Defining a haul-out indicator for removal of crab-pot-strings in Offshore Windfarms under anticipated adverse weather conditions

Rozemeijer, M.J.C.; Cramer, R.; Deetman, B.; Korving, A.


Crab-pot-strings with dahns and Bruce anchors have to be taken out of the water once sea conditions get too adverse. But what is exactly considered too adverse? The potential mobilisation of crab-pot-strings with dahns and Bruce anchors is a complex process not easily captured in a single indicator like wind force. An indicator was developed in this project on the base of the mechanisms of mobilisation (currents and waves hauling on the dahn). The Indicato rHaul-out was assembled based on Maximum extra Water Level as compared to Normaal Amsterdam Peil (= wind surge as a measure for currents), Swell and Average Significant Wave Height over 10 min. periods .A threshold of the Indicator Haul-out was defined for haul-out. When an Indicator Haul-out value is predictedof 445 cm using the predictions at Hoek van Holland and Eurogeul 13, the crab-pot-strings will be hauled out before this value will occur. Wageningen Marine Research will calculate this value during the entire experiment planned in PAWP in 2023. As an extra safeguard, two fishermen will also observe the weather and sea condition predictions. They will also give a warning when, based on their experience, sea condition gets too adverse for safe deployment of the strings. In both cases the project leader of Wageningen Marine Research will open a dialogue with the project leader of Eneco on how to proceed. The project leader of Eneco has the final decision on the obligation for haul-out.