Fatigue Strength Assessment Of Extra Thick Welded Joints In Offshore Structures

Fatigue is one of the most important failure modes in ship and offshore structures. Fatigue is particularly prominent especially in recent years for large offshore structures and the use of high-strength steel. Fatigue design rules for offshore structures are based on research work of relatively small scale welded joints for ease of experimentation. However, extrapolation of the results to bigger and thicker welded joints may not be safe, especially for extra thick joints that are even up to 100 mm thick and more.Based on structural fatigue and fracture theory, this paper combined with the hot-spot stress method of the fatigue assessment rules examine the hot-spot stress analysis procedure and the thickness effect correction method recommended in the current fatigue design rules for extra thick welded joints by using Finite Element (FE) calculation. By FE calculation this paper analyses the relationship between the influence zone of the local stress concentration and the plate thickness. For extra thick welded joints, the influence of different elements selections, elements sizes and positions of the reference points for the stress extrapolation was investigated. The results show that the influence zone of the local stress concentration due to the weld toe is proportional to the plate thickness. It is appropriate to take 0.5t as the distance of first reference point away from the weld toe even for extra thick welded joints. The linear extrapolation method over reference points 0.5t and 1.5t away from the weld toe are applicable to the hot-spot stress calculation of extra thick welded joints.In this paper the thickness effect was studied based on fracture mechanics theory. The stress intensity factor (SIF) of a surface crack at the weld toe, obtained from FE analysis, was used to calculate the fatigue life of welded joints with different plate thicknesses. The results show that the thickness effect correction method is applicable to the fatigue assessment of extra thick welded joints; compared with the numerical results in this paper, the proposed effect exponent in the rules gives a non-conservative result for the thickness effect correction. Further research work is needed to analyze the influence of the joint types and loading modes to the thickness effect factors.