Scientists at the Naval Research Laboratory - Stennis Space Center (NRL-SSC) measured a record-size ocean wave when the eye of Hurricane Ivan passed over NRL moorings deployed last May in the Gulf of Mexico. The possibility of a super wave is often suggested by anecdotal evidence such as damage caused by Hurricane Ivan in September of 2004 to an offshore rig in the Gulf of Mexico that was nearly 80 feet above the ocean surface. Hence, some of the destruction done by Ivan has been attributed to a rogue wave. According to industry and national weather sources, the damage done by waves during Ivan has been on the extreme high end for a category 4 hurricane. Ivan has been the most expensive hurricane ever for the oil and gas industry in the Gulf. The Minerals Management Service (MMS) reported that Ivan amazingly forced evacuation of 75% of the manned platforms in the Gulf (574 platforms) and 59% of the drilling rigs (69 rigs), set adrift 5 rigs and sunk 7 rigs entirely. However, the damage by Hurricane Ivan in the oil fields in the Gulf cannot be measured by how many platforms or rigs were destroyed. The most costly damage is believed to have been made to the underwater pipelines. Aside from obvious leaks, some pipelines were reported to have moved 3000 ft while others were buried under 30 feet of mud and cannot be found. The most extensive damage to the pipelines is attributed to undersea mudslides (equivalent to a snow avalanche) and to extreme waves. The complete findings of this study are published in the August 5, 2005 issue of Science.
During NRL’s Slope to Shelf Energetics and Exchange Dynamics (SEED) field experiment, six current profiler moorings that also contained wave/tide gauges (Sea-Bird Electronics SBE 26) were deployed on the continental shelf at water depths ranging between 60 and 90 meters just west of the DeSoto Canyon, about 100 miles south of Mobile Bay, Alabama. An additional eight deep moorings were deployed down the shelf slope but did not contain wave/tide gauges. Fortuitously, between 8:00 pm CDST and midnight on September 15, the eye of Ivan passed through the center of the array, and almost directly over moorings 2, 5, 8, and 11. Historically, instruments in the ocean do not even survive near misses of such powerful storms, much less direct hits. Fortunately, all of the SEED moorings survived this powerful storm, and provided the best ocean measurements of currents and waves ever obtained directly under a major hurricane.
During the approach of Ivan, a moored buoy (ID 42040), deployed by the National Data Buoy Center (NDBC) near the west side of the SEED array, registered a significant wave height of 16.0 meters (53 ft). Unfortunately, the NDBC buoy broke loose and was set adrift on September 15 at 5:00 pm CDST, just before the arrival of the main force of the hurricane. According to a spokesman at NDBC, this wave height appears to be the largest ever reported by NDBC from a hurricane and comes within a few tenths of a meter of NDBC’s all-time record reported in the North Pacific. Note that the wave heights reported by the NDBC buoys are derived from wave spectra. Buoy measurements do not report time series of surface wave elevations, and hence, maximum individual wave heights can only be statistically postulated from spectrum-derived significant wave heights. The SEED wave/tide gauges, however, provided direct time series measurements of surface wave elevations. The maximum individual crest-to-trough wave heights can be reliably obtained. At mooring 3, located under the most intense winds, the maximum measured wave height was 27.7 meters (91 feet) which was part of a group of large waves with periods of approximately 10 seconds where several waves reached heights of about 20 meters (66 feet). These waves recorded by the NRL SEED gauges are by far the largest waves ever directly measured. Even larger waves could have been missed entirely on the shelf since the surface wave data from the SEED gauges were not closely sampled in time, but were instead sampled at 1 hertz over a 512 second data burst only every 8 hours. Analysis of the wave data with the winds suggests that the wave heights likely exceeded 130 feet near the eye wall of the hurricane. Orbital wave velocities generated by such large waves during Hurricane Ivan (not rogue waves) exceeded 2 meters/second at the ocean bottom (in addition to lower-frequency measured currents that exceeded 1 meter/second) and could certainly have caused much damage to underwater structures and pipelines. The measurement of "super waves" cannot be planned and are indeed very rare. These in-situ measurements made by NRL directly under a category 4 hurricane are very valuable since they can be used to provide an assessment of potential impacts to offshore structures and operations by energetic storm waves.
Donna McKinney | EurekAlert!
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