Restrained Hybrid Buoyant Riser Tower for Deepwater Developments

Event:
DOT
Date:
September 1999
Author:
S. Hatton - 2H. J. Legras - ETPM

Hybrid Risers are considered as a suitable option for Deepwater fields developed using FPSO and subsea well heads because they have low sensitivity to the environmental and operational constraints: they can bear significant currents, sea states and FPSO motions over large water depths (1,000m and over). Furthermore, the surrounding syntactic foam provides high thermal insulation to the production flowlines over the water column.

However, the piping connections located at the top part of the risers are usually underwater, inducing high design, operational and maintenance constraints for the fluid link to the FPSO, an area where the velocity of the production fluid is large and may induce erosion

Moreover, the nominal distance between the FPSO and the Hybrid Riser should be carefully selected to avoid any interference considering the FPSO and the Tower maximum relative displacement. Current loading on umbilicals and free hanging flexible jumpers of different weight over diameter ratios must also be considered for possible interference. The concept of a Restrained Hybrid Buoyant Riser developed by ETPM and 2H [Ref. 1] under a contract for the WADO1 group takes advantage of the recent Hybrid Riser design developments [Ref. 2] but eliminates underwater lines and connectors by linking directly the tower top to the FPSO by means of an articulated yoke structure in air (see figure 1). As a main advantage, this configuration allows to locate all the piping connections above the water level, thus offering potential for reducing maintenance cost. The yoke comprises a buoyancy tank which provides tension to the top of the riser tower and is linked to the FPSO side by a hinge.

The basic concept described here is designed for 2,500m of water depth for environmental conditions typical of those of the Gulf of Guinea. A 1,000m water depth has been also analysed and some basic results for this case will be listed when relevant.

The Restrained Hybrid Buoyant Riser main features are described in the followings.

 

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