Gaute Storhaug

Measurements of Wave Induced Hull Girder Vibrations of an Ore Carrier in Different Trades

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.

Effect of Whipping on Fatigue and Extreme Loading of a 13000TEU Container Vessel in Bow Quartering Seas Based on Model Tests

Many large and ultra large container vessels have entered operation lately and more vessels will enter operation in the coming years. The operational experience is limited and one of the concerns is the additional effect of hull girder vibrations especially from whipping (bow impacts), but also from springing (resonance). Whipping contributes both to increased fatigue and extreme loading, while springing does mainly contribute to increased fatigue loading. MAIB recommended the industry to join forces to investigate the effect of whipping after MSC Napoli, a Post-Panamax container vessel, broke in two in January 2007. This has been followed up by a JIP initiated in 2008 with the following participants: HHI, DNV, BV, CeSOS and Marintek. In 2009 a new design 13000TEU vessel was tested in head seas and reported in [1]. The current paper deals with fatigue and extreme loading of the same vessel, but from realistic quartering sea conditions tested in 2010. Different headings and the effect of wave energy spreading have been investigated and compared to results from head seas. Further, the effect of the vibrations have been investigated on torsion and horizontal bending, as the model is also allowed to vibrate with realistic frequencies in other modes in addition to vertical bending. The findings suggest that changing the course is not effective to reduce the fatigue loading of critical fatigue sensitive details amidships. The effect of wave energy spreading did also not reduce the fatigue loading significantly. For the highest observed vertical bending moments in each sea state and for the three cross sections the wave energy spreading in average reduced the maxima, but for the highest sea state the effect of wave spreading did not consistently give reduced maxima. This is an important aspect when considering the available safety margin that may be reduced by whipping. The whipping gave also a considerable contribution to horizontal bending and torsion. This suggests that validation of numerical tools is urgent with respect to off head sea conditions and that these tools must incorporate the real structural behavior to confirm the importance of the response from torsional and horizontal as well as for vertical vibrations.Copyright

Dynamic Selection of Ship Responses for Estimation of On-Site Directional Wave Spectra

DTU Orbit (10/01/2019) Dynamic selection of ship responses for estimation of on-site directional wave spectra Knowledge of the wave environment in which a ship is operating is crucial for most on-board decision support systems. Previous research has shown that the directional wave spectrum can be estimated by the use of measured global ship responses and a set of transfer functions determined for the specific ship. The approach can either be based on parametric or Bayesian (non-parametric) modelling, and in both cases a set of three ship responses usually provides the best estimation. The optimal response combination of three responses at any time depends on the environmental conditions and the operation of the ship. Since measurements of more than three responses are usually available, a quick, dynamic selection procedure of the three signals best suited for the wave spectrum estimation procedure is essential. In the present paper the concept of a selection method based on a simple pre-estimate of the wave spectrum is suggested. The selection method needs to be robust for what reason a parameterised uni-directional, two-parameter wave spectrum is treated. The parameters included are the zero up-crossing period, the significant wave height and the main wave direction relative to the ship’s heading. The procedure basically seeks to minimise the difference between a set of spectral moments derived from a measured response spectrum and the corresponding response spectrum calculated from the parameterised wave spectrum and the transfer function for any given response. Subsequently, the three responses with the best overall agreement are selected for the actual estimation of the directional wave spectrum. The transfer functions for the ship responses can be determined using different computational methods such as striptheory, 3D panel codes, closed form expressions or model tests. The uncertainty associated with transfer functions depends on the computational method used, relative heading, frequency and type of response. This uncertainty is conceptually taken into account in the selection procedure using the First Order Reliability Method (FORM). However, specific assessment of the uncertainties has not been carried out, but is subject to later studies.

Which Sea States Are Dimensioning for Container Vessels When Whipping Is Included

Four container vessels have broken in two during the last four decades. There may not be one single cause explaining these severe accidents. They all broke in moderate storms, but they did not break in extreme storms in terms of extreme wave heights. How could this happen? This paper addresses one possible contributing effect to all of these accidents, i.e. whipping, and how whipping contribute in different sea states.Whipping, as a sudden hull girder vibration caused by bow flare impacts, can contribute significantly to increase the vertical hull girder bending moments of container vessels, which have high design speeds and pronounced bow flare. Model tests have been carried out based on modern container ship designs covering one Panamax vessel, one Post Panamax vessel and one Ultra Large Container Ship. The tests have been carried out primarily in head seas. The whipping contribution depends on the vessel speed, and the tests have been carried out using realistic speed in each sea state. Lower sea states are more frequent than higher sea states, and lower sea states are associated with higher speeds. Does this speed dependence give other dimensioning sea states when whipping is considered?For all three vessel designs, it is not the highest sea states, which define the dimensioning wave moments when whipping is included. Actually, realistic encountered storms can produce the dimensioning wave bending with whipping. These sea states differ considerably from the sea states, which produce the maximum wave moment without whipping. It is also demonstrated how different trades affect the dimensioning wave bending with whipping.The industry seems most concerned about the effect of whipping for the largest vessels. These model tests demonstrate that the dimensioning moment with whipping for the largest vessel is not the main concern. The tests suggest that the bow flare angle is most important, and these may be high for Post Panamax vessels. The speed is well known to be important, while size in terms of length is not particularly important from these tests.Copyright

Extreme Value Statistics of Large Container Ship Roll

The paper describes a method for prediction of large container ship extreme roll angles occurring during sailing in harsh weather. Rolling is coupled with other ship motions and exhibits highly nonlinear behavior. Risk of losing containers due to a large roll is primary concern for ship transport. Because of nonstationarity and complicated nonlinearities of both waves and ship motions, it is a considerable challenge to model such a phenomenon. In case of extreme motions, the role of nonlinearities dramatically increases, activating effects of second and higher order. Moreover, laboratory tests may also be questioned because of the scaling and the sea state choice. Therefore, data measured on actual ships during their voyages in harsh weather provide a unique insight into statistics of ship motions. The aim of this work is to benchmark state of art method, which makes it possible to extract the necessary information about the extreme response from onboard measured time histories. The method proposed in this paper opens up the possibility to predict simply and efficiently both short- and long-term extreme response statistics.

The Effect of Measured Whipping and Springing on LNG Vessels

Strain measurements of structural members onboard two LNG vessels have been collected for a period of 5 years. The vessels have been sailing in the world wide trade except for the North Pacific area. The time spent in the North Atlantic is about 40%, which is higher than for typical LNG vessels. The vessel speed has been relatively low in average and well below the service speed, still the effect of springing and whipping has been significant on the fatigue and extreme loading. Previously, results from the strain sensors have been presented versus the fatigue and extreme loading on a general level, independent on the environmental conditions. In this paper the focus is more towards how the vessel behaves in wind/waves with respect to springing and whipping in order to understand more of the relationship between accumulated fatigue damage, heading and loading condition.The vessels have been equipped with wave radars and wind sensors. The effect of whipping and springing on accumulated or part fatigue damage versus relative heading is demonstrated. The effect as a function of the wave height/wind speed is shown for selected headings. One of the objectives is to check if the wind sensor can be a useful alternative to the wave sensor to capture the physics. Some of the data is studied for ballast and cargo condition separately in order to see if these loading conditions can be merged, which is desirable from an assessment point of view.Copyright

Assessment of Hull Monitoring Measurements for a Large Blunt Vessel

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.Copyright

First Ocean Going Ships With Springing and Whipping Included in the Ship Design

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.Copyright

Comparison Between a Fatigue Model for Voyage Planning and Measurements of a Container Vessel

This paper presents results from an ongoing research project which aims at developing a numerical tool for route planning of container ships. The objective with the tool is to be able to schedule a route that causes minimum fatigue damage to a vessel before it leaves port. Therefore a new simple fatigue estimation model, only using encountered significant wave height, is proposed for predicting fatigue accumulation of a vessel during a voyage. The formulation of the model is developed based on narrow-band approximation. The significant response height hs, is shown to have a linear relationship with its encountered significant wave height Hs. The zero up-crossing response frequency fz, is represented as the corresponding encountered wave frequency and is expressed as a function of Hs. The capacity and accuracy of the model is illustrated by application on one container vessel’s fatigue damage accumulation, for different voyages, operating in the North Atlantic during 2008. For this vessel, all the necessary data needed in the fatigue model, and for verification of it, was obtained by measurements. The results from the proposed fatigue model are compared with the well-known and accurate rain-flow estimation. The conclusion is that the estimations made using the current fatigue model agree well with the rain-flow method for almost all of the voyages.

Full-Scale Measurements and Hull Monitoring on Ships

Full-scale measurements on ships are an important source of information for the development of ship design rules and guidelines. Several full-scale measurements have been carried out by DNV GL on container ships but also on bulk and ore carriers, LNG carriers, oil tankers, and other ships. Three measurements on container ships are presented and examples for exploitation are given. In contrast to full-scale measurements for research purpose, regular hull monitoring systems give shipboard assistance, in particular, to inform the crew about critical load levels. Next to this, hull monitoring data can also be used for onshore evaluation, e.g., of long-term hull girder loads. Hull monitoring systems are described and examples of application are given. DNV GL rules for the approval of hull monitoring systems are outlined.