Note: Descriptions are shown in the official language in which they were submitted.
CA 02609127 2014-12-01
BEND STIFFENER
FIELD OF THE INVENTION
The present invention is related to bend stiffeners for flexible members. More
particularly the present invention is related to bend stiffeners for use with
cables for
interconnecting one or more seismic stations placed at subsea locations with
suitable
control and monitoring equipment.
TECHNICAL BACKGROUND OF THE INVENTION
A bend stiffener is typically used at cable terminations at cable splice
units, regenerator
units, or the like in subsea cable applications where it is desirable to
improve the load
capacity of the cable and cable termination. A cable termination is typically
installed
with the cable and attached units from a cable ship or other installation
vessel.
Lateral movements of a subsea cable may occur due to environmental loading,
underwater currents or handling during installation and removal of the cable.
In
particular, the cable proximate a splice unit, regenerator or sensor unit will
typically be
subject to large bending forces in the above situations due to a combination
of angular
misalignment and tension. If suitable protective means are not used, the
bending forces
may exceed the bending capacity of the cable causing undesirable tear,
fracture or even
full breakage. It is a common technique to use bend stiffeners to improve the
bending
capacity of cables at such cable terminations, thus increasing the bend radius
of the
bending curve of the cable for a given load on the cable.
For the cable terminations three typical load situations occur. Firstly, the
small loads (2-
20 times the weight of the unit) and large angular displacements (up to 90
degrees)
occurring when the unit is raised vertically from horizontal storage.
Secondly, in
connection with the onboard handling and movement over a running wheel of the
ship
(typical diameter 3-6 m) with strain from the cable etc. (determined by cable
weight,
ocean depth, etc.), the typical load is 1.2-1.5 times the weight of the free
cable in water.
Over the running wheel a typical angular displacement from the axis of 25-40
degrees
may occur. The last situation happens when lifting a unit from the ocean
bottom. If the
unit has been buried (for protection purposes or due to movement of material
on the
ocean bottom) for a long time (10-30 years) large forces will be required to
lift the unit.
The cable is typically drawn almost vertically upwards (at up to 90 degrees to
the
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direction of the cable at the ocean bottom), and the cable and unit is stuck
in nearly
"coagulated" mass. Fortunately, experience indicates that the loosening
process
provides a shaping of the cable which somewhat limits the bending of the
cable.
Normally, it is not required that the cable need to be undamaged when it is
taken out of
the seafloor.
Ocean Bottom Seismic (OBS) systems, which may be at least partly buried on the
ocean
bottom includes hydrophones intended to measure pressure waves in the ocean at
or
near the ocean bottom. The hydrophone is a part of a measuring station which
may also
contain geophones or accelerometers), and is normally protected by a stiff
external
housing. On the ocean floor the station will be left as a unit enclosed by a
mass of loose
sediments from a flushing operation (or ploughing). Sand may also enter into
the open
spaces of the station and partly fill the holes which have been provided for
allowing
entry of pressure waves.
Such sediment mass may become fixed or hardened during time when the
particulate
matter (sand, silt, clay, etc.) packs together. Dependent on the bottom
conditions this
mass may become relatively rigid and have a large resistance to water flowing
through
it. If the compressibility of the volume that the station represents becomes
high (low
bulk modulus) in relation to the surrounding masses the pressure build-up will
require
some time to occur. The hydrophone thus may measure a lower response for
signals of
high frequencies than for static pressures.
OBS systems include a cable and stations having metallic parts necessary for
e.g.
mounting and connecting the different parts to each other, also related to the
hydrophone. These may be subject to corrosion when placed on the ocean bottom.
A
common method of avoiding such problems is to provide cathodic protection.
Sink
anodes are attached to the unit and these serves to prevent the corrosion of
metallic parts
of the station/cable. Cathodic protection is a galvanic process which involves
the
dissociation of water. This process may in some cases cause the formation of
gas. If the
gas is collected in the station (or in the cavity this represents when buried)
it will in
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effect result in a very high compressibility (low bulk module). Measurements
of
pressure waves may thus be strongly affected (se above paragraph).
THE OBJECTIVES
It is thus a primary objective of the present invention to provide a bend
stiffener
suitable for use with ocean bottom cables intersected by subsea units; for
example
seismic stations, splice units, regenerator units, which improves the bending
curve
and/or load distribution along the bend stiffener and the associated end of
the cable
when such a cable with attached units are being installed and removed,
typically to and
from locations at large sea depths. It is also an objective to reduce or
eliminate the
problems related to gas formation, especially from cathodic protection when
the cable
with the units is placed on the sea bottom.
THE INVENTION
According to the invention there is thus provided a bend stiffener having a
first
substantially elongate member having a longitudinal through-going conduit and
a
second member having also a longitudinally through-going conduit. The conduits
are
designed to allocate a cable being terminated at a unit that the bend
stiffener is mounted
onto. The second member is arranged at one end of said first member thereby
effectively extending the bend stiffener in its longitudinal direction. The
first member
has an amount of resilience in order to be able to bend when the bend
stiffener is loaded
in the transverse direction, while the second member is produced from a
material
yielding a second member being more resilient than said first member in order
that the
second member will start bending at transversal loads which are so low that
there is
hardly any bending of said first member. This bend stiffener provides an
improved
bending of the cable at the termination, whereby the risk of damage and
fracture of the
cable during handling is reduced.
According to another aspect of the invention the bend stiffener constitutes an
interface
between the hydrophone and the surroundings. According to this embodiment the
bend
stiffener comprises channels stretching from the second member to the
hydrophone. As
the second member is resilient the pressure waves propagate through the
material and is
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lead by the channels to the hydrophone through the more rigid first member.
From the
first member to the hydrophone the channel is lead through a more rigid
channel so that
it is not affected by gas in the chamber containing the abovementioned metal
parts. The
channel through both members and rigid channel has non-metallic surfaces
against the
sea water, thus this embodiment of the invention provides a bypass avoiding
the areas
being subject to gas formation.
More specifically the invention is characterized as disclosed in the
accompanying
claims.
Further preferable embodiments of the bend stiffener are given in the
dependent claims.
THE DRAWINGS
The invention will below be described in further detail with references to the
accompanying drawings in which illustrates the invention by way of example.
FIG. 1 illustrates an example embodiment of the bend stiffener
according to the
invention.
FIG. 2 illustrates an example embodiment of a bend stiffener according
the
invention as coupled to a hydrophone unit of a seismic station.
FIG. 1 illustrates a bend stiffener according to the present invention which
comprises a
first substantially elongate member 1 having a longitudinal conduit 2 and a
second
member 3 having also a longitudinal conduit 4. The first member 1 may have a
shape
and size typical for conventional bend stiffeners which are dimensioned for
the loads
experienced during handling on a running wheel onboard a cable installation
vessel. At
one longitudinal end 5 of the first member 1 one end 6 of a second member 3 is
connected, thereby effectively extending the length of the bend stiffener 1,3
in its
longitudinal direction.
While the first member 1 can be any type of conventional bend stiffener, the
second
member 3 is preferably a substantially cylindrical, flexible, tubular member,
and is
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further preferably formed from one or more sections of a conventional flexible
hose. It
could also be a type of bend limiter.
The second member is in a preferable embodiment a hose made from a material
which
5 is more "soft" or less resilient then the first member 1. The first
member 1 has an
amount of resilience in order to be able to bend when the bend stiffener is
loaded at high
tension at small angles, while the second member 3 is preferably produced from
a
material yielding a second member 3 which is less resilient than said first
member 3.
The result is that the second member 3 will start yielding/bending at loads
having force
components in a transversal direction which are so low as not to cause
significant
yielding/bending of said first member 1.
The second member 3 may either be made with a smaller cross-sectional
dimension
than said first member 1 in order to provide the lower resilience.
Alternatively, the same
effect can be achieved by producing the second member 3 in a material having a
lower
elastic modulus than the first member, that is, the result will be that said
second member
3 tends to bend more easily than the said first member 1 when experiencing
external
forces (low tension at large angles). In an alternative, said second member 3
could be
made so as to combine a lower cross-sectional dimension and lower elastic
modulus to
achieve the desired effect.
Thus, the second member 3, for example in the form of a hose, will effectively
function
- as a bend stiffener at relatively small loads, for example as experienced
during normal
handling onboard an installation vessel. The use of a second member 3 having
the
properties as stated, also provides improved bending and load distribution
when the
cable and station is raised up from the ocean bottom after long term
deployment,
possibly in a buried state.
Now referring to Fig. 2, when the compound bend stiffener 1,3 according to the
present
invention is used with a seismic station lithe addition of the second member
constitutes a significant increase in the compressive area of the seismic
station 11.
Normally, the external surface of the seismic station 11 is formed by a metal
housing
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and rigid/stiff bend stiffeners. The pressure wave is allowed into the
hydrophone 10
through holes in an outer part of the metal housing. This provides small
openings for
"refill" of water for the build-up of pressure around the hydrophone 10.
With the second member 3 is arranged according to the present invention, this
second
member 3 functions as a large membrane for the hydrophone. Water enclosed by
the
membrane/second member will be forced towards the hydrophone and build up the
pressure to be detected.
The bend stiffener 1,3 is provided with at least one internal, longitudinal
channel 7
being coupled with at least one corresponding channel 8 in the termination
part 9 of the
bend stiffener. When in use, said bend stiffener termination 9 is typically
connected to a
hydrophone 10 of seismic station 11, whereby said channels 7,8 provide a fluid
coupling from water between the second member and the cable to the hydrophone
10.
Thus the said channels 7,8 of the bend stiffener 1,3 according to the
invention functions
as pressure transfer means for the hydrophone 10. The pressure transfer means
extend
from a first end 5 of the first member 1 at which end the second member 3 is
connected
to a second end 12 of the first member 1, for the transfer of pressure changes
from the
region of the second member 3. Hence, when said second member 3 is being
compressed by an external pressure wave, a corresponding pressure wave is
coupled
into the hydrophone 10 of the seismic station 11 via said pressure transfer
means 7,8. In
one embodiment of the bend stiffener according to the invention the pressure
transfer
means comprise longitudinal grooves 7 along an internal wall of the conduit 2.
In yet a
further embodiment of the bend stiffener according to the invention the
pressure transfer
means comprises any number of channels separate from, but substantially
parallel with
said conduit 2.
The second member 3 of the bend stiffener 1,3 is preferably terminated in a
sleeve or
muff 14 at the end most remote from the hydrophone. The second member 3 is
partially
blocked at one end by the sleeve or muff, however, the sleeve or muff is
provided with
small openings or grooves 15 for allowing an interior volume of the bend
stiffener to be
filled with water during deployment at a subsea location. Thus the internal
volumes of
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the bend stiffener and seismic station housing will fill with water when the
cable and
seismic station is lowered to the bottom of the sea. The openings or grooves
15 of the
sleeve or muff 14 preferably have so small dimensions that the inflow of sand
during
burial at the ocean bottom and during rinsing or flushing operations will be
strongly
limited. In order to ease the entry of the bend stiffener into guides,
conduits, braking
systems or other cable handling devices which is used on the installation
vessel, a part
of the external surface of the sleeve or muff 14 is produced with a conical
shape.
Traditionally, the formation of gas from corrosion processes may cause
problems for the
correct operation of hydrophones in seismic stations if the formed gas is
allowed to
gather internally in the housing, and such that the gas communicates with the
liquid near
the hydrophone. This problem can be avoided when using a bend stiffener
according to
the present invention by manufacturing both the first and second members of
the bend
-stiffener from a polymer material and by ensuring the cable arranged in the
conduit 2,4
of the bend stiffener 1,3 is covered by a non-metallic coating, such as a
polymer
coating. Further the interior water filled volumes of the bend stiffener is
coupled to the
liquid volume of the hydrophone via a rigid pipe. Thus, the hydrophone will
measure.
the pressure in a liquid volume where there is no formation of gas provided
all surfaces
capable of enclosing a volume of water and capable of coupling with the
hydrophone
= 20 are adapted to prevent the generation of gas due to corrosion.
The first member of the bend stiffener according to the invention is
manufactured as
conventional bend stiffeners, for example by moulding polyurethane. A metal
part is
normally included in the first member for mechanically anchoring the first
member to
the station/housing. The dimensions and material properties are determined by
the
expected load conditions over the running wheel during pull-up (of large
lengths of
cable).
The longitudinal channels 7 in the first member 1 for allowing pressure
coupling
between the volume enclosed by the second member 3 (hose) and the hydrophone
10
can be produced during the moulding process as grooves on the internal surface
facing
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the cable or as one or more separate channels near the cable. The channels can
all be
made to end in a small pressure collection volume 16 near the base of the bend
stiffener
1,3 where it couples to the hydrophone 10. The pressure collection volume 7
has a
passage 8 for coupling with the hydrophone 10 of the seismic station 11. The
passage 8
for coupling with a hydrophone 10 is arranged so as to be isolated from the
remaining
parts of a seismic station 11 for which the bend stiffener can be used to
protect a
connected flexible cable running from the seismic station 11 to other
equipment (not
shown). The passage can be made from a polymer pipe or pipe-like element which
is
impermeable to hydrogen. Further, the pipe or pipe-like element can be painted
or
otherwise surface treated in order to prevent internal development of gas. The
passage 8
should be tight towards the surroundings in order to prevent gases from any
corrosion
processes from entering into the passage.
The second member 3 of the bend stiffener can be produced from one or more
conventional hoses or it can be moulded or extruded with dimensions to fit the
bend
stiffener. Diameter, wall thickness and material properties are determined by
the initial
handling before and during installation (lifting from basket which imply low
loads and
large bend angles) and coupling to pressure waves. The second member 3 can be
attached to the first member 1 by direct moulding, adhesion or attachment
using
conventional cable clamps on a pipe-sleeve.
The hose can be terminated in a sleeve having small air holes or grooves in
order to
allow water to enter during lowering into the sea. The small holes or grooves
will limit
possible inflow of sand during the subsequent burial/flushing. The sleeve can
be made
conical in order that the hose more easily may enter into conduits/brake
systems which
are used on the installation vessel.
In conclusion the present invention provides a new type of bend stiffener
which is able
to accommodate large variations in bending forces/angles when used to
strengthen a
cable termination at a subsea unit. This is highly desirable when handling
cables, in
particular seismic cables, at large ocean depths. The first member is
optimized for large
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loads during installation and the second member for low loads during lifting
from
baskets etc.
As mentioned above the invention also provides means for avoiding gas
generated by a
metal having cathodic protection disturbing the detection of pressure waves by
the
hydrophone. The second member 3 is made from a resilient material and includes
at
least one channel 18. When pressure waves hit the second member 3 the signal
is
transferred through the second member channel 18 to communicating first member
channel or channels 7. The pressure waves then propagate through a rigid
channel 8 to
the hydrophone 10. In the illustrated example the metal parts having cathodic
protection are positioned in a second chamber 17. This second chamber 17 is
provided
with pressure relief channel to the surrounding environment. The shape and
number of
conduits constituting the channel may vary, from the cylindrical space between
the
channel to separate channels, in which case the receipt of pressure waves to
the
hydrophone is independent of the presence of the cable. According to the
illustrated
example the channels may be constituted by a number of smaller conduits
distributed
around the centre of the member or members.
Gas from the metallic surfaces affecting the hydrophone coupling is thus
avoided,
resulting in an improved hydrophone signal.
The outer end 14 of the second member channel 18 may be provided with channels
15
for slow pressure equalization without letting the received pressure waves out
of the
channel in that direction. The presence or size of the leak channels 15
depends on the
need for avoiding intrusion of sand or other materials from the sea bottom
into the
channels. The cross section of the second member is illustrated as being
circular, but
other shapes may be possible, e.g. flat or elliptic, which may provide
advantages, for
example when the hydrophone and bend stiffener is positioned at the sea bottom
where
it may be covered by more or less hard bottom materials.
To summarize the bend stiffener according to the invention comprises a first
substantially elongate member 1 having a longitudinal conduit 2 and a second
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substantially elongate member 3 having also a longitudinal conduit 4 being
connected
essentially coaxially to an end 5 of said first member 1. Thus the effectively
compound
length of the bend stiffener is extended. The first member 1 has a chosen
amount of
resilience in order to be able to bend when the bend stiffener is loaded at a
large tension,
5 most likely at a small angle, while the second member 3 is designed so as
to have less
resilience than said first member. Thus the second member will typically start
bending
earlier than said first member at a lower tension and allow for bending at a
large angle.
As mentioned above the problems related to gas occurring as a result of
cathodic
10 protection may be avoided by producing the first and second members from
a chosen
polymer material. In addition the cable arranged in the conduit 2,4 of the
bend stiffener
1,3 being should be covered by a non-metallic coating, such as a polymer
coating. This
way the contact between the sea water and the metal parts is limited, both by
the chosen
materials and by the barrier provided by the bend stiffener. Although polymers
are
mentioned as the preferred material according to the invention other materials
may also
be used, as long as they are suitable for avoiding contact between metal and
sea water,
and providing the required resilience depending on the specific use.
The conduits or channels (7,18, 16, 8) acting as waveguides to the hydrophone
are
sealed against the second chamber 17 wherein the gas is generated. This may be
done
by using an 0-ring between the cable 2 and the wall separating the the
pressure
colleting chamber 16 from the second chamber 17, and also a similar seal
related to the
connecting channel 8 between the hydrophone 10 and the pressure collecting
chamber
16. It should also be noted that the two parts of the bend restrictor in some
cases may be
made in one piece having a gradual transition between them.
