For Vattenfall’s offshore wind farm Hollandse Kust Zuid, Femto performed the structural analyses of  monopile covers developed by polyester manufacturer Composite Cover Manufacturing (CCM) and offshore engineering specialist Enersea. Monopiles are the foundation of the wind turbines that rises several meters above sea level. During the construction of the wind farm the covers will be used to protect the platform and constructor workers inside the monopile before the turbine tower is placed.

Mission

Wind farm developer Vattenfall demanded its subcontractors to develop a smart and environmentally friendly solution for the monopile cover. Until now, most monopile covers have been made of steel of aluminium and are designed for one specific offshore wind farm. After finalizing the farm, these covers become a waste product. Of course, these materials can be recycled, but this involves an energy consuming process. Another method is using a tent cover. This option has the risk of being blown away at high speeds contaminating the sea and leave the monopile unprotected. To come up with a new and sustainable concept, Vattenfall gave the subcontractors the freedom both in terms of construction and material choice.

Concept

Enersea and CCM accepted the engineering challenge and came up with a first concept that can be best described as an ingenious birthday cake. The construction consists of different fiber reinforced polyester panels that with a little imagination look like pie slices. The top panel of the cover can be reused for every diameter of offshore monopile currently on the market. Top diameters of monopile typically vary between 6 and 8 meters. By using longer or shorter part on the slope the monopile cover can be tailor made to fit the diameter of the monopile in a new project.

Design

The first step to turn the concept idea in an approved product fulfilling all the requirements was to develop a prototype. From the start, Femto Engineering was involved in the development. The structural feasibility of the concept was proved by defining and carrying out Finite Element Analysis for a great number of load case combinations. The combinations include the loads in different stages (transport, stacked lifting, installation, usages) combined with extreme weather conditions (wind, snow). After feasibility was proved, the concept design was improved based on the insides of the structural analysis and feedback of other stakeholders.

The first prototype was constructed at the Theuws yard in Wieringerwerf mid June 2020. After a positive functional inspection and water test, the prototype was further tested at Sif’s terminal at Maasvlakte 2 in Rotterdam. Sif has been granted the assignment to build to monopiles. The cover was mounted on a mockup monopile so the complete construction could be tested on functionality, load resistance, water tightness and several other aspects. Test outcomes were very promising.

Together with CCM and Enersea, Femto works on the optimisation of the prototype design and focuses on the structural integrity of the monopile cover in every design iteration. Vattenfall will start the construction of Hollandse Kust Zuid in June 2021. Only 5 months to go to optimise the cover, get into serial production and have the first 41 covers ready for installation.

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