Sea Science by Scott McDowell
Rogue waves are not uncommon throughout our world’s oceans, as documented by dozens of examples in this space last month. What’s more surprising is that although large vessels are designed to registry and international specifications, most are vulnerable to the destructive forces of rogue waves, as confirmed by the losses of more than 200 vessels in the past 10 years.
Consider these specifications for static loading on deck hatches: the American Bureau of Shipping (ABS) requires a minimum design of 17 kN per square meter, which is equivalent to 2 metric tons (4,400 pounds) per square meter of hatch area. That sounds substantial — it equals 22 men of 200 pounds each — but this is equivalent to only 6 feet of green water above each square meter of hatch. Really not much.
Forensic analysis of the sunken British MV Derbyshire proved that a rogue wave put more than 201 kN per square meter (equal to 60 feet of water) on the forward deck hatches: 10 times the design load.
It’s easy to visualize large freighters and tankers often burying their bow in the trough of huge waves, and taking up to 30 feet of water on deck.
This loss of the ore-bulk-oil carrier Derbyshire in 1980 prompted a major investigation into the cause, as she was only 3 years old, 965 feet long with a 145-foot beam, and had a crew of 44. She was the largest British ship ever lost at sea.
In 1994, the wreck was located and analyses of undersea photographs acquired by the Woods Hole Oceanographic Institution shed much light on the vessel’s failure. Additional investigation by Douglas Faulkner, a marine architect from the University of Glasgow, concluded that the vessel sank because of structural failure of forward cargo hatches when rogue waves washed over the deck.
A survey of 125 bulk carriers that sank between 1963 and 1996 found that more than 60 percent likely flooded from hatch failure, nine vessels broke in two and the remaining 40 losses are unknown.
Investigators outlined a simple, generic scenario for vessel failures when encountering rogue waves: First, the weight of the vessel causes acceleration down the back of the previous wave into a deep rogue-wave trough. Second, the bow pierces into the lower part of the approaching rogue wave, resulting in thousands of tons of water crashing onto the forward deck hatches of the vessel.
Whether the vessel’s bow can rise before the hatches implode is totally dependent upon the structure of the vessel. The downward pressure on the bow may also cause the vessel to break its back as the midsection rises and the distant stern descends into the trough of a wave that has already passed.
Faulkner recommended a different approach for design specifications of large commercial vessels and offshore platforms, with addition of new “survival design” parameters that exceed the previous requirements for safety during normal storm waves (not rogue waves). It remains to be seen whether the ABS or the International Maritime Organization (IMO) invoke more conservative design specifications that acknowledge the magnitude of rogue-wave forces and probability of occurrence.
Horizontal dynamic forces of rogue waves crashing into the superstructure of vessels also can be devastating. For example, an 80-foot wave will slam a vessel at speeds near 30 knots, implode windows on the bridge level and tear away lifeboats. These consequences are now well documented and warrant a major reassessment of vessel design (survivability) parameters.
Ocean wave modeling is a mature, exacting science but rogue waves defy all the relevant principles. Understanding and predicting these random waves, some as tall as the Statue of Liberty from head to toe, is essential as it would save dozens of lives and millions of dollars in cargo each year.
Consequently, scientists in oceanographic research facilities around the globe have made significant advances in rogue-wave prediction during the past decade, with remarkable discoveries in generation mechanisms including wave-wave interactions (constructive interference); energy focusing, when currents oppose the direction of large waves and intensify wave characteristics; and unique atmospheric processes that cause spatially coherent wind-gust systems that, when moving at the speed of large waves, can amplify the waves to rogue proportions.
Research on this topic is intense and necessitated by commercial vessel owners, international navies, the surface cargo industry and worldwide vessel-insurance companies. Increased pressure must be placed on the IMO, registries and other maritime regulatory organizations for safer design specifications that acknowledge rogue waves.
Hull designs based upon predictions that 85-foot waves occur once in 10,000 years are now obsolete, especially since satellite-borne radar observations have recently shown that such large waves occur twice per day on a worldwide basis.