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Sloshing disadvantage of membrane tankers
By Stan Jones Researcher/writer for the Office of the Federal Coordinator Editor’s note: Parts 1 and 2 of this story ran in the June 8 and June 15 issues. This is a reprint from the Office of the Federal Coordinator, Alaska Natural Gas Transportation Projects, available online at www.arcticgas.gov/lng-carriers-called-floating-pipelines Surf’s up But membrane tankers come with their own disadvantages, perhaps most of important of which is susceptibility to sloshing.
Sloshing refers to waves being generated inside an LNG tank as the vessel rides the ocean.
Given the right combination of sea conditions, tank and vessel characteristics, and LNG volume in a tank, sloshing waves can become so big they damage the tank. This can include not only the walls or cap of the tank, but also the tall vertical pump mast used to move LNG in and out at the dock.
This is a fluid dynamics problem so complex that even the marvels of modern computer technology have yet to produce a complete answer to the challenge of designing a slosh-proof LNG membrane tanker. Nonetheless, some general rules have emerged that seem to work.
One tactic is to use smaller tanks - meaning more tanks are required for a given size vessel. Another is to limit loading levels. The basic rule is that a tank should always be either less than 10 percent full (too little liquid to hurt the tank) or more than 70 percent full (too little open space to allow damaging waves to build up inside the tank). However, operational considerations unrelated to sloshing often rule out running the tanks completely empty, as discussed below.
The forbidden zone between the numbers is known as the barred fill range, a result of both research and experience over the years.
“There’s some science and some practice that comes into it,” said Peter Noble, former chief naval architect for ConocoPhillips, in a December 2013 interview. Noble is now president of the Society of Naval Architects and Marine Engineers and principal adviser with Noble Associates.
The first recorded incident of sloshing damage to an LNG tanker came when the industry was barely a decade old. The vessel involved was the Polar Alaska, one of the two tankers built to export Cook Inlet LNG out of Nikiski. Damage to one of its tanks was discovered when it returned home to Alaska from its very first trip in 1969.
Nearly two years later, the Arctic Tokyo - sister ship to the Polar Alaska - also sustained damage to a single tank after surviving two typhoons in Tokyo Bay.
Neither incident resulted in a threat of sinking or release of any LNG cargo.
In both cases, the damaged tank had been 20 percent full as the ship returned home from delivering its load. That was before the industry figured out that 10 percent was the safest lower loading limit.
As is often the case, the Polar tankers had unloaded and kept some cargo aboard, known as heel. Heel serves two purposes. For one, the fact that some of the gas evaporates from the tanks - no matter how well insulated - helps keep them cold, which speeds up loading for the next cargo.
For another, many LNG tankers recover that evaporating gas, called boil-off, for engine fuel on the trip back to port for the next load.
The first response to the incidents on the Alaska and Tokyo was to reduce heel levels, though not to the specifications used today.
The second response was to study the problem in an attempt to determine what combination of tank design, loading levels and sea conditions would produce sloshing damage. This led to a recommendation that the shape of LNG tanks be altered by reducing the size of the bevels at the corners of the tanks, but no further advice on loading levels.
The problem, however, was far from solved. It cropped up again in 1978 on a 130,000-cubic-meter vessel that was the largest of its kind at the time. This time, all five of its tanks were damaged. Again, the ship was never in danger of sinking and no cargo was released.
The sloshing club convenes The new evidence that sloshing still stalked the LNG shipping industry set off a fresh round of scrutiny and the formation of a group called the “Sloshing Club.”
The club was led by Gaztransport - the French company that had designed the tanks damaged in the 1978 incident - with help from two Japanese shipyards, the American aeronautics and aerospace giant McDonnell Douglas, and a ship classification society (a non-governmental organization that establishes and maintains technical standards for building and operating ships).
The group’s first recommendation was for more study, and a research program was launched. One finding was that changing the shape of LNG tanks, as recommended after the Polar Alaska and Arctic Tokyo incidents, had made the problem worse, not better. It turned out that the bigger bevels in the old design had served to clip the waves and reduce turbulence in the LNG.
Now things were back to about where they were right after the Alaska and Tokyo incidents: use the old tank shape, and keep heel levels low.
Time passed, and hopes grew that the sloshing monster had been driven permanently back into its cave. Wrong again.
“This,” as a 2009 sloshing paper by Gaztransport and Technigaz (successor to Gaztransport as a result of a merger) put it, “is why the information from Navantia shipyard in El Ferrol, Spain, during spring 2006 came as such a shock.”
The news from Navantia shipyard? An LNG tanker called the Cataluña Spirit had dry-docked and three tanks were found to have sloshing damage, apparently from side-to-side waves. The ensuing analysis triggered a re-thinking of how sloshing occurs inside a tank.
Until then, it had been assumed that the biggest waves caused the worst damage by producing the most violent motion of the LNG. Consequently, previous studies had focused on what happened in the stormiest seas.
In fact, the analysis showed, stormy seas were less of a threat than intermediate seas - not too big, not too small. That was because big sloshing waves, rather than slamming into the walls, tended to break in mid-tank, much like big surf breaks offshore rather than hitting the beach. And when the decaying wave finally did hit the wall, it was a mixture of LNG and gas rather than pure LNG.
How, then, could smaller sea waves damage a tank if big waves weren’t the problem?
It turned out that one of the most crucial factors was the period of the ocean waves - the time between crests. If the timing was just right, the LNG wave could start at one wall of the tank, build up and up, then crash into the opposite wall of the steel tank at full power and inflict the kind of damage seen in the Cataluña Spirit.
Not surprisingly, yet more research ensued, resulting in indications that loading tanks just above the lower limit of the forbidden zone might have been the culprit.
The sloshing monster put in its last reported appearance in 2008, this time in a type of vessel with tanks of a design that had never experienced sloshing damage before. A total of three such tankers turned up with sloshing damage.
“Again,” the 2009 paper said, “the LNG industry was strongly surprised.”
Over the years, the practice of not loading below 70 percent had begun to emerge, and ship classification societies like Lloyd’s Register endorsed it in 2009.
Nowadays, the operators of membrane tankers are getting a boost from high technology. At least one company markets a shipboard software product designed to help avoid sloshing in two major ways. One is to analyze weather forecasts and identify the route least likely to take the vessel through slosh-inducing seas. The other is to advise the captain if he starts to encounter such conditions on mitigation actions such as changing course or speed.
“Sloshing is not a major problem in the LNG trade today in ships that are well built, well maintained, and operated in a proper way,” Noble said.
The trip to market and back Once under way, the tanker crew faces the issue of boil-off, as discussed above. LNG tankers don’t have refrigeration units on board to keep the LNG at minus 260, so they rely on the insulation around the tanks.
As no insulation system is perfect, the LNG starts to warm up and some of it evaporates - or boils off. (The current industry standard is to limit boil-off to a rate of 0.1 percent to 0.15 percent a day.) Boil-off cools the rest of the LNG and keeps it liquid, just as evaporating sweat keeps people cool on a hot day.
But how to manage boil-off?
Many LNG tankers use it for engine power. If there’s not enough boil-off for the purpose, they can make more from the LNG cargo with their on-board vaporizers.
But boil-off management is changing. Builders of LNG tankers are switching to fuel-efficient low-speed diesels for power, permitting more of the valuable cargo to reach market. Small onboard liquefaction plants return the boil-off to the tanks as LNG.
Some new tanker engines can even burn a mixture of diesel and LNG, in any ratio.
In the destination port, the tanker unloads using pumps immersed in the tanks, then starts the trip home.
But are they safe? Given the complexity of building and sailing these huge tankers filled with a liquid cargo awaiting the chance to boil off into a flammable gas, what about their safety record?
The LNG tanker industry has essentially no history of such problems. As of 2011, more than 135,000 LNG tanker trips had taken place without a major accident in port or at sea.
While there have been minor LNG spills in loading and unloading operations while in port, none involved fatalities or more than minor damage to the ship.
And, in some cases where disaster might have been expected, none occurred.
In 1979, for example, the El Paso Paul Kayser was heading out of the Mediterranean at 19 knots (22 mph) when it hit a submerged rock outcropping in the Strait of Gibraltar. The result was a 750-foot scar in the hull. While damage to the ship was substantial, none of the LNG was lost.
Experts and the industry alike attribute this safety record to a number of factors. One is that LNG tankers, like oil tankers, have double hulls to reduce the chances that a collision or grounding will breach the cargo space. In the case of LNG tankers, each hull is made of inch-thick steel and they are about 10 feet apart - space that is used for ballast water when needed.
Another factor is the elaborate precautions taken to prevent the LNG from mixing with air to create an explosive combination, as described above. LNG itself is not explosive or flammable.
The layer of insulation that separates the ship’s inner hull from the LNG tanks is filled with nitrogen, so that any leak from the tank will enter an inert atmosphere. Additionally, the insulation is monitored for any intrusion of gas so that immediate action can be taken.
In port, federal rules require safety zones around LNG tankers, as well as around the facilities where they dock.
“LNG has been handled safely for many years and the industry has maintained an enviable safety record,” wrote Michelle Michot Foss, chief energy economist at the University of Texas’s Center for Energy Economics, in a 2012 briefing paper. “Engineering and design and increasing security measures are constantly improved to ensure the safety and security of LNG facilities and ships.”
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