Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention relates to techniques and methods utilized
in laying underwater pipelines. More particularly, the invention
relates to laying pipelines wherein continuous lengths of pipe are
first spooled onto a reel carried by a vessel and are thereafter
unspooled into the water as the vessel proceeds along the pipeline
route.
The methods and techniques described herein are particu-
larly suited for sel~-propelled types of reel pipelaying vessels.
Suitable vessels which would be expected to use the methods and
techniques described herein include drill ships and ore carriers
converted to carry pi.pe spoollng reels and related reel pipelaying
equipment. One such self-propelled vessel constructed specifically
as a reel-type pipelaying ship is described in U.S. Patent 4,230,421,
issued to Charles N. Springett, Dan Abramovich, Stanley T. Uyeda
and E. John Radu; U.S. Patent 4,269,540 issued to Stanley T. Uyeda,
E. John Radu, William J. Talbot, Jr. and Norman Feldman.
The present appliction (and the i~ventive subject matter
described and claimed herein) and the above-listed U.S. Patents are
all owned by Santa Fe International
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Corporation; hereafter the above-listed commonly owned applica-
tions will be referred to as "prior related Santa Fe Inventions-.
Prior to the development by Santa Fe of the self-propel-
led reel ship known in the industry as -Apache'- (the construction
of which is substantially described in the above-listed prior
related Santa Fe application) and which was scheduled to begin
commercial pipelaying operations in late summer of 1979, most
known commercial reel type pipelaying vessels consisted of non-
self-propelled barges towed by a tug. One portable pipelaying
system designed and built by Santa-Fe for use on small supply
boat type vessels for laying small diameter pipelines (up to
4-- I.D.) has been in commercial use off the coast of Australia
since about July, 1978; this portable pipelaying system is
described in U.S. Patent 4,260,?.87 issued to Stanley T. Uyeda
and John H. Cha, and assigned to Santa Fe.
Other patents owned by Santa E'e directed ko and describ-
ing one or more features of reel pipelaying vessels include:
U.S. Patent No. 3,237,438, issued March 1, 1966 to
Prosper A. Tesson;
U.S. Patent No. 3,372,461, issued March 12, 1968 to
Prosper A. Tesson;
U.S. Patent No. 3,630,461, issued December 28, 1971 to
Daniel E. Sugasti, Larry R. Russell, and Fred W. Schaejbe;
U.S. Patent No. 3,641~778, issued February 15, 1972 to
Robert G. Gibson;
U.S. Patent No. 3,680,342, issued August 1, 1972 to
James D. Mott and Richard B. Feazle;
~, l72~r~'~ Jt"
U.S. Patent ~o. 3,712,100 issued January 23, 1973 to
Joe ~1. Key and Larxy Ro Russell; a~
, IJ.S. Patent 3,9.,2,402, i.ssued Septembe~ 28, 197G to
~lexander Craig Lans and Peter Alan Lunde.
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Summary of the Invention
The present invention is concerned with methods and
tec~niqués related to the control of pipelaying operations
from a self-propelled reel .ipelaying vessel. The methods are
con.cerned with 1) con~rolling pipeline geor.~etry as a unction
of pipe entry angle into the ~ater and tension on the pipeline;
2) monitoring the excursion of the pipeline outside certain
defined limits and controlling the pipeline geome~ry based on
such measured excursions; and 3) com~enSating for pipeline
induced turning moments which woula othen~ise tend ~o draw
the pipelaying vessel off course and of $he predetermined
pipeline ri~ht of ~7ay.
; The present invention is primarily applicable to a
self-propelled reel pipe laying vessel, having a reel for
spooling relatively in~lexible pipe thereon, pipe wor~ing and
handling mPans for straightening the pipe as it is unspooled,
pipe guide means fox guiding the straighkened pipe into the
water at a presettable, adjustable exi.t angle, me~ns for
maintaining the pipe under a predetermined adiustable tension,
main vessel drive means, preferably includin~ t~lin screws located
on opposite sides o the vessel longitudinal centerline, and
~orward and aft thruster means located fon~ard and aft,
respectively, of the longitudinal center of the ~essel.
During a pipelaying operation, the pipe hand~ing
equipment and ~ipe ~uide means txanslates across the beam of
the vessel as it follo~s ~or leads) the pipe wrap being unspooled.
In the process of translating the pipe guide means across the beam of
the vessel, turning moments (in the horizontal plane~ are imparted
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to the ve~ssel by the tension in the pipeline. In one ~sp~ct,
there~ore, the invention co~prises a method of compensatin~
for these pipeline tension induced turning moments by
generating a reactive force in opposition to the ?i~eline
tension induced turnin~ moment'to thereby correc~ for deviations
in the vessel's course and to maintain the vessel on course
~long the desired right o way.
A further aspect o~ the me~hod of this invention
comvrises monitoring the an~le of entry of the plpe into the
water relative ~o a nominal horizontal plane re~resenting the
water surace; ~onitoring the angle of-excursion ~Jhich the
pipe makes xelative to a nominal pipe centerline substantially
parallel to the nominal preset anale of entry into the water;
and adjusting the nominal pipeline tension if the monitored
excursion angle remains outside a predetermined permissible
excursion ran~e for at least a significant time period, for
example, greater than the pitching period o the vessel.
A still further aspect of the method of this invention
comprises setting the pipe gui~e means to establisll a desirecl
pipe,exit angle at which the pipeline substantially enters its
, catenary con~iguration before exitincJ the vessel and pipe
guide means; and setting the tensioning means to hold the
pipe under a predetermined nominal ten,sion in conjunction ~Jith
the pipe exit angle, to establish a minimum radius of curvature
o t~e pipe in the sag bend region ~7hich is greater than the
., r.linimum radius to which that pipe may be be~t ~ithout exceeding
its elasticiky limits as it is unspooled and paid out from t~e
ve~sel.
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Brief Descrintion of the Dra~ing
. Fi~ure l is a diagra~matic sketch of a self~pro~elled
rëel pipe laying vessel showing the approximate pipe profi1e
bet~een the vessel and the sea bottom.
Fi~ures 2~-C are diagramma-tic s~e~ches of the vessel
deck, ramp assembly and pipe, in several conditions of pitching
due to sea conditions.
Figure 3 is a diagram~atic ~lan view of the vessel
showing course-correctins force relationships.
Description of Preferred ~bodiments
.
~ ndert7ater pipelines for carryin~ oil or ~as must
meet certain requirements and limlts set by the customer
(pipeline o~mer) and/or yovernmenkal or other regulatory bodies.
It is of prir.lary i~portance that the pipe, as it is being laid
and as it lays on the sea bottom, be subjected ko minimal
residual stress, strain, tension, etc. In terms of pipe laid
b~ the reel me~hod, this means that the pipe as it lays on the
sea bottom must be straight and have substantially no residual
curvature due to spooling or laying. It is also important that
the pipeline be laid close to the nominal right of ~ay. The
"as laid" restrictions are developed as a function o~ a nur~ber
of parameters developed by the pipeline desi~ner, including the
type of sea bed on which the pipe rests, the size and grade
of pipe to be used, the type, amounts, and flow rates of fluid
to be carried by the pipeline, and ~redicted lie s~an of the
pipeline. O~her parameters xelatin~ to, or based on, the
seometry ~shaye) of the pipeline during the pipe laying
operation are developed by the pipe ~ayin~ engineer;. C /
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Additionally, a reel pipelaying vessel and the pipe being
laid are subjected to a number of hydrostatic and h~drodynamic
forces during a pipelaying operation which must be taken into
account and compensated for in order to properly lay pipe so that
it meets the customer and regulatory body requirements. Such
forces include the effects of wind, waves, and current on the
vessel due to its heave, pitch, and roll characteristics.
Self-propelled reel pipelaying ships, including for
example, Apache-type vessles described in the aforesaid -prior
related Santa Fe inventions , have certain distinct advantages
over non-self-propelled pipelaying vessels, either of the reel
pipelaying type or of the -stove piping-- type; the latter technique
involved joining 40 or 80 foot lengths of pipe end to end and mov-
ing the vessel ahead an equivalent distance after each such turn-
ing to thereby effectively pay out pipe from the vessel. Known
commercial vessels employing the -stove pipe-- techni~ue have
generally been vessels which maintain their operational position
by setting out anchors. Auxiliary support ves~els set out the
barge anchors in specified patterns and the barge moves along
the pipeline right of way by hauling in on some anchors and
paying out line on other anchors. In relatively shallow water
(up to about 200 feet deep), sufficient anchor line can be paid
out to allow the barge to move along the right of way 1,500 to
2,500 feet before the anchors must be raised and a new pattern
set. The distance which a stove piping barge can move along the
right of way on a single anchor set pattern decrea~ses as ~ater
depth increases. It is apparent that the limited forward movement
permitted by this anchor setting technique is not at all suitable
for economical reel pipe laying operations.
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Although towed reel pipelaying barges have been
found to be quite adequate for the relatively calm waters of
the Gulf of Mexico offshore of the United States coastline,
they have certain inherent limitations which make them un-
suitable for use in relatively rough waters, such as are found
in the North 5ea or off the coast of South America or Australia.
One of the principal built-in limitations of a towed barge
system resides in the towing connection itself. Unlike a self-
propelled ship, in which the motive source is effectively
connected directly and rigidly to the pipeline (through the
reel), the connection between the towing vessel tmotive source)
and the towed barge (effectively including the pipeline end)
is a flexible one which introduces an additional unpredictable
and uncontrollable factor into the overall system. In rough
water, the barge may be subjected to irregular pulling action
as the tow line tightens or sags with relative movement between
the tug and barge. This may cause the pipeline tension to
exhibit sudden increases and/or decreases in magnitude which
can neither be predicted nor controlled eEectively by the
barge operator(s).
A self-propelled reel type pipelaying ship requires
neither anchors nor tugs as the motive source. Therefore,
compared to stove-piping type barges as described above, a
self-propelled reel pipelaying ship is able to move continuously
down the right of way, stopping only when necessary, for
example, to install anodes as required by the customer and/or
to perform other operations on the pipe, such as coating repair,
etc. Compared to towed reel barges, the self-propelled reel
ship has a significant advantage in that the motive source of
the reel ship can, for practical purposes, be considered to be
fixed with the reel and pipeline end, thereby eliminating
relative movements therebetween due to weather related factors,
as noted above.
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Commercial and practical limitations efec~i~ely
restrict the operating capability of a towed reel barge. One
of the principal requirements in laying pipelines offshore from
a surface vessel is that, in general, adequate tension must be
maintained on the pipe at all significant times. This is
necessary to prevent the sag bend from exceeding certain pre-
determined tolerance limits. The sag bend region of the
pipeline occurs at or near the sea bottom where the pipe curves
back to the horizontal plane as it comes to rest on the sea
bottom. The point at which the pipe touches the bottom is
called the Touchdown Point (TDP). It is important that the
radius of the sag bend curve be kept above the minimum per-
missible radius to which the pipe may be bent without exceeding
elasticity limits in accord with customer requirements. The
pipeline should be kept under sufficient tension at all
significant times during the laying operation to maintain the
proper profile in the pipe between the pipe departure point
from the vessel and the sea bottom on which the pipe rests, and,
in particular, to prevent the sag bend radius from decreasing
to below its allowable minimum.
It has been found that the relationship between the
departure or exit angle (also sometimes called pipe entry angle
into the water) and the required tension can be expressed as an
essentially linear logarithmic relation where the pipe profile
is catenary-shaped in its unsupported length between the vessel
and the sea bottom, substantially as represented in Fig. l; i.e.,
for a given size and grade of pipe and a given lay depth along
the right of way, the tension required to hold the sag bend
radius above the allowable minimum decreases as the departure
angle of the pipe into the water increases. For example, it
is necessary to hold about 250,000 lbs. of tension (250 Kips,
where Kips equals thousands of pounds) on a pipe having an
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outside diameter of 10 3/4" and 3/4" wall thickness laid in
a water depth of 500 eet, if the pipe exit angle is set at
about 26, in order to maintain the sag bend radius above the
allowable minimum, at an exit angle of 58, the same conditions
require ~ tension of about 60 Kips. ~These examplary pipe size
and water depth conditions are typical for North Sea operations~
All known commercial reel type pipelaying barges ko
date have been designated to operate at a relatively fixed
departure angle of between about 6 and 12 (relative to a
nomi~al horizontal plane representing the water surface). At
this shallow exit angle, the tension required to maintain a
catenary shaped pipe profile or deep water (deeper than about
1,000 feet) is typically greater than can be generated by the
barge and tug. The pipe therefore assumes an ~r~ shape (with two
inflection points) in its unsupported length between the barge and
the sea bottom. The ~irst point of inflection, or "overbend",
occurs near the surace as the weight of the pipe imparts a down-
ward force vector to the pipe, forcing it to curve downwardly; the
second point of inflection occurs at the sag bend.
Referring to Figure 1, a feature of "Apache-type" special
reel pipelaying ships isthe adjustable pipe carrying ramp assembly
40 pivotably mounted (generally atthe skern~ to the dec]co~ the
vessel 10, aft o~ the reel 20. The vessel also comprises main propul-
sion propellers 12, one or more forward lateral thrusters 126 and
one or more stern lateral thruskers 122. (Throughout this dis-
closure, reference is made to the main propellers as providing
the requisite forward thrust; it is apparent,however, that other
suitable drive means could be provided to generate the neces-
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2 8 5 9 h''
sary for~ard thxust and the re~erence to "propellers" throughoutthis disclosure is intended to encompass other such suitable
drive means, except where otherwise speci~ically noted.~
Special pipe handling equipment, which may include, for exarnple,
the adjustable radius control member, adjustable straightener
tracks, tensioner tracks, pipe clamping assemblies, guide
roller assemblies, and pipe angle measuring assembly, is
advantageously mounted to the ramp assembly 40.
~ n adiustable ramp assembly o~ this type has not
heretofore been incorporated into any ~kno~n commercial offshore
reel pipelaying vessel, specifically including the supply boat
portable reel s~stem used off the coast of Australia, the two
reel pipelaying towed barges o~ned and used by Santa Fe and/or
Santa Fe's predecessors-in-interest since about 1961 and t~70
competitive reel ~ipelaying barges, one used for a short time
in 1972 or 1973 and the other currently in use in the Gulf of
llexico off the United States coast.
The Apache-type reel pipelaying vessel di~fers ~rom
prior commercial reel pipelaying barges in its abi}ity to
discharge pipe i.nto the ~ater at any desired angle ~ithin its
operating rang~ o~ bet~J~en about 15~ and 65~, preferably between about
18 and 60. The adjustable ramp assembly of an Apache-type reel ship
permits the angle ofentry of the pipe into the water to be preset and
maintained during a pipe lay operation; the ramp assembly guides
the pipe as it enters the water at the preset exit angle As
noted abo~e, all prior kno~m commercial reel pipelaying barges i-
ha~e operated a~ a Cixed, non-variable exit an~le of between about
6 and 12. The adjustable exit angle feature of the Apache-type
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vessel enables it to handle a wider ranye of pipe sizes in a
greater range o~ water depths than was herekoore possible ~ith
fixed low exi~ angle reel pipelaying barges
One of the advantages of an Apache-type adjustable
ra~p assembly for setting the pipe exit angle is the virtual
elimination of the overbend region ~i.e~, the bend region
occurring as the pipe translates do~Jnwardly fromthe relatively hori
zontal plane of the barge ~oward the sea bed in the relatively vertical
planeof the catenar~. Advantageously and preferably, the rarnp angle and
tension are set so that downstream o~ the straightener/tensioner
apparatus, the pipe will be unsupported; thus, pipe exiting the
straightener mechanism and traveling along the ramp assem~ly
will already be in its nominal catenary configuration before and as it
en~ers the water. Preferably, as the pipë moves through the
straightener mechanism toward the water, all or substantially
all of the curvature imparted to the pipe by the reel an~ other
pipe handling elements is removed so that pipe exi~ing fro~ the
straightener mechanism has substantially zero residual stress
and zero residual bending moments.
By inikially settiny the ramp angle and nominal
pipeline tension to virtuall~ eliminate the overbend as a factor
in aetc-~rmining and controlling the final residual pipeline
characteristics, the sag bend (i.e., the bend occurring in
the translation o~ the pipe fxom the vertical to the hori-
zontal plane on the sea bottom) becomes a critical factor in
the control of the pipe as it i5 laid. The sag bend is con-
trolled, at least in part, as a function of the tension main-
tained on the pipe by the functional elements of the pipelaying
vessel, includiny the reel, straightener /tensioner elements
~essel drive assembly, etc. Controllea tension is im~arte~
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to the pipe by (1) the reel through the reel drive mechanism
operating as a dynamic brake, (2) ~he main vessel drive thrust
acting through the vessel main propellers and/or the lakeral
thruster assemblies, and (3} the tensioner assembly, which may
or may not be used, through a regulated tensioning force
established at the beginning of a lay operation and generally
maintained throughout the lay operation.
The desirea pipelaying tension and the desired entry
angle of the pipe into the water are preferably determined on
the basis of information supplied by the pipeline designer.
Such information from the pipeline designer (or customer -
pipeline owner) includes (1) the size of the pipe, including
internal pipe diameter and wall thickness, (2) the type or grade
of pipe, including such information as the pipe material and
minimum yield strength, ~3) maximum allowable stress, strain
and residual tension, and (4) water depth along the pipeline
right of way. An optimum nominal tension and lay angle can
be determined f~om these parameters.
One of the crlteria which has been developed for laying
pipe with an Apache-type vessel is that the maximum allowable
working stress, due to pipelaying operation, in the unsupported
length of pipe between the vessel and the sea bottom should not
be greater than abouk 85% of the minimum yield strength of the
pipe. It is also desirable and preferable to minimize the
tension imparted to the pipe by the vessel while maintaining
operating conditions such that the maximum allowable stress
limit and the maximum allowable residual tension in the pipeline
are not exceeded. This may be accomplished by setting the
ramp assembly angle (and thus the pipe entry angle into the
water) in corljunction with nominal pipe tension such that the
tightest sag bend radius will be achieved wi~hout exceeding the
above-noted stress and residual tensiorl limit.
` . ` ~. ~7~8~9 -~ .
The ramp assembly angle tand thus the pipe entry angle
into the water~ is set at the beginning of the pipelayin~ opera-
tion ~nd ls normally noi changed during the entire lay operation.
I~ is possible to alter the ramp angle during a pipelaying opera-
tion, for example, to account for (appreciable) changes in water
depth. During the pipe-laying operation, control of the pipe as
it ïs being laid is maintained by controlling the tension in the
pipe. Such control is normally achieved through adjustments in
the reel torque and/or tensioner setting and/or in the vessel
forward and/or lateral thrust.
Prior to the start of the pipe~aying operation, the
ramp angle and nominal pipe tension level are established on the
basis of input from the pipeline designer. Also, in the case of
an Apache-type vessel wherein the straightener tracks and the
radius controller section are independently adjustable relative
to each other, the radius controller and the straighteners are
set at predetermined positions rela-tive to each other and to the
ramp assembly aft o the straighteners so that the (preferably
unsupported length of) pipe between the straightener assembly and
at end of the ramp assembly (at the skern guide roller assembly)
will have little or no residual strain between the stra.ightener
assen~ly exit point and the aft end of the ramp assembly.
Und~r certain operating conditions, the "flexi.ble"
towing connection between a reel barge and its tug will not be
adequate to maintain the necessary continuous tension on the
pipeline as it is being laid~ The tug moves independently of
the barge due to wave action. This means that the motive source
which pxovides the forward thrust necessary to maintain tension
on the pipeline is susceptible to uncontrolled variations
relative to the barge and thus to the pipe. Limited excursions
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of this type may be acceptable ~or some sizes of pipe and
some sea conditions. However, the range of permitted
excursions is relatively small and decreases, particularly
with increasing pipe size and increasingly rough sea conditions.
A self-propelled reel ship has the advantage that the
forward thrust producing motive force can be considered to be
coupled directly to the pipe end on board the ship so that
relative movement between the motive source and the pipe end
connected to the vessel is reduced essentially to zero. Further,
external forces produced by waves, winds, current, etc. act on
the pipe and motive source together and at the same time.
Since the mo*ive source and pipe end are substantially directly
coupled, the pipe is more directly responsive and more rapidly
responsive to changes in thrust. The self-propelled ship can
therefore operate in a greater range of sea conditions, and
particularly adverse sea conditions, than can a towed barge.
On a reel pipelaying vessel, it is not possible to
; measure the pipeline tension directl~. There are, however,
several ways to measure the tension indirectly. One such way
is to measure the forward thru~t of the vessel, which is
directly proportional to the tension on the pipe. Increasing
or decreasing the vessel thrust will produce a corresponding
proportional increase or decrease in the tension on the pipe-
line. This can be done by measuring the main propeller shaft
torque or by measuring the force on a thrust bearing against
which the propeller shaft acts.
A second method is to measure the drive motor force
acting on the reel. Neglecting the components of tension pro-
duced primarily by the straightener assembly (and tensioner,
when used), the force exerted by the reel drive motors is
directly proportional to the tension in the pipe; thus, an
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2 8 ~ 9
incre~se cr ~ecrease in the drive motor ~orce produces a
corres~onding increase or decrea~e in the pineline -tension~
The reel motor drive force may be ~,easured by, e.g., load
cells between the ~otor/reel mechanical connection.'
A third practical ~ay to r;leasure pi~eline t~nsion is
basèd on ~easurement of the exit angle of the pipe from the
vessel. It is advantageous and preferable that the pipe angle
be measured ~lith respect both to the hori7-on and to the ramp
angle; the latter measurement is particularly helpful where
the pi~e passes through an exit window defined by a stern
~ui~e roller assen~ly, such as is used on Apache-type vessels.
Figures 2A-C are diagrammatic representations of the
pipelaying ~essel 10, ramp assembly 40 (set at a nominal lay
an~le of about 30 degrees), the stern gui~e roller assembly
54 defining the exit window, and the pipe P. ~igure 2A shows
the relationship between t~le ramp assembly and the pipe when
'the vessel i5 substantially flat in ~he water so that the entry
an~le Al of the pipe into the water (relat.ive to ~ nominal
horizontal plane or axis, such as the horizon) is substantiall~
the sc~ne as the predetermined ramp angle R; Fiyure 2B sho~s the
same relationship when the vessel .is ~itched bow up at an
angle D2 and the pipe P2 enter.s the water at an angle A2; and
Fi~ure 2C shows the same relationship when the vessel is
pitche~ bow down at an angle D3 and pipe P3 enters the water
at an angle ~3. The exit point of the pipe fro~ the straightener/
tensioner asse~bly is designated by refe-rence SE. The pipe is
essentially in fixed relation to the ramp assembly'and the
vessel at point SE. Preferably and aavantageously, sufficient
and ade~uate tension is maintained on the pipe P during the
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laying operation so that the pipe travels in a pa~h substantially
parallel to the ramp and through the guide ~oller assembly 54
substantially unsupported between straightener exit SE and the
*ouchdown point TDP on the sea bottom. Also advantageously and
preferably control of pipelaying operation is maintained so that
angles Al, A2, A3 will be essentially equal.
The stern guide roller assembly provides a pipe
excursion window between the upper and lower guide rollers for
pipe excursion relative to the vessel as a result Gf vessel motion
due to wave action. In one commercial embodiment, the distance
between straightener exit SE and stern guide roller assembly 54
is approximately 45 feet; the distance between the upper and
lower stern guide rollers is approximately four feet. This
permits an angular excursion of the pipe between straightener
exit SE and stern guide roller assembly 54 in a range from about
4.7 for 4 inch OD pipe to about 3.2 for 18 inch OD pipe; that
is, the pipe can move through this range without ~eing subject
to bending moments by the stern guide rollers. Reerring to
Figs. 2B and 2C, angles F2 and F3, respectively, represent the
excursion above and below the nominal centerline of the plpeline
P when it is tensioned to be parallel to the plane of the ramp
assembly ~0.
During the pipelaying operation, the vessel ~oves
forwaxd through the water as a function o~ the thurst generated
by the main vessel drive, reacting against pipe tensioning
forces produced by the pipe handling equipment, including reel
dynamic braking forces, straightener, tensioner, etc. Changes
in or modifications to the rate of forward motion of the vessel,
and thus the rate at which pipe is unspooled from the reel 20
and paid out into the water, may be controlled by adjusting the
dynamic ~raking force exerted by the reel drive mechanism
and/or the amount of thrust generated by the main propellers.
A typical lay rate, i.e., the rate at which pipe is paid
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out fxom the vessel during a lay operation! would be in the
range of 75 - 150 feet per minute. It has been foun~ ~o be
preferable to maintain the forwar~ thrust relatively constant
and to control pipe tension changes through adjustments to
the reel dynamic braking force. Due to the large mass of the
xeel and pipe, it is not possible to effect instantaneous
chan~es in the pay out rate.
As the vessel pitches during a laying operation,
the stern, with the ramp and other related pipe handling
equipment, moves up and down inthe water. The pipe, paid
out ~rom the straightener exit SE at a predetermined rate
which, as noted, cannot be changed instantaneously, also
moves up and down with the vessel. The pipe is subjected to
inertial effects through its underwater suspended length and
on-bottom friction. Due to such inertial effects on the pipe,
the portion of the pipe downstream of straightener exit SE
does not necessarily move with the vessel so that the total
pipe excursion relative to the ramp may be greater than the
stern guide e~cursion window limits. Under conditions where
the bow pitches up by an angle D2, the pipe may be bent around
the upper stern guide roller, as shown in Fig. 2B. Similarly,
when the bow o~ the vessel pitches downwards by an angle D3,
the pipe may be bent aroundthe lower stern guide roller assembly,
as shown in Eig. 2C.
In a commercial embodiment of an Apache-type vessel,
an angle measuring device measures the pipe angle downstream of
the stern guide assembly relative to the ramp assembly 40 and
relative to the horizon. One such angle measuring device is
shown and described in aforesaid British Application Serial No.
791591~. An apparatus for this purpose is manufactured by
Interstate Electronics, Inc.
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In Fig. 2B, refexence E2 ~epresents th~ m~asured
angle o~ e~cursion of the pipe P2 relative ~o the ramp assembly
40 un~er the condition where the ve~sel pitches up by th~ bow
at an angle D~ ~t t]liS pitch angle, the effec~ive exi~ an~le
G2 becomes R (rarnp angle) plus D2 (pitch angle). ~s noted
earlier, it has been found that pipe tension and exit an~le are
inversely proportional; therefore, as the effective exit angle
G2 increases, the tension applied to the pipeline should be
decreased in order to maintain the pipe profile within
acceptable limits. However, since, due to reel and pipe
inertia and other factors, the tension applied to the pipe
cannot be adjusted to directly follow the pitching of the
vessel, the efective tension on the pipe is increased and a
pi~e profile such as shown in Fig 2B results. Under sufficiently
severe conditions of vessel ~itchJ the ~ipe P2 undergoes a
relàtively large excursion so that the pipe excursion angle E2
exceeds the gui~e assembly windcw excursion limit angle F2
In such cases, ~he ~ipe undergoes a bendiny mo~ent about the
upper stern guide xoller. I khis bending moment exceeds the
elastic limit of the pipe, the pipe will under~o plastic bendiny
and will thu~ retain a residual curvature due to such plastic
bending ~7hen it res~s on the bottom.
When the bow of the vessel pitches downward, e.g.,
at an angle D3, a pipe profile such as sho~n in Fig. 2C may
result. In ~his case, the effective exit angle G3 becomos R
tramp angle) minus D3 (pitch angle); in this case, the effective
exit angle is s~aller than the nominal preset ramp angle. In
order to ~aintain a ~roper pipe profile, in bow down pikch
condition, the tension on the pipe should be increased an amount
"C-f~
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. . . . .... .....
2~5~ ~i
sufficient to compensate f~r the decrease in effective e~it
angle. ~lowever, for reasons noted a~ove, it is not Dossi~le to
instant~neously change the tension imparted to the pipe by the
vessel, and particularly by the reel. Therefore, the pipe
undergoes an excursion E3 which may be greater than the excursion
F3 ~er~itted by the stern suide ~.indo~ limits. Under such
conditions, the pipe undergoes a bending mo~ent abou~ the lower
stern guide roller; if this bending moment exceeds the elastic
limit, the ~ipe undergoes plastic bencling and will retain a
residual curvature when it is laid.
The angle measuring device mèasures excursion E2 and
E3 to thereby generate an indication of excessive bending o~ the
pipe on the ramp. ~easurernent of excursion E2 or E3 is parti-
cularly important as an indicator that the pipe is over~tensioned
or undertensioned, irrespective of the itching of the vessel.
~en the vessel ;s pitching, excursions E2 and E3 would be
expected to be relatively short-lived. Measurement of such
short-lived excursions would not provide an accurate indica~
tion of over- or under-tensioning.
A continuous measurement of excursion E2 gxeater than
limit ~2,or meclsured excursions E2 greater than F2 ~hich occur
a significant percent of the ti~e (e.g., greater than the
pitching period of the vessel), even ~hougn such excursions
are not continuous, indicate to the operator that the pipe ls
being held under excessive tension. The operator can then
acljust tl~e reeldynamic braking force to decrease the tension on the
pipe until the angle measuring device measures an excursion E2
less than excursion F2, neglectin~ short-lived excursions due
to vessel pitching. Correspondingly, when the angle measuring
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2 8 ~ 9
device measures an excursion E3 continuously c3rea-ter th~n
excursion limit F3, or greater than F3 a significank percent
oE the ti~e (e.g., ~reater than th~ pitching period of the
v~ssel), even though not continuous, khese constitute indica-
tions that the ~i~e is being held under insuficient tension.
The operator can then increase the tension on the pipe until
the measured excursion E3 becomes less than excursion limit
F3, again neglecting short~lived excursions due to vessel
pitching.
~ Jhen the vessel is pitching, d~e, for example, to sea
conditions, measurins excursions E2 and E3 may produce crroneous
indications o pi~e tension and may make it difficult, if not
practically impossibIe, for the operator to maintain proper tension
ontthe pipe. Therefore, the angle measuring device also measures
the actual exit angle ~ of the pipe (relative to the horizon or
mean water line). Such measurement provides a more accurate
indication of the actual pipe entry angle into the water so that
under varying sea conditions, ~7ith the vessel pitching continu-
ously, the operator can maintain a direct reading o~ the actual
pipe en~ry angle. The operator is then able to maintain the
proper reel dynamic breaking force and provide necessaxy compensation
adjustments based on the ac-tual pipe angle relative to the fi~:ed
horizon, as distinguished from angles measured relative to the
moving and pitching vessel.
The pipe laying operation is also affected by the
fact that the ~ipe traverses across the beam of the vessel as
it is unspooled. This produces a turning moment tending to pull
the vessel oEf course~ This turning moment increases to a
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2 ~ L~
maximu~ at the end of transverse -travel of th~ ~amp assembly,
decreases to zero ~he~ the ramp assembly (and pipeline path) is
aligned ~ith the vessel centerline, and increases to a m~xi~um
in ~he opposite ~irection as the ram2 assen~ly continues moving
to the extreme opposite end o~ its transverse travel.
The turning moment can be quite larye co~pared to the
fonJard thrust generated by the main propellers. E'or example,
in one commercial e~bodi~ent, the ram~ assembly has an athwart-
ship ~ovement range of 21.5 feet. The sha~ts of the main pro-
pellers are located about 20 feet to either side of the vessel
centexline; each produces a ~aximum thrust of 80 Kips. I~hen
pipe is being laid under 100 Kivs tension at an exit angle of
30 degrees, the pipe tension induced turning mo~.ent at each
extreme end of ramp asse~bly travel is on the order of 930
foot Kips. The op~osing turning moment produced by -the main
propeller on ~hat side o~erating at ~aximum thrust is abo~t
1,600 foot l~ips. It will be seen that the pipe tension induced
turning mor~ent may well be a significant percentage (58.9~ in
the example given here) of the drive induced turning ~.oment.
If the pip tension induced turning ~.oment is not compenc:~lted for,
the vessel will be pulled o~f course; this can result in the
pipe being laid out o~ the ri~ht of ~7cly~ ~hich is co~merciAlly
wlacceptable .
The pipeline induced turning mo~ent must be compensated
for in ordex to lay the pipe in a straight line along the right
of way. ~ith twin sc.rew vessels, that is, vessels Pro elled by
two sets of main drive propellers equally spaced on o~posite
sides of the longitudinal centerline o the vessel, it ~ay be ,
possible to overcome the turning ~oment introduced by the
pi~e's pipeline offset relative to the vessel center line by
incre~sing thrust on the pro.eller ~ocated on that side of the
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iffC
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.. .. ,. ,.. .; ,......... . .
~ ~Ç~; .
1 1 7 ~
vessel and /or decreasing thrust on the opposite side main
drive propeller. This has cer~ain inherent disadvantages
because the pipeline induced turning moment continually
varies as the pipeline shifts laterally across the vessel as
it is unspooled.
To compensate for this varying turning moment using
the main drive propellers requires that the thrust of the
drive propellers be varried accordingly, while at the same
time taking into account that the forward component of thrust
must be maintained relatively constant in order to maintain
the proper amount of tension on the pipe at all pertinent
times during the pipelaying operation. Under certain condi-
tions of pipeline tension and forward thrust, the system will
not be able to generate sufficient additional thxust to com-
pensate for the pipeline induced turning moment, especially
when the ramp assembly and pipeline are ak an e~treme end of
transverse displacement.
A second and potentially more commercially preferable
way to compensate for the turning moment introduced by the
pipeline lateral travel comprises utilizing Eorward and aft
lateral thrusters. Examples of such thrusters are shown in
the aforesaid prior related Santa Fe inventions. Also refer
ring to Fig. 3 hereof, an aft thruster tunnel 120 houses the
aft thruster 122; a forward thruster tunnel 124 houses the
forward thruster 126.
The thrusters 122 and 126 can be operated either
manually or automatically in conjunction with, e.g., a
computer operated guidance system, to generate turning moments
which react against the pipeline induced turning moments. The
pipeline introduces a turning moment about the intersection of
the vessel longitudinal axis and re~l shaft axis; the magnitude
of the pipeline induced turning moment is a fun,ctionof the
tension on the pipeline and the pipeline offset
~L 172859 ~)
from the vessel's centerline. ~he vessel ~hrusters yenerate
turning moments about the'aforesaid intersec~i.on a~ the vcssel's
centerline and reel shaft axis ~hlch react against the pipeline
turning mo~ent to maintain the vessel on its pro~er course.
Consideration must also be given to the fact that a
turning moment occurs bet~7een the for~qard vessel thrus'ter(s)
and the pipeline touchdo~ point on the sea botto~. Therefore,
in addition to rotating the vessel abou~ the centerline in~er-
section points, the enLire vessel ~ust be rotated abou~c the
touch~o~n point to maintain the vessel on and parallel to the
right of ~ay. ~his may be accomplished by increasing the thrust
genera~ed by the forward thruster(s) relative to tll2 opoositely
reacting force generated by the aft thruster(s).
~ The amount of thrust required varies as a function of
a num~er of factors, including the lateral position of the
pipeline relative to the vessel's longitudinal axis, the flistance
between the vessel and the touchdot~n ~oint, the pipeline tension
and pipe exit angle. In general, the for~7ard thruster ~ill be
controlled to generate a thrust component Tl in one lateral
direction relative to the vessells longitudinal centerline.
The aft thruster ~ill be controlled to generate a thrust
component T2 i~ the opposite lateral direction relative to the
vessel's longitudlnal centerline. Advantageously and pre~erab]y,
Tl is maintained greater than T2; together, Tl + T2 produce a
turning moment which reacts the pipeline induced turning moment.
The thrus~ generated by the forward thruster there~ore comprises
the additive components of the thrust necessary to react the
pipeline induced turning moment about the vessel axis and the
pipeline induced turning moment about the touchdo~n point
pivot axis. The aft or rear thrus-ter need only react the
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2 8 S ~
pipeline induced turning moment about the vessel axis. The
for~axd thruster therefore imparts a relatively greater l~teral
~hrust co~ponent than the rear thruster to o~JerCome the pipeline
induced turning momen-ts about the vessel piVQt aY~is and about
the touchdown point ~ivot axis to thereby maintain the vessel
on course along the ri~ht of way.
The invention may be e~bodied in other specific
~orms ~Ji~hout departing from the spixit or essential charac~
teristics thereo. The embodiment described above is therefore
to be considered in all respects as illnstrative and not
restrictive, the scope of the invention being indicated by the
herea~er appended claims rather than by the ~oregoing
description, and all changes which come within the meaning
and range of equivalency of the claims are therefore intellded
to be e~brace~ therein.
J
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