Bundle designs ready for +2,000 m
Since the success of hybrid riser towers in the deepwater conditions at the Girassol field offshore Angola, Stolt Offshore has undertaken a development program to improve the HRT concept and extend the range of conditions where the HRT can be applied.
The basic concept still remains the same, in that a number of riser pipes are bundled together around a core pipe, all fabricated and assembled onshore prior to towing the riser bundle to the field. Once at the field, the riser is upended and connected to the seabed via a pre-installed suction anchor. Finally the top of the HRT, which is located at an optimum depth to minimize the effects of strong wave and current action, is connected to the floating production facilities through a series of short flexible pipes.
The concept of a bundled vertical riser had its origins in the Gulf of Mexico, where in 1988 Placid installed a riser bundle on the Green Canyon block 29 development. In 1994, Enserch re-deployed the same riser on its Garden Banks block 388 development. Although this riser system is closer to drilling technology than the Stolt Offshore HRT system, the overall riser tower concept is built upon field-proven technology.
One of the main advantages of the HRT concept is the ability to deal with a majority of the flow assurance issues that are pivotal to the success of today's deepwater developments. These include the prevention of wax and hydrates forming within the riser pipes, which in the past have been solved with chemical dosage systems. As a result, one of the key areas on which the HRT development program has concentrated has been that of flow assurance management. A suite of measures to maintain the required flow conditions within the production risers can now be built into the HRT concept as and when required.
Alternates to the Girassol approach combining both the insulation and buoyancy properties of the syntactic foam have been developed. These include using the syntactic foam elements only for buoyancy, and applying a separate insulation coating to the individual production risers or using pipe-in-pipe products directly as part of the riser bundle. In addition, active heating can be included within the HRT cross-section design using several available methods, such as hot water or electrical heating elements. The HRT can be considered as process equipment capable of controlling the conditions of the production fluids by providing heat, gas lift, and blending fluids and chemicals.
Each Girassol tower was made up from a bundle of 6 x 8-in. riser pipes (four production and two injection) arranged around a structural core pipe, together with a further six small-diameter gas lift and service lines.
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The Girassol field off Angola, which came onstream in 2001, saw the application of three HRTs) in 1,400 m of water. The main driver for the selection of the HRT system was the critical control of the fluid temperature on its journey to the processing facilities onboard the FPSO. Through the insulation provided, the temperature of the fluids stayed above the critical temperature where wax could form inside the pipes and eventually plug the entire line. In addition, during unplanned shutdown conditions, the HRT provides the required cool-down time to implement hydrate mitigation measures.
The Alto Mar Girassol consortium, of which Stolt Offshore was the major partner, designed, fabricated, and installed the three HRTs produced for Girassol. Each Girassol tower was made up from a bundle of 6 x 8-in. riser pipes (four production and two injection) arranged around a structural core pipe, together with a further six small-diameter gas lift and service lines. The bundle was encased with sections of syntactic foam, which provided both the insulation for the well stream fluids and the buoyancy to keep the HRT in a vertical position.
A large buoyancy tank is situated at the top of each of the towers deployed at Girassol to provide additional vertical buoyancy to the tower. The connection of each tank was a complex operation performed in the sheltered water close to the fabrication site. This particular area of the tower design has been studied as part of the HRT development program, with a number of solutions produced to simplify the connection procedure, including a system of sliding buoyancy elements to reduce the need for the tank at the top.
Other parts of the program include the adaptation of the HRT concept for applications with other floating production platforms such as TLPs and semisubmersible platforms, as well as turret moored FPSOs. These are relatively straightforward adaptations of the HRT because of the inherent flexibility built into the overall concept. The extension of the HRT concept to ultra-deepwater conditions beyond 2,000 m has been studied and solutions are now available for implementation.
