Wind turbine generators (WTG) are renewables structures built and installed to harness energy from wind and convert to electricity. Though such renewables structures bring the benefits of reduced greenhouse gas emissions and reliance on fossil fuels, they also present a series of unique installation challenges. This is owing to the size and weight of the structures, the location and limitations of the installation sites, and the requirement of specialised equipment and people.

Mobilisation to remote locations
Renewable energy structures such as wind turbines are installed at locations which provide optimal conditions for harnessing wind energy; they are also often therefore remotely located. Mobilisation of cranes and lifting appliances to remote locations with difficult access is logistically complex and costly. Mobilisation of cranes or crane components along public roads exceeding standard weight and dimensions will require special arrangements and permits. For example, the dimensions and weights of special vehicles used on British roads are regulated by the Road Vehicles (Authorisation of Special Types) (General) Order 2003 (STGO). The timeline required to obtain such permits can vary from a few weeks to months. Therefore, the mobilisation of cranes should be carefully planned. Depending on the country, police escort or designated private escort cars may also be required to raise awareness of the Mobile Crane/ abnormal size crane component on the road.

Crane transportation to remote project locations also often requires travel on narrow unpaved roads with abrupt curves and significant route slopes as well. This calls for a thorough route survey and transportation plan. The movement of cranes between the crane pads within the site must overcome the challenges posed by site tracks built on weak and unstable ground. Wind farms are traditionally located in distant areas with adverse soil conditions. Areas with soft soil conditions pose significant obstacles in constructing strong and stable access roads that must support enormous heavy lift cranes. Over excavation and backfilling to build site tracks suitable for heavy cranes is very costly and time consuming.

Special equipment: Cranes
WTG component’s weight and dimensions are increasing in order to boost their efficiency, necessitating the use of larger cranes on the installation site. Modern WTG hub heights have reached a staggering 170m and the heaviest components can weigh up to 120 tonnes. Heavy lift cranes with high load capacities and long booms are essential to make such lifts possible.
Other important characteristics desired for a crane for wind farm construction are:

• Low ground pressures exerted for different load cases
• Narrow track base to allow the movement of the crane on site tracks between the crane pads
• Outriggers for operating mode (for cranes with narrow track)
• Reduced assembling/disassembling time for moving between the crane pads
• Less space requirement for the assembly
• Operability at higher wind speeds
• Versatile boom configurations
• Ability to move fully/partially erected between the crane pads

Luffing or fixed jib
Mobile cranes for wind farm construction often require specialised jibs, like luffing or fixed jibs, to reach the necessary heights for lifting wind turbine components. These jibs are crucial for accessing and assembling turbine parts, particularly the larger components like nacelles and rotor blades. Extra jibs fixed at an angle may be required to allow the placement of the nacelles with the required clearance.

Blade yoke
Wind turbine blades are challenging to install because of their immense size and weight, the necessity for exact alignment, and the heights at which they must be installed. To lift a WTG and combined with its weight, size, centre of gravity and the height at which it is mounted, makes it a challenging lift. The nacelle lifting requires specialised equipment and careful planning. Specialised lifting beams with multiple attachment points for slings are required to ensure a balanced lift and to guide the nacelle on to the WTG tower.

Nacelle lifting beams/yokes
The WTG nacelle houses the generator and other critical components. The nacelle is typically the heaviest single component of a WTG and combined with its weight, size, centre of gravity and the height at which it is mounted, makes it a challenging lift. The nacelle lifting requires specialised equipment and careful planning. Specialised lifting beams with multiple attachment points for slings are required to ensure a balanced lift and to guide the nacelle on to the WTG tower.

Super lift and outriggers
WTG components require cranes with exceptionally long boom configurations due to the reach required. To erect cranes with long boom configurations, additional systems such as super lift or outriggers are needed to provide sufficient stability even though they may not be required for the operating mode. The requirement of super lift ballast increases the mobilisation cost and assembly time.

Environmental challenges
Adverse weather conditions
Weather conditions can significantly impact crane operations. As the wind speed increases with altitude from ground level, lifting of WTG components to the top of tall towers is incredibly challenging. High winds can have a significant effect on suspended loads, and on the crane itself, leading to an increase in the load radius or side loading on the boom. This adversely affects the crane stability and strength, compromising safety. Most WTG components require accurate positioning and alignment to make the connection with their mounting. High wind speeds can make this operation extremely challenging.

By their very nature, WTG blades present the biggest targets for the wind. A high level of focus is crucial during the final phases when these are raised onto the turbine and fastened to the hub. For loads such as WTG blades which have a low mass in relation to their surface area exposed to wind, the actual permitted speeds are often much lower than the maximum permitted wind speeds according to crane capacity tables. Before every crane operation, maximum permitted wind speed must be calculated as per the crane operating manual. A comparison of permitted wind speed for the specific load lifting operation and the wind speed to be anticipated during crane operation at the highest point of the boom must be carried out.

Vortex-induced vibration (VIV) is one of the other impacts of wind flow surrounding the WTG towers during lifting. This may result in structural fatigue and present lifting safety hazards. In order to suppress VIV, auxiliary devices such as helical strakes can be necessary. Beyond wind, poor visibility due to fog and snow also poses a significant threat to the safety of lifting operations. Similarly, heavy rain reduces the visibility, as well as adversely affecting the ground condition. WTG erection cranes are tall, isolated structures and they are very susceptible to lightning. Lightning strikes can damage the cranes and endanger personnel. WTG component lifting operations cannot be performed in the vicinity of electrical storms. Crane operators must closely monitor weather forecasts (wind speed, direction and lightening risk), wind speed from the anemometer or LiDAR and follow established guidelines and safety protocols.

Ground integrity
Crane stability depends largely upon the integrity of the supporting ground. The two factors contributing to the ground integrity are: soil bearing capacity and ground levelness A detailed assessment of ground conditions is required prior to setting up the crane. All different load cases and increased soil bearing pressure due to wind loading on a long boom should be considered. A hardstand area needs to be constructed at each turbine to provide a stable and level area to position the crane to lift the WTG components. Rated capacity charts of the cranes are developed based on the cranes standing on a level, firm, and uniform supporting surface. Hardstand with additional matting and shimming may be required to get the crane level within manufacturer’s specifications. Hardstand design is influenced by a range of factors including, the site area for the crane, expected bearing pressure under the crawlers/outriggers, soil bearing capacity of the site, lifting, assembly and disassembly area of the crane, dimensions of the outriggers and outrigger plates or crawler tracks and matting, slewing area of the crane and number and positions of auxiliary cranes involved in lifting/assembling.

Load positioning and control during lifting
Wind turbine blades, tower sections and nacelles are required to be lifted to extremely high elevations. Wind at higher elevations may cause swinging, rotation and accidental collisions of the load with the WTG structure or indeed the crane itself. Considering the height of WTG, very long taglines may be required to ensure preferable pulling angles. The surrounding area of the tower will be required to clear of obstacles for the persons guiding load to walk safely with the tagline. Usage of manual taglines are challenging in this situation. Specially designed tugger systems may be required to control the load during lifting and accurately position it during installation.

Limited visibility
One of the significant challenges faced by crane operators is limited visibility. Large components can obstruct the operator’s view during the lift. When lifting WTG components to extremely high elevations, the crane operator will not have a clear visual of the mounting point or landing location. Given this, radio communication between the signallers on the tower and the operator is extremely important. Additional equipment such as CCTV may be required to aid the operator.

Complex lift
Major lifting operations involved in the WTG construction are:

• Upending of tower sections: Multi-crane lift
• Lifting of rotor blades
• Lifting of Nacelle

BS 7121-1:2016 states, “lifting operation where significant hazards have been identified with the load or with the working area or access route of the crane, and the crane is used to lift complex loads or persons, or where two or more cranes are used to lift the load, or where the lifting operation is carried out at a location with exceptional hazards”. As per this definition, most lifting operations of WTG components fall in the category of complex lifts. Complex lifts require a higher level of engineering, planning and supervision. All movements and situations, communications and signalling should be meticulously planned. A method statement detailing the safe system of work (SSOW) for the transportation, movement, assembly, erection use and dismantling of the crane at the site should be developed by the appointed person (AP).

Compliance with safety regulations
Lifting operations in the onshore wind farm construction must comply with the safety regulations in the region and follow the best practices in the industry. In the UK, Provision and Use of Work Equipment Regulations (PUWER) 1998 and the Lifting Operations and Lifting Equipment Regulations (LOLER)1998 are to be complied with to ensure the safety of personnel who use lifting equipment. BS 7121-1 is a British standard that provides a code of practice for the safe use of cranes. It offers guidance on complying with the requirements of LOLER and PUWER. It is mandatory for the crane to have a report of ‘thorough examination’ under LOLER. Crane operators should have a valid Construction Plant Competence Scheme (CPCS) card, driver’s licence based on the crane weight and operator training certificate.

Any crane with a height of 100 meters or more above ground level (AGL) must be notified to the Civil Aviation Authority (CAA). Depending on the distance from an aerodrome and the crane height there can be additional requirements for lighting and marking. CAP 1096 provides guidance on this topic. These regulations are critical to the industry because they ensure the safe and effective use of lifting equipment, which is required for the construction, maintenance, and operation of renewable energy plants.

Conclusion
In conclusion, the heavy lifting of WTG components presents a series of unique challenges spanning complex lifts requiring special equipment, environmental and infrastructure limitations, and regulatory compliance, and safety concerns. Strategic planning, investment in specialised resources, and collaborative partnerships with heavy lift companies and crane manufacturers are therefore essential to address these challenges.

References
1. BS 7121-1:2016-Code of practice for safe use of cranes
2. ESTA-Best practice guide for transport and installation of onshore WTG systems
3. CAP 1096-april-2021
4. Demag CC3800-1 Operating Instructions
5. ICSA N004-Mobile Crane ground preparation for Wind farm construction