Between 18-36 kilometres off the Dutch coast, Vattenfall’s offshore wind farm Hollandse Kust Zuid (HKZ), meaning Dutch Coast South, originated. In June 2021 the construction of this wind farm, which is world’s largest, started. With 140 11MW wind turbines, the wind farm will produce enough fossil free electricity to cover the consumption of over two million households. This enormous project, Vattenfall tried to realize in the most environmentally friendly way.

Monopile cover

One of the aspects Vattenfall wishes to improve in this project, is the sustainability of the monopile (MP) cover. This is a device which protects the inside of the monopile, once it has been installed in the seabed, and specifically the airtight platform which is situated 1.2m underneath the top, inside the monopile. The monopiles in this project will not be equipped with transition pieces but will make a direct connection  with the turbine tower. Every monopile will therefore be equipped  with a flange through which it will be bolted together with the turbine tower. All secondary steel such as ladders and platforms will be hung on the monopile. Courtesy of Vattenfall Dutch teamwork leads to smart & sustainable monopile cover offshore workers will use the airtight platform for among other things, the pull in of inter array cables. The MP cover is placed on top of the monopile and protects them during their job.

Safe and healthy

Without a cover, the work platform inside the monopile can become wet and slippery from rain or splash water and soiled by bird faeces. It is essential for Vattenfall to ensure a safe and healthy working environment for all offshore workers. Therefore, a cover was an absolute requirement. As to functional specifications, the cover needed to be water- and dirt tight to prevent any chance of mould growth and condensation on the inside of the monopile above the airtight platform. With regards to environmental aspects, the cover had to be reusable or easy to recycle to minimize the environmental footprint.

“We wanted a new and sustainable concept and gave our subcontractors the freedom to come up with a new concept, both in terms of construction as well as in material choice.”

Sustainable concept

With Vattenfall’s sustainable mission in mind, they asked Sif – who was granted the fabrication of the monopiles – to come up with a smart and environmentally friendly solution for the covers. Quickly the ball started rolling and different Dutch parties joined forces. Stimulus programme ‘Kansen voor West (KVW)’ meaning Chances for West, CCM BV, polyester manufacturer Theuws and engineering company Enersea started brainstorming and together developed the first concept that resembles an ingenious birthday cake. The construction consists of different fiber reinforced polyester panels and looks a little bit like pieces in a pie. ‘It was not the first material we thought of’, Christian Larsen, project engineer for Vattenfall at HKZ, admits. ‘But we wanted a new and sustainable concept and gave our subcontractors the freedom to come up with a new concept, both in terms of construction as well as in material choice.’  The composite panels are connected to each other with metal strips and bolted together. Hatches for entering the monopile are also included the cover design. Nine strongpoints with a working load limit of 0.5 tonnes are included at the underside of the MP cover that allow the cable contractor to fixate its cables. The strongpoints are located 2m from the MP flange and equally distributed across the entire circumference. The strong points can be loaded to a maximum of six adjacent strongpoints with a maximum of 500 kg each. The cover will have three lifting points and the possibility to install a tripod for cable pull in works. Furthermore, the design of the MP cover will be optimised in such a way that it avoids opportunities for bird nesting on the MP cover.

Following the successful design and construction of this prototype, Vattenfall decided to grant the production order for the complete 140 monopile covers to the parties involved. An exciting challenge all Dutch partners in this project are eager to tackle together with client Vattenfall.