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form Two-Four-Delta, which was struck by an extreme wave while under construction. This was several years after the Alexander Kielland capsized, but the disaster was still a poignant memory for the people with whom I worked.
Since 1980, extensive monitoring of the sea state has been made at Ekofisk and the occurrence of extreme waves 66 feet or higher documented on a number of occasions. Damage has occurred, sometimes resulting in the loss of life. The adjoining oil field at Draupner collected a historic “first,” during a storm on January 1, 1995, when an extreme wave of 84 feet was actually recorded, as depicted in Figure 18.23 An analysis of the sea state at the time this wave occurred gave a significant wave height of 39 feet, so the extreme wave criterion (the ratio of these two numbers) was 2.15 or close enough to the value of 2.2 to characterize the wave as an extreme wave. The crest height was 60 feet with the trough depth only 24 feet, demonstrating the asymmetry of an extreme wave. The steepness of the wave was 0.06, about twice that of the significant waves.
CONSTRUCTIVE INTERFERENCE (SUPERPOSITION)
Another cause of extreme waves is the random superposition of two or more wave trains in a confused sea—wave trains that happen to have one or more wave crests that occur at the same instant, combining to create an extreme wave. Superposition can occur in “normal” seas when there is no apparent storm or unusual winds. This was the situation described earlier in this chapter, as we were sailing to Catalina Island, although the resulting wave could hardly be called extreme.
Ernie Barker is a former merchant mariner who dissolved a successful law practice when in his 50s, bought a 41-foot Kettenburg yawl called Nepenthe, and set sail for Australia. My good friend and neighbor Keith Garrison now owns Nepenthe, and he put me in touch with Barker.
As a kid, Barker always wanted to go to Australia. After serving nine years in the merchant marine, he still hadn’t made it to Australia. In 1950, upon departing from Yokohama, Japan, while still in the merchant marine, they ran into a gale that came crashing in on an entire