[1] Tech Univ Denmark, Dept Wind Energy, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
[2] UiS, Dept Mech & Struct Engn & Mat Sci, N-4036 Stavanger, Norway
[3] Sandia Natl Labs, Wind Energy Technol Dept, POB 5800, Albuquerque, NM 87185 USA
[4] Univ Texas Dallas, Dept Mech Engn, Richardson, TX 75080 USA