Abstract

Oryx installed the world's first production spar platform in the Gulf of Mexico, Vioska Knoll Block 826 in September 1996. The water depth is 1930 ft. A Joint Industry Project was formed to place instruments on the spar to measure its motions, riser relative motion and tensions and mooring line tensions. These instruments were installed in the fall of 1997 along with a separate wave rider buoy and two submerged, tethered current meters. The instruments have been collecting data almost continuously since November of 1997. This paper discusses measured and predicted motions from two storms during September 1998: hurricanes EARLE and GEORGES. The data forms a good basis for validation of analytical tools and confirms the performance of the spar for production. This paper presents a description of the instruments, their performance and an interpretation of measured motions measurements. Some comparisons are made with a time-domain analytical tool, TDSIM6.

Introduction

ORYX Energy Company and CNG Producing Company installed the world's first production spar in September, 1996. The "Neptune" spar was installed in 1930 ft water depth in Vioska Knoll Block 826 approximately 90 miles south of Mobile, Louisiana in the Gulf of Mexico. This is a small oil field, approximately 50 – 75 million equivalent barrels of oil, and the spar represented new technology, which led to a low cost solution required for development of the field1.

The Neptune spar is designed to process 25000 BOPD and 30 MMSCFD. Total topsides facilities weight with a removable workover rig and deck structure is 6600 tons. The hull is a cylinder, 72 ft in diameter and 705 ft long. 55 ft. of the spar extends above the water. The draft is 650 ft. The hull is moored by six taut catenary anchor lines consisting of 4–3/4" chain and spiral strand.

Interest in the spar concept prompted initiation of a Joint Industry Project, led by Scientific Marine Systems, Inc (SMS). to collect and analyze at least one year's worth of data on the global responses of the spar. This year of data collection ended in December 1998. The data collection activity was successful, largely due to a series of events in September which yielded two hurricanes and a high current event with which to evaluate the vortex induced responses of the spar. This data indicated, in general, that the spar performed according to or better than expectations. The data is currently being processed and distributed to JIP participants who will be performing their own verifications.

This paper presents an outline of the data collected, and some preliminary comparisons with predictions carried out by Deep Oil Technology, the developer of the spar production concept.

Instrumentation
Overview.

The instrumentation system installed aboard the Oryx Spar was based on the Amoco Remote Access Portable Instrumentation and Data Storage systems (RAPIDS). In addition to the standard six-degree-of-freedom motions, wave height and wind speed and direction sensors included with RAPIDS, differential GPS, a directional wave buoy and two current profiling buoys were added for this project.

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