Crude oil is stored in salt mines, tanks and oil tankers. On arrival she joined with the frigate in protecting cargo vessels and oil tankers. It is used by many oil tankers, bulk carriers and container ships. Many of these patrols had the support of Boeing 707-3J9C in-flight refueling tankers. Euronav provides complete shipping services in addition to its fleet of large tankers. Harland & Wolff specialised in building tankers and cargo ships at Govan. They specialise in crude oil, LNG and LPG tankers. Tank trucks ("tankers" in the UK) are designed to carry liquids or gases. There are four Boeing KC-137 used as transport and tankers roles. Ammonia is easier to transport safely in tankers than pure hydrogen. Some specialized types of oil tankers have evolved. An all out aerial bombardment with air tankers dropping fire retardant failed. Q Series aircraft are the basis of new, next-generation air tankers. In 2005, worth of oil tankers were sold used. It was one of the few German tankers exporting oil from Romania.