Proceedings of the Tenth (2000) International Offshore and Polar Engineering Conference Seattle, USA, May 28-June 2, 2000 Copyright 2000 by The International Society of Offshore and Polar Engineers ISBN 1-880653-46.X fSet); ISBN 1.880653-47.8 (VoL I); ISSN 1098-6189 (Set)

Dynamics of a Spar Platform

Mehernosh B. Irani, Thomas Rouckout and Robert P. Johnson

Texas A & M Universi ty College Station, TX, U S A

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). Typically, the Spar is a deep draft, large diameter hollow cylinder that is moored by means of conventional slack or taut spread mooring lines. The Spar's deep draft gives it excellent motion characteristics even in extreme sea-states. The Spar platform has been extensively studied numerically and by means of physical model tests. Oryx Energy Company installed the world's first production Spar platform in the Gulf of Mexico in September 1996 in 1930 ft of water (Glanville et al, 1997). 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 (Anonymous, 1999), 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 (Prislin et al, 1999). The Oryx Neptune Spar platform was model tested at the Offshore Technology Research Center wave basin in April 1995 (Johnson, 1995). The wave basin model test program provides data for the Spar response under a range of wave, wind and current conditions in a controlled laboratory environment.

Table 1: Neptune Spar Particulars

Spar Diameter 72 feet 702 feet Overall Depth

Draft 650feet Centerwell 32 x 32 feet

1930 feet Water Depth Moorings Faiflead Position Center of Gravity Center of Buoyancy

6 lines, Chain-Wire-Chain 300 feet above keel 306-314 feet above keel 325 feet above keel

Analytical and numerical work done on the Spar predicted its heave natural period to be around 28 seconds. However the full-scale data do not show significant heave response at the predicted Spar natural period of about 28 seconds. Instead, the full-scale heave response suggests a new natural period of about 10 seconds. It is speculated that this apparent shift in natural frequency of the Spar heave motion is due to the riser system affecting the dynamic characteristics of heave motion.

During the physical model tests performed at the Offshore Technology Research Center, models of the Spar were tested with and without risers. Tests without risers showed noticeable heave responses at the frequency of 28 seconds. The presence of the risers in the model tests do not show any significant shift in riser heave natural period, but the risers introduce additional damping in the system. It must be noted that the field data showed a significant shift in the heave response period. Thus data from the model tests do not show any significant shift to higher frequencies similar to the phenomenon observed in the field data. However, as the model Spar was extensively instrumented, the model test data gives an appreciation of the various sources of

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