On creating a numerical model for estimating spillage in a cutter suction dredger while cutting sand

Abstract

Great Lakes Dredge & Dock (GLDD) is one of the biggest dredging companies in the USA which provides engineering services in dredging and land reclamation. GLDD came up with an idea to develop a working model that can estimate spillage when using a cutter suction dredger (CSD) for cutting soft material like sand. The main reason for this is because not every material that is cut from the seabed contributes to production. A part of the material is lost since it doesn’t enter the suction mouth and contributes to spillage. Spillage can be defined in a number of ways. GLDD defines spillage as “the difference in elevation between the maximum depth at which the dredge cuts (cutting depth) and the depth obtained after dredging (after dredging depth)”. Theoretically, spillage is defined as “the material that is displaced from the seabed which does not enter the suction pipe and thus does not contribute to production”. The aim of this thesis is to identify and describe the types of spillage that occur while dredging soft material like sand and to use this information to create a numerical model that estimates spillage. Four types of spillage are found, namely; soil that disintegrates and is directed away from the cutter head (Type-1), soil that enters the cutter head but not the suction mouth (Type-2), soil that does not enter the cutter head when cutting large bank heights (Type-3) and breaching of the bank after the cutter head has passed (Type-4). The numerical model focuses only on Type-2 spillage. The foundation of the model is based on the theory that the cutter head of a CSD behaves like a combination of an axial and centrifugal pump with an inflow present near the nose and an outflow present near the cutter ring. For simplicity, the cutter head is divided into two slices with the top slice (2) having an inflow and the bottom slice (1) having an outflow. It is assumed that the outflow from the bottom slice circulates back to the top slice and re-enters the cutter head. Equations for pressure and flow are derived from the pump affinity laws for each slice. A constant, β, is a parameter that influences the flow and an initial value is assumed which is later calibrated. Using the concept of the continuity equation, an implicit equation for the height of the bottom slice (w1) is found. In the model, spillage percentage is defined as the ratio of the circulating flow to the total outgoing flow. Real life data from The Texas (GLDD) is used in the model. The constant β is calibrated using spillage values obtained from hydrographic surveys. Sensitivity analyses are performed on the various operational parameters of the dredge as well as on β. To study the influence of β on the relationship between the operational parameters and spillage, a number of β-values are used to plot data. For the validation of the model, the spillage value obtained from the model is compared with the spillage values obtained from the hydrographic surveys. As the spillage is in close proximation with values observed in the industry, the model is said to be working as intended. In order to further validate the model, the results obtained from the model are compared with the results obtained from experiments performed by Marco den Burger and the results are almost similar. Thus, it can be concluded that although the model is simple, it is able to provide realistic values of spillage.