The tensioner cylinders used to support subsea drilling and production risers where they connect to the rig are the size of an 18-wheeler. They are in motion 24/7 and must accommodate 50-ft strokes during storms. With a cost of up to $100,000 to ferry one onshore for refitting and return it to the rig (not including lost production costs), it’s little wonder rig operators insist that every cylinder they install lasts at least 10 years.
That’s a tall order for the seals contained within the system—not only because of the wear requirements, but also the water glycol fluids used in offshore systems. The fluids are more environmentally friendly than mineral oil-based fluids, but they exhibit poor lubricity (especially low film strength and low viscosity), which is detrimental to many seal materials, including rubber.
Redundancy and Wear-Resistant Materials
Successfully creating a sealing system that meets the requirements of an offshore tensioner cylinder involves extensive experience with these types of systems; flexible (yet strong) and wear-resistant seal and wear ring materials that are compatible with water glycol fluids; and inclusion of a secondary seal and wiper.
Key considerations for the primary seal include wear life, friction, and fluid compatibility. A highly wear-resistant seal material is essential to ensure the seal system is leak-free for as many years as possible. However, the seal must also be able to move constantly within the water-based fluid system with as little friction as possible and withstand the aggressive chemicals used to inhibit corrosion.
Seals composed of rubber-based materials tend not to perform effectively in these types of systems. They are prone to compression set and chemical attack, and their friction coefficient is higher than that of polytetrafluoroethylene (PTFE)- and polyethylene (PE)-based seals. The primary seals in tensioner systems are typically composed of PTFE or PE variants and are the first line of defense against leaks.
The secondary seal, which must perform in dry environments for years, is typically a PTFE-based seal due to its inherent low coefficient of friction and wear resistance at high temperatures. Even though the cylinders operate at ambient temperatures, the seal friction when running dry can generate heat, and the seal material must be able to resist this. In real-world environments, PTFE seals have performed very well when put to use as a secondary seal, ready to operate in the event of primary seal leakage.
Considerations Beyond the Seal Itself
Additional to the seal system, and equally important, is the wear ring and rod scraper system. Tensioner cylinders are subjected to high side loads, so keeping the rod and piston concentric with the seal is essential. The wear rings used must be strong enough to resist deformation under dynamic load and resist swell from water absorption, preventing metal-to-metal contact with the cylinder’s metallic components.
The material of choice for tensioner wear rings is a resin-fabric composite, the better to yield a strong, wear-resistant matrix for long life, high-load capability, and compatibility with HFC fluids. Such wear rings have successfully been specified in tensioners using HFC fluids for more than 30 years, and samples reviewed from the field showed very little wear and were not dimensionally changed from swell in HFC fluids. In fact, these resins were originally engineered as a marine bushing and bearing material due to their superior water and wear resistance.
Keeping debris buildup off the rod during retraction helps prevent ingress of contaminants into the sealing area and, ultimately, the hydraulic system. Debris, such as abrasive particulates, can wear away or cut into seals. Rod wipers, typically manufactured out of PTFE or PE compounds, have been developed to scrape aggressive debris such as ice and salt off the rod during retraction.
Debris normally tends to collect at the top of vertically mounted tensioner cylinders at the rod gland and rod interface. Therefore, we designed these scrapers to deflect debris away from these areas.
Robust Filtration and Early Warning Systems
Beyond the primary and secondary seals, wear rings, and scrapers, a successful offshore hydraulic cylinder system must include robust filtration and an early warning system to alert operators to any leaks.
Although environmental seals with scraping elements are included in every tensioner cylinder, fluid cleanliness is essential for a long system life. Particles and sand can exist within a hydraulic system from the fabrication process and must be filtered out to avoid damaging the primary seal. Residual media from blasting, machining burrs, and weld debris are the most common offenders. Any potential for these should be addressed to prevent seal damage.
Early warning systems are in place on virtually every rig to prevent a catastrophic event. In the case of tensioner cylinders, this consists of an evacuation port with a sensor—either between the primary and secondary seals or downstream of the secondary seal. If the sealing system fails, fluid goes first into the port, which alerts the operator to the leak, so the system can be shut down.
Seal, wiper, and wear ring selection for tensioner cylinders centers around four main criteria. First, the primary seal must be highly wear resistant; able to perform well in low lubricity water glycol fluids; and compatible with the chemicals used in the fluid, while the secondary seal must be able to run dry for up to 10 years without degrading to provide redundancy should the primary system fail.
Second, the wiper must keep debris buildup off the rod during stroking and prevent ingression into the sealing area. Third, the wear rings used must be strong enough to resist deformation under dynamic load and resist swell from water absorption, preventing metal-to-metal contact with metallic cylinder components.
Finally, all components should be proven to perform as specified in real-world situations to ensure the rig operator will not incur the high cost of bringing the cylinder onshore to refit the seal system.
Four questions should be asked by offshore operators about potential seal and wiper systems for tensioner cylinders:
- What material is used for the wiper and the primary and secondary seals? Some old-school systems use stacks of seals to provide redundancy, but the seals are typically rubber-based and, thus, are prone to compression set, chemical attack, and high friction.
- Have the materials being used been tested to ensure they are compatible with the chemicals that will be used in the hydraulic fluid? Trelleborg has conducted a comprehensive seal material test program in HFC fluids, and a report is available upon request.
- How long has the seal system lasted without leaking in real-world situations? Systems from Trelleborg have returned from rigs with less than 50% wear after 15 years, for an estimated 30-year extrapolated life.
- What level of filtration is required to ensure that seals within the hydraulic system are not damaged?