The main safety challenges concerning the use of LNG as fuel are related to the explosion risk, extremely low temperatures and tank location.
In its liquid form, LNG is neither explosive, corrosive nor toxic. Under atmospheric pressure it will, however, evaporate to form natural gas at temperatures above -162°C. And natural gas is flammable, as are all hydrocarbons and fuels, and explosive. However, natural gas will only ignite when mixed with air in a ratio of 5%-15% and at a temperature of above 500°C (in an air-fuel mixture of about 10% methane in air, the auto ignition temperature is approximately 540°C, while the auto ignition temperature for diesel oil is in the range of 260°C to 371°C). LNG will be very cold if released, and may thus have a higher density than air even after evaporation. However, at approximately -110°C it becomes buoyant. At ambient temperatures, natural gas has a specific gravity of about 0.6, which means that natural gas vapours are much lighter than air and will rise quickly.
While the high density at low temperatures increases the risk of gas being trapped, the high ignition temperature and limited flammability range show that natural gas is actually not that easy to ignite and that good design can overcome many risk factors.
When skin touches an extremely cold body or LNG, heat is transferred from the skin and organs to the cold body or LNG. This will cause damage to the skin and underlying tissues. The normal functioning of the body may be disturbed by the cooling of internal organs, which will lead to a critical condition called hypo¬thermia. The cooling of the brain or heart is very dangerous. Proper procedures and the use of protective clothing and equipment to prevent any contact with the LNG are hence imperative.
However, the extremely low temperatures are not only hazardous to people. While stainless steel will remain ductile, carbon steel and low alloy steel will become brittle and fractures are likely if they are exposed to such low temperatures. Standard ship steel must therefore be protected and insulated from any possible exposure to LNG.
LNG storage tanks may be located above or below deck. The tank must be at least the lesser of B/5 or 11.5 m from the ship side and never less than 760 mm from the ship bottom. If the tank is above deck, A-60 insulation towards the accommodation, service stations, cargo spaces and machinery spaces is required, and a drip tray below the LNG tank is required if the tank has connections below the liquid level. Further, a water spray system is to be fitted, covering exposed parts of storage tanks, for cooling and fire prevention purposes. For tanks located below deck, the maximum allowable pressure is 10 bar. In addition, a “secondary barrier” is required around the LNG tank, built in stainless steel or an equi¬valent low-temperature-resistant material and designed to withstand maximum pressure build up.
It is generally assumed that LNG-fuelled vessels can be designed in such a way that the LNG tank is not damaged by a collision. This may be a technological design challenge, but the consequences of a ship collision are not expected to impact the overall risk picture.
Large gas carriers typically have a distance of between 2 m and 4 m from the outer shell to cargo. These will resist high energy collisions without any loss of cargo containment. Only a 90° side-on collision with a fully loaded very large oil tanker (VLCC) is considered to have a release of LNG as a likely outcome. It is therefore considered feasible to construct, locate and protect fuel tanks on LNG-fuelled container vessels in order to achieve a sufficiently low risk level.
Hazid for LNG in container ships
The aspects listed below are potential hazards that should be evaluated and taken into account when designing an LNG-fuelled vessel.
Fire/explosion due to the ignition of gas leakage in the:
LNG bunkering station/system
Cold box/tank room
Gas Regulation Unit (GRU) room
Gas engines (incl. piping)
Fire/explosion in other areas/systems escalating to the LNG system
Leakage of liquid LNG causing loss of structural integrity/brittle fracture of structures
Asphyxiation due to the lack of oxygen in the case of a release of LNG/gas in an enclosed or semi-enclosed area
Human contact with surfaces or substances at extremely low temperatures
Very high energy collisions may cause damage to the LNG/gas containment system and, in the most extreme cases, also to the LNG tank
Collision statistics, risk assessment
During the 41-year period from 1970 to 2010, more than 71,500 vessel years were registered in IHS Fairplay. The fleet increased from some 200 vessels in 1970 to almost 5,000 vessels in 2010.
669 collisions were registered, of which 493 occurred during the last 20 years and 350 during the last 10 years. Eight accidents were registered as fatal, of which three occurred during the last 10 years. As can be seen from the figure, an average of approximately one collision occurs per 100 vessel years.
Since the turn of the millennium, there has only been one collision that has led to a total loss. However, even though the 29-year-old vessel was severely damaged and it was not considered economically feasible to repair it, it remained afloat and the structural damage would not have impaired a possible LNG tank or gas fuel system.
A review of reported collisions involving container vessels showed that in 60% of cases the container vessel was the striking vessel, while it was hit in only 40% of cases. A simple geometrical evaluation indicates that the probability of a striking ship hitting a container vessel in the area where a LNG tank might be located is less than 10%, and probably not more than 5%. For the striking vessel to have the potential to penetrate so extensively into the hull of the other vessel so as to represent a threat to a possible LNG tank, it must hit at an angle close to 90°, probably within +/- 15° and at least within +/- 30°. Hence, even without taking the size and speed of the hitting vessel, or the structural resistance of the hull and LNG tank of the struck vessel, into account, the frequency of potentially hazardous collisions related to a possible LNG system is well below once in 10,000 vessel years.