7 minute read
Creating safe lifts to hit renewable targets
1 From Wind power Monthly
Author: Michelle Verkerk, CICA The Crane Industry Council of Australia
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Wind farming is part of the Australian Government’s renewable energy plan to reduce the country’s dependence on fossil fuels. Wind energy is one of Australia’s primary renewable energy sources, generating enough electricity to meet 7.1 per cent of the nation’s total electricity demand .
Cranes are crucial to renewable energy targets, as the weight of current turbine designs require large mobile cranes. As well as the suitability of crane size, serious consideration must also be given to wind speeds and location when planning the lift.
Wind farms are typically erected on hilltops and sea cliffs, which can be difficult to access and pose challenges by wind speeds and ground conditions. The locations are selected to harness wind power, which is excellent for meeting energy objectives but is not without issues regarding lifting objectives.
WIND EFFECT CALCULATIONS
Mobile crane operations are subject to excessive wind forces that pose risks during lifting, erection, servicing, and dismantling of wind turbines as parts to be lifted are usually heavy and have a large surface area. The mass and shape of the load plays a major role in how much influence the wind will have on the lifting operation.
Calculating the wind effect on lifting the wind turbine parts:
⊲ Suppose the wind surface area (multiplying the load surface area by the drag coefficient) of the load is less than the total allowable wind surface area (equals to load weight multiplied by the wind surface area to load ratio specified by the crane manufacturer).
In that case, no further calculations need to be made. ⊲ (More likely) if the wind surface area of a load is greater than the total allowable wind surface area, further calculations need to be made to determine the permissible wind speed. The Permissible Wind Speed can be determined by calculating allowable wind surface area, dynamic wind pressure, wind surface area, wind speed and air density. This resultant permissible wind speed should be compared against the maximum operable wind speed nominated by the manufacturer for the required crane configuration. The manufacturer’s nominated maximum wind speed limit should not be exceeded.
Before starting lift operations, the crane operator should determine the expected maximum wind speed on site (for example, by checking a trusted weather website (http://www.bom.gov. au/vic/observations/melbourne.shtml), if the forecasted maximum wind speed exceeds the permissible wind speed for the lift, the load should not be lifted, and the crane may not be able to be erected. Wind speed on site should be measured and compared with the operator manual’s load chart wind speed or the calculated permissible wind speed to see if it’s safe to operate the crane under the site condition. (It should be noted that the wind speed can vary between ground level and the boom tip or load height.
Using anemometers attached on a boom tip can be an excellent way to assess the conditions.)
Calculating wind load impact on crane operation is complex, and for exceptional cases, the lift designer should seek the crane manufacturer’s instruction on wind load calculation as the crane manufacturers have better knowledge of the static/dynamic stability and the specific crane’s limits.
Other than the calculations, when it comes to wind effect, another area worth noting is freely suspended loads (especially loads with large wind surface areas like wind turbines), as they can swing or rotate in the direction of the wind during the lift. This swing or rotation must be managed or controlled to reduce the lateral load against the crane or the boom.
Riggers/doggers on-site usually use tag lines to control/manage the swing/rotating action caused by the wind. A tagline is a rope attached to a load during a lifting operation to allow the riggers/doggers to control the swinging and/or rotation of a suspended load. They should not be used to pull a load out of its natural suspended line, inducing in-haul or out-haul of the load lines or hold a load against wind forces trying to push it out of line.
WORKING FROM HEIGHTS
Working with wind turbines also requires working at heights, and as always, planning is key to understanding how the risk of falls will be controlled and what to do if a fall does occur. Planning should include:
⊲ Minimising the time and number of persons working at heights ⊲ Ensuring the person/s working at heights are height safety trained ⊲ Using anchorages that are rated to AS/NZS1891.4 or equivalent ⊲ Ensuring compliant full-body harnesses (with trauma relief straps) are used and fitted correctly ⊲ Preventing falls with short or adjustable lanyards and minimising the distance to fall with a retractable lanyard ⊲ Using an energy absorber that will limit shock loads to <6kN ⊲ Having a rescue plan
SHIFTING OVERSIZE AND OVERWEIGHT COMPONENTS
Transport of odd-shaped and overdimensional components is common with the construction of wind farms. Securing the load in a way that both holds the load in position and doesn’t damage the components requires careful planning and route assessment. The load
0.5W
Half the weight of the load sideways (cornering)
0.8W
80% of the weight of the load forwards (braking) (W = weight of the load)
0.2W
If relying on friction to withstand the force in other directions
0.5W
Half the weight of the load rearwards (accelerating, braking in reverse)
0.5W
Half the weight of the load sideways (cornering)
Graphic used with permission from the National Transport Commission
• 0.8 g deceleration in a forward direction • 0.5 g deceleration in a rearward direction • 0.5 g acceleration in a lateral direction • 0.2 g acceleration in a vertical direction if relying on friction to withstand forces in other directions. g means gravitational acceleration or 9.81 m/s2
restraint guide as outlined in the image extracted specifies the transverse, lateral and vertical load restraint requirements.
Once on site, the access roads and construction areas need sufficient area and ground bearing capacity to manoeuvre and unload. Recent weather conditions in South East QLD, Australia serve as a reminder that the suitability of an access road or the hardstand needs to continue to be monitored as excessive rain can cause deterioration.
Specialised rigging is used nowadays to position the various components, removing the need to select correct slings and sling arrangements. The lifting sequence needs to be followed according to the lift procedure.
Attributed to Max Cranes
GROUND CONDITIONS
The CICA Guidance Note: Crane Stability and Ground Pressure and the ICSA: Mobile Crane Ground Preparation for Wind Farm Construction are useful reference documents when considering ground conditions for wind turbine lift planning. Both documents are available from the CICA website.
Concerning ground conditions, the ICSA (International Crane Stakeholder Assembly) Guidance Note focuses on both:
⊲ Access roads - ground bearing pressure for crane weight on the road - route path (e.g., swept path analysis to assist the determination of dimensional and clearance requirements for the crane path). ⊲ Crane working areas - suggestions on hardstand construction - ground bearing pressure for different lifting configurations - ground level requirements for crane working areas
Ground condition inspection should provide permissible ground pressure, and during the planning stage, a geotechnical engineer should conduct this inspection. This inspection should assess the bearing capacity of the ground, including surface conditions, as well as the layers of the ground underneath the surface that could influence ground bearing capacity. Any ground, particularly coastal ground, can have weak layers below the surface, and these underlying layers of weak or soft ground can possibly collapse. The inspection should also outline the estimated settlement due to the load and whether the settlement would cause any instability of the crane during the lift.
If the ground is found to be not suitable, additional measures must be taken before proceeding with the task. These may include, but are not limited to:
⊲ Design measures to reduce imposed loads, i.e. re-sizing the crane mat used, repositioning the crane, reducing task loads (e.g. splitting of loads), and re-selecting the crane. ⊲ Design measures to ensure ground suitability, i.e. soil stabilisation, grouting, and dynamic compaction.
The Federal Labor Government has updated Australia’s emissions reduction obligations under the Paris Agreement to a 43% emissions reduction by 2030, based on 2005 levels, so the renewable energy transition will rely on cranes for many years. Comprehensive planning is vital to ensure the safe lifting of turbines to protect people, equipment, and the environment.
Note: Documents, guides and information provided may vary in your country.
