The Loop Current Experience- Ewing Bank 871

Abstract

Summary An unprecedented intrusion of the Loop Current into the northern Gulf of Mexico severely affected expiration drilling in areas where water depths are asshallow as 450 ft. At Mobil E and P U.S. Inc.'s well, drilling was discontinuedowing to rig offset and resultant high riser angle. Safe disconnect of theriser required modeling of the mooring system and riser, building necessaryequipment, and proper planning. Introduction In Aug. 1989, the Loop Current unexpectedly moved into the northern Gulf of Mexico, severely affecting exploration drilling in several lease block areas. At least six operators were forced to shut down operations. The affected blockswere primarily in deep water; however, strong currents were found at sites withwater depths as shallow as 450 ft. At Mobil's well in 770 ft of water at EwingBank 871, drilling from the Sedco Forex 601 rig was discontinued owing to rigoffset, vortex-induced vibrations, and resultant high riser angle on Aug. 11. The riser was safely disconnected on Aug. 19 and was not reconnected until Aug.27, when current speeds had diminished greatly. Data on currents measured atthe rig were entered into models of the mooring system and riser that were usedat the rig to ensure a safe disconnect. The Loop Current and Eddies The Loop Current is a portion of the Gulf Stream system that enters the Gulfof Mexico between Cuba and the Yucatan, loops through the eastern Gulf of Mexico, and exits between Florida and Cuba. The path of the Loop Current variesconsiderably. At intervals of 6 to 17 months, large clockwisecirculating eddiesbreak from the current, as shown in Fig. 1. Eddies generally translate to thewest and southwest, but occasionally move northwest onto the Louisiana shelf. The Loop Current and its eddies have the potential to produce currents inexcess of 4 knots near the surface and up to 1 knot at depths to 1,000 ft. Tracking the Loop Current and eddies is possible from November through May bypossible from November through May by use of satellite thermal images availableat no cost from government agencies. The images show the strong temperaturegradients that occur at the edge of the Loop Current and eddies. Certainfactors affect the accuracy of thermal images. Cloud cover often obscures thesea surface, and the location of currents can vary with respect to surfacetemperature gradients. If the edge of the Loop Current is near a site ofinterest, detailed tracking must be undertaken from ships. During June through October, the entire Gulf of Mexico becomes uniformly warm at the surface. Therefore, surface contrasts disappear, making satellite imagery of littlevalue and more costly in-situ measurements necessary. Strategic seeding of the Gulf of Mexico with satellite-tracked drifting buoys is used to infer grossmovements, and ship-deployed instruments are required to delineate the edge ofthe highcurrent features. Time Series of Data at the Drillsite. Measurements on the rig, shown in Fig. 2, began on Aug. 17 and continueduntil Sept. 7. Owing to instrument failures, two major data breaks convenientlyseparate the record into three segments for discussion:Aug. 17–19,Aug. 21–28, andSept. 1–7. Currents during Period 1 were strong with aminimum speed of 1.75 knots and a maximum of 2.68 knots, with the directionrelatively steady at about 70 deg. At the beginning of Period 2, currentscontinued to be strong; however, the direction was almost due north. Thisstrong northward flow is questionable because steerage by the bottom topographyis typically in a more eastward direction. Visual estimates from the rig, however, support the northward direction; the direction change is similar to ameander passage. Current speeds decreased linearly from 2.25 knots on Aug. 24at 5 p.m. to a speed of 0.26 knots on Aug. 26 at 7 p.m., a decrease of about0.25 knots every 6 hours. The current direction change was much more abrupt. Direction changed nearly 90 deg., from 356 to 77 deg., in 4 hours (the changemay have been quicker, but data are not available between the two readingsseparated by hours). The reduction in speeds allowed the riser to bereconnected on Aug. 27. The measured current data in Period 3 are indicative ofcurrents normally experienced at the site. Speeds are generally less than 0.5knot, and direction is continually changing in response to tides and/orinertial oscillations. The vector plots indicate a slow westward flowsuperimposed on the rotational currents during this period. JPT P. 1038