A Sensitivity Study of Hull Girder Wave Loads for Ship Shaped Oil Production and Storage Vessels
Authors: Soqstad B.E. 
Published: October 2008

For ship type floating production and storage vessels (FPSO) moored at a permanent location, the environmental loads will be different compared to a conventional merchant ship for which wave bending moment and shear forces are calculated by use of empirical formula established by classification societies.

The effect on the design loads caused by the differences in the environmental parameters applicable in the design of a FPSO versus a merchant vessel will be considered. The environmental parameters are: Long crested compared with short crested waves. Equal probability of occurrence of all heading angles compared with the waves always approaching the vessel's bow. The design of a FPSO is based on a storm with a return period of 100 years, while a merchant vessel is designed to withstand a storm with 20 years return period.

The linear wave theory is not able to distinguish between the sagging and hogging wave induced responses.

This will be taken care of by introducing non-linear correction factors. like non-linear response is caused by e.g. slamming, water on deck and bow flare.

The study will propose a simplified method to predict wave induced vertical mid ship bending moments and shear forces in an early design stage for a FPSO.

1.INTRODUCTION

In ship design it has been common to apply empirical formulae for the mid ship section. When the first turret moored FPSO was designed it became clear that these numerical formulae gave non-conservative estimates on the wave induced bending moments and shear forces, partly due to the requirements laid down in statutory legislation, but also due to the vessel's design and special way of operation mode.

Direct calculations by linear theory were carried out to establish the correct global responses and nonlinear corrections were introduced to distinguish between the sagging and hogging moments and shear forces. The calculations which are basis for this study are in most cases based on 3D sink source theory. However, some calculations are carried out with 2D strip theory. The non-linear correction factors are calculated by using a computer program which calculates the non-linear response of coupled heave/pitch motions and vertical loads in regular head waves. These calculations are carried out in time domain.

Five vessels will be included in the study. One vessel will be a tanker design, three will be typical FPSOS, and the last is called the "Central Value Vessel" This vessel has been applied in an extensive parametric study in connection with non linear corrections. The variation in the vessels' particulars will be investigated with respect to their influence on the linear responses. The effect on long term response due to variation related to the environment will be investigated, such as:Short crested versus long crested seas.Equal probability of the waves approaching the vessel from any direction compared with the waves always approaching the vessel's bow.The consequence of increasing the probability of the occurrence of the response from once every 20 years to once every 100 years.