Hi

For the actual environmental design, there is a whole standard available EN 1991-2-4

ftp://law.resource.org/pub/eur/ibr/en.1991.1.4.2005.pdf

The monopile natural frequency is affected by the soil stiffness. If wind, flow, scouring or other eroding processes are available to remove matter around the monopile, one can expect the soil stiffness to vary with time.

Many wind tower designs are tuned to operate at anti resonance. If the soil stiffness varies, the structure frequency varies. More importantly, the anti-resonant frequency varies a lot.  

If you wish to use FE to simulate a system like this, Code Aster has good abilities for geodynamic simmulation. http://www.code-aster.org/forum2/viewforum.php?id=12

A comment. From what I have read, lock-in, is a phenomenon that is amplitude dependent.

The equation you state is a matching between structural resonance and forcing excitation frequency (von Karman vortex shedding). vind excitation will drive structure vibration mostly around the the system natural frequency, i.e. lower vibration below resonance, highest near resonance, lower above resonance. The highest vibration is found at resonance as the excitation and vibration then may find the best phase closure.

If wind excitation is very strong, lock--in, may occur from the resonance frequency and stay very high up to the next mechnical frequency is hit (if the wind speed increases). Such phenomena exist in corrugated pipes and for heat exchanger tubes. 

Lock-in, as I am accustomed to the concept, is amplitude dependent as once the mechanical vibration energy is so high that it no longer can be damped using linear damping mechnisms, it forces the structure to vibrate at fs and with high amplitude where the amplitide would drop if excitation was not as strong and the system state was linear.

I can only provide one paper on your specific phenomenon

http://web.mit.edu/towtank/www/papers/bourguet_jflstruc_2011.pdf

I see that these authors use lock-in in the same manner as you. I would call it phase-closure. Then again, many roads lead to the city of Rome.

However, the phenomenon is a well know plague for thermowells that protrude into the flow. .http://enu.kz/repository/2009/AIAA-2009-2471.pdf

But - I can point you at three excellent books on the subject of flow-vibration interaction

http://www.win.tue.nl/%7Esjoerdr/papers/boek.pdf

http://www.amazon.co.uk/Acoustics-Physics-Uno-Ingard/dp/1934015083/ref=sr_1_sc_1?ie=UTF8&qid=1390285296&sr=8-1-spell&keywords=uno+ingaard

http://www.amazon.co.uk/Acoustics-Fluid-Structure-Interactions-Cambridge-Monographs/dp/0521054281/ref=sr_1_1?ie=UTF8&qid=1390285863&sr=8-1&keywords=Howe+acoustics

If you are completely new to the subject, start by reading Uno Ingaards book.

If you are looking for more papers on the subject, take a look at these gentlemen's home pages to see if you find anything

http://www.tue.nl/en/university/departments/applied-physics/the-department/staff/detail/ep/e/d/ep-uid/19850005/ep-tab/4/

http://www.win.tue.nl/~sjoerdr/

Sincerely

Claes

Thanks a lot for your answer