Reassessment of Platforms in Cook Inlet,Alaska
Authors: Visser C. 
Published: October 2008

This paper presents the results of an audit of condition surveys and reassessment studies of several aging high consequence platforms located in Cook Inlet, Alaska. The purpose of the study was to demonstrate that these platforms continue to be structurally sajii and have a low risk potential for environmental damage. The effort included,the preparation of a Cook Inlet platform database,the selection effective platforms for more detailed review,audits of condition surveys, and,audits of structural assessments.

All of the selected platforms were found fit for- purpose.

INTRODUCTION

Fifteen offshore platforms are in operation in Cook Inlet, Alaska. These structures are unique because they are exposed to ice loading and are located in a seismically active arm. 1,2* Fourteen of the fifteen platforms were designedand installed in the mid-sixties. Location of the area and of the platforms is shown in Figures I and 2.

The four Cook Inlet operators, i.e., Phillips, Marathon, Shell and Unocal, were requested by the Cook Inlet Regional Citizens Advisory Council to demonstrate that these aging Cook Inlet platforms continue to be structurally safe and do not present an unreasonable risk to the environment. The structural review was accomplished through the preparation of (1) a design and operating database of all the Cook Inlet platforms, and, (2) condition and structural audits of a selected group of five of the platforms. Tworeports document the results. 3,4, Table 1 provides a summary of the assessment audit results.

Specific offshore platform design codes did not exist in the mid-sixties. Since then, the design standards of API RF 2A, for fixed offshore structures, and API RF 2N, for platforms in an ice environment, have come into existence. 5,6,

Concurrent with the platform review, an API task group Developed an assessment procedure for existing platforms. The assessment procedure was subsequently released as anew, draft Section 17 for inclusion in the 21st edition of API RP 2A. At the 1994OTC conference several paperswere presented describing the assessment procedure.7,8,9,10

METHODOLOGY

Procedure

Figure 3 outlines the reassessment procedure for a Cook Inlet platform. The procedure is quite similar to the procedures Developed by the API reassessment task group and those recommended in the seismic requalification document prepared for the API.7'11 The operating Cook Inlet platforms are considered high consequence platforms, meaning that structural failure could have significant life safety and pollution consequences.

Of the four design analyses shown in Figure 3, the screening analysis is the simplest and is also the most conservative, while the ultimate strength analysis is the most complex and the least conservative. The requirements for the design level reassessment are essentially the same as the requirements for the design of a new platform, includingall safety factors. If the structure cannot elastically withstand the design level 200-year earthquake loading, the structure is further analyzed in an ultimate strength analysis to assess global stability or determine the probability of failure in a 1,000-year earthquake.