MOSAS: A Motion and strength Analysis System for Semisubmersible Units and Floating Structures
Authors: van Opstal G.H.C. , Hans D.H.C. , Salomons J.W. , van der Vlies J.A. 
Published: October 2008

A realistic approach to analyzing a floating unit is to elaborate statistically motion and stress-transfer functions for regular waves, using measured wave-energy spectra. Accurate determination of the transfer functions is therefore of utmost importance. This paper presents a computer program in which all necessary computation have been integrated into one all-embracing system. For reasons of efficiency, the program is divided into two parts. The motions part is comprised of the calculation of all quantities of the motion equations and the solution thereof. In the second part, all loading data and structural data are collected, after which a space-frame analysis program is applied to determine stresses. The paper elucidates the structure of the program and its theoretical backgrounds, of which the use of potential theory for computing wave forces is perhaps the most important. The results of the program are illustrated by two examples. For the unit ' STAFLO?, the computed added-mass terms, wave forces and motions are compared with the outcome of model experiments. For a unit of the 'SEDCO-700? type, the motion-transfer as well as some stress-transfer functions are given. As an example of the possibilities of anchoring computations, the program is used to determine the influence of vertical motions on the line tensions. The 'MOSAS? program has proved to be capable of quick and effective analysis. Contrary to previous methods, data handling is restricted to a minimum by complete integration of all computations. This approach reduces the possibility of errors and saves a considerable amount of time.

1. INTRODUCTION

The analysis of motions and stresses for floating units requires a large number of calculations. In view of their complexity, some of these tasks have gradually been computerized. Still, the use of individual programs involves a large amount of data handling and processing, which in fact means poor use of the possibilities of a computer and an unnecessary risk of making essential mistakes. In the 'MOSAS? program all computations have been brought together into one single all-embracing system. The amount of input is restricted to a minimum and the transfer of data between successive calculations is completely computerized. The output is concise, but the storage of all results on disk still leaves the possibility of more detailed analysis. Programming was performed in FORTRAN, which promotes the implementation of the program on most computers of sufficient memory capacity. 'MOSAS? is subdivided into two sections: analysis of the motions and subsequently of the stresses, both for regular long-crested waves. The first step consists in the computation of the static and dynamic properties of the unit, external forces due to wind, current and waves, and the motions for all six degrees of freedom: surge, sway, heave, roll, pitch and yaw. The effect of anchoring can also be taken into account. In second step the tensional and bending stresses in each member are computed. Apart from some input and output subroutines, the stress-analysis part basically comprises the existing 'ICES STRUDL? system. The subdivision described above seemed indicated from an efficiency point of view.