cause storage problems and offloading problems. Maximum boil-off/minimum oil:- In this mode the tank pressures are kept low and there is greater boil-off but a large amount of fuel oil is still used. This decreases the amount of LNG delivered but the cargo will be delivered cold which many ports prefer. 100% gas:- Tank pressures are kept at a similar level to maximum boil off but as this may not be enough to supply all the boilers needs, additional LNG must be "forced" to vaporize. A small pump in one tank supplies LNG to the forcing vaporizer, where it is warmed and vaporized back into a gas that is usable in the boilers. In this mode no fuel oil is used. Recent advances in technology reliquefication plants to be fitted to vessels, allowing the boil off to be reliquefied and returned to the tanks. Because of this, the vessels' operators and builders have been able to contemplate the use of more efficient slow-speed diesel engines (previously most LNG carriers have been steam turbine-powered). Exceptions are the LNG carrier Havfru (built as Venator in 1973), which originally had dual fuel diesel engines, and its sister-ship Century (built as Lucian in 1974), also built with dual fuel gas turbines before being converted to a diesel engine system in 1982. Vessels using Dual or Tri-Fuel Diesel Electric, respectively DFDE/TFDE, propulsion systems are now in service. Recently, there has been interest in a return to propulsion by boil-off gas. This is a result of the IMO 2020 anti-pollution regulation that bans the use of marine fuel oil with a sulfur content greater than 0.5% on ships not fitted with flue-gas scrubbing plant. Space constraints and safety issues typically prevent the installation of such equipment on LNG carriers, forcing them to abandon the use of the lower-cost, high-sulfur fuel oil and switch to low-sulfur fuels that cost more and are in shorter supply. In these circumstances, boil-off gas may become a more attractive option. Consequences of spillage Compared to oil, there is less public concern over spillage of Liquid Natural Gas (LNG) carrying vessels as the gas would quickly vaporize and become atmospheric methane. The LNG sector is known to have a good safety record regarding cargo loss. By 2004, there had been close to 80,000 loaded port transits of LNG carriers with no loss of containment failure. An analysis of several spherical carriers showed