Dynamic control cable for use between a floating structure and a connection point on the seabed

A dynamic control cable string or umbilical intended to hang in catenary form between a connection point on the seabed and a connection point located at the sea surface which can be attached to a floating structure; the umbilical is of the type that is low in weight per running meter and comprises flowlines and optionally current-conducting cables, all of which are twisted around the longitudinal axis of the string and are spaced apart from an axially extending core member and lie in channels in spacers for free axial movement of the cables and conduits in the channels; the core member, throughout the entire length of the control cable string functions as a load-bearing member and, in a portion of the length, all fluid transport and signal communication takes place peripherally relative to the core member.

BACKGROUND OF THE INVENTION 
The present invention relates to a dynamically working umbilical or control 
cable string, intended to hang in a catenary form between a connection 
point on the seabed and a connection point located on the surface and 
which may be attached to a floating structure, which umbilical is of the 
type having low weight per running meter and includes flowlines and 
optionally current conducting cables, all of which are twisted around the 
longitudinal axis of the conduit and lie in channels in spacers for free 
axial movement of the cables and flowlines in the said channels. 
DESCRIPTION OF THE RELATED ART 
Umbilicals of this type are designed for subsea use for the transfer of 
energy, signals and fluids in one or both directions. As used here, the 
designation "umbilical" is intended to embrace a flexible or bendable 
bundle of conduits and cables which comprises many transmission lines such 
as electric cables, for both energy and signals, and fluid transport 
lines, for both liquid and gas. Typically, these transport lines will be 
of steel having a fairly small diameter and may, for example, be used for 
high-pressure hydraulic fluid for actuating equipment such as valves on 
the seabed. Furthermore, conventionally they include a central steel 
conduit of larger diameter for transport of larger amounts of fluid, such 
as methanol for injection into an oil or gas well. One or more of the 
flowlines may also be used for chemicals which are to be injected into a 
formation or for feedback of "spent" fluid. A control cable string of this 
type is exemplified and described in NO. 920689 (WO 93/17176), and among 
persons versed in the art they are known as "umbilicals". However, it is 
not crucial that the umbilical includes electric conductors in the cross 
section, and it may conceivably be a pipe bundle for the transport of 
produced oil and gas to the surface in the same way as risers. 
A previous use of such control cables was between a surface vessel and a 
submersed remote controlled vehicle. 
When an umbilical or control cable is connected to a surface vessel or 
floating structure, the movements of the vessel or structure will be 
transmitted to the umbilical. The metallic pipes will then be subjected, 
to some extent, to great bending and tensile stresses. Naturally, this 
situation is the most unfavourable possible since the bending loads on the 
umbilical will be greatest at the top towards the connection point to the 
floating structure, whilst the cable cross-section of the umbilical in 
this very same area has the greatest tensile load because of the weight of 
the cable hanging down towards the bottom. This gives rise to a situation 
wherein the cable initially is under tensile stress which gives 
correspondingly less of a margin for bending stresses before yield 
stresses appear in the pipe materials. When these bending and tensile 
stresses exceed certain values, local plastic deformations occur and after 
this happens repeatedly, the steel pipes will be vulnerable to fatigue and 
fracture. In order to limit the size of the bending stresses it has been 
customary to provide bend stiffeners on the upper section of the 
umbilical, i.e., on the last 20 to 30 meters of umbilical up towards the 
floating structure. The bend stiffeners are mounted on the outside of the 
umbilical, and generally have an increasing cross-section in the upward 
direction, and are secured as a rule to the termination in the end 
thereof. It will thus be understood that when the movements of the 
floating structure are expected to be substantial, the bend stiffeners 
must also be substantial. Today, these bend stiffeners already have 
considerable dimensions and have almost reached their practically feasible 
outer limits. In addition to the movements of the floating structure, 
movements caused by currents in the water must also be taken into account. 
This affects the umbilical along the length thereof that suspends totally 
or partly free. It is usual to have limiting values for these movements 
too. 
SUMMARY OF THE INVENTION 
One of the main objects of the present invention is to reduce the load on 
the central large steel conduit, and especially in the area in the 
vicinity of the connection point for the floating structure. 
It has also been a desire to provide a method for increasing the weight of 
the umbilical along certain parts or sections of its length, and also a 
way to make attachments on the umbilical so that it can be anchored to an 
attachment or attachments on the seabed or to buoyancy bodies. 
Thus, a new design of the cross-section of the umbilical is provided, 
either along the whole of the part which runs from the seabed up to the 
sea surface or only in the end which runs up towards the connection point 
for the floating structure, so that stresses in the metal pipes and the 
global (geographical) position of the umbilical are kept within the 
limiting values that apply. 
According to the present invention there is provided a dynamically working 
umbilical of the type mentioned by way of introduction which is 
characterized in that the core member throughout the entire length of the 
umbilical functions as a load-bearing member only and all fluid 
transport/communication takes place peripherally relative to the core 
member. offhand, it would be considered unfavourable to move the conduits 
out from the centre account of bending loads. However, this does not have 
any particular significance for the present umbilical because of the 
special construction using spacers designed to have channels for receiving 
current-conducting cables and fluid conduits of small dimensions, where 
the cables and conduits are axially moveable, or "floating", in said 
channels, whilst all the components in the umbilical are twisted or layed 
and behave in principle like a steel rope. In this way great peripheral 
stresses are avoided. 
In one embodiment, the load-bearing member may be a solid rod or stay of a 
suitable material, such as steel, carbon or titanium. 
In a second embodiment, the load-bearing member may be a "stay" built up of 
twisted or layed single wires. 
As one option, the umbilical may be divided into three subsections--one 
flexible section close to the connection point on the surface, one 
submersible section in the vertical part from the connection point on the 
surface and one buoyant section between the submersible section and the 
connection point on the seabed. In this way, the said catenary form is 
achieved. 
In order to obtain the submersible section, weight elements are placed on 
the umbilical and the weight elements are threaded onto the outside of the 
core member of the umbilical. The elements are placed at predetermined 
intervals and the core member is provided with load-bearing means for 
transmitting weight or load from the weight elements to the core member. 
As mentioned previously, it is known to have a control cable string with a 
core member which transports fluid along its entire length whilst it also 
performs a load-bearing function--see the previously mentioned NO 920689 
(WO 93/17176). However, it is new to set apart a certain part of the 
length of the control cable string, especially where the string is exposed 
to bending loads, such as up towards the connection point on the surface, 
where the core member only functions as the load-bearing member and the 
actual fluid transport and any transport of energy and-signal 
communication take place peripherally relative to the core member. 
In the transition between the central fluid flow member and the 
peripherally arranged flowlines there is a manifold element in the form of 
a branch pipe disposed and forming communication between the central 
flowline and the peripheral flowlines. 
The manifold element expediently forms a cavity from where the central 
flowline runs in an axial direction and the various peripheral flowlines 
run out in the opposite basically axial direction. 
The load-bearing central member may be secured to an axial extension of the 
manifold housing with the aid of suitable attachment means, such as 
moulding, a compression sleeve, cold welding, cold-heading, welded 
connections, rivet connections and screw connections. 
The principle used to reduce the load on the centre conduit is to replace 
it with several conduits of smaller dimension, either along the entire 
length of the cable or along the length of the umbilical where the stress 
would otherwise be excessively great. This is done, in the last-mentioned 
case, by ending the large centre conduit in a branch pipe or manifold 
where there are outlets to the smaller conduits. In order that the 
smallest load possible is to be transmitted into the smaller conduits, a 
slightly bendable "rod" is inserted into the centre of the umbilical for 
this to take most of the tensile load. In the detailed construction made 
in the transitions to and from the small conduits, importance is given to 
the increase in flow resistance being as small as possible. 
The method used to increase the weight of the umbilical is to place tubular 
lead elements on the outside of the centre conduits. These elements are 
held in a stable longitudinal position by welding into place attachment 
rings on the centre conduits or on special connecting elements along the 
length of the centre conduit. An intermediate piece is clamped to these 
attachment rings, which at each end rests against the lead elements. 
Between each lead element there is placed a flexible ring which 
distributes the pressure in the contact faces. 
In any position along the umbilical where there is a centre conduit, there 
can be provided an attachment for an external connection such as a 
mooring. A connecting link having two or more longitudinal wings is welded 
inside the centre conduit line. These wings are narrow enough to pass 
through the layer of conduits and/or cables which lie around the centre 
conduit. The top of the wings project outside the outer sheath and a clamp 
is secured to these.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A typical situation where the present umbilical or control cable string 1 
is included as an essential part is shown in FIGS. 1 and 2. In order to 
bring out the details, FIG. 1 is a somewhat more schematic presentation 
than FIG. 2, which shows more a real course of the umbilical 1 between a 
connection point 6 on the surface and a connection point 5 on the seabed. 
The umbilical 1 is roughly divided into three sections, an upper rigid 
section A close to the connection point 6 on the surface, a submersible 
section B at the basically vertical part from said connection point 6 on 
the surface and a buoyant section C between the submersible section B and 
connection point 5 on the seabed. The connection point 6 may be a buoy 
floating on the surface of the water, and can be attached directly or 
indirectly via another cable to a floating structure or a vessel operating 
on the surface. The rigid section A shown in FIG. 1 is of the more 
conventional type with increasing cross-section in the upward direction 
towards the buoy or the connection point 6. The increasing cross-section 
is due to the stiffening members which are applied onto the outside of the 
actual umbilical, which has the same cross-section the whole way between 
the connections points 5 and 6. This rigid section has been introduced 
precisely because of the particularly great bending and tensile stresses 
to which the umbilical is exposed in this area, and is there to take up or 
withstand these stresses. In the new umbilical 1, the rigid section A is 
replaced by a flexible section which handles the great forces at work in 
this area in a different way than the earlier rigid section. 
The submersible section B may to advantage include gravity elements, e.g., 
in the form of leaden weights, which are attached to the umbilical at 
certain predetermined intervals. These gravity elements are only required 
when the umbilical's own weight is such that the umbilical tends to float 
or when the unloaden weight is not sufficient to make the umbilical hang 
in the desired catenary form. 
The floating section C may have buoyancy elements 15 to increase the 
buoyancy along is a certain predetermined portion of the umbilical 1 in 
order to achieve the desired catenary form. 
The connection point 5 on the seabed may be a wellhead or other equipment 
on the seabed with the facility for methanol injection into the production 
stream, or for chemical injection into subterranean formation strata. 
FIG. 4 shows a cross-section through the new umbilical 1. Its structure is 
as follows: A core member 4 constitutes the load-bearing part, in the 
application called the rod or stay, of the umbilical 1 so that the 
essentially axial tensile forces are transmitted through this member 4 and 
only minimal axial forces are transmitted in the other components of the 
umbilical. The core member 4 may to advantage be a steel stay consisting 
of single wires 11. A spacer pipe 16 may, although not necessarily, be 
placed around the core member 4. This may also quite simply be a 
cavity--depending upon whether this cross-section is to be found 
throughout the entire length of the umbilical or only in the flexible 
section A. On the outside of the spacer pipe 16, there is a first set of 
fluid flowlines 2" which are also twisted or layed around the core member 
4 in the longitudinal direction and with a relatively long laying length. 
On the outside of the flowlines 2" inner spacers 8' and outer spacers 8, 
are provided which between them form channels 7 for receiving additional 
flowlines 2 and electric cables 3. It should be noted in particular that 
all the elements mentioned above are twisted in the longitudinal direction 
with a moderate laying length. It should also be noted that a clearance 
exists between the walls of the channels 7 and the flowlines 2 and cables 
3 accommodated in the channels 7, so that the flowlines 2 and the cables 3 
are axially moveable in the channels 7 relative to the spacers 8, 8'. This 
is essential for obtaining a fully flexible umbilical 1. As an outer 
sheath 17, it would be advantageous to use a plastic covering material. 
A conceivable possibility is to construct the length of the entire 
umbilical 1 having the cross-section which is described above. However, in 
practice the umbilical 1 will probably be constructed in a conventional 
manner along the greatest part of its length, i.e., it will have a 
structure identical to that shown in FIG. 5 and described in more detail 
in WO 93/17176, having a centrally located flowline. In such a situation 
only the upper flexible section A will have the cross-section shown in 
FIG. 4 in order to avoid the use of the ever-larger bend stiffeners which 
are indicated in FIG. 1. 
A part of the flexible section A is shown in FIG. 3. In order to pass the 
central fluid flow to the peripheral fluid flowlines, a manifold element 
12 having an internal cavity 13 is installed. The cavity 13 is in fluid 
communication with the central flowline 2' at its axial end and with 
several peripherally arranged flowlines 2 at its other axial end. 
In an axial extension 14 of the manifold housing 12, this is secured to a 
steel stay core member 4 with the aid of suitable attachment means, such 
as moulding, a compression sleeve, cold welding, cold-heading, welded 
connections, rivet connections and screw connections. The extension 14 may 
be in the form of a bar which advantageously is an integral part of the 
manifold housing 12. The number of flowlines 2 and cables 3 may be varied 
according to need and the application of the umbilical. There is nothing 
to prevent the umbilical 1 from having no electric conductors or cables 
whatsoever and consisting of fluid flowlines only. It should be noted that 
the peripherally arranged flowlines 2 are also intended to be twisted 
about the core member 4 along the length of the flexible section A. 
A section totally identical to that shown in FIG. 3 but inverted may be 
provided at the top of the flexible section A close to the connection 
point on the surface, so that the umbilical 1 at the connection site has 
the conventional cross-section. However, this is optional and must be 
adapted to the application in question.