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| Ocean Introduction | Ocean Environment | Wind |
The high point of an ocean wave is called the crest, and the low point is the trough. The distance from crest to trough is of course the wave height, and the distance between successive crests is known as the wavelength. Waves are created by wind blowing on the oceans surface, which steadily adds energy to them. The size of waves thus depends on how hard and for how long the wind blows. Because waves can travel long distances without losing much energy, they may appear when there is no wind, having been produced by some distant storm.
While it may appear that waves consist of water moving linearly, in reality each water particle simply travels in a circle. The water transmits energy without being carried along. This is why small free-floating objects on the surface just bob up and down in waves. Even in huge waves, a piece of driftwood doesn’t get broken, just shaken around a lot.
However, our potential seastead designs are not like this. Objects that are large or heavy don’t just roll with the waves, they resist and must absorb some energy. For them, the ocean is a much more hostile environment. The amount of energy stored in a large wave is quite scary, hence why they occasionally pulverize large ships. You’ll first learn about the biggest waves, and then about our strategies for avoiding them.
Many people think of the tsunami as the most fearsome wave, but that’s a landlubber’s perspective. Generally driven by earthquakes, tsunamis are often unnoticeable in the deep ocean, where they have extremely long wavelengths and low wave heights (several meters at most, usually much less).
As this wave reaches a continental shelf, it piles up, becoming shorter and higher. Only then will it resemble the monsters of legend - and as usual, legend exaggerates. Tsunamis rarely result in giant breaking waves, but are more like very strong, fast tides [USGSTsunami]. While this is very dangerous for coastal structures (as the horror of the 2004 Asian tsunami demonstrated), even if a seastead was close to shore, it would just rise with the water level. The worst consequence would be the mooring system failing or being damaged.
Now we’ll see a case where the storytellers, not the scientists, turned out to be right.
They were struck by a rogue wave - a monstrous wall of water that rose out of nowhere and slammed onto the deck like the fist of god. Ships often don’t survive an onslaught like that. Many sink before anyone on board knows what’s hit them.
[Lawton2001]
On the right is perhaps the only photograph of an elusive phenomenon known as a rogue wave. Not many people would reach for a camera when struck by such a monster, but that’s exactly what Phillipe Lijour did. He was onboard the oil freighter Esso Languedoc in 1980 when it was struck by a rogue. By his estimate, most waves were 5-10m, as you tell from the low seas in the background. The mast visible on the starboard side is 25m above sea level, and the wave is breaking from behind the ship. The wave was at least 20m high, perhaps 30m (since its trough, as well as crest, would be lower than other waves).
Scientists used to dismiss such tales of unusually large waves as mere folklore, like monsters or mermaids. But with the proliferation of oil and gas platforms, some of which record wave data, accumulated observations have finally led to mainstream acceptance of this seafaring “myth” [Lawton2001]. And recent data from the European Space Agency’s ERS satellites has not only re-confirmed the existence of these waves, but indicated that they may be fairly common. Researchers with the MaxWave project computer-analyzed satellite photos from a three-week period in 2001 during which two ships were hit by 30m rogues. They found “ten individual giant waves around the globe above 25 metres in height.” [ESA2004].
These rogue waves are the real dangers in open water. Towering above their neighbors, they are unstable and break quickly, thus containing tremendous power. They sometimes come unexpectedly from a different direction than the prevailing swell, which adds to the surprise and danger. Rogues have been known to ravage coastlines as well, sometimes coming out of calm seas to sweep away unsuspecting victims. Emergency services have warned beachgoers in some areas to be aware of this danger [RogueWarning].
Understanding rogue waves is clearly quite important for marine safety. Hence while their existence has only been accepted for a few decades, a decent-sized body of academic work has sprung up. There was a Rogue Wave conference in 2000 [RogueWaves2000]. Theories about their existence include interference patterns (refraction/diffraction), current/wind interactions, and normal variations in the height of wave groups. These theories are problematic, however. Interference ought to produce a bell-shaped distribution, but high outliers occur much more often than that. Also, in the open ocean it is unclear what would cause an interference pattern. Current interactions don’t explain the many rogue waves in areas without fast-flowing currents. Focusing effects of some type seem to be the most promising. They are difficult to analyze on messy real-life waves, but some non-linear mathematial models have produced focusing and shown promise at replicating the observed distribution [NorwayRogueGroup]. And the MaxWave team is starting a new investigation called WaveAtlas to further study the distribution of rogue waves.
The question of how big waves get and how often they hit is of far more than academic importance. Most offshore platforms and cargo ships have been designed assuming the standard distribution. This is why rogue waves are so dangerous - not just because they are big, but because they are unexpectedly big, and so structures are not designed to handle them. We can see that a seastead intended to last decades must be prepared to withstand these “Monsters of the Deep”.
To design a safe and reasonably-priced seastead, we need to know what wave heights to expect. Most important is the worst-case height. Since this will depend on the exact region and seasons, we’ll just give a general overview, as well as sources for additional information. Wave height is a function of wind strength and the “fetch”, which is the distance over which the wave been building. Oceanographers use a statistic for wave height called the “significant height”, denoted Hs. This is the average of the 1/3 highest waves (from crest to trough) over a given time period (20 minutes for [NDBC] buoys). One wave in a thousand is twice Hs, and about one wave in three-hundred thousand is a so-called “rogue” whose height is two and a half to three times Hs [Bascom1980]. However, this distribution may reflect out-dated assumptions about rogue rarity, and must be investigated further.
The National Data Buoy Center [NDBC], part of the NOAA, is a good source for wave height information. Data from several hundred buoys (not all theirs) is accessible on their website, often with historical records. The highest waves ever recorded by the NDBC buoys were in the North Pacific, near the Aleutian Islands, and had an Hs of 18m. A rogue wave in that storm could have reached a staggering 48m in height. The location is no accident. While 100-foot waves in the North Atlantic are rare enough that it took a “Perfect Storm” to create them, David Gilhousen, a metereologist with the NDBC, says that in the North Pacific “sea waves of that magnitude are something you would see every other year – maybe every year” [Chui2000]. Not the best place to build a seastead.
The highest wave ever accurately assessed at sea was seen from the USS Ramapo on February 6, 1933, and measured at 34m by triangulating on the crows nest. The ship was on passage from the Philippines to California during a hurricane with a wind force measured at 68 knots. The storm lasted 7 days and stretched from Asia to New York, producing strong winds over thousands of miles of unobstructed ocean. Other sources have reported rogue waves of 17.5m (Skourop, North Sea), 26m (1/1/95, North Sea), 28m (1943, North Atlantic, the Queen Elizabeth), and 29m (1995, North Atlantic, the Queen Elizabeth 2) [Lawton2001].
It not only takes strong winds to generate these monster waves, it also takes a long fetch. For this reason, sheltered seas like the Mediterranean and the Caribbean experience smaller waves even during severe storms. The doldrums around the equator, where winds are low, are another area with much smaller waves. Cautious seasteaders may wish to gain some experience in these places before venturing into rougher areas.
“One thing about the sea. Men will get tired, metal will get tired, anything will get tired before the sea gets tired” - A marine engineer’s observation about the tragic collapse of TT-4.
The loss of Texas Tower Four demonstrates the disasters that can occur when a flawed structure encounters the tremendous power of the ocean. The tower was one of three manned radar platforms off the US Atlantic coast numbered Two, Three, and Four (One and Five were planned but never built). The tower, part of the nations air defense system, “was such a spectacular sight that ocean liners veered off course to permit passengers to glimpse it” [TexasTowersRD, p. 95]. Because it was a lonely post, the interior of each tower had libraries, gyms, and rec rooms. Music, movies, and even a daily ration of beer helped entertain the crew.
Things began to go wrong with Four from the beginning. Built in a Portland, Maine shipyard, it was completed in June of 1957. While a convoy was towing it to its permanent location, a sudden storm struck and damaged the tower. (For those with an engineering bent, it tore off the diagonal cross braces on the legs). Because of the cost and time delays involved in towing it back for repair, the tower was installed anyway (the civilian contracter said that the damage could be repaired on-scene). This problem was especially serious because TT-4 had legs more than twice as long as the other towers and needed these missing supports. Divers later installed an extra piece to compensate, but it proved insufficient for the task. Even modest waves caused the structure to tremble (earning it the nickname “Old Shaky”), and over time the bracing was compromised. Again, divers attempted to repair it, but in September of 1960, Hurricane Donna destroyed the patchwork repairs with 132mph winds and 50-foot waves [Ray1965].
The towers lurching worsened and most of those onboard were evacuated, but a skeleton crew remained. They were there to protect the millions of dollars in radar equipment from the Russians, and to maintain the tower while more repairs were attempted. In January of 1961 there were 28 crew, half of them USAF personnel and half a civilian repair team. Warned by radio of an approaching storm, the next cargo ship offered to evacuate the tower, but orders from land said for the men to stay on the tower and the ship to stand by. The chain of command for the towers was poorly designed, and apparently the superior officer on land was unsure of his authority to evacuate the station.
The storm arrived, and while it batterred the waiting cargo ship and the shaking tower with 85mph winds and 35-foot waves, the commander on shore finally decided to evacuate during the next lull. Helicopters from the Coast Guard waited to take off the moment the weather permitted, and the Towers crew frantically cleared off the flight deck. But the weather refused to slacken, and under the incredulous eyes of the cargo ship captain, the radar image of Texas Tower Four disappeared from his screen. All hands were lost. [TexasTowersRD]
It was later determined that one of the three legs had broken under the strain, rendering the tripod of support unstable. it’s important to note that Towers Two and Three survived this and many other Atlantic storms over the next few years, although they were evacuated a number of times to ensure that no repeat tragedy ocurred. Eventually currents weakened the foundations and they were decommissioned. (We’d like to note here that the TT’s weakness was their supporting legs, which our design avoids entirely).
The lesson is not that permanent manned sea structures are impossibly dangerous, but that the ocean does not forgive mistakes. Great care must be taken in the design and building of seasteads to ensure their absence from the annals of marine tragedy. This will increase their expense, but safety - a matter of life and death - is no place for shortcuts.
Because waves are so dangerous, we think it’s quite important to be able to avoid or nullify them. Our preferred design, the spar platform, handles this by lofting the living space above the waves on a tall pillar. While this keeps a seastead from being pummelled by the waves around it, it may also want to avoid big waves in the first place. Since a seasteads only place in the America’s Cup race would be as a buoy, this requires planning well in advance. There are other methods for minimizing this danger as well. Here are the techniques we consider: