Hull Strength Design of a Floating Production Unit Using Long Term Hydrodynamic Response

Abstract

Normal procedure for design of a ship shaped Floating Production Unit (FPU) is to use Rule Based approach recommended by the Classification Societies. In this approach the primary design strength quantities such as longitudinal and transverse wave induced bending moments, shear forces etc. are calculated based on formulae prescribed by the Classification Society Rules. In the Rules, these formulae have been validated for a class of ocean going vessels such as container ships or tankers, as the case may be, for open ocean conditions akin to the Northern Atlantic. Thus, actual hydrodynamic response of the vessel to a defined sea conditions at a location of service need not be calculated. FPUs are different from ocean going vessels in that they are usually fixed at a location for several years of service with the help of some station keeping arrangement such as a mooring system or a dynamic positioning system, except for transit. An alternate approach is to use direct calculations for hydrodynamic response of the vessel to the site specific sea conditions at the operating site. The hull geometry may also be very much different from a conventional tanker due to addition of sponsons that could justify such an approach. In addition, an FPU is likely to support several equipment modules on top of the deck which is different from the loading of usual cargo ships and may demand more accurate calculations. Thus, in order to design the module support footings one is interested to know the actual accelerations at the module center of gravity. Many Classification Societies allow use of this direct approach, but it appears that this route is seldom taken possibly due to complexities of hydrodynamic analysis. Response quantities of interest for the design of the vessel, such as hull bending moment, acceleration, roll angle etc., are selected as the Dominant Load Component (DLC). In general, there are two methods to compute the extreme hydrodynamic response of any DLC, short term method and long term method, the latter being considered as superior and an extension of the former. The wave environment is defined in terms of the probabilities of different sea states at the specific offshore location through a directional scatter diagram. The long-term response refers to the long term Most Probable Extreme Value (MPEV) of the response at a specific probability level of exceedance derived from the short term responses. Usually, the MPEV of the DLC having a Return Period of 100 years, i.e., a probability of occurrence roughly of the order of 10−8.7 is taken as the extreme design response. For each DLC, an equivalent regular wave called the Design Wave is determined which simulates the magnitude of this extreme value of the Dominant Load Component for the purpose of the structural analysis. For the FPU discussed in this paper a detailed 3D Finite Element Model is created and the hydrodynamic loads corresponding to each Design Wave for the DLC are applied for the structural analysis. Gulf of Mexico weather conditions are taken as the operating area. In this paper, selected results from the hydrodynamic approach are reported and compared with similar results from the conventional Rule Based approach. If any difference is found, the possible reasons thereof are discussed. The authors believe that the methodology and results reported in this paper for a specific FPU will help to understand the true behavior of other ship type FPUs for the site specific conditions and the same methodology can be applied for design.