The Gulf of Mexico Loop Current and Deepwater Drilling (1991)

Summary This paper reviews industry knowledge of the Gulf of Mexico circulation feature known as the Loop Current. Data obtained in Green Canyon in 1989indicate how the Loop Current influences drilling operations. The paper alsodiscusses the analytic and operational considerations for drilling-riser andstation-keeping system management when these currents are of concern. Introduction The Loop Current, the principal deepwater current in the Gulf of Mexico, ispart of the north Atlantic Ocean's western-boundary current. It is formed from water that flows into the Caribbean Sea from the western north Atlantic, thesouth Atlantic, and the Mediterranean Sea. The Caribbean Sea water flows intothe gulf through the Yucatan Strait, loops around Cuba, and exits via the Florida Straits, where it becomes source water for the Gulf Stream. Althoughthe north Atlantic's circulation has been studied for more than 50 years, eventhe more recent studies barely touch on circulation within the Gulf of Mexico. Significant questions remain from studies conducted as recently as the early1960, s. Since then, however, researchers from academe and industry have expanded understanding of the Loop Current's characteristics. Loop Current Characteristics Although much remains to be learned, an understanding of several basic aspects of the Loop Current is emerging. First, analysis of satellite data thathave been available since the early 1970's indicates that the mean position ofthe Loop Current lies in the position of the Loop Current lies in the deepwaterregion of the east-central Gulf of Mexico. Vukovich et al. and Auer, whosummarized the periods of 1973–77 and 1980–85, respectively, suggest that theme an position of the Loop Current's western position of the Loop Current'swestern boundary lies between 88 and 89 degrees W, with the north boundary atabout 27 degrees N. This estimate places the mean location just within the southeastern corner of the Atwater Valley leasing area. Vukovich et al. and Auer indicate that a large amount of annual variability has been observed withrespect to this mean position. Second, the Loop Current has a cycle of northwesterly movement within the Gulf of Mexico. Although the forcing mechanisms behind this movement are notwell understood, most research suggests that the cycle is approximately annualin period. Recent analysis of nearly 14 years of Loop Current position data suggests that the movement is bimodal, with dominant periods of both 1 and 21/2 years; however, the data are insufficient to determine the significance ofthe longer period. Although early research suggested that the maximum northward movement occurred during the summer, it is now generally recognized thatnorthward movement can occur at any time during the year. Third, the northward movement is frequently associated with the formation ofeddies that detach from the Loop Current and translate west-southwest acrossthe gulf. Although the northward movement can affect deepwater lease areas, detached eddies pose an even more significant problem because of their tendencyto translate west across drilling regions. Some evidence* suggests that threedetached eddies moved into the Green Canyon lease area between 1979 and1988. Fourth, measurements of near-surface current as high as 4 knots [2 m/s] havebeen recorded in eddies recently detached from the Loop Current. Available datado not indicate that the maximum possible speed in the Loop Current is any lessthan that figure. Current profiles associated with both the Loop Current andits eddies are highly sheared. These high surface speeds drop to about 1 knot[0.5 m/s] at 650 ft [200 m] and 1/2 knot [0.25 m/s] at 130 ft [400 m] depth. Features with lower surface speeds also have lower speeds at depth. Fifth, advances in modeling techniques and high-speed computing have led tothe development of basin-wide models that aid understanding of Gulf of Mexicodeepwater circulation. For example, Refs. 11 and 13 suggest that wind forcingplays a larger role than previously thought, and Ref. 12 suggests that acorrelation may exist between sea level near Florida and the location of the Loop Current. Nevertheless, much research remains to be done to understandfully the mechanisms that drive the movement of the Loop Current and lead toeddy formation. JPT P. 1046