Today’s U.S. offshore wind industry has its roots in the oil and gas industry, along with developments from Europe’s offshore wind industry. This combination has produced our first offshore wind farm near Block Island, Rhode Island, but at a staggering cost of $50 million per turbine. Costs must come down for the U.S. offshore industry to grow. That means it is time to put down innovative roots that offer significant cost reductions.
Andy Filak / Principal / AMFConcepts
The jacket foundation would be made of a geopolymer with basalt rebar giving it a lifespan that would allow supporting several generations of wind turbines.
One way for offshore wind developers to lower installation and operations costs is to look for manufacturers and marine installation companies that offer innovative technologies. One way would be to eliminate costly steel foundations for offshore turbines. Saltwater is high in oxygen making it naturally corrosive to steel. Cathodic protection (CA) is necessary to safeguard steel foundations from erosion. The cost of a CA system requires regular monitoring, periodic maintenance, a reliable external power source, and methods for detecting stray current interference. Using new materials that do not require CA may lead to faster and less costly construction and deployment methods.
In addition, installation companies must call for the design of new vessels that allow manufacturing turbine foundations on board and provide for their deployment. These new, self-powered ocean-going deck barges will have catamaran hulls with dynamic positioning, and rudderless azimuth propulsion units with forward-mounted side thrusters on each hull. Furthermore, active heave compensation will make it easier to deploy the foundations and mount fully assembled and commissioned wind turbines. These vessels will reduce the typical high-day rates and residency time of O&G deployment vessels.
Better ideas for fixed & floating foundations
One hurdle to offshore wind development in the U.S. has been designing a reliable, inexpensive, long-life foundation that would be relatively easy to build and install. Existing steel technology requires highly complicated shore-side construction and at-sea installation procedures with multiple large crane vessels and anchor handling tugs.
Jackets can be slip formed to handle installations in water depths from 20 to 50m. The floating spar buoy foundation would work in water depths greater than 50m.
Such challenges may be solved with these few improvements:
- Advances in composite concrete ideal for marine use
- A new formwork method for manufacturing foundations
- A catamaran ocean-going deck barge for construction and deployment.
Meeting the challenges is possible thanks to four significant developments.
- Omitting the Ordinary Portland Cement (OPC) binder in the composite concrete.
- Replacing the coated-steel rebar with a non-metallic material that will provide a stronger reinforced structural frame. This material includes a cut basalt fiber and high-strength aggregates that make for a high-strength geopolymer concrete mix. Basalt rebar (see definition below) can develop high-strength concrete in tension without a lot of cover or concrete on top.
- Placing the foundations in a completely novel way (the catamaran deck barge).
- Replacing the steel-foundation piles with those made of epoxy-basalt fibers that have the same strength as steel. This combination of advances will increase the longevity of the foundations while simultaneously reducing construction and installation costs.
Novel deck barge & new materials
The new construction and deployment process for deep-water foundations described here allows erecting a wind turbine for a fraction of the current foundation cost. Why? Because the foundations are constructed on a deck barge while it is tied to the quay, saving offshore costs. The barge is then powered out by its own propulsion to the wind farm for deployment.
The top image shows a cross-section of the top of the jacket on which a transition would mount. The bottom cross-section is a five-ft high starter section for a jacket. It would be built on the deck of the deployment vessel with conventional steel formwork.
The greatest threat to existing marine concrete is water, either fresh or salt. With time, water penetrates concrete through unseen cracks and material porosity, and rusts the rebar skeleton. Even proactive rebar has coating failures and deterioration that eventually corrodes the steel. Seawater also directly attacks the chemistry of OPC, causing rapid deterioration.
The OPC binder fails at sea because of its high percentage of calcium compounds. About 70% of the binder comes under attack by the sulfur compounds in saltwater. It essentially rots the concrete. The cement binder in a concrete mix design can occupy up to 20% of the concrete mass.
A more advanced mixture replaces the OPC cement binder in the concrete mix with a geopolymer binder to foil this degradation scenario by minimizing the calcium compounds. Geopolymer cement binders are used commercially for their superior performance over OPC binders. Geopolymers produce a saltwater-resistant material that is stronger, fireproof, and waterproof. Geopolymer binders are formable, bond well to most materials, have minimal expansion or contraction, and are resistant to salts, acids, and alkalis. This will ensure the new foundation substructures will have a minimum of a 100-year life due to their low porosity, high-strength, and heat and dry-cured technology.
Basalt rebar is made from basalt stone found all over the earth. Basalt is a key component enabling the hundred-year minimum durability of the foundation structure. Heating basalt stone to 1,800°F liquefies it, so it can be run through a palladium die to produce soft flexible fibers or threads. The threads are laid in parallel and bonded together with an epoxy, to produce basalt rebar and basalt foundation pilings.
To select a formwork system a wide variety were studied used, globally. Slip forming (like a slow extrusion) is the only method to show significant promise in reducing costs and production timelines for three proposed foundations systems (two fixed and one floating). Analysis showed that it is less costly to build smaller jackets than to build steel monopile foundations. The proposed formwork system produces the most cost-effective solution. The heat curing formwork can achieve a two-foot-per-hour slip. The formwork allows completing any of the three possible foundation systems in under seven days each. Slipping the foundations uses standard methods except for the heated formwork and the trunk placement of the geopolymer concrete.
Construction platform & deployment vessel
It is typically too costly to slip-form the foundations on land and lift them onto a vessel. Therefore, it is necessary to develop a self-propelled catamaran barge. To simplify the overall process, the deck barge serves as a construction platform, transport vessel, and deployment platform all in one. Foundations would be slip-formed at each end of the barge, in front of and behind the mid-ship structure of the bridge castle, while it is moored to the quay with ready access to materials and resources.
Upon completing the onboard construction of two foundation substructures, the barge would power out by its own propulsion to the wind-farm site. At the site, the barge dynamically positions the foundation over the pre-pile center with the azimuth propulsion unit and forward side thrusters on each hull. The foundation is then lowered with its four-active heave compensation units supporting high-capacity synthetic cables, from linear winches over a sheave to four lift points on the foundation. Once over the piles, the foundation is lowered to mud mats (large pads) on the seabed, where the foundation is leveled and grouted.
By using this method, construction and installation costs are a fraction of current steel foundations because there is no need for tow-out tugs, anchor-handling tugs, or specialized heavy-lift crane vessels.
Such new foundations can be installed on a wide range of seafloor conditions, including sand and rock. Foundations can be anchored by driven or drilled piles or, depending on the correct soil, by suction buckets. To maintain a 100-year life goal, fiber (basalt) glass would be used on piles and suction buckets. The fiberglass is one fourth the weight of steel and of equal strength. The piles, produced in 100-foot lengths, allow for greater energy absorption than steel and work well with underwater vibration and air-hammer placement.
No more monopiles
The offshore wind industry in Europe is placing 6-MW turbines on monopiles in 40-m deep water. This is pushing up the cost of a monopile. Rather than producing an individual design for each depth, the offshore industry in the U.S. needs a way to produce jackets and adjust them for all water depths from 20 to 50m. Floating offshore wind turbines are essential to meet the challenge between the fixed foundations in up to 50m depths and floating platforms in over 50m. The steel spar buoy is too costly to produce, transport, and deploy in deep water.
Deployment also includes up-ending and placing ballast, the cost to mount the WTG, and then transport the assembly to the wind farm. All this requires several vessels and tugs with high day rates and long periods on station. However, by slip-forming the spar buoy on the barge described, it can all be done more efficiently on one vessel.
Considerations for decommissioning
The 100-year minimum life of the foundation systems is key to supporting three generations of wind turbines and will play a role in decommissioning. The decommission deposit, required at the start of the wind-farm lease, could be smaller due to their long-term earning potential. The Bureau of Ocean Energy Management Regulation and Enforcement estimates the cost of decommissioning at 60% of the total estimated construction cost of the WTG in the wind farm.
Decommissioning, required within two years after termination of a wind-farm lease, calls for removing all facilities 15-ft below the mud line. This will include the turbine, foundation structures, pipelines, cable and other structures and obstructions. The requirements are similar to O&G offshore site cleanup. Decommission bonds are aimed at reducing, not eliminating, potential later financial liability. Most bond levels are set at three times the expected cost. The product’s 100-year life could reduce these bond level requirements. The cost of decommissioning could be substantially reduced by using the same type of vessel that originally deployed them.
Filed Under: News, Offshore wind
