Abstract

A flexible riser system in Weight Added Wave (WAW) configuration has been developed and installed in a field in the Campos Basin in Brazil, with water depths around 100m. The 11.5 inch ID flexible pipes, used as both production and water injection risers, are tied back to an FPSO with a detachable turret.

Due to the onerous environmental loads combined with excessive FPSO excursion and extreme content density variation of the production risers, the traditional lazy wave and tethered wave configurations were found inapplicable. These configurations were not able to satisfy the required clearances for example between the riser and FPSO hull, the risers and seabed, etc. under the defined service conditions. The traditional Lazy-S configurations were shown feasible but implied a significant cost increase and schedule risk due to the fabrication of the Middle Water Arch (MWA).

As a result, an innovative riser system, WAW, was proposed and subsequently evaluated as a practical alternative solution. The system is based on the traditional wave configuration that incorporates a set of weight chains. The chains are attached to selected buoyancy modules to form a dynamic, self-compensating mechanism that automatically maintains the elevation of the riser configuration within a desirable range, when subjected to excessive changes in weight and FPSO excursion.

This paper presents the technical challenges during the design and detailed engineering stage. As part of the feasibility validation of this system, the effect of the added weight chains over the system's dynamic behaviour was quantified and the dynamic interactive behaviour between the chains and buoyancy modules and seabed was thoroughly investigated. The paper also discusses the WAW buoyancy modules fabrication and special installation procedure. The WAW system was successfully installed and the system is in good production.

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