Modal parameters for structural integrity monitoring of fixed offshore platforms

Abstract

The most commonly used offshore oil-production platforms, the jacket structures, because of their long service in a very hostile environment, require periodic inspection for structural integrity. Present-day diver inspection is highly uneconomical and has certain inherent limitations. To overcome these limitations, it has been proposed to use an instrumented monitoring system based on the dynamic response of the platform. Several investigations were carried out to remove the uncertainties in this method which arose mainly from changes in the mass on the deck and marine growth. One of the important proposals investigated was the use of modal vectors. But these investigations were based on simple analytical and physical models. In order to assess the efficiency of using modal vectors to monitor the structural integrity of the platform, a detailed study was carried out on a real platform. Finite-element and physical models of the platform were used in the investigation. This paper reports on the physical model tests. A geometric model of the platform was tested under electrodynamic and wave excitation, simulating damages in the structure and changing the mass on the deck. In all tests, modal frequencies and modal vectors in different degrees of freedom were determined by autospectral analysis of acceleration response. Baed on the results, a scheme for integrity monitoring of a fixed offshore platform is suggested which, if implemented carefully, can at least minimize, if not dispense with, the uneconomic diver inspection.