July 07, 2015
Safety and Security of LNG Powered Cruise Ships
by Kevin Mueller
On June 15th, 2015 Carnival Corporation & plc entered into a multi-billion dollar (USD) contract with Meyer Werft of Germany to “…build four next-generation cruise ships with the largest guest capacity in the world.” It seems that a year doesn’t go by with larger and larger cruise ships being ordered, with most of the technological advances centered on the passenger’s experience (on-board wave surfing, IMAX theater, elevated activity course, ice skating rinks, bumper cars, and even a sky-diving simulator, just to name a few). This particular agreement is transformative in that the new ships will be powered using liquefied natural gas (LNG) in dual-powered hybrid engines while at sea and in port.
Historically, cruise ships utilize large diesel engines and transmissions (similar to vehicles) to generate the power needed for propulsion. As cruise ships became larger and the demand for electricity substantially increased, diesel-electric engines became the norm with hybrid propulsion systems. While improvements have been made on the propulsion system with more advanced propellers, fundamentally the power system for cruise ships have remained unchanged for years. The technology to power ships using alternative fuels such as LNG has been available for quite some time, but the infrastructure needed for wide-spread operation does not fully exist.
LNG has a lower energy density than petroleum. As a result, greater quantities of LNG are required to be stored on the cruise ship compared to petroleum for long distance journeys. For safety, security, and environmental reasons, LNG liquefaction, degasification, and storage facilities are typically located far away from cities. In order to refuel economically, the LNG would have to be delivered to the cruise ship via a bunkering barge, increasing the risk of accidents and congestion at port.
As a leading engineering consultancy specializing in protection people, operations, and property against structural damage caused by explosions and man-made hazards, using LNG as a fuel source for large cruise ships is of particular interest to Hinman. As compared to the current practice of using petroleum for fuel, LNG requires additional safety measures for the following reasons:
The accidental spillage of LNG (stored around negative 161.5°C) onto sea water (typically around positive 15°C) results in a rapid phase transition that can produce violent physical explosions due to the large difference in temperature. This can severely damage the ship’s hull if not designed properly. For Carnival’s four new cruise ships, it was specifically pointed out that they will “…feature more than 5,000 lower berths...” which are located within the ship’s hull and not the superstructure above. This puts both passengers and crew in harm’s way while refueling at port.
The accidental release of LNG into the atmosphere can cause a large vapor cloud to form over the hull and throughout the superstructure of the cruise ship. If the vapor cloud reaches the proper concentration range (5% to 15%) any ignition source can cause it to burn. The flame would burn back to the source of the vapor cloud, causing widespread destruction and fire spread along its path.
- According to Rolls Royce, large ships typically use ‘Type C’ LNG storage tanks. These are vacuum insulated and hold a pressure of just under 10 bar to push the LNG through the vaporizer and super heater units without using additional externally applied pressure. This minimizes the number of movable parts and increases the lifespan of the LNG fuel system. While rare, if the storage tank were to develop a leak, the storage pressure alone could cause a jet fire to develop that would be difficult to contain using traditional fire protection methods.
The environmental advantages of using LNG over traditional petroleum products are driving the current fuel transformation in the cruise ship industry. The proximity of ports to major cities and waterways around the world increase the risk of petroleum emissions on local communities. Apart from the obvious safety and security concerns listed above, it is prudent and responsible for the cruise ship industry to explore and switch to cleaner burning fuels such as LNG. However, considering the large number of revolving passengers and crew that will temporarily call the cruise ship home, it is important to plan for and mitigate the potential hazards of LNG which can be far greater than traditional petroleum.