Holroyd worm gear system is "Giant Spanner" for unlocking components on sub-sea oil wells.
In one of the most extreme and demanding applications for its gearing systems, Holroyd has manufactured a worm and worm wheel assembly that acts as a giant spanner for removing components from sub-sea oil wells. Used in conjunction with a remotely operated vehicle (ROV), the worm wheel assembly is providing torques of up to 1000Nm in corrosive salt water, at depths of up to 1200 metres.
The Holroyd gearing assembly is an integral part of a component change out tool (CCOT) designed and manufactured by imPROV Ltd an Aberdeen company specialising in oilfield related engineering and manufacturing. The CCOT is deployed from a surface vessel onto a sub-sea well (a Christmas Tree in industry jargon). An ROV is then employed to operate the tool to remove and change components such as oil flow control choke valves and sub-sea control modules.
The role of the Holroyd worm wheel assembly in this procedure is that of a giant spanner. The inner diameter of the worm wheel is machined out to a hexagonal shape, which is 72mm across flats (AF). The resulting form acts as a socket, which engages on the retaining bolts of the sub-sea well, unlocking them under the driving force of a hydraulic motor.
The operation of removing and replacing components at depths of up to 1200metres in corrosive seawater demands high levels of torque, which was one of the main reasons why a worm gearing system was employed. Here, Neil Macdonald of impROV takes up the story. "When we were designing the CCOT, we calculated that 1,000Nm of torque would be required from the mechanism providing the bolt fastening/unfastening function on the sub-sea wells. A worm gear system was the obvious choice to provide this: it could provide the torque multiplication we required from the hydraulic motor, at the same time as meeting the limited space requirements we had for the device. Holroyd were recommended to us as worm gear specialists, so we contacted them and they helped us with the design."
The worm and worm wheel assembly is manufactured from aluminium bronze, which offers excellent resistance to corrosion in marine environments. It is tightly integrated into a fabricated stainless assembly, which is mounted, via lugs and spring assisted bolts, directly beneath the CCOT.
To-date, Holroyd have supplied two of the worm gearing systems, both for use on sub-sea gas fields in the Mediterranean, off the coast of Egypt. A third is also on order; this unit destined for use in the North Sea.