The problem
While sub-sea oil exploration has been going on in the North Sea for 40 years, there has been a limit to how far beneath the surface companies can go to extract much deeper oil and gas reserves. That is changing, with new approaches being successfully applied to develop deeper offshore discoveries of up to 3,000 metres below the surface.

The confidence in the success of these ventures is such that they are now being used in more onerous environments. A good example of this is the Ormen Lange field off the north coast of Norway. It sits in 1,250 metres of water on the edge of the Arctic Circle where the sea temperature can be as low as -2° C. Despite the technological difficulties, the gas from this field could well be used to meet a large part of Britain's energy needs.

There has been one major challenge to deal with, however. Divers cannot carry out vital maintenance at depths of over 250 metres because of higher pressures from both the water and the sub-sea environment. So any malfunctioning equipment needs to be brought to the surface for repair, which is expensive.

Why it was difficult
The sub-sea valves are crucial and must work first time, every time. But at these greater depths pressures of up to 150 bar on sub-sea ball valves are common. That meant that the soft, polymeric seals traditionally used have proved unreliable. One of the most difficult aspects that manufacturers face in supplying sub-sea ball valves is to meet the leakage criteria specified in Factory Acceptance Tests. Ball valves can be tested at pressures above 200 bar, which means that very dense tungsten-carbide coatings are needed for the balls and seals to be leak-tight.

The Stellite solution
Our engineers worked closely with the specialists from Weir Valves & Control UK to develop HVOF coatings that could be deposited consistently to the required standard. These optimised HVOF coating parameters enabled the spraying of very dense coatings with minimal micro-porosity, which met the leak-tight sealing requirements.

The result
During high-pressure leak testing, Weir Valves & Controls has found that the micro-hardness of these coatings offered excellent resistance to abrasion when sand or other piping/process debris is present. These HVOF coatings are more dense and provide a better surface to achieve a satisfactory gas seal compared to other HVOF coatings. The company has also reported that these coatings do not require sealing of the coating surface to achieve the leak-tight sealing requirements between the ball and seat rings.