U.S. patent application number 10/361856 was filed with the patent office on 2003-11-27 for underwater pipe-laying.
Invention is credited to Bianchi, Stefano, Giovannini, Umberto, Signaroldi, Teresio.
Application Number | 20030219313 10/361856 |
Document ID | / |
Family ID | 26314098 |
Filed Date | 2003-11-27 |
United States Patent
Application |
20030219313 |
Kind Code |
A1 |
Giovannini, Umberto ; et
al. |
November 27, 2003 |
Underwater pipe-laying
Abstract
A pipe-laying vessel includes a vessel that is propelled during
pipe-laying; a tower on the vessel; a device for raising a length
of pipe from a deck of the vessel to a position aligned with the
tower; a device for joining such a length of pipe to a pipeline
being laid; and tensioners arranged to grip such a pipeline and to
lower it into the water while maintaining a desired tension in the
pipeline as the pipeline passes through the tensioners; and one or
more clamps arranged to grip the pipeline below the tensioners.
Inventors: |
Giovannini, Umberto; (Milan,
IT) ; Signaroldi, Teresio; (Lodi, IT) ;
Bianchi, Stefano; (Milan, IT) |
Correspondence
Address: |
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
26314098 |
Appl. No.: |
10/361856 |
Filed: |
February 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10361856 |
Feb 11, 2003 |
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09744211 |
Jan 22, 2001 |
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6524030 |
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09744211 |
Jan 22, 2001 |
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PCT/EP99/05201 |
Jul 21, 1999 |
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Current U.S.
Class: |
405/158 ;
405/166 |
Current CPC
Class: |
B63B 1/107 20130101;
F16L 1/19 20130101; B63B 35/03 20130101 |
Class at
Publication: |
405/158 ;
405/166 |
International
Class: |
F16L 001/00; F16L
001/028 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 1998 |
GB |
9816026.0 |
Feb 25, 1999 |
GB |
9904422.4 |
Claims
What is claimed is:
1. A pipe-laying vessel comprising: means for propelling the vessel
during pipe-laying; a tower on the vessel; means for raising a
length of pipe from a deck of the vessel to a position aligned with
the tower; means for joining such a length of pipe to a pipeline
being laid; tensioners arranged to grip such a pipeline and to
lower it into the water while maintaining a desired tension in the
pipeline as the pipeline passes through the tensioners; and one or
more clamps arranged to grip the pipeline below the tensioners.
2. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower on the vessel; a device for raising a
length of pipe from a deck of the vessel to a position aligned with
the tower; a station for joining such a length of pipe to a
pipeline being laid; tensioners arranged to grip such a pipeline
and to lower it into the water while maintaining a desired tension
in the pipeline as the pipeline passes through the tensioners; and
one or more clamps arranged to grip the pipeline below the
tensioners.
3. A pipe-laying vessel for J-laying that is propelled during
pipe-laying, the vessel comprising: a tower on the vessel; means
for raising a length of pipe from a deck of the vessel to a
position aligned with the tower; means for joining such a length of
pipe to a pipeline being laid; crawler-track tensioners arranged to
grip such a pipeline and to lower it into the water while
maintaining a desired tension in the pipeline as the pipeline
passes through the crawler-track tensioners; and one or more clamps
arranged to grip the pipeline below the crawler-track
tensioners.
4. A pipe-laying vessel for J-laying that is propelled during
pipe-laying, the vessel comprising: a tower on the vessel; a device
for raising a length of pipe from a deck of the vessel to a
position aligned with the tower; a station for joining the length
of pipe to a pipeline being laid; crawler-track tensioners arranged
to grip the pipeline and to lower the pipeline into the water while
maintaining a desired tension in the pipeline as the pipeline
passes through the crawler-track tensioners; and one or more clamps
arranged to grip the pipeline below the crawler-track
tensioners.
5. A pipe-laying vessel for J-laying that is propelled during
pipe-laying, the vessel comprising: a tower on the vessel;
longitudinal conveyors for bringing lengths of pipe into a position
generally aligned with the tower; a pipe loader for lifting the
lengths of pipe along the tower; a station for joining the lengths
of pipe to a pipeline being laid; tensioners arranged to grip the
pipeline and to lower the pipeline into the water while maintaining
a desired tension in the pipeline as the pipeline passes through
the tensioners; and one or more clamps arranged to grip the
pipeline below the tensioners.
6. A method of laying pipes from a vessel having a tower,
comprising: repeatedly raising lengths of pipe from a deck of the
vessel to a position aligned with the tower; joining each length of
pipe in turn to a pipeline being laid; gripping the pipeline with
tensioners and lowering the pipeline into the water while
maintaining a desired tension in the pipeline as the pipeline
passes through the tensioners; and gripping the pipeline with one
or more clamps below the tensioners during any interruptions in
laying.
7. A method of J-laying pipes from a vessel having a tower,
comprising: repeatedly raising lengths of pipe from a deck of the
vessel to a position aligned with the tower; joining each length of
pipe in turn to a pipeline being laid; gripping the pipeline with
crawler-track tensioners and lowering the pipeline into the water
while maintaining a desired tension in the pipeline as the pipeline
passes through the crawler-track tensioners; and gripping the
pipeline with one or more clamps below the crawler-track tensioners
during any interruptions in laying.
8. A pipe-laying vessel comprising an upwardly extending tower
assembly defining a path down which the pipe passes as a pipeline
is being laid by the vessel, and a lower guide arrangement for
guiding the pipeline after it has passed down the tower, the lower
guide arrangement including a plurality of sets of guide rollers
spaced apart along the path of the pipeline and defining the
lateral limits of the path, the guide rollers being located such
that they allow some bending of the pipeline as it passes through
the lower guide arrangement.
9. The vessel according to claim 8, in which the plurality of sets
of guide rollers comprises three or more sets of guide rollers.
10. The vessel according to claim 8, in which the plurality of sets
of guide rollers comprises five or more sets of guide rollers.
11. The vessel according to claim 8, in which the guide rollers of
at least one set of rollers include rollers whose axes of rotation,
in a plane perpendicular to the tower, are inclined to one
another.
12. The vessel according to claim 11, in which the guide rollers of
at least one set of rollers extend at least one quarter of a
revolution around the path of the pipeline.
13. The vessel according to claim 11, in which the guide rollers of
at least one set of rollers extend substantially all around the
path of the pipeline.
14. A method of laying a pipeline from a vessel, comprising
lowering the pipeline down an upwardly extending tower assembly of
the vessel and then through a lower guide arrangement, the lower
guide arrangement including a plurality of sets of guide rollers
spaced apart along the path of the pipeline and defining the
lateral limits of the pipeline, the pipeline undergoing some
bending as it passes through the lower guide arrangement.
15. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower on the vessel; an upper pipe joining
station at an upper location along the tower; means for raising a
length of pipe from the deck to a position aligned with the tower
and with the bottom of the length of pipe at substantially the
level of the upper joining station; a lower pipe joining station at
a lower location along the tower, separated from the upper joining
station by substantially the length of one length of pipe; and
means at the upper or lower joining stations for joining a length
of pipe to a pipeline being laid.
16. The pipe-laying vessel according to claim 15, wherein the
vessel comprises means at the upper and lower joining stations for
joining a length of pipe to a pipeline being laid.
17. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower on the vessel; an upper pipe welding
station at an upper location along the tower; means for raising a
length of pipe from the deck to a position aligned with the tower
and with the bottom of the length of pipe at substantially the
level of the upper pipe welding station; and a lower pipe station
at a lower location along the tower, separated from the upper pipe
welding station by substantially the length of one length of pipe,
the lower pipe station being equipped for testing the welding
carried out at the upper pipe welding station.
18. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower on the vessel; an upper pipe welding
station at an upper location along the tower for joining a length
of pipe to a pipeline being laid; means for bringing the lengths of
pipe into a position generally aligned with the tower; means for
lifting the lengths of pipe along the tower with the bottom of the
length of pipe at substantially the level of the upper pipe welding
station; and a lower pipe station at a lower location along the
tower, separated from the upper pipe welding station by
substantially the length of one length of pipe, the lower pipe
station being equipped for testing the welding carried out at the
upper pipe welding station.
19. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower on the vessel; an upper pipe joining
station at an upper location along the tower; a device for raising
a length of pipe from the deck to a position aligned with the tower
and with the bottom of the length of pipe at substantially the
level of the upper joining station; a lower pipe joining station at
a lower location along the tower, separated from the upper joining
station by substantially the length of one length of pipe; and
welders at the upper or lower joining stations for joining a length
of pipe to a pipeline being laid.
20. The pipe-laying vessel according to claim 19, comprising
welders at the upper and lower joining stations for joining a
length of pipe to a pipeline being laid.
21. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower on the vessel; an upper pipe welding
station at an upper location along the tower for joining a length
of pipe to a pipeline being laid; longitudinal conveyors for
bringing the lengths of pipe into a position generally aligned with
the tower; a pipe loader for lifting the lengths of pipe along the
tower with the bottom of the length of pipe at substantially the
level of the upper pipe welding station; and a lower pipe station
at a lower location along the tower, separated from the upper pipe
welding station by substantially the length of one length of pipe,
the lower pipe station being equipped for testing the welding
carried out at the upper pipe welding station.
22. A method of laying pipes from a vessel having a tower, the
vessel comprising: repeatedly raising lengths of pipe from a deck
of the vessel to a position aligned with the tower; and joining
each length of pipe in turn to a pipeline being laid; wherein upper
and lower pipe joining stations are provided on the tower, the
joining of the pipe is carried out at the upper or lower pipe
joining stations and the pipe joining stations are separated by
substantially the length of one length of pipe.
23. The method according to claim 22, wherein the joining is
carried out at the upper and lower pipe joining stations.
24. A method of laying pipes from a vessel having a tower, the
vessel comprising: repeatedly raising lengths of pipe from a deck
of the vessel to a position aligned with the tower; and joining
each length of pipe in turn to a pipeline being laid; wherein an
upper pipe welding station and a lower pipe station are provided on
the tower, the joining of the pipe is carried out at the upper pipe
welding station and the upper pipe welding station and the lower
pipe stations are separated by substantially the length of one
length of pipe.
25. The method according to claim 24, wherein the lower pipe
station is equipped for non-destructive testing and finishing of
the join between the length of pipe and the pipeline being
laid.
26. A method of laying pipes from a vessel having a tower, the
vessel, comprising: repeatedly bringing lengths of pipe into a
position generally aligned with the tower; repeatedly lifting the
lengths of pipe along the tower; and joining each length of pipe in
turn to a pipeline being laid; wherein upper and lower pipe joining
stations are provided on the tower, the joining of the pipe is
carried out at the upper or lower pipe joining stations and the
pipe joining stations are separated by substantially the length of
one length of pipe.
27. The method according to claim 27, wherein the joining is
carried out at the upper and lower pipe joining stations.
28. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: an apparatus for assembling sections of pipe
generally horizontally on the vessel to form longer lengths; a
tower at the bow of the vessel, with respect to an intended
direction of movement during pipe-laying, pivotally mounted so that
it can be angled to the vertical with the top of the tower being
further forward in the intended direction of movement during
pipe-laying than the bottom of the tower; a device for raising a
length of pipe from a deck to a position aligned with the tower; a
station for joining such a length of pipe to a pipeline being laid;
tensioners arranged to grip such a pipeline and to lower the
pipeline into the water while maintaining a desired tension in the
pipeline; one or more clamps arranged to grip the pipeline below
the tensioners; and a lower ramp provided with rollers and arranged
to guide the pipeline as it leaves the vessel.
29. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower at the bow of the vessel, with respect
to an intended direction of movement during pipe-laying, pivotally
mounted so that it can be angled to the vertical with the top of
the tower being further forward in the intended direction of
movement during pipe-laying than the bottom of the tower; means for
raising a length of pipe from a deck of the vessel to a position
aligned with the tower; and means for joining such a length of pipe
to a pipeline being laid; wherein the tower is placed at the
extreme bow of the vessel whereby pipeline being laid by the vessel
is not enclosed by the vessel in the region of the hull of the
vessel.
30. A pipe-laying vessel that is propelled during pipe-laying, the
vessel comprising: a tower at the bow of the vessel, with respect
to an intended direction of movement during pipe-laying, pivotally
mounted so that it can be angled to the vertical with the top of
the tower being further forward in the intended direction of
movement during pipe-laying than the bottom of the tower; a device
for raising a length of pipe from a deck of the vessel to a
position aligned with the tower; and a station for joining such a
length of pipe to a pipeline being laid; wherein the tower is
placed at the extreme bow of the vessel whereby pipeline being laid
by the vessel is not enclosed by the vessel in the region of the
hull of the vessel.
31. A method of laying pipes from a vessel having a tower at the
bow of the vessel with respect to an intended direction of movement
during pipe-laying, pivotally mounted so that it can be angled to
the vertical with the top of the tower being further forward in the
intended direction of movement during pipe-laying than the bottom
of the tower, the method comprising; repeatedly raising lengths of
pipe from a deck of the vessel to a position aligned with the
tower; and joining each length of pipe in turn to a pipeline being
laid; wherein the tower is at the extreme bow of the vessel,
whereby pipeline being laid by the vessel is not enclosed by the
vessel in the region of the hull of the vessel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. Ser. No.
09/744,211, filed on Jan. 22, 2001, which is a national stage
application of PCT/EP99/05201, filed on Jul. 21, 1999, which claims
priority of GB 9816026.0, filed on Jul. 22, 1998 and GB 9904422.4,
filed on Feb. 25, 1999. The contents of all above-identified
applications are hereby incorporated herein by reference.
BACKGROUND OF THE APPLICATION
[0002] The invention relates to the laying of pipelines under
water, especially at sea, from a vessel floating on the water.
[0003] The technique mainly used up to now to lay pipelines in the
sea is that called "S laying". This technique can be described
briefly as follows. The pipe joints, generally 12 metres long, are
transported from a port to a lay-barge by means of supply barges or
supply vessels, and are loaded onto the deck of the lay-barge.
Those pipe joints are added one after the other along a
construction ramp, which is usually horizontal or in some cases has
a small inclination of 5 or 10 degrees (this ramp being called the
"firing line"). On the firing line the operations necessary to
complete the connection of the pipe joints are performed in several
working stations in order to build a continuous pipeline. When a
new joint is added, the barge moves forward and the pipeline,
supported at the stern of the lay-barge by an inclined ramp (or
floating stinger) curves over the stern of the barge down toward
the seabed. The profile of the pipeline, from the lay-vessel to the
seabed, is in the form of a long "S" (from which comes the term "S
lay"). The upper part of the profile is called the "over-bend", and
the lower part is called the "sag-bend". In order to reduce the
stresses on the suspended part of the sealine on its way from the
lay-vessel to the sea bottom, a constant tension is maintained on
the pipeline by means of machines called tensioners. There is a
maximum to the depth of water in which that method can be used. As
the water depth increases, the tensioner pull necessary to maintain
the pipe stress within acceptable values increases dramatically,
and the horizontal bollard pull on the lay vessel increases
correspondingly. A method to reduce the above-mentioned pulls
consists of increasing the angle of descent of the sealine in to
the sea. If the angle is close to vertical (called "J lay") the
necessary tension on the pipe is very close to the weight of a
length of the sealine string equal to the water depth, and the
horizontal component is close to zero. This method has the contrary
limitation that there is a minimum to the depth of water in which
it can be used, because the pipeline must have room to curve
through about 90.degree. to lie on the seabed, and if the pipeline
is too tightly curved it will be over-stressed.
OBJECTS AND SUMMARY
[0004] It is an object of the present invention, considering all
the above mentioned problems, to provide an apparatus able to lay
pipeline in very deep waters as well as in relatively shallow
waters, in conditions favourable for the integrity of the pipe and
with high productivity.
[0005] The invention provides a pipe-laying vessel comprising:
means for propelling the vessel during pipe-laying; means for
assembling sections of pipe generally horizontally on the vessel to
form longer lengths; a tower at the bow of the vessel, with respect
to an intended direction of movement, pivotally mounted so that it
can be angled forwards towards the top; means for raising a length
of pipe from the deck to a position aligned with the tower; means
for joining such a length of pipe to a pipeline being laid;
tensioners arranged to grip such a pipeline and to lower it into
the water while maintaining a desired tension in the pipeline; one
or more clamps arranged to grip the pipeline below the tensioners;
a lower ramp provided with rollers and arranged to guide the
pipeline as it leaves the vessel; and means provided at a rear
portion of the vessel for monitoring the pipe where it touches down
on the water bottom.
[0006] The invention also provides a method of laying pipes from a
vessel, comprising: providing a tower at the bow of the vessel,
with respect to an intended direction of movement, pivotally
mounted so that it can be angled forwards towards the top;
repeatedly assembling sections of pipe in a generally horizontal
position on the vessel to form a longer length, raising that length
of pipe from the deck to a position aligned with the tower, joining
that length of pipe to a pipeline being laid, and gripping the
pipeline with tensioners and lowering it into the water while
maintaining a desired tension in the pipeline; guiding the pipeline
as it leaves the vessel using a lower ramp provided with rollers;
monitoring the pipe from a rear portion of the vessel where it
touches down on the water bottom; and gripping the pipeline with
clamps below the tensioners during any interruptions in laying.
[0007] The mounting of the tower (firing line ramp) at the extreme
bow of the vessel, and the accompanying inclination of the pipeline
axis toward the front of the vessel, greatly facilitate the
operation of passing the pipeline to a platform or other similar
structure when constructing, for example, catenary risers. The end
of the pipeline may be lowered free of the vessel using the
abandonment and recovery winch, and then drawn up to the platform
using one or more winches on the platform, without the need to pass
the pipeline under the vessel or to manoeuvre the vessel out of a
position between the end of the pipeline and the platform. Also,
the pipeline, on its way to the bottom, runs under the vessel and
the touchdown point is relatively close to a point directly below
the stern. That makes it easy to monitor the touchdown and control
the laying operation by means of a remotely operated vehicle
(r.o.v.) based on the pipe-laying vessel without the need for an
extra survey vessel. The monitoring means may comprise means for
operating an r.o.v. from the vessel, and the vessel may include
such a remotely operated vehicle.
[0008] The means for propelling the vessel may comprise propellers
driven by motors on the vessel. Instead, especially in relatively
shallow water, the vessel may be propelled by capstans working on
ropes attached to fixed anchors or the like.
[0009] Having the tower positioned in the bow, rather than
amidships as has previously been proposed, allows a very large
clear deck area for storing pipe containers and for assembling
lengths of pipe. The assembling means advantageously comprises
means for welding four sections of pipe to form a single length,
preferably by welding them together in pairs and then joining the
pairs. Thus, the vessel may be supplied with standard 12.2 metre
joints, and raise 48.8 metre lengths to the tower.
[0010] The tower is advantageously pivotable between a vertical
position and an angle of about 30.degree. forwards. The tower may
also be capable of being lowered to a shallow angle for use as a
launch ramp or stinger in S laying, but is preferably arranged to
be dismounted to allow S laying from that end of the vessel. The
end of the vessel that is the bow during J laying may then become
the stern during S laying.
[0011] The raising means may comprise a cradle for supporting a
length of pipe, pivotally mounted at or near the foot of the tower.
Such a pivoted cradle can be raised and lowered by a rope from the
tower. Preferably, the cradle is only approximately as long as the
maximum length of pipe to be raised, in which case the pipe may be
raised further up the tower by a vertical conveyor.
[0012] The joining means may comprise a first working station,
equipped for actually welding the pipeline, and a second working
station, one pipe length below the first, at which non-destructive
testing and finishing of the join are carried out. One join may
then be welded while the previous one is being tested, with a
consequent increase in the rate of pipe laying. The second working
station may be positioned between the tensioners and the
clamps.
[0013] The tension machines may be a standard type previously used
for S laying. The use of tensioners simplifies and speeds up the
pipeline lowering operation, and allows the passage of anodes,
buckle arrestors, etc. without problems.
[0014] The said clamps may comprise a fixed friction clamp and a
movable friction clamp. Each of the clamps is preferably strong
enough to support the maximum weight of pipeline that the vessel is
capable of laying, even if the pipeline becomes flooded with
water.
[0015] The lower ramp preferably comprises means for monitoring the
load on the pipeline as it passes over the rollers, and is
preferably of adjustable curvature so that the optimum curve can be
imposed on the pipeline for any weight of pipeline in any depth of
water that the vessel is capable of laying.
[0016] Preferably the lower ramp comprises a lower guide
arrangement including a plurality of sets of guide rollers spaced
apart along the path of the pipeline and defining the lateral
limits of the path, the guide rollers being located such that they
allow some bending of the pipeline as it passes through the lower
guide arrangement. By allowing bending of the pipeline as it leaves
the vessel and controlling the bending of the pipeline the
likelihood of over-stressing the pipeline as it leaves the vessel
is reduced. It becomes possible to arrange for the horizontal
forces applied to the pipeline by the vessel during laying to be
spread between the various sets of guide rollers, so that the
stresses introduced at any one set of guide rollers are
reduced.
[0017] It will usually be preferable for the guide rollers to be of
cylindrical shape, but other shapes can be adopted if desired and
it is even possible for revolving tracks to be provided around some
or all of the rollers, should that be desired for some reason.
[0018] Preferably the guide rollers of at least one set of rollers
include rollers whose axes of rotation, in a plane perpendicular to
the tower, are inclined to one another. In that case the guide
rollers surround, at least to some extent, the pipeline and,
thereby, allow for some variation between the alignment of the
vessel and tower on the one hand and the vertical plane containing
the undersea pipeline being laid on the other hand. Preferably the
guide rollers of at least one set of rollers extend at least one
quarter of a revolution around the path of the pipeline; that
allows for a variation of about 90 degrees between the alignment of
the vessel/tower on the one hand and the undersea pipeline on the
other hand. More preferably the guide rollers of at least one set
of rollers extend substantially all around the path of the
pipeline; the guide arrangement is then operative for all
orientations of the vessel relative to the undersea pipeline.
[0019] The lower guide arrangement is preferably of substantially
trumpet shape flaring outwardly in the direction of travel of the
pipeline during laying, the angle of flare increasing in the
direction of travel of the pipeline during laying. In that case,
whichever region of the guide arrangement is acting to guide the
pipeline as it is being laid, has the effect of introducing a
controlled bend into the pipeline and forces applied to the
pipeline are spread between rollers of various sets.
[0020] Whilst it is within the scope of the invention for the guide
rollers to fulfil some tensioning purpose, it is preferred that the
guide rollers are freely rotatable so that substantially the only
force applied to the pipeline by the guide rollers is a lateral
force.
[0021] At least some of the rollers are preferably mounted for
rotation on bearings that are directly or indirectly resiliently
displaceable. It is advantageous to provide the resilience by
allowing the axis of rotation of the roller to change resiliently
rather than by, for example, providing a resilient roller structure
so that the effective direction of the roller changes. It is
possible for each roller to be mounted individually for resilient
displacement but preferably a whole set of rollers is mounted on a
structure that is resiliently displaceable.
[0022] The lower guide arrangement is of particular relevance to
the laying of a relatively rigid pipeline rather than, for example,
a pipeline that is so flexible it can be curved into reels for
storage. Preferably the resistance of the bearings to resilient
displacement is more than 100 kN/m, and more preferably more than
500 kN/m. In an embodiment of the invention described below, the
resistance to displacement is of the order of 5000 kN/m in the case
of certain higher sets of rollers and of the order of 1000 kN/m in
the case of certain lower sets of rollers.
[0023] The bearings are preferably resiliently displaceable by a
distance of more than 50 mm and preferably at least some of the
bearings are resiliently displaceable by a distance of more than
100 mm. In an embodiment of the invention described below, the
higher sets of rollers with a resistance to displacement of 5000
kN/m are able to be displaced 100 mm and the lower sets of rollers
with a resistance to displacement of 1000 kN/m are able to be
displaced 300 mm.
[0024] As will be appreciated, the amount of bending introduced
into the pipeline during its passage through the lower guide
arrangement may be small. One purpose of introducing the bend is to
enable the horizontal forces between the guide rollers and the
pipeline to be distributed evenly between the sets of rollers. In
an embodiment of the invention described below, the amount of
bending of the pipeline is of the order of 0.34 m per 10 m length
of pipeline; in that embodiment the total force applied by the
lower guide arrangement under normal conditions is about 1000
kN.
[0025] The inclination of the tower during use is preferably in the
range of 45.degree. to 90.degree. to the horizontal.
[0026] Preferably three or more sets of guide rollers, and more
preferably five or more sets of guide rollers, are positioned along
the path of the pipeline below sea level. Some sets of guide
rollers are preferably also positioned along the path of the
pipeline above sea level; bending of the pipeline is then able to
begin above sea level and, in the case where the tower is
pivotable, even above the axis of pivoting of the tower. Preferably
the sets of rollers are spaced apart substantially evenly along the
path of the pipeline. The spacing along the path of the pipeline
between adjacent sets of guide rollers is preferably in the range
of 2 m to 15 m and more preferably in the range of 3 m to 10 m. In
an embodiment of the invention described below, the spacing is
about 5 m.
[0027] Preferably the vessel includes means for monitoring the
forces applied to the pipeline by the rollers of the lower guide
arrangement. Preferably a plurality of force monitoring means are
associated with respective sets of guide rollers for monitoring the
forces applied to the pipeline by the respective sets of guide
rollers. The monitoring means may comprise a plurality of load
cells.
[0028] Preferably, forces exerted on the pipeline by one or more of
the guide rollers are monitored and the operation of the vessel
adjusted in dependence upon the monitoring.
[0029] It will be appreciated that the pipe-laying vessel described
above has many advantageous features that are most desirably
combined together to enable the best results to be achieved, but
which in many cases can be used separately from some or all of the
other features, as will now be described more fully.
[0030] A first advantageous feature concerns the provision of the
tower at the bow of the vessel such that pipeline being laid by the
vessel is not enclosed by the vessel in the region of the hull of
the vessel. Thus in a first advantageous aspect the invention
provides a pipe-laying vessel comprising: means for propelling the
vessel during pipe-laying; a tower at the bow of the vessel, with
respect to an intended direction of movement, pivotally mounted so
that it can be angled forwards towards the top; means for raising a
length of pipe from a deck of the vessel to a position aligned with
the tower; and means for joining such a length of pipe to a
pipeline being laid; wherein the tower is placed at the extreme bow
of the vessel. Similarly in the first advantageous aspect the
invention also provides a method of laying pipes from a vessel,
comprising; providing a tower at the bow of the vessel, with
respect to an intended direction of movement, pivotally mounted so
that it can be angled forwards towards the top; repeatedly raising
lengths of pipe from a deck of the vessel to a position aligned
with the tower; and joining each length of pipe in turn to a
pipeline being laid; wherein the tower is at the extreme bow of the
vessel, whereby pipeline being laid by the vessel is not enclosed
by the vessel in the region of the hull of the vessel.
[0031] A second advantageous feature concerns the provision of two
pipe joining stations along the tower spaced apart by one assembled
length of pipe. Such an arrangement provides two distinct
opportunities during pipe laying to make and/or inspect pipe joints
with the result that less work has to be carried out on a given
joint at either one of the two stations; thus the time for which
the joint is required to remain stationary while a joint is made
can be reduced. Thus in a second advantageous aspect the invention
provides a pipe-laying vessel comprising: means for propelling the
vessel during pipe-laying; a tower on the vessel; an upper pipe
joining station at an upper location along the tower; means for
raising a length of pipe from the deck to a position aligned with
the tower and with the bottom of the length of pipe at
substantially the level of the upper joining station; a lower pipe
joining station at a lower location along the tower, separated from
the upper joining station by substantially the length of one length
of pipe; and means at the upper and/or lower joining stations for
joining a length of pipe to a pipeline being laid. The raising
means preferably comprises a first means for bringing the assembled
lengths of pipe into a position generally aligned with the tower
and a second means for lifting the assembled lengths of pipe along
the tower. The second advantageous aspect of the invention also
provides a method of laying pipes from a vessel, comprising:
providing a tower on the vessel; repeatedly raising lengths of pipe
from a deck of the vessel to a position aligned with the tower; and
joining each length of pipe in turn to a pipeline being laid;
wherein upper and lower pipe joining stations are provided on the
tower the joining of the pipe is carried out at the upper and/or
lower pipe joining stations and the pipe joining stations are
separated by substantially the length of one length of pipe. Each
raising step preferably comprises a first step of bringing the
length of pipe into a position generally aligned with the tower and
a second step of lifting the length of pipe along the tower.
[0032] A third advantageous feature of the invention concerns the
provision of tensioners and clamps to maintain the desired grip on
the pipeline. Thus in a third advantageous aspect the invention
provides a pipe-laying vessel comprising: means for propelling the
vessel during pipe-laying; a tower on the vessel; means for raising
a length of pipe from a deck of the vessel to a position aligned
with the tower; means for joining such a length of pipe to a
pipeline being laid, tensioners arranged to grip such a pipeline
and to lower it into the water while maintaining a desired tension
in the pipeline; and one or more clamps arranged to grip the
pipeline below the tensioners. In the third advantageous aspect the
invention also provides a method of laying pipes from a vessel,
comprising: providing a tower on the vessel; repeatedly raising
lengths of pipe from a deck of the vessel to a position aligned
with the tower; joining each length of pipe in turn to a pipeline
being laid; gripping the pipeline with tensioners and lowering it
into the water while maintaining a desired tension in the pipeline;
and gripping the pipeline with one or more clamps below the
tensioners during any interruptions in laying.
[0033] A fourth advantageous feature of the invention concerns the
provision of a lower guide arrangement including a plurality of
sets of guide rollers for guiding the pipeline after it has passed
down the tower. Thus in a fourth advantageous aspect the invention
provides a pipe-laying vessel including an upwardly extending tower
assembly defining a path down which the pipe passes as a pipeline
is being laid by the vessel, and a lower guide arrangement for
guiding the pipeline after it has passed down the tower, the lower
guide arrangement including a plurality of sets of guide rollers
spaced apart along the path of the pipeline and defining the
lateral limits of the path, the guide rollers being located such
that they allow some bending of the pipeline as it passes through
the lower guide arrangement. In the fourth advantageous aspect the
invention also provides a method of laying a pipeline from a
vessel, comprising lowering the pipeline down an upwardly extending
tower assembly of the vessel and then through a lower guide
arrangement, the lower guide arrangement including a plurality of
sets of guide rollers spaced apart along the path of the pipeline
and defining the lateral limits of the pipeline, the pipeline
undergoing some bending as it passes through the lower guide
arrangement.
[0034] It will be appreciated that each of the advantageous
features of the invention described in the four paragraphs
immediately above can be used all together, entirely separately
from one another, or in any combination; they can of course also be
used with any of the preferred features of the invention described
earlier.
[0035] The vessel employed in any of the methods of the invention
may be in any of the forms defined above.
[0036] Certain forms of pipe-laying vessel will now be described by
way of example with reference to the accompanying drawings, in
which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a sectional side elevation view of a pipe-laying
vessel;
[0038] FIG. 2 is a front elevation view of the vessel;
[0039] FIG. 3 is a top plan view of the vessel, partly in
section;
[0040] FIG. 4 is a section through a pipe preparation area of the
vessel, to a larger scale than FIG. 1;
[0041] FIG. 5 is a side elevation view of the tower, to a larger
scale than FIG. 1;
[0042] FIG. 6 is a cross-section through the tower, along the line
6-6 in FIG. 5;
[0043] FIG. 7 is a cross-section through the tower, along the line
7-7 in FIG. 5;
[0044] FIG. 8 is a longitudinal section through part of a lower
ramp portion of the tower;
[0045] FIG. 9 is a cross-section along the line 9-9 in FIG. 8;
[0046] FIG. 10 is a side elevation view of part of the vessel;
[0047] FIG. 11 is a side elevation view of the lower portion of a
tower similar to that shown in FIGS. 5 to 10 but including a
modified form of lower guide arrangement for the pipeline;
[0048] FIG. 12 is an end elevation view of the lower portion of the
tower shown in FIG. 11;
[0049] FIG. 13 is a diagrammatic side view of the lower portion of
the tower shown in FIG. 11 indicating the spatial arrangement of
guide rollers in the modified lower guide arrangement;
[0050] FIG. 14 is a sectional plan view through a lower region of
the modified lower guide arrangement;
[0051] FIG. 15 is a sectional plan view to a larger scale than FIG.
14 through part of the same lower region of the modified lower
guide arrangement;
[0052] FIG. 16A is a sectional view along the line 16-16 in FIG. 15
of a guiding part of the modified lower guide arrangement with the
guiding part in an unstressed condition;
[0053] FIG. 16B is a sectional view along the line 16-16 in FIG. 15
of a guiding part of the modified lower guide arrangement with the
guiding part in a stressed condition;
[0054] FIG. 17A is a sectional plan view through an uppermost
region of the modified lower guide arrangement;
[0055] FIG. 17B is a sectional plan view through an upper region of
the modified lower guide arrangement; and
[0056] FIG. 17C is a plan view to a larger scale of a roller
arrangement of the kind shown in FIGS. 17A and 17B.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0057] Referring to the drawings, and initially to FIGS. 1 to 3,
one pipe-laying vessel, indicated generally by the reference
numeral 1, is a semi-submersible vessel arranged to be propelled
and manoeuvred by propeller thrusters 2. The vessel is equipped
with two large rotatable cranes 3, mounted one on each side of the
bow, which may be of conventional design and, in the interests of
simplicity, will not be further described or shown in detail.
[0058] The cranes 3 carry on board containers 4 of pipes, which are
brought by cargo barges or the like (not shown) and stow them on
the deck on both sides of the ship bulkheads.
[0059] Pipes 5 are then delivered by crawler cranes 6 and conveyors
(not shown) to a double-quadruple joint area, which is accommodated
within a module 7 fixed to the deck of the vessel 1 (see FIGS. 3
and 4). The quadruple joint module 7 is placed on the starboard
side of the firing line.
[0060] The crawler cranes 6 first move the pipes 5 from the
stackers, which may be storage stacks on the vessel or may be
containers in which the pipes are loaded onto the vessel, to a
bevelling station module 8 that is positioned on the port side of
the ship, just aside the center line. After the ends of the pipes 5
have been bevelled, transverse conveyors transfer the pipes to a
double joint area 9 of the quadruple joint module 7, where the
single joints are welded together. The transverse conveyors may
consist of cradles movable along rails running across the vessel.
The pipes 5 may be supported on the cradles by rollers that allow
lengthwise movement of the pipes and can be adjusted to support
pipes of different diameters.
[0061] The quadruple joint module 7 has two different levels. In
the double joint area 9, at the lower level, four pipes 5 at a
time, just arrived from the bevelling station, are conveyed to the
appropriate positions in the module and then welded together in
pairs into double joint strings 10. Welding is carried out by means
of four welding stations, and the welds are X-ray checked at a
fifth working station. If the weld has been performed correctly,
the double joint strings 10 are lifted up to the second level, to
the quadruple joint area 11. Otherwise the joint is moved outside
the joint module structure 7 in order to be repaired or, if it
cannot be repaired, to be cut. At the quadruple joint area 11, two
double joint strings 10 are welded together (by means of four
welding stations) and then checked in a fifth working station (NDT
station) to form a quadruple joint string 12. If the string 12 is
satisfactory, it is transferred horizontally to the center line of
the vessel, where there is a conveyor arranged to move it towards
the bow. If the NDT detects a bad weld, the pipe string is shifted
aside to the port side of the vessel 1 to be repaired or cut. If a
cut is necessary, the quadruple joint 12 is split into four single
joints 5 and then transferred back to the bevelling station 8,
where it is bevelled again before being returned to the welding
process.
[0062] At the bow of the vessel 1, on the center line, between the
two cranes 3, is a lay tower indicated generally by the reference
number 14. The tower 14 is designed to allow simple installation
and removal. The tower 14 is coupled to the hull of the vessel by
means of two hinges 15 capable of varying the operational lay
slope, which is defined by the longitudinal axis of the tower, from
90.degree. to 120.degree. (from the vertical position to 30.degree.
outboard). That movement is necessary to lay various pipe sizes in
different sea depths (from shallow to deep water). The angle of the
tower 14 is determined by a pivoting jacking system, described
below.
[0063] The tower 14 is essentially constructed in three sections as
can be seen in FIG. 5.
[0064] The lowest section or basket 16 is designed to support the
maximum pulling force on the pipe, received by one or more friction
clamps 18. It accommodates the clamps 18 and has at its lower end a
lower ramp 17 carrying one or more terminal rollers 41 that guide
the pipeline 40 as it leaves the vessel. It is preferably designed
so as to be installed and removed by the cranes 3, or by an
auxiliary crane vessel, and stored on the deck or on a barge. The
friction clamps 18 comprise at least a fixed clamp arranged to hold
the sealine 40 securely during interruptions in laying. Preferably,
there is also a clamp movable on hydraulic jacks, which can be used
when laying objects attached to the pipe that are too large or too
irregular to be gripped properly by tensioners.
[0065] The middle section 19 accommodates three tensioners 20
which, in operation, lower the pipeline into the water while
maintaining it at a desired tension, and pipe support rollers to
guide the pipe when the tower 14 is not vertical. The tensioners
are crawler-track tensioners of a sort that has been well known for
some years in S laying, and the pipe support rollers are also of a
type known, per se. They are not described in more detail. The
middle section 19 also accommodates fold-away sheaves (not shown)
for an abandonment and recovery system, when those sheaves are in
their idle condition. An NDT and field joint station 21, with
floating floor, is located at the lower end of the middle section.
A welding station 23, with floating floor, is located at the upper
end of the middle section. The distance between the welding
stations 21 and 23 corresponds approximately to the length of a
quadruple joint string 12 so that the top of a string can be at the
station 23 while the bottom of the same string is at the station
21.
[0066] The abandonment and recovery (A/R) system comprises a double
capstan winch (electrically driven) with its associated reel winder
and with a steel wire rope. The wire rope will be driven to the
fold-away sheave (placed on the middle part of the tower) and then
connected to the pulling head. The A/R system is accommodated on
the main deck in a central position on the center line of the
vessel 1, just beside the quadruple joint module.
[0067] The upper section 22 of the tower 14 is a comparatively
lightly built structure, because it never needs to support the
tension on the pipeline. It accommodates a coupling device, and a
line-up station 24 (FIG. 2). The upper section 22 is of open
construction, in order to allow simple transferring of the string
12 from a pipe loader, described below, to the line-up station 24.
The upper section 22 can be installed onto and removed from the
middle section 19 during on-board installation activity.
[0068] The jacking system that controls the tilting movement of the
tower 14 uses booms 25 that are connected to the upper portion of
the middle section 19 of the tower, on its sternward or inboard
face, and to jacking devices 26 on a support base frame 28
positioned on the deck just astern of the cranes 3. The jacking
system comprises hydraulic cylinders, the pistons of which each
carry a set of four hydraulically-actuated locking pins that engage
in a row of apertures in the booms 25. When the tower is not being
jacked, it is held by similar locking pins that are mounted in
fixed positions on the base frame 28. If the tower is to be moved
by more than the stroke of the hydraulic cylinders, it is held by
the locking pins while the cylinders return for another stroke.
[0069] The pipe string 12 that is waiting horizontally at the
quadruple joint area 11 is transferred by longitudinal conveyors
29A towards a pipe loader 29 at the base of the tower 14. The
longitudinal conveyors may consist of rollers 29A mounted with
their axes oblique to the horizontal, so that they define a
V-shaped envelope, within which the pipe moves. They are movable
towards and away from the center line, so that pipes of different
diameters can be conveyed at a desired height above the deck.
[0070] The pipe loader 29 consists mainly of a truss 30 that
transfers the pipe 12 from the quadruple joint area 11 to the
middle section 19 of the tower 14 by means of its own rotation
around a hinge 31. Because it rotates, the loader can easily
accommodate the different positions that the pipe must reach when
rotated to the several operational laying angles (from 90.degree.
up to 120.degree.). The loader truss 30 is designed to be as light
as possible in order to reduce the inertia of the system. As may be
seen from FIG. 3, the loader truss is on the starboard side of the
firing line, and it accommodates clamping units 32 (FIG. 5) that
firmly grip the pipe string 12 and hold it alongside the truss 30
during the rotation. The pipe string 12 is supplied horizontally by
the roller conveyors 29A from the quadruple joint area 11 to the
loader area, and is then, if necessary, immediately raised by pipe
lifters (not shown in detail) to a position where it can be engaged
by the clamping units 32 on the pipe loader 29. At the proximal end
of the loader 29, a mechanical safety stopper 33 is installed to
prevent the pipe from falling if there is a hydraulic failure of
the clamps 32. In normal use, the pipe does not rest on the stopper
33, in order to avoid the risk of damage to the machined bevel at
the end of the pipe.
[0071] The rotation of the pipe loader 29 is effected by means of
two winches (not shown), both of them mounted in the A-frame 28. A
hoisting winch, with a rope passing over a sheave on the tower,
raises and lowers the pipe loader 29, while a back tension winch
applies a constant tension in the opposite direction, in order to
prevent uncontrolled movement of the pipe loader when it rotates
beyond the vertical position or as a result of the movement of the
vessel. The winch speed is defined in order to perform the loading
activity within the cycle time of the laying operation.
[0072] The function of the loader 29 is only to grab the pipe 12
(by means of the clamps 32) and rotate it to the same angle of tilt
as the tower 14. As soon as the pipe 12 is lying along the tower
14, the loader 29 stops its movement and waits (holding the pipe)
for the lowering of an elevator 34.
[0073] The elevator 34 takes the pipe string 12 from the loader 29
and transfers it to the level of the line-up station 24. The
elevator 34 consists mainly of a trolley running in two railway
tracks 35, positioned on the middle part 19 and the upper part 22
of the tower, on the port side of the center line of the loader 29.
The trolley carries openable clamps 37. As may be best seen from
FIG. 6, when the clamps 37 are fully open they are withdrawn to the
port side of the firing line, so that the loader 29 can raise the
pipe string 12 alongside the elevator 34. When small pipes are
being handled, guiding rollers may also be fitted. At the lower end
of the elevator 34 is installed a safety stop to support the pipe
string if there is any failure of the hydraulic clamps 37. When the
loader 29 arrives at the tower 14 and the elevator 34 is in its
lower position, the clamps 37 grab the pipe 12, and the clamps 32
release it. The elevator 34 then lifts the pipe to the upper
position. Then, the pipe 12 is transferred to transfer clamps
38.
[0074] Three vertically spaced transfer clamps 38 are provided, to
transfer the pipe string 12 from the elevator 34 to the line-up
machine 24: FIG. 6 shows one of the transfer clamps in three
different positions; the clamps are arranged as independent units,
each comprising a rotatable and extensible arm controlled by
transducers, on which is installed a fully opening clamp. When the
clamps are in the external position the elevator transfers the pipe
to them, while when they are in the inner position the line-up
machines 24 move transversely and take the pipe from them. If a
pipe string 12 is delivered by the elevator 34 before the line-up
machines 24 are free to receive it, the transfer clamps 38 can hold
the pipe string in a standby position, as shown in FIG. 6. In
addition, an umbilical may be inserted into the pipe string 12,
and/or the lower end may be pre-heated for welding, while the pipe
string is held in the standby position.
[0075] The line-up machines 24 are necessary for the correct
alignment between the pipe string 12 and the sealine 40. They are
capable of moving the pipe in all three dimensions. They consist of
a mixture of roller clamps, which fix the horizontal position of
the pipe string 12 but allow it to rotate, and at least one
rotatable friction clamp by means of which the orientation of the
pipe about its own axis can be adjusted. Each machine is
structurally independent from the others, but they must be
controlled by means of transducers in order to ensure the correct
alignment of the machines during the alignment of the pipe. Between
the line-up machines 34, guiding rollers are installed in order to
ensure that the pipe 12 is adequately supported during laying, when
the line-up clamps are open, even with the tower 14 in a tilted
position.
[0076] Referring now to FIGS. 8, 9, and 10, the lower ramp 17
carries the terminal rollers 41 in sets of 3 on bogies 42. During
pipe-laying, each bogie 42 is urged into engagement with the
sealine 40 by a hydraulic cylinder 44, pressurized by an
accumulator (not shown), while the load on the bogie 42 is
monitored by a load cell 46. The movement of each bogie is
controlled by a parallel linkage 48, while the fully retracted
position can be set by means of screw-jacks 50. The terminal
rollers 41 thus make it possible both to monitor and to control the
alignment of the sea line 40 as it leaves the vessel. If greater
control is required, several sets of bogies 42 may be mounted, one
above another, as shown in FIG. 10, enabling a controlled
deflection of the sealine, so that it can be laid at a slight angle
to the axis of the tower 14, thus increasing the versatility of the
vessel.
[0077] Below the terminal rollers 41 there are provided
wire-guiding rollers 52. The rollers 52 are mounted on pivot arms
54, and are moved in and out by hydraulic cylinders 56. They are
kept fully retracted during pipe-laying, but are advanced to guide
the wire rope during the abandonment and recovery procedures.
[0078] A remotely operated vehicle 58 is carried on the vessel 1,
for monitoring the touch-down of the sealine 40 on the seabed.
Because the point of touch-down is directly below, or close to
directly below, the stern of the vessel, the ROV may be operated
from the stern deck of the vessel 1 without needing an
inconveniently long tether. Instead, or in addition, the touch-down
may be monitored directly by active or passive sensors 60 mounted
on the rear of the vessel.
[0079] In operation, the pipe 12 is aligned by the line-up machines
24 and held by them while its lower end is welded to the upper end
of the sealine 40 in the welding station 23. Then, the line-up
machines are released, the vessel is moved forwards, the sealine is
fed out by the tensioners 20 by the length of a quadruple joint 12.
Then, the join that has just been welded is tested in the NDT
station 21, while a new joint 12, which has in the meantime been
delivered by the loader 29 and the elevator 34, is fed by the
transfer clamps to the line-up machines 24.
[0080] The tower 14 is constructed in three sections and is
attached to the vessel, and the quadruple joint module 7 is
constructed as one or more largely self-contained modules attached
to the deck, in such a way that the entire pipe-laying equipment
can be assembled or disassembled easily and quickly, allowing
conversion of the vessel as a whole from use as a pipe-laying
vessel to use as an ordinary semi-submersible crane vessel.
[0081] As an example, a vessel such as that shown in the drawings
may have the following dimensions:
1 Length of vessel 200 metres Height of tower 135 m Pipe Diameter
Range (O.D.) from 4" to 32" Max. diameter of objects to be laid 2.5
m with tensioners open Lay depth range for 4" pipe 50 m to 3000 m
Lay depth range for 32" pipe 200 m to 2000 m Maximum lay holding
force (tensioners) 525 metric ton Maximum lay holding 2000 metric
ton force (movable clamp) Lay tower angle 90.degree.-120.degree.
Intermediate pull A/R winch up to 550 metric ton (max.) (double
capstan) High pull A/R winch (linear winch) 2000 metric ton (max.)
Length of joint carry on board 12.2 m Pipe string length (4 .times.
10 to 15 m) 48.8 m Lay phase time 2 min. Maximum operational wave
height 4 m (significant)
[0082] FIGS. 11 to 17C show a modified form of lower guide
arrangement 61 which can be used in place of the lower ramp 17. In
FIGS. 11 to 17C parts corresponding to parts shown in other
drawings are designated by the same reference numerals. As shown in
FIGS. 11 and 12, the arrangement 61 generally comprises a tubular
frame including four longitudinal members 62 and cross-bracing
members 63 joined together to form a substantially rigid structure
that is fixed to the bottom of the tower 14 by four legs (one at
the top end of each of the members 62). The lugs are secured to the
tower by respective pin connections.
[0083] Referring now also to FIGS. 13 to 17C, the structure formed
by the members 62 and 63 serves to support various assemblies at
different levels, including fixed and mobile clamps 18A and 18B
respectively (FIG. 13), three sets of adjustable rollers 64A, 64B
and 64C and six sets of guide rollers 65A to 65F, each set
comprising a ring of rollers as will be described more fully
below.
[0084] The adjustable rollers 64A, 64B and 64C serve a purpose
similar to that of the rollers 52 described with reference to FIGS.
8, 9 and 10. The rollers 64A, 64B, 64C are mounted for radial
movement and moved by respective hydraulic piston and cylinder
arrangements. In the particular example of the invention described
the sets of rollers 64A and 64B are each arranged as shown in plan
view in FIG. 17A and comprise 4 rollers 66 equiangularly spaced
around the pipeline path and the rollers of the set of rollers 64C
are each arranged as shown in plan view in FIG. 17B and comprise
eight equiangularly spaced rollers 67. One purpose of the rollers
64A, 64B, 64C is to keep the pipeline (sealine) 40 within a central
circular region of an adjustable radius so that the clamps 18A and
18B, which are operated only in special situations such as an
emergency, are assured of gripping the pipeline. A further purpose
of the rollers, however, and especially the rollers 64B and 64C is
to allow some initial, controlled, bending of the pipeline even at
their relatively high levels above sea level. Television cameras 69
(FIG. 13) and load sensors associated with the rotational mountings
of the rollers can monitor the passage of the pipeline through the
rollers and the extensions of the hydraulic piston and cylinder
arrangements and the hydraulic pressures therein can also be
monitored and adjusted.
[0085] In FIG. 17C one of the hydraulic piston and cylinder
arrangements is shown by way of example. It will be seen that the
roller 66 or 67 is rotatably mounted on a support 70 and a load pin
71 provides a measurement of the force exerted by the pipeline 40
on the roller 66, 67. The support is connected to the piston of one
of the piston and cylinder arrangements 68 which includes a
pressure transducer 72 for monitoring pressure in the cylinder and
a position transducer 73 for monitoring the position of the piston
relative to the cylinder.
[0086] Signals from the television cameras 69 and the transducers
72, 73 and load pins 71 are all passed back through a multicore
cable 79 having junction boxes 80 to a control station 81 which may
be provided in a tower control room. Control signals for the
operation of the piston and cylinder arrangements 68 are passed
down from the control station 81 to a hydraulic supply and control
valve station 82.
[0087] The arrangement of the six sets of guide rollers 65A to 65F
will now be described in more detail with reference to FIGS. 13,
14, 15, 16A and 16B. For convenience the arrangement of the set of
rollers 65D will first be described, that being the set of rollers
shown in FIG. 14. A steel ring 75 provides the main fixed
structural support for the set of rollers and is of a diameter
suited to the particular set of rollers 65D so as to extend
circumferentially around them. The ring 75 is fixed to the four
longitudinal members 62 of the lower guide arrangement via struts
76. Immediately inside the ring 75 is another ring-shaped member 77
on which the rollers 78 are rotatably mounted. The ring-shaped
member 77 is connected to the ring 75 at four equiangularly spaced
positions around the roller structure via resilient mountings 83
shown in FIGS. 16A and 16B.
[0088] Referring in particular to FIGS. 15, 16A and 16B, each of
the rollers 78 is rotatably mounted at each end on bracket arms 85
which are fixed to the ring-shaped member 77 and project radially
inwardly therefrom. The ring 75 has supports 86 welded to it at the
four locations of the resilient mountings 83 and each support 86
has an upper arm 87 and a lower arm 88 which projects radially
inwardly over the ring-shaped member 77. The arms 87, 88 are each
joined to the member 77 by a respective block 89 of elastomeric
material. FIG. 16A shows the blocks 89 in their unstressed state,
whilst FIG. 16B shows the blocks 89 in a stressed state following
radially outward movement of a roller 78 (and therefore
corresponding movement of the member 77) as a result of the force
exerted by the pipeline 40. It can be seen from FIG. 16B that the
blocks 89 undergo shear strain to accommodate the movement.
[0089] Load cells (not shown) are associated with each of the sets
of guide rollers 65A to 65F and signals from the load cells are
passed back to the control station 81 via the junction boxes 80 and
the cable 79. Signals from the load cells can be used by a
controller to alter the pipe laying operation or adjust the
direction or speed of travel of the vessel or the like.
[0090] Operation of the guide rollers during laying of a pipeline
will now be described. In order to simplify the description, it
will be assumed that the tower is oriented vertically, but it
should be understood that the guide arrangement operates in
substantially the same way when the tower is inclined. Also, for
ease of description, it will in the first place be assumed that the
vessel is travelling directly above the path on which the pipeline
is being laid and is aligned with the path.
[0091] In order that the curvature of the pipeline in the region of
the seabed should not be excessive, it is important that during
laying of the pipeline a horizontal force is applied to the
pipeline by the vessel in the direction in which the pipe is being
laid and that a tensioning force is also applied. At the same time
the force must be applied in a way that does not cause undue local
stress in the pipeline.
[0092] Consequently, it is desirable that each of the sets of
rollers 65A to 65F apply a horizontal force to the pipeline and,
desirably, each set of rollers applies substantially the same
force. That is achieved in the embodiment of the invention by
arranging the sets of rollers so that they are positioned along a
curved path allowing a degree of controlled bending of the pipeline
as it passes through the sets of rollers. The resilient mounting of
the rollers further assists in promoting an even application of
loads amongst the various sets of rollers.
[0093] An especially valuable feature of the design of the guide
arrangement is that each set of guide rollers completely surrounds
the pipeline. That is important in allowing the vessel to be at any
angle to the path of the pipeline as may be desirable or essential
when laying a pipeline in a significant current.
[0094] In a particular example of the invention, that may be
employed in the case of the particular example of vessel described
above, the sets of guide rollers 65A to 65F are spaced apart along
the cable path at intervals of 5.2 m with the top set of rollers
65A above sea level and all the other sets below sea level. In that
case the spacing between the circumferences of diametrically
opposed rollers in each set is as follows:
2 Roller Set Spacing (m) 65A 2.44 65B 3.54 65C 5.0 65D 6.79 65E
8.96 65F 11.48
[0095] The mountings of the sets of rollers 65A to 65C are
relatively stiff and they provide the rings 77 of the sets with a
radial stiffness of about 5,000 kN/m with (with a maximum
displacement of 100 mm), whilst the mountings of the sets of
rollers 65D to 65F are less stiff and provide the rings 77 of those
sets with a radial stiffness of about 1000 kN/m (with a maximum
displacement of 300 mm). The total load typically applied to the
pipeline by all six sets of rollers is of the order of 1000 kN
during normal operation, resulting in a force of about 170 kN
between each roller and the pipeline.
* * * * *