Economic Optimal Reserve Strength for a Jacket Structure

Authors

Gerhard Ersdal, Oliver Kübler, Michael H. Faber, John D. Sørensen, Sverre Haver and Ivar Langen

Abstract

This paper evaluates the optimal Reserve Strength Ratio (RSR) of a jacket structure based on a model taking into account the cost of field development, income from production and possible costs of failure. The failure probability related to this RSR is calculated, and compared with acceptance criteria given in existing regulations relevant for offshore facilities located on the Norwegian continental shelf. The cost of the jacket structure includes an estimate of construction cost and the cost of increasing the strength of the jacket to higher reserve strength. The cost of failure takes into account consequences of material losses and loss of lives. The sensitivity of the economic optimal reserve strength to the different types of uncertainty is studied. This includes a study with only aleatory uncertainties, a study with aleatory and epistemic uncertainties included. Evaluations of the optimal reserve strength ratio (RSR) of jacket structures are typically performed without looking at the possibility of member failures and the possibility of wave in deck loading. This paper will address these two possible additional failure modes to make the risk model more accurate, and to evaluate the effect on the conclusion. The present study shows that, with the legislative requirements imposed on offshore operators on the Norwegian continental shelf, the optimal design of fixed jacket structures is governed by the minimum safety level implicit in the regulation and standards. The optimal RSR will only in a few cases indicate higher values of RSR. The economical (life cycle benefit) optimal failure probabilities can result in a less conservative design, but also an increased fatality risk for the individual worker.

Published in/by

Proceedings 2nd International ASRANet Colloquium, Barcelona, Spain, July 5-7, 2004.