Third Generation Deepwater Hybrid Risers

World Wide Deepwater Technologies, IBC
June 1999
S. Hatton, F. Lim

As oil exploration and production move into deeper water there is a need to develop technically viable and cost effective risers. A candidate riser arrangement is the hybrid riser system, which is the basis for a number of developments offshore West Africa.

Hybrid risers consist of a vertical bundle of steel pipes supported by external buoyancy. Flexible jumpers connecting between the top of the riser and the vessel are used to accommodate relative motion between the vessel and riser bundle.

The first generation hybrid risers, which were installed in the Gulf of Mexico, were designed for a large number of flow paths and were installed through the moonpool of a drilling vessel. However, due to the large size, design complexity and weight of these systems installation was very costly.

The second-generation hybrid riser design, proposed for Girassol, is fabricated at an onshore site with installation by tow out and upending. This approach provides significant cost reductions as a result of weight savings, design simplification and reduction of installation schedule.

The disadvantages of both first and second-generation hybrids are:

  • Design complexity of the bundle
  • Installation risk
  • Low field development flexibility

The Concentric Offset Riser (COR), developed by 2H Offshore, may be considered to be a 3rd generation Hybrid riser. It offers many of the benefits of the earlier concepts but, having a simple bundle design, can be quickly and economically installed from a drilling vessel. This greatly reduces risk and the smaller bundle arrangement increases development flexibility. The following paper describes the system arrangement, preliminary analysis results and costing data.

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