Whipping Investigations Based on Large-Scale Measurements and Experimental Fatigue Testing
- Adrian Kahl (DNV GL) | Wolfgang Fricke (Hamburg University of Technology) | Hans Paetzold (Hamburg University of Technology) | Hubertus von Selle (DNV GL)
- Document ID
- International Society of Offshore and Polar Engineers
- International Journal of Offshore and Polar Engineering
- Publication Date
- December 2015
- Document Type
- Journal Paper
- 247 - 254
- 2015. The International Society of Offshore and Polar Engineers
- wave load, longitudinal strength, Hull girder, whipping, fatigue test, fatigue damage
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- 49 since 2007
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Wave loads and slamming loads acting on the ship’s hull lead to the superposition of low-frequency wave-induced stresses and high-frequency stresses from whipping as well as springing effects. Whipping, mainly caused by slamming loads, may impact the ultimate strength of the ship’s hull. Both whipping and springing increase the fatigue damage of structural details of the ship’s hull. Fatigue damage assessment typically relies on rainflow counting and the Palmgren–Miner rule for linear damage accumulation, but it is not yet proven whether this approach works well for superimposed high- and low-frequency stresses. Loads recorded in full-scale measurements onboard a containership are the basis of this investigation. Fatigue assessment of measured stress shows a significant damage increase due to high-frequency contribution. However, it has to be emphasized that this is not reflected by observed damages of the fleet in service. To get more insight into the fatigue damaging mechanism of combined low- and high-frequency loads, fatigue tests have been performed in cooperation between DNV GL and TUHH. Transverse stiffeners on a continuous plate have been selected for tests as a representative structural detail. The Palmgren–Miner rule will be verified by test series with measured unfiltered load sequences as well as with low-pass filtered load sequences.
Ship hulls at sea are not only subjected to bending stresses due to waves but also to stresses caused by wave impacts (slamming) and subsequent vibrations (whipping). Such impacts can occur at the bottom of the forebody, which emerges in rough seas and subsequently hits the water surface with a certain relative velocity. The frequency of whipping vibrations is typically much higher than that of wave-induced bending, being determined by the lowest natural frequencies of the hull girder.
As bottom slamming occurs mainly at lower draft, usually in ballast condition, where wave-induced bending stresses are smaller than at full draft, additional whipping stresses were not taken into account during design in the past, according to the longitudinal strength assessment procedure that was internationally unified by the International Association of Classification Societies (IACS, 1978).
The situation has changed during the past years. Modern containerships are characterized by pronounced flare at the bow and stern, which causes more frequent slamming impacts on them, particularly at design draft. The first indications that the associated whipping stresses can be relatively high and may have to be considered during design were found during long-term measurements in the early 1990s on four containerships sailing in the North Atlantic (Hansen, 1993). These measurements showed significant differences between three containerships having moderate bow flare and a fourth one with pronounced bow flare and a smaller length of 167 m, which showed larger pitch motions in North Atlantic waves. The observed bending moments were 30% higher than those expected from the design values, and the number of load cycles was doubled. The conclusion was that the revised longitudinal strength requirements (IACS, 1989) agree better with the observations than the previous ones. Furthermore, an increased wave bending moment was introduced in the rules of Germanischer Lloyd (GL, 1991) for ships with relatively pronounced bow flare and high speed.
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