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High Accuracy Torque Multiplier for Wet Valve Actuation

Tuesday, Aug 05, 2008

 Norbar Torque tools has developed an innovative torque multiplierfor the subsea tools market. Designed as an adapter to allow API17D Class 4 intervention torque tools to generate the torquerequired to handle Class 7 applications, the new torque multiplierdelivers a higher level of accuracy than alternative solutions, andincreased flexibility in use through its added-value features andits ability to be configured in two standard lengths.

 The gear unit has been designed as an adapter for torque tools usedon subsea Christmas trees – the assemblies of control valves,gauges and chokes that control oil and gas flow at a well head.Installed on the ocean floor at depths of up to 3000m, these‘wet trees’ provide the ultimate challenge for thetorque tools designed to actuate the various valves.

 The valves come in various sizes with different torquerequirements, with the typically used subsea products ranging fromClass 1 valves with maximum torques of 68Nm up to Class 7 withtorques of 34,000Nm. Clearly it would be impractical to holddifferent tools for each valve, or to standardise only on thelargest tool on the off-chance of having to accommodate a Class 7valve now and again. So the gear adapter has become popular as ameans of extending a Class 4 tool (which would generate up to2,700Nm of torque) up to Class 7.

 To date, the trade-off for this added convenience has beenaccuracy, with extended Class 4 tools failing to offer the precisetorques of dedicated Class 7 tools. But Norbar’s innovativedesign has overcome this problem, enabling a Class 7 torque outputto be generated from a Class 4 torque input to an accuracy ofbetter than ±5%.

 “Our Norwegian customer came to us to see if we thought wecould improve on the existing designs,” says Norbar’sTim Bloor. “We looked at the problems with the existingproducts, and went back to basics. The result was two prototypeunits that we believe deliver far greater accuracy than the currentgear adapters on the market.”

 The function of the gear is to operate as a torque multiplier, butthe ratios have to be exactly right to achieve the correct torqueand the specified speed from the Class 4 tool. At the same time,the gear has to be able to function reliably in a subseaenvironment at extreme temperatures and pressures. It also has tobe easy to handle. Norbar was able to meet all of theserequirements.

 The design of the gear to match the accuracy requirements was notrivial matter. Inherent gearbox inefficiencies mean that thenumber of turns that the input has to make to achieve onerevolution of the output is not the same as the torquemultiplication ratio. A key aspect of the design, then, was toengineer each gear stage to give an exact velocity ratio whichwould deliver the required torque multiplication at an accuracy ofbetter than ±5%.

 The company’s newly developed torque multiplier is designedto withstand the corrosive environment and is pressure compensatedto allow operation at typical subsea pressures. It is designed tobe suitable for use with the standard Class 4 torque tools that,operated by ROVs, are used to remotely actuate the wet valves. There is also a restriction on the weight that ROVs can manipulateso considerable design work has gone in to keeping the tool weightdown to an acceptable level.

 The design recognises that there are two standard tool lengths ingeneral use, with different companies preferring either one or theother. With the Norbar design, the nominal length can be quicklychanged from short to long by adding or removing an adjustmentshaft under the adapter sleeve. The result is a tool that is highlyversatile.

 As well as the improved accuracy and length options, the multiplierbuilds in a host of special features that will make it even moreattractive in the subsea torque tools market. These include alifting handle that has been developed from the ground up to makethe tool as easy as possible to manoeuvre. Attention to detail isalso demonstrated in the fact that the tool is coloured orange– the last colour to fade at increasing depths of water.

 The new torque multiplier is currently under test withNorbar’s Norwegian customer serving the North Sea Oil market,but the expectation is that the product will prove an idealsolution in this demanding application. “It’s been asignificant investment on our part, but if we get it right then themarket potential is huge,” says Bloor. “It seems tohave been favourably accepted, and we’re confident that theextensive testing it is currently undergoing will prove ourdesign.”

 Norbar will then be able to offer a standard, high accuracysolution to the various subsea valve tool manufacturers worldwide,enabling those companies in turn to offer tools of significantlysuperior performance and accuracy to the multitude of oil and gascompanies across the globe with subsea operations.

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