Richard D’Souza, Richard B Offshore LLC
The floating offshore wind industry is at a cross-roads. While it has done a good job of demonstrating the technical viability of floating offshore wind farms by successfully deploying and operating demonstrator and pilot wind farms, it has been unable to translate them to economies of scale. Current projected levelized cost of energy (LCOE) for floating wind farms remains two to three times greater than the aspirational threshold of fixed offshore wind farms LCOE’s. There is pressure to reduce LCOE of pre-commercial wind farms by at least half to enable sanction of utility scale-floating wind farms. The reduction must be achieved without introducing novel or unproven technologies.
In 2014, when oil and gas prices collapsed, the offshore industry had to reduce breakeven cost of new developments from $70/barrel to $35/barrel. Of several levers that were used to accomplish this goal, standardization, simplification, and serialization of floating production systems were key. Several major and independent oil and gas operators achieved significant capex reductions with a ‘design one build many’ approach, standardizing a hull type (FPSO, semisubmersible), simplifying specifications, and employing creative contracting and supply chain strategies. As a result, most floating oil and gas deepwater developments today have breakeven prices under $40/barrel. Several of these levers are already being employed by the wind industry but have not been fully exploited.
A floating offshore wind farm’s mooring system life cycle cost (design, procure, install, operate, maintain) is a significant percentage of its total life cycle cost. The goal here is to describe some opportunities that could be employed to significantly reduce the life cycle cost of a floating wind farm mooring system, implementable in the near term. The mooring system described below is concept agnostic as to the floating offshore wind turbine (FOWT) platform.
Mooring concepts
Inserting a low stiffness synthetic rope segment, such as polyester or nylon, between the platform and ground chain of a mooring system has the potential to provide significant cost savings versus all chain, chain wire or chain HMPE systems. These have constituted most of the mooring systems in FOWTs to date. The savings will apply across the applicable bandwidth of water depths of floating offshore wind farms.
Nylon versus polyester
In the past, polyester rope was a breakthrough for floating oil and gas platforms. These types of systems enabled moorings to be cost effectively installed in progressively deeper waters. Nylon rope has significantly lower stiffness than polyester rope. Traditionally, nylon rope had not been considered for oil and gas platform permanent moorings mainly due to its low fatigue life. To date, it has been mostly used for hawser moorings due to its ability to handle snap loadings. However, interest in nylon rope for floating offshore moorings was rekindled due to its low stiffness properties. These properties provided the incentive to create a nylon rope that was more rugged and had better fatigue properties than those used for hawser moorings. The low dynamic stiffness of nylon ropes reduces peak tensions, and the size of the steel components (chain, anchor) increases fatigue life to a greater extent than polyester rope. This combination results in potentially substantial procurement and installation cost reductions.
Quantifying the savings
Several recent studies have quantified these savings. One study conducted a techno-economic analysis of nylon versus polyester taut moorings for a 15MW FOWT in 150 m water depth, in harsh environmental conditions. For a 50-year extreme environmental condition scenario, the mean tensions of nylon and polyester ropes are almost same, but the peak and standard deviation tensions of nylon rope are about 62% and 48% that of polyester rope. Significant reductions of peak mooring line tensions and standard deviation of tensions lead to considerably reduced anchor sizes. For a 100-unit wind farm, the estimated total installed cost saving of nylon as compared to polyester rope moorings was of the order of 40% to 45%. Savings will depend on water depth, turbine size and number of units. The savings will be greater versus all chain or chain/wire moorings.
Qualifying nylon
The industry has begun the process of technically qualifying nylon rope for permanent moorings. The first step was focused on rope construction, making it like polyester parallel strand ropes used in oil and gas platform moorings. The next was to demonstrate fatigue characteristics of this rope construction relative to chain and polyester. One study group performed numerous tests to characterize fatigue behavior of nylon ropes and compare it with chain and polyester rope. The study concluded that newly developed nylon ropes have better fatigue characteristics than chain.
Further qualification was provided by Bexco, a rope and mooring manufacturer, while researching and developing nylon rope for permanent moorings. These studies were carried out in three programs that included protype testing in the Floatgen damping pool barge demonstration project (first power 2018), with six of their ropes installed in the moorings. Bexco expects to obtain technical qualification by 2026.
Shared anchors
Shared anchors are possible on wind farm arrays where a single anchor can share multiple mooring lines, thus reducing the total number of anchors and total installed cost of the wind farm mooring system. At present, the only feasible anchors that can resist multi-directional loads are piled or gravity anchors. Technical feasibility of multi-directional loaded suction piles has been extensively studied and validated. Equinor was the first operator to deploy shared anchors on its eleven turbine Hywind Tampen pre-commercial wind farm. In so doing they were able to reduce the number of suction piles from 33 (if the turbines were uncoupled) to 19, by sharing anchors. The result is a 40% reduction in suction pile anchor fabrication and installation cost for the wind farm array, or approximately 30% of the total installed cost of the wind farm mooring system. The savings will be greater for larger mooring arrays.
The successful functioning of Hywind Tampen shared anchors will provide proof-of-concept and go a long way to increasing confidence in shared anchors, and lower insurance and financing premiums. Shared anchors will be best suited for wind farms in water depths greater than 500 m, which will provide the recommended separation between turbines for maximum array production efficiency, without increasing mooring scope. The use of shared anchors will be limited to geotechnical conditions suited for suction or driven piles. Industry has also looked at shared moorings as well as shared anchors and moorings to further drive down costs. But these options greatly increase overall risk and complexity and are many years away from being given serious consideration.
Challenges ahead
Installing moorings for large floating wind farms with fifty or more FOWTs is a major challenge for the industry. It will require a new mind set to manage logistics, supply chain issues, staging yards and most importantly a new generation of installation vessels that can efficiently preinstall multiple anchors and mooring lines in a one-to-two-year span during available weather windows. The floating oil and gas industry has pioneered vessels like the BOA SubC and the Jumbo Fairplayer that can be adapted for this role. In a recent presentation (Van der Weil, 2024) showed how the Fairplayer pre-installed the Vito semisubmersible chain-polyester-chain suction pile mooring systems in 1200m of water in two campaigns. The first was to install the twelve suction piles and anchor chain and the second was to pre-lay the twelve mooring lines and inline tensioners. An installation vessel like the Jumbo Fairplayer has the versatility and capability to pre-install moorings for a fifty FOWT wind farm in the North Sea over a two-year period. Several installation contractors are working on new generation vessels tailored to the demands of serial installation of wind farm moorings. To meet the floating wind industry’s aggressive goal of installing 40GW of floating wind power in the next five to ten years will require an urgent commitment to build several of these new generation installation vessels.
Reducing IMRR risk
One key concern is the need to reduce inspection, monitoring, repair and replacement (IMRR) risk for these mooring systems. The oil and gas industry has developed guidelines for life cycle inspection and maintenance of floating platform mooring systems that are continuously updated to reflect new operational information on failures and technologies. These are codified in recommendations and practices such as API 2MIM, ABS 2018, and DNV-ST-0119, among others, that have served the industry well.
The floating wind industry is leveraging knowledge and experience gained from the offshore oil and gas industry. The difference is the sheer volume of mooring lines in a single wind farm (300 or more for a 100-unit farm) versus twelve to sixteen for a single oil and gas platform. Presently there is no single global governing operation and maintenance standard for floating wind farms. Several international classification societies have developed standards (ABS 195, DNV-ST-0119, LR GN2) that incorporate existing rules, but require further development. In addition, several floating wind consortia (Carbon Trust, 2023; WFO, 2022) are critically assessing floating wind mooring reliability and integrity. These studies are factoring in the difference between floating oil and gas and floating wind. Currently there are few insurers willing to underwrite the risk of this developing market because of the variety of FOWTs, mooring systems, innovation, and lack of operational track record.
The subject of floating wind mooring IMMR is too vast to cover here but some of the highlights of initiatives that could significantly reduce cost and risk are presented below.
A mooring integrity management program is intended to detect conditions outside the original design envelope through regular inspection and monitoring so that changes can be identified, and remedial actions taken. This can be achieved either by a rule-based prescriptive approach or a risk-based predictive approach. The scale of a wind farm mooring integrity program lends itself to a risk-based approach that considers the likelihood of damage and potential consequences of various elements of the mooring system to focus resources and inspection frequency on the higher risk levels. Studies have shown that the risk-based inspection approach has the potential to save 40% to 50% of the cost of a rule-based approach without compromising mooring integrity.
The oil and gas industry has developed many inspection platforms to remotely and efficiently monitor the condition of mooring systems including ROVs and autonomous underwater vehicles (AUVs).
There are direct mooring tension measurement devices such as load cells, but these have had long-term reliability and accuracy issues, and will be cost prohibitive for farm-scale moorings. The alternative is to use sensors such as inclinometers, accelerometers on the platform as well as metocean monitoring, and differential global positioning systems. When coupled with digital twins (digital replicants of the farm), these technologies can continuously and accurately track mooring line tensions, fatigue, and position of each platform in the array. Coupled with artificial intelligence and data-driven machine learning models, these systems can be programmed to detect anomalies in near real time so that pre-emptive actions can be taken before a failure occurs. The pairing of virtual and physical on manned and unmanned oil and gas platforms has minimized downtime, and reduced maintenance costs and led to improved safety outcomes over the asset lifecycle. The technology is relatively inexpensive to implement, provided the most reliable and robust instrumentation is deployed.
A key finding of the floating wind JIP (Carbon Trust, 2023) was that manufacturing defects and installation damage threats are independent of loads experienced by the mooring system. These threats will be mitigated as increased installation experience is obtained and applied following large-scale commercial installations.
Field experience to date
The WindFloat Atlantic semisubmersible pilot wind farm offshore Portugal is entering its fourth year of service. In an interview with Offshore magazine (Offshore, April 2024) the head of Floating Foundations at Ocean Winds discussed operating experience and lessons learned to transfer to future commercial projects.
Real-time information from the three FOWTs enabled the operations team to track the floater position, and the hull trim system and turbine, and employ floater communications to optimize turbine performance. Anomalies detected by the condition monitoring data alerted the operation team to take pre-emptive action to address emerging issues before they could escalate to failures. In addition, key enabling robotic technologies, capable of reducing LCOE by minimizing the use of divers and support vessels for inspection and maintenance, were successfully tested.
Looking forward
The floating wind industry is nascent, but in about fifteen years has achieved the technical capabilities needed to harness the energy of the strong and persistent winds that prevail in waters beyond the commercial limits of fixed offshore wind farms. It is poised to develop industrial scale wind farms that will eventually be at the vanguard of the transition to renewable energy. Much of the progress has come by adapting technologies and lessons learned from the floating oil and gas industry. It does however still face the need to lower the LCOE by a factor of two or three in short order, if it is to displace conventional sources of electrical energy.
The floating offshore wind industry has developed many creative ideas over these past fifteen years. During that time, it has narrowed the technology funnel to a few concepts that lean heavily on oil and gas technology while adapting to the significant drivers unique to floating wind. After concept standardization and scaling for larger turbines and wind farms, the biggest cost element for many wind farms will be the mooring system. There are solutions available on the market today that can play a key role in helping these offshore wind energy systems reduce their field development costs and their LCOE.
Acknowledgment
Based on an excerpt from the paper, “Gulf of Mexico Deepwater Mooring Experience Applied to Floating Offshore Wind Turbines,” presented at the SNAME Maritime Convention 2024, held on Oct. 14-16, 2024, in Norfolk, Virginia.
