CA1172859A - Method of laying offshore pipeline from a reel carrying vessel - Google Patents
Method of laying offshore pipeline from a reel carrying vesselInfo
- Publication number
- CA1172859A CA1172859A CA000411523A CA411523A CA1172859A CA 1172859 A CA1172859 A CA 1172859A CA 000411523 A CA000411523 A CA 000411523A CA 411523 A CA411523 A CA 411523A CA 1172859 A CA1172859 A CA 1172859A
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- CA
- Canada
- Prior art keywords
- pipe
- vessel
- pipeline
- reel
- tension
- 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.)
- Expired
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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
~ ~ ~ 7 28~
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 sel~-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 pi.pe spoollng 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 issued to Stanley T. Uyeda, E. John Radu, William J. Talbot, Jr. and Norman Feldman.
The present appliction (and the i~ventive subject matter described and claimed herein) and the above-listed U.S. Patents are all owned by Santa Fe International - ``` i 1~285~
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,?.87 issued to Stanley T. Uyeda and John H. Cha, and assigned to Santa Fe.
Other patents owned by Santa E'e directed ko 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 Fred 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;
~, l72~r~'~ Jt"
U.S. Patent ~o. 3,712,100 issued January 23, 1973 to Joe ~1. Key and Larxy Ro Russell; a~
, IJ.S. Patent 3,9.,2,402, i.ssued Septembe~ 28, 197G to ~lexander Craig Lans and Peter Alan Lunde.
.' ', ' ~.
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1 112~159 ~
Summary of the Invention The present invention is concerned with methods and tec~niqués related to the control of pipelaying operations from a self-propelled reel .ipelaying vessel. The methods are con.cerned with 1) con~rolling pipeline geor.~etry as a unction of pipe entry angle into the ~ater and tension on the pipeline;
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 sel~-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 pi.pe spoollng 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 issued to Stanley T. Uyeda, E. John Radu, William J. Talbot, Jr. and Norman Feldman.
The present appliction (and the i~ventive subject matter described and claimed herein) and the above-listed U.S. Patents are all owned by Santa Fe International - ``` i 1~285~
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,?.87 issued to Stanley T. Uyeda and John H. Cha, and assigned to Santa Fe.
Other patents owned by Santa E'e directed ko 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 Fred 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;
~, l72~r~'~ Jt"
U.S. Patent ~o. 3,712,100 issued January 23, 1973 to Joe ~1. Key and Larxy Ro Russell; a~
, IJ.S. Patent 3,9.,2,402, i.ssued Septembe~ 28, 197G to ~lexander Craig Lans and Peter Alan Lunde.
.' ', ' ~.
" . ~7'C`
1 112~159 ~
Summary of the Invention The present invention is concerned with methods and tec~niqués related to the control of pipelaying operations from a self-propelled reel .ipelaying vessel. The methods are con.cerned with 1) con~rolling pipeline geor.~etry as a unction of pipe entry angle into the ~ater and tension on the pipeline;
2) monitoring the excursion of the pipeline outside certain defined limits and controlling the pipeline geome~ry based on such measured excursions; and 3) com~enSating for pipeline induced turning moments which woula othen~ise tend ~o draw the pipelaying vessel off course and of $he predetermined pipeline ri~ht of ~7ay.
; The present invention is primarily applicable to a self-propelled reel pipe laying vessel, having a reel for spooling relatively in~lexible pipe thereon, pipe wor~ing and handling mPans for straightening the pipe as it is unspooled, pipe guide means fox guiding the straighkened pipe into the water at a presettable, adjustable exi.t angle, me~ns for maintaining the pipe under a predetermined adiustable tension, main vessel drive means, preferably includin~ t~lin screws located on opposite sides o the vessel longitudinal centerline, and ~orward and aft thruster means located fon~ard and aft, respectively, of the longitudinal center of the ~essel.
During a pipelaying operation, the pipe hand~ing equipment and ~ipe ~uide means txanslates across the beam of the vessel as it follo~s ~or leads) the pipe wrap being unspooled.
In the process of translating the pipe guide means across the beam of the vessel, turning moments (in the horizontal plane~ are imparted - 5 . .
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~ ~7~8~ ~
to the ve~ssel by the tension in the pipeline. In one ~sp~ct, there~ore, the invention co~prises a method of compensatin~
for these pipeline tension induced turning moments by generating a reactive force in opposition to the ?i~eline tension induced turnin~ moment'to thereby correc~ for deviations in the vessel's course and to maintain the vessel on course ~long the desired right o way.
A further aspect o~ the me~hod of this invention comvrises monitoring the an~le of entry of the plpe into the water relative ~o a nominal horizontal plane re~resenting the water surace; ~onitoring the angle of-excursion ~Jhich the pipe makes xelative to a nominal pipe centerline substantially parallel to the nominal preset anale of entry into the water;
and adjusting the nominal pipeline tension if the monitored excursion angle remains outside a predetermined permissible excursion ran~e for at least a significant time period, for example, greater than the pitching period o the vessel.
A still further aspect of the method of this invention comprises setting the pipe gui~e means to establisll a desirecl pipe,exit angle at which the pipeline substantially enters its , catenary con~iguration before exitincJ the vessel and pipe guide means; and setting the tensioning means to hold the pipe under a predetermined nominal ten,sion in conjunction ~Jith the pipe exit angle, to establish a minimum radius of curvature o t~e pipe in the sag bend region ~7hich is greater than the ., r.linimum radius to which that pipe may be be~t ~ithout exceeding its elasticiky limits as it is unspooled and paid out from t~e ve~sel.
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. ` ~ l L72~59 `~
Brief Descrintion of the Dra~ing . Fi~ure l is a diagra~matic sketch of a self~pro~elled rëel pipe laying vessel showing the approximate pipe profi1e bet~een the vessel and the sea bottom.
Fi~ures 2~-C are diagramma-tic s~e~ches of the vessel deck, ramp assembly and pipe, in several conditions of pitching due to sea conditions.
Figure 3 is a diagram~atic ~lan view of the vessel showing course-correctins force relationships.
Description of Preferred ~bodiments .
~ ndert7ater pipelines for carryin~ oil or ~as must meet certain requirements and limlts set by the customer (pipeline o~mer) and/or yovernmenkal or other regulatory bodies.
It is of prir.lary i~portance that the pipe, as it is being laid and as it lays on the sea bottom, be subjected ko minimal residual stress, strain, tension, etc. In terms of pipe laid b~ 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 important that the pipeline be laid close to the nominal right of ~ay. The "as laid" restrictions are developed as a function o~ a nur~ber of parameters developed by the pipeline desi~ner, 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 ~redicted lie s~an of the pipeline. O~her parameters xelatin~ to, or based on, the seometry ~shaye) of the pipeline during the pipe laying operation are developed by the pipe ~ayin~ engineer;. C /
- 7 - ~ ~
'. . ',' ''. . ~fC
8 ~ ~
. .
Additionally, a reel pipelaying vessel and the pipe being laid are subjected to a number of hydrostatic and h~drodynamic 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-- techni~ue have generally been vessels which maintain their operational position by setting out anchors. Auxiliary support ves~els 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 paid 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 decrea~ses as ~ater depth increases. It is apparent that the limited forward movement permitted by this anchor setting technique is not at all suitable for economical reel pipe laying operations.
~ ~Z8.~
,. .
Although towed reel pipelaying barges have been found to be quite adequate for the relatively calm waters of the Gulf of Mexico offshore of the United States coastline, they have certain inherent limitations which make them un-suitable for use in relatively rough waters, such as are found in the North 5ea or off the coast of South America or Australia.
One of the principal built-in limitations of a towed barge system resides in the towing connection itself. Unlike a self-propelled ship, in which the motive source is effectively connected directly and rigidly to the pipeline (through the reel), the connection between the towing vessel tmotive 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. This may cause the pipeline tension to exhibit sudden increases and/or decreases in magnitude which can neither be predicted nor controlled eEectively 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 right of way, stopping only when necessary, for example, to install anodes as required by the customer and/or to perform other operations on the pipe, such as coating repair, etc. Compared to towed reel barges, the self-propelled reel ship has a significant advantage in that the motive source of the reel ship can, for practical purposes, be considered to be fixed with the reel and pipeline end, thereby eliminating relative movements therebetween due to weather related factors, as noted above.
1 ~7~
Commercial and practical limitations efec~i~ely restrict the operating capability of a towed reel barge. One of the principal requirements in laying pipelines offshore from a surface vessel is that, in general, adequate tension must be maintained on the pipe at all significant times. This is necessary to prevent the sag bend from exceeding certain pre-determined tolerance 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 the Touchdown Point (TDP). It is important that the radius of the sag bend curve be kept above the minimum per-missible 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 the departure 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 profile 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 way, 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 Kips, where Kips equals thousands of pounds) on a pipe having an 2 ~ ?
outside diameter of 10 3/4" and 3/4" wall thickness laid in a water depth of 500 eet, if the pipe exit angle is set at about 26, in order to maintain the sag bend radius above the allowable minimum, at an exit angle of 58, the same conditions require ~ 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 ko date have been designated to operate at a relatively fixed departure angle of between about 6 and 12 (relative to a nomi~al horizontal plane representing the water surface). At this shallow exit angle, the tension required to maintain a catenary shaped pipe profile or 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 ~irst point of inflection, or "overbend", occurs near the surace 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 feature of "Apache-type" special reel pipelaying ships isthe adjustable pipe carrying ramp assembly 40 pivotably mounted (generally atthe skern~ to the dec]co~ the vessel 10, aft o~ the reel 20. The vessel also comprises main propul-sion propellers 12, one or more forward lateral thrusters 126 and one or more stern lateral thruskers 122. (Throughout this dis-closure, reference is made to the main propellers as providing the requisite forward thrust; it is apparent,however, that other suitable drive means could be provided to generate the neces-2 8 5 9 h'' sary for~ard thxust and the re~erence to "propellers" throughoutthis disclosure is intended to encompass other such suitable drive means, except where otherwise speci~ically noted.~
Special pipe handling equipment, which may include, for exarnple, the adjustable radius control member, adjustable straightener tracks, tensioner tracks, pipe clamping assemblies, guide roller assemblies, and pipe angle measuring assembly, is advantageously mounted to the ramp assembly 40.
~ n adiustable ramp assembly o~ this type has not heretofore been incorporated into any ~kno~n commercial offshore reel pipelaying vessel, specifically including the supply boat portable reel s~stem used off the coast of Australia, the two reel pipelaying towed barges o~ned and used by Santa Fe and/or Santa Fe's predecessors-in-interest since about 1961 and t~70 competitive reel ~ipelaying barges, one used for a short time in 1972 or 1973 and the other currently in use in the Gulf of llexico off the United States coast.
The Apache-type reel pipelaying vessel di~fers ~rom prior commercial reel pipelaying barges in its abi}ity to discharge pipe i.nto the ~ater at any desired angle ~ithin its operating rang~ o~ bet~J~en 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 the water to be 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 abo~e, all prior kno~m commercial reel pipelaying barges i-ha~e operated a~ a Cixed, non-variable exit an~le of between about 6 and 12. The adjustable exit angle feature of the Apache-type . -,1~-. ~
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vessel enables it to handle a wider ranye of pipe sizes in a greater range o~ water depths than was herekoore possible ~ith fixed low exi~ angle reel pipelaying barges One of the advantages of an Apache-type 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~Jnwardly fromthe relatively hori zontal plane of the barge ~oward the sea bed in the relatively vertical planeof the catenar~. Advantageously and preferably, the rarnp angle and tension are set so that downstream o~ the straightener/tensioner apparatus, the pipe will be unsupported; thus, pipe exiting the straightener mechanism and traveling along the ramp assem~ly 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 an~ other pipe handling elements is removed so that pipe exi~ing fro~ the straightener mechanism has substantially zero residual stress and zero residual bending moments.
By inikially settiny the ramp angle and nominal pipeline tension to virtuall~ eliminate the overbend as a factor in aetc-~rmining and controlling the final residual pipeline characteristics, the sag bend (i.e., the bend occurring in the translation o~ the pipe fxom the vertical to the hori-zontal plane on the sea bottom) becomes a critical factor in the control of the pipe as it i5 laid. The sag bend is con-trolled, at least in part, as a function of the tension main-tained on the pipe by the functional elements of the pipelaying vessel, includiny the reel, straightener /tensioner elements ~essel drive assembly, etc. Controllea tension is im~arte~
; The present invention is primarily applicable to a self-propelled reel pipe laying vessel, having a reel for spooling relatively in~lexible pipe thereon, pipe wor~ing and handling mPans for straightening the pipe as it is unspooled, pipe guide means fox guiding the straighkened pipe into the water at a presettable, adjustable exi.t angle, me~ns for maintaining the pipe under a predetermined adiustable tension, main vessel drive means, preferably includin~ t~lin screws located on opposite sides o the vessel longitudinal centerline, and ~orward and aft thruster means located fon~ard and aft, respectively, of the longitudinal center of the ~essel.
During a pipelaying operation, the pipe hand~ing equipment and ~ipe ~uide means txanslates across the beam of the vessel as it follo~s ~or leads) the pipe wrap being unspooled.
In the process of translating the pipe guide means across the beam of the vessel, turning moments (in the horizontal plane~ are imparted - 5 . .
~fC
~ ~7~8~ ~
to the ve~ssel by the tension in the pipeline. In one ~sp~ct, there~ore, the invention co~prises a method of compensatin~
for these pipeline tension induced turning moments by generating a reactive force in opposition to the ?i~eline tension induced turnin~ moment'to thereby correc~ for deviations in the vessel's course and to maintain the vessel on course ~long the desired right o way.
A further aspect o~ the me~hod of this invention comvrises monitoring the an~le of entry of the plpe into the water relative ~o a nominal horizontal plane re~resenting the water surace; ~onitoring the angle of-excursion ~Jhich the pipe makes xelative to a nominal pipe centerline substantially parallel to the nominal preset anale of entry into the water;
and adjusting the nominal pipeline tension if the monitored excursion angle remains outside a predetermined permissible excursion ran~e for at least a significant time period, for example, greater than the pitching period o the vessel.
A still further aspect of the method of this invention comprises setting the pipe gui~e means to establisll a desirecl pipe,exit angle at which the pipeline substantially enters its , catenary con~iguration before exitincJ the vessel and pipe guide means; and setting the tensioning means to hold the pipe under a predetermined nominal ten,sion in conjunction ~Jith the pipe exit angle, to establish a minimum radius of curvature o t~e pipe in the sag bend region ~7hich is greater than the ., r.linimum radius to which that pipe may be be~t ~ithout exceeding its elasticiky limits as it is unspooled and paid out from t~e ve~sel.
~ 7~
Jh~C
. ` ~ l L72~59 `~
Brief Descrintion of the Dra~ing . Fi~ure l is a diagra~matic sketch of a self~pro~elled rëel pipe laying vessel showing the approximate pipe profi1e bet~een the vessel and the sea bottom.
Fi~ures 2~-C are diagramma-tic s~e~ches of the vessel deck, ramp assembly and pipe, in several conditions of pitching due to sea conditions.
Figure 3 is a diagram~atic ~lan view of the vessel showing course-correctins force relationships.
Description of Preferred ~bodiments .
~ ndert7ater pipelines for carryin~ oil or ~as must meet certain requirements and limlts set by the customer (pipeline o~mer) and/or yovernmenkal or other regulatory bodies.
It is of prir.lary i~portance that the pipe, as it is being laid and as it lays on the sea bottom, be subjected ko minimal residual stress, strain, tension, etc. In terms of pipe laid b~ 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 important that the pipeline be laid close to the nominal right of ~ay. The "as laid" restrictions are developed as a function o~ a nur~ber of parameters developed by the pipeline desi~ner, 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 ~redicted lie s~an of the pipeline. O~her parameters xelatin~ to, or based on, the seometry ~shaye) of the pipeline during the pipe laying operation are developed by the pipe ~ayin~ engineer;. C /
- 7 - ~ ~
'. . ',' ''. . ~fC
8 ~ ~
. .
Additionally, a reel pipelaying vessel and the pipe being laid are subjected to a number of hydrostatic and h~drodynamic 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-- techni~ue have generally been vessels which maintain their operational position by setting out anchors. Auxiliary support ves~els 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 paid 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 decrea~ses as ~ater depth increases. It is apparent that the limited forward movement permitted by this anchor setting technique is not at all suitable for economical reel pipe laying operations.
~ ~Z8.~
,. .
Although towed reel pipelaying barges have been found to be quite adequate for the relatively calm waters of the Gulf of Mexico offshore of the United States coastline, they have certain inherent limitations which make them un-suitable for use in relatively rough waters, such as are found in the North 5ea or off the coast of South America or Australia.
One of the principal built-in limitations of a towed barge system resides in the towing connection itself. Unlike a self-propelled ship, in which the motive source is effectively connected directly and rigidly to the pipeline (through the reel), the connection between the towing vessel tmotive 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. This may cause the pipeline tension to exhibit sudden increases and/or decreases in magnitude which can neither be predicted nor controlled eEectively 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 right of way, stopping only when necessary, for example, to install anodes as required by the customer and/or to perform other operations on the pipe, such as coating repair, etc. Compared to towed reel barges, the self-propelled reel ship has a significant advantage in that the motive source of the reel ship can, for practical purposes, be considered to be fixed with the reel and pipeline end, thereby eliminating relative movements therebetween due to weather related factors, as noted above.
1 ~7~
Commercial and practical limitations efec~i~ely restrict the operating capability of a towed reel barge. One of the principal requirements in laying pipelines offshore from a surface vessel is that, in general, adequate tension must be maintained on the pipe at all significant times. This is necessary to prevent the sag bend from exceeding certain pre-determined tolerance 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 the Touchdown Point (TDP). It is important that the radius of the sag bend curve be kept above the minimum per-missible 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 the departure 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 profile 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 way, 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 Kips, where Kips equals thousands of pounds) on a pipe having an 2 ~ ?
outside diameter of 10 3/4" and 3/4" wall thickness laid in a water depth of 500 eet, if the pipe exit angle is set at about 26, in order to maintain the sag bend radius above the allowable minimum, at an exit angle of 58, the same conditions require ~ 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 ko date have been designated to operate at a relatively fixed departure angle of between about 6 and 12 (relative to a nomi~al horizontal plane representing the water surface). At this shallow exit angle, the tension required to maintain a catenary shaped pipe profile or 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 ~irst point of inflection, or "overbend", occurs near the surace 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 feature of "Apache-type" special reel pipelaying ships isthe adjustable pipe carrying ramp assembly 40 pivotably mounted (generally atthe skern~ to the dec]co~ the vessel 10, aft o~ the reel 20. The vessel also comprises main propul-sion propellers 12, one or more forward lateral thrusters 126 and one or more stern lateral thruskers 122. (Throughout this dis-closure, reference is made to the main propellers as providing the requisite forward thrust; it is apparent,however, that other suitable drive means could be provided to generate the neces-2 8 5 9 h'' sary for~ard thxust and the re~erence to "propellers" throughoutthis disclosure is intended to encompass other such suitable drive means, except where otherwise speci~ically noted.~
Special pipe handling equipment, which may include, for exarnple, the adjustable radius control member, adjustable straightener tracks, tensioner tracks, pipe clamping assemblies, guide roller assemblies, and pipe angle measuring assembly, is advantageously mounted to the ramp assembly 40.
~ n adiustable ramp assembly o~ this type has not heretofore been incorporated into any ~kno~n commercial offshore reel pipelaying vessel, specifically including the supply boat portable reel s~stem used off the coast of Australia, the two reel pipelaying towed barges o~ned and used by Santa Fe and/or Santa Fe's predecessors-in-interest since about 1961 and t~70 competitive reel ~ipelaying barges, one used for a short time in 1972 or 1973 and the other currently in use in the Gulf of llexico off the United States coast.
The Apache-type reel pipelaying vessel di~fers ~rom prior commercial reel pipelaying barges in its abi}ity to discharge pipe i.nto the ~ater at any desired angle ~ithin its operating rang~ o~ bet~J~en 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 the water to be 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 abo~e, all prior kno~m commercial reel pipelaying barges i-ha~e operated a~ a Cixed, non-variable exit an~le of between about 6 and 12. The adjustable exit angle feature of the Apache-type . -,1~-. ~
'' , , ~
- ~ .
..
vessel enables it to handle a wider ranye of pipe sizes in a greater range o~ water depths than was herekoore possible ~ith fixed low exi~ angle reel pipelaying barges One of the advantages of an Apache-type 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~Jnwardly fromthe relatively hori zontal plane of the barge ~oward the sea bed in the relatively vertical planeof the catenar~. Advantageously and preferably, the rarnp angle and tension are set so that downstream o~ the straightener/tensioner apparatus, the pipe will be unsupported; thus, pipe exiting the straightener mechanism and traveling along the ramp assem~ly 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 an~ other pipe handling elements is removed so that pipe exi~ing fro~ the straightener mechanism has substantially zero residual stress and zero residual bending moments.
By inikially settiny the ramp angle and nominal pipeline tension to virtuall~ eliminate the overbend as a factor in aetc-~rmining and controlling the final residual pipeline characteristics, the sag bend (i.e., the bend occurring in the translation o~ the pipe fxom the vertical to the hori-zontal plane on the sea bottom) becomes a critical factor in the control of the pipe as it i5 laid. The sag bend is con-trolled, at least in part, as a function of the tension main-tained on the pipe by the functional elements of the pipelaying vessel, includiny the reel, straightener /tensioner elements ~essel drive assembly, etc. Controllea tension is im~arte~
3 5 ~
to the pipe by (1) the reel through the reel drive mechanism operating as a dynamic brake, (2) ~he main vessel drive thrust acting through the vessel main propellers and/or the lakeral thruster assemblies, and (3} the tensioner assembly, which may or may not be used, through a regulated tensioning force established at the beginning of a lay operation and generally maintained throughout the lay operation.
The desirea pipelaying tension and the desired entry angle of the pipe into the water are preferably determined on the basis of information supplied by the pipeline designer.
Such information from the pipeline designer (or customer -pipeline owner) includes (1) the size of the pipe, including internal pipe diameter and wall thickness, (2) the type or grade of pipe, including such information as the pipe material and minimum yield strength, ~3) maximum allowable stress, strain and residual tension, and (4) water depth along the pipeline right of way. An optimum nominal tension and lay angle can be determined f~om these parameters.
One of the crlteria which has been developed for laying pipe with an Apache-type vessel is that the maximum allowable working stress, due to pipelaying operation, in the unsupported length of pipe between the vessel and the sea bottom should not be greater than abouk 85% of the minimum yield strength of 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 pipeline are not exceeded. This may be accomplished by setting the ramp assembly angle (and thus the pipe entry angle into the water) in corljunction with nominal pipe tension such that the tightest sag bend radius will be achieved wi~hout exceeding the above-noted stress and residual tensiorl limit.
` . ` ~. ~7~8~9 -~ .
The ramp assembly angle tand thus the pipe entry angle into the water~ is set at the beginning of the pipelayin~ opera-tion ~nd ls normally noi changed during the entire lay operation.
I~ 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 adjustments in the reel torque and/or 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 rela-tive to each other and to the ramp assembly aft o the straighteners so that the (preferably unsupported length of) pipe between the straightener assembly and at end of the ramp assembly (at the skern guide roller assembly) will have little or no residual strain between the stra.ightener assen~ly exit point and the aft end of the ramp assembly.
Und~r certain operating conditions, the "flexi.ble"
towing connection between a reel barge and its tug will not be adequate to 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 pxovides 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 - 15 - ~f ~C
.
of this type may be acceptable ~or some sizes of pipe and some sea conditions. However, the range 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 considered to be coupled directly to the pipe end on board the ship so that relative movement between 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 mo*ive source and pipe end are substantially directly coupled, the pipe is more directly responsive and more rapidly responsive to changes in thrust. The self-propelled ship can therefore operate in a greater range of 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 directl~. There are, however, several ways to measure the tension indirectly. One such way is to measure the forward thru~t 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 pipe-line. This can be done by measuring the main propeller shaft torque or by measuring the force on a thrust bearing against 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 primarily by the straightener assembly (and tensioner, when used), the force exerted by the reel drive motors is directly proportional to the tension in the pipe; thus, an .' " ' 1~ ' 2 8 ~ 9 incre~se cr ~ecrease in the drive motor ~orce produces a corres~onding increase or decrea~e in the pineline -tension~
The reel motor drive force may be ~,easured by, e.g., load cells between the ~otor/reel mechanical connection.' A third practical ~ay to r;leasure pi~eline t~nsion is basèd on ~easurement of the exit angle of the pipe from the vessel. It is advantageous and preferable that the pipe angle be measured ~lith respect both to the hori7-on and to the ramp angle; the latter measurement is particularly helpful where the pi~e passes through an exit window defined by a stern ~ui~e 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 gui~e roller assembly 54 defining the exit window, and the pipe P. ~igure 2A shows the relationship between t~le ramp assembly and the pipe when 'the vessel i5 substantially flat in ~he water so that the entry an~le Al of the pipe into the water (relat.ive to ~ nominal horizontal plane or axis, such as the horizon) is substantiall~
the sc~ne as the predetermined ramp angle R; Fiyure 2B sho~s the same relationship when the vessel .is ~itched bow up at an angle D2 and the pipe P2 enter.s the water at an angle A2; and Fi~ure 2C shows the same relationship when the vessel is pitche~ bow 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 designated by refe-rence SE. The pipe is essentially in fixed relation to the ramp assembly'and the vessel at point SE. Preferably and aavantageously, sufficient and ade~uate tension is maintained on the pipe P during the .' -' ~
', ' ~
.
; ~7~
laying operation so that the pipe travels in a pa~h substantially parallel to the ramp and through the guide ~oller assembly 54 substantially unsupported between straightener exit SE and the *ouchdown point TDP on the sea bottom. Also advantageously and preferably 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 Gf vessel motion due to wave action. In one commercial embodiment, the distance between straightener exit SE and stern guide roller assembly 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
to the pipe by (1) the reel through the reel drive mechanism operating as a dynamic brake, (2) ~he main vessel drive thrust acting through the vessel main propellers and/or the lakeral thruster assemblies, and (3} the tensioner assembly, which may or may not be used, through a regulated tensioning force established at the beginning of a lay operation and generally maintained throughout the lay operation.
The desirea pipelaying tension and the desired entry angle of the pipe into the water are preferably determined on the basis of information supplied by the pipeline designer.
Such information from the pipeline designer (or customer -pipeline owner) includes (1) the size of the pipe, including internal pipe diameter and wall thickness, (2) the type or grade of pipe, including such information as the pipe material and minimum yield strength, ~3) maximum allowable stress, strain and residual tension, and (4) water depth along the pipeline right of way. An optimum nominal tension and lay angle can be determined f~om these parameters.
One of the crlteria which has been developed for laying pipe with an Apache-type vessel is that the maximum allowable working stress, due to pipelaying operation, in the unsupported length of pipe between the vessel and the sea bottom should not be greater than abouk 85% of the minimum yield strength of 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 pipeline are not exceeded. This may be accomplished by setting the ramp assembly angle (and thus the pipe entry angle into the water) in corljunction with nominal pipe tension such that the tightest sag bend radius will be achieved wi~hout exceeding the above-noted stress and residual tensiorl limit.
` . ` ~. ~7~8~9 -~ .
The ramp assembly angle tand thus the pipe entry angle into the water~ is set at the beginning of the pipelayin~ opera-tion ~nd ls normally noi changed during the entire lay operation.
I~ 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 adjustments in the reel torque and/or 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 rela-tive to each other and to the ramp assembly aft o the straighteners so that the (preferably unsupported length of) pipe between the straightener assembly and at end of the ramp assembly (at the skern guide roller assembly) will have little or no residual strain between the stra.ightener assen~ly exit point and the aft end of the ramp assembly.
Und~r certain operating conditions, the "flexi.ble"
towing connection between a reel barge and its tug will not be adequate to 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 pxovides 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 - 15 - ~f ~C
.
of this type may be acceptable ~or some sizes of pipe and some sea conditions. However, the range 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 considered to be coupled directly to the pipe end on board the ship so that relative movement between 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 mo*ive source and pipe end are substantially directly coupled, the pipe is more directly responsive and more rapidly responsive to changes in thrust. The self-propelled ship can therefore operate in a greater range of 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 directl~. There are, however, several ways to measure the tension indirectly. One such way is to measure the forward thru~t 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 pipe-line. This can be done by measuring the main propeller shaft torque or by measuring the force on a thrust bearing against 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 primarily by the straightener assembly (and tensioner, when used), the force exerted by the reel drive motors is directly proportional to the tension in the pipe; thus, an .' " ' 1~ ' 2 8 ~ 9 incre~se cr ~ecrease in the drive motor ~orce produces a corres~onding increase or decrea~e in the pineline -tension~
The reel motor drive force may be ~,easured by, e.g., load cells between the ~otor/reel mechanical connection.' A third practical ~ay to r;leasure pi~eline t~nsion is basèd on ~easurement of the exit angle of the pipe from the vessel. It is advantageous and preferable that the pipe angle be measured ~lith respect both to the hori7-on and to the ramp angle; the latter measurement is particularly helpful where the pi~e passes through an exit window defined by a stern ~ui~e 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 gui~e roller assembly 54 defining the exit window, and the pipe P. ~igure 2A shows the relationship between t~le ramp assembly and the pipe when 'the vessel i5 substantially flat in ~he water so that the entry an~le Al of the pipe into the water (relat.ive to ~ nominal horizontal plane or axis, such as the horizon) is substantiall~
the sc~ne as the predetermined ramp angle R; Fiyure 2B sho~s the same relationship when the vessel .is ~itched bow up at an angle D2 and the pipe P2 enter.s the water at an angle A2; and Fi~ure 2C shows the same relationship when the vessel is pitche~ bow 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 designated by refe-rence SE. The pipe is essentially in fixed relation to the ramp assembly'and the vessel at point SE. Preferably and aavantageously, sufficient and ade~uate tension is maintained on the pipe P during the .' -' ~
', ' ~
.
; ~7~
laying operation so that the pipe travels in a pa~h substantially parallel to the ramp and through the guide ~oller assembly 54 substantially unsupported between straightener exit SE and the *ouchdown point TDP on the sea bottom. Also advantageously and preferably 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 Gf vessel motion due to wave action. In one commercial embodiment, the distance between straightener exit SE and stern guide roller assembly 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 inch OD pipe; that is, the pipe can move through this range without ~eing subject to bending moments by the stern guide rollers. Reerring to Figs. 2B and 2C, angles F2 and F3, respectively, represent the excursion above and below the nominal centerline of the plpeline P when it is tensioned to be parallel to the plane of the ramp assembly ~0.
During the pipelaying operation, the vessel ~oves forwaxd through the water as a function o~ the thurst 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 ~raking force exerted by the reel drive mechanism and/or the amount of thrust generated by the main propellers.
A typical lay rate, i.e., the rate at which pipe is paid 2 ~ ~ 9 out fxom the vessel during a lay operation! would be in the range of 75 - 150 feet per minute. It has been foun~ ~o be preferable to maintain the forwar~ thrust relatively constant and to control pipe tension changes through adjustments to the reel dynamic braking force. Due to the large mass of the xeel and pipe, it is not possible to effect instantaneous chan~es 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 down inthe 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 effects on the pipe, the portion of the pipe downstream of straightener exit SE
does not necessarily move with the vessel so that the total pipe excursion relative to the ramp may be greater than the stern guide e~cursion 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 o~ the vessel pitches downwards by an angle D3, the pipe may be bent aroundthe lower stern guide roller assembly, as shown in Eig. 2C.
In a commercial embodiment of an Apache-type vessel, an angle measuring device measures the pipe angle downstream of the stern guide assembly relative to the ramp assembly 40 and relative to the horizon. One such angle measuring device is shown and described in aforesaid British Application Serial No.
791591~. An apparatus for this purpose is manufactured by Interstate Electronics, Inc.
-` ~ 1. 728 5~
In Fig. 2B, refexence E2 ~epresents th~ m~asured angle o~ e~cursion of the pipe P2 relative ~o the ramp assembly 40 un~er the condition where the ve~sel pitches up by th~ bow at an angle D~ ~t t]liS pitch angle, the effec~ive exi~ an~le G2 becomes R (rarnp angle) plus D2 (pitch angle). ~s noted earlier, it has been found that pipe tension and exit an~le are inversely proportional; 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 efective tension on the pipe is increased and a pi~e profile such as shown in Fig 2B results. Under sufficiently severe conditions of vessel ~itchJ the ~ipe P2 undergoes a relàtively large excursion so that the pipe excursion angle E2 exceeds the gui~e assembly windcw excursion limit angle F2 In such cases, ~he ~ipe undergoes a bendiny mo~ent about the upper stern guide xoller. I khis bending moment exceeds the elastic limit of the pipe, the pipe will under~o plastic bendiny and will thu~ retain a residual curvature due to such plastic bending ~7hen it res~s on the bottom.
When the bow of the vessel pitches downward, e.g., at an angle D3, a pipe profile such as sho~n in Fig. 2C may result. In ~his case, the effective exit angle G3 becomos R
tramp angle) minus D3 (pitch angle); in this case, the effective exit angle is s~aller than the nominal preset ramp angle. In order to ~aintain a ~roper pipe profile, in bow down pikch condition, the tension on the pipe should be increased an amount "C-f~
. .' .. , ~f7C
. . . . .... .....
2~5~ ~i sufficient to compensate f~r the decrease in effective e~it angle. ~lowever, for reasons noted a~ove, it is not Dossi~le to instant~neously 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 suide ~.indo~ limits. Under such conditions, the pipe undergoes a bending mo~ent abou~ the lower stern guide roller; if this bending moment exceeds the elastic limit, the ~ipe undergoes plastic bencling and will retain a residual curvature when it is laid.
The angle measuring device mèasures excursion E2 and E3 to thereby generate an indication of excessive bending o~ the pipe on the ramp. ~easurernent of excursion E2 or E3 is parti-cularly important as an indicator that the pipe is over~tensioned or undertensioned, irrespective of the itching of the vessel.
~en the vessel ;s pitching, excursions E2 and E3 would be expected to be relatively short-lived. Measurement of such short-lived excursions would not provide an accurate indica~
tion of over- or under-tensioning.
A continuous measurement of excursion E2 gxeater than limit ~2,or meclsured excursions E2 greater than F2 ~hich occur a significant percent of the ti~e (e.g., greater than the pitching period of the vessel), even ~hougn such excursions are not continuous, indicate to the operator that the pipe ls being held under excessive tension. The operator can then acljust tl~e reeldynamic braking force to decrease the tension on the pipe until the angle measuring device measures an excursion E2 less than excursion F2, neglectin~ short-lived excursions due to vessel pitching. Correspondingly, when the angle measuring ~i'' ~ 21 -: ..... , ~C
, . . . .
., ~ ... .
2 8 ~ 9 device measures an excursion E3 continuously c3rea-ter th~n excursion limit F3, or greater than F3 a significank percent oE the ti~e (e.g., ~reater than th~ pitching period of the v~ssel), even though not continuous, khese constitute indica-tions that the ~i~e is being held under insuficient 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.
~ Jhen the vessel is pitching, d~e, for example, to sea conditions, measurins excursions E2 and E3 may produce crroneous indications o pi~e tension and may make it difficult, if not practically impossibIe, for the operator 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 varying sea conditions, ~7ith the vessel pitching continu-ously, the operator can maintain a direct reading o~ the actual pipe en~ry angle. The operator is then able to maintain the proper reel dynamic breaking force and provide necessaxy compensation adjustments based on the ac-tual pipe angle relative to the fi~:ed 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 ~ipe traverses across the beam of the vessel as it is unspooled. This produces a turning moment tending to pull the vessel oEf course~ This turning moment increases to a ' 2?
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2 ~ L~
maximu~ at the end of transverse -travel of th~ ~amp assembly, decreases to zero ~he~ the ramp assembly (and pipeline path) is aligned ~ith the vessel centerline, and increases to a m~xi~um in ~he opposite ~irection as the ram2 assen~ly continues moving to the extreme opposite end o~ its transverse travel.
The turning moment can be quite larye co~pared to the fonJard thrust generated by the main propellers. E'or example, in one commercial e~bodi~ent, the ram~ assembly has an athwart-ship ~ovement range of 21.5 feet. The sha~ts of the main pro-pellers are located about 20 feet to either side of the vessel centexline; each produces a ~aximum thrust of 80 Kips. I~hen pipe is being laid under 100 Kivs tension at an exit angle of 30 degrees, the pipe tension induced turning mo~.ent at each extreme end of ramp asse~bly travel is on the order of 930 foot Kips. The op~osing turning moment produced by -the main propeller on ~hat side o~erating at ~aximum thrust is abo~t 1,600 foot l~ips. It will be seen that the pipe tension induced turning mor~ent may well be a significant percentage (58.9~ in the example given here) of the drive induced turning ~.oment.
If the pip tension induced turning ~.oment is not compenc:~lted for, the vessel will be pulled o~f course; this can result in the pipe being laid out o~ the ri~ht of ~7cly~ ~hich is co~merciAlly wlacceptable .
The pipeline induced turning mo~ent must be compensated for in ordex to lay the pipe in a straight line along the right of way. ~ith twin sc.rew vessels, that is, vessels Pro elled by two sets of main drive propellers equally spaced on o~posite sides of the longitudinal centerline o the vessel, it ~ay be , possible to overcome the turning ~oment introduced by the pi~e's pipeline offset relative to the vessel center line by incre~sing thrust on the pro.eller ~ocated on that side of the _ 23 _ ~/
iffC
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1 1 7 ~
vessel and /or decreasing thrust on the opposite side main drive propeller. This has cer~ain inherent disadvantages because the pipeline induced turning moment continually varies as the pipeline shifts laterally across the vessel 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 thxust to com-pensate for the pipeline induced turning moment, especially when the ramp assembly and pipeline are ak an e~treme 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 Eorward 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 re~l shaft axis; the magnitude of the pipeline induced turning moment is a fun,ctionof the tension on the pipeline and the pipeline offset ~L 172859 ~) from the vessel's centerline. ~he vessel ~hrusters yenerate turning moments about the'aforesaid intersec~i.on a~ the vcssel's centerline and reel shaft axis ~hlch react against the pipeline turning mo~ent to maintain the vessel on its pro~er course.
Consideration must also be given to the fact that a turning moment occurs bet~7een the for~qard vessel thrus'ter(s) and the pipeline touchdo~ point on the sea botto~. Therefore, in addition to rotating the vessel abou~ the centerline in~er-section points, the enLire vessel ~ust be rotated abou~c the touch~o~n point to maintain the vessel on and parallel to the right of ~ay. ~his may be accomplished by increasing the thrust genera~ed by the forward thruster(s) relative to tll2 opoositely 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 position of the pipeline relative to the vessel's longitudinal axis, the flistance between the vessel and the touchdot~n ~oint, the pipeline tension and pipe exit angle. In general, the for~7ard thruster ~ill be controlled to generate a thrust component Tl in one lateral direction relative to the vessells longitudinal centerline.
The aft thruster ~ill be controlled to generate a thrust component T2 i~ the opposite lateral direction relative to the vessel's longitudlnal centerline. Advantageously and pre~erab]y, Tl is maintained greater than T2; together, Tl + T2 produce a turning moment which reacts the pipeline induced turning moment.
The thrus~ generated by the forward thruster there~ore 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 touchdo~n point pivot axis. The aft or rear thrus-ter need only react the ' - 25 ~
" ', . . . ' - . ~f~C
~.............. . . - - .. ... . . ..... . ... ... . ... . .. . .. . .
2 8 S ~
pipeline induced turning moment about the vessel axis. The for~axd thruster therefore imparts a relatively greater l~teral ~hrust co~ponent than the rear thruster to o~JerCome the pipeline induced turning momen-ts about the vessel piVQt aY~is and about the touchdown point ~ivot axis to thereby maintain the vessel on course along the ri~ht of way.
The invention may be e~bodied in other specific ~orms ~Ji~hout departing from the spixit or essential charac~
teristics thereo. The embodiment described above is therefore to be considered in all respects as illnstrative and not restrictive, the scope of the invention being indicated by the herea~er appended claims rather than by the ~oregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intellded to be e~brace~ therein.
J
- 26 ~
3~C
.. . .- . - - . .
During the pipelaying operation, the vessel ~oves forwaxd through the water as a function o~ the thurst 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 ~raking force exerted by the reel drive mechanism and/or the amount of thrust generated by the main propellers.
A typical lay rate, i.e., the rate at which pipe is paid 2 ~ ~ 9 out fxom the vessel during a lay operation! would be in the range of 75 - 150 feet per minute. It has been foun~ ~o be preferable to maintain the forwar~ thrust relatively constant and to control pipe tension changes through adjustments to the reel dynamic braking force. Due to the large mass of the xeel and pipe, it is not possible to effect instantaneous chan~es 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 down inthe 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 effects on the pipe, the portion of the pipe downstream of straightener exit SE
does not necessarily move with the vessel so that the total pipe excursion relative to the ramp may be greater than the stern guide e~cursion 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 o~ the vessel pitches downwards by an angle D3, the pipe may be bent aroundthe lower stern guide roller assembly, as shown in Eig. 2C.
In a commercial embodiment of an Apache-type vessel, an angle measuring device measures the pipe angle downstream of the stern guide assembly relative to the ramp assembly 40 and relative to the horizon. One such angle measuring device is shown and described in aforesaid British Application Serial No.
791591~. An apparatus for this purpose is manufactured by Interstate Electronics, Inc.
-` ~ 1. 728 5~
In Fig. 2B, refexence E2 ~epresents th~ m~asured angle o~ e~cursion of the pipe P2 relative ~o the ramp assembly 40 un~er the condition where the ve~sel pitches up by th~ bow at an angle D~ ~t t]liS pitch angle, the effec~ive exi~ an~le G2 becomes R (rarnp angle) plus D2 (pitch angle). ~s noted earlier, it has been found that pipe tension and exit an~le are inversely proportional; 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 efective tension on the pipe is increased and a pi~e profile such as shown in Fig 2B results. Under sufficiently severe conditions of vessel ~itchJ the ~ipe P2 undergoes a relàtively large excursion so that the pipe excursion angle E2 exceeds the gui~e assembly windcw excursion limit angle F2 In such cases, ~he ~ipe undergoes a bendiny mo~ent about the upper stern guide xoller. I khis bending moment exceeds the elastic limit of the pipe, the pipe will under~o plastic bendiny and will thu~ retain a residual curvature due to such plastic bending ~7hen it res~s on the bottom.
When the bow of the vessel pitches downward, e.g., at an angle D3, a pipe profile such as sho~n in Fig. 2C may result. In ~his case, the effective exit angle G3 becomos R
tramp angle) minus D3 (pitch angle); in this case, the effective exit angle is s~aller than the nominal preset ramp angle. In order to ~aintain a ~roper pipe profile, in bow down pikch condition, the tension on the pipe should be increased an amount "C-f~
. .' .. , ~f7C
. . . . .... .....
2~5~ ~i sufficient to compensate f~r the decrease in effective e~it angle. ~lowever, for reasons noted a~ove, it is not Dossi~le to instant~neously 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 suide ~.indo~ limits. Under such conditions, the pipe undergoes a bending mo~ent abou~ the lower stern guide roller; if this bending moment exceeds the elastic limit, the ~ipe undergoes plastic bencling and will retain a residual curvature when it is laid.
The angle measuring device mèasures excursion E2 and E3 to thereby generate an indication of excessive bending o~ the pipe on the ramp. ~easurernent of excursion E2 or E3 is parti-cularly important as an indicator that the pipe is over~tensioned or undertensioned, irrespective of the itching of the vessel.
~en the vessel ;s pitching, excursions E2 and E3 would be expected to be relatively short-lived. Measurement of such short-lived excursions would not provide an accurate indica~
tion of over- or under-tensioning.
A continuous measurement of excursion E2 gxeater than limit ~2,or meclsured excursions E2 greater than F2 ~hich occur a significant percent of the ti~e (e.g., greater than the pitching period of the vessel), even ~hougn such excursions are not continuous, indicate to the operator that the pipe ls being held under excessive tension. The operator can then acljust tl~e reeldynamic braking force to decrease the tension on the pipe until the angle measuring device measures an excursion E2 less than excursion F2, neglectin~ short-lived excursions due to vessel pitching. Correspondingly, when the angle measuring ~i'' ~ 21 -: ..... , ~C
, . . . .
., ~ ... .
2 8 ~ 9 device measures an excursion E3 continuously c3rea-ter th~n excursion limit F3, or greater than F3 a significank percent oE the ti~e (e.g., ~reater than th~ pitching period of the v~ssel), even though not continuous, khese constitute indica-tions that the ~i~e is being held under insuficient 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.
~ Jhen the vessel is pitching, d~e, for example, to sea conditions, measurins excursions E2 and E3 may produce crroneous indications o pi~e tension and may make it difficult, if not practically impossibIe, for the operator 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 varying sea conditions, ~7ith the vessel pitching continu-ously, the operator can maintain a direct reading o~ the actual pipe en~ry angle. The operator is then able to maintain the proper reel dynamic breaking force and provide necessaxy compensation adjustments based on the ac-tual pipe angle relative to the fi~:ed 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 ~ipe traverses across the beam of the vessel as it is unspooled. This produces a turning moment tending to pull the vessel oEf course~ This turning moment increases to a ' 2?
., ~;~7 - .
.. . ... ..
2 ~ L~
maximu~ at the end of transverse -travel of th~ ~amp assembly, decreases to zero ~he~ the ramp assembly (and pipeline path) is aligned ~ith the vessel centerline, and increases to a m~xi~um in ~he opposite ~irection as the ram2 assen~ly continues moving to the extreme opposite end o~ its transverse travel.
The turning moment can be quite larye co~pared to the fonJard thrust generated by the main propellers. E'or example, in one commercial e~bodi~ent, the ram~ assembly has an athwart-ship ~ovement range of 21.5 feet. The sha~ts of the main pro-pellers are located about 20 feet to either side of the vessel centexline; each produces a ~aximum thrust of 80 Kips. I~hen pipe is being laid under 100 Kivs tension at an exit angle of 30 degrees, the pipe tension induced turning mo~.ent at each extreme end of ramp asse~bly travel is on the order of 930 foot Kips. The op~osing turning moment produced by -the main propeller on ~hat side o~erating at ~aximum thrust is abo~t 1,600 foot l~ips. It will be seen that the pipe tension induced turning mor~ent may well be a significant percentage (58.9~ in the example given here) of the drive induced turning ~.oment.
If the pip tension induced turning ~.oment is not compenc:~lted for, the vessel will be pulled o~f course; this can result in the pipe being laid out o~ the ri~ht of ~7cly~ ~hich is co~merciAlly wlacceptable .
The pipeline induced turning mo~ent must be compensated for in ordex to lay the pipe in a straight line along the right of way. ~ith twin sc.rew vessels, that is, vessels Pro elled by two sets of main drive propellers equally spaced on o~posite sides of the longitudinal centerline o the vessel, it ~ay be , possible to overcome the turning ~oment introduced by the pi~e's pipeline offset relative to the vessel center line by incre~sing thrust on the pro.eller ~ocated on that side of the _ 23 _ ~/
iffC
. . . . : .
.. .. ,. ,.. .; ,......... . .
~ ~Ç~; .
1 1 7 ~
vessel and /or decreasing thrust on the opposite side main drive propeller. This has cer~ain inherent disadvantages because the pipeline induced turning moment continually varies as the pipeline shifts laterally across the vessel 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 thxust to com-pensate for the pipeline induced turning moment, especially when the ramp assembly and pipeline are ak an e~treme 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 Eorward 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 re~l shaft axis; the magnitude of the pipeline induced turning moment is a fun,ctionof the tension on the pipeline and the pipeline offset ~L 172859 ~) from the vessel's centerline. ~he vessel ~hrusters yenerate turning moments about the'aforesaid intersec~i.on a~ the vcssel's centerline and reel shaft axis ~hlch react against the pipeline turning mo~ent to maintain the vessel on its pro~er course.
Consideration must also be given to the fact that a turning moment occurs bet~7een the for~qard vessel thrus'ter(s) and the pipeline touchdo~ point on the sea botto~. Therefore, in addition to rotating the vessel abou~ the centerline in~er-section points, the enLire vessel ~ust be rotated abou~c the touch~o~n point to maintain the vessel on and parallel to the right of ~ay. ~his may be accomplished by increasing the thrust genera~ed by the forward thruster(s) relative to tll2 opoositely 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 position of the pipeline relative to the vessel's longitudinal axis, the flistance between the vessel and the touchdot~n ~oint, the pipeline tension and pipe exit angle. In general, the for~7ard thruster ~ill be controlled to generate a thrust component Tl in one lateral direction relative to the vessells longitudinal centerline.
The aft thruster ~ill be controlled to generate a thrust component T2 i~ the opposite lateral direction relative to the vessel's longitudlnal centerline. Advantageously and pre~erab]y, Tl is maintained greater than T2; together, Tl + T2 produce a turning moment which reacts the pipeline induced turning moment.
The thrus~ generated by the forward thruster there~ore 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 touchdo~n point pivot axis. The aft or rear thrus-ter need only react the ' - 25 ~
" ', . . . ' - . ~f~C
~.............. . . - - .. ... . . ..... . ... ... . ... . .. . .. . .
2 8 S ~
pipeline induced turning moment about the vessel axis. The for~axd thruster therefore imparts a relatively greater l~teral ~hrust co~ponent than the rear thruster to o~JerCome the pipeline induced turning momen-ts about the vessel piVQt aY~is and about the touchdown point ~ivot axis to thereby maintain the vessel on course along the ri~ht of way.
The invention may be e~bodied in other specific ~orms ~Ji~hout departing from the spixit or essential charac~
teristics thereo. The embodiment described above is therefore to be considered in all respects as illnstrative and not restrictive, the scope of the invention being indicated by the herea~er appended claims rather than by the ~oregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intellded to be e~brace~ therein.
J
- 26 ~
3~C
.. . .- . - - . .
Claims (3)
1. 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 spool-ing 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 main-taining 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:
providing a reel mounted on the self-propelled vessel with pipe spooled thereon;
unreeling the pipe from the reel;
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 opposi-tion 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 in the horizontal plane.
providing a reel mounted on the self-propelled vessel with pipe spooled thereon;
unreeling the pipe from the reel;
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 opposi-tion 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 in the horizontal plane.
2. A method according to claim 1, further comprising:
generating said opposing reactive force by increasing the thrust generated by the main vessel drive on the same transverse side of the vessel as the pipe handling means and/or decreasing the thrust generated by the main vessel drive on the oppo-site 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 transverse side of the vessel as the pipe handling means and/or decreasing the thrust generated by the main vessel drive on the oppo-site transverse side of the vessel from the pipe handling means.
3. A method according to claim 1 or 2, further comprising:
generating a thrust T1 in one substantially lateral direction with said forward thruster and generat-ing a thrust T2 in a substantially opposite lateral direction with said aft thruster, where T1 is greater than T2 and where T1 plus T2 pro-duce 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 generat-ing a thrust T2 in a substantially opposite lateral direction with said aft thruster, where T1 is greater than T2 and where T1 plus T2 pro-duce a turning moment which at least substantially reacts the pipeline induced turning moment to thereby correct for said course deviations.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000411523A CA1172859A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
Applications Claiming Priority (2)
| 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 |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000358052A Division CA1147566A (en) | 1980-08-12 | 1980-08-12 | Method of laying offshore pipeline from a reel carrying vessel |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1172859A true CA1172859A (en) | 1984-08-21 |
Family
ID=25669127
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000411523A Expired CA1172859A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1172859A (en) |
-
1982
- 1982-09-15 CA CA000411523A patent/CA1172859A/en not_active Expired
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