Seismic Reassessment of Maui A

Abstract

ABSTRACT The improvement in techniques and data used to define seismic exposure has resulted in redefinition of the seismic demand for many existing structures. Maintaining these structures for continued safe operation will require structural reassessment for seismic conditions. This paper presents the details ofa seismic reassessment of Maui A, an existing platform located off the west coast of New Zealand. Redefinition of seismic criteria for a new satellite platform resulted in a reevaluation of the existing Maui A drilling, production, and quarters (PDQ) and platform. Site hazard, geotechnical and structural analyses were performed which demonstrated the adequacy of Maui A for extended service. INTRODUCTION Background Shell Todd Oil Services (STOS) operates the Maui A PDQ platform for production of gas and condensate the Maui field, 1ocated approximately 34 km off the exposed west coast of the North Island of New Zealand (Figure 1). Maui A was designed in 1974 for a service life of 30 years and has been in continuous operation since 1979. In order to optimize production from the Maui field, a second platform was required to be installed 15 km offshore from Maui A, and to be onstream by Quarter 1 1993. Conceptual studies resulted in the configuration of Maui B as an unmanned satellite connected by pipeline to Maui A, which would in turn be modified to receive and process the Maui B wellstream fluids. In parallel, design criteria were reviewed resulting in the development of new strength and ductility level earthquake ground motion spectra for the Maui site. These spectra were developed utilizing data and methods regarding historical regional seismicity, attenuation relationships and local site effects that were not available at. the time of the development of seismic criteria for Maui A. The resulting Maui B seismic criteria were significantly more severe than those used for the original design of Maui A. Subsequent response spectrum analyses of Maui A indicated the potential for significant overstress in both the structure and in the topsides framing when subjected to strength level earthquake (SLE) loadings. Designed as a "K" braced structure, the Maui A substructure does not conform with current API recommendations for ductile response [1]. This fact, plus the presence ofa number of main framing members that had been damaged during the installation phase, heightened concern over the likely structural performance under the revised ductility level event (DLE) even though DLE analyses had been performed during the original design [2]. Study Objectives The Operator subsequently initiated a structured reassessment program to comprehensively review all aspects affecting Maui A seismic performance, these being the seismic demand, structure capacity, and performance criteria. Seismichazard analyses were performed for the site to determine explicit horizontal and vertical uniform risk spectra and to evaluate the original basis for the development of seismic source model parameters [3]. The ultimate capacity of the structure, including the effects of localized member damage, was assessed through nonlinear structural analyses using both pushover and time domain techniques. Performance criteria were reviewed using a risk-based approach.