Dear Sirous, if there are 20 to 30 turbines minimum recomendet number of boreholes should be minimum one for each turbine. The number and depth of drilling can be estimated with  reccomendations in Eurocode 7.Sometimes maybe more or less investigations can be accepted, but it depends on the foundation media and its heterogeneity. If the  foundation media is composed of soil  with low bearing capacity, the CPT test can replace drilling, or some  geophysical methods can support driling. Also, pile testing is necassry to checked validity of design approach.Any how, from the practice it is  known that the cost of investigations for most risky structurs as dams are not higher than 2 to 5 percenst of structure costs.  I don't have data for cost of some types of wind turbines, but I suggest to folow the logic that investment in investigations is not so high compared with cost of possible consequences.  

The question is how to balance the cost of foundation and cost of information. Where do we stop and say we have enough data?

Dear Sirous,

Very good question. One approach in considering high degree of soil property (and other uncertain parameters) is to use a reliability based design optimization (RBDO) approach. 

We can obtain the optimal designs for different values of probability of failure (or reliability index). Plotting the costs versus reliability index provides us with information on relation between costs and associated reliabilities. The cost of obtaining additional data can also be considered.

It is up to decision makers to choose one design based in their preferences.

These articles deal with RBDO approach in pile foundations.

Overgård, I. E. (2015). Reliability-based Design of a Monopile Foundation for Offshore Wind Turbines based on CPT Data.

Zhao, W. Y., Xu, Z. J., & Zheng, J. J. (2014). Optimization design of piles subjected to horizontal loads based on reliability theory. Journal of Central South University, 21, 2928-2934.

Best regards,

Behrouz

P.S. my brother's name is Sirous as well.

http://link.springer.com/article/10.1007%2Fs11771-014-2259-y

Hi Behrouz,

Would you please convey my admiration to your mum and dad for their choice of name for their children.  

Thank you for your answer and references.  I will read them and come back to you. Dr David Cathie  of  Cathie associates  (http://www.cathie-associates.com/why-us/) asked me this question. He is looking for an economic valuation.  This can be done using the Real Option analysis to a certain extent (see my recent publication).  The problem is,  foundation on its own has no underlying value. However, if it is part of a wind turbine, then entire system can bring in cash flow.  For example a house has value as you can rent it to get some cash. But its foundation, or its walls and roof on their own don't have value. Perhaps they have some salvage value.  When you put foundations, walls, floors and roof in right order, then you get something which has value, a house. 

One way  to answer this question is to assume the cost of turbine and shaft in a wind turbine is deterministic and only the foundation cost is stochastic.  I am unsure if the reliability method can tell me the reasonable level of expenditure for acquiring more  data. As you see I am still grappling with the problem and trying to get my head around it. Nowhere close to answer. 

Sirous

Dear all,

Thank you for your contribution. I need to explain more hoping to get more opinon on this question. 

My question is one of the decision making that needs to be made in the context of the foundation design for offshore renewable projects. We start with very limited ground data, then collect more progressively with geophysical and geotechnical surveys. In the middle of the data collection, the concept or FEED design has to happen (so with partial data). Then the invitation to tender is made  and the EPCI contracts start, taking over the data already collected and usually completing it. But the contract price will be affected by the limited data in one way or another.

The question is how the EPCI contractor should price the foundation so that to have chance staying  in the game, or loosing money due to lack data. This is a question of decision making under uncertainty.  The contractor must make a decision now and knows that soil data is inadequate. 

Sirous

Dear Sirous,

It is very heavy decision, because all the time the contractor is in the circle of uncertainty.  Usual opinion is that the lack of data leads to design solution which is on a safety side, but you cannot know this if you don't have data for all zone of interaction. 

The question is if the contract type allows you to prepare in a first phase (or parallel with construction) on the foundations where you don't have enough data control drilling and to redesign the solution or now. In fact, no one can be sure how large is the risk of overestimation or underestimation knowing that soil data is inadequate. 

Dear Milrod,

Thank you for your contribution. 

This is exactly the question that EPCI contractors are facing every time they bid. Some go bankrupt  but most don't . This is due to the fact that every contractor is facing the same problem, thus they allow this uncertainty in their price- some more some less.  If they don't they  allow for uncertainties, there is a chance of loss. 

This question is asked by a clever contractor who wants to know how much money to allow for such uncertainties; enough to cover the likely loss, but not too much to lose the bid.  

I am not so sure if there is an answer to this question, but I haven't given up yet.

Cyrus the great

Pareto principle may be helpful.

The Pareto principle has many applications in quality control.It is the basis for the Pareto chart, one of the key tools used in total quality control and six sigma. The Pareto principle serves as a baseline for ABC-analysis and XYZ-analysis, widely used in logistics and procurement for the purpose of optimizing stock of goods, as well as costs of keeping and replenishing that stock. Rushton, Oxley & Croucher (2000), pp. 107–108.

The Pareto principle was also mentioned in the book 24/8 - The Secret for being Mega-Effective by Achieving More in Less Time by Amit Offir. Offir claims that if you want to function as a one-stop shop, simply focus on the 20% of what is important in a project and that way you will save a lot of time and energy.

sometimes we should keep our distance to problems then we could survey it totally.

Hi Mahmood,

Thank you for your post.  

PS: The one who is great is under a stone near Shiraz. This one just carries the name not the glory. 

Dear Sirous,

I believe, that you have a misconception. A wind turbine is without foundation without value! You can them badly in the hand hold, if it turns !? The service life of the foundation will normally exceed that of the wind turbine. The profit from offshore installations have a dependency on the costs of foundation . It may be that wind farms display no longer profitable due to high foundation costs.

As an attachment I have added you a dissertation, that this is excellent describes and a computer program provides. Unfortunately, the dissertation is in German,  but the summary is also available in English, so that you can capture the content immediately, at least. Perhaps you can sit down with the author in conjunction.

I hope have helped you, a little.

Best regards

Michael Lersow

Dear Michael,

Thank you for your contribution.  I highly appreciate your and other friends thoughts on this issue. 

You are absolutely right that a wind turbine without foundation doesn't bring in cash flow hence no economic value.

Lets look the problem this way. For the sake of simplicity assume that a wind turbine consists of three parts, a turbine, a shaft and a foundation. The value of each is their salvage value, which is much less than the investment.   If you put these three items together, and in right order of course,  you get something which can bring in cash flow. 

One avenue that I am exploring is assuming the cost of shaft and turbine is fixed, but the cost of the foundation is uncertain.  This allows to perform financial assessment ( such as NPV and Real option valuation.  If you assume the environmental loads (wave and wind) are also uncertain then you add another layer of complexity.  Perhaps a Monte Carlo simulation on NPV is a better way to get some insight.

Dear Sirous,

I have designed and constructed some wind farms, especially in and on abandoned surface mine sites. My part was the foundation in a very bad building ground but the location has very good wind conditions. The tender referred only to the foundation. This has to consider the very bad building ground,, and the costs for foundation has been very high. The wind farms works effective and brings profit. The owner has payed for both foundation and windmill. So, now are the windmills worned, the foundation is o.K. If the site should be sold, which price you will be take??? Naturally a high price and you'll get it !!! Because the buyer judged the site as a whole and a profit perspective operates. You must see: site, foundation and windmill together and always under the point of view:  the profit-perspective of the whole facility. No facility is the same how the other

I hope, I could help you a little

Michael Lersow

Thanks Micheal. Very good points to think in over. Keep them coming.