Tilting windmills

A new market segment for the lifting industry is appearing on the horizon – the offshore erection of giant windmill parks.

Windmill producers have developed, or are in the process of developing, 2MW to 2.5MW wind turbines. These turbines are simply so big that onshore erection is no longer economically viable. The architecture of the onshore landscape does not allow the turbines to produce enough energy to give a profitable return on investment.

Offshore, however, the wind is much better received by the turbine, and there are enough windy days to make turbines a profitable business. And as there are no complaints from neighbours 25km off the coast, the largest possible turbine is the obvious choice.

It is a daunting task to erect these turbines. The hub height is between 50m and 60m, and the turbine (or nacelle) weighs 80t to 100t. The blades give the turbine a diameter of 80m, and total weight is approximately 230t to 250t with tower, nacelle, hub and blades. These weights do not begin to compare to the weights of the oil industry’s offshore modules, but the turbines are extremely delicate and the slightest deflection or bump may damage the nacelle car body, split a blade or dent the tower. Costing $2m to $3m each, and with the difficulties delivering new parts or indeed a new turbine if wrecked or lost at sea, the ability to guarantee safe delivery on time is essential to the customer. The method used to erect the turbines must therefore be fast, safe, efficient and economically viable.

The European Union’s policy on alternative energy has recently been updated, and the onus is now placed on the producers and end users of energy to buy at least 20% of their energy from non-fossil fuel sources or face a fine. This policy makes it more attractive for electricity producers to move into the wind business. As electricity producers are already in fierce competition over prices in the new, deregulated electricity market, the size and capacity of the turbines are important factors in the price passed on to end users.

The choice to look to the sea is therefore obvious. There is plenty of space, plenty of wind and no problems with neighbours. The only question is: how do we actually get those huge windmills out there? Not all offshore conditions work in favour of wind turbines. Where you have good wind factors, you also have frequent, high waves – the very thing you don't want when you are lifting from a floating platform or crane. A very heavy load, such as an offshore module or a jacket, has an enormous amount of inertia due to its weight, therefore you can position it at a reasonable wave height. But even an 80t load has very little inertia compared with the crane lifting it, making it extremely sensitive to motions due to waves or wind. Every movement from the crane barge will be transferred to the lift, thus making it a very delicate task to position it on a 4.5m diameter foundation, with two or three people standing on it.

To add to the difficulties of lifting, even assembling the turbine offshore is very difficult. Imagine being on a free floating barge with a crane on it, and then trying to lift a 30m tower section from horizontal to vertical, with all other components for the turbine lashed down around you. You are predestined to bump into something.

These are some of the reasons why most crane companies have decided to assemble the turbines onshore, then load them upright onto a barge, float them to the site and finally lift them on to the foundation in a single 250t section.

This, however, requires the turbine manufacturer to redesign a number of components. The slew ring must be able to hold a weight of 130t as the tower is attached to the nacelle when lifted. The gearbox and generator, which are normally used for lifting the nacelle, must be made much stronger in order to carry the added weight of the tower. The nacelle frame must be stronger, and so on. This in turn increases the cost of the turbine, thus making it less attractive as an investment.

The fact that both on and offshore cranes are needed, along with tugs and barges, makes it even less likely to be a financial success, not withstanding the fact that it seems unlikely that anyone can get away with carrying six turbines with a tip height of 140m and a centre of gravity 65m above sea level. A wind turbine is, if nothing else, defined by its ability to attract wind, and six of them can make a sea journey like this extremely exciting.

A second method of erecting the turbines includes a small jack-up rig and a large lattice boom crane. This way, you lift the crane barge out of the water and you have a firm stand on the sea bed. The problem is, however, that the barge itself is still floating, and those familiar with lifting operations on oil platforms will know that only minor waves can make it a very dangerous and bumpy operation to lift a container from a supply vessel. Imagine the damage you can inflict on the fibreglass car body of the nacelle from a single touchdown on the barge when lifting. It will require major work on the nacelle to replace the car body, and thus delay the time of delivery considerably.

This method can have further drawbacks, as has been seen previously in the more hostile waters off the Danish west coast, where a small jack-up rig capsized and sunk during installation of measuring equipment. The current over the sandbanks was so strong, that the surface below one of the supporting legs was washed away within hours, and the jack-up was lost to the sea. This could very well happen during lifting operations and it is possible, that a number of offshore sites are in areas similar to this, with strong currents, high tidal waters and other factors that have to be taken into consideration when erecting turbines.

One of the more peculiar concepts for lifting wind turbines has been to use a blimp or balloon to position the different parts on the foundation, and weighing about 100t at the most, this seems feasible, although wind and other factors concerning the lifting method should be determined. The site conditions will have a great influence on the ability to deliver the turbines to a set date. This criteria is crucial to the electricity producers, as their government grants come tied with conditions. The producer – the electricity company – must have the turbines up and running before a fixed date to receive the grant, or else the project is not eligible for alternative energy source funding. This is outlined by the Danish ministry of environment, for example, in its energy plan for the year 2030. Here it is stated that a total of 5,500MW of energy should be produced by wind turbines, of which 4,000MW should be from turbines erected offshore.

Danish power companies have plans to build six offshore windmill parks, each generating 150MW. These must be built by 2004 to qualify for government grants. In other European countries – Sweden, the Netherlands, Finland and Ireland for example – there are similarly ambitious plans.

A wind farm that generates 150MW is estimated to cost between $170m and $240m, so aggregating all the various plans around Europe leads to a market with considerable appeal. The funding is substantial, and the fixed delivery date non-negotiable, therefore contractors will face serious claims for any delays occurring, regardless of the reasons.

The total offshore market can only be roughly estimated, but if a good method of erection is found, the more uncertain projects could be made feasible, giving a total in Europe and Scandinavia of more than 30,000 MW over the next three decades.

What is the best way to lift offshore wind turbines? Is there a solution using existing technology? Or is a new invention needed to make this market segment financially viable – and, if so, how much will it cost? We know by now that the price of erecting the rather small offshore turbines already in production (600kW), has been extremely high and – so rumour has it – that they only got out there thanks to heavy subsidies. This will not continue in the future as the electricity market has been deregulated.

We also know that a number of both on and offshore crane rental companies are putting a large effort into being the one to come up with the solution.

What will the solution be? Ideally it would look like erecting a turbine onshore, at the same price, or less, and using the same amount of time. Is it at all possible to find a solution that is economically viable? Probably.

But the point is, to use the words of Pete Stoof, director of Mammoet Innovation & Engineering: “You have to think of the problem as moving a load from A to B, not as a question of what size the crane should be.” In other words, redefine the process, how could the task be performed, could the existing equipment be used in a different manner, and do we need to design new tools to solve the problem at all? The wind turbine industry has to a large extent pushed the sizes of cranes, particularly telescopic ones, upwards. With the offshore market now evolving, it will be another trendsetter for the lifting industry, since it is here that the largest cranes and hoists will be demanded in the near future.