In 1974, the semisubmersible Transocean 3 overturned and sank in the North Sea.
The semisubmersible jacked down onits pontoons for towing, was about to drill for Mobil when the accident happens. The rig was caught in an unusually long beam sea. When the wave length equalled the distance between the rig legs, it caused an oscillation which loosened the stiffening mechanism. A leg broke free, the rig overturned, and sank.