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INTRODUCTION

Technological innovations, as well as economic and environmental conditions, are causing an increased interest in both life extension and modification of existing offshore platforms. These factors require reassessment of existing offshore platforms to determine their fitness for purpose. Natural questions which arise during this process are:

• What are the minimum acceptable life and environmental safety objectives that must be achieved?

• Does the platform meet these safety objectives ?

• If it does not, how can the platform be modified to meet these safety objectives?

• How can it be verified that the minimum life and environmental safety objectives are met?

The procedure by which these questions are answered is termed SEISMIC SAFETY REQUALIFICATION. For the purposes of requalification, the platform is taken to be in its current location and condition with any proposed repairs, modifications and/or seismic alterations completed.

Seismic safety requalification of an existing platform must address two fundamental issues:

1. The life safety of platform occupants.

2. The environmental implications of release of any petroleum products or production fluids.

The overall seismic performance of a requalified platform may be gauged by the level of life and environmental safety provided.

An independent Panel was formed by the American Petroleum Institute in 1991 to address technical issues related to the seismic safety requalification of existing offshore platforms. The charge to the Panel was to prepare a written document that provides a rational basis for the seismic safety requalification of offshore platforms, focusing specifically on issues related to performance objectives and requalification methodologies. The Panel was asked to relate recommended performance objectives and requalification methodologies for offshore platforms to those for onshore structures such as buildings, lifelines and other facilities.

The Panel consisted of four members: Wilfred Iwan, Chairman, C. Allin Cornell, George Housner, and Charles Thiel. All are specialists in earthquake engineering with extensive experience in practice, research, standards development, and public policy development on seismic safety issues. A Technical Advisory Committee was appointed by the American Petroleum Institute to advise the Panel on technical and industry specific issues. The Committee was chaired by Mr. Griff Lee and had four other members, Michael J. K. Craig, Jack T. Irick, James R. Lloyd, and David J. Wisch. In addition, representatives from state and federal regulatory agencies, universities, and other firms provided advice. Appendix A gives details on the Panel, affiliations of the Advisory Committee and names of others who contributed.

The Panel's deliberations were carried out with complete independence. Recommendations were developed through advice, research, and the informed opinion of the Panel. The Panel received no directions, implicit or explicit, from the petroleum production industry or governmental regulators.

The Panel has developed recommendations for minimum acceptable levels of performance that are consistent with the public's safety expectations and procedures by which these levels may be demonstrated. These levels are consistent with present (1992) land-based practices, particularly for buildings.

The Panel has uncovered no safety or environmental issues associated with the seismic safety requalification process that indicate that platforms should be subject to risk criteria that are more restrictive than those for other industrial facilities. Thus, use of the seismic performance expectations developed for similar structures was deemed appropriate to meet the public's safety expectations.

The process of requalification presents a number of technical and safety policy issues that are not addressed in current offshore guidelines or regulations. The American Petroleum Institute (API) Recommended Practices Volume 2A (RP 2A) provides an effective technical tool for both the designer and regulator to ascertain if minimum seismic performance can be expected of a new structural design. The current 19th edition of RP 2A incorporates recent advances in knowledge of the seismic hazard posed by offshore sites and of how materials and structural systems perform under seismic loadings. Newer editions of the RPA are considerably advanced compared to older editions and to the practices by which many older platforms were designed and constructed. Therefore practices that were the state-of-the-art when an existing platform was designed and constructed may no longer be preferred, or in some cases allowed. The standards of care in current use for new design may not therefore be directly applicable for making retrofitting decisions regarding platform safety.

The Panel's recommendations do not include considerations of the economic costs of seismic retrofitting or of the benefit of the achieved level of seismic performance; only minimum acceptable life and environmental safety are considered. Whether or not an action is cost beneficial did not bear directly on the Panel's deliberations. Nonetheless, it is the Panel's professional opinion that the owner should make explicit cost-benefit-risk analyses not only for earthquakes, but for all severe loadings and accidents - in particular, for severe storm wind-wave-current loads. The result may well be that it is beneficial to design for a lower level of seismic safety risks than that permitted under the Panel's recommendations.

The decisions made by owners based on cost and economic return are properly theirs, as is the decision to design for a lower level of risk. The benefits may be significant. Incorporating earthquake risk issues into financial decisions may yield better informed economic decisions. The Panel believes that such analyses will provide additional incentive to reducing the seismic risk of platforms, perhaps significantly below the minimum recommendations for public safety contained in this document.

The Panel believes that implementation of its recommended seismic requalification procedure will responsibly meet the public's seismic life safety and environmental safety needs. Seismic safety requalification performed under the guidelines recommended in this report should yield platforms whose seismic performance will be comparable to the balance of commercial and industrial buildings onshore.

The balance of this report provides a detailed discussion of the recommended life and environmental safety objectives for seismic safety requalification and of the procedure recommended to demonstrate that these objectives have been met.