Fatigue strength Comparative study of knuckle joints in LNG carrier by different approaches of classification society’s rules

Abstract

The cumulative damage due to fluctuating loads leads to fatigue fracture which is the main cause of the fracture of offshore vessels and structures. Knuckle joints are the most critical area due to its susceptibility to fatigue failure. This is mainly due to a high-stress concentration at knuckle joints. Also, the area is inaccessible for inspection and repair due to cargo tank containment arrangement. In this report, the spectral fatigue analysis is presented for 148k, Moss type spherical tank LNG carrier. The study is focused on the fatigue damage evaluation of hopper knuckle joint details by full spectral fatigue analysis.

The full spectral fatigue analysis involves the computations of hydrodynamic response, global structural analysis, local structural analysis and calculation of fatigue damage. The structural response is assessed by performing linear FE–analysis with a linear material response. In order to simulate structural response, a linear hydrodynamic analysis using unit wave amplitude is carried out to simulate the wave-induced loads on the LNG carrier, which is followed by a linear FE global analysis to assess stress transfer function. The wave loading is calculated by linear hydrodynamic analysis is based on 3D diffraction theory. The wave amplitude of 1.0m considering wave headings from 0 to 360 degrees with an increment of maximum 30 degrees is used to calculate the ship response. For each wave heading 25 wave frequencies are included to describe the shape of the transfer functions. The inertia loads, internal and external pressures are calculated in the hydrodynamic analysis and transferred directly to the global structural model. Direct wave load computations by the numerical method improve the accuracy of the calculated loads compared to the approach of using the classification society‘s formulae. Two type of loading cases i.e. full load and normal ballast condition are considered for the damage calculation. For each heading of sea state, fatigue damage is calculated by combing the hotspot transfer functions with stress cycle (S-N) curve data and wave scatter diagram.

Fatigue damage computations involve design variables such as S-N curve data, wave scatter data, wave spectrum, etc. Current rules of classification societies DNV GL and ABS are used to evaluate the fatigue damage of knuckle joint. In general, the spectral fatigue calculation is cumbersome due to time-consuming calculation process. However, the study also provides information about the procedures involved in the spectral fatigue calculation