Seismic Considerations in Design and Assessment of Platform Topside Facilities
Authors: Visser Robert C. 
Published: December 2007

This paper evaluates the potential risks to the topside facility of an offshore platform from an earthquake and describes what to look for during the topside design and/or a requalification process. There has, to-date, been no recorded earthquake damage to either the topsides or a platform support structure, primarily because no platforms have been exposed to a significant earthquake. California, Alaska, Japan, China and Peru are some of the active seismic areas with current offshore oil and gas development activities. From both quantitative and qualitative analyses it is concluded that the risk of topside earthquake damage is low, provided all equipment and piping are adequately restrained and the drilling rig and other slender structures are designed for the appropriate earthquake loading. Comprehensive seismic design and assessment guidance for topside facilities are provided in the twentieth edition of API RP 2A.

Introduction

The seismic safety re-assessment of an offshore platform involves the evaluation of the adequacy of a number of platform components and management systems. At the 1996 Offshore Technology Conference a number of papers were presented dealing with the structural design and/or reassessment of platform support structures located in active seismic areas. This paper deals with the operational considerations of the seismic safety assessment. The key word is "operational" and in this context it includes the design, operation and/or re-qualification of the platform topsides.

Topside equipment is defined to encompass the drilling rig and associated equipment, all process related equipment, safety and control equipment, cranes, flare booms, quarters, pipeline risers and well conductors. The paper reviews and briefly discusses the potential earthquake hazards to each of these topside components.

The earthquake risk to the topside equipment is quite often ignored. In fact, there is very little specific information in the literature regarding the design of topside facilities for earthquake loading. Thoughts on potential further investigations or research that may be beneficial to reduce, or better understand, the topside risks from earthquakes conclude the paper.

Risk Assessment

Risk assessment is a generic term covering a wide range of techniques used to assess the level of safety by considering both the magnitude of harm or damage and the likelihood of such harm occurring. Such assessments can be qualitative or quantitative or a mixture of the two. Comprehensive probabilistic risk-based assessments have been used as part of the re-qualification procedure on a few West Coast platforms.

The potential risk to an offshore operation comes from many different external factors. The distribution of platform loss causes is shown in Figure 1. A platform can be damaged or destroyed by storms, by a collision, by a blowout, by a fire or explosion in the process facility, and by an earthquake. Platform risks from storms, collisions, blowouts and fires and explosions are reasonably well defined. An extensive database of accidents has been accumulated over the past twenty years by the Minerals Management Service from the almost 4,000 operating platforms in the Gulf of Mexico. As noted on Figure 1 the majority of platform losses are from storms, i.e., hurricanes.

High activity seismic environments where platforms are located include offshore California, Alaska, Japan, China and Peru. There has, to the best of the author's knowledge, not been any significant earthquake damage to an offshore platform. This record is probably not due to good design, but to the fact that, to-date, no offshore platforms have been exposed to a major earthquake. The absence of earthquake impact may lead to complacency, which can be an obstacle to recognizing earthquake hazards and initiating consequence mitigation.

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