Calculation of a Wind Turbine Pole: Scope and Sequence

Input data

The calculation starts from data that the standards do not contain: they come from the turbine manufacturer, the customer and the site.

• from the turbine manufacturer: the mass of the nacelle with the rotor, the rotor diameter, the speed range, the axial force on the rotor in operation and in extreme wind, the dimensions and design of the top flange, the position of the centre of mass, the requirements for pole stiffness;
• from the site: the wind and ice regions, the terrain type, the altitude, the relief, the ground conditions;
• from the customer: the height required, the service life, the consequence class, the requirements for access and maintenance.

Scope of the calculation

The calculation goes in stages, and each relies on the results of the previous one.

Loads from the turbine

The rotor loads are set by the turbine manufacturer. Several design situations are usually considered: operation with shutdown and emergency braking, the extreme wind the turbine is rated for, and the extreme wind under the standard for the construction region with the rotor stopped. For each situation the force on the rotor, the moments and the torque at the top joint of the pole are determined separately.

Wind on the shaft is calculated under DBN V.1.2-2, and the combinations take the most unfavourable of the situations. The second limit state group is considered separately under characteristic loads.

What often governs the section

For wind turbine poles strength often has a margin, and the section size is governed by stiffness requirements: the permitted displacements of the top, the head tilt and the position of the first natural frequency relative to the rotor rotation frequencies. So stiffness is checked early, not after the section has been chosen for strength.

Limits of the calculation

The calculation needs a fatigue load spectrum from the manufacturer, so the fatigue check of welds and flange joints is done from it. Erection and transport loads, weakening by openings and the effect of the foundation on the stiffness of the base are considered separately.

Each pole is calculated for a specific turbine and a specific installation point, so the article describes the scope of the calculation and does not give ready dimensions. What to send for a calculation is described in Wind Turbine Towers.

Standards and validity

• DBN V.1.2-2:2006 “Loads and actions. Design standards”;
• DBN V.2.6-198:2014 “Steel structures. Design standards”;
• DSTU EN 1090-2: execution of steel structures; DSTU EN ISO 1461: hot-dip galvanising;
• DBN V.2.1-10:2018 “Bases and foundations of buildings and structures”: foundation.

The validity of the documents is checked before the work starts, and numbers from reference tables are taken from the current edition.