Advanced Applications of Template Offshore Platform Capacity Analysis Tools: A Fast Ultimate Strength Analysis Tool
Authors: Brooks Allen M. , Digre Kris A. 
Published: October 2008

Since 1993, Shell has been an active participant in a joint industry/government sponsored project (JIP) to develop the TOPCAT computer program. Initially, the program's intended use was to quickly estimate the ultimate lateral load capacity of steel template-type offshore platforms subjected to wind, wave, and current forces. As the development of the program matured and the scope of the project expanded, a variety of applications became obvious, far exceeding initial expectations. The purpose of this paper is to describe how TOPCAT has been utilized and a vision of potential future uses for this software.

Introduction

Topcat has been calibrated and shown to be an acceptable tool to estimate minimum ultimate strength capacity.1 Actual use of TOPCAT includes: Verifying that the performance and safety level of shallow and deepwater structures, designed in accordance with API RP 2A 20th edition,2 will be equal to or greater than that achieved when 19th edition3 loading is used; Assessing an existing platform in accordance with API RP 2A Supplement 1 Section 174 due to a wave hitting the cellar deck; Determining the reserve strength of a minimum, tripod structure in deep water. Results from these applications are included in this paper. Anticipated future applications include use as a rapid screening tool to reduce the number of detailed ultimate strength analyses required and as an optimizing tool during detailed jacket design as well as for further Section 17 assessments when a 'first line' ultimate strength analysis is required.

Performance Verification

In July 1993, the American Petroleum Institute (API) released the 20th edition of its' Recommended Practice, RP 2A. The most significant change from previous editions of RP 2A was the directional application of wnd, wave, and current in determining lateral loading on the structure. This resulted in reduced loading on a structure's 'backside' or north side. This situation, in Industry's opinion, is a more realistic assumption of the structure's 100-year loading than the previous omnidirectional application of environmental loading used in the 19th and earlier editions of RP 2A.5

In February 1997, the MMS issued Notice-to-Lessee (NTL) No. 97-05. This NTL stated that all Operators "must clearly demonstrate that the performance and safety level of the structure will be equal to or greater than that achieved when the 19th edition reference level loading is used."

As Shell had numerous platforms this NTL would apply to, the information in Table 1 was presented to the MMS. Using this table, it was clearly demonstrated that the performance and safety level of any structure designed to the 20th edition of API RP 2A will be equal to or greater than that achieved when the 19th edition level loading is used.

Since the introduction of API RP 2A 20th edition, Shell has performed numerous ultimate strength analyses reviewing the effects of the directional loading. Safety level is reviewed on a minimum Reserve Strength Ratio (RSR) basis consistent with the industry practice. Table 1 presents results from several platforms.