A Shaffer-type tensioner consists of a cylinder and accumulator with a sheave assembly at both ends. It maintains constant tension on the cable by extending or retracting to change the distance between the sheaves as waves raise and lower the drilling platform.
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Building cylinders with the required capacity and stroke that will operate reliably in the unforgiving environment of an offshore platform, meet the stringent requirements of various government maritime agencies, and conform to American Petroleum Institute standards is anything but simple. To add to the challenge, the cylinder systems must be as light and compact as possible to conserve valuable deck space and cargo capacity on the platform.
Eaton’s hydraulics plant at Eindhoven, Netherlands, has been a supplier of specialized cylinder/accumulator assemblies for WLRT cylinders for more than 12 years. More recently, Eaton’s Decatur, Ala. plant emerged as an additional manufacturing site for WLRT cylinder/accumulator assemblies for offshore platforms.
Eaton produces several sizes of WLRT cylinders, capable of providing between 200 kips (200,000 lbf) to 250 kips (250,000 lbf) of force each. The cylinders range in size from 18.5 in. (470 mm) bore with a 15.75 in. (400 mm) diameter rod, to 21.26 in. (540 mm) bore with an 18.9 in. (480 mm) diameter rod. Most WLRT designs have a stroke of around 150 in. (3.8 m), and weigh between 24,000 lb (10,886 kg), including 240 gal (909 l) of hydraulic fluid, to 28,000 lb (12,700 kg) with 305 gal (1155 l) of fluid.
The high-pressure portion of the cylinder system operates at 3000 psi. The low-pressure end is used only to provide fluid for lubrication and to keep the seals conditioned. “This is essentially a ‘push’ application,” says Eaton’s senior cylinder engineer Sudarshan Sharma, “so the relatively large rod size in relation to the cylinder bore is beneficial and does not adversely affect the cylinder performance.”
The cylinders are of welded construction with a welded-on cap on the high-pressure end. A welding system is designed to accommodate the large diameter, thick-walled tubes that become the cylinder barrel. “Once a weld procedure is qualified,” Sharma continues, “the system reproduces the weld with a high degree of repeatability. The critical operating parameters such as voltage, current, and heat input are keyed in before starting the weld. The operator monitors the weld and makes sure that there is enough wire and flux in the feeders, but unless some problem is encountered, the weld itself is done entirely by the machine.”
Sealing water-glycol