Rig-Mounted Instrumentation System Measurement of a Gulf of Mexico Loop Current Eddy Event
Authors: Magnell B.A. , Schanzle J.A. , Bowers G.A. , Brubaker J.P. , Settoon D.M. 
Published: October 2008

A complete meteorological and oceanographic measurement system was successfully deployed from a semisubmersible drilling rig at two deepwater well sites in the Gulf of Mexico. During one deployment, a major Loop Current eddy event wasrecorded, including full water column current profiles. Observations recorded may prompt therevision of present extremal design currents in the Gulf of Mexico.

INTRODUCTION

In 1988, Texaco held interest in over 130 deepwater leases in the Gulf of Mexico in water depths ranging from 1,500 to 9,000 feet. In preparation for drilling and production operations in these areas, Texaco created a deepwater development task force to identify and address areas of technological concern. One of the areas identified as requiring further research was the accurate definition of environmental design criteria, particularly currents. Deepwater structures, like those in shallow water, must be designed to resist the most probable combination of extreme environmental forces, including winds, waves, and currents. However, current becomes a greater contributing factor with deepwater structures due to the larger portion of the structure in the water. Information on current magnitude, directionality, and frequency throughout the water column was found to be sparse and poorly defined.

PROGRAM PLANNING

In 1988, Texaco embarked on a measurement program to gather a comprehensive meteorological and oceanographic data set in deep water. Thepurpose of the program was to collect simultaneouswind, wave, and current data at deepwater sites in order to:support ongoing drilling operations, andimprove Texaco's knowledge of these phenomena in deep water, both individually and collectively, for use in assessing design criteria for future developments. Several concepts of data collection were considered, including:joint industry projects,acquisition of data sets from other operators,a waverider buoy in conjunction with a second subsurface buoy supporting a string of current meters, anda full suite of sensors mounted on a semisubmersible drilling rig. No available joint industry projects fulfilled Texaco's deepwater requirements. In general, availability of data sets from other operators was limited and in most instances an exchange of similar data was preferred. At that time, Texaco had no in-house deepwater data sets available for trade. Most government data in the public domain for this region consisted of current measurements not deemed usable for predicting full water column current profiles. The dual buoy concept was ruled out due to its inherently higher risk of loss at sea, mooring costs, and the problem of obtaining near-surface currents with a moored current meter. Furthermore, simultaneous meteorological measurements from a nearby rig would have been necessary to complete the data set.

As of 1988, Texaco's planned drilling schedule projected at least 2 years of deepwater drilling in the Gulf of Mexico. Due to the availability of the contracted drilling rigs for use as a stable deployment base, a rig-mounted system was chosen. This choice was also economical because no additional vessel or mooring costs were required todeploy the sensors. EG&G WASC Oceanographic Services was contracted to design, fabricate, install, and maintain a rig-based metocean system.