Statistical Verification of Predicted Loading and Ultimate Strength Against Observed Storm Damage for an Offshore Structure
Authors: van de Graaf J.W. , Tromans P.S. 
Published: October 2008

A partially damaged structure in the Gulf of Mexico has been studied. The extreme hurricane loading has been predicted probabilistically. A parameter study has been made in order to identify the magnitude and direction of loading that would cause the damage. The failure sequence and ultimate collapse load were calculated using non-linear push-over technology. Finally, the likelihood of experiencing the damage severity observed has been calculated. The results provide confidence in the models used.

INTRODUCTION

In the Gulf of Mexico a few of the older platforms have experienced damage or have collapsed during severe hurricanes. These damaged platforms offer an ideal opportunity to study structural collapse behaviour, loading and reliability. This paper outlines a first attempt at such a rational evaluation. A partially damaged structure has been selected and analysed with the most advanced models for load and strength analysis presently available. The models include a new theory for extreme random wave kinematics, the effect of current blockage and recently developed technology for static push-over analysis. The predicted extreme environmental loading, progressive collapse sequences and ultimate platform strength are evaluated against observed damage. Such studies form an essential stepping stone towards predicting probabilities of failure and quantifying the reliability of offshore platforms.

STRUCTURE AND DESCRIPTION OF DAMAGE

The structure is an unmanned self-contained drilling production platform, Fig. 1. It was designed in the early sixties for a 55 ft wave and no current. The jacket stands in 213 ft (64.9 m) of water and has base dimensions of 171.75 ft (52.4 m) by 92.75 ft (28.0 m). It is supported by eight main piles (dia. 42 in., w.t. 1 in.), which are driven to a penetration of 250 ft. The piles pass through the 46 in. dia. by 1/2 in. w.t. legs and are welded off at the jacket top. The jacket supports twelve 24 in. dia. conductors and other appurtenances. It is Platform E of the AIM-4 Research Programme.

Damage to the jacket was discovered after a hurricane passed nearby. The damage was restricted to the top bay and is shown in Fig. 2. It comprised:Damaged legs Al and A3, local leg shell buckling occurring at -10 ft elevation, just below the corrosion wrap and the joint can.One bent vertical diagonal member in row 2 between elevations I and II.Two bent and cracked vertical diagonal members in row B between elevations I and II.

After the damage was discovered, the platform was repaired and strengthened.

NON-LINEAR FINITE ELEMENT MODEL

A non-linear model of the structure was developed using the USFOS program (ref. 1). This program uses a beam element formulation valid for large displacements but restricted to small strains. Geometric non-linearity at the element level is included by the additional second order axial strain term. Geometric non-linearity at the global system level is included by updating the geometry of the structure at each load step.