The tank punch test
Innovative technology, a growing supply infrastructure and new international standards have built a strong case for LNG, long considered too hazardous, as a general ship fuel. A recent study looks at the collision safety of LNG membrane tanks.
LNG is the cleanest fossil fuel option available to shipping; further reductions of the CO2 output can only be achieved with fuels from renewable sources. While LNG carriers have been sailing the world’s oceans for decades with a very good safety record, the remainder of the shipping industry and its insurance partners rightly demand clarity regarding the risks of carrying LNG on board, in particular in grounding or collision incidents.
In spite of increased ocean traffic, collisions are much rarer today than they were decades ago, thanks to improved navigating, ship locating and traffic management technologies. But incidents do occur, especially in busy sea areas such as the North Sea or the South China Sea, and container ships are somewhat more vulnerable than other ship types due to their more slender contour.
Because of its cryogenic properties and its flammability in air, LNG requires a storage tank system that remains completely tight in an accident. To find out what would happen to an LNG-powered container ship in a collision, DNV GL, the Hamburg University of Technology, and the French LNG containment system specialist GTT launched a joint research project. The collision risk study investigated a hypothetical 18,000 TEU container ship with GTT Mark III stainless-steel membrane LNG fuel tanks designed according to the requirements of the new IGF code, which specifies safety criteria for LNG as a ship fuel, including the minimum distance between the outer shell and the LNG tank. Membrane tank systems make the best use of the space available in a ship’s hull. Their volumetric efficiency and reduced steel weight can lower the total vessel cost and CAPEX for large tanks compared to Type-C tank solutions.
Probabilistic risk assessment
The ship under investigation was assumed to trade on a typical route between Asia and Europe. Collision statistics for all ship types along this route were used to estimate the probability and consequences of a worst-case collision impact: over a twelve-year period, the analysis concluded, 470 collisions involving container ships should be expected, and 20 of these incidents would result in a rupture of the inner hull.
A membrane tank system is composed of various layers of insulation and reinforcement materials which are directly connected to the inner structure of the ship and can absorb some of the impact energy in a collision. The ship design investigated had a double-hull width of 2.5 metres distance.
The collision scenario derived from this data was subjected to both numerical and experimental evaluation. Before any mechanical tests were performed, the deflection behaviour of the inner hull was calculated through finite-element analysis assuming the bow of another vessel striking the most critical hull area of the LNG-powered ship at a 90-degree angle – the most forceful type of collision – at a speed of 10 knots. The size chosen for the striking ship was about 30 per cent smaller than the impacted vessel because its sharper contours represent a more critical collision scenario. To simplify the calculations the impacted vessel was assumed to be stationary; hydrodynamic effects were ignored. According to the FEM calculations, outer fracture will occur at a penetration depth of about 1 metre, and inner hull fracture at about 3 metres as the bulbous bow of the impacting vessel hits the container ship
Membrane tank flexibility ensures safety
The structure of the GTT stainless-steel tank membrane is of critical importance for the collision behaviour. A grid of evenly spaced knots and corrugations in the austenitic steel sheeting stiffens the tank wall while allowing it to react to the large temperature differences LNG containment systems must withstand. The same structure also acts as an energy-absorbing feature in a collision: the corrugations simply yield to impact pressure by “unfolding”, giving the tank wall additional flexibility. This greatly increases the survivability of these tank systems, which have withstood major deformations in grounding and other incidents without leaking.
To verify the FEM results, a lab experiment was performed at Hamburg University of Technology (TUHH) using a 3.75 by 3.75 metre mock-up of the GTT membrane tank wall welded to a horizontal supporting frame, which assumed the role of the inner hull of the ship. The impact of the bulbous bow was simulated by a spherical steel body moving downwards into the centre of the mock-up. The bulbous bow dummy was rigid enough to resist deformation, which would absorb some of the impact energy.
The mock-up was indented by approximately 0.8 metres. On a real container ship with a membrane LNG fuel tank 24 metres long, this would be equivalent to an eight-metre penetration into the inner hull; the amount of impact energy absorbed without rupture would be close to 400 MJ. These experimental results, which were largely consistent with the FEM calculations, cover the majority of historical collision energy values collected for the relevant route.
Some deviations between the test and numerical simulation data occurred during the first run of FE calculations. TUHH is working on fine-tuning the calibration of numerical simulations.
Adding some reinforcements to the ship’s side structure near the LNG tanks would further increase collision resistance. By comparison, grounding incidents are less likely to cause major penetration since the double bottom of these large container vessels is more than two metres tall, and historically very few groundings have exceeded this height at which the inner hull would be affected.
In the unlikely event of an actual rupture of an LNG membrane tank, the vaporizing LNG could catch fire. In contrast to HFO spills and fires, however, an LNG fire would last hours rather than days and cause no direct environmental damage. On the other hand, an on-board fire with another cause does not necessarily pose a major risk to the LNG system, which is well protected by insulation, safety valves and vent masts. In fact, the most hazardous aspect of using LNG as a ship fuel is bunkering operations.
Membrane-type containment systems have demonstrated their exceptional efficiency and safety on board LNG carriers in more than 15,000 accumulated years of experience without any loss of cargo. Since the LNG containment system is much smaller on a container vessel, the hull surface area vulnerable to a collision is likewise smaller, which – compared to an LNG carrier – reduces the relative risk.