It will be the first of several massive wind farms which are larger than 500MW each (meaning they can produce close to 2 TWh of electricity per year), and a price tag in the EUR 1.5-2 billion range. That price can be split roughly in 3 similar-sized bits, being (i) the turbines, (ii) the marine construction work, including the foundations, and (iii) the rest, including development costs, electrical equipment (high voltage transformer station and cables) and, if relevant, financing costs. The size of the different fractions can vary depending on the distance to shore, water depth, site conditions and regulatory framework (in Germany, for instance, you don't need to build the high voltage export cable as this has to be taken care of by the grid operator).

:: ::

Construction typically starts with installation of the foundations, laying of the intra-array cables, and installation of the foundations. The offshore transformer station and main export cable are independent tasks which are typically done as early as possible as their absence prevents the operation of the wind farm - but these are "long lead items" - ie it takes time to order them and build them.

:: ::

Offshore transformer station

In this case, the project is so vast that it has 2 transformer platforms and 3 export cables. Here's one of the platforms. It is roughly 30 meters by 30 meters by 30 meters:

:: ::

Foundations

Today, almost all of the foundations - the parts of the structure that are hammered, or anchored on the sea floor and carry the turbines - have been installed, as have most of the "transition pieces" - the intermediate part of the structure which sits between the foundation and the turbine, ensuring that the bit where the turbines are built is perfectly horizontal, and including the boat landing, cable tubes and sometimes some of the electrical equipment.

Here are some of the foundations with nothing on top:

(In the back, you can see the foundation laying vessel, a huge boat, but we were not allowed to approach, as it was engaged in manoeuvring with its anchors and the security distance was quite large)

Here's a transition piece:

:: ::

The turbines

And the same with the turbines on top:

Most of the turbines were still in the storage area at the port:

You can see the bottom part of the towers on the right, and nacelles on the left. Blades are also on site, but not visible on that picture

:: ::

As the first picture above suggests, a few of the turbines have been erected already, which means that there was activity across the different tasks on the wind farm, allowing it to see various bits of the construction. What is striking is the number of vessels engaged in work on such a wind farm; beyond a number of smaller crew vessels and various tug boats (not shown here), here's a sample.

Hotel boat

A hotel vessel for the construction crews, located on the edge of the wind farm. Workers typically spend 2 or 3 weeks in a row over there. With the site 20 miles from shore, you can save 3 hours of transfer per day for workers - plus they are not woozy or seasick when they get to their tasks...

It's like a cruise ship, except that there is no alcohol on board...

Transport vessel

>

This is a simple transport vessel, bringing the turbines from Denmark to the staging port. You can see the two tower sections (the bottom one is vertical, the second one is horizontal) and blades. I would guess the nacelles are transported by a separate boat.

Cable laying vessel

And here is a cable laying vessel. Notice the platform on the front of the boat, which can carry several kilometers of rolled up cable (in that case, the boat was not doing any installation):

With turbines typically situated 500-800 meters from each other (with larger distances in the prevailing wind directions, to limit wake effects), you typically need more internal cabling than export cable, and you need to know exactly where you put these cables, to avoid any damage (in a famous incident in an earlier wind farm, one of the vessels installing turbines jacked up through one of the cables, damaging it...)

:: ::

Installation vessels

The project was using at least 3 jack-ups at that point. Jackups are boats which are able to "stand" on the seafloor thanks to retractable legs which can be lowered to the seabed and carry the boat. Their usefulness is constrained, naturally, by the size of their legs (which limits the water depths they can be used in) and their carrying capacity, both in terms of useful loading area on board and of the capacity of their crane(s). The jackups used by the project were quite different:

The first one was a simple barge with no means of propulsion: it needs to be tugged to site. It was on its way to the construction site. We overtook it when we went there ourselves, and it still hadn't arrived when we left: the trip requires roughly 6 hours for such a barge. It can carry 3 full turbines:

As you can note, the tower sections have been pre-assembled onshore. Any work which can be done onshore is a huge time and cost saver as things are always more complicated offshore. In this case, blades must be installed individually.

The second jackup is a self-propelled vessel (ie it can move with its own engines); it can carry 2 turbine sets. It has strong lifting capacity, but the positioning of the crane between the legs makes it more difficult to load the blades onto the vessel in the harbor.

A third jackup vessel was available; note how the cranes are located on the "legs," giving them a lot more freedom to move. However that boat is also relatively small.

:: ::

There have been two philosophies to build offshore wind farms: consider that it is a wind farm project that happens to be offshore, or consider that it is marine construction work which happens to include wind turbines. In either case, construction has been complicated until recently by the lack of adequate vessels and the need to make do with equipment which had not been specifically designed for the tasks involved in the offshore installation of wind turbines. A lot of oil&gas people scoff at the wind industry people when they explain how difficult it has been to build the turbines: after all, the few hundred tons which need to be lifted when the various large bits of a turbine are being installed are rather puny compared to the massive structures built for offshore oil&gas platforms; but oil&gas people tend to underestimate (i) the logistics involved in transporting and building dozens, and now hundreds, of identical structures at sea, and (ii) the specific requirements of below-centimeter tolerances to position multi-hundred ton items 80 meters above the sea...

The land requirements in harbors for storage (but with reinforced quays to bear the loads), the carrying capacity of the vessels (to transport the parts 20km - and in forthcoming projects 100km out at sea), the transfer times, the crane specifics (to install towers, nacelles and blades, which are very different animals) make this industry a very unique one, and it is now just beginning to work out the best way to do things - and of course, all sites have different characteristics (depth, distance to shore, nature of the subsea soil, size of the turbines selected) which make standardised, universal equipment a pipe dream...

But it's happening - and it's keeping an increasing number of people busy.

There are worries about how quickly the supply chain will ramp up to accommodate the massive investment plans being contemplated; ironically, I still think that the technical side will be easier to take care of than building up the commercial teams of the various key players (turbine manufacturers, project developers, financing institutions) which must put in place the highly complex contractual framework for these massive, and still risky, projects. Right now, it seems it's the same people on almost every other project...