Technical Field of the Invention

The present invention relates to a conduit system for transferring hydrocarbons, power or electrical/optical signals from the seabed to a floating structure in the shallow water when exposed to the environmental loadings from wind, wave and current.

Background of the Invention

hi offshore oil and gas production a variety of buoyant structures are used to support oil and gas processing equipment, storage facilities or other facilities that condition fluids which are being transferred between the seabed and the buoyant structure. Such buoyant structures includes monohull vessels type structures known as FSO (Floating Storage Offloading) or FPSO's (Floating Production Storage Offloading) , semi submersibles, tension leg platforms, caisson structures including spar types, CALM (Crude and a loading buoy) buoys etc. Common for all these structures is that when exposed to a combination of wind, wave and current the structures will displace in the horizontal and vertical plane. Conduits between such structures and the seabed will therefore need to incorporate some means of accommodating this relative movement.

Flexible pipes constructed from a combination of rubber, plastics and metallic elements are frequently used in applications to overcome the above aforementioned relative displacements. Such pipe structures are commonly known as "Flexible Pipe" and when applied for the purpose of transferring flowing media between the seabed and the sea surface the pipe structure is referred to as a "Flexible Riser".

Umbilicals constructed from small bore pipes and/or electrical cables and/or optical cables together with possible armor wiring and external thermoplastic sheeting are frequently used to transfer electrical or hydraulic power and/or electrical or optical control signals between the seabed and the sea surface. The umbilical used in such a dynamic application is commonly referred to as a "Dynamic Umbilical". In the description of the invention the focus will be on its applicability to Flexible Riser, however, the invention equally apply to Dynamic Umbilicals.

A number of configurations are used when suspending the flexible pipe between the seabed and the buoyant unit. The main riser configurations are those known by names of "Free Hanging

Catenary", "Chinese Lantern", "Steep S", "Steep Wave", "Lazy S", "Lazy Wave" and "Pliant

Wave". The shape of the various riser configurations is attained by use of concentrated or distributed buoyancy elements fixed to the riser and sometimes tethered back to the seabed as well as weight elements and tether lines to seabed anchors.

In all these configurations the flexible pipe is continues from the unit to the seabed except for any joints at some mid water depth required to join flexible pipes of different construction is so required or joining made via shot length of hard pipe over support structures such as mid water arches employed in for instance the "Steep S" and "Lazy S" configurations.

In particular shallow water and harsh environment a large proportion of the total riser length is exposed to hydrodynamic loads from the wave action and any buoy structures used in producing the riser configuration is equally exposed to said wave loading. This in combination with the excursions of the surface unit can lead to extreme dynamic movements of the flexible pipe with over stressing, over bending of the pipe or unacceptable compressive loads in the flexible pipe.

Also buoyant support structures may become unstable and exhibit unacceptable dynamic motions rendering the system unfeasible.

A further complication may arise in shallow water applications when the riser system is used in combination with a surface unit that may at times be over the riser system, such as for instance with a turret moored monohull vessel that will weather vane in response to the prevailing environmental loadings. In this instance the available height from the seabed that can be used to generate a riser configuration is limited by the requirement to provide clearance to the bottom of the vessel above, which will experience vertical motions up and down in addition to the water currents.

Summary of the Invention

The object of the present invention is to improve the response of the flexible pipe such that a flexible pipe based system can be used in harsher and shallower water applications.

Unlike the known riser configurations where the riser is terminated at the seabed in close proximity to the mudline in the current invention the riser is suspended from a rigid structure extend up from the seabed to somewhere at mid water depth. The connection at this point is fixed and will not be subject to any significant movement even when exposed to extreme environmental loadings. The flexible pipe in its simplest configuration is suspended in a catenary configuration between the mid water connection point and the surface unit. This arrangement was proposed by KeIm et al refer US Patent US 7,040,841 Bl.. However, in the current invention the further buoyancy beads are added to the flexible pipe to provide one or more inverse catenary shapes along the length of the riser, which will further greatly enhance the relative movements that can be accommodated between the surface unit and the mid water fixity points. Buoyancy beads may also be attached to the flexible pipe to add weight locally to change the shape of the configuration and the riser's dynamic response if required. Subject to the seabed conditions the riser may be allowed to touch the seabed and external wear modules may be attached to the flexible pipe in the touch down area should this feature be required.

Brief Description of the Drawings

Other features and advantages of the present invention will be clear from reading the description hereafter, given by way of non-limitative example, with reference to the accompanying drawing wherein:

Figure 1 illustrates a conduit system of the present invention.

Brief Description of the Invention

Other features and advantages will become more apparent form the description of one preferred embodiment of the invention and from the appended drawing depicting the invention.

The present invention relates to a conduit system to carry liquid or gas or a combination of such media i.e. hydrocarbons and or power and or electrical/optical signals to or from a buoyant unit 1 at or near the sea surface and the seabed 2. In case of hydrocarbons the flow is carried via a flexible pipe 3 attached to the buoyant unit 1 via a flanged or similar type pipe connection. The flexible pipe is attached to a fixed point 4 elevated substantially above the seabed mudline 5. The conduit for the media from the fix point 4 to the seabed connection point 6 is via fixed or flexible pipe 7 as the case may be.

The fixed or flexible pipe 7 is supported of a riser support 8 preferably a rigid structure that can be constructed from steel, concrete or other rigid construction material. The riser support 8 is fixed to seabed and the structures foundation can be drilled or driven piles 9 connected to the structure such suction cans 10 or gravity type foundation or a combination of such foundations.

The fixed or flexible pipe 7 may include valve arrangement 11 for isolation and manifolding if more than one riser is used, pig launcher or pig receiver. Since such devices can be installed at mid water depth it may allow diver intervention using air diving technique rather than saturation diving required at greater depth.

The flexible pipe 3 may be fitted with weight modules 12 to change the catenary shape and/or improve the dynamic response of the flexible pipe when subject to environmental loadings and movements of the surface unit 1. The flexible pipe 3 may be allowed to touch the seabed and can if required be fitted with external wear protection 13.

To accommodate even larger excursions of the buoyant unit 1 the flexible pipe will be fitted with buoyancy beads 14 to create one or more inverse catenary shapes along the length of the flexible pipe. These inverse catenary shapes greatly enhance the excursion envelope of the surface unit the system can accommodate as compared to a riser hanging in a simple catenary shape as proposed in prior art.

The flexible pipe 3 may require bend stiffener 15 to provide a gradual increase in bend stiffness and so as to not over bend or kink the pipe at or close to its attachment points. The pipe may also be supported by an arch or a gutter structure 16 to avoid the flexible pipe being over bend in the in and out of plane directions. The tether 17 may be required to anchor and prevent the flexible pipe 3 from over float.

One advantage of the present invention is that it increases the excursions and movements that the flexible pipe can accommodate with out exceeding its allowable mechanical properties for stress and bending. This reduces the requirement to the mooring arrangement required for the surface unit and also allows the system to be deployed in more shallow and harsh environment.

Another advantage with the current invention is that it reduces the length of flexible pipe to be used and since flexible pipe is more expensive than normal rigid pipe the invention offers savings on flexible pipe material, which can be significant particular when a large number of risers are configured as proposed.

The proposed invention is also less sensitive to changes in the density of the fluid being conveyed inside the flexible pipe and so can for instance readily accommodate slug flow where the flow regime consist of gas flow separated by liquid slugs flowing through the system