Vol 21 No 4 (02-2012)
CLP Power Hong Kong Limited installs an offshore data mast


Background

As Hong Kong depends on imported natural gas and coal to generate electricity, CLP Power Hong Kong Limited (CLP) plans to promote sustainable development and the use of renewable energy in Hong Kong. Since 2006, CLP has been studying the feasibility of building an offshore wind farm in the southeastern waters of Hong Kong to generate electricity from wind power. CLP will first spend a year for data collection and evaluation, to assess the cost and efficiency of the wind farm by building a data mast in 2012 with a cost of HK$70 million. Environmental data such as wind speed, wave, temperature, humidity and air pressure will be collected.

CLP selected southeastern waters of Hong Kong as the location of the offshore wind farm. The area of the proposed site is approximately 16km2 which is located 9km east of Clearwater Bay Peninsula. In the project, up to 67 wind turbines of 3MW or 40 turbines of 5MW will generate electricity for over 80,000 households every year. Electricity generated in the turbines will be transmitted to offshore transformer platform through 33kV cables and to the land through 132kV cables.

Due to the lack of experience in constructing a large-scale offshore wind farm in Hong Kong, CLP will install an offshore data mast to collect sufficient data of the site for 1 year before finalizing design of the wind farm such as distribution, height and number of turbines. Installation of the data mast will be executed in mid-2012 when the sea conditions are relatively stable.

Data Mast

The 79m high data mast will be inserted into water for about 30m and 12m into the seabed. It will stand about 37m above sea level. Various instruments will be located on a platform 15m above sea level, including a Light Detection and Ranging (LIDAR) system to measure wind speed and wind direction over 100m above the platform to accurately estimate the wind power at turbines’ height. To further obtain wind direction and wind speed, wind vanes and cup anemometer at three levels will be installed on the data mast.

Other environmental data such as wave, temperature, air pressure, humidity and lightning will be collected by corresponding environmental data sensors. Movements of suction caisson foundation must be considered as one of the factors in the study, thus structural movement detection devices will monitor the movements in compliance with government requirements. Data collected will be transmitted to data storage device on the platform first, then to onshore servers through a wireless communication system. It is noted that the automatic operation of the data mast will be powered by a solar power supply system.

Navigational aids such as aviation lights, radar reflectors, marine lanterns and fog horns will be installed at the data mast to avoid collisions with aircrafts or ships under all weather conditions. CLP will also implement lightning protection on the top of the data mast to prevent damages.

Construction of foundation

The construction of the data mast will be carried out according to international standards for offshore infrastructure. Besides, the foundation of the data mast will be installed by suction caisson technology for the first time in Hong Kong. The installation process will be carried out in the following order: data mast, substructure and foundation will be constructed in a fabrication yard specialized in making offshore oil drilling platforms. Then the components will be loaded onto cargo barge and transported from Shekou, Shenzhen to the installation location, where an installation vessel will assist in installing the data mast. The tripod foundation will then be vertically lowered into the sea and immersed into seabed by self-weight. Afterwards, water will be pumped out from the caisson that differential water pressure will push the foundation into the seabed 12m deep. Once the foundation is installed, superstructure will be connected to the substructure, which completes the installation process.

Suction caisson technology is commonly adopted in constructing offshore infrastructure in deep water such as offshore natural gas and oil drilling platforms. This kind of technology is suitable to be used in deep waters with soft seabed. Compared to conventional piling method, the suction caisson technology is less time-consuming that only 2 days are needed to install the data mast. Since no dredging works are involved in the installation process, the technology is more environmental friendly than the traditional one. Upon completion of the 1-year study, the data mast will be removed from the seabed in reverse order of the installation process: the superstructure will be removed while water will be pumped into the caisson when installation vessel will pull the substructure and foundation out of the seabed. It is noted that no construction wastes will be generated during the processes and influence to the environment will be minimized. If an offshore wind farm is proved feasible in Hong Kong, CLP will adopt the same method to install offshore wind turbines.

Challenges

The proposed location of the wind farm is one of the windiest locations in Hong Kong where lacks shelters from islands, so wind farm will face great wind and waves. In addition, the area is situated a few kilometers away from shores that will add complexities to construction works. Besides, CLP plans to install the data mast within the period from April to September, when the sea conditions are relatively stable but it collides with typhoon season in Hong Kong. To ensure the construction process is safe, CLP will monitor sea conditions and weather beforehand. Suction caisson technology will be adopted on a suitable day to shorten the installation time.

Another challenge of the installation is the data mast will be immersed into the sea 30m deep. The seabed is soft and it will be relatively challenging if piling works are adopted. Together with dredging works, the conventional method will place undesirable effect to the seabed and nearby ecology. CLP decided to use suction caisson technology for the first time in Hong Kong to reduce impact on the environment according to features of the location.

The construction will be complex and time-consuming since this will be the first time to use such advanced technology in Hong Kong. CLP had already conducted an in-situ test on a trial suction caisson in 2008. To ensure the result was reliable, the test was monitored by Buildings Department and Environmental Protection Department. Sea water sample and conditions of the foundation were also recorded by CLP to prove the test had met standards of Environmental Impact Assessment Ordinance. 45 days after the installation, a tension test was carried out to show the location is suitable for adopting such technology and in compliance with construction standards of Buildings Department. The results had shown CLP is capable of building offshore wind farm.

Since installation vessels are vital in building offshore wind farm, natural gas and oil drilling platforms, the great demand and limited supply of the vessels require a long booking time. CLP will book the vessels in advance.

Conclusion

According to experience from other offshore wind farms around the world, the wind farm in Hong Kong will take about 8 to 10 years to develop if it is proved feasible and supported by the general public and the government. It will provide 20 to 25 years of renewable energy after completion in 2016. With the new source of clean electricity, the annual emission of more than 300,000 tons of carbon dioxide will be reduced.



Courtesy acknowledgement:
CLP Power Hong Kong Limited
for providing the above information and photos