Design Criteria for Seakeeping and Stability of Fishing Vessels in Regular Waves
Examensarbete för masterexamen
Gómez Rojas, Nestor Juan de Dios
The design requirements for new fishing vessels tend to be mostly based on existing fishing vessels. The design stage normally does not include an initial study of the seakeeping and dynamic stability of the vessels, due to the lack of time and budget and of documented criteria for this vessel segment. This study focuses on the compliance of a selected stern trawler fishing vessel with general existing seakeeping and parametric rolling criteria (e.g., ISO, 1997; Lewis, 1989; Nordforsk, 1987) to (a) select suitable seakeeping criteria for fishing vessels and (b) propose an assessment methodology to optimize such criteria, using the software SHIPFLOW Motions. To test compliance, a set of typical working conditions (e.g., trawling, lightship, fully loaded) for the selected fishing vessel has been computed employing a commercial computational fluid dynamics (CFD) software tool, SHIPFLOW, which is based on non-linear potential flow theory. The results (ship responses or motions) have been post-processed through a code developed in MATLAB (RBSMC). Both seakeeping and the second-generation IMO stability criteria (specifically parametric roll due to its occurrence in this type of vessel despite its geometrical characteristics that differ from tankers and container vessels, which are the most susceptible to the phenomenon) have been assessed through a systematic study of the influence of the centre of gravity over the ship responses under different working conditions. This approach allowed to understand the behaviour of this vessel, where its responses were highly dependent on the working conditions and wave characteristics. It was seen that the resonance of the studied motions (heave, pitch, and roll) take place at longer wavelengths than a tanker or a container. The centre of gravity location highly influences the responses. It was found that the trawling condition was one of the most affected ones, presenting high amplitudes of pitch and roll. Instead of applying an external force for modelling the trawling force, the trawling condition was modelled through shifting the position of the centre of gravity. Under this working condition, the vessel showed high pitch and roll angles of low acceleration that surpassed the acceptable limits. Meanwhile, in other conditions, the responses complied with the criteria. The lightship condition was the most sensitive condition in terms of parametric roll, where the variation of GM (metacentric radius) is susceptible to the occurrence of parametric roll, and the right waves could trigger the phenomenon. In general, the roll damping coefficient of a fishing vessel is high enough to prevent parametric rolling. The selected criteria often resulted in margins which were larger than the responses of the vessel except for the trawling condition for which the limiting values of the criteria were smaller than the obtained responses. The presented methodology and the customized criteria in this work can be used for evaluation of fishing vessels in the design stage.
Seakeeping , Regular Waves , Fishing Trawlers , Parametric Rolling , SHIPFLOW , IMO Second Generation Intact Stability Criteria