Hybrid Risers

Hybrid Risers
January 1995
S. Hatton

Ship shaped and semi-submersible floating production systems have been shown to be highly cost effective field development solutions. Flexible risers have been widely adopted on these developments since their flexibility allows large vessel excursions to be accommodated without overstressing.

Flexible riser technology is now well established however, costs are relatively high due to the complex pipe manufacturing process and materials used. As water depth increases, flexible riser costs become an increasing proportion of the total development cost. Additionally, flexible pipe may be considered limited with respect to high temperature and pressure capability.

This has lead to the development of a number of deepwater riser concepts that utilise all or a majority of rigid pipe. The main objective of these developments has been to reduce the length of flexible pipe required, which is up to 10 times the cost of steel pipe, in order to reduce total riser costs.

The hybrid riser is one of these concepts. Whilst a number of hybrid configurations have been developed over recent years only one has been installed. This was by Placid Oil Company in 1988, in the relatively calm waters of the Gulf of Mexico (Green Canyon 29, Water depth 469m).

Evaluation of alternate hybrid riser configurations and design approaches clearly shows that cost effectiveness, when compared to flexible risers in the water depth range 400-600m, can only be achieved by careful design optimisation.

The following paper discusses an approach to hybrid riser design and presents a design, developed by 2H Offshore Engineering Limited, that is cost effective in water depths as low as 400m and is also suitable for an extreme environment typical of the Atlantic Frontier.

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