Wind Turbine Construction
As wind turbines have gotten larger the cost of electricity produced per megawatt-hour has decreased. But the logistics of construction have become more difficult. Many of the components are now so large that transporting them requires police escort and rerouting of traffic. The large cranes that are required to assemble the turbines from the trucked-in pieces are themselves trucked to the site in pieces. But, for offshore installations, component transportation can operate at the more appropriate scale of shipping.
At sea, the wind is stronger and steadier. Floating wind turbines, it is reported, could produce the total electricity demand of the planet many times over. Many of the transportation factors of on-land installations could be eliminated for installation at sea. There are other factors at sea that can be more expensive, but some of those may be amenable to improvement. Here we are trying to outline a minimum cost approach.
We can arbitrarily separate our floating wind turbine inquiry into four separate study areas:
Seabed anchorages and tethering systems.
Concrete foundations from coastal quarries.
Turbine assembly at large harbor facilities.
Joining technology - turbine to foundation, at sea.
Anchorages: At the location of any future wind turbine array, the sea bottom anchorages would be installed ahead of the foundation arrival. Appropriate anchorage at every site will depend on the nature of the seabed. Concrete anchorage blocks could be floated out, sunk, and heaped with rock (see anchor block sketch). Steel ship anchors could be employed. Tiedowns might be drilled into the sea floor so that the weight of the sea floor provides the required restraint (see submersible drill rig sketch).
Tethering Loops: In any case floats could be attached by loops of light rope to the seafloor anchorages. When the foundations arrive, the light ropes would be used to pull heavy rope loops or steel cable loops into place to secure the foundations. Throughout the life of the wind turbine, loops will allow easy replacement of the tethering lines.
Concrete Foundations: Several considerations argue for concrete foundations. Steel corrodes in sea water. Protective coatings eventually flake off, causing pollution. Mining iron ore, smelting, rolling, and fabricating are all energy intensive. For floating foundations, steel is an expensive choice; and it will eventually deteriorate.
In considering the foundation for a floating wind turbine, stability is paramount, and inertia provides stability. The heavier the foundation the more innately stable. Concrete is heavy, cheap, and available around the world. Importantly, concrete does not corrode in sea water and protective coatings are not required. Some Roman concrete survives to this day.
Hard rock mountains meet the sea all over the planet. At some of those coastal junctures, quarries have been established to mine the rock and make sand and gravel. The economics of these operations are so good that shiploads can be delivered to metropolitan areas thousands of kilometers away.
Coastal quarries are our starting points. Concrete is four-fifths sand and aggregate. Foundations for floating wind turbines could be most economically manufactured right at coastal aggregate quarries. Another starting point is no rebar. We are building for an unlimited service life. The only steel would be the anchor bolts that attach the turbine to the foundation. And these would be stainless or better.
There is no limit to the possibilities of the design of concrete foundations for floating wind turbines, so our candidate design is just for discussion. Assume a cluster of six round concrete chambers. Manufacture could be straightforward using modular forms. Launch could be by box jacking techniques.
Candidate concrete foundation manufacturing procedure:
1. A heavy, dead flat, “launch slab” is poured. A jacking backstop is built behind.
2. The jacking slab is covered with bentonite slurry and that is then covered with plastic sheeting.
3. The bottom slab of the foundation is poured.
4. Using modular steel forms, a configuration of six hollow cylinders is raised on the bottom slab.
5. Anchor bolts are placed in the central area.
6. Six silo covers, strong enough to withstand wave impact are installed.
7. Several post tensioning bands are wrapped around the top slab and tightened.
8. The foundation is outfitted with instrumentation, pumps, etc.
9. The completed foundation is jacked off the launch slab into deep water and moved to floating storage. Any number of completed foundations could be stored, floating in protected waters.
Positioning: The foundations are manufactured from concrete, the cheapest construction material, at the cheapest source of the main ingredients. As needed, each foundation is pulled from storage and towed straight to their permanent location. Floats from the pre-positioned seafloor anchorages are gathered. The light loop lines are used to pull into place the permanent anchorage loops. A balanced tensioning is performed. Next the turbines must be built onto the foundations.
Comment re assembly at sea: It is, of course, possible to bring all the individual components to the site for assembly at sea. Jack-up crane ships are used to build turbines onsite in fixed bottom installations. In those cases, the turbine foundation and the jacked-up crane-ship are unmoving. Still, fitting the blades at height, in the breeze, is tricky. A similar procedure, with both crane and foundation floating, would be formidable; not impossible, but difficult and expensive. An easier method is needed.
Turbine Assembly: The components of today’s wind turbines come in shiploads from around the world. There are many requirements for a wind turbine assembly area. First, a docking facility for seagoing ships. Second, storage area for shiploads of large components and a work area for turbine assembly. Also, a sufficient skilled workforce must be available. Such facilities are not likely to be coincident with coastal quarries. So, generally, the foundations and the completed turbines will come from separate facilities. To minimize cost, they should meet and be joined at sea, at their final permanent location.
Joining Technology, NO: For the floating wind turbines of the future, to assemble onsite at sea the crane will be floating and the foundation will be floating. The turbines of the future will be higher. Fitting a 115 m (about 377 ft) long blade to the turbine hub 150 m (about 492 ft) up in the breeze when both the crane and the tower are oscillating should not be anybody’s first choice. A better way would be to assemble the turbine complete, blades and all, at the dock, in the harbor, and bring it to the site whole.
Joining Technology, YES: The transport vessel has the turbine in horizontal position. The tower is 150 meters feet long with three 115 meter blades. The transport ship has the capability to raise the tower to vertical.
Both the foundation and the transporter each weigh many thousands of tons. Each is moving on the waves. In comparison to the magnitudes of these uncoordinated moving masses, the parts that must be joined are relatively fragile. A careful procedure is called for. We must prevent breakage at the tower pickup point or at the tower to foundation base plate.
The overriding hazardous condition is the random movements of the floating foundation and the tower carrying vessel. Both are massive. A major simplification to the tower installation procedure would be to lock the two structures together so that they move as one. Then, the erection of the tower would be fairly straightforward. But totally unifying those two structures seems just too difficult. We leave that as a desirable possibility, and meanwhile investigate a compromise procedure. Let’s just lock them together around the roll axis. They would still pitch independently.
Engaging the roll restraint system and erecting the turbine would consist of several steps.
1. The floating foundation is built circular. This allows the circular post-stressing tendons to create a uniform radial compression. The transport vessel has a similarly radiused convex bumper. The only foundation modification from minimal is the adding of additional foundation bolts to provide alternate anchorages for the “female hinge half”.
2. The transport vessel noses up to the floating foundation. Adjusting for the wind and waves, the relative orientation of the transport vessel and the foundation can be at any of twelve positions, whichever is closest to optimal. A small crane on the transport vessel drops three loads into place on the top of the foundation. This is the three piece “female hinge half”
3. First, the crane places the two hinge back pieces onto their hold-down bolts. Personnel leave the nuts slightly loose on the bolts pending total assembly. The hinge front piece is craned into position and bolted to the back pieces. All the bolts are tightened in a developed sequence until the total assembly is solid and is bolted rigidly to the foundation.
4. Two large, very rigid arms swing out from the transport vessel and clamp onto both the outboard ends of the female hinge assembly. The clamping mechanism is a hydraulically powered large male taper into a large female taper. The two floating structures are now locked together for roll and the centerline of the vessel is aimed at the centerline of the tower mounting flange.
5. The “male hinge half” was attached to the bottom of the tower at assembly in port. The tower rests on a cushioned bed in a cradle that can slide longitudinally a small amount. The cradle is raised by an exceptionally large, trunnion mounted, telescopic, hydraulic cylinder. It is sea water powered. Yes, it is huge, but it is much simpler than a giant floating crane.
6. Raising the hydraulic cylinder pivots the male hinge half on the bottom of the tower. When the tapered male parts align with the tapered female hinge openings the longitudinal slide is activated and the two hinge halves slide together. Immediately tapered hinge pins are hydraulically inserted. Note: the tower arrives strapped to a cushioned bed. The straps are released at the hinge joining to allow a small amount of “squirm” as the tapered hinge pins slowly force alignment. When the hinge pins are home, the straps are retightened.
7. At this point the clamp axis, the hinge axis, and the trunnion axis are all parallel. The two floating structures are locked for roll but they are pitching independently. The cylinder raises the wind turbine to near vertical. Openings on the tower base plate pivot down onto steel guidance cones on the foundation. But the raising cylinder must hold back from closing the gap between the tower and the foundation, lest movement of one or the other of the floating structures close the gap too forcefully.
8. The last proviso is a ring of several “catch cylinders” on the female hinge. These extend and hook into a grabber ring that extends around the back of the tower. They gently lower the tower the last few degrees of rotation as the straps holding the tower to the cradle release. The foundation bolts are tightened all around.
9. The male hinge half is unbolted from the tower. The transport vessel unclamps itself from the female hinge half. The three parts of the female hinge half are unbolted from the foundation. All the parts are craned back onto the transporter. The ship and the foundation are separated.
10. The transport vessel returns to harbor for another wind turbine.