Lely Wind Farm

by W van Zanten CADDET Dutch National Team

Introduction

In 1994, after four years of preparation, the first off-shore wind farm in the Netherlands was erected. The "Lely" wind farm comprises four NedWind 500 kW wind turbines, of rotor diameter 40.77 m. The turbines are placed on unique single-pile foundations. An interesting engineering aspect of the project is the interaction between the foundations and the turbine.

Legal Procedures

Wind farms in the Netherlands require a building permit and a permit for environmental acceptance. Lely was our first off-shore wind farm, so no judgement criteria were available to help the authorities. The project gave the opportunity to develop these criteria.
 An environmental research programme was developed which played an important role in the legal process. The environmental programme was split into two parts. Part one was carried out before construction began. The site was examined and the following information recorded:

  • landscape structure;
  • use of water by vessels;
  • existing pipes in the soil;
  • influence of the turbines on sailing ships, birds and fish on the site.

Experts in each of these fields assisted in defining the investigations.
 The second part of the programme was carried out in the summer of 1995, after construction of the wind farm, when the ecological investigations of part one were repeated.
 In reporting the results, special attention will be given to the opinion of the many users of the waterway. By comparing the results of the first and the second parts of the programme, conclusions can be drawn about how the wind farm influences its environment. This helps to develop criteria for evaluating future wind farms.

Siting

Based on a study of the landscape and technical demands, a location along the dyke north of the town of Medemblik was chosen. Procedures to obtain the necessary building permit then began, and were completed in under ten months. Objections from local conservation organisations and large shipping companies were dealt with by referring to the environmental research programme. All points of complaint were investigated, so the environmental research programme turned out to be the oil in the wheels of the legal procedures.
 In the end, no serious objections to the project were found. There are hardly any added risks to shipping, the site is not an important one for birds and the wind farm's influence on fish-life is expected to be very small. The environmental research programme took about 4% of the total project costs.

Design and Construction

Four 500 kW NedWind wind turbines make up the off-shore wind farm. They are located 800 m from the shore at 200 m centre-to-centre distances. The local water depth varies between 5 to 10 m.

 The regional utility, who are principals in the contract, preferred a single-pile foundation. The dimensions of the foundation piles are, 3.2-3.7 m in diameter, 35 mm wall thickness and 30 m in length.
 The wind turbine foundations must withstand both dynamic and static loads. The dynamic criteria are normally formulated by the designer of the wind generator in terms of stiffness of the foundation, to avoid dynamic amplification. The static loadings of the foundation are determined by the dead-weight of the structure, ice load and ship impact.
 Ice loads are easy to describe in general terms but difficult to quantify accurately. The loads executed on a structure depend on the expected thickness of the ice, the compressive strength and the contact area. For Lely wind farm, the ice load is 1,100 kN.
 Although ship impact is a rather dynamic event, it is dealt with in civil engineering as a static load. The energy to be absorbed by the structure is a result of the ship's mass and the square of the ship's velocity. The forces involved in the impact depend on the stiffness of the point of impact. In order to limit the impact force to an acceptable level, an easily deformable structure is used. In this case, the absorption capacity of the maintenance platform around the pile is employed. The platform will be hit first and, with its plastic deformation characteristics, will absorb the ship's impact at an acceptable force.
 The load frequency on the foundation is dictated by the rotation speed of the generator in use, which is 32.2 rpm. The rotor frequency is therefore 32.2/60 = 0.54 Hz. The blade-passage frequency for the two-blade installation is twice this frequency; ie 1.08 Hz.
 The natural frequency of the structure should be well above, in between, or below these frequencies. The natural frequency of tower and generator, assuming an infinitely stiff foundation, was calculated as 0.83 Hz, which is within the criteria. Interaction between foundation piles and the soil necessitates the use of different piles at distinct locations.
 The wind turbines are standard NedWind 500 kW machines. Their quality should meet the requirements of the site in fresh water. Because of the project's pilot character, several special features are built into the turbines, with the aim of reducing maintenance costs. The feasibility of these features will be examined during the first year of operation. Other aims of the project include:

  • to reduce travelling to the wind farm;
  • to reduce the use of cranes on the site (in the water);
  • to preserve the environment surrounding the wind farm;
  • to preserve a safe environment for shipping movements to the port of Medemblik.

A research and measurement programme will be executed, to record the experience gained from this project in all aspects of maintenance, performance and durability of projects at an off-shore site.
For more information contact the CADDET Dutch National Team in Sittard.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

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