Alternative counterweights: an unusual use of the auxiliary crane

Use of the tailing crane as counterweight. Picture copyright Helling GmbH & Liebherr

If you are familiar with the huge cranes used in wind farm construction, you will know that a lot of time is consumed rigging and derigging the main crane.

Part of the effort is in the manipulation of the ballast (the counterweight needed to operate safely). The cranes use a modular ballast concept – different steel slabs are added or removed depending on the task to be performed.

These slabs are very heavy – the ones used by Liebherr for instance weight 10.000 Kg each and depending on the crane configuration and the type of lift you can have more than 20 slabs.

Several transport trucks back and forth trips are also needed to move these slabs around the wind farm.

It is important to know that usually the majority of the ballast is needed when the main boom is lowered to the ground (for instance to be dismantled) or it is lifted before the start of the erection works.

I have discovered that a German crane company (Helling GmbH) has developed an alternative technique. It seems that this solution has the blessing and approval of Liebherr, so maybe you want to consider it for your project.

Basically they used one of the hydraulic auxiliary crane as ballast.

In wind farm construction there always at least one “tailing” crane, helping the main crane rotating the wind turbine components in the correct position or assembling and disassembling the main crane itself.

They have had the idea to connect the auxiliary crane to the main crane (in this case, an LR 1600/2 erecting a Senvion turbine with hybrid tower).

The solution worked perfectly, and the 150 meter boom was lowered and lifted without problems.

Wind farm testing and commissioning

This is a short (and incomplete) summary of the main test which are usually performed in a wind farm.

Test can be divided in 3 categories: factory tests, site tests and performance tests.

Some test are performed before the start of the construction works, others during construction and commissioning and others when the wind farm is completed and producing power during the defect liability period.

Factory tests

These tests, usually called FAT (Factory Acceptance Tests) are performed during the manufacturing of the WTGs and the other main equipment of the wind farm (such as the substation main transformer).

On the WTGs side, the most usual one are:

  • Test on towers (dimensional inspection, coating, non-destructive reports, etc.)
  • Electrical components (generator, transformer, converter system, etc.)
  • Mechanical components (gear box, yaw and pitch systems, etc.)

For the BoP, you will test at the very least the main transformer and possibly the MV cables.

Site acceptance tests

Site  acceptance tests can be divided in test on commissioning and test on completion.

The “commissioning” of a wind turbine is a setoff activities performed to confirm that the wind turbine has been correctly installed and it’s ready for energy production. You normally need to have  the grid connection to do the commissioning – this means that the wind farm substation (or the connection to the grid) should be ready.

A very long list of items is checked at this point. Some of the key ones are run test with the WTG connected and producing power, verification of protection systems, test of power measurements, plus many mechanical tests.

Basically, you want the turbine to work and produce many hours in a row (200, 300 or more) without faults. It can lead to delays if not enough wind is available to perform the test.

There is also a separate commissioning for the main  transformer, the substation (protection systems, power measure equipment, MV switchgear) and the cables.

Test on completions are usually for the full wind farm.

The whole system has to work without failures for many hours generating power. Among other things you want to confirm that the main transformer can evacuate correctly all the power without overheating, abnormal losses, etc..

SCADA system is assessed as well.

Performance test

This group include test like availability, power curve and acoustic noise level.

“Availability” of the whole wind farm is assessed.

Availability means that the wind farm (and each and every wind turbine) is operating for a relevant percentage of time (95%, 97% or even more depending on the contract).

Power curve is the relation between the wind and the output of the wind turbine. It is critical that the WTG produce as much as expected – otherwise the basic assumptions behind the business model of the project will be wrong.