Structural Integrity Management for a Large Pre-Stressed Concrete Floating Production Unit

Offshore Floating Production Units, usually deployed under long-term plan, handle the field production so they cannot be easily removed for dry-docking and repair. In order to constantly analyze and monitor the condition of the units, a tailor-made methodology has been developed and implemented since 2004 for the Integrity Management of our Floating Units currently in operation. The paper gives a description of this methodology, and then focuses on how the methodology was deployed for the large pre-stressed concrete Floating Production Unit (FPU) located offshore Congo on N’KOSSA field. This FPU is the largest existing pre-stressed concrete Floating Production Unit, built with high performance concrete, installed offshore since 1996 in 170 m water depth. She has now accumulated 10 years of production. A significant part of the methodology is based on a full Finite Element Model (FEM) with non-linear analysis capacity for the concrete structure, incorporating a description of passive and active steel. There is often an anchored perception that a concrete unit is something not requiring attention once installed. This paper shows otherwise, underlining the complexity of modeling the highly non linear characteristics of pre-stressed high performance concrete and degradation modes.