Currently, the conventional wave loading is the only effect considered in fatigue assessment of ships. Det Norske Veritas (DNV) has recently confirmed that fatigue damage from wave induced vibrations may be of similar magnitude as from the conventional wave loading (Moe et al., 2005, RINA, International Conference, Design and Operation of Bulk Carriers, London, Oct. 18–19, pp. 57–85). A 40% contribution to the total fatigue damage in deck amidships is documented through extensive measurements onboard an ore carrier (the reference ship) trading in the North Atlantic. The effect of strengthening the vessel, i.e., increasing the natural frequency by 10%, is ineffective in reducing the relative magnitude of the vibration damage. The wave induced vibration, often referred to as whipping and/or springing, also contributes to fatigue damage for other ship types and trades (Moe et al.). This paper considers the effect of trade. It indicates when the wave induced vibrations should be accounted for in the design phase with respect to fatigue damage. A second ore carrier (the target ship) is monitored with respect to the wave induced hull vibrations and their fatigue effect. Stress records from strain sensors located in the midship deck region are supplemented by wave radar and wind records. Based on the measurements, the vibration stress response and associated vibration induced fatigue damage are determined for varying wind and wave forces and relative headings. While the reference ship operates in the Canada to Europe ore trade, the target ship trades between Canada and Europe, Brazil and Europe, and South Africa and Europe. A procedure is suggested by Moe et al. to estimate the long term fatigue damage for different trades by utilizing the measured data from the reference ship. The vibration and wave damage are considered separately. By comparing the measured wave environment and the DNV North Atlantic scatter diagram, the effect of routing indicated a reduction of the fatigue damage by one-third. A slightly revised procedure is applied to estimate the effect of trade for the second ore carrier, comparing the long term predicted fatigue damage with the measured fatigue damage. The importance of trade is confirmed. However, the relative contribution of the vibration damage is shown to increase in less harsh environments. The target ship vibrates more than the reference ship for the same trade and Beaufort strength. The vibration damage of the target ship constitutes 56% of the total measured damage, and the high natural frequency is observed to have no significant effect.

It is well known that ships vibrate due to waves. The wave induced vibrations of the hull girder are referred to as springing (resonance) and whipping (transient vibration from impacts). These vibrations contribute to the fatigue damage of fatigue sensitive details. An Ore Carrier of 400 000 dwt is currently being built by DSME, and at time of delivery, it will be the world’s largest bulk (ore) carrier. The scantlings of large ships must be carefully designed with respect to global loading, and when extending the design beyond experience, it is also wise to consider all aspects that may affect operation and the life time costs. The vessel will also enter a long term contract and is therefore to be evaluated for 30 year Brazil-China operation. In order to minimize the risk of fatigue damage, the vessel is designed according to DNV’s class notation CSA-2 requiring direct calculations of the loading and strength. Further it has been requested to include the effect of springing and whipping in the design. Reliable numerical tools for assessing the additional fatigue effect of vibrations are non-existing. DNV has, however, developed an empirical guidance on how the additional effect may be taken into account based on previous development projects related to the effect of vibrations on large ore carriers Due to the size and route of operation of the new design, it has, however, been required by the owner to carry out model tests in both ballast and cargo condition in order to quantify the contribution from vibration. The results from this project have been used for verification and further calibration of DNV’s existing empirical guidance. A test program has been designed for the purpose of evaluating the consequence in head seas for the Brazil to China trade. Full scale measurements from previous development projects of ore carriers and model tests have been utilized to convert the current model tests results into estimated full scale results for the 400 000 dwt vessels. It is further important to carefully consider how the vibrations are to be included in the design verification, and to develop a procedure for taking into account the vibrations which results in reasonable scantlings based on in-service experience with similar designs and trades. This procedure has been developed, and a structural verification has been carried out for the design. The final outcome of the model test was in line with previous experience and in overall agreement with DNV’s empirical guidance, showing a significant contribution from vibrations to the fatigue damage. The springing/whipping vibrations more than doubled the fatigue damage compared to fatigue evaluation of the isolated wave induced loading. The cargo condition vibrated relatively more than experienced on smaller vessels. Various sources to establish the wave conditions for the Brazil to China ore trade were used, and the different sources resulted in significant differences in the predicted fatigue life of the design.

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Currently, the conventional wave loading is the only effect considered in fatigue assessment of ships. DNV has recently confirmed that fatigue damage from wave induced vibrations may be of similar magnitude as from the conventional wave loading (Moe et al. 2005). A 40% contribution to the total fatigue damage in deck amidships is documented through extensive measurements onboard an ore carrier (the reference ship) trading in the North Atlantic. The effect of strengthening the vessel, increasing the natural frequency by 10%, is ineffective to reduce the relative magnitude of the vibration damage. The wave induced vibration, often referred to as whipping and/or springing, does contribute to fatigue damage also for other ship types and trades (Moe et al. 2005). This paper considers the effect of trade. It indicates when the wave induced vibrations should be accounted for in the design phase with respect to fatigue damage. A second ore carrier (the target ship) is monitored with respect to the wave induced hull vibrations and their fatigue effect. Stress records from strain sensors located in the midship deck region are supplemented by wave radar and wind records. Based on the measurements, the vibration stress response and associated vibration induced fatigue damage are determined for varying wind- and wave forces and relative headings. While the reference ship operates in the Canada to Europe ore trade, the target ship trades between Canada and Europe, Brazil and Europe, and South Africa and Europe. A procedure is suggested by Moe et al. (2005) to estimate the long term fatigue damage for different trades by utilizing the measured data from the reference ship. The vibration and wave damage are considered separately. By comparing the measured wave environment and the DNV North Atlantic scatter diagram, the effect of routing indicated a reduction of the fatigue damage by one third. A slightly revised procedure is applied to estimate the effect of trade for the second ore carrier, comparing the long term predicted fatigue damage with the measured fatigue damage. The importance of trade is confirmed. However, the relative contribution of the vibration damage is shown to increase in less harsh environments. The target ship vibrates more than the reference ship for the same trade and Beaufort strength. The vibration damage of the target ship constitutes 56% of the total measured damage, and the high natural frequency is observed to have no significant effect.