Wind turbines foundation design and optimisation in the context of profitability of investments in wind farms.
Never before has the question of optimizing the foundations for wind turbines been as valid as today, in the context of searching for the best model of financing and highest profitability of investments in wind farms. When deciding to undertake such a project, investors must specify the expected rate of return and time frame for investment in advance. Most often, the financing is based on a feed-in tariffs (FiTs), feed-in premium (FiP), tenders, tradable green certificates (TGS), Power Purchase Agreement or Contracts for Difference (CfDs).
In recent years, more and more is heard about investments without subsidies, based only on a fixed price for the energy generated, so-called “black energy”.
Regardless of finding the best model of profitability of a wind farm, the interest of each investor is to optimize the project implementation costs. The better the wind farm project is prepared, thought out and planned, the more money can be saved. The biggest reductions in investment costs can be obtained at the stage of wind project implementation, in which one of the most significant expenses is the cost of the foundation itself.
The optimization of the foundation for the tower of the wind turbine should first of all be made with rationality, based on knowledge and experience, because the basic goal must be the failure-free construction. It is ill-advised if the marketing pressure of winning orders leads to balancing on the edge of security.
Designing the foundation of wind turbines is one of the most difficult engineering tasks. They can not be designed by engineers without experience in this narrow specialization and without proper, specific knowledge related to the effects of wind, dynamic variable loads and material fatigue. It is necessary to skillfully combine and adapt international guidelines, recommendations and requirements of turbine manufacturers as well as national regulations, local conditions and design standards.
Each time the investor faces a dilemma as to how to properly balance the funds allocated for the investment in the project phase. One should be aware that in the context of optimizing the foundations of wind farms, a good understanding of the ground conditions has a prominent impact on the searching for the cheapest and technically justified solution.
Wind turbine design and optimization step 1: Soil investigation
Wind farms are Energy Facilities, and their subsoil research should be treated in a special way. Detailed geotechnical investigations and a geological report with the recommendations of an expert must be made for such objects. The geologist expert present on the construction site, has the best, real picture of local ground conditions. It is recommended to carry out field tests to a depth equal to approximately 1.5 multiplied by foundation diameter, with the most frequent boring depth at about 20-25 m. It is important to consider the degree of difficulty of the foundation cooperation with the ground while running the process of the soil investigations.
It is recommended that on the basis of field and laboratory tests, at least the following geotechnical parameters should be determined:
• type of soil, divided into lithological layers,
• maximum level of groundwater table,
• parameters describing the physical properties of the soil,
• effective strength parameters of the soil,
• static and dynamic deformation modules,
• Poisson ratio.
It is a good practice not to limit yourself to one particular test, especially when it comes to the dynamic parameters of the ground. Practice shows that research results may differ significantly, giving an inconsistent picture of the real profile of the ground. Developers and investors of wind farms, commissioning soil tests at the construction design stage, often have a limited budget for this purpose. The cost difference between detailed and basic soil investigation is probably only a small part of the entire investment. The project, which will be the result of good research, can bring even from 10% to 15% percent of savings in the final settlement. Additionally, it should be remembered that if the reliability of the ground survey is too low, in the next phase it will have to be repeated and paid for again. Savings at the stage of ground survey may contribute to the increase in the cost of laying foundations in the final effect. Inadequate parameters to the real conditions cause the design of ground reinforcement or piling in cases where such kind of solutions could be avoided.
Wind turbine design and optimization step 2: Projects
According to ICE 61400-1, the lifespan of a wind turbine is 20 years. Sometimes, at the request of the investor, this time can be extended up to 30 years. During this period, the following variants of the construction work may occur: assembly, disassembly, start-up, energy production, extreme wind (occurrence about 5 times per 231 million cycles), failure and others. On the basis of these variants, impacts transferred to the subsoil are determined, in the form of vertical force Fz, horizontal force Fxy and moments of: twisting Mr and overturning Mxy.
Fig.1. Impacts transmitted to the ground surface
There are three main calculation cases:
• extreme wind (50-years gust),
• emergency loads, untypical work, defect or destruction of the turbine,
• permanent load, in which case no detachment of the foundation is allowed, for gravity foundation – often referred to as “no gapping”; or no tensile forces in piles.
All of these three cases in the appropriate computational combinations can be dimensioning and affect the size and type of the foundation and its reinforcement. In addition, fatigue loads have a significant effect on the amount and shape of the reinforcement. For the permanent load case, static and dynamic rotational stiffness are also verified. This analysis aims to exclude the possibility of a resonance effect.
The geotechnical designer should have a lot of knowledge about the soil’s behavior along with the cooperating foundation. It is the ground conditions determined by geotechnical research that mainly impact the cost of reinforcing ground or piling if necessary.
Correct geotechnical analysis is crucial in the design and optimization of wind turbine foundations. Due to the non-standard work of these structures subjected to variable, cyclically occurring dynamic loads, the interpretation of ground conditions differs from the standard analysis. In addition to checking limit values of stresses, differential settlements, slip and travel conditions, such conditions as the maximum compressed area (including no lift-off case), no traction in piles and minimal dynamic stiffness for rotation and horizontal displacement, have to be verified.
Further than the dynamic aspect, the presence of buoyancy or its lack is an extremely important parameter affecting the design. Foundations due to buoyancy may differ from each other in terms of volume by up to> 35%. With awareness how important it is on the investment costs, it is recommended in every case to thoroughly investigate the water pressure levels, eg with piezometers. You can additionally refer to archival surveys in a given area.
The variability of the water situation over the 20-year period may turn out to be quite large. In this context, it is also helpful to analyze the situation and altitude of the investment area and to identify possible watercourses or the risk of water accumulation. If possible, the problem of water buoyancy can be compensated by elevating the foundation. Of course, in such a case a number of multi-disciplinary conditions, such as the Environmental Decision, Local Spatial Development Plan, Decision of Civil and Military Aviation, turbine manufacturer’s guidelines and other local conditions have to be taken into account. One should be aware of the fact that wind loads given by the turbine supplier refer to a specific height and its change by elevating the foundation may affect the value of these loads.
In a situation where the soil does not exhibit sufficient bearing capacity, it is necessary to modify it to reach required parameters. For example: by soil replacement under the foundation (cost efficient up to about 1.5 meters deep), designing foundation on piles or direct foundation on soil improvement. For weak soils occurring up to a depth of about 8-12 meters below ground level, it is usually cheaper to strengthen the ground. Piling is most often preferred for larger depths. It is also a good practice to take into account investor’s expectations and to conduct an active market research aiming at finding the best technically and time and cost-efficient alternative.
Foundations placed on a ground supported by columns and a pile foundation differs from each other in a way of cooperation. Both of these cases result in completely different project approaches, change of assumptions and calculation checks. Columns and foundation part of the so-called transmission layer, causing that these two structures work independently [Fig. 5]. The piles with the foundation are connected by means of anchoring reinforcement, which creates a uniform and cooperating system.
Wind turbine design and optimization : Summary
In most cases, the lack of sufficient knowledge by designers ends with a significant oversized design of the structure, which entails a significant increase in investment costs. Optimal design for wind turbine foundations is a complicated issue and in the case of difficult ground conditions requires close multi-branch cooperation. Due to the complexity of static and dynamic complications of the above-mentioned constructions, the role of designers and the geotechnical experts is to attract investors’ attention to the issues related to soil investigations, which constitute the input material for the design of wind turbine tower’s foundations. A proper design guarantees a cost-efficient solution and can bring up to several million euros savings on a large investment.