Abstract

The offshore industry is presently developing a new recommended practice (RP) that will focus on the Structural Integrity Management (SIM) of existing offshore structures. The proposed API RP 2SIM will be a significant change to existing practice and provide considerably more in-depth guidance for maintaining existing platforms than is available in the present API RP 2A. The key concept of proposed RP will be the use of Risk-Based inspection strategies, which will require the engineer to understand the platform's likelihood of failure and consequence of such a failure. Additionally RP 2SIM will, for the first time, provide the engineer with fitnessfor- purpose acceptance criteria against the platform's ultimate load capacity, measured as the Reserve Strength Ratio (RSR). To take full advantage of RP 2SIM provisions, the engineer will require knowledge of the likelihood of platform failure, which is best determined through an understanding of the platform's ultimate strength.

This paper provides an overview of ultimate strength assessments and their role in understanding the structural system response to extreme loads for defining appropriate risk-based inspection strategies and for demonstrating fitnessfor- purpose. The paper also reviews future recommended practices (RPs) and regulations, and provides several informative studies to further demonstrate the role of ultimate strength assessments in the SIM of offshore structures.

Introduction

SIM is an ongoing life-cycle process for ensuring the continued fitness-for-purpose of offshore structures. The SIM process has evolved over the last 25 years to provide industry and regulatory authorities a means to ensure the continued safe and reliable operation of the aging fleet of offshore platforms around the world. RP developments, in the form of a proposed new API RP for the SIM of offshore structures, will allow the engineer to use ultimate strength assessments to gain an understanding of the behavior of the structural system. This valuable information can provide a role for the development of risk-based strategies, including setting appropriate intervals between inspections and selecting areas for inspection. The information can also be used to demonstrate fitness-forpurpose and assess the need for risk reduction and/or mitigation.

Offshore structures are traditionally designed on a component-by-component basis, such that under all combinations of design loading every component in the structure has a utilization ratio, derived using the strength formulations from the API RP 2A, of unity or less. However, it is recognized that fixed offshore structures are usually redundant and have a number of different load paths such that failure of one member is unlikely to lead to catastrophic structural collapse, provided that adequate redundancy is available. By utilizing this inherent redundancy found in most offshore structures the likelihood of failure of a platform in an extreme event can be determined.

During the life-cycle of an offshore structure the ultimate capacity is an important attribute that affects the SIM strategy, and can significantly influence the risk levels and operational costs.

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