A large blunt vessel was designed for a longer target life than usual in world-wide trade. Due to experience with whipping and springing, special attention to these effects was also made during the design and approval in order to ensure satisfactory strength of the vessel. The vessel was consequently strengthened beyond the minimum industry standard, and it has now been operated for several years.
The owner/operator who observed wave induced vibrations (whipping/springing) onboard, equipped the vessel with a hull monitoring system from a recognized supplier. After a few years of measurements, the data was sent to DNV for assessment of the effect of the vibrations and the consequence for the particular design.
The data shows that the vessel has been trading in more demanding areas than assumed during design, but the environment is less severe than the North Atlantic. The measured fatigue life based on a stress concentration factor of 2.0 has been estimated to be well below the design life, so special attention to cracks need to be taken from now on if the trade remains the same in the future. No cracks have, however, been identified so far during inspection.
The vessel has also experienced two severe storms. The maximum loading level has been higher than ever assessed by DNV before based on hull monitoring data of blunt vessels. The rule of thumb value of 20% increase on extreme loading for blunt vessels due to whipping has been exceeded. The wave bending moment according to IACS URS11 has also been exceeded without whipping. The ultimate collapse strength has been assessed and compared to the measured dynamic loading and allowable still water loading. When whipping is assumed fully effective to contribute to collapse, the safety margin is still above 1.0, but on the borderline of what is desirable. However, if the vessel had not been strengthened beyond the original design due to the concern of whipping and springing, the safety margin would have been below 1.0. This may be the first documentation of a vessel that has been saved from breaking in two due to addressing springing and whipping properly during design.