Offshore facilities are of different types and differ depending on the seabed, water depth and the prevailing climatic conditions. Up to a depth of 100 metres, movable jack-up rigs are used. These consist of a hull supported by retractable structures (legs). These rest on the sea floor, leaving the hull 15-20 metres above the water surface so waves and tides cannot affect it. For depths up to 1,500 metres, floating platforms are used. These, once moored, rest on submerged hull structures. For greater depths (up to 3,300 metres) drillships are utlised. These are equipped with an opening in the hull through which telescopic pipes (risers), which connect the floating structure to the wellhead, pass. Drillships can operate without fixed moorings, maintaining their position using dynamic systems with numerous computer-controlled propellers that generate thrust in different directions.