CRS research is as varied as the CRS members. Traditionally, powering, maneuvering and hydro-structural response are high on the CRS research agenda. CFD, optimisation and data science tools are developed and applied with the objective to improve the design, building and operation of ships. The publications listed below give an impression of typical CRS research. If you wish to receive a full report/paper, please fill in the contact form.

Abstract

Design loads for extreme wave events on ships, such as slamming and green water, are hard to define. These events depend on details in the incoming waves, ship motions and structure layout, which requires high-fidelity tools such as CFD or experiments to obtain the correct loads. These tools (presently) do not have the capability to fully resolve the long-term statistics of rare events in all metocean conditions over the ship’s lifetime. The idea of ‘screening’ is to use lower-fidelity numerical methods to identify the occurrence of extreme load events based on an indicator. A good indicator has a strong correlation to the design load, but is easier to calculate. A high-fidelity tool can then be used to find the loads in these events. The low-fidelity statistics and the high-fidelity loads can be combined to define a design load and its probability. The present study compares different numerical screening indicators for green water loads on a containership against experiments. The quality of the identification of the critical events and the required computational time served as comparison metrics. This showed that screening both with potential flow tools and with coarse mesh CFD tools is feasible, provided the indicator, grid, time step and wave input settings are well chosen. The results from coarse mesh CFD are slightly better than from potential flow, but the computational costs are much higher. The results also show that the peaks and steepness of the relative wave elevation around the bow are suitable green water load indicators, as well as the undisturbed wave crests at the bow. Fine mesh CFD calculations were done for the identified events based on an example indicator, which resulted in a green water load distribution very close to that of the experiments. This study shows that screening could potentially reduce the required high-fidelity modelling time with up to ∼90% compared to common practice.

Keywords: screening, critical events, extreme events, waves, green water, design loads, multi-fidelity approach, potential flow, coarse mesh CFD

Abstract

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.

Keywords: parametric roll; numerical simulations; direct stability assessment; statistical comparison; deterministic validation

Abstract

This paper provides an overview of several validation studies that were carried out for a Boundary Element Method (BEM) for ducted propellers. These studies comprise validation of open water characteristics, pressure distributions, blade forces at inclined inflow and steady and unsteady cavity extent. The results of the BEM are compared with model test data, RANS results and full scale cavitation observations.

Keywords: Ducted propellers, Boundary Element Method (BEM), Validation.

Abstract

Numerical seakeeping codes for ships at forward speed in waves are often validated or tuned based on experiments, which makes knowledge about the experimental variability essential. This variability was evaluated using repeat tests during a state-of-the-art seakeeping campaign. A steep wave condition over the longitudinal basin axis (waveA) and a less steep oblique wave condition (waveB) were studied. Overall similarity as well as individual crest height, steepnesses and timing variability are discussed, because ship response is not equally sensitive for every point in the wave time series. The variability of the measured incoming wave crests and their timing increases with distance from the wave generator for waveA. The crest height variability for waveB is lower and more constant over the basin length (because the propagation distance to the model is constant in oblique waves and wave breaking is less likely). It was shown that only a small part of the variability close to the wave generator is caused by ‘input’ uncertainties such as the accuracy of the wave generator flap motions, measurement carriage position, their synchronisation and measurement accuracy. The rest of the variability is caused by wave and basin effects, such as wave breaking instabilities and small residual wave-induced currents from previous tests. The latter depend on previous wave conditions, which requires
further study. Further work on the influence of the wave variability on the variability of ship motions, relative wave elevation along a ship and impact loads on deck of a ship at forward speed will be presented in a next publication.

Keywords: seakeeping, waves, variability, repeatability, forward speed, experiments, basin, validation, accuracy, wave generation, tests.

Abstract

In order to validate numerical results or verify design choices using experiments, knowledge about the experimental variability is essential. This variability was evaluated for seakeeping tests at forward speed with a model in a steep wave condition over the long axis of a basin and in a less steep oblique wave condition, in a commonly applied test procedure. The incoming wave and response variability was evaluated using deterministic repeat tests. The results for incoming waves at some distance before the model have been published al-ready; the present study discusses the model responses. Overall time trace simi-larity as well as the amplitude and timing variability of individual wave crests and response peaks were studied, after assessing the input uncertainties. The re-sponse variability increases with distance from the wave generator for the wave crest height and (relative) ship motion peaks. The variability of the impact loads on a deck structure is large with a lot of scatter. Small wave-induced currents may build up differences in wave propagation speed between the repeat runs, which means that the seakeeping variability partly depends on previous wave conditions. A proportional relation could be identified between most response peaks and the corresponding incoming wave peaks. The timing variability of the response peaks follows from that of the incoming wave crests. Unfortu-nately, there is no direct relation between the response amplitude variability and that of the corresponding wave crest. The presented results can be used as refer-ence for the typical variability of free-sailing seakeeping experiments.

Keywords: Seakeeping, Waves, Variability, Repeatability, Experiments, Basin, Validation, Accuracy, Wave generation, Forward speed, Tests, Ship mo-tions, Relative wave elevation, Impact loads.

Abstract

The determination of roll extrema distribution for ships and offshore structures is a matter of great concern. In order to take into account the nonlinearities of roll motion, a lot of effort is devoted to the evaluation of the linear and the nonlinear damping coefficients. However, it should be noted that the way to use these coefficients in the overall design loop is equally important, and that inaccurate post-processing could spoil the effort invested in the evaluation of the linear and nonlinear roll damping. The objective of this paper is to investigate different methods to assess roll extrema distribution under nonlinear roll damping. In this study, the original model to represent the roll response consists in an ordinary differential equation with a nonlinear (quadratic) damping. All the other terms, such as the restoring force, are assumed linear through the whole paper. The time domain solution of this differential equation is the reference solution for the rest of the paper. Unfortunately, this resolution becomes CPU time consuming when long term roll value is required. Classically, a method of equivalent linearization is applied. This technique is well suited to frequency domain but unfortunately has a strong tendency to overestimate roll motions. To circumvent these issues, two different approaches (the Linearize & Match method introduced by Duthoit [3] and the equivalent design wave method) are presented, applied to two cases (a simple one degree of freedom illustrative case and a 6 degree of freedom realistic application case) and proved to give very good results for the estimation of the roll extrema distribution at almost no calculation cost.

Keywords: Roll, nonlinear roll damping, statistics.

Abstract

Experiments have been carried out with a model of the KCS container vessel. The model tests focussed on three out of five stability failure modes of the Second Generation Intact Stability Criteria that are currently being developed by the IMO. This paper focusses on two aspects of the prediction of the risk on parametric roll in regular waves. The first aspect is a check on the assumption of the IMO that simulation programs exist that properly can predict the risk on parametric roll; the second aspect is the effect of the roll damping model on the predicted parametric roll amplitudes.
The first aspect has been investigated by asking members of the CRS community1 to do simulations using proprietary programs. Five members responded to this request. The paper shows that a prediction of the roll damping based on exclusively geometrical information results in quite different answers. If the coefficients of a quadratic damping model are fixed in the input, the predictions of parametric roll angles in regular waves as a function of the wave amplitude are quite close for the different simulation programs. However, there is a significant discrepancy between simulations and experimental results with respect to the threshold wave amplitude at which the parametric roll phenomenon starts. An investigation in the modelling of the damping shows that this has some effect, but it does not explain the large difference. A final conclusion is, that the studied simulation programs will benefit from further improvements to predict all aspects of parametric roll events accurately. A good understanding of these aspects is considered important for a reliable Direct Stability Assessment.

Keywords: Parametric roll, roll damping, simulation, direct stability assessment.

Abstract

Slamming of ships is an important research topic in ship hydrodynamics and global structural assessment for many decades. To date, the prediction of slamming loads on a ship still is mainly restricted by the complexity of real-time scenarios including the actual attitude of the vessel, the detailed free surface pattern immediately around the ship, and the sensitivity of the slamming pressures to the local geometrical properties of the hull surface. The complex, three-dimensional interactions between the full ship and the wave field can only be feasible using Computational Fluid Dynamics (CFD). In the present study, CFD models were created for a moving ship in either head waves or oblique waves. The CFD models were set up using an in-house code, which was developed based on OpenFOAM®, supplemented with secondary developed tools. The CFD models adopted a framework including the continuity equation for incompressible fluids, the Reynolds-Averaged Navier-Stokes (RANS) equations for solving the momentum distribution of the flow field, and the Volume of Fluid (VOF) method for resolving the free surface. A total of 29 slamming events were simulated for the head-sea case within an acceptable time frame. The CFD results for the head-sea case showed excellent agreement with the corresponding model experimental data. This is especially true for the prediction of local pressure variations at the lower locations of the hull and the prediction of vertical forces on the ship segments. For oblique seas, the CFD results showed good comparisons with the model test data for the heaving, pitching motions, the hull pressure patterns, and the vertical forces on the ship segments. Higher discrepancies are evident in the comparison of rolling motions and transverse forces on ship segments. Further improvement of the CFD model for oblique waves is necessary for the future.

Abstract

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. Five
members of the Cooperative Research Ships (CRS) community participated in a comparison study where numerical simulations and model experiments
were compared for three of the five failure modes. This paper focusses on the results for the parametric roll failure mode in regular and irregular head
waves.

Keywords: : CRS Dynasty, ​parametric roll, head seas, regular waves, irregular waves, time domain simulation, model test