proceed to the discharge port. During passage various boil-off management strategies can be used. Boil-off gas can be burned in boilers to provide propulsion, or it can be re-liquefied and returned to the cargo tanks, depending on the design of the vessel. Once in the discharge port, the cargo is pumped ashore using the cargo pumps. As the tank empties, the vapour space is filled by either gas from ashore or by vaporising some cargo in the cargo vaporiser. Either the vessel can be pumped out as far as possible, with the last being pumped out with spray pumps, or some cargo can be retained on board as a "heel". It is normal practice to keep onboard 5% to 10% of the cargo after discharge in one tank. This is referred to as the heel and this is used to cool down the remaining tanks that have no heel before loading. This must be done gradually otherwise the tanks will be cold shocked if loaded directly into warm tanks. Cool-down can take roughly 20 hours on a Moss vessel (and 10–12 hours on a membrane type vessel), so carrying a heel allows cool-down to be done before the vessel reaches port giving a significant time saving. If all the cargo is pumped ashore, then on the ballast passage the tanks will warm up to ambient temperature, returning the vessel to a gassed up and warm state. The vessel can then be cooled again for loading. If the vessel is to return to a gas free state, the tanks must be warmed up by using the gas heaters to circulate warm gas. Once the tanks are warmed up, the inert gas plant is used to remove the methane from the tanks. Once the tanks are methane free, the inert gas plant is switched to dry air production, which is used to remove all the inert gas from the tanks until they have a safe working atmosphere. Transportation of natural gas both in the form of LNG and by pipeline causes greenhouse gas emissions, but in different ways. With pipelines, most of the emissions stem from the production of steel pipe; with LNG most of the emissions stem from liquefaction. For both pipelines and LNG, propulsion causes additional emissions (pressurization of the pipeline, propulsion of the LNG tanker). Containment systems Today there are four containment systems in use for new build vessels.