Nonlinear Analysis and Tests Comparison of Motion And Dynamic Structural Response for Two Gravity Platforms in Floating Conditions

Abstract

1. ABSTRACT The paper describes the basic principles followed for the dynamic analysis of the behaviors of floating framed structures. The main theoretical results calculated by a computer program system based on a non linear dynamic method of analysis for two self floating, gravity type, drilling and production platforms (200 meter and 90 meter water depth platforms of original Kenmare design), are presented together with experimental results performed on models of said platforms at the Netherlands Ship Model Basin. From the analysis and tests comparison made on the above models, the requirement to adopt a non linear method of analysis, which is not normally used for such type of calculations, is demonstrated. 2. INTRODUCTION One of the most important problems in the design of floating offshore structures is the dynamic behavior in floating condition. This is obvious for semi submersible drilling platforms, where structural damages and operational delays can be minimized by means of analytical studies in the design stage, but it is significant also for fixed offshore platforms of the type designed by Kenmare, due to the method of installation. In fact for gravity platforms /18/, sometimes subject to long oceanic trips from the construction to the installation site, the floating condition loads are not less significant for the design than those m the standing on bottom condition. Of course, for this type of platform the floating condition is limited in time, and some problems, e. g. fatigue while floating, are not involved. However an incorrect design may cause great difficulties during towing and lead, in extreme conditions, to the loss of the unit. It should be remembered that the great eight above the sea level of this type of platform, towed in upright position, coupled to bad dynamic performance, may cause dynamic stresses, in the upper portion, higher than the maximum stress level reached after installation. Tecnomare S. p.A. has faced these problems during the design of its 200 m W. D. gravity type platform /18/ and, more recently, in the development of four 90 m W.D. gravity type platforms to be constructed in Northern France and installed offshore Congo, after an ocean voyage of about 6000 miles, during a period of 3 months. For this reason, being there involved non linear phenomena very significant for such structures, the non linear computer system DISMAR, presented in the following pages, has been developed. The main applications of the system together with model tests comparison are presented and discussed. 3. CRITICAL ANALYSIS OF CURRENTLY EMPLOYED TECHNIQUES 3. 1. Quasi-static Method Quasi-static method of analysis, widely employed in the past, may be schematized in the following main steps, shown in fig. 3:Structure is assumed to have no motion.Sea state is simulated as a regular, long crested sine wave system; wave height and period are selected on a given probability level.Forces on the restrained structure, due to this wave system and to the associated current and wind speeds, are calculated.