Compact footprint HPU maximises rig deck space

Upgrading the riser tensioning system on the Ensco 102 jackup drilling rig has meant installing a new hydraulic power unit, the specification for which posed some interesting challenges for engineers at Claxton, the company chosen to design and build it, as project engineer Will Robinson explains.

“Claxton was contracted by the Ensco 102 to drill an exploration well in the North Sea and stipulated that drilling operations at the given location had to be possible under 100-yr storm conditions. A riser analysis determined that to drill the required well safely the rig would have to be capable of pulling 400t of tension on the riser. Additionally, the hydraulic cylinders within the tensioning system would have to be able to cope with a cyclic displacement (sometimes termed a ‘squat’) of 8 in. within a four second time period.”

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The innovative HPU designed by Claxton

In order to accommodate this degree of rapid movement the Claxton project team had to incorporate an unusual amount of hydraulic energy storage into the system. They calculated the unit would require 12 accumulator bottles, each with a volume of 37 litres. Although this was not considered to be such a problem in itself, it did create a headache for the designers who knew the entire unit had to be small enough to fit into a space on the rig no bigger than 1.8 m wide by 2.0 m long and 2.0 m high.

In fact, packed into the frame of the unit are the 12 accumulator bottles, a 450-litre fluid reservoir, four 40:1 hydropneumatic pumps (which provide the basic power for the system) plus all the requisite hoses and valves, which are all roughly twice the usual bore to handle the necessarily higher fluid flow rates through the unit.

“It took quite some ingenuity to get all the equipment into the frame,” says Robinson.

One of features of the design that helped significantly was the use of a flat, stainless steel control plate for the unit in place of a normal console. This saved space and made it easier for the hydraulic fitter, as he was able to fit the valves and pipework to the control plate while it was on the bench and then offer it up to the unit afterwards.

The unit is also designed to last. As painting was never going to be possible after the unit was assembled, the unit was built and tested and then stripped down for the application of a high-performance paint system before being rebuilt, an exercise that added three weeks to the production schedule.

The unit was recently fitted to the rig on location by Claxton engineers. As there are now four Ensco jackups that benefit from Claxton riser tensioning systems, the last word really ought to go to the client. Speaking on its behalf, James Foreman, the Ensco 102’s barge engineer, said, “The new hydraulic power unit fits well and the crew are finding it easy to operate. The upgrade to the tensioning system is allowing us to use the jackup to drill a well that would normally have required a semisubmersible rig.”

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