Experience in Requalification of Offshore Platforms Installed in the Bay of Campeche
Authors: Perez Faustino , Lopez Alejando , Barrera Procoro , Inda Gregorio , Ortega Roberto 
Published: October 2008

The present work reports requalification results of 33 platforms installed in the Bay of Campeche (BOC), Offshore Mexico, based on simplified and detailed ultimate strength analyses. These analyses have been carried out according with the Transitory Criteria (TC) developed for the BOC and the API RP 2A 20th edition.

Taking into account that simplified structural analysis performed with a software called Template Offshore Platform Capacity Assessment Tools (TOPCAT) are quick and conservative as compared with sophisticated computer programs, PEMEX - IMP decided to use this software. In general terms, results obtained with TOPCAT agree well with those from the sophisticated software.

Introduction

As an offshore platforms becomes older it is more difficult to comply with modern codes, due to accumulated damages load increments, changes of service and so on. On the other hand codes are stricter than the ones used in the design of old platforms. A structure that does not comply with a code can not be replaced with a new one, because this implies time and cost, among other technical problems. Fortunately API issued Supplement 1, which is applicable to requalification of existing offshore platforms.

As a result of the first non-linear ultimate strength analyses carried out for PEMEX according with Supplement 1, Petroleos Mexicanos (PEMEX) and Instituto Mexicano del Petroleo (IMP) developed the TC for design of new offshore platforms and requalification of existing structures in the BOC.

API RP 2A Supplement 1 and the TC change the engineering practices, including the non-linear ultimate strength analysis as a compulsory task when assessing an existing structure that does not pass the design level analysis or does not have adequate deck height. This change requires reliable software for ultimate strength analysis.

For planning and taking decisions purposes PEMEX needed some structural assessments in a short period of time, too short to carry out detailed structural analyses. Therefore, TOPCAT was used to assess nine platforms considering that this software gives quick and conservative results as compared with those from sophisticated software.

Nine platforms were assessed in one week by a group of three structural engineers. With sophisticated software this nine structures could not be assessed in this time. For this reason and the results obtained with the simplified ultimate strength tools, PEMEX - IMP have carried out several analyses. However, TOPCAT itself can not replace the sophisticated computer programs.

Requalification of 33 platforms was conducted, analyses were carried out with simplified and sophisticated tools and results of both tools are very close.

TOPCAT has the capability to model damages on the structural members such as dents and out of straightness, as well as the capability to consider grout repair of week members. Based on the above mentioned if necessary one can recommend to grout repair structural members. In some cases, it is recommended to perform detailed analyses, because detailed analysis could be less expensive that grouting members.