Green Water on FPSO Analyzed by a Coupled Potential-Flow-NS-VOF Method

Abstract

The nowadays frequent use of FPSOs for offshore oil production in areas prone to green water events has increased the industrys focus on wave-induced impact loads as an important design parameter. This is a complex hydrodynamic problem where simplified engineering methods are often used in connection with model testing. Various efforts have been presented during the recent 10–15 years to establish reasonably good industry design tools, while the use of fully nonlinear methods and CFD is still in its development. The main focus of this paper is to investigate the potential of a simplified coupled method between a potential theory based Green Water engineer tool (Kinema3) and the commercial CFD tool Star-CCM+ based on its Navier-Stokes Solver (NS) and the Volume of Fluid (VOF) method. Results from a case study application on a large FPSO are validated against model test data. The case study contains analyses of the FPSO in long crested regular seas, both in fixed and in moored conditions. Three different heading directions are included. The approach for modeling green water events uses a Finite-Volume-VOF method with a complex velocity inlet boundary condition. Thus the Kinema3 engineering tool is used to generate simplified spatio-temporal inlet conditions from the relative wave elevation and wave kinematics at the bulwark, based on linear potential theory combined with nonlinear random wave kinematics. The VOF method is then used to model the detailed flow on deck, including impact forces on deck structures. Kinema3 can also generate simplified estimates for the peak water height, velocity as well as impact force values assuming an extended dam-break approach together with a simplified, local 2D deck layout, and comparisons to the CFD results show an overall fairly good agreement although flow details on deck can of course not be expected to be modeled that well. Comparisons of the above results to model test data show good agreement both for the relative wave height, water height and impact force level, in regular and irregular waves. Detailed time histories, including force rise time, from the coupled Kinema3 - Star-CCM+ CFD simulation analysis are quite similar to the measured ones. The CPU time consumption for the coupled simulation is moderate compared to a full CFD simulation of the FPSO in waves. Hence the achieved calculation time and the simplicity of the simulation setup of the numerical simulation makes this method an interesting candidate for industrial use. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.

3D Numerical Simulations of Green Water Impact on Forward-Speed Wigley Hull Using Open Source Codes

“…where m denotes the ship mass; z is displacement in heave; F z represents the total force in the vertical direction; I yy denotes moment of inertia in pitch; θ represents angle in pitch, N is moment of force in pitch. The Newmark-β method [32] with γ = 0.25 and β = 0.5 was used to solve Equations ( 13) and (14). Mesh deformation was achieved by moving the node points of the mesh according to the ship motion and without changing the mesh topology.…”

“…In this numerical idealisation the green water event is divided into four different stages and each of them is modeled separately using distinct methodologies. The freeboard exceedance is used as input for a dam-break model [10][11][12][13][14] or as a solution to the shallow water equations [15,16] that can simulate hydrodynamic behaviour of on-deck flows. The green water loading on deck structures is approximated either analytically by the similarity method [17] and Wagner-type analysis [18] or empirically by utilizing extensive scale model tests [3].…”

Study on green water prediction for FPSOs by a practical numerical approach

“…Based on this approach, many efforts have been made to derive analytical and empirical formulas to predict hydrodynamic loads caused by green water flow, which can be found in Buchner (1995Buchner ( , 2002, Hamoudi and Varyani (1994), Ogawa (2003), Greco (2001), Kapsenberg and de Kat (2000), and Stansberg and Berget (2009). Alternatively, there have also been researches that used CFD methods for the local interaction between green water flow and structures, including Pham and Varyani (2005), Shibata and Koshizuka (2007), and Pakozdi et al (2014).…”

“…The software Kinema was further developed to extend applicable wave directions and was used to study green water on FPSOs in Santos Basin (Schiller et al, 2014). In Pakozdi et al (2014), the selected relative wave elevation and freeboard exceedance (corrected by Kinema3) were used to setup local CFD simulation for critical green water event by using VOF-based software Star-CCM+. Greco et al (2015) proposed a 3D domain-decomposition approach, combining a weakly nonlinear potential flow solver with a 2D shallow-water approximation of shipped water.…”

Prediction of wave-induced loads on ships: Progress and challenges

“…It is concluded that the differences in water ingress predictions are mainly due to free surface capturing by the VOF method and much refined mesh requirement in the region of open holds of the vessel. Pakozdi et al (2014) investigated the viability of a simplified coupled method between a potential theory based green water engineering tool, Kinema3, and the commercial CFD tool Star-CCM+. Results from a case study application on a large FPSO are validated against model test data.…”