Wind turbines foundation design

Wind turbines foundation design is an art (and a job) by itself. Here you have a very quick overview of the process for shallow foundation (Patrick & Henderson foundation, widely used in the USA, follow a totally different approach) .

The inputs for the foundation design are the result of the geological survey and the design loads.

The geological survey gives the structural engineer key parameters to define the soil: internal friction angle, cohesion, density, Young modulus, shear modulus, etc. The number and type of parameters will depend on the type of materials below the foundation: expansive clay will need different test compared to sand or rock.

Design loads are provided by the manufacturer of the WTG, who has standard documents with the different type of loads (usually available data includes operational loads, extreme normal loads and extreme abnormal loads, together with fatigue calculation data).

Numerous codes, national laws and international standard are used in the design process. These are the most commonly used:

 

 

Following the IEC-61400, the design load cases used to verify the structural integrity of a wind turbine shall be calculated by combining:

  • Normal design situations and appropriate normal or extreme external conditions;
  • Fault design situations and appropriate external conditions;
  • Transportation, installation and maintenance design situations and appropriate external conditions.

The design load cases must be analyzed at fatigue (F) or at ultimate loads (U). Ultimate loads are defined as normal (N), abnormal (A) or transport and erection.

Several load combination are checked: for every ULS (ultimate limit state) and SLS (serviceability limit state) the correct partial factor is used, to diminish stabilizing forces and incrementing destabilizing forces. The same load type can have a different partial load safety factor, depending on the verification made. For instance, in the case of overall stability, the bending moment at the tower base has a factor of 1.35 for normal loads and 1.1 for abnormal loads.

Partial safety factors are defined by the IEC as:

Seismic loads must be analyzed separately and they are normally combined with the turbine operational loads. It is of critical importance to recognize that seismic plus operational loads may in some cases govern the tower and foundation design.

The main requirements to be satisfied are:

 

  • Not to exceed the bearing capacity of the soil: the soil pressure must be lower than the allowable bearing pressure.
  • Not to overturn (i.e. there is no rotation around the edge)
  • Not to slide (i.e. there is no horizontal movement)
  • Not to exceed the maximum differential settlement provided by the manufacturer during the life of the structure (normally few millimeters/m). The differential settlements are normally calculated using a finite elements models software, simulating the composition of the different layers of materials below the foundation.
  • To comply with the minimum dynamic rotational stiffness given by the manufacturer (to limit the potential coupling phenomena with the rotating parts of the WTG).
  • The compressed area below the foundation is normally assumed to be 100% for operational load and at least 50% for other loads cases.

 

The first step is to define a tentative geometry: than, if the various checks are satisfied (overturn, slide, bearing capacity, etc.) a detailed analysis with a finite elements software is made, to define the amount of reinforcement needed: from nodal stress distribution along the most unfavorable positions of the cross section, necessary mechanic capacity is calculated,and the amount of steel (mechanical capacity) necessary to withstand the calculated stresses is compared with the amount of steel placed in the section. Clearly, the first must be lower than the second.

Soil reaction transmitted to the foundation is modeled with vertical non linear springs

The calculation can sometimes lead to the conclusion that a shallow foundation is not feasible, due to low bearing capacity, insufficient rotational stiffness or many other possible factors. In these cases, a soil improvement is made or a deep (piled) foundation is calculated. These alternative solution are normally quite expensive: depending on the country and the technology needed, the extra cost can vary from 50% to 100% and more.

AutoTURN special transport Nooteboom vehicle library

Here comes my collection of AutoTURN Nooteboom trucks for wind blades transport.

I’ve created them starting from several drawings, technical documentation and brochures I’ve found trough the years for the Teletrailer and Telestep. Of course I can’t guarantee they are correct, but until now they have worked – no truck has been stuck in a bend.

The main problem you will see if you work with AutoTURN is that you can’t use the reverse direction with this type of vehicle, although in real world situation we use it quite often: for this reason when I need to simulate it I use a standard truck, only “stretched” to the length of a Teletrailer.

The V100 is available in 2 version, with vertical and horizontal (that is, lying) blade as I’ve seen it transported in both ways.

AutoTURN Nooteboom Teletrailer vehicle library

Wind turbines blades materials

A fundamental aspect in the wind blades design is the choice of the correct materials, because several parameters (weight, load and fatigue behaviour, physical properties, etc.) are influenced by this basic decision.

Several years ago materials as varied as wood, steel and aluminium where used to produce wind blades.

 

The first wind blades where made using wood, a low cost, low density material with a good resistance to fatigue. However, wood has a strong anisotropy and a huge potential variation of his properties, together with a notable tendency to water absorption that can lead to a reduction of resistance.

NASA MOD-0 prototype (a 38 metre diameter downwind two-bladed rotor developed in 1975) was initially made with aluminium blades, but soon due to structural problems wood blades were installed.

Wood was used in the 80’s in small (< 10 m.) blades, and can be still seen today as part of composite materials together with epoxy resins.

 

Steel was used in the first years of the 80’s in turbines as Growian ("Große Windkraftanlage" – big wind turbine), a Goliath for his times with steel spars and fibreglass skin. The structural properties of this material are exceptional, while the main drawback is its elevated density, leading to an increase of inertial and gravitational loads.

 

Aluminium was the initial choice for the MOD-0 prototype. Blades were developed by Lockheed, but were removed do to problems at the root caused by the rapidly changing loads (the WTG was downwind). It has a low density and a good resistance to corrosion, but a low fatigue resistance. It has been traditionally used in vertical axis wind turbines.

 

Today, the materials commonly used are fibreglass polyester and fibreglass epoxy (called GRP, glass reinforced plastic).

GRP was previously used in the naval sector: among his advantages, good structural and fatigue resistance, fabrication versatility. Other interesting physical properties of this material is that it has a low electrical conductivity (useful if the blade is struck by a lightning).

 

The evolution of the sector is leading to the introductions of new materials, as Kevlar and carbon fibre (CFRP, carbon fiber reinforced plastic) which are lighter but still too expensive for mass production.

 

It must be observed that several variables influence the choice of the materials: cost, weight, resistance, number of blades of the rotor (3 blades WTGs with fixed hubs are more robust than 2 blades WTGs with tilting hubs). An important parameter is the specific weight, the relation between the weight of the rotor and the swept area (in kg/m²).

Big manufacturers such as Enercon, Vestas, Gamesa, etc. produce blades by themselves, while others buy blades produced by specialized manufacturers on the market.

Wind blade special transport: 45 degrees lifter for montainous roads

This is a blade special transport system that I’ve seen used by Enercon, for instance for Europe highest wind turbine in Europe, in Switzerland, but is not so common otherwise.

It is a hydraulic lifter, which allows lifting the wind blade up to approximately 45º.

Doing so it can guarantee important saving on the civil works, above all in mountainous areas where important earthworks have to be realized to reach the wind park.

It has to be used together with a 5 axle low loader, and its price is around 100.000€.

The biggest problem is that in many countries it couldn’t be used on the public roads, due to restriction the maximum height of the load (around 5 meters).

By the way I guess that special permit are granted in some countries (for instance I’ve seen several picture where this solution was used in public roads in Germany and Switzerland).

Liebherr cranes AutoCAD DWG blocks

My collection of AutoCAD blocks of several Liebherr cranes(both standard and narrow crane) that we use in our project.

Hope you can find them useful. More cranes here: Terex/Demag cranes and Manitowoc cranes AutoCAD DWG blocks.

 

Liebherr LTM 1090 crane autoCAD block

Liebherr LTM 1130 crane autoCAD block

Liebherr LTM 1350 crane autoCAD block

Liebherr LR 1400 crane autoCAD block

Liebherr LR 1400 2W narrow crane autoCAD block

Liebherr LTM 1500 8.1 crane autoCAD block

Liebherr 1600 W narrow track autoCAD block

Liebherr 1750 crane AutoCAD block

Liebherr LTM 11200 - 9.1 crane AutoCAD block

Liebherr LR 11350 crane AutoCAD DWG Block

Liebherr LR 1750

 

Wind farm standard layout

The components of a typical, utility size wind farm are:

  • One or more rows of turbine perpendiculars to the prevailing wind direction, with a distance between them of 2 or more diameters. For instance, if the diameter of the rotor is 100 meters, the distance would be from 200 to 300 meters. This distance is necessary to avoid induced turbolences to the rotor, than can affect the life of the generator. The number of turbine installed depend of the chosen model: a wind analysis is made, to define the WTG with the best fit for the location.
  • An underground power collection system, normally around 20kV-30kV. Sometimes an above ground medium voltage power collection system is used, although it happens very rarely due to the visual impact. Here there is a trade off between greater spacing between the WTGs (=more production) and greater cables length (=more energy losses and cable cost).
  • Civil works (roads, crane pads, foundations)
  • A substation transformer
  • An operation and management (O&M) building, to store spare parts and perform maintenance. Can be integrated in the substation.
  • A SCADA system, to collect and transmit data from the wind farm to the stakeholders.
  • A high voltage interconnection line to the grid. The distance from the substation to the grid can vary greatly, and if the wind resource is sufficiently high longer distance can be feasible. The interconnection point to the utility line can be co-located in the substation: in this case (frequent when the wind farm connects to an existing substation) there is no need for an overhead line.

Floating roads on peat

Floating roads are a solution developed in Scotland during the construction of several Km of internal wind farms roads on very poor, organic materials such as peat.

It is an interesting constructive technology, developed on a very compressible, decayed material normally considered as the worst possible subgrade for road construction. In a normal highway project, peat would be removed

Usually floating roads are stabilised haul roads with one or two layers of geogrid. This approach leads to several advantages:

 

  • Reduced road thickness (and subsequently less weight)
  • Better distribution of pressure
  • Untouched surface layer, lesser impact on vegetation
  • Less construction material needed
  • Usually cheaper

 

The suggested approach to floating roads construction is based on six steps:

 

  1. Carrying out a detailed survey to define the hydrology of the area and the paet type (according to the “Von Post” system, there are 10 possible classification for this material depending on the decomposition level)
  2. Identifying the value for in situ peat strength
  3. Estimate the expected traffic loading
  4. Design the road
  5. Monitoring the construction
  6. Recording action and outcomes for future projects

 

Among the design assumptions, it’s relevant to remark that a rut with a depth up to 10 cm is to be expected.

The first step is an in depth site investigation, where several parameters are recorded: peat depth and classification, side slope angle, hydrology and permeability. Many in situ test are available, from probing and sampling to more sophisticated techniques such as ball penetrometer or Mexecone penetrometer and ground penetrating radar. After, in laboratory, water and organic content can be defined, together with vane testing and direct simple shear tests.

When all the data is collected, normally is the geogrid manufacturer who will design the cross section of the road, deciding the type of grid (there are several dimensions possible – they must match with the aggregate size), the number of layers and the height of the compacted stones.

The designer will use semi empirical rules, using as inputs shear strength of the peat, weight of the road and expected number of equivalent axes.

It is standard practice to use at least 2 layers of geogrid: a lower one directly on the existing material, and an upper layer on the top of the embankment, used as a support for the controlled granulometry crushed gravel.