The use of flexible dynamic risers in conjunction with floating production systems is rapidly becoming the preferred method for developing small- to medium-sized offshore oil- and gasfields. The systems are based on the following types of units:
lack-up
ship/tanker
semisubmersible
tension-leg platform
As of April 1987, some 75% of the estimated total 32 floating production systems in operation worldwide were fitted with flexible riser pipes, with an additional ten units of an advanced stage of design/construction.
The majority of existing systems are located offshore Brazil, the Far East and in the Mediterranean The use of flexible riser systems in harsh environmental conditions has, however, gradually gained acceptance, partly due to the Hamilton Argyll, Sun Balmoral and Golar Nor Petrojarl projects. Amerada Hess has, for instance, confirmed the use of flexible riser pipes for the Rob Roy/Ivanhoe Field development, with Sovereign expected to follow suit when developing the Emerald Field.
An advance on the Marathon Kakap FPSO system is the use of a semisubmersible production unit linked to a lightweight wellhead platform via a series of flexible riser pipes. The concept is shown in Fig 1 and has been put forward for Statoil's Veslefrikk and Norsk Hydro's Oseberg phase two field developments.
A more novel application for flexible riser pipes is Placid Oil's US Green Canyon project, featuring a hybrid riser (Fig. 1 is available in full paper) system. Flexible risers are used to link the floater to a 420 m buoyant rigid riser in order to enhance the compliancy of the system.
Plans for using flexible riser pipes to tie-in additional remote satellite wells to a tension-leg platform is a further manifestation of the increased acceptance of flexible pipes. Both the TLP proposed for Saga's Snorre Field and the TLWP destined for Conoco's Green Canyon Jolliet Field will feature such systems.
To assist the engineer in the initial stages of design, this chapter sets out to define the component parts of a flexible dynamic riser system, and also provides guidelines for the selection of system configurations and methods for design. Aspects of installation, as well as system inspection, maintenance and repair, are then briefly discussed.
A dynamic flexible riser system will generally comprise of any of the following component parts.
flexible riser pipe including end terminations
upper end connector and release system
subsurface buoys or buoyancy modules
riser bases or pipeline end manifolds (PLEM)
The selection and design of each component part is closely interlinked with the overall design of the riser system. It is therefore essential that the design engineer has a good understanding of the specific function of each component part as well as their influence on the global behaviour of the system. The system should also be designed in close cooperation with the flexible pipe supplier, to ensure that loads and the geometry of the pipe are maintained within acceptable limits
