Dave Rowley
Offspring International
Synthetic fiber rope is widely accepted for moorings beyond 1,000 m (3,281 ft) water depth. Yet there are few machines designed to test fiber rope, and none dedicated specifically to deepwater ropes. This, allied to the high cost of testing, has led to the paucity of authoritative data on rope properties for different fiber types and rope constructions.
Lankhorst Ropes Offshore Division has developed a deepwater fiber rope and connector test machine designed to reveal the impact of new materials and rope constructions on mechanical and fatigue behaviour. The machine is at its fiber rope production facility in Portugal.
The purpose of a deepwater mooring system is to hold the production unit on station. However, this imposes fluctuating fiber elongation and tension-tension fatigue loads on the mooring lines. Long- term (more than 25 years) deepwater moorings will be exposed to millions of small waves which translate into more than 50 million cyclic loads. This continuous cycling has the potential to cause real fatigue damage, although the steel components in a mooring system actually suffer from fatigue damage more than the fiber rope.
Performance of fiber rope is more difficult to predict than mooring chain or wire rope performance. Fiber rope is visco-elastic, so its properties are history and time dependent. For example, the load placed on the rope, and the time since the load was applied, influence the rope’s stiffness characteristics. The rope will recover some of its elasticity after the load is removed, but the degree of recovery is time dependent. These variable stiffness characteristics lead some people to regard fiber rope mooring system design as a “black art.”
Rope test variables
A test rope comprises a length of rope with an eye spliced at each end, which are attached to pins. Between the splices is a free length of rope. Test results from the full test sample length (i.e., pin to pin data) is of little use, as the test sample may comprise 5 m (16.4 ft) x soft eyes, 5 m x splice, and 2 m x free rope; whereas the tether in service may comprise 5 m x soft eyes, 5 m x splice, and 1,990 m (6,529 ft) x free rope, so the pin-to-pin data will be severely distorted when extrapolated.
Very accurate extensometers must, therefore, be attached to the free rope. The longer the free rope section on the test sample, the further the extensometer can be positioned away from the tail of the splice. Any transitional disturbance effects, plus the longer the extensometer can be, minimize the extrapolation factor.
To accept a longer test sample requires longer test bed length. More importantly, the longer sample will stretch more. A 10% extension or stroke on a 10-m (32.8-ft) sample is only 1 m (3.28 ft), but on a 20-m (65.6-ft) sample is 2 m (6.5 ft). To facilitate longer samples, the Lankhorst test machine uses longer hydraulic rams.
When simulating wave frequency loadings, the cycle speed is constant regardless of sample length, so if a 10-m (33-ft) test sample is used, the ram will move 1 m in 15 seconds, while the longer sample will need to move 2 m in 15 sec. to have the same effect on the rope. This means the test machine needs higher power to achieve the dynamics of moving the hydraulic ram through this distance. The Lankhorst test machine can accept 20-m samples, has a primary cylinder rated at 1,200 metric tons (1,323 tons) with a 3-m (9.8-ft) stroke, and a secondary cylinder of 1.5 m (4.9 ft) length coupled to a 350-kW power pack.
Understanding a deepwater synthetic rope’s stiffness and degree of stretch under maximum load are essential to predict in-service/life performance. The ability to accurately control peak loading during rope testing is, therefore, critical and more difficult than apparent at first glance. The elasticity of the rope is such that at target load it will continually creep against time.
Maintaining the target load means that the control system has to continually self-correct – 2% variance is typical for existing fiber and wire rope test machines, while the Lankhorst machine control system maintains target loads within 10kN. This makes a significant difference when assessing the accuracy of test data.
Recently, the rope test machine received its Certificate of Calibration from the National Physical Laboratory (UK). When analyzed in accordance with ISO 7500-1:2004, test results for the calibration are within the Class 0.5 classification limits, compared with Class 1.0 limits for most rope test machines.
The deepwater fiber rope and connector test machine is designed to reveal the impact of new materials on rope construction.
Deepwater ropes are routinely loaded to approx. 40% of MBL to pre-stretch the rope during installation, so that “out of the box” storm offsets are minimized. More accurate information on the degree of pre-loading required will avoid the high cost and safety issues surrounding excessive pre-loading during offshore installation.
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