This paper describes the design of an underwater inspection system to be used during the construction of the storm surge barrier in the Eastern ScheIdt, which is the last dam of the Dutch Delta Project to be completed. The dam design chosen, was a storm surge barrier consisting of monolithic piers placed on foundation mattresses, while the seabed between them is raised by a sill construction. This way a frame has been constructed, within which steel sliding gates can be raised and lowered. The main tasks for the underwater inspection system designed by NeCOS are determination of the condition of the mattresses after positioning and detection of sand on the surface of the mattresses. All these inspections are carried out underwater.
The inspection system basically consists of inspection sensors mounted on a seabed crawler and a control and monitoring centre on board a support vessel. The operation of the inspection system, which will take approximately two to three years, is integrated in the working cycles of construction vessels. Among them a mattress laying pontoon and a pier lifting barge. The system operates in a depth of 3050 meters and work is expected to start May 1982.
The main inspection sensors are six television cameras, fitted with especially designed Perspex lenses and four high frequency acoustic transducers, in order to detect a sand thickness with a resolution of 0.5 cm. The underwater visibility in the Eastern ScheIdt is very bad (sometimes less than 10 cm) thus explaining the choice of Perspex lenses acting as a clear vision unit. Data from the inspection sensors is collected through the umbilical and processed real time as well as off line by means of a video-processor and a minicomputer. The crawler has track and wheel drive either can be selected during operational use, this to avoid damage to the mattresses while turning. The choice of a seabed crawler was dictated by the prevailing strong currents and by the fact that the targets are on the seabed.
The use of underwater inspection systems during the construction of barriers in tidal channels is not very common. Most underwater inspection systems are employed for platform or pipeline inspection to serve the offshore oil and gas exploration. The storm surge barrier in the Eastern ScheIdt will be built with the aid of prefabricated parts like foundation mattresses, piers and gates. This design was chosen in order to keep the critical construction offshore as short as possible.
The construction elements are positioned by means of special equipment, such as the pier lifting and mattress laying barge. In order to inspect the condition of mattresses and piers before and after positioning, an underwater inspection system is required. The construction phase of the storm surge barrier will take approximately three years, which together with the special inspection requirements justifies the design of a special remotely operated underwater inspection system.
