Development of a method for prediction of water ballast tank coating failure
Examensarbete för masterexamen
Naval architecture and ocean engineering (MPNAV), MSc
Failure of ballast tank coatings is an increasing problem in today’s shipping industry. The consequences of coating failures (fracture) are corrosion of the steel which can lead to early occurrence of fatigue cracks and fast fatigue crack propagation. Inspections of coatings in water ballast tanks of ships have shown that fracture of many coatings occurs earlier than their design life (which is around 8-12 years), although the coatings have been maintained regularly and properly. From a financial perspective, maintenance and repair of a coating is very expensive (810 million USD for the US marine shipping industry in 2006) and should be reduced. This can be achieved by better methods that can assist in the identification of critical locations in water ballast tanks regarding coating failure. With such a method, the preparation for maintenance and repair procedures can be made more efficiently. The aim of this thesis is to suggest a method (simulation procedure) which can be used to identify local areas inside a ballast tank where coating fracture may occur as a result of large strains and ageing of the coating’s mechanical properties. The methodology proposed is to use a finite element sub-modelling technique that involves full ship analysis to find local areas which thereafter are refined to better represent ballast tank areas and locations with high strain levels. These areas are then modelled into more detailed local models in order to perform non-linear finite element analyses where the ship steel is allowed to exceed its yield stress in e.g. cut-outs. Fracture of a coating is assumed to occur when the strain level of the ship steel exceeds the fracture strain of the aged coating. Different sea state conditions are simulated where the significant wave height, the wave heading angle and the ship’s speed are varied. Six different local areas are studied in detail with regard to the strain response of the steel (substrate). The computed strains in the substrate are compared against fracture strains of un-aged and aged water ballast tank coating samples from uniaxial tensile tests. The results show that the strain in the substrate exceeds the fracture strain of aged coatings in several locations, i.e. the coating will fracture. The most critical areas and locations of the case study vessel are at the bottom of the ballast tank, in the cut-outs that intersect a longitudinal stiffener and a transverse web frame. It is concluded that the method proposed is suitable for finding local areas where the strain of the substrate exceeds the fracture strain of the coating. It is also concluded that high mesh density in local details is necessary in this method in order to carry out realistic simulations and resolve the strain gradients in cut-out details and thereby be able to compare strains in the substrate with coating fracture strains.
Farkostteknik , Hållbar utveckling , Produktion , Transport , Vehicle Engineering , Sustainable Development , Production , Transport