ABSTRACT

Oryx Energy Company installed the word's first production Spar platform in the Gulf of Mexico in September 1996 in 1930 ft of water. In order to obtain better understanding of the dynamic behavior of the Spar platform, a field monitoring program was commissioned under a Joint Industry Project involving 12 participants. A suite of instruments was installed to measure platform motions, riser and mooring line tensions as well as environmental conditions such as currents, waves and wind. Full-scale measurements from this field study provide valuable data for verifying the predicted Spar responses. The Oryx Neptune Spar platform was also model tested at the Offshore Technology Research Center wave basin. Analytical and numerical simulations predicted the Spar natural period in heave to be about 28 seconds. However, the field data does not show significant heave response of the Spar at the predicted 28 second natural period. The 16 production risers of the Oryx Neptune Spar terminate in buoyancy cans that float independently from the Spar platform. It is speculated that the additional damping and stiffness contributions from the risers/buoyancy cans/platform interactions cause this discrepancy in the predicted and measured Spar heave response. The model test data includes measurements of the riser top tensions and riser relative heave motion. The model test data provides valuable insights into some of the riser/buoyancy cans/Spar hull interactions. The paper presents an analysis of the model test data to study the influence of the risers on the Spar heave motion. An analysis of the riser/Spar hull interaction forces and its effects on the platform heave motion is presented.

INTRODUCTION

The deep-draft cylindrical Spar has found increasing use for deepwater production, drilling and storage platform (Halkyard, 1996).

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