Indeed, the Netherlands-based Maritime Research Institute describes sloshing, on its website, as “the most emerging technical issue and concern of the LNG industry associated with the application of membrane technology.”
Sloshing happens when a ship carrying liquid cargo moves in waves. The ship motions excite sloshing, which in turn further affects the vessel’s motions and vice versa.
Leon Adegeest, managing director of Amarcon and developer of OCTOPUS-Onboard, which includes sloshing warning and avoidance features, has spoken to a number of captains to try to understand the sloshing phenomenon.
“Think of walking with a bowl of soup and how difficult it is to stop the liquid moving. Captains have told me you can’t miss a sloshing event on board a vessel. When it starts to roll, it’s like a gun firing beside your ear, they say,” he explains.
“The impact of the liquid gas hitting the sides of the tank is very violent. It can really damage the inside – maybe not the first time but definitely after several times. Gas and cracks in tanks is not a nice combination,” says Adegeest.
Besides damaging the tank, sloshing can make a vessel move in dangerous ways. As Adegeest says, “The big vessels have 40x46 meter-wide tanks without a bulkhead in the middle, so when there is sloshing, the vessels starts to ‘dance’. I’ve seen videos at an oil company showing a tanker moving violently beside the terminal.”
Assessing the strength of containment systems during sloshing is a complex process still the subject of extensive international research. Two modern trends that contribute to sloshing are:
– Most tank vessels carrying LNG nowadays generally do so with larger tanks than in the past.
– The production of LNG is moving towards offshore regions, where so-called floating LNGs (FLNGs) operate. Shell’s Prelude is an example of such a mega-production facility. These FLNG’s are unloaded by transferring the LNG to tankers coming alongside. These tankers are exposed to wave and swell conditions and will always have phases during the loading operation when the tanks are not full .