Patent Application
20080196899
The present invention relates to a marine riser tower, of the type used in the transport of hydrocarbon fluids (gas and/or oil) from offshore wells. The riser tower typically includes a number of conduits for the transport of fluids. In particular it relates to apparatus for buoyancy tensioning of offshore deepwater structures. It finds particular application in tensioning a slender, vertical or near-vertical, bottom-anchored, submarine structure, such as a riser or a bundle of risers (which may, or may not, include a structural member) or an umbilical.
Tensioning is the act of ensuring that a marine structure doesn't experience excursions from its nominal upright position that would fall outside the acceptable limits, even in extreme weather conditions, the said limits being possibly defined with reference to the occurring sea state. There should always be sufficient tension to ensure stability, no matter the weight of the structure and the weight of the pipelines/risers hanging off the structure.
The structure may form part of a so-called hybrid riser, having an upper and/or lower portions (“jumpers”) made of flexible conduit. U.S. Pat. No. 6,082,391 (Stolt/Doris) proposes a particular Hybrid Riser Tower consisting of an empty central core, supporting a bundle of riser pipes, some used for oil production some used for water and gas injection. This type of tower has been developed and deployed for example in the Girassol field off Angola. Insulating material in the form of syntactic foam blocks surrounds the core and the pipes and separates the hot and cold fluid conduits. Further background has been published in papers “Hybrid Riser Tower: from Functional Specification to Cost per Unit Length” by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, 18 Oct. 2001 and “Girassol Field Development—Total Elf Fina—Riser Tower Installation” OTC 2002 number 14211 by Vincent Alliot & Olivier Carré. Updated versions of such risers have been proposed in WO 02/053869 A1, from which it is known to use a vertical riser bundle where the production lines are individually insulated and where the syntactic foam function is buoyancy only.
It is also known, on the Wanaea & Cossack field in Australia, for Woodside, for example, to have flexible riser jumpers each supported by buoyancy foam elements which are clamped to each flexible jumper. Buoyancy foam suppliers such as the CRP Group have developed clamps to attach the buoyancy elements on flexible and umbilical lines.
However, such a system presents some drawbacks: Firstly, there is the substantial cost of individual buoyancy elements and clamps (made in titanium). There is no spare buoyancy, unless there are some spare foam buoyancy elements and associated removable ballast weight placed on the riser tower structure. Furthermore it is necessary to provide sufficient buoyancy along the riser bundle to compensate for the weight of the bundle with the pipe full of water. Also, the buoyancy elements are required to be added to the jumpers on board the vessel and consequently the installation procedure to connect the positively buoyant flexible jumper onto the tower structure is complicated and time consuming. There is also the potential problem of riser jumper clashes which requires the separation of the riser jumper connections at the riser tower top. This requires the need to enlarge the structure at the riser tower top which could potentially create fatigue problems at the interface with the bundle. This increase in the vertical bundle diameter would degrade the dynamic behaviour of the riser tower when it is surface towed.
The present invention attempts to alleviate some or all of such drawbacks.
In a first aspect of the invention there is provided a marine riser apparatus for use in the production of hydrocarbons from offshore wells, said riser tower comprising one or more rigid conduits supported in a tower structure and extending from a connecting structure on the seabed to a point below the sea surface and wherein there is provided one or more flexible conduits extending from said tower structure to connect said tower structure to a surface structure, and wherein there is farther provided a buoyancy device attached to said tower structure, such that said buoyancy device is located above and exerts a buoyancy force on said riser tower and wherein said buoyancy device also supports an intermediate section of at least one of said one or more flexible conduits.
Said tower structure may comprise a plurality of rigid conduits arranged around a structural core. Alternatively some conduits may be located inside a tubular core. Preferably there is also provided the same number of flexible conduits as rigid conduits such that a flexible conduit connects each rigid conduit to the surface structure.
Said buoyancy device may comprise a tank, such as a steel pressure tank, or syntactic foam elements, or both and may be attached to said tower structure by at least one tether. Preferably two tethers are used. Said buoyancy device may initially be ballasted to provide spare buoyancy when required.
Preferably, said buoyancy device also incorporates a support device for the support of said flexible conduits. Said support device may be provided with guides for each flexible conduit in order to minimise clashing. The guides may be replaced by clamping devices combined with bend stiffeners mounted on the flexible conduit structure to optimize the breath of the support device and improve the dynamic response of the structure under the pulling action of the flexible jumpers.
This configuration allows the connection of the flexible jumpers from above directly to the tower structure with or without any intermediate pieces Therefore there is no need for the gooseneck which simplifies the installation.
Preferably said buoyancy force is exerted on the riser through a combination of said at least one tether and said flexible conduits. In one embodiment there is further provided adjustment means to enable adjustment of the tension imparted on said tower structure by said flexible conduits and/or the tether(s). This is particularly preferable since compression loads should not be exerted on the flexible conduits, and the provision of adjustment means which allow the adjustment of the tension of the flexible lines once connected to the tower structure helps to prevent this. There may be provided separate adjustment means for each flexible conduit and/or for each tether. Said adjustment means may be provided on the support device and may consist of hydraulic or mechanical jacks. In an alternative embodiment the flexible conduits may be tensioned by inducing a tilt in a top part of the tower structure by selective ballasting of the buoyancy device. The buoyancy device may comprise at least two tanks or a tank with at least two chambers and each of the tanks/chambers may be selectively ballasted relative to each other, or one tank/chamber may be ballasted only.
The tower structure may optionally further comprise top buoyancy. This may be in the form of a steel tank or foam located around the core at the top of the tower structure. There also may be, additionally or in place of the top buoyancy, buoyancy located substantially along the full length of the tower structure, or alternatively at strategic points along its length.
In a further aspect of the invention there is provided a method of installing a marine riser apparatus according to a first aspect of the invention comprising:
Other embodiments of this method are as disclosed in the appended claims.
Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:
a-5d shows the support arch/buoyancy tank from the front, side, top and isometric views respectively;
This buoyancy tank/support arch 7 is attached to the top of the riser tower bundle 2 by tethers 8. A number of the jumpers 3a rest on the buoyancy tank/support arch 7, depending on the number of riser lines. If there are only a few then all may rest on the arch 7, however if there are many, it may be difficult to accommodate all the jumpers 3a 3b on the support arch and it may be appropriate to have the smaller lines 3b kept in a simple catenary.
In use, the riser tower bundle 2 extends approximately vertically from the well head and is tensioned via the tethers 8 by the buoyancy force acting on the tank 7. There may also be foam provided along the length of the riser tower bundle 2, in order to aid buoyancy as well as foam or steel tank top riser buoyancy on the top of the bundle 1 itself. The buoyancy tank/support arch 7 is designed to be ballasted and consequently can be de-ballasted to provide adequate spare buoyancy when required.
The buoyancy tank/support arch 7, in this embodiment, also incorporates devices 41 to allow independent tension adjustment of each jumper and tether. This support arch tension adjustment of the jumpers and tethers allows optimisation of the way the top tension is transferred to the riser tower bundle 2. It also presents an additional reliability in that the buoyancy tank/support arch 7 is connected to the riser tower by several mechanical links and potentially the role of the vertical tethers 8 can be minimised in operating conditions throughout the design life of the system.
a-5d shows the buoyancy tank/support arch 7 in greater detail from the front, side, top and isometric views respectively. From this it can be clearly seen that the tank/support arch 7 of this embodiment actually comprises two steel tanks 7a, 7b and support arch 7c. Jumper guides 40 are incorporated on the arch 7c which control the jumpers 3a and prevent them from clashing. The jumpers 3a are attached to the top of the riser tower bundle 2 and each one is fed over a jumper guide 40 of the buoyancy tank/support arch 7 which splay out, keeping the jumpers 3a from one another between the buoyancy tank/support arch 7 and the FPSO 5. Each one of the guides has an adjustment device 41 mounted to it.
An alternative arrangement to adjust the tension in the jumpers in depicted in
A particular advantage of this concept is that it allows the installation of both the riser vertical bundle and buoyancy device/support arch in one single operation. The buoyancy device/support arch, the riser bundle and tether line(s) are assembled together at the fabrication yard prior to surface tow operation. The installation operation is then based on the operation as used on the Girassol field (refer to OTC 2002 number 14211 “Girassol Field Development—Total Elf Fina—Riser Tower Installation”) and can be described as follows:
The invention is not limited to the above described embodiments, and other embodiments can be envisaged without departing from the spirit and scope of the invention. Namely, other forms of adjustment means or other methods than those described may be used to keep the flexible conduits tensioned. Also the steps of the installation method may be achieved in a different order where appropriate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0409361.3 | Apr 2004 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2005/005244 | 4/26/2005 | WO | 00 | 12/10/2007 |
