Simulation Modeling of a Ship Propulsion System in Wavesfor Control Purposes

The article deals with a simulation approach to the representation of the ship motions in waves, interacting with the propulsion system behavior (diesel engine and propeller). The final goal is the development of a simulator, as complete as possible, that allows the analysis of the main engine thermodynamics in different sea conditions, also in the unfavorable event of dynamic instability of the hull, and the correct management of the other propulsion components. This latter aspect is particularly interesting in some of the last new energy solutions for decarbonization of ships, concerning, for example, auxiliary electric motors, powered by batteries, to support the traditional diesel-mechanical propulsion (especially in heavy weather conditions). From this point of view, a proper analysis of the engine dynamic performance, affected by particular sea states, is fundamental for a smart management and control of shaft generators/auxiliary electric motors, batteries, etc. To this end, the work presents and highlights the main features of a ship simulator, suitable for the study of the new propulsion solutions that are emerging in maritime transport. Some representative results will point out the complex non-linear behavior of the propulsion plant in waves. Moreover, a parametric roll scenario will be investigated, in order to highlight the capability of the conceived simulator in modeling the effects of the dynamic instability of the hull on the propulsion plant.

CONTROLLING THE PARAMETRIC ROLL OF A CONTAINER SHIP MODEL BY CHANGING THE HEADING ANGLE

Parametric roll on ships is an auto-parametric resonance phenomenon whose onset causes a sudden rise in roll oscillations leading to dangerous situations for the ship, the cargo and the crew. In the paper, we have numerically investigated the effect of modifying the heading angle on the roll amplitudes. We followed three strategies. In the first, we allowed the heading angle to decrease with a constant angular acceleration so that the encounter frequency has left the dangerous region of the resonance. However, this option involves changing the course of the ship in the long run, which is of course a shortcoming. In the second strategy, we changed the heading angle up and down around an average value that generates large roll amplitudes, by using different periodic sinusoidal or triangular profiles. The beneficial effect of this action is to keep the course, even if at the cost of a momentary delay. We noticed that both control techniques listed above generally managed to significantly reduce the roll amplitudes if certain thresholds have been exceeded. As a last idea for decreasing the parametric roll amplitudes, we used the combined effect of ship forward speed and heading angle change.

Numerical Study on Ship Parametric Roll in Head Waves

Ship parametric roll is one of the main reasons for marine accidents and is introduced into the second-generation intact stability criteria by the International Maritime Organization (IMO) recently. In this paper, a 6-DOF three-dimensional time-domain model based on the IRF (Impulse Response Function) method is constructed to predict large-amplitude ship motions and investigate the phenomenon of parametric roll in head waves as well as major factors. The F-K forces and the restoring forces are calculated on the instantaneous wet surface while the radiation and diffraction forces are kept linear and transformed from frequency-domain results calculated with the three-dimensional Havelock form translating-pulsating source green function method. The proposed weakly nonlinear time-domain model is used to simulate motions of the C11 containership, which predicts the occurrence of the parametric roll successfully and shows a good agreement with the experimental data in amplitude. The inner mechanism of parametric roll is revealed by investigating the time-history and resonance frequencies of restoring forces and coefficients numerically.

Second Generation Intact Stability Criteria Fallout on Naval Ships Limiting KG Curves

The International Maritime Organization (IMO) finalized the Second Generation Intact Stability Criteria (SGISC), in February 2020. They are intended to be included in Part A of the 2008 International Code on Intact Stability in the following years. The SGISC consider five modes of dynamic stability failure in waves: parametric roll, pure loss of stability, surf-riding/broaching to, dead ship condition and excessive acceleration. In this paper, two semi-displacement, round bilge and transom stern hull forms, the parent hull of the Systematic Series D and the ONR Tumblehome, i.e. typical naval hull forms, are examined. Although naval ships are not directly impacted by SGISC, they are sensitive to dynamic stability failure phenomena due to their geometry and range of service speeds. The procedures to assess the ship vulnerability to the dead ship condition and excessive acceleration criteria, referring to the latest drafts of the criteria (SDC 7/5, 2019), were implemented in Matlab®,. The limiting KG curves associated with this set of criteria were obtained for each vessel. The minimum allowable KG curve associated with the excessive acceleration criterion was compared with the maximum allowable KG curve associated with dead ship condition, to investigate the existence of a safe operational area.

Sensitivity Analysis in Parametric Rolling of a Modern Cruise Ship Using Numerical Simulations in 6-DOF

Parametric roll is a nonlinear phenomenon that can result in large roll angles coupled with significant pitch motions. These motions might induce large loads on the ship structure, and compromise the safety of the crew and the cargo. The severity of the motions might reach to such levels that capsizing might occur. In this study sensitivity analysis in parametric rolling of a modern cruise ship is investigated using numerical simulations. Several parameters were considered as sources of uncertainty such as the combined effect of GM and roll radius of gyration, roll damping, ship speed, and fin characteristics. In terms of fin characteristics, fin angle rate and maximum angle, fin area and aspect ratio, and fin gains were investigated. Additionally, the non-ergodicity of parametric roll was studied as well as the effect of simulation duration on the statistics of parametric roll. The simulations were carried out with a hybrid time-domain seakeeping and manoeuvring code. The time-domain code was used in combination with a strip-theory based frequency-domain program in order to calculate diffraction and radiation forces as well as added-mass. The time-domain code was able simulate the dynamic behavior of a steered ship in 6-DOF, where the motions can be large up to the moment of capsize.

Influence of Systematic Hull Shape Variations on Ship Stability Performances in Waves

The sensitivity of ship stability performance in waves to geometric variation has been investigated by means of a simulation-based design framework. The study was devoted to assess the influence of hull geometry variations on some stability failure modes, namely, parametric roll (PR) and pure loss of stability (PLS). The application has been developed by using a representative model of a postpanamax container vessel. PR and PLS phenomena have been investigated by the application of second-generation intact stability criteria (SGISc). The initial multidimensional design space has been filled by 500 design configurations identified by means of a design of experiments approach. A method developed in-house, combining the subdivision surface and free-form deformation approaches, has been used to create the whole set of design alternatives. The generated design configurations have been assessed analyzing the results derived from application of the first- and the second-level SGIS vulnerability criteria for both the selected stability failure modes. To strengthen the correlation behaviors, the design space has then been further explored by using 10k design configurations exploiting the capabilities of a surrogate model-based approximation, relying on a Gaussian process formulation. The study has been focused on the correlations among the variables and the response functions, i.e., the outcomes of the SGIS vulnerability criteria. The significance, in terms of effects, of each geometry shape variable has been investigated. Results have been discussed in the light of the SGISc structure, to provide further insight into this innovative safety framework for a modern approach to intact stability. 1. Introduction In the last 10 years, the development of the so-called second-generation intact stability criteria (SGISc) has been one of the most engaging topics addressed by the Sub-Committee on Safety Design and Construction (SDC) of the International Maritime Organization (IMO).