Over the past decade, Contact Energy Ltd. has found several valuable applications for synthetic rope to replace steel cable at its 464-MW Clyde Power Station in New Zealand.
By Aidan Smith and Grant Campbell
Synthetic ropes have been used for high-strength applications, such as in the marine industry, since the 1990s. Synthetic ropes are effectively “plastic” as compared to natural fiber or metal ropes of the past, and the benefits are similar to plastics used in other industries: they are lightweight and corrosion-free. This article discusses several applications of these ropes at the 464-MW Clyde Power Station on the Clutha River in New Zealand. Use of this type of rope allows the company to decrease maintenance requirements and the weight of the rope itself, making them easier for staff to handle.
Contact Energy Ltd. operates and maintains two hydropower stations, Clyde and Roxburgh, which are on the Clutha River in Central Otago, New Zealand. In total, 12 units provide 752 MW of capacity.
The Clyde scheme is run-of-river with an average flow of 500 m3/sec and a plant factor of 50%. The plant was commissioned in 1992.
When construction began on Clyde, steel cables were still in prominent use for heavy lifting applications. However, problems occurred when carrying out specific tasks for routine checks, such as the steel ropes developing a “set” or kink, making the ropes unusable.
Performing stator inspections
When the time came to conduct the first inspection of the stator with the rotor out in 2005, the limitations of the existing steel cables became apparent. The problem faced was the accuracy of the length of the lifting cables, which would cause the load to be lifted on an angle. Due to the fine tolerance between the stator and the rotor, alternative lifting options were explored.
At Roxburgh, the crane connects directly to the rotor via a steel beam, which is an option that was explored for Clyde, but these beams are expensive when compared to using a sling. Contact Energy chose Dyneema rope as a cheaper option and also because it provided other benefits, such as its ease of use.
Dyneema rope is made from ultra-high molecular weight polyethylene (UHMWPE), which gives a high strength for minimum weight. Long molecular chains transfer the load on the rope throughout the polymer backbone. This high molecular weight also makes Dyneema extremely resistant to chemicals and solvents that would normally cause corrosion. Dyneema’s mechanical properties are not affected by water and its density of 980 kg/m3 means it is neutrally buoyant in water. Due to being a polymer, its strength increases at sub-ambient temperatures and decreases at higher temperatures. Synthetic ropes have a maximum long duration temperature of 70 degrees Celsius and no minimum working temperature.
Two Dyneema ropes were purchased, 51 mm diameter and 27 m long rated at a working load of 43 tonnes. The ropes were pretested at the factory and tested again by a certifying company before any major lifts were carried out. The 300-tonne lift was arranged with 16 falls in total, giving a load of 18.75 tonnes per fall.
Dyneema specifies that the minimum radius of lifting points should be five times that of the radius of the rope, and care should be taken to avoid sharp edges. At Clyde, the ropes are stored in a powerhouse stairway, where they are well out of the way of anything that could compromise their lifting integrity.
Spillway gate lifting cable problems
As the flow varies depending on the season, the spillway gates at Clyde get used in normal operation every year. By 2002, the steel cables used to lift the four spillway gates were beginning to corrode at the underwater connection joint, apparently due to damage that occurred during the original splicing operation to make the joint. When identified, it was initially treated with grease to keep the water out, but the corrosion continued and it became evident in 2006 that the cables needed replaced.
Coincidentally at this time a Contact Energy technician, Pete, who is a glider pilot, had been researching replacing the steel cable used to winch and tow the gliders into the air. Their existing steel cable was prone to breaking and tangling due to being of a small diameter (6 mm) and more than 2,000 m long. He recommended to the gliding club that the cable be replaced with Dyneema, 6 mm in diameter. They accepted his recommendation and the cable was replaced in 2005 and is still in use today.
The trades staff at the Clyde station were now used to synthetic rope, so they were open to the idea of replacing the spillway gate lifting cables with Dyneema. Research, however, identified an issue that needed to be addressed. The synthetic ropes are covered in a protective layer due to debris that can collect on the water in this area. A concern was whether the ropes would fit into the existing winch drum grooves, but with the protective cover they were the same diameter as the original steel cables.
The total capacity of the new Dyneema cables is 480 tonnes (120 tonne per cable), which when compared to the steel cables only reduced the capacity by 3%.
The synthetic ropes were twice the cost of steel cables but the maintenance was estimated to be less, and this has proven to be the case, as neither repairs to the skinplate nor lubrication of the rope have been required.
When the ropes were installed in 2007, the gate was allowed to hang on the ropes (with the stoplogs in place) for two weeks. While Dyneema does not stretch, the ropes will unwind a little with the load on. Once this had taken place, the gates were zeroed into position and have not changed since then. Regular inspections over the next three years showed the ropes were performing as expected, so the steel cables on the remaining three spillway gates were similarly changed in 2010. After more than 10 years, there is no sign of deterioration and no additional corrosion on the skin plate where contact is made.
Maintaining the intake gates
When maintenance is being carried out in the penstock, the stoplogs are placed in front of the intake gate. There is inevitably some water leakage through these and if the gate is closed, the space between them fills up to lake level. If the gate is then opened, the water will be discharged down the penstock into the spiral case and through the turbine. The consequences of this happening during an outage could be fatal.
Permitting restrictions were put in place, limiting work in the spiral case while work was being carried out at the gate. At Roxburgh, the gate can be blocked up on wood, to allow the water to drain. This was not possible at Clyde due to the angle of the penstock where the gate seals. We considered installing valves at the bottom of the gate, which would allow the water to drain when the gate was closed. This came with operational issues and a reluctance to penetrate the skin plate of the gate. One of our tradespeople, Bob, came up with the idea of “hanging” the gates with a 300 mm opening under them so that the water always drained when on an outage. Permanently installing two stoplog beams at dam deck level meant ropes could be attached that would be connected to pins on the gate and suspend it. Dyneema rope was again selected for this application.
These were installed in 2015 and 2016 and have been used, allowing us to carry out work on the gate and in the scroll case at the same time. The gates weigh 32 tonnes, so the two ropes are rated to a minimum breaking load of 178 tonnes and a safe working load (SWL) of 35.5 tonnes. The Dyneema ropes have a safety factor of 5 at the SWL and are 45 mm in diameter.
Once an outage starts and the penstock is dewatered, the ropes are attached to the beams at dam deck level and the gate is slowly lowered to a point where the gate connection pins are made. The gate is further lowered until the ropes take the weight. At this point the gate is 300 mm off the seal and water can freely drain down the penstock.
An additional advantage was using the same setup but with shorter ropes, allowing us to take the link out of the connection between the ram and the gate to get the gate into its maintenance position. The original method used spragging beams inserted into block-outs in the gate. These beams were heavy, difficult to maneuver and time-consuming to put in place. We believe this has provided us with a safer and more practical solution with a much reduced chance of human error causing an incident.
Based on these results, the design to use Dyneema rope to support the two low-level dewatering gates at Roxburgh, each weighing 80 tonnes, is under way.
Aidan Smith was a mechanical engineering student working onsite during his university holidays, and Grant Campbell is asset manager at Contact Energy.