Integrated Fatigue Analysis of the GAP Structural System

MURPHY Sabah Oil Co. Ltd. has developed the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a Spar Dry Tree Unit (DTU). Fluids are transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system. The GAP is an interesting combination of mooring (tether chains), dynamic steel riser (carrier pipe and flowlines) and steel structures (towheads). Design codes and standards usually address the design of these components separately. One of the challenges of the GAP project is to have a consistent design philosophy for all the components so that the GAP can be treated as an integrated system with homogeneous quality and safety levels. GAP component fatigue analysis is a good example of integrated system design. In the GAP, fatigue loading is applied by the floaters, through the tether chains, to the towheads into the carrier pipe. The fatigue analysis of individual GAP components cannot be performed in isolation — it must be the result of an integrated GAP fatigue analysis. A global model of the GAP is built with towheads modelled as rigid bodies and tether chains and carrier pipe modelled as dynamic lines. This model is used to obtain time series of loads on all components of the GAP. The fatigue of each component is calculated using the same methodology based on stress Response Amplitude Operators (RAOs) for a selected number of combinations of FPSO headings, wave directions, FPSO drafts and fluid densities. This methodology is classical for chains and steel pipes. It is less classical to apply such a detailed methodology for large structures like towheads. The towhead structures are key components that provide connection between tether chains and carrier pipe, flexible jumpers and steel flow lines, carrier pipe and decoupling overhead buoyancy tank. As such, the fatigue analysis of the towhead is as critical as for the tether chains and the carrier pipe. Finite element models of the towheads have been subjected to unit loads from all components attached to them and from the dynamic fluid pressures generated by unit towhead accelerations. Using the loads extracted from hydrodynamics calculations on the global GAP model and the matrix of stress for unit loads, the time domain approach is kept throughout the complete structural assessment of the towheads. This is in order to maintain a high degree of accuracy in the stress prediction. Given the criticality of the carrier pipe, a very detailed Engineering Criticality Assessment (ECA) is performed to define flaw acceptance criteria to be used during the Non Destructive Examination (NDE) campaign.