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Technical Talk “General Overview of Offshore Wind”
Reported by David Leslie, Chapter Secretary
On November 18, 2021, Jeff Fodiak and Tim Reiher, engineers from Mayflower Wind, delivered a technical talk to the Providence Section, Ocean Engineering Chapter, titled “General Overview of Offshore Wind.”
Jeff is the Electrical Systems Lead for Mayflower and a senior member of IEEE and the IEEE Power & Energy Society. His areas of expertise cover design of AC and DC electrical systems, design of offshore array and export cable systems, submarine and onshore cables, reliability analysis, due diligence, project strategy, planning requirements for electrical infrastructure, and power systems simulations.
Tim is the Offshore Export Array Cables Package Manager at Mayflower Wind. He worked previously for Shell Renewables and Energy Solutions. He has worked on planning, engineering, and execution of deep-water projects in the Gulf of Mexico and has strong experience in delivering large scale, complex projects in the offshore environment.
Mayflower Wind is a joint venture between Shell New Energies and Ocean Winds (which is itself a joint venture of EDP Renewables and ENGIE). The US Clean Energy plan proposes to develop 30,000 MW of offshore wind by 2030. There are 16 active federal lease areas off the Atlantic Coast with a total offshore wind pipeline of more than 35,000 MW to date. 1 MW is enough to power 1,000 homes. Mayflower Wind will be the largest contributor towards the Commonwealth of Massachusetts net-zero emissions goal. The Mayflower project itself will eliminate over 4 million metric tons of greenhouse gas (GHGs) annually.
During the talk, Jeff and Tim spoke about their experiences in the Offshore Wind Industry and the opportunities which are ahead, as they see it. They gave an overview of offshore wind project components from wind farm design, cable routing and installation methods, landfall considerations, and survey and site assessment.
The Mayflower Wind project comprises 127,388 acres of outer continental shelf lease area south of Martha’s Vineyard, Massachusetts. Up to 149 wind turbine generators (WTG) will be installed there along with offshore substation platforms (OSP). Turbines will be placed on a grid at 1 nm x 1 nm spacing. In order to maximize the potential of the lease area capacity, Mayflower is using dual export cable routes to deliver the power to electricity customers via interconnection points (POI) at Falmouth and Somerset, Massachusetts, providing 1,200 MW at each location. There is the potential for ~2,400 MW depending on technologies.
Several different offshore transmission technologies and topologies are available, depending on the power export requirements and length of the offshore cable section.
In the first “Medium Voltage AC Option,” each WTG has a turbine that generates at a low level, usually less than 1 kV, and a transformer that can boost the voltage to ~34.5 kV or 69 kV for the recent, larger turbines. Cables from all WTG are combined and then transmitted to shore at 69 kV. The combined power can be about 80-100 MW max for each 69 kV cable. There are no offshore substation platforms, and the WTG string connects directly to the grid POI onshore. This option is suitable for nearshore projects less than 400 MW, and also for some deep-water floating projects.
A second “High Voltage AC Option” (HVAC) requires one or more offshore substations. These can be “traditional” OSPs with 2 or 3 transformers, plus 2 or 3 reactors, and 3-5 decks with enclosed rooms. The WTG transmit to the OSP at 69 kV, but voltage is stepped up at the OSP. Power may be exported at 138, 230, 345 kV. The option is suitable for export cable lengths up to ~120 km.
A third “High voltage DC Option” (HVDC) requires an offshore converter platform. The OSP has medium voltage to high voltage transformers, as well as an AC-to-DC converter. This is a significantly larger and more expensive OSP than an offshore AC platform. It is typically used for offshore distances > 120 km, and for power of 900-1,200+ MW. At distances of 120 km or so this solution is cheaper. It is suitable for the larger power range, not the small (400 MW) projects.
The choice of offshore transmission technology depends also on AC and DC cable performance characteristics. HVAC is more efficient for short distance power delivery and HVDC more efficient for longer distances, HVAC has more redundancy than HVDC in terms of cable failures. HVAC requires more separately installed cables for power capacity beyond 300-400 MW. The capital cost of HVAC is less than HVDC for shorter distances, as HVDC has higher substation costs but lower cable costs.
Examples were shown of cable cross-sections. The HVDC cable bundle (~1200 MW) contained two armored wire bundles (Cu, Al) for power, and an optical fiber, all contained in a 12-inch max. diameter oval encasement. The HVAC cable (~400 MW) contained three conductor bundles (Cu, Al) and an optical fiber bundle, all contained within a 12-inch diameter circular encasement.
Once the transmission technology is chosen, the hard work begins of determining how best to get the cables to shore. This stage is the “Siting and Routing Assessment.” Desktop planning must consider the seabed with varying water depths, constraints and hazards, ecological sensitivities and existing infrastructure (telecommunication cables, gas pipelines, water pipelines). Options may be narrowed down if there are areas reserved for naval operations or anchorage, or prime areas for fishing activities. Cable installation operations are more complicated in shallow waters.
After initial desktop planning, survey data is acquired to “ground truth” the seabed route characteristics, which were assumed in the desktop studies. For further cable design and installation engineering, consideration is given to water depths, seabed slopes, and soil types/characteristics. Potential areas of archaeological sensitivity need to be avoided. Potential hazards such as boulders and sand waves need to be identified and characterized along the route. A “cable corridor” (500 m -1000 m wide) is surveyed to allow for “micro-routing” for avoidance of hazards and ecological sensitivity.
Surveys are conducted to acquire geophysical data, geotechnical core data, and benthic data within both the array lease area and along the offshore cable routes. Examples of survey types include Vessel-based geophysical surveys (multibeam bathymetry echo-sounder, side scan sonar, sub-bottom profiler, and gradiometer), Aircraft based LIDAR (light detection and ranging) bathymetry surveys, Vessel-based geotechnical surveys (Vibracores (shallow), CPTs (cone penetration testing, shallow or deep), Bore-holes (deep)), and Vessel based benthic and eelgrass surveys for understanding habitat for ecological considerations.
Cable installation may involve both seabed preparation and cable burial.: Grapnel runs, UXO (unexploded ordnance) clearance, dredging, and boulder clearance may all be performed to prepare the seabed. Cable burial methods depend on the seabed type and can include the use of vertical injector, mechanical plow, Jetting plow / sled / ROV, Pre-Cut Plow, and Horizontal Directional Drilling (HDD).
HDD is employed as a landfall method to avoid impacts to sensitive nearshore environmental resources, including beaches. Permanent surface impacts will be minimal. Onshore cable vaults will be buried. Once onshore, cables will be conducted to onshore substations, typically while buried in trenches. The construction profile for a duct bank in a roadway will have conduits containing the two cables separated horizontally by about 15 inches at a depth of about 4-5 ft. Those conduits are encased in concrete, above which is a 30-inch-thick layer of FTB (fluidized thermal backfill), an 8-inch layer of concrete and about 6 inches of asphalt and binder. Alternatively, the cables may be directly buried without the use of conduits. An example showed two HVDC land cables buried about 15 inches apart at a depth of about 4 ft in a layer of thermal sand zone backfill, topped by a larger layer of native backfill.
Mayflower Wind conducted surveys in the lease and cable export areas in 2019, 2020 and 2021. The Mayflower Project itself is currently in a stage of development for which the construction and operations plan (COP) has been submitted and the program is undergoing environmental review. The proposed design includes a combination of the technologies described above. Two export cable routes are proposed.
An HVDC cable will run from Offshore to Onshore at Brayton Point: Turbines, inter array cables and OSP (HVDC Converter station) will be on the Outer Continental Shelf. The offshore underground export cable will make terminal landfall at Brayton Point at an onshore HVDC Converter station where DC-to-AC conversion is accomplished. From there they travel underground to a Point of Interconnection with the ISO New England grid system.
An HVAC cable will run from Offshore to Onshore Falmouth: Turbines, inter array cables and OSP (HVAC station) will be on the Outer Continental Shelf. The offshore underground export cable will make landfall and continue underground to an Onshore substation. The power will be transmitted from there to the POI interconnection switching station via overhead lines. At the POI it will be connected to the grid.
The route from the offshore lease area to Brayton Point is about 170 km and to Falmouth it is about 90 km. This dictated the choice of transmission technology. Dividing the power was driven by grid access. Connecting more than 1200 MW at a single connection point with a single circuit is difficult and, in fact, is not allowed because it is a system security issue.
This talk was an excellent technical talk for our chapter and resulted in an extensive period of post-talk questions.
- What is the nature of “thermal sand”?
- Will the fiber optic cables be used for distributed temperature or acoustic sensing (DTS, DAS)?
- Will there be any significant EM radiation from the cables that might affect fish or people in any way?
- Will there be continued acoustic or vibrational monitoring during installation and operation?
- Could the offshore installation be a host for further oceanographic monitoring equipment – AUVs?
- Has the choice of turbine technology been finalized?
- How is a field joint/splice performed offshore on these massive cables?
- What factors went into determining the planned grid spacing?
With regard to the grid spacing issue, Chris Hardy from Mayflower Wind commented that all the New England lease holders worked with the US Coast Guard to come up with the 1 nautical mile spacing solution, which offers some of the widest transit lanes available anywhere in the world compared to other wind farms. This solution may be specific to this lease area given the history of concern and the history of fishing in and around New Bedford, Rhode Island and Long Island. Mayflower is proud of it because it does offer greater navigational safety for mariners.
Student support activities
Reported by Nick Hall-Patch, Chapter Secretary
Over the last two years, the IEEE OES Victoria Chapter has been assisting with the funding for students’ ocean engineering capstone projects at the University of Victoria, in British Columbia, Canada. This support has already resulted in three short articles by students in the June, September and December 2020 issues of the Beacon, describing the results from their work. We are pleased to submit yet another student report in this issue of the Beacon (see the article in the page 41).