Yes, providing the following is met -

• the tank is not so full as to create a possible spillage
• the gas is contained in a cylinder with valves which can be closed when the vehicle is on the vessel (where possible closed before they board the vessel)
• only properly piped and fitted cylinders will be allowed. All cylinders should be properly secured to the vehicle

As per MCA MGN341, vehicles propelled by LPG / LNG should have gas storage and associated systems built to standard equivalent to the Road Vehicles Regulations 1986. 

Hence, vehicle conversions need to be done with an ‘approved conversion kit’, designed for that type of vehicle AND fitted by a competent mechanic.