No abstract

Motion analysis of platforms operating in deep waters is quite a challenge. In order to obtain a realistic motion, the analysis must be performed as an integrated system of risers, mooring lines, and the platform. A fully coupled analysis of such a system is very time consuming and modeling simplifications, both structural and hydrodynamic, are often applied thereby introducing uncertainty in the accuracy of the results. For accurate vessel motions such fully coupled analysis may not really be warranted. It is rather beneficial to use a simplified procedure to include the effects of the risers and mooring lines and, if possible, be more accurate in the hydrodynamic modeling.A simple procedure has been developed in the form of a computer program, NICDAF, (Non-linear Integrated Coupled Dynamic Analysis of Floaters). The coupling effects of the risers and mooring lines are included based on their quasistatic configurations at each time step instead of their actual dynamic equilibrium configurations. The case study results show that full coupling of dynamic equilibrium is not important for accurate platform motion, but it is important for the riser or mooring line motions. Details of the development of the procedure and the case study analysis results from ABAQUS and NICDAF are presented.

The use of non-weathervaning floating production units is found feasible for fields in the deep waters of West Africa. The lack of pipeline infrastructure in these remote areas requires export of produced fluid from an offloading system. The transfer of product from the production vessel to offloading buoy is carried out by large diameter suspended flexible flow-lines. The flow-lines are configured in a double wave shape, "W" concept, by introducing distributed buoyancy along the middle section of the lines.The paper presents an alternative concept, a naturally suspended double catenary ("U" configuration) rigid steel flow line. A preliminary, but comprehensive, study was conducted, which considered strength and fatigue performance, technical merits and demerits, economic gains and losses, and safety and reliability concerns between the two concepts. The results of the study confirmed the feasibility and potential savings of the proposed "U" concept.

Steel catenary risers (SCR) have been used to connect the export lines to the Morpeth TLP platform. To date, this is the shallowest water depth employment of SCR. The majority of the risers, in particular, the touchdown and sag-bend have been installed by employing the "S" lay technique, another first for SCR installation. Significant design effort was invested to understand the issues involved and resolving them, in a method to demonstrate the safety and reliability of such an application. Design effort consisted of an improved analysis model, non-linear modeling of the seabed and flex-joint, fatigue testing of full scale pre-strained riser joins, development of a VIV strake sizing procedure, and the extensive use of fracture mechanics and Engineering Critical Assessment.