Hurricane Georges Hindcast Assessment of LeTourneau 116-C and 82-SD-C Jackups
Authors: Stock D.J. , Lewis D.R. , Baucke T.C. , Hsu H. Y. 
Published: October 2008

Two LeTourneau designed jackups, a 116-C and an 82-SD-C, were in the direct path of Hurricane Georges in late September 1998 and both survived without damage. The eye of the category four hurricane passed just west of the two rigs, exposing them to the most severe environmental conditions. Using hindcast storm predictions and measured current data, the rigs structural performances are evaluated using SNAME Technical and Research Bulletin 5-5A, Site Specific Assessment of Jackup Unit (Reference 1) procedures. The analytical study attempts to benchmark the demands placed on the jackups with the satisfactory performance observed.

Introduction

The jackups were both in contract to perform workover on Gulf of Mexico fixed platforms and were evacuated with the cantilevers stowed prior to arrival of Georges. Both rigs were preloaded as defined in their operating manuals, which developed only partial spudcan penetrations at the competent sand sites. The storm center passed by 25 miles to the west, which caused the rigs to experience severe storm excitation for 10 hours. The 82-SD-C rotated about 20 degrees from its original heading, but neither rig exhibited any evidence of damage or overstress within the critical leg-hull connection.

The rigs are LeTourneau designs, each with teardrop chords (SNAME Type 4). The larger 116-C has square legs and is able to operate in deeper water with a higher elevated weight. The 82-SD-C has three chords per leg (equilateral leg plan section). Both rigs use pinions to transmit the hull vertical load to the legs (no leg chocks). Upper and lower guides provide the moment transfer at the leg-hull connection. The chord cross sections on both rigs are the same LeTourneau "teardrop" style (triangular) without additional strengthening. The 116-C has a 90ksi yield strength chord, while the 82-SDC has 85ksi rack and 70ksi side and backplates. Neither chord section has added cover plates or rack strapping within the critical region of the leg-hull connection.

The original goal of this project was to evaluate the rigs according to SNAME Technical and Research Bulletin 5-5A, Site Specific Assessment of Jackup Unit (subsequently called the "TR") procedures for 10 year, Hurricane Georges and 50 year storm conditions. Hurricane Georges was a 30 to 35 year event. It was originally anticipated the rigs would pass the 10 year site assessment, but fail both Georges and the 50 year conditions. Unfortunately, the rigs also failed the 10 year event using the standard TR procedures with the prescribed 1.25 environmental load factor on static and dynamic effects. Further work on the 10 year and 50 year events were abandoned, but additional study was conducted of the Georges conditions with the goal of determining actual performance.

The Georges environmental conditions were developed by combination of a hindcast prediction performed by Oceanweather, Inc (Reference 2) and measured current velocities from the DeSoto Canyon Eddy Intrusion Study conduced by SAIC (Reference 3, sponsored by the MMS). The actual current readings were within four miles of the 116-C site and 15 miles from the 82-SD-C.