Numerical Test Tank: Simulation of Ocean Engineering Problems by Computational Fluid Dynamics

Abstract

Abstract With the increase in knowledge in computer-based simulation methods and recent advances in high-speed computing, solutions to complex fluid-structure interaction problems, such as transient wave impact on fixed and floating structures, can now be studied in more detail. Numerical results, combined with model test measurements for verification, form the basis of the concept of the Numerical Test Tank (NTT) concept. Sample results of the application of Computational Fluid Dynamics (CFD) methods to the solution of two important ocean engineering problems are presented to illustrate the present methodology, and are then compared to model test results. Specifically, CFD solutions are obtained for green water impact on a full-scale, FPSO hull and deck structure. The present numerical method employs a fully three-dimensional, transient, viscous Navier-Stokes solver based on the FLOW3D/ 96 CFD code. The results of the simulation are shown to produce at least qualitative agreement with model test results with respect to the shape of the incident, reflected and on deck- transient surface wave profiles. Some differences between the simulation and test results arise due to details of the simulation set-up, such as cell sizes, cell distribution, convergence criteria, turbulence model, and boundary conditions. Additionally, CFD methods are used to compute current force coefficients for a scale model of a 200,000 DWT tanker. Computed force coefficient values are Froude-scaled and compared to OCIMF recommendations, which are based on Froude-scaled model test data. Preliminary CFD results indicate basic agreement with OCIMF data, however future computations of coefficients for a full-scale hull, and further analysis of test data will be necessary in order to investigate viscous scale effects in more detail. The apparent utility of the Numerical Test Tank, as illustrated by the example solutions presented, indicates that this computational approach holds promise for improving detailed knowledge of design loads for fixed and floating structures subject to severe wave and current loading. Introduction The design of FPSO systems for harsh wave environments often requires model testing of green water impact on deck structures. In 1993, SOFEC conducted 100-yr survival tests of the 140,000 DWT FPSO for AOPC's Liuhua field at MARINTEK; Figs. la, 1b and 1c show a typical sequence of on-deck green water flow and impact. During these tests, a limited number of instruments were used to determine impact loads on the turret surround structure and the breakwater protecting the process modules. The measured loads and volume of water which passed over the breakwater were sufficiently large so as to warrant a second set of green water tests at MARIN (Fig. 6). These tests were specifically designed to study wave impact and breakwater optimization. In September 1996, typhoon Sally passed within 10 nautical miles of the Liuhua FPSO, generating waves (88 feet) and winds (112 knots) associated with a 200-yr event.