1. Centre for Marine Technology and Engineering (CENTEC), Instituto Superior Técnico, Technical University of Lisbon, Lisboa 1049-001, Portugal e-mail:
2. Centre for Ships and Ocean Structures and Department of Mathematical Sciences, Norwegian University of Science and Technology, Trondheim NO-7491, Norway e-mail:
3. Department of Marine Technology, Norwegian University of Science and Technology, Trondheim NO-7491, Norway e-mail: