MODERN POSITIONING SYSTEM PROVIDES FOR AN INEXPENSIVE MATRESS-LAYING VESSEL AT THE VENICE SURGE BARRIER

摘要

The Venice lagoon has to be protected against storm surges and sea level rising due to global wanning. The lagoon entrances will receive dams and barrier doors. To improve the soil stability at the entrances, mattresses have to be installed. The laying of mattresses is well known from the Eastern Scheldt project, but was very expensive in construction and operation. By using the same philosophy, but adapted to modern systems, this specialized installation takes only a fraction of the costs. This makes the technology available for more lagoons and estuaries threatened by storm surges.The mattresses are deployed from a pontoon, carrying a mattress storage cylinder 2m under water. These 4cm thick and 10m wide mattresses are made of geotextile, filled with sand and gravel. They are approximately 150m long, max 250m. Six winches position the pontoon at the required location. However, the mattress is subject to the current and waves as it is rolled down the cylinder. Therefore, the position of the pontoon is not directly related to the eventual position of the mattress. Still the mattresses have to be laid with a horizontal accuracy of ca. 20cm. Especially the adverse conditions at the entrances with strong currents (2.5m/s) and waves (2.3m); make it very challenging to develop a reliable system meeting these specifications.The pontoon position comes from DGPS-signals and is used together in a survey system to compare this with the required position. The difference is used in a PLC to control the winches and minimize the offset. Specially developed algorithms calculate the forces on the pontoon, the pulling forces of the winches and the behaviour of the suspended mattress. The resulting signals instruct the winch controllers with the corresponding actions.Inspection with a multi-beam echosounder reveals that the mattresses are laid down within the required accuracy.