CA1156059A - Method of laying offshore pipeline from a reel carrying vessel - Google Patents
Method of laying offshore pipeline from a reel carrying vesselInfo
- Publication number
- CA1156059A CA1156059A CA000411522A CA411522A CA1156059A CA 1156059 A CA1156059 A CA 1156059A CA 000411522 A CA000411522 A CA 000411522A CA 411522 A CA411522 A CA 411522A CA 1156059 A CA1156059 A CA 1156059A
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- pipe
- vessel
- pipeline
- angle
- excursion
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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 excursion; 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 excursion; 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
1 155(~
Back~round of the Invention -This invention relates to techniques and ~ethods 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, lssued to Charles N. Springett, Dan Abramovich, Stanley T. Uyeda and ~. John Radu; U.S. Patent ~,269,540 issued t~ Stanley T. Uyecla, E. John Radu, William J. Talho~, Jr. and Narman ~'~ldrnan.
The pr~sent appli~tion ~and the inventive sub~e~t matter de~cribed an~ ~laimed hereln) and the above lis~ed U.S. ~atents are all owned by Santa Fe In~ernational ~, ~ 1 5~0~
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 Dani~l E. Sugasti, Larry R. Russell, and E'red W. Schae~be;
U.S. Patent No. 3,641,778, is9ued February 15, 19/2 to Robert G. Gibson;
V.S. Patent No. 3,6~0,342, i~sued August 1, 1972 to James D. Mott and Richard B. Feazle;
u O ~i g U.S. Patent l~lo. 3,712,100 issuea January 23, 1973 to ~oe ~I. Ke~ and Larry ~. Russelli an~
U.S. Patent 3,9~,2,~02, issued Septenb~3r 28, 1976 to Alexander Craig Lang and Peter Alan Lunde.
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-? ~) 1 1 56~59 Summary oE -the Invention The present invention is concerned wi~h ~ethods ~nd techniques related to the control of pipelaying operations from a self-proPellecl reel ~ipelaying vessel. The methods are concerned with 1) controlling pipeline geor.letry as a function of pipe entry angle into the wate.r an~ tension on the pipelinei
Back~round of the Invention -This invention relates to techniques and ~ethods 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, lssued to Charles N. Springett, Dan Abramovich, Stanley T. Uyeda and ~. John Radu; U.S. Patent ~,269,540 issued t~ Stanley T. Uyecla, E. John Radu, William J. Talho~, Jr. and Narman ~'~ldrnan.
The pr~sent appli~tion ~and the inventive sub~e~t matter de~cribed an~ ~laimed hereln) and the above lis~ed U.S. ~atents are all owned by Santa Fe In~ernational ~, ~ 1 5~0~
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 Dani~l E. Sugasti, Larry R. Russell, and E'red W. Schae~be;
U.S. Patent No. 3,641,778, is9ued February 15, 19/2 to Robert G. Gibson;
V.S. Patent No. 3,6~0,342, i~sued August 1, 1972 to James D. Mott and Richard B. Feazle;
u O ~i g U.S. Patent l~lo. 3,712,100 issuea January 23, 1973 to ~oe ~I. Ke~ and Larry ~. Russelli an~
U.S. Patent 3,9~,2,~02, issued Septenb~3r 28, 1976 to Alexander Craig Lang and Peter Alan Lunde.
.
_ ~ _ .. .. ., . . . , .... ... .. ~ . . ~ . .. . . . . .. ... . .
-? ~) 1 1 56~59 Summary oE -the Invention The present invention is concerned wi~h ~ethods ~nd techniques related to the control of pipelaying operations from a self-proPellecl reel ~ipelaying vessel. The methods are concerned with 1) controlling pipeline geor.letry as a function of pipe entry angle into the wate.r an~ tension on the pipelinei
2) moni~oring the excursion of the pipeline outside certain de~ined limits and controlling the pipeline ~eometry based on such measured excursions; and 3) compenSating for pipeline induced turning moments which would otherwise tend to draw the pipelaying vessel oPf course and off the predetermined pi?eline right of ~7ay.
1 The ~resent invention is primarily applicable to a self-propelled reel pipe laying vessel, having a reel for spooling relatively inLlexible pipe thereon, pipe wor~ing and handling mPans for straightening the pipe as it iS unspooled, pipe guide ~eans for guiding the straightened pipe into the ~ater at a presettable, adjustable exit angle, means for maintaining the pipe under a predeterminecl adjustable tension, main vessel drive means, pre~erably including twin screw~ located on opposite sides o~ the vessel longitudinal centerline, and ~orward and aft thruster means located forward and aft, respec~i.vely, of the longitudinal center of the vessel.
During a p:i~elaying operation, the pipe handling q~ui~nt and ~ipe ~uide means txansl~tes acros~ ~he beam o~
~hQ ves~el as lt .~0110~3 ~or leads) the pipc wrap b~ing unsyooled.
Inthe proce~s o~ tran~ ing the pipe guide means ~cross ~he beam o~
the v~el, ~U~ning momen~s ~ln ~he ~orlzQnt~l plane) are impar~cd .
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to tho vcssel by the tensioll in tlle pipeline. In one aspect, therefore, thc inventioll col~rises a method of compensatin~
for these pipeline tension induced turniny moments by generating a reactive force in opposition to the ~i~eline tension induced turnin~ moment to thereby correct for deviations in the ve~sel's course and to maintain t'ne vessel on course alon~ the desired right of way.
A further aspect o~ the method of this invention comprises monitoring the an~le of entry of the pipe into the water rela~ive to a nominal horizontal plane re~resenting the water surface; ~onitorin~ the angle of excursion which the pipe makés relative to a nominal ~ipe centerline substantially parallel to the nominal preset angle of en~ry into the water;
and adjusting the nominal pipeline tension if the monitored excursion angle remains outside a predetermined permissible excursion ran~e ~or at least a si~nificant time period, for example, greater than the ~itching period o the vessel.
A still further aspect of the method of this invention comprises setting the pipe guide means to establish a desired pipe exit an~le at which the pipeline substantially enters its catenary con~iguration before exiting the vessel and pipe yuide means; and settin~ the tensioning means to hold the pipe under a predetermined nominal tension in conjunction with the pipe exit angle, to establish a minimum radius o~ curvature o~ the pipe in the sa~ bend reglon which is great~r than the rlinimum r~dius to whic~ th~t pipe may be ben~ ~7ithou-t exceedin~
lt~ el~ ici~y limi-ts as it is uns~ool~d ana pald out ~om tlle ve~sel.
1 ~5~059 Brlef cscri~tion of the Dr~ing ~ Figure 1 is a diaqra~matic sketch Q~ a self-prooelled reel pipe laying vessel sho~ing the ap~roximate pipe profile bet~een the vessel and the sea bot-~om.
Fi~ures 2~-C are diagrammatic s~etches o the vessel dec~, ramp assembly and pipe, in several conditions o-f pitching due to sea condi~ions.
Figure 3 is a diagrar~tic ~lan view o the vessel showing course-correcting force relationships.
Description of Preferred ~bodiments ~nde~later pipelines for carrying oil or gas must meet certain requirements and li~its set by the customer ~pipeline o~ner) and/or governmental or other regulatory bodies.
It is of prir.lary i~portance that the pipe, as it is b~ing laid and as it lays on the sea bottom, be subjected to minimal residual stress, strain, tension, etc. In terms of pi~e laid by the reel method, this means that the pipe as it la~s on the sea bottom must be straight and have substantially no residual curvatux~ clue to spooling or laying. It is al~o important tha~
the pipeline be laid close to the nominal right of way. The "as laid" restrictions are developed as a ~unction of a nu~ber of parameters developed by the pipeline designer, including the ~ype o~ s~a bed cn Which ~h~ pipe x~sts, ~he sizq and ~rade v o~ pipe ~o he Usqcl, ~h~ -type, amoun~, and ~low ra~q~ o~ ~luid ~o b~ ~arriad ~y ~he plp~llne~ and pxedicted li~q ~p3n of ~he plpelln~ hqr paramek~rs xela~in~ ~/ or basq~ ~n, the ~e~me~ry ~hap~) o~ the pipeline durlncJ ~he ~ in~
~p~xa~iorl ~x~ cl~vel~p~d b~ th~ pip~ layin~ entJinecx~
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1 1S605~
Additionally~ a reel pipelaying vessel and the pipe being lald are su~jected to a number of hydros~atic 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 or 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 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 ~et. Th~ distance which a ~tove pipln~ bar~e can moYe along the right o~ way on a slngle anchor ~et pattRrn decrea^~es as water dep~h in~r~ase~. It is apparent that the limited ~orward movement permitted by thi~ anchor setting technique is not at all su:Ltable ~or economiaal reel pipe laying opera~ions.
! a 1 1 5~059 Althou~h ~ow~ r~el pipelayin~ baryes have been found to be cluitc adequate for -thc relatively calm waters o khe Gulf of rlexico ofs}lore of the United States coastline, the~ have cextain inherent limitations which make -then unsuitable for use in relativel~ rouyh waters, such as are found in the Mor~h Sea or o~ the coast of South ~merica or Australia. One of the principal btli]~-in limita-tions o~ a towed bar~e system. resides in the towing connection itself. Unli]~e a self-propelled ship, in ~hich the rnotive source i5 effectively connected directly and rigidly to the pipeline (through the~reel), the connection between the towing vessel (motive source) and the towed bar~e (e~ectively includlng the pipeline end) is a flexible one ~hich 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 rel~tive moverrent between the tug and bar~e. This ~ay cause the pipeline tension ~o exhibit sudden increases and/or decreases in ~nagnitude t~hich can neither be predicted norco~trolled e~ectively by the barge operator(s).
A self-propelled reel type pipelaying ship requires neither anchors nor tugs as the mo~ive source. There~ore, compared to stove-piping type bar~es as described above, a sel~-propelled reel pipelaying ship is able to move con-tinuously down the ri~ht of way, stopping only when necessary, for e~ample, to install anodes as required by the customer and/or ~o perform othQr oper~tions on the pipe, such as coating r~pair, etc.
Cornparecl to ~o~d re~l barges, th~ selE-propelled reel ~hlp~
h~ a significant ad~antage in that the mo~ive souxce o~ the reel ship c~n, for prac~:Lcal purpos~s, be consider~d ~n b~ ~ixed with ~he x~el and pipelin~ end, -khereb~ el:lmina~inq ~la~ive movem~n~s ~hcr.e~ek~eell ~lue -t~ ~Jeather ~ela-tccl ~ac~c~rs, as rlo~d above.
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Co~ercial and practical limitations effectively restrict thc operatin~ capability of a to~ed reel barye. One of the principal requirements in l~ying pipelines of~shore from a sur~ace vessel is tha~ 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 predeterminecl 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 bot~om is called t~?e Touchdown Point (TDP).
It i5 impor~ant that the radius o~ the sag bend curve be kept above the mini~um permissible radius to which the pipe may be bent ~ithout exceeding elasticity limits i~ 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 bet~?een the departure or exit angle (also sometimes called pipe entry angle into the water~ and the required tension can be e~pressed as an essentially linear logari~hmic relation where ~he pipe pro~ile i5 catenary~sh~ped in i~s unsupported length betw~en the vessel and th~? ~,e~ ~?ottom, subs~antially as represented in Fig. l; i.e., ~?or ~ givqn ~i~e and grad~ of pipe and a given l~y d~pth along kh~ xigh~ o~ ~ay, the tension required to hold ~e sag bend r~diu~ above ~he allo~able minimum decxeases as tlie departure ~ngle o$ the pipe into the wat~r increases. E~or ex~mple, it is neces~a~y to hold about 250,000 lbs. o~ tension ~250 ~ip5, ~Jhere "~ipS" eC~ S ~hC?~ anClS O~ pOUllC S) on a ~ e hav;incJ an .
1 ~ 5 ~ O ~(J
outside diameter of 10 3/4" and 3/4" wall thickness laid in a water depth of 500 ~eet, if the pipe exit angle i5 set at abou~ 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 conclitions 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 o 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 "t" shape (with t~Jo 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.
Re~erring to Figure l, a feature of "Apache-type" special reel pipela~ing ships is the adjustable pipe carrying ramp assembly 40 pivotably mounted (generally at the stern) to the deck of 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 la*eral thxusters 1~2. ~Throughout this dis-closure, re~qrence is made ka the main propel:Lers as providin~
~he re~uisite ~orward thxu~t7 it i5 apparent,however, tha-t other suitable drive me~ns oould be provided to ~enerate the neces~
l 1 5~051.3 sary for~/carcl-thru~;t ancl the reference to "propellers" throughout t:hi~; ~isclosure is in~ended to encompass other such suitable drive means, exce~t where othen~ise specificall~ noted~) Special pipe hand:Linc3 equipment, which may include, for example, the adjustable radius control me~er, adjustable stra.i~lltener tracks, tensioner tracks, pipe claMping assemblies, ~uide roller assemblies, and pipe angle measuring assembly, is advantageously mounted to the ramp assembly 40.
~ n adjustable ramp assemhly o~ this type has not heretofore been incorporated into an~ known commercicll offshore reel pipelayin~ vessel, specifically including -the supply boat portable reel system used o~f the coast of ~ustralia, the tt~o reel pipelaying towed barses ot~ned and used by Santa Fe and/or Santa Fe's predecessors-in-interest since about 1961 and tt~o com2etitive reel pi~ela~ing barges, one used for a short time in 1972 or 1973 and the other currently in use in the Gulf o~
I5exico 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 ~ater at any desired an~le ~ithin its operatin~ ran~e o~ bett;een ahout 15~ and 65~, pre~erably 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 maintalned during a pip~ lay operation; the ramp assem~ly ~uides the pipe as i~ enters th~ wa~er at the preset exi~ an~:Le. As noted aboYe, all prior kno~n commercial rqel pipelayin~ h~r~qs ha~q opqra~ed at a fixed, non-variable exit an~l~ o~ b~ qen abou~
6 and 12 ~ The adjus~able cxi~ an~le ~ea~uxe o~ ~he ~pache-type .
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1 1 5 ~ {~
v~ssel ena}~les it ~o handle a ~lider range of pipe sizes in a (~reater ranse of ~ater cl-pth~ than was herctofore ~o~sible ~ith fixed lo~ e~it anylc rcel pipelaying bar~es.
One of the advantages of an Apache-type adjustable ral~p ass~mbly ~or settin~ th~ pipe exit angle is the virtual elimination of the ovcrbend region (i e., the bend re~ion occurring as ~he pipe translates downwardly from the relatively hori-zontal plane of the bar~e ~oward the seabed in the relatively vertical planeof the catenar~. Advantageously an~ preferably, the ramp angle and tension are set so that downstream o~ the straightener/tellsioner apparatus, the pipe will be unsupported7 thus, pipe exiting the straightener mechanism and traveling along the ramp assembly will already ~e in its nominal caténary configuration before and as it en~ers the ~ater Preferably, as the pipe 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 resiclual bending moments.
By initiall~ setting the ra~p angle and nominal pipeline tension to virtually eliminate the overbend as a factor in ~eter~inincJ and controlling the ~inal residual pipeline characteristics, the saJ bend ~i.e., the bend occurring in the translation o~ the pipe ~xom thq vertical to the hori-zonta~ plan~ on the sea bo~kom) becomes a critlcal fackor in kh~ conkrol o~ the pi~e as it i5 lald. ~he ~g bend is con-trolled, at l~a~t in ~ark, as ~ function o~ the kellslon main~
tained on ~h~ pipe by ~he ~unctional element~ of khc pipela~in~
vessel, including ~he xeel, straicgh~ener ftensioner e~ ents vessel clriv~ assembly, etc. Cont~o.lled tension .is impart~.
6 ~ ~ 9 to t-h~ pipe by (l) the r~el throughthe reel drive mechanism operating as .a dynami.c brake, (2) th~ main vessel drive thrust acting through the vessel main propellers and/or the lateral thruster assel~lies, and (3) the -tensioner assernbly, which may or may not be used, through a re~ulated tensioning force estabiished at the beginning 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 ~referably determined on khe basis of informati.on supplied by th~ pipeline designer.
Such informa'cion from tlle pipeline designer (or customer - pipeline owner) includes (l) the size o~ the pipe, including internal pipe diam~ter and ~Jall thic~ness, (2) the type or grade of pi~e, including such in~ormation as the pipe material and minimu~ yield strength, (3) maximum allowable stress, strain and residual tension, and (4~ water depth along the pipeline - righ~ of way. An optimum nominal tension ~nd lay angle can be determined from these para~.eters.
One of the criteria which has been developed ~or 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 an-d thesea bottom shouldnot be greater than about 85~ of the minimum yield strength of the pipe.
I~ i.s also de~irable and preferable to minimize the kension imparted ko thq pipe by the vessel while maintaining operating conditlons such that the maxi.~um allo~7~ble stress linll-t and ~he maximu~ allowable residual tension in th~ ~ipeline ~re not exceed~d. llh.is may be accomplished by setting -khe r~mp assembly ~n~ ncl thu~ the pipe en.txy an~le into the s~ater) in conjunction w.~th nominal pipe tension such ~h~ ~he ~i~htest sag bend radius will be ~cl1ieve~ ~7it}1011~ excec~ing thc .~hove ne~cd ~sl~rer-:s ~ncl x~sldu~l te/)siGn lir1it.
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115~5~
The ramp assembly angle (and thus the pipe entry angle into the water) is set a~ the becJinning of the pipelayin~ opera-tion and is normally not change~ 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 is 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 set~ing 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 (preferably 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 "~lexible"
towing connection between a reel barge and its tu~ will not be ~dequa~e to maintain ~he ne~e~sary continuous tension on ~he plpelin~ a~ i~ ls being lald. ~he tu~ move~ indqpenden~ly o~
th~ harge du~ to wave action. ~his means that th~ motive source which provides the ~on~axd thrust necessary to malntain tension q ~n the pip~llne is susceptible ~o uncontrolled variations rqla~iv~ ~o the barge and thus to the pipe. Limite~ excursions .
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of this -type may be acceptable for some sizes of pipe and some sea conditions. ~lowever, 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 motive 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 directly. There are, however, several ways to measure the tension indirectly. One such way is to meaQure 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 ~he pipeline.
~hi~ can be done by measurlng the main propeller sha~t torque or by measurin~ the force on a thrust bearing against which the propellex sha~t ~cts.
A second method i5 to measure the drive motor ~orce acting on the reel. Neglecting the aomponents of tension pro duced primarily by the strai~htener assembly ~and tensioner~
when used), the force exerted by the reel drive motors is directly proportional to the ten~ion in the pipe; thus, an ; 0 5 9 increase o~ decreasc in the (Irlve .~otor force produccs a corres~ondilg increase or decrease in the pipeline terlsion.
The reel motor drive force ma~ bc r.~easured by, e.g., load cells between the ~otor/reel mechan.ical connection.
A third prac~ical Jay to r~leasure pipeline tension is based on ~.easurement of the exit an~le of the pipe from the vessel. It is advantacJeous and preferable that the piPe angle be measured ~Jith respect both to the hori70n and to the ramp angle; the latter measurc~.ent is particularly helpful where the pi~e passes throu~h an exit window defined by a stern ~uide xoll~r assenbly, such as is used on Apache-type vessels.
Figures 2A-C are diagral~atic representations of the pipelaying vessel 10, rar~p asser~bly 40 (set at a nor~.inal lay an~gle of about 30 degrees), the stern gui~e roller assembly 54 defining the exit windo~7, and the pipe P. Fisure 2A shows the relationship between the ramp assembly and the pipe when the vessel is substantially ~lat in the water so that the entry an~le Al of the pipe into the water (relative to a nominal horizontal plane or axis, such as the horizon) is sub~antially the same as the prede~ermined ramP angle ~; Figure 2B shows the same relationship when the vessel is pitched bo~ up at an angle D2 and the pipe P2 enters the wa~er at an angle A2; and Fi~ure 2C ~hows the same relationship when the vessel is pitched bow down at an angle ~3 and pipe P3 ~nters the wa~er a-~ ~n a~ 3~ ~'he exi.t poin~ o~ the ~ipe ~ro~ ~he straigh~ener/
-~ensloner ~se~bly is desi~nated by re~erence SE. q'he pipe is ~ssQn~lally ln ~ d relatio;n to ~h~ raMp a~çmhly ~nd the vessel a~ point S~. PreEexahly and advant~geousl~, su~Eicient and adequ~lte tension i~ ma.in~ained on ~he pipe ~ durincJ -~}le .
? ~ o ~ ~ ~
laying oper~tion so that the pipe travels in a path substantially paral1~1 to the ramp an~l through the guide roller assembly 54 sub-stantially unsupported between straightener exit SE and the touch-down point TDP on the sea bottom. Also a~vantageously and prefer-ably control oE pipelaying operation is maintained so that angles ~l' A2, A3 will be essentially equal.
The stexn guide roller assemhly provides a pipe excursion ~indow 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 ~ommercial embodiment, the distance between straight-ener exit SE and stern guide roller asserrbly 54 is approximately 45 feet; the distanc~ between the upper and lower stern guide rollers is approximately four ~eet. This permit~ 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 with-out being subject to bending moments by the stern guide rollers.
Referring to Figs. 2B and 2C, angles F2 and F3, respectively, re-present the excursion above and below the nominal centerline of the pipeline P when it is tensioned to he parallel to the plane o~ the ramp assembly 40.
During the pipelaying operation, the vessel moves forward through the water as a function o~ the thrust generated by the main vessel drive, reacting against pipe tensioning forces produced by the pipe handling equipment, includlng reel dyn~ic ~r~king ~orces, s~xaightener, tensianer, etc. Changes in or ~od1~lcations to the rate o~ ~orward maklon o~ the vessel, and thu~ the ra~e a~ which pipe is unspooled ~rom the reel ~0 and p~id out into ~le wa~er, may be controlled by adjus~ing the dynamic brakln~ force exert~d by the re~l drive mechanism and/or ~he amount o~ ~hrust generated by the main propellers.
A ~ypical lay rate, i.e., the rate at which ' 1 1 5~()59 yipe is p~iQ out fro~ the vessel during a lay opcration, would be in the rang~ of 75 - 150 fe~t per minute. It has been found to be preferable to r~aintain the Eorwarcl tllrus-t rel~tively constant a~d to control pipe tension chclnges through adjustn!ents to the reel dynamic braking force. Due to the large mass of the reel and pipe, it is not possible to effect instantaneous changes in the pay out rate.
As the vessel pitches during a laying o~eration, the stern, with the rar~p and other related pipe handling e~uipment, moves up and down in the wate,r. The pipe, paid out from 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 unden7ater suspended length and on-bottom ~riction. Due to such inertial effects on the pipe, the portion of tlle pipe downstream of straiyh-tener e~it SE
does not nece~sarily ~ove ~ith the ves~ei so that the total piwe oxcursion relative to the ra~p may be greater than the stern guide excursion windo~ limits. Under conc~itions where the bo~ pitches up by an angle D2, the pipe ~a~ be bent around the upper stern guide roller, as shown in Fig. 2B. S:imilarly, when the bot~ of the ves~el pitches do~mwards by an angle D3, the pipe ~a~ be bent aroun~ the lo~er stern guide rolLer assembly, as sho~m in Fig. 2C.
In ~ commerci~l e~odiment of an Apache-t~pe vesseL, ~n an~lo me~suring device measures the pipe an~le downstream o~
~hq ~ern ~uide assembly relative to the ramp assembly 40 and r~lative to the horizon. Onc Su~h anqle moa~urin~ device is ~ho~m an~ d~scribecl in ~E~res~id ~ri.~ish P.pplic~ion Sexial ~o.
~91591~ ~n apparatus ~or this purpose is m~nu~ac~ured by IntC X~ t a ~ Eloctror-lics, :Inc.
.
,' , .
... ..... .. . . . . .. .
r. . . , . _ . .
l 15~059 In Fis. 2~, rcference E2 represents the measured an~Jle of c~cursion o~ the pipe P2 xelative to the ram~ assembly ~0 under the condition wher~ the vessel pitches ~p by thc bow at an ansle D~ ~ this pitcll ancJle, the effective exit an~le G2 become~ R (rarnp anyle) plus D2 (pi-tch angle). ~s noted earlier, it has been found that pipe tcnsion and exit an~le are invers~ly proportional; therefore, as t~?e e~fective exit angle G2 increases, the tension applied to the pipeline should be decreased in order to maintain the pipe ~rofile within acceptable limits. ~owever, since, due to reel and pipe .inertia ~nd other factors, the tcnsion applied to the pipe cannot be acljusted to directly follo~l the pitching of the , vessel, the effective tension on the pipe is increased and a pi~e pro~ile such as sho~n in Fig 2B results. Under su~ficiently 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 ~i~e undergoes a bending moment about the upper stern guide roller. If this bending moment exceeds the elastic limit of the pipe, the pi~e will under~o plastic bending and ~ill thus ratain a resldual curvature due to such plastic bending ~7hen it rests on the bottom.
11hen the bow o~ the vessel pitches down~ard, e.g., at an angle D3, a pipe pro~ile such as shown in Fi~. 2C may re~ult~ In this case, -the e~fec~ive ~xit angle G3 becornqs ~
~ramp ~ngle) minu~ D3 (pi~ch an~le); in ~his ca~e, the ~fqc~ive exi~ an~le is 3~aller ~han ~he nominal pr~se-k rarnp an~lQ~ In orde~ ~ r~laintaln a proper pipe pxo~ , in bow d~wn pitch condi~ion, ~he ~ension ~n th~ pip~ shou].d be incr~a~ed an amount .: . - .. ; ....
i 1 5~ ~S~J
"
sufEicient to co~pensate for the decrease in cffective c~it anc~le. ~lo~cver, for reasons noted above, it is not ~ossib]c to instant~neously change the -tension impar-ted to the pipe by the vcssel, and particularly by the reel. There~ore, -the pipe under~oes ~n excursion E3 whlch ~ay be greater than the excurs;on F3 ~err~itted by the stern guide ~.indo~ limits. Under such conditions, the pipe undercJoe~ a bending mor~ent about the lower stexn guide roller; if this bending moment exceeds the elastic limit, the ~ipe under~oes plastic bending and will retain a residual curvature ~Ihen it is laid.
The angle measuring device measures excursion E2 and E3 to thereby generate an in~ication of excessive bending of the pipe OII the ramp. ~.easurernent of excursion E2 or ~3 is parti-cularly important as an indicator that the pipe is over-tensioned or undertensio~ed, irrespec-tive o~ the ~itching of the vessel.
~en the vessel is pitching, excursions E2 and E3 would be expecLed to be relatively short-livea. Measurement of such short-lived excursions t;~ould not provide an accurate indica-tion of over- or under-tensioning.
A can-kinuous measurement of excursic~n E2 greater than limit F2~ormeasured excursions E2 greater than F2 ~hich occur a significant percent of the time ~e.g., greater than the pitching period o~ the vessel), even though such excursions ~re not continuous, indicate to the operator that the pipe is bqinc3 hqld under excqssive tension. ~he opexa~or can then acljusk tll~ reeldynamicbraking ~oxce to decrease th~ tension on the pipe ~til ~h~ an~le measuring device measures an excursion E2 l~ss than eXCursion ~, neglectirl~ short-livecl excursiorls du~
to v~ssel plkchiny. Correspon~irlgly, when the an~le measuring - 21 ~
,/ 1 1 5 ~ ~ 5 9 .1~
device ~easures an e~cursion E3 continuously greater than excursion lir~nit F3, or yrcater than F3 a si~nificant percent o~ the tire (e.g., ~reater than th~ pitchiny ~eriod of the vessel)/ even thouyh not continuous, these constitute indica-tions that the pipe is being held under însuE~icient 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 exc~rsions due to vessel pitching~
~ 1hen the vessel is pitching, aue, for exa~ple, to sea conditions, measurin~ excursions E2 and E3 may produce erroneous indications of pipe tension and may make it dificult, if not ~ractically i~possibIe, for the operator to maintain proper tension ontthe pipe. Lherefore, 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 thP actual pipe entry angle into the water so that under varying sea conditions, with the vessel pitching continu-ously, the operator can main-tain a direct reading of the actual pipe entry angle. The operator is then able to rnaintain the proper reel dynamic breaking ~orce and provide necessary compensation adjustments basea on the ac~ual pipe angle relative to the fi~:ed hori20n, as distinyuished From an~les measured rel~tive to the moving and pitchin~ vessel.
- The pipe laying operation is also ~fectqd by the fac~ tha~ the pipe tr~Vqrses across the beam o~ the vessel as i~ is unspoole~. This produGes a turniny momen~ tending to pull the vessel o-E~ ~ourse. This turnin~ moment increases to a ?,?
.. . ...
., ... ~..
// ~
ll56o~cJ
m~xi.rnu~ at the en~ oE t:r~nsverse travel of the ramp assembly, CreaSe5 to 7,ero ~hen the r~rnp a~isembly (and ~ipeline p~th) is aligned t1itl1 the vessel centerline, ~nd increases to a ~aximum in tlle op~osite direction as the ramp assem~ly continues movin~
-to the extrcme opposite end of its transverse -travel.
The turning moment can be qui te larye co~pa~ed to the fon1ard thrust generated by the main propellers. For example, in one commercial embodi}~ent, the ramD 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 ~laximum thrust of 80 Kips. IYhen pipe is being laid under 100 Ki~s tension at an exit angle of 30 de~rees, the pipe tension induced turning mo~.ent at each extreme end of ram~ assembly travel is on the order of 930 foot Kips. The o~osing turninq moment produced by the main pro~eller on ~hat side o~erating at maximum thrust is about 1,600 foot iCips. It will be seen tllat the pipe tension induced turning moment may well be a significant percentage (58.9~ in the example given here) o~ the drive induced turniny ~oment If the pipe tension induced turning ~.oment is not compensc~ted for, the vessel will be pulled off course; this can result in the pipe bein~ laid out o~ the riyht of ~ay, ~hich-is c~mmercially accepkable.
The pipeline induced tUrninCJ mo~ent must be compensated - ~or .in order to lay the pipe in ~ s~raicJht l.ine alon~ the right o~ way. ~Yith twin scre~ vessels, that is, vcssels pro~elled by ~wo sets oE ~in drive p~o~ellers equa:Lly sp~ced ~n o~posite si~e~ o~ t.he lonclitudinal centerline of the vess~l~ ik ~ay be pussibl~ to overcome the ~u~niny ~o~en~ in~roduced by -khe pipe's pipeline ol.~set rela~lve ~o ~he ve~el center line by .i.ncre~sincJ thrust on the propeller :located on that side of th~
. : . ...
- , . . .~.;, I 1 S605~
vessel and ~or decreasing thrust on the opposite side main drive propeller. This has certain inherent disadvantages because the pipeline induced turniny 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 re~uires that the thrust of the drive propellers be varried accordingly, while at the same time taklng 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 turning 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 o~ such thrusters are shown in the a~oresaid prior rela~ed Santa Fe inventions. Also refer-rin~ to Fig. 3 hereo~, an aft thruster tunnel 120 houses the A~`t thruster 122; a ~orward thruster tunnel 12~ houses the forward thrustex 126.
The thruster~ 122 and 126 can be operated either manuall~ or automatically in conjunction with, e.g., a computer ope~ated guidance system, to generate turnlng 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 axis, the magnitude of the pipeline induced turning moment is a functionof the tension vn the pipeline and the pipelin~ offset / ~56~5`9 ~
from the vcssel's centerline. TJle vessel thrusters gener~te turnincJ moments about the aEoresaid intersection of the vcssel's centcrline and reel shaft axis which react against the ~ipeline turnin~ mo~ent to maintain the vessel on its pro~ex course.
Consideration must also be given to the fact that aturning ~oment occurs be-tween the forward vessel thrus'ter(s) and the pipeline touchdown point on the sea bottom. Therefore, in addition to rotating the vesse:L abou~ the centerline inter-sec-tion pointY, the en~ire vessel ~ust be rotatecl about the touchdo~n poin-t to maintain the vessel on and parallel to the right o~ ~ay. This mcl~ be accom~lished by increasing the thrust generated by the forward thrus-ter(s) relative to -lle op~ositely reaoti}lg force generated by the aft thruster(s).
~ The a~ount of thrust required varies as a f~mction of a num~er of factors, including the lateral ~osition of the pipeline relative to the vessel's longitudinal axis, the ~istance between the vessel and the touchdo~n ~oint, the ~ipeline 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' 5 lon~itudinal c~nterline. ~dvantageously and pxeferably, Tl ls m~in~ained greater than T2; ~o~ether, q'l ~ 'r2 produce a kurnin~ mom@nt which reaats the pipeline in~uced -~uxning momenk.
The thrus~ ~enerated by the ~orward thrustex there~ore comprise~
thq ~dditive componen~s of the thru~ necessary to xeact the ~ipeline induced turnin~ mom@nt about -the ves~el axis and the pipeline induced ~urni.n~ momen~ about the touchdo~n point pivot axis. ~he a~t or rear thrus~er nsed onl~ reack the .
-:
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r~,~ .. : . . . .
1 5 ~ 0 5 ~i h ~ ' pipeline indu~ed turning moment about the vessel axis. ~he for~Jaxd thrus~r therefore imparts a r~latively yreater lateral thrust component than the rear thruster to o~ercome the pipeline induced turni~g mo~ents about the vessel pivot axis and about the touchdotJn poin-t pivot axis to thereby maintain the vessel on course along the right of way.
The invention may be erbodied in other specific ~orms ~ithout de2arting fror~ the spirit or essential charac-teristics thereof. The embodi~ent described above is therefore to be considerecl in all respects as illustrative and not restrictive, the scope o~ the invention being indicatcd b~ the hereafter appended clairns 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 e~raced therein.
~,.,,',,, ,, ~ ' ..
1 The ~resent invention is primarily applicable to a self-propelled reel pipe laying vessel, having a reel for spooling relatively inLlexible pipe thereon, pipe wor~ing and handling mPans for straightening the pipe as it iS unspooled, pipe guide ~eans for guiding the straightened pipe into the ~ater at a presettable, adjustable exit angle, means for maintaining the pipe under a predeterminecl adjustable tension, main vessel drive means, pre~erably including twin screw~ located on opposite sides o~ the vessel longitudinal centerline, and ~orward and aft thruster means located forward and aft, respec~i.vely, of the longitudinal center of the vessel.
During a p:i~elaying operation, the pipe handling q~ui~nt and ~ipe ~uide means txansl~tes acros~ ~he beam o~
~hQ ves~el as lt .~0110~3 ~or leads) the pipc wrap b~ing unsyooled.
Inthe proce~s o~ tran~ ing the pipe guide means ~cross ~he beam o~
the v~el, ~U~ning momen~s ~ln ~he ~orlzQnt~l plane) are impar~cd .
..
~t,, ; 0 5 ~J
to tho vcssel by the tensioll in tlle pipeline. In one aspect, therefore, thc inventioll col~rises a method of compensatin~
for these pipeline tension induced turniny moments by generating a reactive force in opposition to the ~i~eline tension induced turnin~ moment to thereby correct for deviations in the ve~sel's course and to maintain t'ne vessel on course alon~ the desired right of way.
A further aspect o~ the method of this invention comprises monitoring the an~le of entry of the pipe into the water rela~ive to a nominal horizontal plane re~resenting the water surface; ~onitorin~ the angle of excursion which the pipe makés relative to a nominal ~ipe centerline substantially parallel to the nominal preset angle of en~ry into the water;
and adjusting the nominal pipeline tension if the monitored excursion angle remains outside a predetermined permissible excursion ran~e ~or at least a si~nificant time period, for example, greater than the ~itching period o the vessel.
A still further aspect of the method of this invention comprises setting the pipe guide means to establish a desired pipe exit an~le at which the pipeline substantially enters its catenary con~iguration before exiting the vessel and pipe yuide means; and settin~ the tensioning means to hold the pipe under a predetermined nominal tension in conjunction with the pipe exit angle, to establish a minimum radius o~ curvature o~ the pipe in the sa~ bend reglon which is great~r than the rlinimum r~dius to whic~ th~t pipe may be ben~ ~7ithou-t exceedin~
lt~ el~ ici~y limi-ts as it is uns~ool~d ana pald out ~om tlle ve~sel.
1 ~5~059 Brlef cscri~tion of the Dr~ing ~ Figure 1 is a diaqra~matic sketch Q~ a self-prooelled reel pipe laying vessel sho~ing the ap~roximate pipe profile bet~een the vessel and the sea bot-~om.
Fi~ures 2~-C are diagrammatic s~etches o the vessel dec~, ramp assembly and pipe, in several conditions o-f pitching due to sea condi~ions.
Figure 3 is a diagrar~tic ~lan view o the vessel showing course-correcting force relationships.
Description of Preferred ~bodiments ~nde~later pipelines for carrying oil or gas must meet certain requirements and li~its set by the customer ~pipeline o~ner) and/or governmental or other regulatory bodies.
It is of prir.lary i~portance that the pipe, as it is b~ing laid and as it lays on the sea bottom, be subjected to minimal residual stress, strain, tension, etc. In terms of pi~e laid by the reel method, this means that the pipe as it la~s on the sea bottom must be straight and have substantially no residual curvatux~ clue to spooling or laying. It is al~o important tha~
the pipeline be laid close to the nominal right of way. The "as laid" restrictions are developed as a ~unction of a nu~ber of parameters developed by the pipeline designer, including the ~ype o~ s~a bed cn Which ~h~ pipe x~sts, ~he sizq and ~rade v o~ pipe ~o he Usqcl, ~h~ -type, amoun~, and ~low ra~q~ o~ ~luid ~o b~ ~arriad ~y ~he plp~llne~ and pxedicted li~q ~p3n of ~he plpelln~ hqr paramek~rs xela~in~ ~/ or basq~ ~n, the ~e~me~ry ~hap~) o~ the pipeline durlncJ ~he ~ in~
~p~xa~iorl ~x~ cl~vel~p~d b~ th~ pip~ layin~ entJinecx~
.
1 1S605~
Additionally~ a reel pipelaying vessel and the pipe being lald are su~jected to a number of hydros~atic 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 or 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 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 ~et. Th~ distance which a ~tove pipln~ bar~e can moYe along the right o~ way on a slngle anchor ~et pattRrn decrea^~es as water dep~h in~r~ase~. It is apparent that the limited ~orward movement permitted by thi~ anchor setting technique is not at all su:Ltable ~or economiaal reel pipe laying opera~ions.
! a 1 1 5~059 Althou~h ~ow~ r~el pipelayin~ baryes have been found to be cluitc adequate for -thc relatively calm waters o khe Gulf of rlexico ofs}lore of the United States coastline, the~ have cextain inherent limitations which make -then unsuitable for use in relativel~ rouyh waters, such as are found in the Mor~h Sea or o~ the coast of South ~merica or Australia. One of the principal btli]~-in limita-tions o~ a towed bar~e system. resides in the towing connection itself. Unli]~e a self-propelled ship, in ~hich the rnotive source i5 effectively connected directly and rigidly to the pipeline (through the~reel), the connection between the towing vessel (motive source) and the towed bar~e (e~ectively includlng the pipeline end) is a flexible one ~hich 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 rel~tive moverrent between the tug and bar~e. This ~ay cause the pipeline tension ~o exhibit sudden increases and/or decreases in ~nagnitude t~hich can neither be predicted norco~trolled e~ectively by the barge operator(s).
A self-propelled reel type pipelaying ship requires neither anchors nor tugs as the mo~ive source. There~ore, compared to stove-piping type bar~es as described above, a sel~-propelled reel pipelaying ship is able to move con-tinuously down the ri~ht of way, stopping only when necessary, for e~ample, to install anodes as required by the customer and/or ~o perform othQr oper~tions on the pipe, such as coating r~pair, etc.
Cornparecl to ~o~d re~l barges, th~ selE-propelled reel ~hlp~
h~ a significant ad~antage in that the mo~ive souxce o~ the reel ship c~n, for prac~:Lcal purpos~s, be consider~d ~n b~ ~ixed with ~he x~el and pipelin~ end, -khereb~ el:lmina~inq ~la~ive movem~n~s ~hcr.e~ek~eell ~lue -t~ ~Jeather ~ela-tccl ~ac~c~rs, as rlo~d above.
9 _ .
.
.
Co~ercial and practical limitations effectively restrict thc operatin~ capability of a to~ed reel barye. One of the principal requirements in l~ying pipelines of~shore from a sur~ace vessel is tha~ 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 predeterminecl 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 bot~om is called t~?e Touchdown Point (TDP).
It i5 impor~ant that the radius o~ the sag bend curve be kept above the mini~um permissible radius to which the pipe may be bent ~ithout exceeding elasticity limits i~ 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 bet~?een the departure or exit angle (also sometimes called pipe entry angle into the water~ and the required tension can be e~pressed as an essentially linear logari~hmic relation where ~he pipe pro~ile i5 catenary~sh~ped in i~s unsupported length betw~en the vessel and th~? ~,e~ ~?ottom, subs~antially as represented in Fig. l; i.e., ~?or ~ givqn ~i~e and grad~ of pipe and a given l~y d~pth along kh~ xigh~ o~ ~ay, the tension required to hold ~e sag bend r~diu~ above ~he allo~able minimum decxeases as tlie departure ~ngle o$ the pipe into the wat~r increases. E~or ex~mple, it is neces~a~y to hold about 250,000 lbs. o~ tension ~250 ~ip5, ~Jhere "~ipS" eC~ S ~hC?~ anClS O~ pOUllC S) on a ~ e hav;incJ an .
1 ~ 5 ~ O ~(J
outside diameter of 10 3/4" and 3/4" wall thickness laid in a water depth of 500 ~eet, if the pipe exit angle i5 set at abou~ 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 conclitions 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 o 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 "t" shape (with t~Jo 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.
Re~erring to Figure l, a feature of "Apache-type" special reel pipela~ing ships is the adjustable pipe carrying ramp assembly 40 pivotably mounted (generally at the stern) to the deck of 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 la*eral thxusters 1~2. ~Throughout this dis-closure, re~qrence is made ka the main propel:Lers as providin~
~he re~uisite ~orward thxu~t7 it i5 apparent,however, tha-t other suitable drive me~ns oould be provided to ~enerate the neces~
l 1 5~051.3 sary for~/carcl-thru~;t ancl the reference to "propellers" throughout t:hi~; ~isclosure is in~ended to encompass other such suitable drive means, exce~t where othen~ise specificall~ noted~) Special pipe hand:Linc3 equipment, which may include, for example, the adjustable radius control me~er, adjustable stra.i~lltener tracks, tensioner tracks, pipe claMping assemblies, ~uide roller assemblies, and pipe angle measuring assembly, is advantageously mounted to the ramp assembly 40.
~ n adjustable ramp assemhly o~ this type has not heretofore been incorporated into an~ known commercicll offshore reel pipelayin~ vessel, specifically including -the supply boat portable reel system used o~f the coast of ~ustralia, the tt~o reel pipelaying towed barses ot~ned and used by Santa Fe and/or Santa Fe's predecessors-in-interest since about 1961 and tt~o com2etitive reel pi~ela~ing barges, one used for a short time in 1972 or 1973 and the other currently in use in the Gulf o~
I5exico 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 ~ater at any desired an~le ~ithin its operatin~ ran~e o~ bett;een ahout 15~ and 65~, pre~erably 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 maintalned during a pip~ lay operation; the ramp assem~ly ~uides the pipe as i~ enters th~ wa~er at the preset exi~ an~:Le. As noted aboYe, all prior kno~n commercial rqel pipelayin~ h~r~qs ha~q opqra~ed at a fixed, non-variable exit an~l~ o~ b~ qen abou~
6 and 12 ~ The adjus~able cxi~ an~le ~ea~uxe o~ ~he ~pache-type .
" .
1 1 5 ~ {~
v~ssel ena}~les it ~o handle a ~lider range of pipe sizes in a (~reater ranse of ~ater cl-pth~ than was herctofore ~o~sible ~ith fixed lo~ e~it anylc rcel pipelaying bar~es.
One of the advantages of an Apache-type adjustable ral~p ass~mbly ~or settin~ th~ pipe exit angle is the virtual elimination of the ovcrbend region (i e., the bend re~ion occurring as ~he pipe translates downwardly from the relatively hori-zontal plane of the bar~e ~oward the seabed in the relatively vertical planeof the catenar~. Advantageously an~ preferably, the ramp angle and tension are set so that downstream o~ the straightener/tellsioner apparatus, the pipe will be unsupported7 thus, pipe exiting the straightener mechanism and traveling along the ramp assembly will already ~e in its nominal caténary configuration before and as it en~ers the ~ater Preferably, as the pipe 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 resiclual bending moments.
By initiall~ setting the ra~p angle and nominal pipeline tension to virtually eliminate the overbend as a factor in ~eter~inincJ and controlling the ~inal residual pipeline characteristics, the saJ bend ~i.e., the bend occurring in the translation o~ the pipe ~xom thq vertical to the hori-zonta~ plan~ on the sea bo~kom) becomes a critlcal fackor in kh~ conkrol o~ the pi~e as it i5 lald. ~he ~g bend is con-trolled, at l~a~t in ~ark, as ~ function o~ the kellslon main~
tained on ~h~ pipe by ~he ~unctional element~ of khc pipela~in~
vessel, including ~he xeel, straicgh~ener ftensioner e~ ents vessel clriv~ assembly, etc. Cont~o.lled tension .is impart~.
6 ~ ~ 9 to t-h~ pipe by (l) the r~el throughthe reel drive mechanism operating as .a dynami.c brake, (2) th~ main vessel drive thrust acting through the vessel main propellers and/or the lateral thruster assel~lies, and (3) the -tensioner assernbly, which may or may not be used, through a re~ulated tensioning force estabiished at the beginning 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 ~referably determined on khe basis of informati.on supplied by th~ pipeline designer.
Such informa'cion from tlle pipeline designer (or customer - pipeline owner) includes (l) the size o~ the pipe, including internal pipe diam~ter and ~Jall thic~ness, (2) the type or grade of pi~e, including such in~ormation as the pipe material and minimu~ yield strength, (3) maximum allowable stress, strain and residual tension, and (4~ water depth along the pipeline - righ~ of way. An optimum nominal tension ~nd lay angle can be determined from these para~.eters.
One of the criteria which has been developed ~or 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 an-d thesea bottom shouldnot be greater than about 85~ of the minimum yield strength of the pipe.
I~ i.s also de~irable and preferable to minimize the kension imparted ko thq pipe by the vessel while maintaining operating conditlons such that the maxi.~um allo~7~ble stress linll-t and ~he maximu~ allowable residual tension in th~ ~ipeline ~re not exceed~d. llh.is may be accomplished by setting -khe r~mp assembly ~n~ ncl thu~ the pipe en.txy an~le into the s~ater) in conjunction w.~th nominal pipe tension such ~h~ ~he ~i~htest sag bend radius will be ~cl1ieve~ ~7it}1011~ excec~ing thc .~hove ne~cd ~sl~rer-:s ~ncl x~sldu~l te/)siGn lir1it.
. .. ~ - -.
115~5~
The ramp assembly angle (and thus the pipe entry angle into the water) is set a~ the becJinning of the pipelayin~ opera-tion and is normally not change~ 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 is 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 set~ing 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 (preferably 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 "~lexible"
towing connection between a reel barge and its tu~ will not be ~dequa~e to maintain ~he ne~e~sary continuous tension on ~he plpelin~ a~ i~ ls being lald. ~he tu~ move~ indqpenden~ly o~
th~ harge du~ to wave action. ~his means that th~ motive source which provides the ~on~axd thrust necessary to malntain tension q ~n the pip~llne is susceptible ~o uncontrolled variations rqla~iv~ ~o the barge and thus to the pipe. Limite~ excursions .
~ 15~0~ ~J
of this -type may be acceptable for some sizes of pipe and some sea conditions. ~lowever, 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 motive 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 directly. There are, however, several ways to measure the tension indirectly. One such way is to meaQure 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 ~he pipeline.
~hi~ can be done by measurlng the main propeller sha~t torque or by measurin~ the force on a thrust bearing against which the propellex sha~t ~cts.
A second method i5 to measure the drive motor ~orce acting on the reel. Neglecting the aomponents of tension pro duced primarily by the strai~htener assembly ~and tensioner~
when used), the force exerted by the reel drive motors is directly proportional to the ten~ion in the pipe; thus, an ; 0 5 9 increase o~ decreasc in the (Irlve .~otor force produccs a corres~ondilg increase or decrease in the pipeline terlsion.
The reel motor drive force ma~ bc r.~easured by, e.g., load cells between the ~otor/reel mechan.ical connection.
A third prac~ical Jay to r~leasure pipeline tension is based on ~.easurement of the exit an~le of the pipe from the vessel. It is advantacJeous and preferable that the piPe angle be measured ~Jith respect both to the hori70n and to the ramp angle; the latter measurc~.ent is particularly helpful where the pi~e passes throu~h an exit window defined by a stern ~uide xoll~r assenbly, such as is used on Apache-type vessels.
Figures 2A-C are diagral~atic representations of the pipelaying vessel 10, rar~p asser~bly 40 (set at a nor~.inal lay an~gle of about 30 degrees), the stern gui~e roller assembly 54 defining the exit windo~7, and the pipe P. Fisure 2A shows the relationship between the ramp assembly and the pipe when the vessel is substantially ~lat in the water so that the entry an~le Al of the pipe into the water (relative to a nominal horizontal plane or axis, such as the horizon) is sub~antially the same as the prede~ermined ramP angle ~; Figure 2B shows the same relationship when the vessel is pitched bo~ up at an angle D2 and the pipe P2 enters the wa~er at an angle A2; and Fi~ure 2C ~hows the same relationship when the vessel is pitched bow down at an angle ~3 and pipe P3 ~nters the wa~er a-~ ~n a~ 3~ ~'he exi.t poin~ o~ the ~ipe ~ro~ ~he straigh~ener/
-~ensloner ~se~bly is desi~nated by re~erence SE. q'he pipe is ~ssQn~lally ln ~ d relatio;n to ~h~ raMp a~çmhly ~nd the vessel a~ point S~. PreEexahly and advant~geousl~, su~Eicient and adequ~lte tension i~ ma.in~ained on ~he pipe ~ durincJ -~}le .
? ~ o ~ ~ ~
laying oper~tion so that the pipe travels in a path substantially paral1~1 to the ramp an~l through the guide roller assembly 54 sub-stantially unsupported between straightener exit SE and the touch-down point TDP on the sea bottom. Also a~vantageously and prefer-ably control oE pipelaying operation is maintained so that angles ~l' A2, A3 will be essentially equal.
The stexn guide roller assemhly provides a pipe excursion ~indow 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 ~ommercial embodiment, the distance between straight-ener exit SE and stern guide roller asserrbly 54 is approximately 45 feet; the distanc~ between the upper and lower stern guide rollers is approximately four ~eet. This permit~ 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 with-out being subject to bending moments by the stern guide rollers.
Referring to Figs. 2B and 2C, angles F2 and F3, respectively, re-present the excursion above and below the nominal centerline of the pipeline P when it is tensioned to he parallel to the plane o~ the ramp assembly 40.
During the pipelaying operation, the vessel moves forward through the water as a function o~ the thrust generated by the main vessel drive, reacting against pipe tensioning forces produced by the pipe handling equipment, includlng reel dyn~ic ~r~king ~orces, s~xaightener, tensianer, etc. Changes in or ~od1~lcations to the rate o~ ~orward maklon o~ the vessel, and thu~ the ra~e a~ which pipe is unspooled ~rom the reel ~0 and p~id out into ~le wa~er, may be controlled by adjus~ing the dynamic brakln~ force exert~d by the re~l drive mechanism and/or ~he amount o~ ~hrust generated by the main propellers.
A ~ypical lay rate, i.e., the rate at which ' 1 1 5~()59 yipe is p~iQ out fro~ the vessel during a lay opcration, would be in the rang~ of 75 - 150 fe~t per minute. It has been found to be preferable to r~aintain the Eorwarcl tllrus-t rel~tively constant a~d to control pipe tension chclnges through adjustn!ents to the reel dynamic braking force. Due to the large mass of the reel and pipe, it is not possible to effect instantaneous changes in the pay out rate.
As the vessel pitches during a laying o~eration, the stern, with the rar~p and other related pipe handling e~uipment, moves up and down in the wate,r. The pipe, paid out from 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 unden7ater suspended length and on-bottom ~riction. Due to such inertial effects on the pipe, the portion of tlle pipe downstream of straiyh-tener e~it SE
does not nece~sarily ~ove ~ith the ves~ei so that the total piwe oxcursion relative to the ra~p may be greater than the stern guide excursion windo~ limits. Under conc~itions where the bo~ pitches up by an angle D2, the pipe ~a~ be bent around the upper stern guide roller, as shown in Fig. 2B. S:imilarly, when the bot~ of the ves~el pitches do~mwards by an angle D3, the pipe ~a~ be bent aroun~ the lo~er stern guide rolLer assembly, as sho~m in Fig. 2C.
In ~ commerci~l e~odiment of an Apache-t~pe vesseL, ~n an~lo me~suring device measures the pipe an~le downstream o~
~hq ~ern ~uide assembly relative to the ramp assembly 40 and r~lative to the horizon. Onc Su~h anqle moa~urin~ device is ~ho~m an~ d~scribecl in ~E~res~id ~ri.~ish P.pplic~ion Sexial ~o.
~91591~ ~n apparatus ~or this purpose is m~nu~ac~ured by IntC X~ t a ~ Eloctror-lics, :Inc.
.
,' , .
... ..... .. . . . . .. .
r. . . , . _ . .
l 15~059 In Fis. 2~, rcference E2 represents the measured an~Jle of c~cursion o~ the pipe P2 xelative to the ram~ assembly ~0 under the condition wher~ the vessel pitches ~p by thc bow at an ansle D~ ~ this pitcll ancJle, the effective exit an~le G2 become~ R (rarnp anyle) plus D2 (pi-tch angle). ~s noted earlier, it has been found that pipe tcnsion and exit an~le are invers~ly proportional; therefore, as t~?e e~fective exit angle G2 increases, the tension applied to the pipeline should be decreased in order to maintain the pipe ~rofile within acceptable limits. ~owever, since, due to reel and pipe .inertia ~nd other factors, the tcnsion applied to the pipe cannot be acljusted to directly follo~l the pitching of the , vessel, the effective tension on the pipe is increased and a pi~e pro~ile such as sho~n in Fig 2B results. Under su~ficiently 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 ~i~e undergoes a bending moment about the upper stern guide roller. If this bending moment exceeds the elastic limit of the pipe, the pi~e will under~o plastic bending and ~ill thus ratain a resldual curvature due to such plastic bending ~7hen it rests on the bottom.
11hen the bow o~ the vessel pitches down~ard, e.g., at an angle D3, a pipe pro~ile such as shown in Fi~. 2C may re~ult~ In this case, -the e~fec~ive ~xit angle G3 becornqs ~
~ramp ~ngle) minu~ D3 (pi~ch an~le); in ~his ca~e, the ~fqc~ive exi~ an~le is 3~aller ~han ~he nominal pr~se-k rarnp an~lQ~ In orde~ ~ r~laintaln a proper pipe pxo~ , in bow d~wn pitch condi~ion, ~he ~ension ~n th~ pip~ shou].d be incr~a~ed an amount .: . - .. ; ....
i 1 5~ ~S~J
"
sufEicient to co~pensate for the decrease in cffective c~it anc~le. ~lo~cver, for reasons noted above, it is not ~ossib]c to instant~neously change the -tension impar-ted to the pipe by the vcssel, and particularly by the reel. There~ore, -the pipe under~oes ~n excursion E3 whlch ~ay be greater than the excurs;on F3 ~err~itted by the stern guide ~.indo~ limits. Under such conditions, the pipe undercJoe~ a bending mor~ent about the lower stexn guide roller; if this bending moment exceeds the elastic limit, the ~ipe under~oes plastic bending and will retain a residual curvature ~Ihen it is laid.
The angle measuring device measures excursion E2 and E3 to thereby generate an in~ication of excessive bending of the pipe OII the ramp. ~.easurernent of excursion E2 or ~3 is parti-cularly important as an indicator that the pipe is over-tensioned or undertensio~ed, irrespec-tive o~ the ~itching of the vessel.
~en the vessel is pitching, excursions E2 and E3 would be expecLed to be relatively short-livea. Measurement of such short-lived excursions t;~ould not provide an accurate indica-tion of over- or under-tensioning.
A can-kinuous measurement of excursic~n E2 greater than limit F2~ormeasured excursions E2 greater than F2 ~hich occur a significant percent of the time ~e.g., greater than the pitching period o~ the vessel), even though such excursions ~re not continuous, indicate to the operator that the pipe is bqinc3 hqld under excqssive tension. ~he opexa~or can then acljusk tll~ reeldynamicbraking ~oxce to decrease th~ tension on the pipe ~til ~h~ an~le measuring device measures an excursion E2 l~ss than eXCursion ~, neglectirl~ short-livecl excursiorls du~
to v~ssel plkchiny. Correspon~irlgly, when the an~le measuring - 21 ~
,/ 1 1 5 ~ ~ 5 9 .1~
device ~easures an e~cursion E3 continuously greater than excursion lir~nit F3, or yrcater than F3 a si~nificant percent o~ the tire (e.g., ~reater than th~ pitchiny ~eriod of the vessel)/ even thouyh not continuous, these constitute indica-tions that the pipe is being held under însuE~icient 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 exc~rsions due to vessel pitching~
~ 1hen the vessel is pitching, aue, for exa~ple, to sea conditions, measurin~ excursions E2 and E3 may produce erroneous indications of pipe tension and may make it dificult, if not ~ractically i~possibIe, for the operator to maintain proper tension ontthe pipe. Lherefore, 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 thP actual pipe entry angle into the water so that under varying sea conditions, with the vessel pitching continu-ously, the operator can main-tain a direct reading of the actual pipe entry angle. The operator is then able to rnaintain the proper reel dynamic breaking ~orce and provide necessary compensation adjustments basea on the ac~ual pipe angle relative to the fi~:ed hori20n, as distinyuished From an~les measured rel~tive to the moving and pitchin~ vessel.
- The pipe laying operation is also ~fectqd by the fac~ tha~ the pipe tr~Vqrses across the beam o~ the vessel as i~ is unspoole~. This produGes a turniny momen~ tending to pull the vessel o-E~ ~ourse. This turnin~ moment increases to a ?,?
.. . ...
., ... ~..
// ~
ll56o~cJ
m~xi.rnu~ at the en~ oE t:r~nsverse travel of the ramp assembly, CreaSe5 to 7,ero ~hen the r~rnp a~isembly (and ~ipeline p~th) is aligned t1itl1 the vessel centerline, ~nd increases to a ~aximum in tlle op~osite direction as the ramp assem~ly continues movin~
-to the extrcme opposite end of its transverse -travel.
The turning moment can be qui te larye co~pa~ed to the fon1ard thrust generated by the main propellers. For example, in one commercial embodi}~ent, the ramD 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 ~laximum thrust of 80 Kips. IYhen pipe is being laid under 100 Ki~s tension at an exit angle of 30 de~rees, the pipe tension induced turning mo~.ent at each extreme end of ram~ assembly travel is on the order of 930 foot Kips. The o~osing turninq moment produced by the main pro~eller on ~hat side o~erating at maximum thrust is about 1,600 foot iCips. It will be seen tllat the pipe tension induced turning moment may well be a significant percentage (58.9~ in the example given here) o~ the drive induced turniny ~oment If the pipe tension induced turning ~.oment is not compensc~ted for, the vessel will be pulled off course; this can result in the pipe bein~ laid out o~ the riyht of ~ay, ~hich-is c~mmercially accepkable.
The pipeline induced tUrninCJ mo~ent must be compensated - ~or .in order to lay the pipe in ~ s~raicJht l.ine alon~ the right o~ way. ~Yith twin scre~ vessels, that is, vcssels pro~elled by ~wo sets oE ~in drive p~o~ellers equa:Lly sp~ced ~n o~posite si~e~ o~ t.he lonclitudinal centerline of the vess~l~ ik ~ay be pussibl~ to overcome the ~u~niny ~o~en~ in~roduced by -khe pipe's pipeline ol.~set rela~lve ~o ~he ve~el center line by .i.ncre~sincJ thrust on the propeller :located on that side of th~
. : . ...
- , . . .~.;, I 1 S605~
vessel and ~or decreasing thrust on the opposite side main drive propeller. This has certain inherent disadvantages because the pipeline induced turniny 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 re~uires that the thrust of the drive propellers be varried accordingly, while at the same time taklng 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 turning 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 o~ such thrusters are shown in the a~oresaid prior rela~ed Santa Fe inventions. Also refer-rin~ to Fig. 3 hereo~, an aft thruster tunnel 120 houses the A~`t thruster 122; a ~orward thruster tunnel 12~ houses the forward thrustex 126.
The thruster~ 122 and 126 can be operated either manuall~ or automatically in conjunction with, e.g., a computer ope~ated guidance system, to generate turnlng 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 axis, the magnitude of the pipeline induced turning moment is a functionof the tension vn the pipeline and the pipelin~ offset / ~56~5`9 ~
from the vcssel's centerline. TJle vessel thrusters gener~te turnincJ moments about the aEoresaid intersection of the vcssel's centcrline and reel shaft axis which react against the ~ipeline turnin~ mo~ent to maintain the vessel on its pro~ex course.
Consideration must also be given to the fact that aturning ~oment occurs be-tween the forward vessel thrus'ter(s) and the pipeline touchdown point on the sea bottom. Therefore, in addition to rotating the vesse:L abou~ the centerline inter-sec-tion pointY, the en~ire vessel ~ust be rotatecl about the touchdo~n poin-t to maintain the vessel on and parallel to the right o~ ~ay. This mcl~ be accom~lished by increasing the thrust generated by the forward thrus-ter(s) relative to -lle op~ositely reaoti}lg force generated by the aft thruster(s).
~ The a~ount of thrust required varies as a f~mction of a num~er of factors, including the lateral ~osition of the pipeline relative to the vessel's longitudinal axis, the ~istance between the vessel and the touchdo~n ~oint, the ~ipeline 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' 5 lon~itudinal c~nterline. ~dvantageously and pxeferably, Tl ls m~in~ained greater than T2; ~o~ether, q'l ~ 'r2 produce a kurnin~ mom@nt which reaats the pipeline in~uced -~uxning momenk.
The thrus~ ~enerated by the ~orward thrustex there~ore comprise~
thq ~dditive componen~s of the thru~ necessary to xeact the ~ipeline induced turnin~ mom@nt about -the ves~el axis and the pipeline induced ~urni.n~ momen~ about the touchdo~n point pivot axis. ~he a~t or rear thrus~er nsed onl~ reack the .
-:
. . . . .
r~,~ .. : . . . .
1 5 ~ 0 5 ~i h ~ ' pipeline indu~ed turning moment about the vessel axis. ~he for~Jaxd thrus~r therefore imparts a r~latively yreater lateral thrust component than the rear thruster to o~ercome the pipeline induced turni~g mo~ents about the vessel pivot axis and about the touchdotJn poin-t pivot axis to thereby maintain the vessel on course along the right of way.
The invention may be erbodied in other specific ~orms ~ithout de2arting fror~ the spirit or essential charac-teristics thereof. The embodi~ent described above is therefore to be considerecl in all respects as illustrative and not restrictive, the scope o~ the invention being indicatcd b~ the hereafter appended clairns 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 e~raced therein.
~,.,,',,, ,, ~ ' ..
Claims (3)
1. A method of laying pipe offshore from a self-propelled reel pipelaying 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 pipe entry angle, the pipe handling means including tensioning means for maintaining the pipe under a predetermined adjustable tension, said method comprising the steps of:
monitoring the angle of entry of the pipe into the water relative to a nominal horizontal plane representing the water surface;
monitoring the angle of excursion which the pipe makes relative to a nominal pipe centerline substantially parallel to the nominal present angle of entry into the water; and adjusting the nominal pipeline tension if the monitored excursion angle remains outside a predetermined permissible excursion range for at least a significant time period greater than the pitching period of the vessel.
monitoring the angle of entry of the pipe into the water relative to a nominal horizontal plane representing the water surface;
monitoring the angle of excursion which the pipe makes relative to a nominal pipe centerline substantially parallel to the nominal present angle of entry into the water; and adjusting the nominal pipeline tension if the monitored excursion angle remains outside a predetermined permissible excursion range for at least a significant time period greater than the pitching period of the vessel.
2. A method according to claim 1 further comprising:
increasing the nominal pipeline tension it- the monitored excursion angle is less than the predetermined excursion range for at least said significant time period.
increasing the nominal pipeline tension it- the monitored excursion angle is less than the predetermined excursion range for at least said significant time period.
3. A method according to claim 1 or 2 further comprising:
decreasing the nominal pipeline tension if the monitored excursion angel is greater than the predetermined permitted excursion range for at least said significant time period.
decreasing the nominal pipeline tension if the monitored excursion angel is greater than the predetermined permitted excursion range for at least said significant time period.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000411522A CA1156059A (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 |
| CA000411522A CA1156059A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1156059A true CA1156059A (en) | 1983-11-01 |
Family
ID=25669126
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000411522A Expired CA1156059A (en) | 1980-08-12 | 1982-09-15 | Method of laying offshore pipeline from a reel carrying vessel |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1156059A (en) |
-
1982
- 1982-09-15 CA CA000411522A patent/CA1156059A/en not_active Expired
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