Fatigue Strength Assessment for Bulk Carrier According to Common Structural Rules

Abstract

Structures of the bulk carriers are generally prone to fatigue due to high cyclic loads mainly caused by waves and changing loading conditions. During the past decades, a great number of fatigue failures occurred in ship structures, particularly in the areas built in higher tensile steel. Fatigue and corrosion are noted as predominant factors which contribute to the structural failure observed on a ship in service. Therefore, fatigue is an important criterion during design. The increasing demands placed on bulk carrier safety have reinforced the commitment of regulatory bodies to look for higher design standards and to improve the overall approach to design criteria. IACS has developed, for the first time, a unified complete set of Common Structural Rules for Bulk Carriers (CSR-BC). New CSR rules implement highly technical advanced methods to establish new criteria applied in a consistent manner. The problem of the fatigue strength is addressed in the present thesis where the hopper tank – inner bottom knuckle and longitudinal stiffener end connections being the structural details of 30,000 DWT double side skin bulk carrier are analyzed according to the IACS CSR-BC. The fatigue assessment procedure within new CSR for bulk carriers are based on the following hypothesis: S-N curve approach, Palmgren-Miner‟s linear cumulative damage, dynamic fatigue load tuned on 10-4 probability level in north Atlantic, design life 25 years, net scantling, load combination factors, fatigue loading conditions and Weibull probability distribution with two parameters for the long-term stress ranges load history. Global finite element model followed by submodels according to IACS Rules for the corresponding locations is proposed to obtain the hot spot stresses for fatigue assessment. The primary mission is to develop a finite element model that can give the most accurate predictions of fatigue strength in the critical details for the relevant bulk carrier under CSR-BC with feasible computation and working efforts. GL-POSEIDON and ANSYS codes were used for the finite element analysis. POSEIDON provides specific tools for structural design, geometric and finite element modeling of bulk carriers under CSR-BC. ANSYS gives several options for fatigue analysis, post-processor and submodelling techniques. Critical review of design loads for fatigue assessment complying with relating to CSR-BC such as external and internal design pressure for different load conditions is accomplished and calculated using the Excel sheet. Coarse FEM model for the three midship cargo holds is created using four node shell element considering all loading conditions, full, alternate, normal ballast and heavy ballast and all load cases required by the Rules. Fine local FEM model of the hopper inner bottom knuckle and longitudinal stiffener web frame connection at midship cargo hold are built applying a four node shell element and fine mesh size t x t where t is the inner bottom plate thickness. The displacement from global model is interpolated to the boundaries of local models. The fatigue hot spot stresses for hopper inner bottom knuckle are extrapolated based on the DNV recommendations for fatigue assessment. In addition, the geometry stress concentration factors for hopper inner bottom knuckle and longitudinal-web frame are estimated and compared with the IACS Rules values. The method of checking fatigue life for the relevant detail by Excel is also presented. The result is shown that the corresponding total fatigue damage for the hopper inner bottom knuckle is less than the limit criteria of cumulative damage, and the detail has an acceptable fatigue life for 25 years operation in North Atlantic wave environment. The geometry stress concentration factor for that detail is higher than the simple estimation value calculated by CSR. The alternate loading condition has shown the highest stress range and elementary fatigue damage. In addition, the heavy ballast loading condition has high local mean stress and contributes significantly to the fatigue damage.