Structural design of self-unloading mining ship and unsteady flow computations over its vertical riser

Abstract

The Inter Ocean Metal organization (IOM) has proposed some initial considerations which can be used in developing the design of a polymetallic nodules mining ship. The requirements for cargo volume and deadweight are studied on the basis of assumed production output. A novel concept for cargo discharging to a shuttle transport ship by selfunloading is proposed by IOM company, the analysis of cargo characteristics showed that there are several issues which affect the subdivision of the ship and the approach to the problem of arranging its structure. Accidental stress or damage caused during the loading or discharging of self-unloading mining ship could ultimately lead to catastrophic structural failure.

The primary aim of this thesis is to perform a structural design of the concerned mining ship by means calculating the shear forces and bending moments in any load or ballast condition and to asses these against the assigned maximum permissible values. When we consider the primary hull structure, the two most important load effects are the still water and the wave-induced shear force and bending moments. The reference is made to the cross section by creating a scantling of proposal structural arrangement according to the DNV Rules given by Nauticus Hull software used for this study. The values of load effects in the various components of the ship structure show a good correspondence to the requirements of DNV (Det Norske Veritas) classification society. One component related to mining applications that can be subjected to large deformations is the riser which is controlled by a lifting system installed in the mining support vessel.

The secondary aim is to find a means of alleviating vortex-shedding-induced vibration of a deep-ocean mining riser with 0.7 m outer diameter and 5,000 m in length, and to deduce on the power requirements for towing the whole riser through the water depth in vertical position. An unsteady analysis of flow was performed around three riser configurations at different Reynolds number, for their effectiveness in reducing vortex- shedding intensity. The computational implementation was accomplished by the commercial software Fluent Ansys and the flow was analyzed by means of studying pressures, velocities, vorticities distributions, also lift and drag forces variations and torsional moment applied on the risers. The numerical results of drag coefficient and Strouhal number which signifies the vortex shedding in function of Reynolds number are coincident with the previous experiments.