Three years ago I was approached by a wind turbine manufacturer and asked if I could provide a solution for installing wind turbines offshore. Turbine manufacturers had been looking for offshore solutions for some time, without promising results.
The offshore lifting industry was focused on oil and gas. Equipment was slow and depended on fair weather. The turbine industry needed a high number of installations in a short period. Prices did not encourage turbine manufacturers to continue offshore development as the number of ‘weather days’ (when the crane cannot be operated) and mobilisation rates were geared to the oil and gas industry. The turbine industry needed a solution that had a high availability rate and a low number of weather days in a fairly short season.
I sat down at home and after a couple of hours came up with a solution that is now offered by A2SEA A/S, a company set up to execute my plan and install turbines offshore.
From the outset the turbine manufacturers liked my proposal and I started the long and difficult task of marketing the idea which is now known as the craneship concept. In the first four months I only had a front elevation drawing of the craneship to show, and granted, this would have been insufficient today. It only worked because I found a market niche at exactly the right time.
The main reason for success was that I had thought through all the main aspects of the installation process. Turbine manufacturers could not identify any weak points where they could question my methods. This, they later told me, was the most convincing part of my presentations to them. I never had to say ‘I will find out later.’
Having convinced the turbine manufacturers of the concept, the challenge of finding investors began. This was not easy. At the time only dotcom investments were of interest and conventional industry was not offering comparable staggering returns. Nevertheless starting capital was secured and A2SEA was set up in April 2000.
In August 2000 tendering began for the Horns Rev project, the biggest offshore wind farm in the world, consisting of 80 turbines 16km off the west coast of Denmark.
To prove that the system worked and show exactly what the environmental impacts would be, we did a tank test. In this way we proved our technology. A2SEA finally secured the turbine installation contract for Horns Rev, and in June 2001 we started converting the first of two special purpose vessels for turbine erection.
The A2SEA concept
When operating cranes offshore it is crucial, due to the buoyancy of the vessel, the crane lifting capacities etc., to have a stable and horizontal work platform. The list and heel of the vessel could endanger anyone operating on deck during lifting, as well as the receiving banksmen.
Stability and quick, safe turbine loading and transport cannot be done by equipment normally used in the offshore industry, where every operation is a one-off and has to be carefully planned. Existing equipment is, without exception, slow and inflexible in operation. It is not easily moved from site to site and to work as planned it normally requires a number of support vessels. In short, four parameters have to be met to successfully carry out an offshore wind turbine erection.
1. Safety. To work safely is an absolute requirement and is of paramount importance. No personnel or property can acceptably be lost to the sea. Crucial is the ability to safely lift and position components from the supply vessel to the foundation. This problem must be solved before leaving port at all.
2. Availability. The ability to work in the most adverse conditions determines the best concept. Ideally the only factor stopping the erection process should be high winds. In this way it would mean the same conditions as for erecting turbines onshore. Nobody can do better than that. The chosen system should be able to be positioned firmly on site and be able to move between jobsite and harbour even in bad weather.
3. Speed. The aim is to minimise the time between loading, transporting, erecting and repositioning the turbine. The normal offshore operation is a one-off so the project will be planned with a reasonable amount of slack time for the actual transport. With as many as 80 turbines to be erected in one season, this is not possible. The concept, therefore, must be fast and self-contained, with as few links in the supply chain as possible. This way there is less to go wrong and more work can be done on site.
4. Cost. The only way to reach an acceptable level is to reduce the cost per unit. This can only be done if more units are transported at a time.
Meeting the parameters
To work safely, the platform must be stable at all times, so it follows that whatever the type of platform, it must have supporting legs or another stabilising device. We fitted our vessel with four retractable legs. But for the platform to be extremely mobile, it must be able to relocate regardless of weather conditions. This precludes the use of a normal jack-up platform, as it needs calm waters in which to jack up and down. The jacking device must therefore be able to work around that problem.
The second problem encountered in a jack up or down situation is the phenomenon of bottom slamming. This occurs when a wave passes under the hull of the platform or vessel. A wave with a height of 1.5m has enough power to damage or puncture the bottom face of the hull. To avoid this we designed a wave compensation system so that the vessel would never encounter this problem.
Speed is essential but a jack-up platform supplied by tugs and barges is extremely susceptible to bad weather. The maximum speed achieved by these craft is between about 3 and 6 knots at best. This is not enough to secure uninterrupted supply lines to and from site. It was therefore obvious to look at a vessel capable of a minimum speed of 10 or 12 knots – only seagoing craft can do that.
Price determines the success of the system. While safety and other issues cannot be compromised, price determines whether the system sells so it is important to achieve the lowest possible price. The only way to reduce price is to transport more turbines per trip, something a jack-up platform cannot do. A cargo vessel is the only type to have sufficient loading space to transport five or six turbines.
When a cargo vessel is loaded, the draft increases substantially which works against the often shallow waters in which the turbines are erected. Nevertheless, the system was designed to be fitted on an existing cargo vessel, with the length and draft exactly large enough to negotiate a minimum depth of 2.5m.
The solution
An existing self-sustained container ship (3,300t dead weight) is fitted with a lattice boom crane and four (2×2) tension-controlled legs to stabilise the vessel during crane operations. The vessel can sail at an average speed of 12 knots, cutting travelling and increasing the operational range considerably. Since components and equipment can be loaded on the ship, no other supply vessels are needed.
The main crane is for turbine assembly. Auxiliary craneage, in the form of two standard container handling cranes already on the vessel, is used to handle components, whether in the assembly or preassembly process. Each of the secondary cranes has a maximum capacity of 40t at a 12m radius. They act as tailing cranes when tilting up tower sections and as preassembly cranes when assembling the rotor. At the quayside these cranes can also be used for load outs where turbine parts just need to be presented along the quay (straight from road transport). Up to six 2MW class wind turbines can be carried and installed.
On arrival at the turbine site offshore, the tension-controlled legs are dropped to the seabed and the vessel lifted by about 1m out of the water using hydraulic winches on the legs to avoid buoyancy effects from passing waves. In this way the vessel is stable and can still jack down even in the toughest working conditions the crane can cope with. This is essential, as the problem relating to stability during jacking up and down is then solved. A jack-up rig can neither work nor move in the the same weather conditions as our vessel with compensation gear.
It is envisaged that our concept will be operational in harsher conditions than traditional jack-up systems, resulting in an ‘increased construction weather window’. Even if the weather window was the same, better use can be made of it compared with using traditional methods.
The lattice boom crane, a 450t capacity Demag CC 2500, is mounted on a 12m pedestal on the port side sponson. It is near-standard and can be disconnected from the vessel, for example, at the end of the offshore season in October. When fitted with its crawler tracks, it can work as a normal land-based crane, for example, to assemble onshore wind turbines.
Once the crane is mobilised, installation of the turbine parts can begin. Tower sections are lifted from deck, placed on the foundation and bolted in place. The nacelle is lifted from the cargo hold, placed on the tower top and bolted in place. The hub can either be installed along with the nacelle and blades or it can be installed separately, or the whole rotor can be assembled on the deck and lifted into position as a single unit. This flexibility means that erection procedures are available to suit all makes and models of wind turbine on the market today.
