Analysis of TLP VIV responses to Eddy Currents
Authors: Leverette Steve , Rijken Oriol , Dooley William , Thompson Hugh 
Published: October 2008

Recent experiences with eddy currents in the Gulf of Mexico have highlighted the dynamic VIV responses of risers and tendons. However, it has also been observed that excitation of these members can lead to subsequent excitation of the entire floating platform structure, including both single column and multi-column TLP's. This paper presents an analysis that has been performed on two GOM TLP's to evaluate the impact of both conventional warm core eddies, and of deep submerged jets (otherwise known as cold core eddies), on the complete structural system.

The analysis includes a combination of traditional tools used in concert: VIV analysis to establish tendon/riser response; a non-linear coupled analysis to establish tendon end effects on the platform; and a full structural dynamic FE analysis to compute stress responses within the structure. The analysis sequence has been validated against full scale data.

This work is unique in that the effects of VIV are shown to affect the entire platform system, and not just the tendons and risers. As platforms are designed for deeper locations in the GOM which are increasingly subject to loop currents and eddies, such effects will have to be considered. It is concluded that the coupled dynamics of the entire structural system are increasingly important to design in deep water.

Introduction

During the month of April 2001, a Loop Current eddy in the Gulf of Mexico (Millennium) provided high currents at a number of locations in the Green Canyon area. This current feature directly affected the Agip Allegheny Field location (GC 254) and the ChevronTexaco Typhoon Field location (GC237) with currents that are thought to be close to design level severity. The Allegheny field development has a SeaStar tension leg platform, which was installed in 1999. The Typhoon field at that time had drilling operations ongoing in preparation to receiving a SeaStar tension leg platform, which was subsequently installed in June 2001.

As a consequence of observed platform response due to the high current conditions at Allegheny, the following studies were instituted for the Allegheny and Typhoon SeaStars. The objective of the work is to improve the metocean estimates of eddy current, and to evaluate the effects of eddy current VIV on tendons, and the subsequent vibration excitation on the combined tendon/hull/deck system. The calculation used eddy speed probability distributions and profile-with-depth estimates based on the CASE JIP GEM4 program. The structural response simulations implemented a sequence of state-of-the-art analysis programs. The final result was calculation of predicted fatigue damage rates for the structure, and motions assessment for functionality resulting from tendon VIV excitation.

This work was jointly supported by Atlantia Offshore Limited, Agip Petroleum Co. Inc., and ChevronTexaco.

Figure 1 Allegheny SeaStar TLP(AVAILABLE IN FULL PAPER)

Allegheny Field Observations

This paper makes frequent references to two Field Developments in the Gulf of Mexico. These are 1) the Allegheny Field Development, Green Canyon 254, waterdepth approximately 3200 ft, operated by Agip Petroleum Co. Inc., and 2) the Typhoon Field Development, GC236-237, waterdepth 2100 ft and operated by ChevronTexaco.