Offshore Wind in Asia: Typhoons, Cyclones, Earthquakes
Meeting the specific challenges in Asian Offshore Wind Projects
Asia is picking up speed and conquering the offshore wind energy market step by step. But difficult environmental conditions in Asia confront developers, manufacturers and independent consultants with a number of unfamiliar challenges. Find out what specific challenges these are and how stakeholders deal with them.
The “Global Offshore Wind Report 2020”, released by the Global Wind Energy Council (GWEC) last August, predicts significant market opportunities for new offshore wind energy projects in Asia-Pacific over the next decade. While GWEC expects an average annual growth rate in Asia of 1.7 percent in the first half, the forecast for the second half of the decade sounds even more promising with a likely increase to 8.4 percent. Between today and 2030, GWEC identifies the following countries to be the top five markets in this region for new installations: China (52 GW), Taiwan (10.5 GW), South Korea (7.9 GW), Japan (7.4 GW) and Vietnam (5.2 GW).
GWEC also underlines that success in these markets requires the implementation of experience and lessons learned from the last 30 years of offshore wind deployment in Europe (Source: https://gwec.net/global-offshore-wind-report-2020/). However, European offshore wind energy experts have to deal with a number of different and unfamiliar meteorological and geological conditions in the Asian-Pacific markets.
In today’s INSIGHT blog post, Sven Bicker, managing director of Deutsche WindGuard’s offshore division, and Martin Strack, head of the consulting division’s wind resource assessment department, explain what these differences are and how project planners, consultants and manufacturers cope with the associated technical challenges.
Environmental conditions facilitate offshore
wind energy market development
At the beginning of offshore wind energy activities in Europe, there were two major variables which influenced the development of the market: water depth and soil condition. These two factors vary across the European seas. For example, in Spain and Portugal, water depths made offshore wind projects not very attractive at first, so developers focused on the shallower coastal areas in the North Sea and Baltic Sea for the first commercial projects.
First projects in the Baltic Sea were implemented on gravity base foundations. In the North Sea, in coastal areas of Denmark and UK with favourable ground conditions, monopile foundations with 2 to 3MW wind energy turbines marked the starting point. As of 2010, also greater water depths were captured with turbines larger than 5MW, mostly on monopiles. Feasibility was, however, limited by producibility constraints.
In the North Sea, severe but predictable extreme sea states became a design driver especially for deeper waters, while in the Baltic Sea, potential ice loading and varying soil conditions challenged the designers’ work.
However, environmental conditions and structural design challenges had been experienced in the oil and gas industry before. Thus, related expertise was already available.
For offshore wind energy generation in Europe the simple rule applied: Where the national regulatory framework offered favourable incentives and the environmental conditions were conducive, the offshore market grew. Where the political framework or the environment was complicated, offshore wind energy did not take off, as was the case in France or the Mediterranean.
In Europe, high average wind speeds and predictable meteorological events allow for a balanced proportion between dimensioning of the wind turbine and its long-term operational strain.
Meteorological and geological events represent challenging conditions for offshore wind energy
In Asia, the situation is very different. In some parts, such as India, average annual wind speeds are much lower than in Europe, resulting in lower expected energy yields as well. On the other hand, the occurrence of extreme wind and wave events is exorbitantly higher than in European waters. They are caused by very local, hard to predict weather phenomena such as cyclones and typhoons.
This exposes developers and consultants with European backgrounds to major challenges, because proven methods of calculating wind and hydrodynamic loading need to be adapted or even newly developed for state-of-the-art processes. Formerly, type class design was sufficient to achieve verification of turbine site suitability for Europe, where the 10-minute-average wind speed can reach a maximum between 40 and 45 m/s. In Asia, however, wind speeds can reach an average of 60m/s or more in extreme meteorological conditions. While the latest addition to IEC 61400-1, the new “tropical” class introduced in 2019, considers extreme wind speeds up to 57 m/s, it is questionable whether this will be sufficient for all Asian-Pacific locations.
In addition, local high wind phenomena develop severe extreme sea states, which might not follow the same principles as those in the North Sea on which European offshore wind energy experience is based. In areas with steep gradients of water depth near to shore, this will often result in breaking wave situations as a further challenge in load evaluation and design.
The location of occurrence, route and force of tropical storms are unpredictable. For example, in Taiwan most typhoons develop south of the island, head towards its east coast and weaken on their way across land before they reach the west coast. Some, however, take a different route and may hit the wind parks off the western coast significantly harder.
Local geological characteristics in combination with earthquake hazards represent the second factor that has major impact on the way an offshore wind energy farm has to be designed and constructed. In this field, European projects of the past three decades have offered practically no experience. As earthquakes are a negligible threat throughout Europe, it has never been necessary to take them into consideration at all.
Meteorological and geological characteristics influencing offshore wind energy projects in Asia per country
Tropical storms developing above the Indian Ocean are called cyclones. In the northern parts, Bay of Bengal and Arabian Sea, they typically occur in May and June as well as October and November, before and after the summer monsoon season. With its 7,600 kilometres long coastline, India is particularly vulnerable and belongs to those regions on earth that are worst affected by cyclones. Wave heights of ten metes are not rare in this type of tropical storm.
The Indian peninsula sits on the Indian tectonic plate which started colliding with and submerging under the Eurasian plate from the south about 40 Million years ago. This movement continues to date, causing earthquakes in the entire region. At the same time, the movements of the Indian and the Arabian plates are slightly different regarding speed and direction, which represents a second source of seismic activity.
Approximately 30 percent of all tropical cyclones form within the western Pacific, making it the most active region on earth, also featuring a high storm frequency with significant peaks between July and November. The most powerful typhoons originate most frequently from the north-west Pacific. Depending on the typhoon’s track, be it straight or curved, it is likely to affect some part of the Chinese coastline.
With a coastal length of approximately 14,500 kilometres, China’s opportunities to install new offshore capacities are ideal. Geological conditions and water depths vary along the coastline, representing diverse challenges and requiring different approaches regarding the most suitable foundation method for each individual wind farm site.
China is situated on the eastern edge of the giant Eurasian plate. In this specific region, this tectonic plate is exposed to pressure from two sides: While the Pacific plate submerges under the Eurasian plate from the east, the Indian plate pushes from the south. Both effects together create an individual Chinese earthquake belt causing a lot of seismic activity around the year.
The point of origin of most typhoons in the north-western Pacific Ocean is located just south-east of Taiwan. In combination with an increased occurrence of winds blowing in a north-western direction in summer, this means that Taiwan is exactly on the route of several heavy tropical storms a year, typically occurring between July and September. Most of them hit the eastern and southern parts of Taiwan first and lose strength on their way across land before they reach the west coast. Some, however, take a different route and may hit the west coast significantly harder. Globally seen, typhoons in the north-western Pacific are among the strongest and most destructive storms in terms of wind speeds and diameter.
Geological conditions in the Taiwan Strait between mainland China and Taiwan are very complex. Located on the western Pacific earthquake belt, the island is often subject to seismic activity resulting from interaction between the Eurasian and the Philippine tectonic plates. While the Taiwan Strait is characterized by rather shallow water depths with a maximum of 150 meters, its ocean bed mainly consists of sand and clay. These components make it a rather soft foundation soil with a high risk of deformation or liquefaction caused by earthquakes.
The sea bed on the opposite side of the island is very different. From the Philippine Sea, one of the deepest oceans on earth, it ascends very steeply towards Taiwan’s east shore. This causes high and powerful breaking waves representing specific challenges for the planning and construction of offshore wind parks.
Japan is regularly haunted by typhoons with high wind speeds causing severe damage when hitting land. An average of 30 typhoons are counted in the course of the year, the main typhoon season being between July and October. Approximately half of them don’t approach the coast any closer than 300 kilometres but are still able to develop destructive force. If they hit land, they typically reach Japans most southern regions first and weaken on their way further north to Japan’s largest island. However, recent typhoons like “Hagibis” and “Haishen” have shown different courses, windspeeds of around 300 kilometres per hour and extreme destruction.
Japan is located in one of the world’s seismically most active areas at the conjunction of four tectonic plates: the Pacific, Eurasian, Philippine and North American plates. These plates permanently touch, rub and butt each other, leading to a steady increase of tension. When this pressure releases, earthquakes and tsunamis occur as a result. As pressure builds up over time and may release very suddenly, these events are not predictable. In Japan, 5,000 earthquakes on average are counted every year.
With the Sea of Japan to the west and the Pacific Ocean to the east, steeply declining sea beds and remarkable water depths already close to the shore make this area a difficult terrain for any kind of offshore construction. Planners are especially challenged with the task of finding the best foundation method for this environment.
Like Japan and Taiwan, South Korea is regularly on the path of a number of typhoons originating from the north-western Pacific Ocean. However, many of them just bypass the Korean peninsula on their way across the Sea of Japan and have just peripheral effects on South Korea. The main typhoon period of the year in this area is between June and October with a peak in August and September. It actually rarely happens that a severe tropical storm makes landfall in South Korea. The most exposed areas of the country are in the south.
The Korean peninsula is surrounded by the Yellow Sea to the west, the Sea of Japan to the east and the East China Sea to the south. A wide strip of shallow waters up to a depth of 50 meters can be found along the west coast providing options for offshore wind farm sites in reasonable distance from the coastline. However, large tidal differences, fast currents and soft soil represent challenges in some areas. Along the east coast, the sea bed declines steeply in most areas and leads to significant water depths already quite close to the coastline.
The Korean peninsula is part of the stable Eurasian tectonic plate and is therefore in a better position than its seismically very active neighbours. Earthquakes occur less frequently and with lower intensity in South Korea than in China and Japan. Thus, South Korea is considered a low seismic hazard area.
Because Vietnam’s coastline is more than 3,000 kilometres long, the country’s offshore wind potential is significantly higher than its onshore opportunities. At the same time, due to its specific geographical location, Vietnam is quite often affected by extreme weather events. On average, between ten and 20 typhoons roll over the Vietnamese coastline every year – mostly between May and November. It has been observed that the risk of landfall seems to be smaller along the southern coast as compared to the central and northern parts of the coast. And it is definitely smaller than in the more north-eastern Asian countries.
Major parts of the Vietnamese coast feature shallow water depths and relatively consistent soil conditions. The northern part of the country is located along the Gulf of Tonkin featuring a plateau of shallow waters not deeper than 50 meters in reasonable distance from the shore. The same applies to the southernmost part of the country. Especially along the south-central coast, the seabed is much steeper with notably deeper waters in a short distance from the coast.
Vietnam’s northwest, located at the junction between the South China and the Indochina tectonic plates, faces a high risk of seismic activity, while in all other parts of the country this risk is rather insignificant.
Facing tropical storms with experience and local expertise
In recent years, Deutsche WindGuard’s site assessment experts have been working extensively in several onshore projects in Taiwan, Indonesia, South-Korea, India, China, Japan and Vietnam. Supporting clients entering these markets as well as collaborating with local companies both offered them valuable experience regarding local conditions. This knowledge now also serves them as a foundation for all new offshore projects in the Asian-Pacific region.
As wind fields in these markets are in general less homogeneous than in Europe, this sets up higher requirements for wind measurement and wind field modelling, but it also represents challenges for creating wind farm layouts and assessing design parameters. Innovative measuring methods like floating or scanning LiDAR help solving many issues.
Since cyclones and typhoons and the resulting hydrodynamic conditions are hardly forecastable, Deutsche WindGuard’s consultants cooperate with local specialist who have evaluated tracks, forces and impacts of historical storm events of the past 50 years. This data is incorporated into all parts of the site and design assessments. Whenever available, longer term measurements from masts or turbines in the neighbourhood of the designated site are used to complement such rather theoretical analyses and to increase confidence in the results.
Engineering approaches typically enable the most efficient design while accepting a potential remaining risk of damage during extreme meteorological conditions. The protection of life and limb is the top priority. Even during a tropical storm, wind energy generators shall not represent any danger to people and environment, but a certain amount of repair on the plant is tolerable.
Incorporating the impact of earthquake shocks from bottom to tip
While the latter basically applies to the risk of damage caused by seismic activity as well, earthquakes require a number of additional measures throughout the entire offshore wind energy project.
The wind energy plant is typically laid out to continue operating even in high wind speeds and extreme wave activities and also to absorb respective vibrations when in idling mode. Seismic activity, however, does not have any correlation to the meteorological conditions. Thus, earthquakes can hit the plant in any operating state. And unlike in weather phenomena, the impact is highly site specific and depends on the dynamic reaction of the overall structure. These reactions may be completely different from what the turbine was initially designed and verified for.
The risk of an earthquake event must therefore be implemented into every detail of the plant concept. Seismic activity puts the entire plant into motion – with different severity degrees depending on magnitude and soil condition. While forces of 0.5 to 1g usually occur in the overall acceleration during normal operation of the wind energy generator, earthquakes can result in significantly higher loads on any part of the plant as well as unintended directions of movement. This may lead to severe dissonances between moving and static parts. Additionally, this could cause the turbine to exceed its acceleration limits. Therefore, the software must react accordingly and activate different scenarios. An emergency stop, for example, is indispensable.
Rotor blade positions in the idling mode, actually intended to prevent stimulation by the wind, must be designed to withstand extreme earthquake shocks, to name just one more example. All this is relatively new to turbine and project developers focussed on European projects the last decades. And approved processes for calculation and design need to be adapted and further improved. This is necessary to derive the most realistic and economic design assumptions, but also as part of the project certification, where manufacturers have to deliver a proof of site suitability for the turbine in addition to the original type certificate.
The consultant’s role at the interface between disciplines
Beyond that, offshore wind energy projects in Asia-Pacific require knowledge from additional areas of expertise, as different ground conditions such as bed rock, clay, chalk or sand as well as challenging wave action or water depths represent further influencing factors. While a monopile may be a suitable foundation in one region, the higher water depth in another may require a jacket or gravity base foundation. Floating solutions for waters deeper than 50 meters, on the other hand, are not directly subject to damage by earthquakes but have to consider their consequences such as extreme wave activity or even tsunamis.
Deutsche WindGuard’s offshore consultants therefore also work at the interface between wind energy turbine manufacturers and foundation designers and
As current or planned wind energy projects off the shores of China, Taiwan, India, South Korea, Japan and Vietnam are picking up speed, Deutsche WindGuard’s offshore specialists deepen their insight into the conditions of the various markets and their base of experience every day – to gear up for tomorrow’s challenges.
In order of appearance:
- Offshoring Concept: jirsak/stock.adobe.com, #53348159
- Offshore wind turbines farm , Jeju island, South Korea: SYEOLS/stock.adobe.com, #183949932
- Landscape of western Jeju island at sunset with offshore wind turbines: syeols/shutterstock.com, #1514400881
- Offshore Wind Turbine Construction: DJ/stock.adobe.com, #272243952
- Construction of wind power in Shanghai offshore wind farm: chuyuss/shutterstock.com, #138446858
- National Flags: GreenOptixstock.adobe.com, #214914637
- Ship sailing in rough sea around offshore wind farm turbines: Ian Dyball/stock.adobe.com,#242559747
- Typhoon Talim is heading towards China and Taiwan: lavizzara/stock.adobe.com,#171798878
- Seismograph predicting earthquakes with precision: Negro Elkha/stock.adobe.com,#373368070