Good to see you posting again Stephan.
Not long back I set up a QA/QC/NDE program at a wind tower shop. The amount of weld depends on the tower design. Currently the majority of these towers are built in four to five can sections. The client in question had several contracts to build these towers, and through them, I had the opportunity to see several other shops as well.
Your caveat of "highest quality welds" is a current industry problem. Quality in general is a problem industry wide. This is also drawing the ire of green friendly persons who no longer back wind as they once did.http://www.youtube.com/watch?v=CqEccgR0q-o&feature=relatedhttp://realneo.us/blog/jeff-buster/wind-turbine-gears-are-problem-in-lackawannahttp://www.wind-watch.org/news/2008/01/15/vestas-turbine-collapse/http://www.businessweek.com/globalbiz/content/aug2007/gb20070824_562452.htm?chan=globalbiz_europe+index+page_top+storieshttp://news.bbc.co.uk/2/hi/uk_news/england/cumbria/7168275.stmhttp://www.2004ewec.info/files/23_1400_keijitakahara_01.pdfhttp://www.journallive.co.uk/north-east-news/todays-news/2008/01/12/safety-fears-as-turbines-toppled-61634-20342104/
I could go on, but I think you've got the idea.
In my opinion, there are four primary considerations outside the grid considerations.
1. The turbine blades themselves
2. The gear box and generator
3. The tower that the turbine assembly rest on
4. The sit preperation and base.
Composites are the current wave for the blades. It also so happens to be something that there is not a lot of precedence to go on. It was thought that using tech from aircraft props was the fix all. That has turned out to be a false assumption. It's like building a 1/10th scale model. Scaling up to 100% of the desired size does not always work. There are many concerns for the blades, but getting the blades to hold together is the primary one. The tech on that scale is fairly new, the inspection criteria is also fairly new. Therefore there is a learning curve to it all, and one that is currently being faced by many wind companies as we speak. there are Homogeneity issues in that isotropic/anisotropic substrate interfaces are causing failures due to design issues, and lack of understanding as to how to properly inspect the final product.
The gear box and generator designs have been seriously complicated. There have been many problems with the machining in question, but for the most part, it's been an engineering issue.
The welding and inspection issues for the towers themselves are another story entirely. To this day, there is not a single world wide agreement on how to go about designing and welding a tower. I've witnessed some companies using D1.1 as the basis, but at the same time, cherry picking which verse they wanted to use to the point they may as well have used no standard at all. For all the facilities I've visited, there seemed to be one prevalent problem when it comes to engineering, welding, and inspection.
There are some high quality welds out there, but there is a disturbing number out there that are not. Without an enforced standard, I suspect there will be a lot more turbines meet the fate of the one depicted in the first link. At one of these facilities I noted not a single welder had been qualified. The joints themselve had not been tested, and that it was basically willy nilly get ir done that caused a tremendous problem for this company. As for the inspection, it wasn't much better.
There were some that could meet the quality concerns, but could not meet the production schedule.
The single most problematic issue is where do you store this power? To this day, that hasn't been answered. If the power could be stored and released later.
Until that problem is resolved, the idea of green energy will be just that, an ideal. Wind, Solar, Tidal, all of them face that one common problem. If you could store the power produced during off peak hours, and deliver it during peak hours, then green energy could in fact take over.
It's late here, so going to turn in. BTW, each of the tower segments are typically made up of 5 to 6 individual cans with a transverse weld in it. each tower section has a flange on either end. For a 200+ foot tower, the base would be in the neibhorhood of 12 to 16 feet and getting narrower as it goes up. The width of the individual can also narrows as it gets on up into the higher segments. By the time it gets to the top, it's been stepped down to the neibhorhood of 8-10 feet in diameter. That should give you a close approximation of how much Butt welding goes into it. The individual platforms and gear add more weld, but they are typically fillet welds for those.
My opinion for what it's worth,