Nearshore wind farms are normally built 5 to 15 kilometers from the coast. The water depth is generally less than 10-20 meters. Compared to offshore wind farms more than 20 km from the coast, the development of nearshore wind farms saves a lot of money. Nearshore wind farms can use existing onshore infrastructure to transmit electricity to consumers. Construction costs for turbine foundations, cables, grid connections and maintenance are all reduced compared to offshore wind projects.
The Asian nearshore wind market has huge potential. Take Vietnam for example, it has 3,000 kilometers of coastline. Wind resources in Vietnam is concentrated in the southern central coastal region (BinhThuan Province) and the southern coastal region (Treong, Bac Liu and SoC Trang provinces). Vietnam has more than 20nearshorewind farms are in operation now.
Nearshore projects present unique challenges. Most of the offshore wind turbines available in the market are designed to high wind speed region (IEC Class I). The nearshore sites in Asian (e.g China, Japan, Vietnam) are mainly medium or low wind speed site (annual mean wind speed:7~8m/s), which make it not economic to install large offshore wind turbines. Unlike offshore wind farms, the nearshore have very shallow water, making it difficult for vessel or large offshore equipment to reach and work. However the onshore wind turbines are not suitable to nearshore either. The marine environment (e.gwaves, tides, currents, high corrosion etc.) plays an important role in the design and construction of nearshore wind farms, which is not considered relevant for onshore wind farms.The specific site conditions of nearshore requires innovative technical solutions.
On turbine side, adaption of onshore turbine to nearshore environmental condition seems so far, the most optimal technical solution. It requires design modification of standard onshore turbines to cope with marine environment. The coating, cooling system is necessary to be upgraded. Additional filters, sealings, dehumidifiers are required to ensure the salinity environment not damage the components.
"The marine environment plays an important role in the design and construction of nearshore wind farms, which is not considered relevant for onshore wind farms"
For foundation design, in addition to the marine environment, geotechnical conditions and soil profile are also important factors to be considered in the structure design. The types of foundations commonly used for nearshore wind farm in Vietnam are monopile and multiple pile cap.Piles with length more than 30 meters are usually drilled through the muddy seabed.In some cases, the foundation is connected to (or even between) shore by bridge or walkway.China's nearshore wind farmsare in the intertidal zone of the East China Sea, where foundation types include monopile, gravity foundation and jacket structure. Japan's nearshore wind projects are built along the coast of the Sea of Japan on a monopileand gravity base in water depths of up to 20 meters.
The optimization of structure design is an important factor affecting the project CAPEX. There are different approachto design nearshore supporting structures. In some projects, wind turbine OEM and foundation designers carry out an integrated wind-wave analysis and an iterative design process to optimize tower and foundation based on site specific conditions. This will help optimize structural costs, but it will take a long time and require sufficient metcoean&geotechnicaldata and close cooperation between parties. Uncoupled analysis and design approach is also appliable for project with shallow water andstiff foundation. It gives certain conservatism but reduce design lead-time.
For nearshore project, real-time monitoring on the operating status of the wind turbineand supporting structure become critical. For example, the operation measurements on supporting structure can be obtained to detect initial structural cracks and predict the crack propagation to assess potential risks and perform just-in-time maintenance. Real time monitoring and preventive maintenance will reduce the OPEX of the nearshore wind farms.
