Oscar Karl Huh
Coastal Studies Institute
Louisiana State University
Baton Rouge, La.
Kenneth J. Schaudt
Marathon Oil Co.
Houston
With the onset of drilling in water depths in excess of 300 m, detection and location of the boundaries of high-speed current zones has become important in preventing downtime in drilling and production operations.
Although the moorings and overall structures are usually designed to withstand the expected 4-knot currents, certain operations and hardware are adversely affected by such velocities. Costly delays may occur when such currents move into lease areas.
CURRENT ACTIVITY
In July and August 1989, drilling operations throughout the Green Canyon, Ewing Banks, Mississippi Canyon, and Ship Shoal offshore lease areas were struck by 23 knot surface currents as the Loop Current surged northward during the separation of the Nelson Eddy. This caused the suspension of various deepwater drilling operations.
We understand that some operators may have experienced downtimes in excess of 2 weeks.
We were able to detect the location and boundaries of the high-velocity current zone using the National Oceanic & Atmospheric Administration (NOAA) satellite Advanced Very High Resolution Radiometer (Avhrr) imagery. This was accomplished even under environmental conditions which were adverse for radiometry of the sea surface.
Inferences developed from satellite imagery alone were independently corroborated by ship-borne measurements of currents using expendable current probes.
Strong currents such as these have been encountered previously, measured by the Eddy Joint Industry Project in 1983, 1985, 1986, and 1988 as well as by Texas A&M University surveys in the 1950s and 1960s.
Similarly, the intrusion of this current onto the Louisiana shelf, while unusual, was not unprecedented. Strong currents caused by the Loop Current or its associated eddies have previously been observed in water depths as shallow as 1,000 ft, and every few years may approach within 20 miles of the sites affected during 1989.
The Loop Current (Fig. 1) is the "river" of Atlantic equatorial water flowing through the Yucatan Strait into the Gulf. It loops through the Gulf with a clockwise motion prior to exiting via the Florida Strait where it forms the Gulf Stream of the Atlantic. It is a variable current, which at times penetrates deeply northward into the Gulf and at other times flows just north of Cuba into the Florida Strait.
During deep penetrations into the Gulf, the northern portion often separates from the Loop, forming large (200-400 km) clockwise rotating eddies with current speeds ranging up to 4 knots.
Once detached, these eddies will slowly drift westward to decay in the Western Gulf. The lifespan of an eddy from its initial detachment in the eastern or central Gulf to its decay in the western Gulf may take up to 1 year.
Because some offshore operations will be disrupted by the high-speed currents, detection, tracking, and preparation are the simplest solutions to this potentially costly problem.
SATELLITE SYSTEM
The large spatial scales and time variability of these oceanic features make them difficult as well as costly to detect and map. Because deepwater drilling and construction operations may be disrupted by these current systems, oil industry oceanographers have developed a suite of techniques to track these current systems.
The operational NOAA satellite system provides two powerful tracking tools: the Avhrr, an imaging sensor capable of mapping sea surface temperatures over the entire Gulf, and System Argos, a data collection system to track the velocity of drifting ocean buoys.
Because the Loop Current and its associated eddies consist of fast-flowing equatorial waters, it is warmer than the ambient waters in the Gulf of Mexico during the late autumn through late spring. The NOAA satellite system thus provides oceanographers with Avhrr thermal "pictures" of the sea surface temperature (circulation) patterns as well as ARGOS buoy track-current speed, vectors, from which the broad scale current patterns can be deduced.
Once the broad scale circulation is known, vessel reconnaissance surveys are often conducted to delineate the intensity of the features nearest drilling operations.
Oceanographers have for many years been using thermal infrared satellite data in mapping the Loop Current and associated eddies. However, it has been effective only in late fall, winter, and early spring, when atmospheric cold fronts create the following two conditions important for satellite measurements:
- The regional cooling chills the shallow or shallowly stratified surface waters below the temperature of the warm, deep waters of the Loop Current. Cold fronts thus create temperature differences between the different currents/water types (Fig. 2).
- The fronts also radically reduce the atmospheric water vapor over the Gulf. Low levels of atmospheric water vapor improve the quality and reliability of thermal infrared imagery.
Years of experience with satellite thermal infrared imagery have indicated that summer conditions prohibit satellite detection of sea surface temperature (SST) gradient features that mark the edges of the Loop Current and eddies.
The high concentrations of atmospheric water vapor absorb and obliterate temperature details in the Gulf-wide thermal radiation field. Additionally, the predominant solar heating creates an SST field that is flat (uniform across the Gulf) with no ocean temperature features to detect.
Cloud cover, of course, completely obliterates ocean SST features.
CURRENTS OF AUGUST 1989
In late July 1989, strong currents associated with the Loop Current (or the Nelson Eddy), moved into the active drilling leases in Green Canyon and Ewing Bank area, as far north as latitude 28 N. These were eastward flowing currents with velocities apparently ranging between 2 and 4 knots.
They untypically persisted for weeks into September 1989.
Oceanographic evidence indicated that the currents had been poised in the central Gulf for several months prior to this northward surge. We are not certain yet whether this intrusion of high-speed currents was a surge of the Loop Current itself, or an event associated with the detachment of the Nelson Eddy from the Loop Current. Investigators search for the answer to this question as study of the extensive oceanographic data set continues.
SATELLITE DATA
When the problem currents were encountered by industry in July and August 1989, experiments were conducted at Louisiana State University (LSU) using the thermal infrared data of the Avhrr. The expectations were not very high considering the summer conditions which include: high levels of atmospheric water vapor; extensive, though broken cloud cover; and the flat (uniform) sea surface temperature field produced by the predominant regional solar heating.
These experiments were conducted using the new LSU Coastal Studies Institute NOAA satellite earth station.
The Terascan System, by the SeaSpace Corp., San Diego, consists of a tracking antenna, receiver, computers, and specialized software to acquire and analyze NOAA satellite data received directly from the overpassing satellites.
An efficient archiving system allows LSU to capture and store satellite data over a large portion of the north Western Hemisphere 6-12 times daily. This automated facility has been in operation since June 28, 1988.
An experimental analysis was conducted of a series of the noisy, little-used midinfrared thermal sensor data of the Avhrr.
From the subtle features detected, an interpretation and depiction of the high-velocity, surface-flow features was derived.
In an Aug. 10 image, very pronounced east/west oriented stream lines of cold water (2 C. lower than ambient) in the sea surface temperature field were detected just north of the affected blocks. From an image obtained on Aug. 19, the western margin of the Loop Current can be seen as a streamer of cooler water extending from near the Yucatan Strait to a position some 70 nautical miles, south of Grand Isle, La., where it turns sharply to the east (Figs. 3a and b).
On Aug. 20, another image was recorded showing streamlines in the SST field describing an arcuate pattern that also connects the north/northwest trending pattern to the south with the east/west trending pattern to the north (Figs. 4a and b).
The image fragments are integrated to describe a Loop Current circulation pattern as shown in Fig. 5. Surface current vectors, consistent with the interpretation of the surface temperature pattern, were independently measured using expendable current probes (XCP's) from the RN Gyre (Fig. 6).
The surface temperature features shown had been observed previously in the Fast Eddy during the summer of 1985 (J. Hawkins, Norda, Bay St. Louis, Miss., personal communication to K. Schaudt).
The structure observed in these images is explained through surface temperature and current measurements collected from the RN Gyre during a cooperative oil industry, Mineral Management Service (MMS), and Texas A&M survey at this time. This survey revealed that the surface temperatures were depressed by at least 0.4 C. in the region of highest currents. The satellite evidence indicated temperatures depressed by at least 0.4 C. in the region of highest currents.
Satellite detection of the front was thus made, indicating the presence of cool "up-welled" water at the strongly baroclinic edge of the high-velocity current zones.
The surface current data obtained independently by the ship and satellite investigators closely matched each other in pattern and location (Figs. 5 and 6). From these observations some tentative but potentially useful conclusions may be drawn:
- In summer the detection of the high-speed current activity of the Loop Current and associated rings are most likely accomplished by detection of streamlines of cool up-welled water at the current margins, rather than areal water mass differences as seen routinely in winter.
- This summertime detection occurred using specially processed NOAA Avhrr thermal infrared data in both dry and moist air masses, as the SST streamlines were detected well south of the southernmost penetration of the unusual atmospheric "cold" front.
This suggests that detection of high-speed current activity in the Gulf may be possible throughout the summer season.
- The extreme current activity experienced by industry was due to the impingement of the Loop Current/Nelson Eddy directly on the Continental Slope.
- Two important questions remain to be answered. First, can this approach of analyzing specially processed thermal images and time-composites of them reliably detect high-velocity currents in the Gulf in summer? (To be addressed by the remote sensing community.)
Second, what caused this unusual northwestward penetration of the Loop Current/Nelson Eddy in August 1989? (To be addressed by the oil industry.)
ACKNOWLEDGMENTS
The surface data of August 1989 were collected through a cooperative effort by Texas A&M University; the Eddy Joint Industry Project (which is administered by Conoco); and 12 organizations, including Shell, Conoco, ARCO, Mobil, Marathon, Exxon, BP, Amoco, Texaco, Chevron, AGIP, and the MMS.
Without the assistance of Dr. D. Biggs of Texas A&M, this survey would not have been possible. Special appreciation goes to David Wilensky, system manager of the Terascan System, LSU Earth Scan Laboratory, for specialized software support.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.