Fatigue Reassessment of the Cerveza and Cerveza Ligera Platforms for Use in New Deepwater Developments
Authors: Sheppard Robert E. , Miller Tom M. , Marshall Peter W. , Johnson Gayle S. 
Published: October 2008

This paper describes fatigue reassessments performed on the Spirit Energy 76 Cerveza and Cerveza Ligera platforms, two of the deepest water fixed base platforms ever installed. The paper emphasizes how special issues were dealt with related to reassessment of existing platforms for potential use as a host platform for future deepwater developments. Specific issues included appropriate factors of safety for existing fatigue damage, dealing with varied payloads used during the life of the platforms and projected for future use as other deepwater fields are developed, and dealing with actual conditions that have differed substantially from original design conditions. The issue of changing payloads is particularly important for deepwater fixed platforms due to the shift in structure periods that tends to make the structures more fatigue prone. Many new deepwater developments will be tied back to existing installations. One of the issues that must be dealt with is the effects of the changed in-service condition with regards to fatigue service life of the platform. In many cases, the platform has been in place for almost its original design life, and there is a desire to justify an extended life. In so doing, there is a need to account for the actual conditions that the platform has experienced, rather than basing the assessment on design conditions. This paper will describe how one operating company has dealt with many of the issues that were encountered with regards to fatigue. These two flexible deepwater platforms are somewhat sensitive to changes in the fatigue environment. Varied payloads have been accounted for by running several analyses and combining results. A method of interpolating results has been developed for addressing future payloads that might be considered for different field development scenarios. The application of factors of safety and existing damage is presented. Requalification philosophy relevant to this type of application is also discussed. There is currently a strong emphasis on deepwater developments in the Gulf of Mexico and elsewhere in the world. The use of existing facilities will certainly play a key role in the feasibility and development of deepwater fields [1]. Many operators will be forced to continually deal with issues such as those addressed in this paper. Reassessment of platforms will be necessary to not only satisfy regulatory requirements, but also to provide assurance that critical platforms will remain available to be part of the new deepwater developments.

INTRODUCTION

As production operations move into deeper water in the Gulf of Mexico, jackets such as Cerveza and Cerveza Ligera are good "tie-backs" for deeper water operations such as satellite wells or a minimal production facility located on a Spar or FPS, with the main production performed on the jacket. This will mean a substantially larger deck payload for the platforms to accommodate increased production rates, as well as possible additional tieback risers and export risers that will increase hydrodynamic loads. Fatigue is particularly important for Cerveza and Cerveza Ligera (Ligera) because the water depth of nearly 1000 feet increases the platforms' dynamic response. Potential increases in deck weight will affect the platforms' dynamic characteristics, increasing their fundamental periods and making them more fatigue prone.