"MOSAS" : a Motion and Strength Analysis System for Semi-Submersible Units and Floating Structures
Authors: Hans D. , van Opstal G.H.C. , Salomons J.W. , van der Vlies J.A. 
Published: December 2007

This paper was prepared for the SPE-European Spring Meeting 1974 of the Society of Petroleum Engineers of AIME, held in Amsterdam, the Netherlands, May 29–30, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Netherland Section of the Society of Petroleum Engineers, P. O. Box 228, The Hague, the Netherlands. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

A realistic approach to analysing a floating unit is to elaborate statistically motion and stress-transfer functions for regular waves, using measure wave-energy spectra. Accurate determination of the transfer functions is therefore of utmost importance. This paper presents a computer program in which all necessary computations 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 potential theory for computing wave forces is perhaps the most important. 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 moth 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, Ne 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.

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 computerised. 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 art 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 computerised. 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 Programming was performed in FORTRAN, which promotes the implementation of the program on Most promotes the implementation of the program on Most computers of sufficient memory capacity.