kind of ship 'on station', particularly during uncertain weather, made the diving dangerous, problematic and seasonal. Furthermore, seabed operations usually entailed the raising and lowering of heavy equipment, and most such vessels were not equipped for this task. This is when the dedicated commercial diving support vessel emerged. These were often built from scratch or heavily converted pipe carriers or other utility ships. The key components of the diving support vessel are: Dynamic Positioning – Controlled by a computer with input from position reference systems (DGPS, Transponders, Light Taut Wires or RadaScan), it will maintain the ship's position over a dive site by using multi-directional thrusters, other sensors would compensate for swell, tide and prevailing wind. Saturation diving system – For diving operations below 50 m, a mixture of helium and oxygen (heliox) is required to eliminate the narcotic effect of nitrogen under pressure. For extended diving operations at depth, saturation diving is the preferred approach. A saturation system would be installed within the ship. A diving bell would transport the divers between the saturation system and the work site lowered through a 'moon pool' in the bottom of the ship, usually with a support structure 'cursor' to support the diving bell through the turbulent waters near the surface. There are a number of support systems for the saturation system on a diving support vessel, usually including a remotely operated vehicle ROV and heavy lifting equipment. Modern diving support vessels Most of the vessels currently in the North Sea have been built in the 1980s. The semi-submersible fleet, the Uncle John and similar, have proven to be too expensive to maintain and too slow to move between fields. Therefore, most existing designs are monohull vessels with either a one or a twin bell dive system. There has been little innovation since the 1980s. However, driven by high oil prices since 2004, the market for subsea developments in the North Sea has grown significantly. This has led to a scarcity of diving support vessels and has driven the price up. Thus, contractors have ordered a number of newbuild vessels which are expected to enter the market in 2008. More recent vessels are designed and built to support both diving activities and remotely operated vehicles (ROVs) operations with dedicated hangar and LARS for ROV's, and to support seismic survey operations and cable-laying operations. They may have carry 80 to 150 project personnel