A Quasi-Static Approach for Transportation Analysis of Offshore Platforms

Abstract This paper descri bes a computer model to predictthe maximll11 probable stress as well as the cumulative fatigue damage at any joint in a jacket or its ti edowns that may occur duri ng the transportation of an offshore platform by a barge. The dynamic transportation process is represented by a series of quasi-static processes in a frequency domain. The barge/jacket" interaction is analysed taking into account the barge flexibility. The method utilizes a description of the wave spectra of various anticipated sea states encountered during transportation, their probability of occurrence and the voyage is duration. The wave induced forces on the barge and the resulting barge/jacket assembly motions are calculated for unit amplitude waves wi th frequenci es spanni ng the wave spectrum frequency range. The computed accelerations are used to determine the inertial forces acting on the jacket members. The barge/jacket assembly is in instantaneous equilibrium under the acti on of the inertial forces and the pressure forces acti ng on the barge. These forces are used to cal cul ate the stresses resulting from the unit amplitude waves, or stress RAO's, taking into account the flexibility of the barge. The stress spectrum is obtained from the stress RAO's is and the wave spectrum. Assuming a Rayleigh distribution for the stress peaks, the maximum probabl e al ternati ng stress amplitude is computed from the moments of the stress spectrum, the duration of the storm, and the mean period of stress oscillations. A probabilistic procedure is used to derive the cumulative fati gue damage ratio from the computed stress spectra, an assumed stress range probability distribution, and an appropriate S-N curve (applying Mi ner I s rul e). Computed fati gue damage rati os for different sea states are added after being weighted according to their probability of occurrence. Introduction Offshore structures are manufactured in "on-shore" fabrication yards which may be distant from the installation site. Transportation of the structure from the fabri cati on yard to the install ati on site is done by means of a barge, as shown in Fi gure 1. The time requi red for the transportati on can be as much as a few weeks. During transportati on, the barge is subjected to wave forces whi ch, in the event of a storm, can be considerable. The jacket loaded barge assembly responds to wave forces by oscillating in different directions. These oscillations give rise to inertial forces which act on structural members. These inertial forces as well as those from the bargejj acket i nteracti on give rise to stresses that are cyclic by nature, and can reduce the fati gue 1ife of the jacket.