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