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.

Statistical Analysis on the Extreme Events of Big Waves Under Wave Climate Change Around Taiwan Waters

This study aimed to analyze the statistical characteristics of wave heights, wave energy and wave steepness, in order to investigate the wave climate changes around Taiwan Waters, especially for extreme events of big waves. The operational observation of Taiwan sea waves was initiated by the Central Weather Bureau in 1998; however, due to insufficient data length and low data space coverage, the data are unable to serve as references for long-term wave climate change research. Hence, this study adopted the SWAN (Simulation of Wave in Nearshore) Numerical Wave Hindcasting Method, which is a common method used in many studies, to hindcast the history of a wave field. The re-analysis on wind field data of the last 60 years (1948∼2008), published by the National Centers for Environmental Prediction (NCEP), was employed to make the wind field grid consistent with the hindcast wave field grid. Moreover, the Typhoon Wind Field Grid Down Scaling technique proposed by Winter & Chiou (2007) was applied to interpolate a U10 analysis field that better fits an actual typhoon wind field. The hindcast wave data were compared and validated with directional spectra, which were observed by the meteorological/oceanographic data buoys set up by the Central Weather Bureau and Water Resources Agency since 1997. Longdong, Hualien and Hsinchu Stations were chosen to represent the wave characteristics of sea areas around the island of Taiwan. According to observation data, model parameters were adjusted so that the hindcast results could be closer to observed data in Taiwan sea areas.

Stochastic Response and Stability Analysis of Two-Point Mooring System

The stochastic response and stability of a two-point mooring system are investigated for random sea state represented by the P-M sea spectrum. The two point mooring system is modeled as a SDOF system having only stiffness nonlinearity; drag nonlinearity is represented by an equivalent linear damping. Since no parametric excitation exists and only the linear damping is assumed to be present in the system, only a local stability analysis is sufficient for the system. This is performed using a perturbation technique and the Infante’s method. The analysis requires the mean square response of the system, which may be obtained in various ways. In the present study, the method using van-der-Pol transformation and F-P-K equation is used to obtain the probability density function of the response under the random wave forces. From the moment of the probability density function, the mean square response is obtained. Stability of the system is represented by an inequality condition expressed as a function of some important parameters. A two point mooring system is analysed as an illustrative example for a water depth of 141.5 m and a sea state represented by PM spectrum with 16 m significant height. It is shown that for certain combinations of parameter values, stability of two point mooring system may not be achieved.

Tsunami Attack Evaluation and Countermeasures for Moored Small Ships in a Port

There is increasing concern over the possibility of the occurrence of huge Toukai, Tonankai, and Nankai earthquakes in the Nankai Trough, located in the Pacific Ocean off Japan. It is estimated that there is a 50% probability of a tsunami being generated by an earthquake in this area over the next 30 years. A tsunami attack on the coast of Japan would not only increase the sea level, but would also create strong horizontal flows in bays and ports. In particular, along the coastal area of Osaka Bay, Japan, there are numerous small ports where pleasure ships and/or fishery boats are moored. Any small ships moored in these ports would be subject to violent motion by strong lateral flows, even if they are moored to each other. As a consequence, the mooring lines would be cut, ships would move uncontrollably, and causing damage to facilities. Thus, it is very important from the viewpoint of marine-disaster prevention to gain a better understanding of the motion of small vessels originating in tsunami flows and to develop countermeasures in the face of a tsunami attack. In this paper, basic analyses of the ship motions resulting from a tsunami attack are carried out, and possible countermeasures are investigated. First, we describe mathematical models approximating the flow of tsunami and the abrupt maneuvers of relatively small vessels while being moored. Next, numerical simulations of the lateral motion of moored ships resulting from a tsunami attack on a fishery port located in the Osaka Bay are carried out. Additionally, the possibility of relatively high tension to be generated along the mooring lines is evaluated. Finally, the results of computer simulations demonstrate that such hazardous phenomena can be addressed with appropriate countermeasures.

Statistical Analysis of a Set of Basin Waves

In a recent paper the effect of variations in calibrated wave parameters on wave crest and height distributions was analyzed (OMAE2010-20304, [1]). Theoretical distribution functions were compared to wave measurements with a variation in water depth, wave seed (group spectrum) and location of measurement for the same initial power spectrum. The wave crest distribution of the shallow water waves exceeded both second-order and Rayleigh distribution. Whereas, in intermediate water depth the measured crests followed the second order distribution. The distributions of the measured waves showed that different wave seeds result in the same wave height and crest distributions. Measured wave heights were lower closer to the wave maker. In this paper the results of the continued statistical analysis of basin waves are presented with focus on wave steepness and their influence on wave height and wave crest distributions. Furthermore, the sampling variability of the presented cases is assessed.

A Ship Flotation Control System Based on Pump-Driven Tank

Extensive development in ship motion control strategies and systems in recent decades has called for higher requirements in control system accuracy and reliability. In this paper, a ship flotation control system based on pump-driven active tank is established. A special case is discussed, where the ship is heeling under an asymmetric loading either by structural damage or asymmetric consumption of ammunition. The purpose of the control system is to keep the ship in upright floating position in waves by transferring liquid between the tanks. Kalman filter is designed to eliminate the wave disturbance, in order to identify the heeling angle caused by asymmetric loading change. The effect of wave disturbance at different wave encounter angles, wave heights, as well as ship speeds is analyzed. Tuning of filter parameters such as initial state variables, initial error covariance and noise covariance is performed to achieve better filtering performance for different parameters of waves and ship motion. To verify the control model, simulation is conducted for a 3340t ship and the simulation results are compared with the theoretical calculations. The research results show the applicability and efficiency of Kalman filter. The concept of the control system presented in the paper is helpful to improve ship stability and safety when ship upright floating condition is disturbed.

Higher Order Wave-Current Elevations in Deep Water

Wave-current interaction refers to the interaction between surface gravity waves and ocean current flow. This interaction implies an exchange of energy, i.e. both the waves and the current are affected. The present paper describes the calculation of wave elevations in higher order unidirectional, irregular waves with a uniform current in deep water. Results for regular waves are compared with those obtained for Stokes second and third order waves with uniform current according to the method described by Fenton [1]. The results for higher order wave elevations in irregular waves have been compared with waves and current generated in a model test basin and reasonable agreement has been found.

Wave Height Measurements Behind Submerged Lens-Shaped Structures

The ability of submerged lens-shaped structures to focus linear surface waves in deep water is explored through a series of experimental tests in a wave making basin. Three lenses were designed using a combination of linear strip theory and a surface wave analogy to geometrical optics. Two of these lenses were designed to focus waves of a single wavelength of 0.482 m (18.97 in.), one with a focal length to lens width ratio (f-number) of 2.0 and the other with an f-number of 0.5. The third lens was designed to function as a compound lens that could focus a range of wavelengths of between 0.39 m (15.37 in.) and 0.694 m (27.32 in.) at an f-number of 2.0. Using resistance wave height gauges, the sensitivity of wave height at the focus to variations in wavelength from between 0.39 m (15.37 in.) to 0.61 m (24.01 in.) was experimentally measured for all three lenses; the sensitivity of wave height at the focus to variations of lens depths of submergence spanning the range of between 0.75 to 1.25 times the design submergence depth was also explored for the two simple lenses. It was found that the linear strip theory and geometrical optics approach predicted the wave amplification to within ten percent at the design wavelengths and depths, but that the longitudinal position of the experimentally observed focal lengths differed substantially from that expected, by as much as a factor of 2.2 for an f-number of 0.5. Additionally, while the theory predicted a single focal point for each lens, multiple focal points were found to exist behind the compound lens.

Laboratory Observations on the Evolution of Deep-Water Wave Packets

New experimental data for the evolution of deep-water wave packets has been presented. The present experimental data shows that the local maximum steepness for extreme waves is significantly above the criterion of the limiting Stokes waves. The wavelet spectra of the wave groups around the breaking locations indicate that the energy of higher harmonics can be generated quickly before wave breaking and mainly concentrate at the part of the wave fronts. After wave breaking, however, these higher harmonics energy is dissipated immediately. Furthermore, the variations of local peak frequency have also been examined. It is found that frequency downshift increases with the increase of initial steepness and wave packet size.