CFD Investigation into Seakeeping Performance of a Training Ship

The numerous ship accidents at sea have usually resulted in tremendous loss and casualties. To prevent such disastrous accidents, a comprehensive investigation into reliable prediction of seakeeping performance of a ship is necessarily required. This paper presents computational fluid dynamics (CFD) analysis on seakeeping performance of a training ship (full scale model) quantified through a Response of Amplitude Operators (RAO) for heave and pitch motions. The effects of wavelengths, wave directions and ship forward velocities have been accordingly taken into account. In general, the results revealed that the shorter wavelengths (l/L ? 1.0) have insignificant effect to the heave and pitch motions performance of the training ship, which means that the ship has good seakeeping behavior. However, the further increase of wavelength was proportional with the increase of RAO for her heave and pitch motions; whilst it may lead to degrade her seakeeping quality. In addition, the vertical motions behavior in the following-seas dealt with higher RAO as compared with case of the head-seas condition. Similarly, the subsequent increase of the ship forward velocity was prone to relatively increase of the RAO for her heave and pitch motions especially at l/L ? 2.0. It was merely concluded that this seakeeping prediction using CFD approach provides useful outcomes in the preliminary design stage for safety assessment of the training ship navigation during sailing.

A Comparison of Numerical Simulations and Model Experiments on Parametric Roll in Irregular Seas

The recently finalised Second Generation Intact Stability Criteria (SGISC), produced by the International Maritime Organisation (IMO), contain a level 3 assessment, the so-called Direct Stability Assessment (DSA). This assessment can be carried out using either model experiments or simulations. The fact that such a choice is given implies that the methods are equivalent in accuracy. This assumption has been verified, for one case, by the Cooperative Research Ships (CRS) community. The verification was based on new model experiments and calculated results, using four different programs owned by different CRS members. Results of the verification of the parametric roll failure mode in regular waves were published before, but this study concerns results in irregular seas. The experimental and numerical results are compared in both probabilistic and deterministic manners. The probabilistic comparison showed that the simulation programs considered are sometimes conservative and sometimes non-conservative in the prediction of the probability of an extreme value. The deterministic comparison in head seas showed that parametric roll events were predicted in the simulations in a wave train that showed no sign of important roll events in the measurement. The deterministic comparison in the following seas, on the other hand, showed an accurate fit of experimental and numerical results. It is suggested that predictions could possibly be improved by adding non-linear diffraction forces to the numerical model.

Probability of Ship High-Runs from Phase-Space Data

A clustering scheme has been applied for capturing qualitatively different surge motion patterns in the phase space. The scheme enables the identification of "high-run" incidents as soon as such motions are triggered and while their phenomenology has not yet been well developed. A "high run" is a surf-riding-like behavior, appearing in irregular following seas. The concept of finite-time coherent sets is exploited for deriving estimates of the probability of high-runs. The method is verified by identifying independently the corresponding hyperbolic Lagrangian coherent structures; then, consistency is sought between the two approaches. An important feature of the method is that it does not rely on the use of some empirical criterion for the high-run threshold, such as one based on the exceedance of an arbitrary high-speed level. Despite its computational burden, the proposed scheme offers "objective" statistical information on a ship's high-run tendency that can be used for benchmarking simpler (approximative) probability calculation schemes. 1. Introduction Current efforts to assess a ship's tendency for abnormal behavior in extreme seas are still limited from our inadequate grasp of the full variety of nonlinear ship motion phenomena that could be realized in an irregular seaway. A classification of these motion patterns would provide a sound basis for developing probabilistic calculation methods of ship operability and safety in extreme seas. A few recent research efforts in our group have been related to this target. In one case, it was endeavored to distinguish ship high-runs from ordinary surging, by engaging the concept of instantaneous wave celerity (Spyrou et al. 2014). In another, the derivation of a practical metric for the probability of high-run was pursued (Belenky et al. 2016). Also, high-run and broaching-to statistics were produced through a direct approach based on assigning prescriptive exceedance thresholds (Spyrou et al. 2016b). Moreover, the theory of surf-riding was extended for bichromatic waves, revealing some rather unexpected types of motion (Spyrou et al. 2018). Even richer phenomena could be conjectured for a multifrequency environment.

Quantitative Evaluation of Ship Operational Effect in Actually Encountered Sea States

Since ships are being operated under consideration of the safety for lives and properties, economical reasons and so on, the sea states in natural phenomena and those actually encountered by ships are thought to be different, the latter has some effects of human operational factors (called as “ship operational effect”). Evaluating the ship operational effect in detail is important to consider rational wave design loads for hull structure strength. The purpose of this study is to evaluate the ship operational effect in actually encountered sea states quantitatively. As the first report, comparison was made between IACS Rec.34 (a kind of the observed sea states in natural phenomena) and forecasted sea states corresponding to AIS data of ships (a kind of the sea states data actually encountered by ships) on the North Atlantic. Comparisons among the encountered significant wave heights by merchant ships such as bulk carriers, oil tankers and container ships and those specified in IACS Rec. 34 were carried out. Furthermore, the wave headings regarding the encountered waves were investigated. Finally, the relationships between encountered significant wave heights and ship speeds were derived to confirm the ship operational effect. It was confirmed from the results that the actually encountered wave heights were smaller than those in IACS Rec. 34, through comparing the exceedance probability of the significant wave heights for each type of ships and IACS Rec. 34. The exceedance probability in the encountered beam seas is relatively lower compared with those in the encountered head and following seas. The results also show that ship speeds decrease when the encountered wave heights become larger.

Probability of Ship High-Runs from Phase-Space Data

A clustering scheme has been applied for capturing qualitatively different surge motion patterns in the phase space. The scheme enables the identification of “high-run” incidents as soon as such motions are triggered and while their phenomenology has not yet been well developed. A “high run” is a surf-riding–like behavior, appearing in irregular following seas. The concept of finite-time coherent sets is exploited for deriving estimates of the probability of high-runs. The method is verified by identifying independently the corresponding hyperbolic Lagrangian coherent structures; then, consistency is sought between the two approaches. An important feature of the method is that it does not rely on the use of some empirical criterion for the high-run threshold, such as one based on the exceedance of an arbitrary high-speed level. Despite its computational burden, the proposed scheme offers “objective” statistical information on a ship’s high-run tendency that can be used for benchmarking simpler (approximative) probability calculation schemes.

Probability of Ship High-Runs from Phase-Space Data

A clustering scheme has been applied for capturing qualitatively different surge motion patterns in the phase space. The scheme enables the identification of “high-run” incidents as soon as such motions are triggered and while their phenomenology has not yet been well developed. A “high run” is a surf-riding–like behavior, appearing in irregular following seas. The concept of finite-time coherent sets is exploited for deriving estimates of the probability of high-runs. The method is verified by identifying independently the corresponding hyperbolic Lagrangian coherent structures; then, consistency is sought between the two approaches. An important feature of the method is that it does not rely on the use of some empirical criterion for the high-run threshold, such as one based on the exceedance of an arbitrary high-speed level. Despite its computational burden, the proposed scheme offers “objective” statistical information on a ship’s high-run tendency that can be used for benchmarking simpler (approximative) probability calculation schemes.

Experimental and Numerical Study for Gap Resonance of Drillship Moonpool in Waves With/Without Forward Speed

Experimental and numerical studies are carried out to examine the moonpool gap resonance for a drillship at both stationary position and forward speed conditions. The moonpool size and draft are also changed to study their effects for the gap resonance phenomenon. An OpenFOAM based CFD model is developed and the numerical results show good agreement with model tests. Both piston and sloshing modes gap resonances are clearly observed. The study shows that the resonance frequency and RAO of the wave elevation inside the moonpool are subject to the effects of moonpool length, drill ship draft and ship forward speed. The model test shows that moonpool elevation RAO generally significantly increases in head seas and noticeably decreases in following seas condition. It is interesting to notice that the wave flume sidewall significantly depresses the moonpool elevation RAO at a certain frequency regardless of moonpool length and draft. Further study shows that the presence of the flume sidewall results in a trapped mode that coincides with the moonpool piston mode resonance at zero speed. This depresses the peak of the moonpool resonance, which occurs at the same frequency.

Kinematics and dynamics of a novel 3-degree-of-freedom wave energy converter

A wide range of wave energy converter technologies has been proposed so far. Oscillating body systems are an important class of wave energy converters, which typically harvest wave energy from a single degree-of-freedom response. This article presents a novel 3-degree-of-freedom (heave, pitch and roll) wave energy converter which extracts and converts wave power efficiently. The 3-degree-of-freedom mechanism is invented to absorb and convert wave energy, no matter which direction the waves propagate. The hydraulic power take-off system comprises novel energy conversion devices which can be superposed to realize high-power conversion to produce electricity. First, the power conversion principle of the wave energy converter is proposed, and the kinematics of the 3-degree-of-freedom mechanism is derived. Then, the governing equations of the 3-degree-of-freedom wave energy converter are established. Linear time-domain simulations are performed to calculate instantaneous and mean power outputs. The numerical results show that the rated power absorbed by the wave energy converter is up to 4.2 MW and the efficiency is over 80%. And the power matrix of the wave energy converter is obtained from a range of simulations under different sea states. Finally, directional performance of the wave energy converter is investigated, and the numerical results show that the wave energy converter can operate across a range of incident wave directions, but is most efficient in head, beam and following seas.