Split-hopper barges

Abstract

A split hopper barge is a dredging vessel that can split over its longitudinal axis to discharge its cargo. To allow for such an operation, the barge consists of two half-hulls that are connected by hinges at the top and hydraulic cylinders at the bottom. Any forces that act at the interface between the half-hulls have to be transferred by the hinges and cylinders as a result. The aim of this thesis is to develop a method to determine the workability limits of split hopper barges in irregular seas. For this workability of the barge, the design limits of the hinges and hydraulic cylinders are assumed to be governing. A quasi-static model is formulated to determine the effects of roll, different types of cargo, opening the barge and discharging the cargo on the forces in the hinges and cylinders. The results of the quasi-static model are a set of governing load cases for the split hopper barge. From the quasi-static model, it is found that the forces of interest are the largest for a closed split hopper barge and that either a solid or a liquid cargo will yield the governing load case. To account for the dynamics in irregular sea conditions, a calculation method is proposed based on Ansys AQWA. Since AQWA is not able to account for liquid cargo motions, its effects are incorporated separately. Using WAMIT simulations, the influences of the liquid cargo to the added mass and stiffness of the barge are determined. Based on the WAMIT results the AQWA model is adjusted. Superimposing the results from the quasi-static and dynamic models, the maximum forces in the hinges and hydraulic cylinders are obtained for the governing load cases. By comparing the obtained maximum forces to the design forces, a conclusion on the workability limits of the split hopper barge is drawn.
For the barge carrying a solid cargo only one resonance frequency is found, while, due to the liquid cargo motions, the barge with a liquid cargo has two resonance frequencies. As a result, a larger number of combinations of wave directions and periods yield considerable forces. For a 3-hour-extreme with a significant wave height of 3 meters, the governing hydraulic cylinder forces are found for a barge carrying a liquid cargo. The maximum resulting hinge forces however, are found for a barge carrying a solid cargo. Because the limits of the hinges are exceeded before the limits of the hydraulic cylinders, it is concluded that the workability limits of the split hopper barge in irregular waves are most limited, and therefore governing, for a barge carrying a solid cargo.