Ultimate limit state of corroded double bottom tanker after grounding events

Abstract

Grounding accidents will inevitably continue to happen, but hopefully the resulting damage can be mitigated by including crashworthiness considerations in the structural design. A lot of research has already been done to investigate the effect of different ground shapes, penetration depths, ship types (e.g. double or single bottom and stiffened or unstiffened structure), collision velocity, and collision position (e.g. under a transverse bulkhead, a longitudinal bulkhead, or an unstiffened plate section). Typically, the experiments and finite element analyses that are performed are conducted under the assumption that the steel has not succumbed to the harshness of the environment that ships operate in. Therefore, the objective of this thesis is to investigate the effects of corrosion on a structure’s resistance to penetration and damage in the event of a ship grounding. Furthermore, the ultimate hull girder strength and residual strength of the hull after various grounding scenarios is investigated. Nonlinear finite element analysis simulations are conducted in Abaqus/Explicit (Dassault Systèmes, 2020). Displacement controlled grounding simulations are done with two different ground geometries and with three different tanker ages, namely zero, 16, and 25 years. For the aged vessels, the degradation that results from general corrosion is accounted for using two different methods. One method is to just reduce the thicknesses of structural members, while keeping the as-built material properties. The other method is to reduce the thicknesses and to also adjust the constitutive material model so that it more accurately represents the material properties of corroded steel. Reaction forces on the rock are measured during the simulations to determine the impact of adjusting the constitutive material model for the aged tanker models. Ultimate limit state analyses are then carried out with the intact and damaged models to quantify the effects of corrosion on a ship’s ultimate hull girder strength. It is shown that accounting for the material properties of corroded steel is vital to avoid overestimation of the ship’s crashworthiness. If the thicknesses are reduced without changing the material model, in some cases, there is less grounding damage compared to the damage seen on the model with a non-corroded material model and full as-built thicknesses. This is especially apparent for the ground geometry with a wide and flat contact area.