- 4.1 Function. Tensioner units are used to apply vertical force to the top of the marine drilling riser to control its stresses and displacements. The units are normally located on the drilling vessel near the periphery of the drillfloor. They provide nearly constant axial tension to the riser while the floating drilling vessel moves vertically and laterally in response to the wind, waves and current.
- 4.2 Typical Design. Tensioner units use a hydraulic ram with a large volume, air-filled accumulator to maintain near constant pressure/tension on the line. One end of the line, which may be wire rope or chain, is attached at the tensioner and the other is attached to the outer barrel of the telescopic joint. Typically, a four-part line reeving system is used so that the piston stroke is equal to Vi of the vessel heave. The number and rating of tensioner units used will determine the total capacity of the tensioner system. The tension applied by each unit can be varied up to its design capacity by increasing or decreasing the applied air pressure. The tensioner system should be capable of providing sufficient tension based upon the maximum rated water depth, maximum expected mud weight, and other loadings determined from riser analyses.
Emerging designs for tensioner systems are described in Appendix C, Section C.l.
- 4.3 Selection Criteria. Some important considerations for designing an effective tensioner system are:
- The fleet angle. The idler sheaves should be placed so as to minimize the fleet angle. This maximizes the vertical component of tension, minimizes the horizontal component, and increases wireline life.
Because of the fleet angle, the vertical tension applied to the outer barrel of the telescopic joint is less than the tension supplied by the tensioner system. A reduction factor (see Section 3.3.2) should be used to reconcile these parameters.
- Wireline life. Wireline life is a function of many parameters including wire rope construction, sheave diameter, applied tension, operating circumstances relating to travel, etc. See API RP 9B.
- Accumulators and Air Pressure Vessels. Each tensioner unit should have an accumulator that is large enough to store a volume of hydraulic fluid greater than the cylinder volume. Large air pressure vessels will reduce pressure changes caused by the compression and expansion of the stored air as the tensioner strokes in and out.
- Fluid and air flow requirements. Properly sized lines will reduce tension variations caused by piping system pressure losses.
A list of hydraulic fluids compatible with the tensioner units should be specified by the tensioner manufacturer.
- Friction and Inertia losses. Seal friction, sheave friction, and inertia of sheaves, wire rope, tensioner rods, and pistons all contribute to variations in the wireline tension.
- Dynamic Tension Limit (DTL). Tensioner ratings are defined differently by various manufacturers. This document defines a dynamic tension limit as the maximum allowable pressure multiplied by the effective hydraulic area, divided by the number of line parts:
where PA = maximum allowable system operating pressure
Acyl= effective hydraulic area
NLp = number of line parts
All components in a riser system installation, including piping, should be designed for the maximum allowable working pressure. See ASME UG 125-136 for relief valve setting criteria.
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