A new design of seal can be more easily installed in wellheads, says Tim Rushton
The need to drill deeper for oil, with a corresponding increase in pressures and temperatures, makes working in an already harsh environment even more challenging. At the same time, the current spot-market price of oil is putting even greater emphasis on the need to control operating costs.
Meeting these seemingly opposing demands is never easy, but an innovative design of wellhead seal from SKF looks set to play an important role in helping oil companies drill deeper while keeping costs in check.
Called the Locking T-seal, the product is based on a proven design that has been developed from the fluid power sector, where it is used to prevent the piston seals in large bore cylinders becoming displaced or misaligned during installation. SKF has now adapted and patented this design, using specially developed materials to meet the exact needs of the upstream oil sector.
The seal is especially useful in wellheads, where it is difficult to locate seals used with backup rings precisely during installation. Traditional S-seals and spring-backed seals are often hidden from view, making it impossible to determine if they have seated correctly and thus leading to subsequent damage and failure of the seal assembly. Additionally, the steel backup springs used in standard S and FS seals can break apart during installation; if this occurs it can be an expensive and potentially dangerous operation to remove the broken parts from the well-hole.
The Locking T-seals feature specially designed retention ridges that snap into place on installation, helping to prevent rotation or displacement and mechanically locking the anti-extrusion backup rings into the correct orientation. The locking action makes it quick and simple to install each seal, without the risk of damage to the seal or other system components, while providing a secure connection that minimises the risk of leakage and environmental impact.
The Locking T-seal can also be used in dynamic reciprocating motion, as well as high-pressure static connections.
Full seal range
A further development from SKF is a full range of industry approved S and FS wellhead seals. These are designed to withstand ever-higher pressure and temperature, while meeting tougher regulatory requirements. The pressure between casing layers can be in excess of 10,000psi – meaning that the seals must be manufactured and tested to very high standards. Tubing and rough casing diameters can also vary widely, and wellhead seals must be able to compensate for these variations.
Although S seals are used extensively in wellhead components, FS seals have a thicker cross section and are used for larger extrusion gaps and rougher casing and tubing surfaces. Both are capable of withstanding very high pressure.
SKF has tested its S and FS seals to API 6A-17D standards, which ensures they can handle the high pressure, temperature and chemical resistance needed for wellheads. It believes it is currently the only seal company doing validation tests for wellhead conditions.
S and FS seals are very well established. A key factor in their performance is a set of new materials from SKF that can withstand the harsh conditions of the wellhead. High pressure gases can penetrate into seal materials and a combination of chemical attack and rapid gas decompression (RGD) can then cause severe damage.
The new material, called SKF Ecorubber-H85A-b-ED, is resistant to RGD and long-term ageing, and has been certified to demanding standards (NOR-SOK M710).
One potential problem with S and FS seals – as mentioned earlier – is that they incorporate metal springs, which can separate from the elastomer at high pressures. SKF has overcome this by developing a manufacturing technology that ensures a very strong bond between the rubber and metal. The S and FS seals may look identical to any other – but SKF’s underlying technology helps them to handle more extreme conditions.
SKF is best known as a bearings company, but its manufacturing expertise and deep knowledge of the oil & gas sector underpin a range of high quality seals that offer real benefit to wellhead applications.
Tim Rushton is with SKF.
