The first Selantic® Tethers were installed more than 20 years ago, and some of these still remain in operation today. Typical applications for Selantic® Tethers include long-term anchoring in the seabed to subsea buoy mooring systems (like Mid-Water Arch (MWA) tethering systems); mooring of flexible risers and umbilicals; and subsea buoys and pipelines. Tethers are also deployed for other offshore installations such as wave, wind, or in current energy plants. No other fiber tether solution has achieved this design specification.
Positioning of subsea buoyant structures is demanding due to the harsh and remote undersea location. Material and construction decisions are critical to success, when engineering subsea tethers that deliver a design life greater than 20 years in environments battling corrosion, marine growth and particle ingress. In addition, vertical positioning of a subsea buoyant structure for 20 years is very different from horizontal positioning of floating structures like FPSO’s.
The DNV GL design specification confirms that Cortland’s Selantic® fiber Tethers meet the requirements and are certified for subsea anchoring and tethering applications. They can be individually engineered to meet client requirements, including very tight length tolerances with strengths exceeding 3000Te. Cortland has also developed a system for ROV-friendly connection / disconnection of their Selantic® tethers, as described in patent WO2013165253A1.
The weight penalty associated with using chain or steel wire rope for anchoring flexible risers and umbilicals makes fiber tether solutions more efficient and cost effective. This is due in part to the reduced buoyancy requirement, as well as improved handling and installation requirements. High performance fibers can be up to seven times as strong, weight for weight, as traditional steel wire rope; and do not corrode.
Aramid fibers used in Cortland’s Selantic® Tethers have so far been the only available material that has the field-proven performance for this application due to their low creep rate (non-reversible elongation), and long term fatigue performance. Testing activities on Selantic® Tethers retired after 15 years of use have verified this within the given design limits.
Another key performance characteristic of Selantic® Tethers includes high visibility outer jackets to enhance the safety level in operations. In addition, the design of the jackets and load bearing core minimizes the long term effect on performance from particle ingress and marine growth.
Eldar Lien, Technical Manager for Cortland, stated, “Analyses and tests of tethers that have been in service for 15 years confirm that design lifetimes of up to 50 years are achievable. It is all about design and choice of right materials.
“Failure of the tethers during operation is something that does not occur,” continued Lien. “The risk for failure is mainly related to installation issues, and caused by interference with ROV systems. Full scale fatigue testing has confirmed that the Selantic® Tethers outperform steel systems many times over.”
Remi Asnes, Norway Country Leader for Cortland, added, “We have been through a long and extremely thorough process in connection with this DNV GL Design Specification. It is not only the product and our manufacturing process that has been subject to rigorous testing and documentation, but also our inspection and test procedures along with our quality assurance plan. This confirms our offering of a tether solution that complies with the highest standards of safety, quality and efficiency.”