A Modular Unified Model of Ship Manoeuvering in Irregular Wind Generated Waves

Abstract

With the introduction of the Energy Efficiency Design Index, concerns have been raised to the survivability of ships with small engine power to maintain manoeuvring capabilities in adverse weather conditions. Therefore, Sui (2021) developed a simulation model from ‘tank to wake’ of a manoeuvring ship in irregular waves to advice a minimum power limit for regulation purposes. The original model consists of the Kijima model extended with the mean second order wave drift forces of a VLCC tanker published by (Yasukawa et al., 2019).
The research reported in this thesis is conducted as part of this work. Therefore, the aim is to improve the fidelity of hydrodynamics. Therefore, a modular unified model is established where the manoeuvring forces of Kijima model are included as non-linear damping in the Cummings equation. Whereas only the mean second order wave drift forces were included in the original model, the first and second order wave excitation forces, the radiation forces, and the restoring forces are included in the new model. The convolution integrals of the radiation damping forces are circumvented with state space models with the identification method of Perez & Fossen (2009). The input of the forces are the force response amplitude operators, the quadratic transfer functions, and the added mass and damping coefficients in the frequency domain obtained from the diffraction analysis performed on a barge in Ansys Aqwa, because the hull geometry of the benchmark ship is unknown.
Moreover, a wave generation model is included capable of generating sea surface realizations for irregular waves from multiple wave spectra. In this study, irregular long crested wind generated waves are generated from a JONSWAP spectrum. The JONSWAP spectrum is generated based on the wind growth curves and thus depend on the wind speed. Consequently, the significant wave height and the peak period of the spectrum are consistent.
The model is validated against the measurements of the benchmark turning trail and compared to the original model. The original model is slightly more accurate. Nevertheless, both results are considered acceptable, and, it is concluded that that the fidelity of the models is similar.
Thereof, the model is used to simulate a ship escaping an increasing storm based on the case of the Pasha Bulker. Therefore, a turn from beam to head waves starting at a low velocity is simulated in sea state 7, 8, and 9. In the original simulation, the ship failed in sea state 9. With the new model, the ship is able to perform all turns. Therefore, it is concluded that the ship is not underpowered. Consequently, the engine power is reduced. With half installed power the ship fails to escape the storm and it is endangered.