Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters (e.g. diameters may range from 0.05 m up to 0.6 m). Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including polymer, and/or metallic, and/or composite layers. For example, a pipe body may include polymer and metal layers, or polymer and composite layers, or polymer, metal and composite layers.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 meters)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Also, underwater currents and turbulence can be more severe at increased depths and can cause increased movement of the riser in the water. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. For example, a flexible pipe may be required to operate with external pressures ranging from 0.1 MPa to 30 MPa acting on the pipe.
One technique which has been attempted in the past to in some way alleviate the above-mentioned problem is the addition of buoyancy aids at predetermined locations along the length of a riser. The buoyancy aids provide an upwards lift to counteract the weight of the riser, effectively taking a portion of the weight of the riser, at various points along its length. Employment of buoyancy aids involves a relatively lower installation cost compared to some other configurations, such as a mid-water arch structure, and also allows a relatively faster installation time. Examples of known riser configurations using buoyancy aids to support the riser's middle section are shown in FIGS. 1a and 1b, which show the ‘steep wave’ configuration and the ‘lazy wave’ configuration, respectively. In these configurations, there is provided a riser assembly 200 suitable for transporting production fluid such as oil and/or gas and/or water from a subsea location to a floating facility 202 such as a platform or buoy or ship. The riser may be provided as a flexible riser, i.e. including a flexible pipe, or as a composite riser, or a metallic riser, and includes discrete buoyancy modules 204 affixed thereto. The positioning of the buoyancy modules and flexible pipe can be arranged to give a steep wave configuration 2061 or a lazy wave configuration 2062.
Wave riser configurations as shown in FIGS. 1a and 1b may also be used in shallow water applications so as to allow for excursions of the vessel from the point where the riser contacts the sea bed.
During use, a riser may be subject to dynamic loading due to conditions such as motion of a vessel or platform on the sea surface. Surge and heave motion of such surface vessel can cause curvature changes in a riser configuration. Strong currents may also have a similar effect. It is generally advantageous to prevent shape changes or control such changes within predetermined limits. The attachment of buoyancy modules, for example in a wave configuration, is one technique for creating a pre-determined nominal shape without constraining the pipe, although the effects of surface motion or current motion are still significant in the upper section of the riser and in the areas of the sag and hog bends of the wave configuration. A mid-water arch system has a comparatively higher degree of control and constraint on the pipe, as the pipe is typically clamped to a buoyancy module which has guides running across it for the pipe to lie in. However the size and weight of such mid-water arches is such that the costs of design, manufacture and installation can be very high indeed.
In addition, when a riser is located within a relatively short distance of an adjacent underwater structure (e.g. another riser extending to the same turret of a vessel), wave motion of the surface vessel and/or motion from strong currents may cause the riser to collide with the adjacent underwater structure. This can lead to damage of the flexible pipe and may also damage adjacent flexible pipes. The damage may be minor in nature but nonetheless lead to a reduced lifetime of the riser, or the damage may be more severe, requiring emergency repair work.
WO2009/063163, incorporated herein by reference, discloses a flexible pipe including weight chains secured to a number of buoyancy modules on the pipe. The chains hang from the buoyancy modules, extending downwards to the sea bed and having an end portion lying on the sea bed.
Another known arrangement employs a mid-water arch structure (as briefly mentioned above), where a riser is laid over and attached to the mid-water arch, so that the weight of the riser in the water is partially taken by the mid-water arch, reducing tension loading and degrees of freedom of the pipe. These structures tend to be difficult to install because they are secured to an anchor or gravity base on the seabed in a specific location, and also very expensive to install.
It would be useful to provide an improvement or an alternative to the above-mentioned arrangements.