Stress Analysis of Turret Interacting with Ship Hull Structure

Abstract

The trend in offshore oil and gas industry has been that new oil and gas fields are more remote in terms of water depth and in distance from existing infrastructure. The high price for oil and gas drives the industry to develop fields in harsh environment and record breaking water depths. Fixed offshore structures are not feasible for ultra deep water depths and may also be less profitable or unprofitable for moderate depths. This is due to the high cost of laying export pipelines in remote areas or because marginal fields only requires production facilities for a few years. Floating production storage and offloading systems, FPSOs, has been used by the offshore industry since the late 1970s but the usage has rapidly increased over the last two decades. One of the benefits of using FPSO‘s is that export can be  done by shuttle tankers, thus no pipeline infrastructure is required. The FPSO itself are not depth sensitive and the challenges related to greater water depths are mainly considering the riser and mooring system. The use of FPSO‘s for marginal fields are also beneficial because of short installation time and  mobility. Therefore it can also easily be reused for new fields.

The methods for designing the hull of an FPSO has been somewhat mixed between design rules for ships and design rules for offshore structures. Since the ship rules are not based on specific site parameters and the design rules for offshore structures has mainly been focusing on fixed, tension leg systems or semi submersible platforms, neither one of the two methods has addressed the whole aspect of designing an FPSO. It is actually just recently that several of the main class societies and international standards have released new design codes for floating production systems. Based on the requirements in DNV-OS-C102 and the results available from Wamit a worksheet has been made in Mathcad. The worksheet sums the contribution from each panel and gives the resulting bending moment and shear forces as a hydrodynamic transfer function for the wanted section. Further a structural model with the extent of tank 1 and 2 has been made in Ansys. The sea pressures established in the Mathcad spreadsheet (based on Wamit results), are transferred onto the Ansys model. ULS analysis in Ansys of full load and ballast condition has been preformed for a selected ULS design wave.