1. Technical University of Denmark, Department of Mechanical Engineering, Nils Koppels Alle, Build. 403, DK-2800 Lyngby, Denmark; Centre for Ships and Ocean Structures, Norwegian University of Science and Technology, N-7491Trondheim, Norway e-mail:
2. Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Wind Energy Division, Frederiksborgvej 300, DK-4000 Roskilde, Denmark e-mail: