In other cases, fatigue cracks from

In other cases, fatigue cracks from welded details have lead to severe consequences and loss of life. Figure 2.2 shows the semi-submersible Alexander Kielland that capsized in the North Sea in March 1980. Although the disaster was a consequence of several unfortunate circumstances, the root cause was a fatigue crack in an attachment weld at the surface of a brace member. The brace member is denoted D6 in Figure 2.2 and the crack has started from the fillet weld between a hydrophone support and the brace. After the crack had propagated through the wall thickness of the brace, it continued to grow along the circumference of the tube splitting up the main cross section of the brace. Before the final fracture the crack was over 1 meter long. The accident had 123 fatalities and initiated a large research effort on fatigue in Norway in order to obtain a better understanding of the fatigue damage problem of welded joints. Based on the increased knowledge of the fatigue behavior, improvements were made regarding both detailed fatigue design and inspection planning for offshore structures. A final failure case is shown in Figure 2.3. A large fatigue crack in the deck area in the mid-ships section of a tanker can be seen. These types of cracks may cause a rupture of the cross section of the hull beam. This may endanger the life of crew members and cause environmental damage to the sea and the shore.