Note: Descriptions are shown in the official language in which they were submitted.
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REEL PIPE LAYING AND RESPOOLING METHODS
, BACKGROUND OF THE INVENTION
~ his 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 alonq
the pipeline route. Still more particularly, the invention
relates to techniques associated with the laying of such pipe- ;
lines, including attachment of sacrificial anodes, effecting
repairs to the coating and/or Dipe during the laying process,
and with respooling the pipeline, either as part of
the primary pipelaying operation or as part of a pipeline
salvage operation.
The methods and techniques described herein are appli-
cable to several types of reel pipelaying vessels. Suitable
vessels which would be expected to use the methods and techniques
described herein include towed barges, converted drill ships
and/or ore carriers, and specially designed and constructed
self-propelled reel type pipelaying ships. Towed barges
capable of utilizing the methods and techniques described
hexe have included barges owned and operated by Santa Fe
International Corporation (hereafter "Santa Fe") and/or Santa
Fe's predecessors-in-interest
The first known commercial reel type pipelaying barge,
called the U-303, was operated by AquaticContractors and Engineers,
Inc. a predecessor-in-interest to Santa Fe.~ The U-303 utilized
a large vertical axis reel, permanently mounted on a barge and
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having horizontally oriented flanges (generally referred to
in the trade as a "horizontal reelnl. A combined straightener- -
level winder was employed for spooling pipe onto the reel and
for straightening pipe as it was unspooled. The U-303 first
laid ~ipe commercially in September,1961, in the Gulf of
Meixco off the coast of Louisiana and was used successfully
during the 1960's to lay several million linear feet of pipe
of up to 6" diameter. The U-303 reel pipelaying barge is
described in British Patent No. 1,077,331 ~and in corresponding
U.S. Patent Nos. 3,237,438 issued March 1, 1966 to Prosper
A. Tesson and 3,372,461 issued March 12, 1968 to Prosper A.
Tesson.
The successor to the U-303, currently being operated
in the-Gulf of Mexico bySanta Fe and known in the trade as the
nChickasaw", was built and operated by Fluor Ocean Services,
Inc., a successor-in-interest to Aquatic Contractors and Engi-
neers, Inc., and a predecessor-in-interest to Santa Fe. The
~Chickasaw" also utilizes a large horizontal reel, permanently
mounted to the barge such that it is not readily movable from
one carrier vessel to another. Various aspects of the "Chickasaw~
are described in the following patents:
British Patent No. 1,312,592 and corresponding U.S.
Patent Nos.: 3,630,461, issued December 2R, 1971 to Daniel E.
Sugasti, Larry R. Russell, and Fred W. Schaejbe; 3,680,342,
issued August 1, 1972 to James D. Mott and Richard B. Feazle;
and 3,712,100 issued January 23, 1973 to Joe W. Key and Larry
R. Russell; and
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S88~
British Patent No. 1,312,593 and corresponding U.5.
Patent No. 3,641,778, issued Ee~ y ',, 1972 to Robert G.
Gibson.
A portable pipelaying system designed and built by
Santa Fe for use on small supply boat type vessels for laying
; 5 small diameter pipelines (up to 4" nominal diameter) has been in com-
me~rcial use off the coast of Australia since about July, 1978; this
portable pipelaying system is described in U.S. Patent
No. 4,260,287, issued April 7, 1981 from U.S. Patent
Application No. 909,260, filed May 24, 1978 in the names of
Stanley T. Uyeda and John H Cha, as inventors. -
Santa Fe has developed and built a self-propelled reel
ship known in the industry as "Apache" which began commercial
pipelaying operations in August, 1979. Features of Apache-type
` pipelaying ships are described in the following British and
U.S. patent and applications:
British Patent No. 1,507,959 and corresponding U.S.
` Patent No. 3,982,402, issued September 28, 1976, in the names
of Alexander Craig Lang and Peter Alan Lunde, as inventors;
.~ , .
U.S. Patent No. 4,230,421, issued October 28, 1980
from U.S. Patent Application No. 903,180, filed
May 5, 1978 in the names of Charles N. Springett, Dan Abramovich,
Stanley T. Uyeda and E. John Radu, as inventors;
U.S. Patent N. 4,269,540, issued May 26, 1981
from.U.S. Patent Application No. 903,181, filed
May 5, 1978 and 35,216, filed May 2, 1979 as a Continuation-
in-Part of said Serial No. 903,181, each in the names of Stanley
T. Uyeda, E. John Radu, William J. Talbot, Jr. and Norman
~eldman, as inventors.
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The present disclosure (and inventive subject matter
described and claimed herein) and the abo~e-listed British and
U~S. patents are all owned by Santa Fe.
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SUMMARY OF THE INVENTION
.
The present invention was developed to satisfy the
requirements of commercial pipelaying operations. More
parti~ularly, the invention was developed to meet the require-
ments of pipeline owners and various governmental and industry-
established regulatory bo~ies or agencies while at the same
time making the reel pipelaying technique commercially practical,
economical and competitive with other pipe~laying methods, such as
the laying technique known in the trade as "stove-piping." This
invention also has particularly advantageous utility in such cases
where it is desired to remove a pipeline from one location and relay
it elsewhere. Reuse of the same pipeline can be environmentally
advantageous; e.g., natural resources are conserved, pipe is not
- left on the sea bottom as "trash", etc. Such reuse can also
15 have economic advantages. The cost of constructing a pipeline
of a desired length is generally substantially greater than
the cost of respooling a previously laid pipeline of similar
length. Therefore the effective cost of the "new" pipeline
is substantiallly reduced in the case where a pipeline is
reused.
In one aspect, the invention is directed to a method
of intermittently stopping and restarting the laying of pipe
offshore from a reel pipelaying vessel having a reel for
spooling relatively inflexiblepipe thereon, pipe working
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and handling means for straightening the pipe as it is unspooled
from the reel, vessel motive source means for imparting a for-
ward thrust to the vessel, and a reel drive mechanism associated
with the reel for imparting at least a dynamic braking force
to the reel in an unspooling mode, said method comprising the
steps of:
a. stopping the forward motion of the vessel in the
direction of pipelaying by (1) gradually r~ducing the forward
thrust imparted to the vessel by the vessel motive source, and
(2) concurrently gradually increasing the dynamic braking
force imparted to the reel by the reel drive mechanism (3) to
cause the reel to decrease its rotational rate toward zero
while continuing to unspool pipe and to thereby maintain
sufficient tension in the pipeline to hold the pipeline
in a desired profile;
b. locking the reelagainst further rotation in the
direction of unspooling when the vessel has stopped its
forward motion;
c. thereafter controlling the forward thrust of the
vessel motive source to maintain the vessel in a station
keeping mode and to maintain the pipeline under said suf~i-
cient tension to hold the desired pipe profile while the
vessel is in said station keeping mode;
d. performing any necessary operations on the pipe
and/or coating while the vessel is in said station keeping
~25 mode;
e. thereafter resuming forward motion of the vessel
in the pipelaying direction by (1) disengaging the means
locking the reelagainSt the rotation in the unspooling
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~ .rection, (~2) concurrently substantially increasing
the ~dynamic braking force imparted to the reel by the
reel drive méchanism such that the dynamic braking
force exceeds any increases in the forward thrust af the
; vessel's motive source at least up to the pipelaying
thrust level required to move the vessel in the forward
- direction at a desired laying speed, (3) gradually
increasing the forward thrust imparted to the vessel
by the vessel motive source up to said pipelaying
thrust level, and (4) thereafter gradually reducing the
~;~10 dynamic braking force of the reel drive mechanism at least
~,, .
until the forward thrust imparted to the vessel by the
:' vessel motive source exceeds the dynamic braking force of
: the reel drive mechanism and the vessel begins to move
forward.
~15 In a further aspect, the invention concerns a
i= method of intermittently stopping the laying of pipe off-
"~ i
.~. shore from a reel pipelaying vessel, thereafter respopling
a portion of the laid pipeline, and thereafter resuming the
laying of pipe, said reel pipelaying vessel having a reel for
`.2~ spooling relatively inflexible pipe thereon, pipe working and
handling means for straightening the pipe as it is unspooled
from the reel, vessel motive source means for imparting a
forward thrust to the vessel, and a reel drive mechanism
.~~ associated with the reel for alternately driving the reel
~25 in a spooling mode and imparting a dynamic braking force to
the reel in an unspooling mode, said method comprising the steps
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. a; stopping the forward motion of the vessel in the
direction of pipelaying by (1) gradually reducing the forward
thrust imparted to the vessel by the vessel motive
. source, and (2) concurrently gradually increasing
11
~ 5 the dynamic braking force imparted to the reel by
: the reel drive mechanism, t3) to cause the reel
to decrease its rotational rate toward zero while
. continuing to unspool pipe and to thereby maintain
sufficient tension in the pipeline to hold the pipe-
line in a desired profile;
b. locking the reel against further rotation in the
. directi~.n of unspooling when the vessel has stopped its
.; forward motion;
:~ c. thereafter controlling the forward thrust of the
~. 15 vessel motive source to maintain the vessel in a station
: keeping mode and to maintain the pipe~line under said
sufficient te~sion to. hold the desired pipe profile while
. the vessel is in said station keeping mode;
d. thereafter respooling at least a portion of the
`'j 20 laid pipeline by (1) switching the reel drive mechanism
. from unspooling mode to spooling mode (2) adjusting
. the straightening means for a no-load condition whereby
;~ the straightening means imparts substantially no
straightening force to the pipe passing therethrough in
the respooling direction, (3) gradually increasing drive
torque imparted to the reel by the reel drive mechanism to
` overcome the forward thrust imparted by the vessel motive
, source and begin rotating the reel in the spool-up direction,
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. ~S~thereafter respooling at least a portion of the laid
pipeline, (6) thereafter gradually reducing the reel drive
torque to a value which substantially balances the forward
thrust imparted by the vessel motive source to thereby
decrease the rate of spooling rotation of the reel toward zero,
(7) thereafter again locking the reel against further rotation
. in the unspooling direction, and (8) thereafter switching the
reel drive mechanism from spooling-up mode to unspooling
mode;
. 10 e. thereafter resumiNg forward motion of the vessel
inthQ pipelaying direction by (1) disengaging the means
~ locking the reel against the rotation in the unspooling
K:' direction, (2) concurrently substantially increasing the dynamic
Y~. braking force imparted to the reel by the reel drive mechanism
' 15 such that the dynamic braking force exceeds any increases
... .
.l in the forward thrust of .the vessel's motive source at least
up to the pipelaying thrust level required to move the vessel
in the forward direction at a desired laying speed, (3) gradually
increasing the forward thrust imparted to the vessel by the
.. 20 vessel motive source up to said pipelaying thrust level,
and (4) thereafter gradually reducing the dynamic braking
force of the reel drive mechanism at least until the forward
thrust imparted to the vessel by the vessel motive source
exceeds the dynamic braking force of the reel drive mechanism
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.. 25 and the vessel begins to move forward.
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. ~ In a still further aspect, the invention comprises a
`. method of spooling at least a portion of a previously laid
. . pipeline onto a reel pipelaying vessel, said pipelaying vessel
having a reel for spooling relatively inflexible pipe thereon,
- S pipe working and handling means for straightening the pipe
as it is unspooled from the reel, vessel motive source means
for imparting a forward thrust to the vessel, and a reel
drive mechanism associated with the reel for alternately driving
the reel in a spooling mode and imparting a dynamic braking
,t
force to the reel in an unspooling mode, said method comprisi~g the
` steps of:
a. controlling the forward thrust of the vessel motive
source to maintain the vessel in a station keeping mode and to
maintain the pipeline under said sufficient tension to hold
.... . .
:~ 15 the desired pipe profile while the vessel is in said station
::~ keeping mode;
~;~ b. locking the reel against rotation in the direction
, of unspooling when the vessel is in said station keeping mode
.: with the one end of the pipeline secured to the reel; and
c. spooling at least a portion of the pipeline onto
the reel by (1) placing the reel drive mechanism into a spooling
~;
mode, (2) adjusting the straightening means for a no-load condi-
. tion whereby the straightening means imparts substantially no
~ straiqhtening force to the pipe passing therethrough in the
::- 25 spooling direction, (3) gradually increasing drive torque im-
.~ parted to the reel by the reel drive mechanism to overcome
~,i, the forward thrust imparted by the vessel motive source and
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begin rotating the reel in the spool-up direction, (4) there-
aftèr spooling at least a portion of the laid pipeline, (6)
' thereafter gradually reducing the reel drive torque to a value
. which substantially balances the forward thrust imparted
5 by the vessel motive source to thereby decrease the rate of
spooling rotation of the reel toward zero when the desired
amount of pipeline has been spooled onto the reel, and
. (7) thereafter again locking the reel against further rotation.
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~RIEF DESC~IPTION OF THE ACCOMPANYING DRAWINGS
:.
. Figures lA-B show plan and side views of a U-303
type reel pipelaying barge, as described in British Patent
No. 1,077,331.
. 5 Figures 2A-B show plan and side views of a "Chickasaw"type reel pipelaying barge, as described in British Patent
No. 1,312,592.
.
Figure 3 shows a perspective view of an Apache-type
'~ adjustable pipe ramp assembly on which is located various
. 10 pipe handling equipment, as described in U.S. Patent
No. 4,230,421.
,
Figure 4 shows a diagrammatic side view of a reel
.; type pipelaying vessel and the profile of the pipe between
the vessel and the sea bottom
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DESCRIPTION OF PREFERRED EMBODIMENTS
In the following descriptisn of the several types
of reel pipelaying vessels shown in the drawing figures
similar elements will be designated by the same reference
numerals. All reel type pipelaying vessels have certain fea-
tures in common, including a hull 10, on the deck of which is
mounted a pipe carrying reel 12. One or ~ore pipelines of
lengths sufficient to satisfy customex job requirements may
be spooled onto reel 12 (up to the design capacity of the
reel). Although only a "horizontal!' type reel is shown in detail,
Apache-type vessels employ a "vertical" reel (i.e~, the reel
having vertically oriented flanges and mounted for rotation
.,i,.,
about a generally horizontal axis) and operate (for purposes
of the herein described invention) in substantially the same
way as horizontal reel vessels.
Pipe handling equipment is located along the pipe
path downstream of the reel in the unspooling direction. Such
pipe handling equipment may include a level winder/straightening
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; means 16 and separate tensioning means 18 or a combined level
;~ 20 wind/straightening/tensioning apparatus 20 as on Apache-~ype
vessels.
Additional features of the vessels shown in Figures 1-3
are described in greater detail in one or more of the above-
listed Santa Fe owned British and U~S. patents and applications.
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11588 f 2
~ ~Commercially laid underwater pipelines for carrying oil
or ga~ must meet certain requirements and limits set by the
- customer (pipeline owner) and/or governmental or other regu-
latory bodies. For example, the pipe, as it is being laid and
as it layson the sea bottom, should be subjected to minimal
, 5 residual stress, strain, tension, etc. This means that the
pipe as it lays on the sea bottom should be straight and have
substantially no residual curvature due t~ the spooling or
.'~
, laying processes. The "as laid" restrictions are a function
of a number of parameters developed by the pipeline designer,
1~ including the type of sea bed on which the pipe rests, the
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~j; size and grade of pipe to be used, the type, amounts, and flow
Yl rates of fluid to be carried by the pipeline, and predicted
.. `4. life span of the pipeline. Other parameters relating to, or
based on, the geometry (shape) of the pipeline during the pipe
laying operation (and taking owner supplied parameters into
considerationj are developed by the pipelaying engineers.
The pipe laying engineers must also take into considera-
tion the type of coating on the pipe to be laid in a given
operation when developlng the pipe laying guideline para-
meters for use by the operating personnel on board the pipe-
`I laying vessel.
¦ In addition, many commercial pipeline owners require
¦ that sacrificial anodes be att`ached to pipe laid offshore to
¦ inhibit the corrosive effect of the salt water on the pipe.
The pipeline owner may require that anodes be attached at
specified intervals, ranging genèrally between 300 feet and
1,000 feet.
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In the "stovepiping" method of laying pipe, anodes
are pre-attached to certain pipe sections that are then
welded in place at the on-board end of the already-laid
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pipeline portion. In the reel pipelaying method, on the other
hand, the thin film coated pipe travels through various pipe
S handling equipment which must make smooth contact with the
moving coated outer surface of the pipe to avoid tearing the
coating and to adhere as closely as possible to the customer
~. ...
developed "as-laid" requirements. Therefore the anodes must
- be attached at required intervals aft of the pipe handling
equipment, including the straightener/tensioner mechanism,
as the pipeline is being unspooled and before it leaves the
~'~ vessel. It normally takes about 3 to 5 minutes to attach a
sacrificial anode to the pipe, requiring that the vessel be
" stopped. Thus, when laying a 15,000 foot line with a require-
ment that anodes be attached every 500 feet, the vessel may
have to be stopped 30 times during the lay operation to permit
,, application of anodes.
~' The novel techniques described herein of bringing the
:" i
pipelaying vessel to a stop during a pipelaying operation
;~ 20 and thereafter restarting the vessel to continue laying pipe
., ;................................. ..
are applicable as well to situations in which the laying opera-
tion must be temporarily halted to allow repairs to be made
to the pipe and/or coating. In many cases, minor repairs to
the coating can be effected without stopping the laying opera-
tion; such minor coating repairs are made "on the fly" withthe pipeline continuously passing through one or more repair
stations before it enters the water. SometimesJ more
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,! .' ` extensive repairs must be made, which require that the laying
operation be stopped e.g., to permit a damaged pipe section
-to be cut out and the cut ends rejoined, welded and recoated.
... .
i The nature and time required for repairs of this type pre-
clude their being done "on the fly".
The anode locations are marked on the pipe and the
, . . .
; anodes are applied at a specific anode application station
on the vessel along the pipe path. Coating repair locations
- may also be premarked (e.g., when noted during the original
spooling-on operation) or be visually or electronically observable to
the operational personnel watching the pipe as it is unspooled.
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; In order to meet the above-noted pipe laying requirements,
a balance must be maintained between the tension on the pipeline
~- and the vessel's motive source, e.g., main propulsion units
on a self-propelled ship or the tug(s) powering a towed
barge. (Hereinafter, references to the vessel's-"motive source"
will be taken to mean generally the~means employed to provide
a forward thrust componen~ to move the vessel in the forward
. ,~, .
direction along the rîght-of-way). The person in charge of
the pipe laying operation (generally and hereafter in this
,:~
, disclosure called the "superintendent") must effectively control
the thrust of the motive source to (1) maintain adequate tension
on the pipe at all significant times during the laying operation
and (2) avoid excessive tension on the pipeline when the reel
; 25 is braked to a stop, as during an anode application operation.
,, .
As noted above, an important aspect of stopping the
pipelaying vessel during a laying operation is the requirement
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that, in general, adequate tension must be maintained on the pipe
at all significant times. This is necessary to prevent the "sag
bend" from exceedingcertain predetermined tolerance limits. As
i shown in Fig. 4, the "sag bend" region of the pipeline occurs at or
~ 5- near`the sea bottom where the pipe curves back to the horizontal plan~
`t,~,.' as it comes to rest on the sea bottom. The approximate point
at which the pipe touches the bottom is called the touchdown
point (TDP). The radius of the sag bend should normally be
greater than the minimum permissible radius to which the pipe
L0 may be bent without exceeding elasticity limits in accordance
i with customer requirements. The pipeline should normally be
kept under sufficient tension during the laying operation to
maintain the proper desired pipe profile between the pipe
~,~i departure point from the vessel and the sea bottom on which
,,. ,j . .
~15 the pipe rests, and, in particular, to prevent the sag bend
radius from decreasing below its allowable minimum. For
~ .s
' convenience, hereafter, a "desired pipe profile" is one in
~ . .
; which, at all significant times, the maximum allowable working
stress, due to the pipelaying operation, in the unsupported
length of pipe between the vessel and the sea bottom will not
normally be greater than about 85% of the minimum vield strength
~ of the pipe.
; Also as noted above, another important requirement
; associated with stopping the pipelaying vessel during a laying
~25 operation concerns the avoidance of imparting excess tension
.~ i
to the pipe. Controlled tension is imparted to the pipe by
one or more of (1~ the reel through the reel drive mechanism
; operating as a dynamic brake, (2) the main vessel drive thrust,
~ generated by the towing tug or on board main drive source
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- (the vessel's "motive source"), and (3) pipe handling equipment,
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; including a straightener or straightener/tensioner assembly,
~ such as described, for example, in one or more of the above-
;~ referenced prior related Santa Fe patents/applications.
. . . ~
In order to satisfy the requirement that adequate
tension be maintained on the pipe while excess tension be
avoided at all significant times during the pipelaying opera-
tion, certain procedures have been develo~ed by Santa Fe
and/or Santa Fe's predecessors-in-interest while operating the
. .
U-303 and/or "Chickasaw". These operating steps are unique
to the technique of laying pipe by the reel method, stopping
the reel laying vessel during the làying operation, and sub-
~; sequently restarting the vessel. Additional techniques described
herein pertain to retrieving pipe from the sea bottom, e.g.,
during a salvage operation.
A. Stopping the pipelaying operation.
When it is necessary to stop a pipeline laying operationin midstream, as, for example, to apply a sacrificial anode
to the pipeline, the pipelaying vessel must be brought to a
gradual stop to avoid a rapid sharp increase in tension which
20 could result in damage to the pipe and/or coating. When it
is observed that the premarked anode application location (or
- p;peline section requiring repair) is about to be unspooled,
the superintendent orders a gradual reduction in the forward
thrust of the vessel to thereby reduce the forward momentum
of the,pipelaying vessel. On a towed barge, the superinten-
dent orders the towing tug to reduce its forward thrust;
on a self-propelled vessel, the superintendent orders a reduc-
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tion in the forward thrust of the main drive engines.
Reducing the forward thrust produces a reduction in the
tension holding the pipe in its desired profile. In order
to maintain the desired pipe profile as the forward motion of
the vessel is reduced to zero, the superintendent orders a
corresponding increase inthe dynamic braking force exerted by
the reel drive mechanism. The combination of reduced forward
thrust and increased braking force serves to maintain the pipe
under adequate tension and thus hold the proper profile.
As the forward thrust of the vessel is reduced and the braking
lO force of the reel increased, the vessel continues to lay pipe
at a gradually decreasing lay rate. The superintendent must
therefore begin the stopping sequence well in advance so that
when the vessel comes to a complete stop, the portion of the
pipe to be worked on will be located at -the appropriate work
station.
As soon as the vessel comes to a stop and the lay rate
has been reduced to zero, the reel is "dogged" or locked in
a known manner to prevent Eurther unspooling of pipe. The
superintendent then orders any necessar~ adjustments to the
vessel's motive source to modify the forward thrust as required
to hold the vessel in a station-keeping position, while main-
taining a desired pipe profile. Surges due to sudden changes
in the vessel thrust or reel braking force can produce increases
in pipeline tension which exceed the maximum allowable stress/
residual tension limit requirements imposed by the pipeline
owner; an abrupt decrease in the vessel thrust or reel braking
force can produce decreases in pipeline tension and cause the
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sag bend radius (see Fig. 4) to decrease below the minimum
permissible radius for the particular pipeline, possibly even
resulting in severe damage to the pipe and/or coating due to
buckling. It is therefore desirable and preferable to avoid
r~ such surges. Adjustments to the vessel's forward thrust
S and/or reel braking forces should therefore be made gradually
~r~'.' to avoid surging actions as much as possible.
Once the reel has been dogged or locked to prevent further
rotation in the direction of unspooling, anode application or
repair operationscan be performed on the stopped pipe.
B. Respooling p_pe.
If the pipeline is to be respooled, the reel drive
mechanism is changed over from a dynamic braking mode to a
driving mode. It is important that the reel be dogged before
this switch over to maintain the pipe under adequate tension
at all significant times. The vessel motive source thrust is
held at the minimum level necessary to maintain the pipelaying
, vessel in a station-keeping attitude.
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- Before beginning to respool pipe, the straightener mechanism
is preferably retracted to a no-load (or spool up) position so
that it exerts no reverse bending forceon the pipe. Preferably,
therefore, no straightening of the pipe will take place during
; the respooling mode, since the pipe which is being respooled
had previously been straightened.
. .
The tensioner mechanism may or may not be retracted to
a no-load position, depending upon whether it is necessary or
- desirable to impart tension to the pipe in addition to or
alternatively to reel imparted tension. If a power driven
tensioner mechanism is employed, and such mechanism is to be
; used to impart tension to the pipe in a respooling mode, the
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- tension`er drive mechanism will generally be reversed to drive
the tensionér mechanism in the direction of respooling pipe
, movement at a rate which produces the desired respooling
:~ tensi~on or part thereof.
As the next step in a respooling operation, while maintain-
ing station-keeping thrust on the vessel drive source, and while the
,~ reel remains dogged,the superintendent orders a gradual increase
in power to the reel drive mechanism. The pipeline tension will
' thus be reacte'd directly to the reel drive mechanism. When
the reel drive power becomes greater than the forward thrust
of the vessel motive source so that' the reel just begins to
~- move in the spool up direction, the dog(s) holding the reel
~'! against rotation in the unspooling direction are disengaged.
Disengagemen~ of the reel dog(s) should not take place until
after the reel has begun to move in order that there will be'
;` no sudden loss of tension and/or surging in the pipeline.
; Respooling tension may in fact be greater than laying
tension and is considerably greater than the initial spooling
tension, that is the tension on the pipe as it is spooled onto
the reel at the pipeline base. To avoid damaging the pipe due
to the additional tension imparted during the respooling opera-
tion a special technique, called "crossover" spooling, is
advantageously and preferably used. One such crossover spool-
'1 ing method is described in detail in British Patent No.
1,601,730, issued from British Patent Application No.
22,080/78, filed May 24, 1978, titled "METHOD AND APPARATUS
'' FOR SPOOLING PIPE ON A REEL".
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- Pipe is respooled by generating sufficient torque
in the reel to overcome the forwardly directed station-keeping
thrust of the vessel motive source and thereby pull the pipe-
laying vessel back down the right-of-way. The forward thrust
imparted to the pipelaying vessel by the vessel motive source
should preferably not be so great as to impose a torque on
the reel that exceeds the capacity of the reel drive mechanism.
Preferably and advantageously the forward thrust of the vessel
motive source is set to be sufficiently less than the maximum
permitted reel torque so that the respooling rate may be con-
trolled by adjustments to the reel torque through the reel
drive mechanism.
Under certain conditions, the force required to respool
the pipe may be greater than the maximum torque which can
be generated in the reel by the reel drive mechanism while
holding the pipe under sufficient tension to maintain the
desired pipe profile. In order to maintain such adequate
tension on the pipe during the respooling operation, even
though the capacity of the reel drive mechanism is exceeded,
the tensioner mechanism may be used to provide the additional
force required to respool the pipe.
One such circumstance under which the total requiIed
respooling force is greater than that which can be generated
by the reel alone while maintaining the desired pipe profile
occurs when relatively large diameter pipe (e.g. 12" I.D.-
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18'' I.D.) is to be respooled from relatively deep water
~(e.g. l,OOO foot water depths or greater). At such times,
a power driven tensioner apparatus can be used to impart an
additive force component to the respooling force generated
S by the reel drive mechanism to produce the total force
- necessary to respool the pipe.
Another circumstance which may require the use of a
...;
power driven tensioner during the respooling process occurs
when the total force needed to respool the pipe can create an
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^ 10 overtension in the pipe which could result in damage to the
; pipeand~or pipe coating as the pipe is being respooled onto
the reel. Overtensioning may be avoided by using the powered
tensioner to impart a portion of the force required to raise
~ .,.
_ _ the pipe, with the balance of the required force being taken
lS up by the reel drive mechanism. In this way, the portion of
the pipe between the reel and tensioner mechanism will be
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subject to only the force imparted by the reel; the total force
needed to raise the pipe is only applied downstream of the ten-
sioner. For example, if the combination of water depth and
` 20 pipe diameter is such as to require a respooling tension of
' about 200,000 pounds, but the pipe can only safely withstand a
maximum spooling tension at the reel on the order of 150,000 pounds
~i'; (per customer~pipeline owner specification based on such factors as
j `'!'
pipe size, grade, coating, etc.), the tensioner mechanism can
be set to impart 100,000 pounds of force and the remaining
100,000 pounds can be impartedby the reel drive mechanism.
Thus, the portion of the pipeline downstream of the tensioner
will be subject to a total respooling tension of about
200,000 pounds; however, between the tensioner and the reel,
the pipe will only "see" a tension on the order of 100,000
pounds, well within the safe operating limits of the pipe and
reel.
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115~8 f 2
' ~ When the desired amount of pipe has been respooled,
thè drive torque imparted to the reel by the reel drive
mechanism is reduced to balance the forward thrust imparted
by the vessel motive source. To avoid tension surges when
terminating the respooling operation, power to the reel drive
mechanism should be gradually reduced to gradually decrease
the reel torque and thereby gradually bring the reel imparted
tension into balance with the forward thrust imparted by the
vessel motive source. If the reel imparted tension had
been supplemented by a powered tensioner during the respooling
mode, it will generally also be necessary to reduce the tensioner
force in order to reduce the pipeline tension to a value
balancing that of the forward thrust component of the vessel
-- motive source. The superintendent must anticipate termination
and order the gradual reduction in power well before the
respooling termination point.
When the pipeline tension is reduced to a value which
just balances the forward thrust on the vessel so that the reel
has stopped respooling pipe, the reel is redogged to lock it
against rotation in the unspooling direction. The reel drive
mechanism is then changed over from the driving mode to a
dynamic braking mode. At this time, the straightener mechanism
~, is reset to straighten the pipe as it is unspooled. If a
tensioner mechanism is used, it too, is reset to impart the
desired amount of laying tension to the pipe.
C. Restarting the pipelaying operation_
The pipelaying operation is restarted by gradually
increasing the forward thrust imparted by the vessel's motive
source. The reel dog is disengaged before increasing the
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forward thrust of the vessel's motive source but after the
superintendent has been assured that the vessel is in the desired
station-keeping mode and pipeline tension is properly balanced
against the vessel's forward thrust of the vessel motive
S source up to the thrust level required to move the vessel
along the pipeline right-of-way at the desired laying speed.
The dynamic braking force of the reel drive mechanism
is established greater than the forward thrust of the vessel
motive source; the vessel will therefore remain on station
as the motive source thrust is gradually increased up to
pipelaying thrust, that is, the thrust necessary to maintain
the vessel underway at a desired pipelaying speed with the pipe
under a predetermined tension as established by the dynamic
braking force of the reel drive mechanism. The superintendent
then orders a very gradual reduction inthe dynamic braking
force of the reel drive mechanism until the vessel's forward
, thrust begins to exceed the back tension imparted by the reel
,~" drive mechanism so that the vessel begins~ to mo~ve forward and the
~l reel begins to turn to pay out pipe. The dynamic braking
;¦ 20 force of the reel drive mechanism can thereafter be reduced
at a more rapid rate commensurate with the vessel's ability
¦ to accelerate to bring the vessel up to normal pipelaying
speed while holding the pipe under sufficient tension to
maintain the desired pipe profile.
1 25 If anodes are to be applied to the pipeline, the same
¦ procedures are followed as described above except that the
steps associated with respooling the pipe (section "B"
above) may be Qm-it~ç-d~ Since the vessel must be stopped
for 3 to 5 minutes while anodes are applied, all of the above
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described steps associated with stopping and restarting the
pipelaying operation (sections A and C above) are followed.
Minor repairs to the pipe coating may be made "on the
fly" at a reduced lay rate. When, during the unspooling
proce~ure, it becomes apparent before or just as the pipe
comes off the reel, that a minor coating repair will be
; necessary, the superintendent orders an increase in the dynamic
braking force of the reel drive mechanism, thereby increasing
the back tension on fhe pipe and reducing the net forward mo-
tion of the vessel as long as the forward thrust of the
vessel motive source is not increased (e.g., vessel forward
thrust is held constant). The lay rate is therefore reduced
to enable such minor repairs to be made. For more extensive
repairs, the pipelaying operation must be stopped as per the
procedures descrlbed above.
When the pipelaying operation is temporarily stopped,
e.g., for anode application, major pipe repair, etc., the ten-
sion on the pipeline is preferably and advantageously main-
tained between upper and lower operating limits. The lower
limit constitutes the minimum acceptable tension which will
maintain the desired pipe profile; the maximum tension
limit constitutes the maximum tension under which the pipeline
; may be held for a given type, size and grade of pipe. The
reel drive mechanism, in its dynamic braking mode, constitutes
the primary control mechanism for maintaining the pipeline
tension within the above-noted upper and lower limits. The
vessel's motive source is used in conjunction with the reel
;~ drive mechanism control to provide a nominal thrust/tension
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balànce about which the reel drive mechanism may be varied
for maximum tension control within these limits. Thus the
combination of the reel drive mechanism and vessel motive
source must provide the capability for (a) station-keeping for
anode application and (b) adjustable net forward motion
for making coating repairs "on the fly" while paying out
pipe and not exceeding the above-stated mechanical limitations
of the pipe, while permitting control of pipeline variables
, through the reel drive mechanism.
It is further noted that the respooling procedures
- (section "B" above) are employed as part of a pipe salvage
operation after the pipeline end has been picked up and secured
to the reel or the stub end of a pipeline remaining on the reel
, from a prior operation.
,~ 15 The invention may be embodied in other specific
forms without departing from the essential characteristics
thereof. The embodiment described above is therefore to be
`'~ - considered in all respects as illustrative and not restrictive,
the scope of the invention being indicated by the hereafter
, 20 appended claims 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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