Dredging at Sea

ABSTRACT Dredging at sea can take many forms, determined by the purpose of the dredging job and the sea and weather conditions in the dredging area. The main problems are connected with the movement in waves and swells of the surface vessels supporting the dredging equipment. For a number of sea-dredging operations, satisfactory methods and equipment have been developed; other applications still are waiting for the economical solution. This paper describes some concepts that could lead to the development of both technically and economically feasible equipment. SCOPE OF PAPER Dredging can be defined as raising material from the bottom of a water-covered area to the surface and moving it over some horizontal distance. This simple definition covers a very wide range of activities, both with respect to the geographical place and ambient conditions in which a dredging operation can take place and with respect to the goal or purpose of the activity. Dredging jobs can be as simple and straightforward as making or deepening drainage canals in a flat landscape, or as tricky and hazardous as dredging for diamonds in the sea offshore the notorious Southwest African coast. They can range from a 50-cycle/hr operation to a multimillion Yd/annum venture. It will be clear that no standard solution is available for such widely varying problems. It is safe to say that dredging is one of the most complex technologies of modern man and that dredge design and construction are among the most challenging jobs any engineer could possibly wish for. The scope of this paper does not permit a comprehensive treatment of all problems that face the dredge designer and dredge operator, when a dredging operation at sea is contemplated. The authors, therefore, limit themselves to a discussion of' the principal applications and types of equipment, the areas where sea dredging is different from inland dredging, and possibilities to design for the additional difficulties. WHY DREDGING AT SEA? Let us first consider the reasons why people want to undertake the difficult task of dredging at sea. One good reason f'or dredging is the fact that the sea is not deep enough for our purposes. This is a frequent occurrence in this era of increasing world traffic of many commodities, transported in ships of ever increasing size. To provide access to ports, channels have to be deepened and the increased depth has to be maintained. A typical example of this type of dredging job is the creation of a tanker channel in the North Sea, running over a distance of' 40 miles from the port of Rotterdam and requiring the removal of many millions cuyd of soil.