentially to zero. Further, external forces produced by waves, winds, current, etc. act on the pipe and motive source together and at the same time. Since the motive source and pipe end are substantially directly coupled, the p~pe is more directly responsive and more rapidly r~sponsive to changes in thrust. The self-propelied ship can therefore operate in a greater range o sea conditions, and particularly adverse sea conditions, than can a towed barge.
- On a reel pipelaying vessel, it is not possible to measure the pipeline tension directly. There are, however, several ways to measure the tension indirectly. One such way is to ~easure the forward thrust of the vessel, which is directly proportional to the tension on the pipe. Increasing or decreasing the vessel thrust will produce a corresponding proportional increase or decrease in the tension on the pipeline.
This can be aone by measuring the main propeller shaft torque or by ~easuring the force on a thrust bearing agalnst which the propeller shaft acts.
A second method is to measure the drive motor force acting on the reel. Neglecting the components of tension pro-duced prir~arily by the straightener assembly (and tensioner, when used), the orce e,~erted by the reel drive motors is directly proportional to the tensl~on in the pipe; thus, an 1~7S66 , increase or decrease in the drive motor force produces a corresponding increase or decrease in the pineline tension.
The reel motor drive force may be measured by, e.g., load cells between the motor/reel mechanical connection.' A third practical t~ay to measure pipeline tension is based on ~easurement of the exit angle of the pipe from the vessel. It is advantageous and preferable that the pipe angle be measured with respect both to the hori~on and to the ramp angle; the latter measurement is particularly helpful ~here the pi~e passes through an exit window defined by a stern ~uide roller assen~ly, such as is used on Apache-type vessels.
Figures 2A-C are diagrammatic representations of the pipelaying ~essel 10, ramp assembly 40 (set at a nominal lay an~le of about 30 degrees), the stern guide roller assembly 54 defining the exit windo~, and the pipe P. Figure 2A shows the relationship between the ramp assembly and the pipe when 'the vessel is substantially flat in the water so that the entry ~ngle Al of the pipe into the water trelative to a nominal horizontal plane or axis, such as the horizon) is substantially the same as the predetermined ra~p angle R; Figure 2B shows the same relationship when the vessel is pitched bow up at an angle D2 and the pipe P2 enters the water at an angle A2; and Figure 2C shows the same relationship when the vessel is pitched ~ow down at an angle D3 and pipe P3 enters the water at an angle ~3. The exit point of the pipe fro~ the straightener/
tensioner asse~bly is desi~nated by reference SE. The pipe is essentially in fixed relation to the ramp assembly'and the vessel at point SE. Preferably and advantageously, sufficient and adequate tension is maintained on the pipe P during the ,'' ` ' ~
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.-~ 1147S66 laying operation so that the pipe travels in a path substantiallyparallel to the ramp and through the guide roller assembly 54 sub-stantially unsupported between straightener exit SE and the touch-down point TDP on the sea bottom. Also advantageously and prefer-ably control of pipelaying operation is maintained so that angles Al, A2, A3 will be essentially equal.
The stern guide roller assembly provides a pipe excursion window between the upper and lower guide rollers for pipe excursion relative to the vessel as a result of vessel motion due to wave action. In one commercial embodiment, the distance between straight-ener exit SE and stern guide roller asse~bly 54 is approximately 45 feet; the distance between the upper and lower stern guide rollers is approximately four feet. This permits an angular excursion of the pipe between straightener exit SE and stern guide roller assembly 54 in a range from about 4.7 for 4 inch OD pipe to about 3.2 for 18 inah OD pipe; that is, the pipe can move through this range with-out being subject to bending moments by the stern guide rollers.
Referring to Figs. 2B and 2C, angles ~2 and F3, respectively, re-present the excursion above and below the nominal centerline of the pipeline P when it is tensioned to be parallel to the plane of the ramp assembly 40.
During the pipelaying operation, the vessel moves forward through the water as a function of the thrust generated by the main vessel drive, reacting against pipe tensioning forces produced by the pipe handling equipment, including reel dynamic braking forces, straightener, tensioner, etc. Changes in or modifications to the rate of forward motion of the vessel, and thus the rate at which pipe is unspooled from the reel 20 and paid out into the water, may be controlled by adjusting the dynamic braking force exerted by the reel drive mechanism and/or the amount of thrust generated by the main propellers.
A ~ypical lay rate, i.e., the rate at which - - 18 -.
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pipe is paia out fro~ the vessel during a lay op~ration, would be in~the range of 75 - 150 feet per minute. It has been found to be preferable to ~aintain the forward thrust relatively constant and to control pi e tension changes through adjustments to the reel dynamic braking force. Due to the large mass of the reel and pipe, it is not possib'le to effect instantaneous changes in the pay out rate.
As the vessel pitches during a laying operation, the stern, with the ramp and other related pipe handling equipment, moves up and do~n in the water. The pipe, paid out ~rom the straightener exit SE at a~predetermined rate which, as noted, cannot be changed instantaneously, also moves up and down with the vessel. The pipe is subjected to inertial effects through its underwater suspended length and on-bottom friction. Due to such inertial e~fects on the pipe, the'portion of the pipe downstrèam of straightener exit SE
does not necessarily move with the vessel so that the total pipe excursion relative to the ra~p may be ~reater than the stern guide excursion window limits. Under conditions where the bow pitches up by an angle D2, the pipe may be bent around the upper stern guide roller, as shown in Fig. 2B. Similarly, when the bow of the vessel pitches do~^mwards by an angle D3, ' the pipe may be bent around the 10~7er stern guide roller assembly, as sho~m in Fig. 2C.
In a commercial embodiment of an Apache-type vessel, an angle measuring device measures the pipe ansle do~mstream of the s~ern guide assembly relative to the ramp assembly 40 and relative to the horizon. One such angle measuring device is sho~m and described in aforesaid British ApPlication Serial No.
791S91~. An apparatus ~or this'purpose is manufactured by Interstate Electronics, Inc. ' "~
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~47566 In Fig. 2B, reference E2 represen-ts the measured angle of excursion of the pipe P2 relative to the ram~ assembly 40 under the condition where the ve~sel pitches up by the bow at an anyle D~ ~t this pitch angle, the effective exit angle G2 becomes R (ramp angle) plus D2 (pitch angle). ~s noted earlier, it has been founa that pipe tension and exit angle are inversely propoxtional; therefore, as the effective exit angle G2 increases, the tension applied to the pipeline should be decreased in order to maintain the pipe profile within acceptable limits. However, since, due ~to reel and pipe inertia and other factors, the tension applied to the pipe cannot be adjusted to directly follow the pitching of the vessel, the effective tension on the pipe is increased and a pi~e profile such as shown in Fig 2B results. Under sufficiently severe conditions of vessel pitch, the ~ipe P2 undergoes a relatively larse excursion so that the pipe excursion angle E2 exceeds the guide assembly window excursion limit angle F2 In such cases, the pipe undergoes a bending morent about the upper stern guide roller. If this bending moment exceeds the elastic limit of the pipel the pipe will undergo plastic bending and will thus retain a residual curvature due to such plastic bending ~hen i~ rests on the bottom.
~ hen the bow o the vessel pitches downward, e.g., at an angle D3, a pipe profile such as sho~n in Fig. 2C may result. In this case, the effectlve exit angle G3 becomes R
tramp angle) minus D3 (pitch angle); in this case, the effective exit angle is smaller than- the nominal preset ramp angle. In order to maintain a ~roper~pipe profile, in bow down pitch condition, the tension on the pipe should be increased an amount - C'~
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sufficient to compellsate for the aecrease in effective e~it angle. Ilo~ever, for reasons noted above, it is not Dossiblc to instantaneously change the tension imparted to the pipe by the vessel, and particularly by the reel. Therefore, the pipe undergoes an excursion E3 which may be greater than the excursion F3 ~er~itted by the stern guide ~indow limits. Under such conditions, the pipe under~oes a hending moment abou~ the lower stern guide roller; if this bending moment exceeds the elastic limit, the pipe undergoes plastic bending and will retain a residual curvature when it is laid.
The angle measuring device measures excursion E2 and E3 to thereby generate an indication of excessive bending of the pipe on the ramp. ~leasurernent of excursion E2 or E3 is parti-cularly important as an indicntor that the pipe is over-tensioned or undertensioned, irrespective of the ~itching of the vessel.
When the vessel is pitching, excursions E2 and E3 would be expected to be relatively short-lived. Measurement of such short-llve~ excursions would not provide an accurate indica-tion of over- or under-tensioning.
A c~ntinuous measurement of excursion E2 greater than limit F2~or measured excursions E2 greater than F2 which occur a significant percent of the ti~e (e.g., greater than the pi~ching period of the vessel), even thoug'n such excursions are not continuous,-indicate to the operator that the pipe is being hel~ under excessive tension. The operator can then adjust the reeldynamic braking force to decrease thetension on the pipe until the angle measuring device measures an excursion E2 less than excursion F2, neglectin~ short-lived excursions due to vessel pitchiny. Correspondingly, when the angle measuring - 21 -.
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~ 1147566 ~
device measures an excursion E3 continuously greater than excursion limit F3, or grcater than F3 a significant percent of the tir..e (e.g., greater than th~ pitching period of the vessel), even though not continuous, these constitute indica-tions that -~he pipe is being held under insufficient tension.
The~operator can then increase the tension on the pipe until the measured excursion E3 becomes less than excursion limit F3, again neglecting short~lived excursions due to vessel pitching.
When the vessel is pitching, d~ue, for example, to sea conditions, measurins excursions E2 and E3 may produce erroneous -indications of pipe tension and may make it difficult, if not practically im.possibIe, for the oDerator to maintain proper tension ontthe pipe. Therefore, the angle measuring device also measures the actual exit angle ~ of the pipe (relative to the horizon or mean water line). Such measurement provides a more accurate indication of the actual pipe entry angle into the water so that under vaxying sea conditions, with the vessel pitching continu-ously, the operator can maintain a direct reading of the actual pipe entry angle. The operator is then able to maintain the proper reel dynamic breaking force and provide necessary compensation adjustments based on the actual pipe angle relative to the fixed horizon, as distinguished from angles measured relative to the moving and pitching vessel.
The pipe laying operation is also affected by the fact that the pipe traverses across the beam of the vessel as it is unspooled. This produces a turning moment tending to pull the vessel off course. This turning moment increases to a ~ ?~
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~ 1147S~;6 ~ ~
maximu~ at the end of transverse travel of the ramp assen~ly, decreases to zero t~hen the ramp assembly (and pipeline pa~h) is aligned with the vessel centerline, and increases to a maxi~um in the opposite ~irection as the ramp assembly continues movincJ
to the extreme opposite end of its transverse travel.
The turning moment can be quite large compared to the fon~ard thrust generated by the main propellers. For example, in one commercial embodiment, the ramp assembly has an athwart-ship ~ovement ran~e of 21.5 feet. The shafts of the main pro-pellers are located about 20 feet to either side of the vessel centerline; each produces a maximum thrust of 80 Kips. ~hen pipe is being laid under 100 Xips tension at an exit angle of 30 degrees, the pipe tension induced turning mom.ent at each extreme end of ramp asse~bly travel is on the order of 930 foot Xips. ~he opposing turning moment produced by the main propeller on that side operating at maximum thrust is about 1,600 foot ,Cips. It will be seen that the pipe tension induced tùrning mo~ent may well be a significant percentage (58.9~ in the example given here) of the drive induced turning moment.
If the pipe tension induced turning ~..oment is not compensated for, the vessel will be pulled off course; this can result in the pipe being laid out of the ri~ht of way, ~hich is co~mercially unacce~table.
- The pipeline induced turning moment must be compensated or in order to lay the pipe in a straight line along the right of way. .With twin screw vessels, that is, vessels propelled by two sets of main drive propellers equally spaced on o?posite sides of the longitudinal centerline of the vessel, it may be possible to overcome the turning ~oment introduced by the pipe's pipeline oset relative to the vessel center line by increasins thrust on the propeller located on that side of the rf~
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vessel and /or decreasing thrust on the opposite side main drive propeller. This has certain inherent disadvantages because the pipeline induced turning moment continually varies as the pipeline shifts laterally across the ~essel as it is unspooled.
To compensate for this varying turning moment using the main drive propellers requires that the thrust of the drive propellers be varried accordingly, while at the same time taking into account that the forward component of thrust must be maintained relatively constant in order to maintain the proper amount of tension on the pipe at all pertinent times during the pipelaying operation. Under certain condi-tions of pipeline tension and forward thrust, the system will not be able to generate sufficient additional thrust to com-pensate for the pipeline induced turni~g moment, especially when the ramp assembly and pipeline are at an extreme end of transverse displacement.
A second and potentially more commercially preferable way to compensate for the turning moment introduced by the pipeline lateral travel comprises utilizing forward and aft lateral thrusters. Examples of such thrusters are shown in the aforesaid prior related Santa Fe inventions. Also refer-ring to Fig. 3 hereof, an aft thruster tunnel 120 houses the aft thruster 122; a forward thruster tunnel 124 houses the forward thruster 126.
The thrusters 122 and 126 can be operated either manually or automatically in conjunction with, e.g., a computer operated guidance system, to generate turning moments which react against the pipeline induced turning moments. The pipeline introduces a turning moment about the intersection of the vessel longitudinal axis and reel shaft axisi the magnitude of the pipeline induced turning moment is a fu~ctionof the tension on the pipeline and the pipeline offset /- ~ 1147S66 ~
from the vessel's centerline. The vessel thrusters generate turning moments about the aforesaid intersection of the vcssel's centerline and reel shaft axis which react against the pipeline turning moment to maintain the vessel on its proper course.
Consideration must also be given to the fact that a turning moment occurs between the forward vessel thruster(s) and the pipeline touchdown point on the sea botto~. Therefore, in addition to rotating the vessel abou~ the centerline inter-section points, the entire vessel MUSt be rotated about the touch~own point to maintain the vessel on and paxallel to the right of way. This may be accom~lished by increasing the thrust generated by the forward thruster(s) relative to tlle oppositely reacting force generated by the aft thruster(s).
~ The amount of thrust required varies as a function of a num~er of factors, including the lateral positio~ of the pipeline relative to the vessel's longitudinal axis, the distance between the vessel and the touchdown point, the pipeline tension and pipe exit angle. In general, the forward thruster will be controlled to generate a thrust component Tl in one lateral direction relative to the vessel's longitudinal centerline.
The aft thruster will be controlled to generate a thrust component T2 in the opposite lateral direction relative to the vessel's longitudinal centerline. ~dvantageously and preferably, Tl is maintained greater than T2; together, Tl ~ T2 produce a turning moment ~ich reacts the pipeline induced turning moment.
The thrust generated by the forward thruster therefoxe comprises the additive components of the thrust necessary to react the pipeline induced turning moment about the vessel axis and the pipeline induced turning moment about the touchdown point pivot axis. The aft or rear thruster need only react the . :' , - lf~-~1~75~6 ~`
pipe:Line induced turning moment about the vessel axis. The for~7ard thruster there~ore imparts a relatively gre~ter lateral thrust com~onent than the rear thruster to overcome the pipeline induced turning moments about the vessel pivot axis and about the touchdown point pivot axis to thereby maintain the vessel on course along the right of way.
The invention may be e~bodied in o~her specific orms without departing from the spirit or essential charac~
teristics thereof. The embodiment described above is therefore to be considered in all respects as ill~strative and not restrictive, the scope of the invention being indicated by the hereafter appended clai~s 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.
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~ The present invention is primarily applicable to a self-propelled reel pipe laying vessel, having a reel for spooling relatively inflexible pipe thereon, pipe working and ha~dling means for straightening the pipe as it is unspooled, pipe guide ~eans for guiding the straightened pipe into the water at a presettable, adjustable exit angle, means for maintaining the pipe under a predetermined adiustable tension, main vessel drive means, preferably including twin screws ?ocated on opposite sides of the vessel longitudinal centerline, and forward and aft thruster means located orward and aft, respectively, of the longitudinal center of the vessel.
During a pielaying operation, the pipe handling equipment and pipe guide means translates across the beam of the vessel as it follows ~or leads) the pipe wrap being unspooled.
Inthe process of translating the pipe guide means across the beam of the vessel, turning moments (in the horizontal plane) are imparted .
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119~7566 to the vessel by the tension in the pipeline. In one aspect, there~ore, the invention comprises a method of compensating for these pipeline tension induced turning moments by generating a reactive force in opposition to the pi~eline tension induced turning moment to thereby correct for deviations in the vessel's course and to maintain the vessel on course along the desired right of way.
A further aspect of ~he method of this invention com~rises monitoring the angle of entry of the pipe into the water relative to a nominal horizontal plane representing the water surace; monitoring the angle of excursion ~7hich the pipe makes relative to a nominal pipe centerline substantially parallel to the nominal preset angle of entry into the water;
and adjusting the nominal pipeline tension if the ~onitored excursion angle remains outside a predetermined pérmissible excursion range for at least a significant time period, for example, greater than the pitching period of the vessel~
A still further aspect of the method of this invention comprises setting the pipe guide means to establish a desired pipe exit angle at which the pipeline substantially enters its catenary configuration before exiting the vessel and pipe guide means; and setting the tensioning means to hold the pipe under a predetermined nominal ten.sion in conjunction with the pipe exit angle, to establish a minimum radius of curvature of the pipe in the sag bend region which is greater than the minimum radius to which that pipe may be bent without exceeding its elasticity llmits as it is unspooled and paid out from the vessel.
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' Brief Descri~tion of the Drawing ' Flgure 1 is a diagra~matic sketch of a sel~-pro~elled reel pipe laying vessel showing the ap~roximate pipe profile bet~7een the vessel and the sea bottom.
Fi~ures 2~-C are diagrammatic sketches of the vessel deck, ramp assembly and pipe, in several conditions of pitching due to sea conditions.
Figure 3 is a aiagram~atic plan view of the vessel showing course-correcting force relationships.
', Description of Preferred Embodiments ' ~ndenJater pipelines for carrying oil or gas must meet certain requirements and limits set by the customer tpipeline owner) and/or governmental or other regulatory bodies.
It is of prir.lary importance that the pipe, as it is being laid and as it lays on the sea bottom, be subjected to minimal residual stress, strain, tension, etc. In terms of pipe laid by the reel me~hod, this means that the pipe as it lays on the sea bottom must be straight and have substantially no residual curvature due to spooling or laying. It is also im~ortant that the pipeline'be laid close to the nominal right of way. The "as laid"' restrictions are developed as a function of a nu~ber 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 parametexs relatin~ to, or based on, the geometry (shape) of the pipeline during the pipe laying operation are developed by the pipe ~ayin~ engineers.
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' ' 7 .. .... , ~C''---AdditionallY, a reel pipelaying vessel and the pipe being laid are subjected to a number of hydrostatic and hydrodynamic forces during a pipelaying operation which must be taken into account and compensated for in order to properly lay pipe so that it meets the customer and regulatory body requirements. Such forces include the effects of wind, waves, and current on the vessel due to its heave, pitch, and roll characteristics.
Self-propelled reel pipelaying ships, including for example, Apache-type vessles described in the aforesaid prior related Santa Fe inventions , have certain distinct advantages over non-self-propelled pipelaying vessels, either of the reel pipelaying type or of the -stove piping-- type; the latter technique involved joining 40 or 80 foot lengths of pipe end to end and mov-ing the vessel ahead an equivalent distance after each such turn-ing to thereby effectively pay out pipe from the vessel. Known commercial vessels employing the -stove pipe-- technique have generally been vessels which maintain their operational position by setting out anchors. Auxiliary support vessels set out the barge anchors in specified patterns and the barge moves along the pipeline right of way by hauling in on some anchors and paying out line on other anchors. In relatively shallow water (up to about 200 feet deep), sufficient anchor line can be pald out to allow the barge to move along the right of way 1,500 to 2,500 feet before the anchors must be raised and a new pattern set. The distance which a stove piping barge can move along the right of way on a single anchor set pattern decre~ses as water depth increases. It is apparent that the limited for~ard movement permitted by this anchor setting technique is not at all suitable for economical reel pipe laying operations.
~ Althou~h towed leel pipelaying bar~es have been found to be quite adequate for the relatively calm waters of th2 Gulf of ~lexico offshore of the United States coastline, they Itlave certain inherent limitations wbich make then unsuitable for use in relatively rough waters, such as are found in the Morth Sea or off the coast of South ~erica or Australi~. One of the principal built-in limitations of a towed bar~e syste~. resides in the towing connection itself. Unlike a self-propelled shipt in which the motive source is effectively connected directly and rigidly to the pipeline (through the reel), the connection between the towing vessel (motive source) and the towed barge (effectively including the pipeline end) is a flexible one which introduces an additional unpredictable and uncontrollable factor into the overall system. In rough water, the barge may be subjected to irregular pulling action as the tow line . ~ .
tightens or sags with relative movement between the tug and barge. ~his may cause the pipeline tension to exhibit sudden increases and/or decreases in ~agnitude which can neither be predicted norcontrolled ef~ectively by the barge operator(s).
A self-propelled reel type pipelaying ship requires neither anchors nor tugs as the motive source. Therefore, compared to stove-piping type barges as described above, a self-propelled reel pipelaying ship is able to move continuously down the risht of way, stopping only ~hen necessary, for example, to install anodes as re~uired by the customer and/or to perform other opera,ions on the pipe, such as coating repair, etc.
Comparecl to towed reel barges, the self-propelled reel ship has a signi~icant advantage in that the motive source of the reel ship can, for practical purposes, be considerea to be i~ed with the reel and pipeline end, thereby eliminating relative movcments ~herebetween due to weather related fac~ors, as no~ed above.
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475~:i6 ~
Co~mercial and practical limitations effectively restrict the operatin~ capability of a towed reel barge. One of the principal requirements in l~ying pipelines offshore from a surface vessel is tha~ in general,aaequate tension must be maintained on the pipe at all significanttimes. This is necessary to prevent the "sag bend" from exceeding certain predetermined toler-ance limits. 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 point at which the pipe touches the bottom is called t~e Touchdown Point (TDPj.
It is important that the radius of the sag bend curvè be kept above the mini~um permissible radius to which the pipe may be bent without exceeding elasticity limits in accord with customer requirements. The pipeline should be kept under sufficient tension at all significant times during the laying operation to maintain the proper profile in the pipe 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 to below its allowable minimum.-It has been found that the relationship between thedeparture or exit angle (also sometimes called pipe entry angle into the water~ and the required tension can be expressed as an essentially linear logarithmic relation where the pipe pro~ile is catenary-shaped in its unsupported length between the vessel and the sea bottom, substantially as represented in Fig. l; i.e., for a given size and grade of pipe and a given lay depth along the right of ~ay, the tension required to hold the sag bend radius above the allowable minimum decreases as the departure angle of the pipe into the water increases. For example, it is necessary to hold about 250,000 lbs. of tension (250 ~ips, where "Kips" equals thousands of pounds) on a ~ipe havin~ an ' ' .. , '' .
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~14~5~6 ~) outside diam~ter of 10 3/4" and 3/~" wall thickness laid in a water depth of 500 feet, if the pipe exit angle is set at aboùt 26, in order to maintain the sag bend radius above the allowable minimum, at an exit angle of 58, the same conditions require a tension of about 60 Kips. (These examplary pipe size and water depth conditions are typical for North Sea operations.) All known commercial reel type pipelaying barges to date have been designated to operate at a relatively fixed departure angle of between about 6 and 12 (relative to a nominal horizontal plane representing the water surface). At this shallow exit angle, the tension required to maintain a catenary shaped pipe profile for deep water (deeper than about 1,000 feet) is typically greater than can be generated by the barge and tug. The pipe therefore assumes an "r" shape (with two inflection points) in its unsupported length between the barge and the sea bottom. The first point of inflection, or "overbend", occurs near the surface as the weight of the pipe imparts a down-ward force vector to the pipe, forcing it to curve downwardly; the second point of inflection occurs at the sag bend.
Referring to Figure 1, a ~eature of"Apache-type" special reel pipelaying ships isthe adjustable pipe carrying ramp assembly 4~ pivotably mounted (generally at the stern) to the deck of the vessel 10, aft of thereel 20. The vessel also comprises main propul-sion propellers 12, one or more forward lateral thrusters 126 and one or more stern lateral thrusters 122. ~Throughout this dis--closure, reference is made to the maln propellers as providing the requisite forward thrust; it is apparent,however,that other suitable drive means could be provided to generate the neces-,0~
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1147566, .
sary for~ard thrust ~nd the re~erence to "propellers" throughoutthis disclosure is intended to encompass other such suitable drive means, except where otherwise specifically noted.) Special pipe handling equipment, which may include, for example, the adjustable radius control member, adjustable straightener tracks, tensioner tracks, pipe clamping assemblies, guide roller assemblies, and pipe angle measuring assembly, is advantageously ~ounted to the ramp assembly 40.
~ n adjustable ramp assembly of this type has not heretofore been incorporated into any ~kno~n commercial offshore reel pipelaying vessel, specifically including the supply boat portable reel system used off the coast of Australia, the t~o r~el pipelaying towed barges owned and used by Santa Fe and/or Santa Fe's predecessors-in-interest since about 1961 and two competitive reel pipelaying barges, one used for a short time in 1972 or 1973 and the other currently in use in the Gulf of ~exico off the United States coast.
The Apache-type reel pipelaying vessel di~fers from prior commercial reel pipelaying barges in its ability to discharge pipe into the water at any desired angle within its operating range of between about 15 and 65, preferably between about 18 and 60. The adjustable ramp assembly of an Apache-type reel ship permits the angle ofentry of the pipe into thewater tobe preset and maintained during a pipe lay operation; the ramp assembly guides - the pipe as it enters the water at the preset exit angle. As noted aboye, all prior known commercial xeel pipelaying barges haYe operated at a fixed, non-variable exit angle of between about 6 and 12. The adjustable exit angle fea~ure of the Apache-type - ~C ..
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vessel enables it to handle a ~ider ranye of pipe sizes in a greater range of water depths than was heretofore possible with fixed lo~ exit angle reel pipelaying barges.
One of the advantages of an ~pache-t~pe adjustable ra~p assembly for setting the pipe exit angle is the virtual elimination of the overbend region (i e~, the bend region occurring as the pipe translates do~mwardly fromtherelatively hori-zontal plane of the barge toward the seabed in therelatively vertical planeof the catenar~. Advantageously andpreferably, the ramp angle and tension are set so that downstream of the straightener/tensioner , apparatus, the pipe will be unsupported; thus, pipe exiting the straightener mechanism and traveling along the ramp assembly will already be in its nominal catenary configuration before and as it en~ers the water. Preferably, as the pipë moves through the straightener mechanism toward the water, all or substantially all of the curvature imparted to the pipe by the reel and other pipe handling elements is removed so that pipe exiting from the straightener mechanism has substantially zero residual stress and zero residual bending moments.
By initially setting the ramp angle and nomlnal pipeline tension to virtually eliminate the overbend as a factor in determining and controlling the final residual pipeline characteristics, the sag bend (i.e., the bend occurring in the translation of the pipe from the vertical to the hori-zontal plane on the sea bottom) becomes a critical factor in the control o~ the pipe as it is laid. The sag bend is con-trolled, at least in par~, as a function of the tension main-tained on the pipe by the functional elements of the pipelaying vessel, including the xeel, straightener /tensioner elements vessel drive assembly, etc. Controlled tension is imparted ~-C
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1147566 ~`
to the pipe by ~1) the reel throuyhthereeldrive mec~lanismoperating as a dynamic brake, (2) the main vessel drive thrust acting through the vessel main propellers and/or the lateral thruster assemblies, and (3) the tensioner assembly, which may or may not be used, throuyh a rec3ulated tensionin~ force established at the beginnlng of a lay operation and generally maintained throughout the lay operation.
The desired pipelaying tension and the desired entry angle of the pipe into the water are preferably determined on the basis o~ information supplied by th~ pipeline designer.
Such information from the pipeline designer (or customer - pipeline owner) includes (1) the size of the pipe, including internal pipe diameter and ~all thic~ness, (2) the type or grade of pipe, including such information as the pi~e material and minimu~ yield strength, (3) maximum allowable strèss, strain and residual tension, and (4) water depth along the pipeline right of way. An optimum nominal tension and lay anc31e can be determined from these parameters.
One of the criteria which has been developed for laying pip~withan Apache-type vessel is that the maximum allowable working stress, due to pipelayingoperation,in the unsupported length ofpipe between the vessel and thesea bottom should not be greater than about 85% of the minimum yield strength o~ the pipe.
It is also desirable and preferable to minimize the tension imparted to the pipe by the vessel while maintaining operating conditions such that the maximum allowable stress limit and the maximum allowable residual tension in the ~ipeline are not exceeded. This may be accomplished by setting the ramp assembly angle (and thus the pipe entry angle into the water) in conjunction with nominal pipe tension such that the tightest sag bend radius will bP achieved without exceedinc3 the a~ove-noted stress ~nd residual tension li~it.
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` J ~147S66 The ramp assembly angle (and thus the pipe entry angle lnto the water) is set at the b~ginning of the pipelaying opera-tion and is normally not changed during the entire lay operation.
It is possible to alter the ramp angle during a-pipelaying opera-tion, for example, to account for (appreciable) changes in water depth. During the pipe-laying operation, control of the pipe as it ïs being laid is maintained by controlling the tension in the pipe. Such control is normally achieved through ad~ustments in the reel tor~ue andjor tensioner setting and/or in the vessel forward and/or lateral thrust.
Prior to the start of the pipe~aying operation, the ramp angle and nominal pipe tension level are established on the basis of input from the pipeline designer. Also, in the case of an Apache-type vessel wherein the straightener tracks and the radius controller section are independently adjustable relative to each other, the radius controller and the straighteners are set at predetermined positions relative to each other and to the ramp assembly aft of the straighteners so that the tpreferably unsupported length of) pipe between the straightener assembly and aft end of the ramp assembly (at the stern guide roller assembly) will have little or no residual strain between the straightener assembly exit point and the aft end of the ramp assembly.
Under certain operating conditions, the "flexible"
towing connection between a reel barge and its tug will not be adequateito maintain the necessary continuous tension on the pipeline as it is being laid. The tug moves independently of the barge due to wave action. This means that the motive source which provides the forward thrust necessary to maintain tension on the pipeline is susceptible to uncontrolled variations relative to the barge and thus to the pipe. Limited excursions '- ' ' ~
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~47566 ~i`
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of tllis type may be acceptable for some sizes of pipe and some sea conditions. Ho~Jever, the ranse of permitted excursions is relatively small and decreases, particularly with increasing pipe size and increasingly rough sea conditions.
- ~ A self-propelled reel ship has the advantage that the forward thrust producing motive force can be considerea to be coupled directly to the pipe end on board the ship so that relative movement be`tween the motive source and the pipe end connected to the vessel is reduced essentially to zero. Further, external forces produced by waves, winds, current, etc. act on the pipe and motive source together and at the same time. Since the motive source and pipe end are substantially directly coupled, the p~pe is more directly responsive and more rapidly r~sponsive to changes in thrust. The self-propelied ship can therefore operate in a greater range o sea conditions, and particularly adverse sea conditions, than can a towed barge.
- On a reel pipelaying vessel, it is not possible to measure the pipeline tension directly. There are, however, several ways to measure the tension indirectly. One such way is to ~easure the forward thrust of the vessel, which is directly proportional to the tension on the pipe. Increasing or decreasing the vessel thrust will produce a corresponding proportional increase or decrease in the tension on the pipeline.
This can be aone by measuring the main propeller shaft torque or by ~easuring the force on a thrust bearing agalnst which the propeller shaft acts.
A second method is to measure the drive motor force acting on the reel. Neglecting the components of tension pro-duced prir~arily by the straightener assembly (and tensioner, when used), the orce e,~erted by the reel drive motors is directly proportional to the tensl~on in the pipe; thus, an 1~7S66 , increase or decrease in the drive motor force produces a corresponding increase or decrease in the pineline tension.
The reel motor drive force may be measured by, e.g., load cells between the motor/reel mechanical connection.' A third practical t~ay to measure pipeline tension is based on ~easurement of the exit angle of the pipe from the vessel. It is advantageous and preferable that the pipe angle be measured with respect both to the hori~on and to the ramp angle; the latter measurement is particularly helpful ~here the pi~e passes through an exit window defined by a stern ~uide roller assen~ly, such as is used on Apache-type vessels.
Figures 2A-C are diagrammatic representations of the pipelaying ~essel 10, ramp assembly 40 (set at a nominal lay an~le of about 30 degrees), the stern guide roller assembly 54 defining the exit windo~, and the pipe P. Figure 2A shows the relationship between the ramp assembly and the pipe when 'the vessel is substantially flat in the water so that the entry ~ngle Al of the pipe into the water trelative to a nominal horizontal plane or axis, such as the horizon) is substantially the same as the predetermined ra~p angle R; Figure 2B shows the same relationship when the vessel is pitched bow up at an angle D2 and the pipe P2 enters the water at an angle A2; and Figure 2C shows the same relationship when the vessel is pitched ~ow down at an angle D3 and pipe P3 enters the water at an angle ~3. The exit point of the pipe fro~ the straightener/
tensioner asse~bly is desi~nated by reference SE. The pipe is essentially in fixed relation to the ramp assembly'and the vessel at point SE. Preferably and advantageously, sufficient and adequate tension is maintained on the pipe P during the ,'' ` ' ~
." ,' `' ~
.-~ 1147S66 laying operation so that the pipe travels in a path substantiallyparallel to the ramp and through the guide roller assembly 54 sub-stantially unsupported between straightener exit SE and the touch-down point TDP on the sea bottom. Also advantageously and prefer-ably control of pipelaying operation is maintained so that angles Al, A2, A3 will be essentially equal.
The stern guide roller assembly provides a pipe excursion window between the upper and lower guide rollers for pipe excursion relative to the vessel as a result of vessel motion due to wave action. In one commercial embodiment, the distance between straight-ener exit SE and stern guide roller asse~bly 54 is approximately 45 feet; the distance between the upper and lower stern guide rollers is approximately four feet. This permits an angular excursion of the pipe between straightener exit SE and stern guide roller assembly 54 in a range from about 4.7 for 4 inch OD pipe to about 3.2 for 18 inah OD pipe; that is, the pipe can move through this range with-out being subject to bending moments by the stern guide rollers.
Referring to Figs. 2B and 2C, angles ~2 and F3, respectively, re-present the excursion above and below the nominal centerline of the pipeline P when it is tensioned to be parallel to the plane of the ramp assembly 40.
During the pipelaying operation, the vessel moves forward through the water as a function of the thrust generated by the main vessel drive, reacting against pipe tensioning forces produced by the pipe handling equipment, including reel dynamic braking forces, straightener, tensioner, etc. Changes in or modifications to the rate of forward motion of the vessel, and thus the rate at which pipe is unspooled from the reel 20 and paid out into the water, may be controlled by adjusting the dynamic braking force exerted by the reel drive mechanism and/or the amount of thrust generated by the main propellers.
A ~ypical lay rate, i.e., the rate at which - - 18 -.
k~
pipe is paia out fro~ the vessel during a lay op~ration, would be in~the range of 75 - 150 feet per minute. It has been found to be preferable to ~aintain the forward thrust relatively constant and to control pi e tension changes through adjustments to the reel dynamic braking force. Due to the large mass of the reel and pipe, it is not possib'le to effect instantaneous changes in the pay out rate.
As the vessel pitches during a laying operation, the stern, with the ramp and other related pipe handling equipment, moves up and do~n in the water. The pipe, paid out ~rom the straightener exit SE at a~predetermined rate which, as noted, cannot be changed instantaneously, also moves up and down with the vessel. The pipe is subjected to inertial effects through its underwater suspended length and on-bottom friction. Due to such inertial e~fects on the pipe, the'portion of the pipe downstrèam of straightener exit SE
does not necessarily move with the vessel so that the total pipe excursion relative to the ra~p may be ~reater than the stern guide excursion window limits. Under conditions where the bow pitches up by an angle D2, the pipe may be bent around the upper stern guide roller, as shown in Fig. 2B. Similarly, when the bow of the vessel pitches do~^mwards by an angle D3, ' the pipe may be bent around the 10~7er stern guide roller assembly, as sho~m in Fig. 2C.
In a commercial embodiment of an Apache-type vessel, an angle measuring device measures the pipe ansle do~mstream of the s~ern guide assembly relative to the ramp assembly 40 and relative to the horizon. One such angle measuring device is sho~m and described in aforesaid British ApPlication Serial No.
791S91~. An apparatus ~or this'purpose is manufactured by Interstate Electronics, Inc. ' "~
6C~
' - 19 - ~ ~
. .. ' ., ' . '' `' ' , ~ .
-... ~: .. ..... - . .. .
.- , _ . : - -. . . . . . .
';
~47566 In Fig. 2B, reference E2 represen-ts the measured angle of excursion of the pipe P2 relative to the ram~ assembly 40 under the condition where the ve~sel pitches up by the bow at an anyle D~ ~t this pitch angle, the effective exit angle G2 becomes R (ramp angle) plus D2 (pitch angle). ~s noted earlier, it has been founa that pipe tension and exit angle are inversely propoxtional; therefore, as the effective exit angle G2 increases, the tension applied to the pipeline should be decreased in order to maintain the pipe profile within acceptable limits. However, since, due ~to reel and pipe inertia and other factors, the tension applied to the pipe cannot be adjusted to directly follow the pitching of the vessel, the effective tension on the pipe is increased and a pi~e profile such as shown in Fig 2B results. Under sufficiently severe conditions of vessel pitch, the ~ipe P2 undergoes a relatively larse excursion so that the pipe excursion angle E2 exceeds the guide assembly window excursion limit angle F2 In such cases, the pipe undergoes a bending morent about the upper stern guide roller. If this bending moment exceeds the elastic limit of the pipel the pipe will undergo plastic bending and will thus retain a residual curvature due to such plastic bending ~hen i~ rests on the bottom.
~ hen the bow o the vessel pitches downward, e.g., at an angle D3, a pipe profile such as sho~n in Fig. 2C may result. In this case, the effectlve exit angle G3 becomes R
tramp angle) minus D3 (pitch angle); in this case, the effective exit angle is smaller than- the nominal preset ramp angle. In order to maintain a ~roper~pipe profile, in bow down pitch condition, the tension on the pipe should be increased an amount - C'~
- 20 ~
,' ~
.... . ~C
.' ~ '' , ' -~ .
sufficient to compellsate for the aecrease in effective e~it angle. Ilo~ever, for reasons noted above, it is not Dossiblc to instantaneously change the tension imparted to the pipe by the vessel, and particularly by the reel. Therefore, the pipe undergoes an excursion E3 which may be greater than the excursion F3 ~er~itted by the stern guide ~indow limits. Under such conditions, the pipe under~oes a hending moment abou~ the lower stern guide roller; if this bending moment exceeds the elastic limit, the pipe undergoes plastic bending and will retain a residual curvature when it is laid.
The angle measuring device measures excursion E2 and E3 to thereby generate an indication of excessive bending of the pipe on the ramp. ~leasurernent of excursion E2 or E3 is parti-cularly important as an indicntor that the pipe is over-tensioned or undertensioned, irrespective of the ~itching of the vessel.
When the vessel is pitching, excursions E2 and E3 would be expected to be relatively short-lived. Measurement of such short-llve~ excursions would not provide an accurate indica-tion of over- or under-tensioning.
A c~ntinuous measurement of excursion E2 greater than limit F2~or measured excursions E2 greater than F2 which occur a significant percent of the ti~e (e.g., greater than the pi~ching period of the vessel), even thoug'n such excursions are not continuous,-indicate to the operator that the pipe is being hel~ under excessive tension. The operator can then adjust the reeldynamic braking force to decrease thetension on the pipe until the angle measuring device measures an excursion E2 less than excursion F2, neglectin~ short-lived excursions due to vessel pitchiny. Correspondingly, when the angle measuring - 21 -.
., ','. , . , ~C
.. ~ . . .... . ... . ..
~ 1147566 ~
device measures an excursion E3 continuously greater than excursion limit F3, or grcater than F3 a significant percent of the tir..e (e.g., greater than th~ pitching period of the vessel), even though not continuous, these constitute indica-tions that -~he pipe is being held under insufficient tension.
The~operator can then increase the tension on the pipe until the measured excursion E3 becomes less than excursion limit F3, again neglecting short~lived excursions due to vessel pitching.
When the vessel is pitching, d~ue, for example, to sea conditions, measurins excursions E2 and E3 may produce erroneous -indications of pipe tension and may make it difficult, if not practically im.possibIe, for the oDerator to maintain proper tension ontthe pipe. Therefore, the angle measuring device also measures the actual exit angle ~ of the pipe (relative to the horizon or mean water line). Such measurement provides a more accurate indication of the actual pipe entry angle into the water so that under vaxying sea conditions, with the vessel pitching continu-ously, the operator can maintain a direct reading of the actual pipe entry angle. The operator is then able to maintain the proper reel dynamic breaking force and provide necessary compensation adjustments based on the actual pipe angle relative to the fixed horizon, as distinguished from angles measured relative to the moving and pitching vessel.
The pipe laying operation is also affected by the fact that the pipe traverses across the beam of the vessel as it is unspooled. This produces a turning moment tending to pull the vessel off course. This turning moment increases to a ~ ?~
'' ' ~ ;rf,~C
. . . - . ' . - ' ., i ~ .
1,J ~
~ _ . _, .. _ ~ . . . _ . _ ... _ __ __ . __ _ _ .. _ _ ._ . _ . . .. _ . __ _._ _. __, .. .
~ 1147S~;6 ~ ~
maximu~ at the end of transverse travel of the ramp assen~ly, decreases to zero t~hen the ramp assembly (and pipeline pa~h) is aligned with the vessel centerline, and increases to a maxi~um in the opposite ~irection as the ramp assembly continues movincJ
to the extreme opposite end of its transverse travel.
The turning moment can be quite large compared to the fon~ard thrust generated by the main propellers. For example, in one commercial embodiment, the ramp assembly has an athwart-ship ~ovement ran~e of 21.5 feet. The shafts of the main pro-pellers are located about 20 feet to either side of the vessel centerline; each produces a maximum thrust of 80 Kips. ~hen pipe is being laid under 100 Xips tension at an exit angle of 30 degrees, the pipe tension induced turning mom.ent at each extreme end of ramp asse~bly travel is on the order of 930 foot Xips. ~he opposing turning moment produced by the main propeller on that side operating at maximum thrust is about 1,600 foot ,Cips. It will be seen that the pipe tension induced tùrning mo~ent may well be a significant percentage (58.9~ in the example given here) of the drive induced turning moment.
If the pipe tension induced turning ~..oment is not compensated for, the vessel will be pulled off course; this can result in the pipe being laid out of the ri~ht of way, ~hich is co~mercially unacce~table.
- The pipeline induced turning moment must be compensated or in order to lay the pipe in a straight line along the right of way. .With twin screw vessels, that is, vessels propelled by two sets of main drive propellers equally spaced on o?posite sides of the longitudinal centerline of the vessel, it may be possible to overcome the turning ~oment introduced by the pipe's pipeline oset relative to the vessel center line by increasins thrust on the propeller located on that side of the rf~
. 23 _ ~
. ~........... . ' ' . ~ffC
: . - . - .
.
vessel and /or decreasing thrust on the opposite side main drive propeller. This has certain inherent disadvantages because the pipeline induced turning moment continually varies as the pipeline shifts laterally across the ~essel as it is unspooled.
To compensate for this varying turning moment using the main drive propellers requires that the thrust of the drive propellers be varried accordingly, while at the same time taking into account that the forward component of thrust must be maintained relatively constant in order to maintain the proper amount of tension on the pipe at all pertinent times during the pipelaying operation. Under certain condi-tions of pipeline tension and forward thrust, the system will not be able to generate sufficient additional thrust to com-pensate for the pipeline induced turni~g moment, especially when the ramp assembly and pipeline are at an extreme end of transverse displacement.
A second and potentially more commercially preferable way to compensate for the turning moment introduced by the pipeline lateral travel comprises utilizing forward and aft lateral thrusters. Examples of such thrusters are shown in the aforesaid prior related Santa Fe inventions. Also refer-ring to Fig. 3 hereof, an aft thruster tunnel 120 houses the aft thruster 122; a forward thruster tunnel 124 houses the forward thruster 126.
The thrusters 122 and 126 can be operated either manually or automatically in conjunction with, e.g., a computer operated guidance system, to generate turning moments which react against the pipeline induced turning moments. The pipeline introduces a turning moment about the intersection of the vessel longitudinal axis and reel shaft axisi the magnitude of the pipeline induced turning moment is a fu~ctionof the tension on the pipeline and the pipeline offset /- ~ 1147S66 ~
from the vessel's centerline. The vessel thrusters generate turning moments about the aforesaid intersection of the vcssel's centerline and reel shaft axis which react against the pipeline turning moment to maintain the vessel on its proper course.
Consideration must also be given to the fact that a turning moment occurs between the forward vessel thruster(s) and the pipeline touchdown point on the sea botto~. Therefore, in addition to rotating the vessel abou~ the centerline inter-section points, the entire vessel MUSt be rotated about the touch~own point to maintain the vessel on and paxallel to the right of way. This may be accom~lished by increasing the thrust generated by the forward thruster(s) relative to tlle oppositely reacting force generated by the aft thruster(s).
~ The amount of thrust required varies as a function of a num~er of factors, including the lateral positio~ of the pipeline relative to the vessel's longitudinal axis, the distance between the vessel and the touchdown point, the pipeline tension and pipe exit angle. In general, the forward thruster will be controlled to generate a thrust component Tl in one lateral direction relative to the vessel's longitudinal centerline.
The aft thruster will be controlled to generate a thrust component T2 in the opposite lateral direction relative to the vessel's longitudinal centerline. ~dvantageously and preferably, Tl is maintained greater than T2; together, Tl ~ T2 produce a turning moment ~ich reacts the pipeline induced turning moment.
The thrust generated by the forward thruster therefoxe comprises the additive components of the thrust necessary to react the pipeline induced turning moment about the vessel axis and the pipeline induced turning moment about the touchdown point pivot axis. The aft or rear thruster need only react the . :' , - lf~-~1~75~6 ~`
pipe:Line induced turning moment about the vessel axis. The for~7ard thruster there~ore imparts a relatively gre~ter lateral thrust com~onent than the rear thruster to overcome the pipeline induced turning moments about the vessel pivot axis and about the touchdown point pivot axis to thereby maintain the vessel on course along the right of way.
The invention may be e~bodied in o~her specific orms without departing from the spirit or essential charac~
teristics thereof. The embodiment described above is therefore to be considered in all respects as ill~strative and not restrictive, the scope of the invention being indicated by the hereafter appended clai~s 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.
.
... ' ' ~
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. ., .:.
Claims (9)
1. A method of laying pipe offshore from a self-propelled reel pipe laying vessel, said vessel having self-propulsion means, a reel for spooling relatively inflexible pipe thereon, and pipe handling means for straightening the pipe as it is unspooled and for guiding the straightened pipe into the water at a presettable adjustable exit angle, the pipe handling means including tensioning means for main-taining the pipe under a predetermined adjustable tension, said method comprising the steps of:
setting the pipe handling means to establish a desired pipe exit angle at which the pipeline substantially enters its catenary configuration before exiting the vessel and pipe handling means; and setting the tensioning means to hold the pipe under a predetermined nominal tension in conjunction with the pipe exit angle, to establish a minimum radius of curvature of the pipe in the sag bend region which is greater than the minimum radius to which that pipe may be bent without exceeding its elasticity limits as it is unspooled and paid out from the vessel.
setting the pipe handling means to establish a desired pipe exit angle at which the pipeline substantially enters its catenary configuration before exiting the vessel and pipe handling means; and setting the tensioning means to hold the pipe under a predetermined nominal tension in conjunction with the pipe exit angle, to establish a minimum radius of curvature of the pipe in the sag bend region which is greater than the minimum radius to which that pipe may be bent without exceeding its elasticity limits as it is unspooled and paid out from the vessel.
2. A method according to claim 1, further comprising:
setting and maintaining a pipe exit angle and nominal pipeline tension to maintain the allowable working stress in the unsupported length of pipe between the vessel and sea bottom at less than the maximum working stress to which the pipe may be permissibly subjected.
setting and maintaining a pipe exit angle and nominal pipeline tension to maintain the allowable working stress in the unsupported length of pipe between the vessel and sea bottom at less than the maximum working stress to which the pipe may be permissibly subjected.
3. A method according to claim 1, further comprising:
setting and maintaining a pipe exit angle and nominal pipeline tension to maintain the allowable working stress in the unsupported length of pipe between the vessel and the sea bottom at not greater than about 85% of the maximum working stress to which the pipe may be permissibly subjected.
setting and maintaining a pipe exit angle and nominal pipeline tension to maintain the allowable working stress in the unsupported length of pipe between the vessel and the sea bottom at not greater than about 85% of the maximum working stress to which the pipe may be permissibly subjected.
4. A method according to claim 1, 2 or 3, further comprising:
setting the tensioning means to hold the pipe under the minimum tension required for a given exit angle to maintain the pipe within maximum allowable pipe stress limits and within maximum allowable residual tension limits for that pipe.
setting the tensioning means to hold the pipe under the minimum tension required for a given exit angle to maintain the pipe within maximum allowable pipe stress limits and within maximum allowable residual tension limits for that pipe.
5. A method according to claim 1, 2 or 3, further comprising:
setting, in combination, the pipe handling means, including the tensioning means, to establish the highest pipe exit angle and the smallest nominal tension in the pipeline at which (a) the sag bend radius of the pipe remains above the minimum radius of curvature to which the pipe may be bent without exceeding its elasticity limits, (b) the maximum allowable stress limit of the pipe is not exceeded, and (c) the maximum allowable residual tension in the pipeline as laid is not exceeded.
setting, in combination, the pipe handling means, including the tensioning means, to establish the highest pipe exit angle and the smallest nominal tension in the pipeline at which (a) the sag bend radius of the pipe remains above the minimum radius of curvature to which the pipe may be bent without exceeding its elasticity limits, (b) the maximum allowable stress limit of the pipe is not exceeded, and (c) the maximum allowable residual tension in the pipeline as laid is not exceeded.
6. A method according to claim 1, 2 or 3, further comprising:
setting the pipe handling means to establish a desired pipe exit angle between about 20° and 60° relative to a nominal horizontal plane representing the water surface.
setting the pipe handling means to establish a desired pipe exit angle between about 20° and 60° relative to a nominal horizontal plane representing the water surface.
7. A method of laying pipe offshore from a self-propelled reel pipe laying vessel, said vessel having self-propulsion means, including a pair of main vessel drive means located on opposite sides of the vessel longitudinal centerline; a reel for spooling relatively inflexible pipe thereon; pipe handling means for straightening the pipe as it is unspooled and for guiding the straightened pipe into the water at a presettable adjustable exit angle, the pipe handling means including tensioning means for maintaining the pipe under a predetermined adjustable tension;
and forward and aft thruster means located forward and aft, respectively, of the transverse center of the vessel; said method comprising the steps of:
setting the pipe handling means to establish a desired pipe exit angle at which the pipeline substantially enters its catenary configuration before exiting the vessel and pipe handling means;
setting the tensioning means to hold the pipe under a predetermined nominal tension in conjunction with the pipe exit angle, to establish a minimum radius of curvature of the pipe in the sag bend region which is greater than the minimum radius to which that pipe may be bent without exceeding its elasticity limits as it is unspooled and paid out from the vessel;
translating the pipe handling means laterally across the beam of the vessel as pipe is unspooled from the reel; and, compensating for pipeline tension induced turning moments by generating a reactive force in opposition to the pipeline tension induced turning moment to thereby correct for deviations in the vessel's course caused by said pipeline tension induced turning moment.
and forward and aft thruster means located forward and aft, respectively, of the transverse center of the vessel; said method comprising the steps of:
setting the pipe handling means to establish a desired pipe exit angle at which the pipeline substantially enters its catenary configuration before exiting the vessel and pipe handling means;
setting the tensioning means to hold the pipe under a predetermined nominal tension in conjunction with the pipe exit angle, to establish a minimum radius of curvature of the pipe in the sag bend region which is greater than the minimum radius to which that pipe may be bent without exceeding its elasticity limits as it is unspooled and paid out from the vessel;
translating the pipe handling means laterally across the beam of the vessel as pipe is unspooled from the reel; and, compensating for pipeline tension induced turning moments by generating a reactive force in opposition to the pipeline tension induced turning moment to thereby correct for deviations in the vessel's course caused by said pipeline tension induced turning moment.
8. A method according to claim 7, further comprising:
generating said opposing reactive force by increasing the thrust generated by the main vessel drive on the same tranverse side of the vessel as the pipe handling means and/or decreasing the thrust generated by the main vessel drive on the opposite transverse side of the vessel from the pipe handling means.
generating said opposing reactive force by increasing the thrust generated by the main vessel drive on the same tranverse side of the vessel as the pipe handling means and/or decreasing the thrust generated by the main vessel drive on the opposite transverse side of the vessel from the pipe handling means.
9. A method according to claim 7 or 8, further comprising:
generating a thrust T1 in one substantially lateral direction with said forward thruster and generating a thrust T2 in a substantially opposite lateral direction with said aft thruster, where T1 is greater than T2 and where T1plus T2 produce a turning moment which at least substantially reacts the pipeline induced turning moment to thereby correct for said course deviations.
generating a thrust T1 in one substantially lateral direction with said forward thruster and generating a thrust T2 in a substantially opposite lateral direction with said aft thruster, where T1 is greater than T2 and where T1plus T2 produce a turning moment which at least substantially reacts the pipeline induced turning moment to thereby correct for said course deviations.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000358052A CA1147566A (en) | 1980-08-12 | 1980-08-12 | Method of laying offshore pipeline from a reel carrying vessel |
| CA000411523A CA1172859A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
| CA000411522A CA1156059A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000358052A CA1147566A (en) | 1980-08-12 | 1980-08-12 | Method of laying offshore pipeline from a reel carrying vessel |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000411523A Division CA1172859A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1147566A true CA1147566A (en) | 1983-06-07 |
Family
ID=4117629
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000358052A Expired CA1147566A (en) | 1980-08-12 | 1980-08-12 | Method of laying offshore pipeline from a reel carrying vessel |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1147566A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115258073A (en) * | 2022-08-09 | 2022-11-01 | 武汉理工大学 | Track tracking method of ship towing system under environmental interference |
-
1980
- 1980-08-12 CA CA000358052A patent/CA1147566A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115258073A (en) * | 2022-08-09 | 2022-11-01 | 武汉理工大学 | Track tracking method of ship towing system under environmental interference |
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