First monopile-caisson hybrid foundation installed at Chinese offshore wind farm

On 19 April, the first hybrid foundation combining a large-diameter monopile and caisson was installed at an offshore wind farm off the Fujian province in China.

According to the China-based engineering company that designed and patented the new hybrid monopile-caisson solution, the offshore wind project comprises a few different types of wind turbine support structures, including monopile and piled cap foundations.

PowerChina Huadong Engineering Corporation Limited (HDEC) told Offshore Energy in early May that their novel monopile-caisson foundation was installed at the Putian Pinghai Bay Phase II offshore wind farm.

More monopile-caissons are planned to be installed in the later stage of the project, HDEC said.

The 246 MW offshore wind farm, currently under construction some 10 kilometres off the coast of the Fujian province, will comprise 41 wind turbines. Water depths at the project site range from 16 to 20 metres below mean sea level (MSL).

The project, which is the second phase of the Putian Pinghai Bay development, is expected to be completed by the end of this year.

The first 50 MW phase was commissioned in 2016, while Putian Pinghai Bay Phase III is nearing construction.

HDEC’s solution was brought in as a response to the project schedule being affected by unexpected issues emerging during construction, due to an unfavourable geotechnical condition at the site.

Namely, the soil layer is very shallow, so the foundations have to be installed into the rock, HDEC explained. “However, offshore rock drilling or piling in the weathered rock layer caused many unexpected issues like the hole collapse, jamming of a drilling tool, pile buckling, etc.”, the company said.

To put the work on a fast track and meet the project deadline, HDEC decided to shorten the monopile length and avoid the rock drilling operations. The company manufactured and installed the new hybrid foundation in cooperation with the offshore wind project developer Fujian Zhongmin Offshore Wind Corporation, Third Harbor Engineering Corporation and NOED.

“This new foundation type is a promising solution to the problems we are faced with in Fujian, and will hopefully accelerate the project schedule with much lower costs”, said Dr. Li Wei, HDEC’s chief engineer for renewable energy.

According to HDEC, offshore wind projects in the Fujian province have been significantly influenced by short weather windows and unfavourable seabed conditions, which require rock drilling.