Experimental and Numerical Investigation of Green Water Occurrence for KRISO Container Ship

Abstract

The occurrence of green water on the deck of Korea Research Institute of Ships & Ocean Engineering (KRISO) container ship is investigated using model test experiments and a fully coupled impulse response function (IRF)–computational fluid dynamics (CFD)–based numerical approach. In the experimental study, green water pressure over the deck and superstructure is investigated for different regular head wave conditions (wavelength/ship length ratio: .8–1.5) and vessel speeds (Froude number: .055–.166). The impact pressure on the deck is found to be highest at a wavelength/ ship length ratio of 1.2 and increases drastically with the increase in Froude number. The variation of green water pressure with wave steepness is linear for points on the forward deck and quadratic for the superstructure. In the second part, a coupled IRF-CFD–based numerical method is developed in which the global hydrodynamic forces such as radiation–diffraction and Froude–Krylov force are computed using a potential flow solver, whereas the local pressure due to the shipping water impact is computed using CFD and added as an external force. Comparisons of vessel motions and green water pressures with experiments indicate that the coupled IRF-CFD method can be a robust and efficient tool to predict shipping water loads on ships. Introduction The prediction of green water shipped on the deck and the resulting impact on ships and offshore structures have been an important research topic in the last couple of decades because it is directly related to the safety of the structure at sea. Historically, shipping of water on the deck had caused many serious damages to the bow and superstructure, and some of them have even capsized; e.g., the Schiehallion Floating Production Storage and Offloading (FPSO) was damaged severely in November 1998 because of steep waves. These incidents resulted in considerable research on this topic in the past two decades. The primary focus has been on the understanding of critical aspects and the accurate prediction of green water loading which requires proper modeling of green water occurrence. Also, accurate calculation of the green water loads on the deck and superstructure is very important for structural design load calculation. However, this is a nontrivial task because it depends on many physical and geometrical factors such as wave steepness, the wavelength- to-ship length ratio, hull shape around the bow region, and vessel speed. Hence, efforts have been made to develop suitable numerical models that may consider most of these aspects under necessary underlying assumptions.