U.S. patent application number 11/752186 was filed with the patent office on 2008-01-24 for buoyancy element and module.
Invention is credited to Robert Gibson.
Application Number | 20080017385 11/752186 |
Document ID | / |
Family ID | 26246554 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080017385 |
Kind Code |
A1 |
Gibson; Robert |
January 24, 2008 |
BUOYANCY ELEMENT AND MODULE
Abstract
A buoyancy element (200) is disclosed which comprises a moulded
body of plastics-composite material incorporating reinforcement
(202) comprising at least one elongate flexible member or filament,
embedded in the body and adapted to retain fragments of the
buoyancy module together following structural failure of the
module.
Inventors: |
Gibson; Robert; (Bolton,
GB) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
1279 OAKMEAD PARKWAY
SUNNYVALE
CA
94085-4040
US
|
Family ID: |
26246554 |
Appl. No.: |
11/752186 |
Filed: |
May 22, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10486490 |
Aug 20, 2004 |
7214114 |
|
|
PCT/GB02/04212 |
Sep 16, 2002 |
|
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11752186 |
May 22, 2007 |
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Current U.S.
Class: |
166/350 ;
166/335 |
Current CPC
Class: |
E21B 17/012
20130101 |
Class at
Publication: |
166/350 ;
166/335 |
International
Class: |
E21B 19/00 20060101
E21B019/00; E21B 41/00 20060101 E21B041/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2001 |
GB |
0122377.5 |
Feb 13, 2002 |
GB |
0203398.3 |
Claims
1. A buoyancy module for mounting on an underwater conduit in a
string comprising two or more such modules arranged end-to-end, the
buoyancy module being provided with means for transmitting force to
its neighbouring module in the string in a direction along the
length of the conduit while facilitating angular deflection of the
module relative to its neighbour.
2. A buoyancy module as claimed in claim 1 wherein the means for
transmitting force is formed by an end face at the buoyancy module,
which is tapered or curved to facilitate angular deflection of the
module relative to its neighbour.
3. A buoyancy module as claimed in claim 1 wherein the means for
transmitting force comprises a resilient spacer for placement
between end faces of the module and its neighbour.
4. A buoyancy module as claimed in claim 3, in which the resilient
spacer is formed of polyurethane.
5. A buoyancy module as claimed in claim 3, in which the resilient
spacer covers substantially the entire end face of the buoyancy
module.
6. A buoyancy module as claimed in claim 3, in which the resilient
spacer is a moulding formed in situ upon end face of the buoyancy
module.
7. A buoyancy module as claimed in claim 3, in which thickness of
the resilient spacer is between 1 inch and 6 inches.
8. A riser tower comprising multiple buoyancy modules as claimed in
claim 3.
9. A buoyancy module for mounting on an underwater conduit in a
string comprises two or more modules arranged end-to-end, the
buoyancy module being provided with a resilient spacer for
transmitting force to its neighbouring module in the string in a
direction along the length of the conduit while facilitating
angular deflection of the module relative to its neighbour.
10. A buoyancy module as claimed in claim 9, in which the resilient
spacer is formed of polyurethane.
11. A buoyancy module as claimed in claim 9, in which the resilient
spacer covers substantially the entire end face of the module.
12. A buoyancy module as claimed in claim 9, in which the resilient
spacer is a moulding formed in situ upon an end face of the
buoyancy module.
13. A buoyancy module as claimed in claim 12, in which thickness of
the resilient spacer is between 1 and 6 inches.
14. A buoyancy module for mounting on an underwater conduit in a
string comprises two or more such modules arranged end-to-end, the
buoyancy module having an end face for transmitting force to its
neighbouring module in the string in a direction along the length
of the conduit, the said end face being tapered or curved to
facilitate angular deflection of the module relative to its
neighbour.
15. A riser tower comprising multiple buoyancy modules as claimed
in claim 9.
Description
[0001] The present invention relates to buoyancy modules and
particularly to buoyancy modules for attachment to a sub-sea
couduit such as a riser used in offshore drilling operations.
[0002] In offshore drilling operations e.g. oil extraction, a drill
string is guided between sea floor and surface within a marine
drilling riser.
[0003] The riser is normally assembled from a number of similar
sections or "joints". These joints are usually manufactured using
carbon steel as the principle construction material. In deep
waters, the use of steel in combination with the extended length of
the drilling riser produces a structure which has a significant
weight in water. In order to prevent the string from buckling, it
is supported by the surface vessel through a set of riser
tensioners. However, in order to ensure that the required tension
lies within reasonable bounds, the net weight in water of the riser
is reduced by adding subsurface buoyancy. The tensions to be
supported by the surface vessel are thereby reduced.
[0004] This buoyancy is added to the riser joints in the form of
discrete modules. The modules themselves are constructed from low
density composite foams such as syntactic foam. These materials
have a limited structural strength and their use in what is a very
demanding environment, where rough handling occurs, has led to
difficulties being encountered due to module damage.
[0005] The handling, deployment and recovery of damaged buoyancy
modules has given rise to operator concerns with regard to the
safety of drilling personnel.
[0006] The buoyancy modules are typically configured as elongate
cylinders. Conventionally each module is supplied as two similar,
generally semi-circular halves which are in turn known as buoyancy
elements. A typical buoyancy module 10 is illustrated in FIG. 1 and
comprises first and second buoyancy elements 12, 14 which together
define an axial recess or cavity 16 receiving and fitting to a
drilling riser 18. Studs 20 pass through the elements 12, 14 and
secure one to the other to retain the module 10 on the riser 18.
Alternatively, elements 12, 14 could be secured to one another by
circumferential straps surrounding both modules and received in
annular recesses 22 defined within the buoyancy module 10.
Additional axial recesses 24 may be provided through the module 10
to accommodate auxiliary lines 25 (when present) which form part of
the riser bundle. Further recesses may be provided to accommodate
guidance equipment.
[0007] A "string" comprising several buoyancy modules juxtaposed
and abutting at their end faces is in practice fitted to a riser
and constrained against axial motion by half shell clamps fitted at
the outermost end of the string.
[0008] The buoyancy elements are normally constructed with a
low-density syntactic foam core encapsulated within a protective
external skin.
[0009] Problems have been encountered when handling drilling riser
joints with buoyancy modules attached:- [0010] a) extreme local
loadings have been sustained by the buoyancy elements causing
smaller sections to be broken away from the main element structure.
These extreme local loadings are normally caused when an object or
structure impacts with a buoyancy module during handling; [0011] b)
extreme global loadings have been sustained by the buoyancy modules
which have led to major failures of the element structure (e.g.
significant cracks or, in extreme cases, the element being broken
into two sections). These loadings have normally been generated as
the riser joint has bent during offshore handling.
[0012] It is desired to reduce the likelihood of buoyancy element
structural failure.
[0013] It is additionally or alternatively desired to reduce the
dangers and problems posed by buoyancy element structural
failure.
[0014] In accordance with a first aspect of the present invention,
there is a buoyancy element comprising a moulded body of
plastics-composite material incorporating reinforcement comprising
at least one elongate, flexible member or comprising elongate,
flexible filaments, embedded in the body and adapted to retain
fragments of the buoyancy element together following structural
failure of the module.
[0015] In this way the dangers associated with buoyancy module
failure can be reduced and the module may, if it fails in situ, be
retained together for retrieval or repair.
[0016] It has been found, somewhat unexpectedly, that such
reinforcement can dramatically increase the strength and the
deformation) which can be accommodated prior to breakage.
[0017] The term "filament" should be understood in this context to
refer to a material comprising thin elongate, flexible strands or
members.
[0018] Preferably the reinforcement has a pre-treatment whereby
absorption of the plastics material of the body by the
reinforcement is prevented.
[0019] In this respect the reinforcement is to be contrasted with
e.g. conventional glass or carbon fibre reinforcement of plastics
mouldings, wherein the reinforcing fibres are securely bonded to,
and effectively integrated in, the surrounding plastics mouldings.
In buoyancy elements according to this preferred feature of the
present invention the properties of the reinforcement--particularly
its flexibility and in some embodiments also its elasticity--are
advantageously retained.
[0020] Preferably, the reinforcement comprises a branched network
of members or filaments. A branched network can securely anchor
itself in the buoyancy element even if not firmly bonded to it. The
preferred form of such reinforcement is a mesh.
[0021] The most preferred material for the reinforcement is nylon,
more specifically a knotless nylon mesh. In the absence of
pre-treatment the fibrous nylon filaments would absorb resin during
moulding of the buoyancy element, thereby becoming bonded to the
surrounding moulding and losing their inate flexibility. By
pre-treating the nylon such absorption and bonding are prevented.
Experiments have shown this to be highly advantageous with regard
to the strength and resistance to breakage of the buoyancy
element.
[0022] In the event that exceptional loading nonetheless leads to
breakage of the buoyancy element the reinforcement can serve to
retain the pieces of the broken element together in one unit, an
important safety consideration. Because its flexibility and in some
embodiments elasticity is retained in the moulding process the
reinforcing filaments resist being broken along with the
surrounding moulding, the invention again offering advantages in
this respect over more convention fibre reinforced materials.
[0023] The reinforcement is preferably arranged in a layer at or
adjacent the surface of the moulding. In the most preferred
embodiment the buoyancy element comprises an outer skin of fibre
reinforced material and the reinforcement according to the present
invention is arranged in a layer beneath this skin. The fibre
reinforcement may be of conventional type such as glass or
carbon.
[0024] The reinforcement is most preferably pre-treated by soaking
in oil prior to moulding of the buoyancy element. In this way
absorption and bonding between the moulding and the reinforcement
contained therein are avoided.
[0025] Preferably the reinforcement is non water degradable. Water
may enter the buoyancy element and it is especially preferred that
the reinforcement should not be destroyed by the action of salt
water. Nylon is again a highly suitable material in this
respect.
[0026] The reinforcement may comprise at least one elongate, linear
tendon.
[0027] The tendon is preferably substantially straight.
[0028] Preferably, the tendon is provided with an external skin and
separated thereby from the surrounding plastics-composite
material.
[0029] In this way absorption of resin during moulding by the
tendon is prevented, preserving the tendon's mechanical
properties.
[0030] Preferably the skin comprises a material which is softened
at temperatures created by heat given off upon curing of the
plastics material of the body.
[0031] Preferably, the tendon extends along an axial direction of
the buoyancy element.
[0032] Preferably, the tendon extends along substantially the full
length of the buoyancy element. In accordance with a second aspect
of the present invention there is a buoyancy module for mounting on
an underwater conduit, the module comprising at least two buoyancy
elements for assembly around the conduit such that the conduit is
received in an elongate cavity defined between the buoyancy
elements, and a pair of spacer elements which are separated from
each other along the length of the cavity, have surfaces for
seating upon the riser or conduit, and project inwardly from a wall
of the cavity to thereby separate the cavity wall from the riser or
conduit, the spacer elements comprising resilient material such
that their seating surfaces are able to deflect to conform to
curvature of the conduit and so reduce bending moment exerted on
the buoyancy module.
[0033] The spacer elements may each comprise a separate component
from the buoyancy elements, e.g. an annular collar.
[0034] The spacers may be integrally formed with moulded buoyancy
elements, the resilient material being incorporated during
moulding.
[0035] In accordance with a third aspect of the present invention,
there is a buoyancy module for mounting on an underwater conduit in
a string comprising two or more such modules arranged end-to-end,
the buoyancy module being provided with means for transmitting
force to its neighbouring module in the string in a direction along
the length of the conduit while facilitating angular deflection of
the module relative to its neighbour.
[0036] In a particularly preferred embodiment the means for
transmitting force to the neighbouring module is formed by an end
face of the buoyancy module, which is tapered or curved to
facilitate angular deflection of the module relative to its
neighbour. The end face may for example be frusto-conical or
radiussed.
[0037] In a further preferred embodiment, the means for
transmitting force to the neighbouring module comprises a resilient
spacer for placement between end faces of the module and its
neighbour. The spacer is preferably annular.
[0038] In accordance with a fourth aspect of the present invention,
there is a buoyancy module for mounting on an underwater conduit,
the module comprising at least two buoyancy elements for assembly
around the conduit such that the conduit is received in a cavity
defined therebetween, and the buoyancy elements comprising moulded
plastics composite bodies incorporating reinforcing framework, mesh
or members whereby following structural failure of the buoyancy
module fragments thereof are retained together.
[0039] Specific embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0040] FIG. 1 is a view along a radial direction of a known
buoyancy module mounted upon a riser, internal features of the
module being shown in phantom;
[0041] FIGS. 1a-1c are respectively an end view of, and two radical
sections through, the known buoyancy module along arrows A-A, B-B
and C-C of FIG. 1;
[0042] FIG. 2 is a partly cut-away view along a radial direction of
a buoyancy element embodying an aspect of the present invention,
mounted on a riser;
[0043] FIG. 3 is a radial section through the buoyancy element
illustrated in FIG. 2 along arrows A-A;
[0044] FIG. 4 is a partly cut-away view along a radial direction of
a further buoyancy element embodying the present invention mounted
on a riser;
[0045] FIG. 5 is a radial section through the buoyancy element
illustrated in FIG. 4 along arrows B-B;
[0046] FIG. 6 is a partly cut-away view along a radial direction of
a further buoyancy element embodying the present invention mounted
upon a riser.
[0047] FIG. 7 is a radial section through the buoyancy element
illustrated in FIG. 6 along arrows A-A;
[0048] FIG. 8 is a partly cut-away view along a radial direction of
yet a further buoyancy element embodying the present invention,
mounted upon a riser;
[0049] FIG. 9 is a radial section through the buoyancy element
illustrated in FIG. 8 along the arrows B-B;
[0050] FIG. 10 is a perspective illustration of a further buoyancy
element embodying an aspect of the present invention, partly cut
away to reveal internal structure thereof;
[0051] FIG. 11 is a cross section through the buoyancy element
illustrated in FIG. 10;
[0052] FIG. 12 is axial section through a further buoyancy module
of known type, mounted upon a riser;
[0053] FIG. 13 is an axial section through a buoyancy module
embodying an aspect of the present invention;
[0054] FIG. 14 is an axial section through neighbouring portions of
a pair of buoyancy modules of known type mounted in conventional
manner upon a riser;
[0055] FIG. 14 is an axial section trough neighbouring portions of
a pair of buoyancy modules mounted upon a riser with, in accordance
with an aspect of the invention, a buffer disposed between the
modules;
[0056] FIG. 15a is a enlarged view of a portion of the buffer;
[0057] FIG. 16 is a plan view of a mesh used in certain embodiments
of the invention;
[0058] FIG. 17 is a side view of an end region of a buoyancy module
embodying an aspect of the present invention: and
[0059] FIG. 18 is a similar side view of a further buoyancy module
embodying an aspect of the present invention.
[0060] It has been recognised by the inventors that one way to
reduce the danger posed by structural failures of buoyancy modules,
and to enable retrieval and repair of the modules, is to hold a
fractured buoyancy element together until it can be retrieved for
repair or replacement.
[0061] FIGS. 2 and 3 illustrate a buoyancy element 200 which, in
accordance with an aspect of the present invention incorporates an
external security mesh formed as a flexible structural mesh 202
within the element's external skin 204. The illustrated mesh covers
the entire area of the skin 204. Alternatively partial coverage may
be utilized.
[0062] The purpose of the mesh 202 is as follows:
[0063] a) to retain small pieces of foam material which may become
detached from the body of the buoyancy element; and/or
[0064] b) to hold large sections of the buoyancy element together
in the case of catastrophic failure of the buoyancy element.
[0065] FIGS. 4 and 5 illustrate a further buoyancy element 300
which, in accordance with a further aspect of the present
invention, incorporates an internal security structure of mesh 302.
This takes the form of a 3-dimensional, random or regular,
structural space frame, which partially occupies the space within
the element external skin.
[0066] As with the external securing mesh, the function of the
structure is to hold the buoyancy element structure together whilst
in a fractured condition.
[0067] FIGS. 10 and 11 illustrate in a little more detail the
currently favoured embodiment of this aspect of the invention. The
illustrated buoyancy element 600 is again moulded from syntactic
foam. In a conventional manner the element has an outermost skin of
fibreglass. Immediately beneath this is a security mesh 602 formed
of knotless nylon.
[0068] The nylon of the mesh is fibrous and would absorb the
syntactic foam were it not for a pre-treatment stage in which the
mesh is soaked in oil. The currently preferred material is a
millimetre square mesh. The mesh is in this embodiment a knotless
mesh formed from sheet material. A repeat pattern of 30 millimetres
is suitable, although this dimension is not critical. With
reference to FIG. 16, the mesh 700 has a taut direction 702 along
which it is relatively stiff under tension and a flexible direction
704 along which it is less still under tension. The mesh is
installed with its flexible direction 702 lying generally along the
length of the buoyancy element--i.e. with this direction of the
mesh axially aligned with respect to the element.
[0069] The moulding procedure involves cutting the mesh to fit the
outer circumference of the buoyancy elements. Sufficient pieces of
mesh are cut to line the entire outside diameter of the mould. The
setting is then immersed in mineral oil for 5-8 minutes to fully
saturate it, and hung to allow excess oil to drip off. Release
agent is applied to the mould followed by lining thereof with a
glass fibre mat, to form the integral outer skin of the buoyancy
element. The reinforcing mesh is then laid upon the glass fibre mat
and secured thereto, staples being the preferred means of securing.
Macrospheres 604 partially fill the mould, serving to reduce
overall density of the finished buoyancy element, and a known
syntactic foam resin is poured into the mould in a low pressure
environment (reduced air pressure preventing the formation of air
bubbles in the moulding). The syntactic foam is in this embodiment
a mixture of an epoxy and small microspheres which serve to reduce
the density of the foam. Such materials are in themselves well
known.
[0070] Were it not for the oil pre-treatment, the low pressure
environment in which moulding takes place would promote absorption
of the resin by the nylon mesh. Without the pre-treatment the mesh
would become integrated with the surrounding material and would
lose its flexibility and elasticity, becoming hardened by absorbed
plastics material. Due to the pre-treatment, the mesh retains its
flexibility and elasticity and is not bonded to the surrounding
syntactic foam, which can be verified by breaking a sample of the
moulding and observing that the mesh is released thereby from the
moulding.
[0071] Importantly the mesh serves a twofold purpose. Firstly it
significantly increases the strength of the buoyancy element.
Secondly the mesh is resistant to breakage and, following
structural failure of the buoyancy element, can retain the broken
pieces together as a unit and thereby prevent them from causing
injury e.g. by falling.
[0072] As in a vehicle windscreen, which uses layers of relatively
soft plastics material between harder layers of toughened glass to
spread the loading due to impacts and so prevent breakage, the
properties of the, relatively hard, syntactic foam and the flexible
mesh within it are complementary, the mesh serving to distribute
loading through the buoyancy element and resist structural
failure.
[0073] FIGS. 6 and 7 illustrate yet a further buoyancy element 400
which, in accordance with the present invention, incorporates a set
of tendons 402 in or just below the external skin of the element.
The tendons 42 are linear structural members generally aligned with
the longitudinal axis of the buoyancy element. The purpose of these
components is to retain buoyancy element sections together in the
case of catastrophic failure. Their effectiveness in the case of
local limited damage to the buoyancy element is likely to be
limited.
[0074] The buoyancy element 500 illustrated in FIGS. 8 and 9, also
embodying an aspect of the present invention, incorporates tendons
502 located within the body of the buoyancy element, some distance
below the external skin. Again these are at least generally axially
aligned. Their purpose is the same as that of the tendons 402
illustrated in FIG. 6. The plastics composite used in the buoyancy
elements 400 and 500 is in both cases such as to bond to the
internal reinforcement (tendons 402, 502).
[0075] The currently preferred form of tendon comprises a KEVLAR
(registered trade mark) strap 540 which is 2 millimetres thick and
50-250, or more preferably 60-150, millimetres wide with its own
form of pre-treatment--an external plastics skin 512 of EVA. Such
straps are currently used in attaching certain clamps to undersea
tubulars. They possess high tensile strength and elasticity, and
are flexible. The plastics skin of the tendon prevents absorption
of resin by the tendon itself and so allows the tendon to maintain
its flexibility and elasticity. It is found that the elevated
temperatures produced upon curing of the syntactic foam softens the
tendon's plastics skin, producing a secure bond between the foam
and the tendon. Problems of de-lamination (an important issue in
modules for deep sea use, where invasion of salt water can produce
de-lamination) are consequently reduced.
[0076] An alternative form of tendon comprises nylon rope.
Diameters of 5-25 millimetres are preferred. It is believed that
oil pre-treatment of larger diameter ropes would not be appropriate
since the oil may not penetrate to the rope's centre. Hence a
pre-treatment involving plastics coating of the rope would be
utilized to prevent resin absorption.
[0077] An alternative/additional strategy for preventing buoyancy
module failure is to prevent the module from becoming
over-stressed.
[0078] One source of stress is curvature of the riser upon which
the module is mounted. As FIG. 12 illustrates, buoyancy elements
100 are normally supplied with support pads 102. The pads are
integrally formed circumferential upstands or flanges located
towards either end of the buoyancy module and projecting radially
inwardly therefrom to set upon the riser 104.
[0079] The purpose of the pads is to provide a gap 105 between the
external surface of the riser and the internal surface of the
buoyancy element. When the riser pipe deflects during handling, the
presence of the annular gap is intended to prevent contact with the
element and in turn prevent load being transferred to this
structure.
[0080] However, as the pads have a finite length, they cannot be
considered to be point supports. Due to this, as the riser pipe
deflects and assumes a curvature, a bending moment will be passed
from the tubular to the buoyancy element via the support pad.
[0081] In the embodiment of the present invention illustrated in
FIG. 13, the integrally formed support pads 102 of the known
arrangement are replaced by flexible mountings 110. These may be of
resilient material and may be separate components from the elements
100, attached thereto, or may be semi-integral to the element
structure (e.g. being formed in the same moulding process but
containing a material of greater resilience than the element as a
whole). As FIG. 13 illustrates, the effect is that contact surfaces
of the mountings 110, seated upon the riser 104, can deflect to
conform to curvature of the riser and hence minimise bending moment
exerted on the buoyancy module. The annular gap 112 between the
buoyancy module and the riser 104 is chosen to avoid contact
between the riser 104 and the module's inner surface 114, based on
the anticipated riser curvature. The axial position of the pads is
chosen to maximise their effectiveness.
[0082] A further approach to the problem of buoyancy module
integrity involves consideration of force between end faces of the
modules. A single joint of a sub-sea riser is normally fitted with
between 3 and 6 buoyancy modules (i.e. 6 to 12 buoyancy elements).
The modules are mounted in direct contact with each other (i.e.
adjacent buoyancy modules butt together without any intermediate
gap being present). However, there is a gap present between the end
face of the outermost module and the riser joint connecting flange.
In order to prevent the buoyancy modules from moving axially
(either during handling or in operation) an end clamp (or stop
collar) is fitted against the exposed face of the module
string.
[0083] As an alternative to this arrangement, a spacer collar may
be fitted between the riser joint end flange and the end face of
the buoyancy module.
[0084] When the assembly has been completed, the buoyancy module
string can be considered to be held in position rigidly (i.e.
relative axial movement with respect to the riser pipe is not
possible).
[0085] FIG. 14 illustrates in axial section portions of a
neighbouring pair of buoyancy modules 101 with abutting end faces
120. It will be apparent that deflection of the riser joint, e.g.
during handling, causes loads to be passed between the two
modules.
[0086] The presence of these loadings may either lead to:-
[0087] a) failure of the buoyancy element structure local to the
end face; or
[0088] b) an increase in the general stress level carried by the
element structure which may contribute to the global failure of the
buoyancy element.
[0089] In an embodiment of the present invention the end faces are
shaped to reduce local loading at the end faces upon riser
deflection. This may be achieved by shaping the end face 120 with a
taper (e.g. by making the end face frusto-conical as seen in FIG.
17) or a radius as seen in FIG. 18.
[0090] Contact at the interface between adjoining end faces so as
to properly transmit stresses due to module weight and buoyancy
remains a requirement of the module design.
[0091] FIGS. 15 and 15a illustrate how, in accordance with a
further aspect of the present invention, a resilient end face
buffer 122 may be incorporated between the end faces 120 of
adjoining buoyancy modules 101. In this particular embodiment the
buffer 122 comprises an annulus of resilient material.
[0092] In the present embodiment the buffer 122 is made of
polyurethane. Other suitably resilient materials could be
substituted. The buffer 122 may be bonded in place on the end face
of the buoyancy module 101. Alternatively it could be moulded in
situ. For example this can be achieved by firstly moulding the main
body of the buoyancy module 101 (e.g. from epoxy syntactic foam)
and then mounting a further mould on the end face of the buoyancy
module 101 to receive and form the material of the resilient
buffer. In the currently favoured embodiment, the buffer material
covers substantially the entire end face of the buoyancy module
101. It has a thickness, in current embodiments, of approximately
2.5 inches, although thickness and stiffness of the buffer can be
chosen in accordance with factors such as module size, string
length--and consequent buoyancy force--etc.
[0093] A particularly important application of this aspect of the
invention arises in connection with riser towers. These are
distinguished from the type of riser/buoyancy structures discussed
above in that all of the force created by the buoyancy mounted on
the riser is referred to the user through a single connection at
the top of the riser. The periodic clamps used in the arrangements
discussed above are dispensed with. Forces are simply transmitted
along an extended chain of abutting buoyancy modules and each
module must therefore withstand the entire buoyancy force created
by all of the modules beneath it. The modules at the top are thus
subject to the greatest forces and these can exceed 1250 tonnes in
tall riser towers. The buffers 122 assist in distributing pressure
at the interface between neighbouring buoyancy modules 101 over
their end faces, thus preventing extreme local pressures due to
bending which might otherwise cause module failure. Since the
forces acting on modules at the top of the tower are greater than
those acting on modules at its foot, the buffers used on modules
toward the top of the tower may differ from those used lower down.
In the currently favoured embodiment, the dimensions of the buffer
do not change throughout the length of the tower, but two different
grades of polyurethane are used, a stiffer grade being applied to
the buoyancy modules for mounting toward the top of the tower.
[0094] It will be apparent that certain of the strategies explained
above for improving buoyancy module performance may be implemented
in combination with each other. Hence for example a module
reinforced as explained with reference to any of FIGS. 1 to 11
could utilise resilient support pads and/or means for reducing
loading from one module upon another.
* * * * *