Constructability Assessment for a North Sea Energy Island
WODCON XXIII - Dredging is changing - The Practice. The Science. The Business.
D.C. Roukema, D.E.G. Bos, A.M. Roskam
"A constructability assessment is performed for an energy island located in the Doggerbank area of the North Sea. As the basis for the study a tentative island scope and dimensions of key elements are used. Given the harsh conditions of the North Sea, the construction methods, equipment and logistics chosen in this assessment are primarily based on intrinsic robustness. The workability is defined for 3 periods occuring each year. A summer period of 6 months and a winter period of 4 months. Statistically the months of December and January contain the most adverse conditions of the year and are therefore considered idle time. Construction of the rubble mound sea defences is not considered possible without protection. For this purpose a temporary cobble barrier is proposed, which is relocated along with the construction work front. Ultimately the cobbles are re-used in the cobble beach proposed as defence for the south of the island. Early availability of a work harbour should allow optimal supply and handling of construction materials. Reclamation works, comprising ~150 million m3, are mainly performed by a number of TSHD's, well within available market capacity. For rock gradings and quarry run (in total ~4 million ton) a stock yard storage equal to half a year of production capacity is located nearby the work harbour. The concrete armour units (24-34 ton X blocs and ~100 ton cubes with ~10 ton cubes as underlayer) are casted in a prefab yard in a North Sea port, shipped to the island, and stored in a stock yard nearby the work harbour. The sea defences are constructed simultaneously by marine and by land-based equipment, with a total work front of ~600 m, and progress of ~50 m/wk. The amount of equipment required is large, and without exception amongst the largest available in the industry. Special purpose equipment for handling and placing the ~100 ton armour cubes is proposed. Total construction time is ~7.5 years. Options to reduce this are limited, given time needed for preparations and the early works, and the size and amount of equipment already deployed. Reversely, constraints from for instance licenses could seriously reduce construction possibilities, so much care and attention is needed in the preparations for license applications. Caissons may be considered as an alternative to rubble mound sea defences. However, suitable weather windows for towing to site and immersion onto the foundation bed may not be very frequent, even in the summer. Also the construction of the foundation bed, with a top layer of gravel within tight vertical tolerances, is not particularly easy at open sea. The hydraulic and geotechnical design of the caisson, with the associated rock platform and berm, is considered a critical engineering challenge, given the unforgiving nature of the concept. Irrespective of the sea defence concept chosen, already during the design phase the constructability (practical and reliable construction methods and logistics) must be considered as well. This study provides examples of such methods."
Keywords: marine construction, logistics, workability, planning, reclamation, rock supply, concrete armour