Note: Descriptions are shown in the official language in which they were submitted.
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;
- 3
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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.. .. ., . . . , .... ... .. ~ . . ~ . .. . . . . .. ... . .
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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
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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..
~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.
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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.
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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 _ .
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.
1 1 5 6 0 5 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
-- 10 --
.
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~
-- 11 --
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~.
- 13 -
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
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~ 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
- 17 -
.
? ~ 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
- 18 -
'
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.
- 19 -
.
,' , .
... ..... .. . . . . .. .
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
- 24 -
/ ~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
- 25
.
-:
. . . . .
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.
~,.,,',,, ,, ~ ' ..
