wind industry needs
improved foundations 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.
The top left image shows a crosssection 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.
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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. W
APRIL 2018
4/18/18 5:01 PM