1. STOR-i Centre for Doctoral Training, Department of Mathematics and Statistics, Lancaster University , Lancaster , UK
2. Department of Mathematics and Statistics, Lancaster University , Lancaster , UK
3. Department of Mathematics, Imperial College , Huxley Building, 180 Queen’s Gate, London SW7 2AZ , UK
4. MetOcean Research Ltd. , New Plymouth , New Zealand
5. HAW MetOcean ApS DK-1860 , Frederiksberg C , Denmark
6. Shell Research Ltd. , London , UK