Showing papers in "Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme in 2003"

A Strongly-Nonlinear Model for Water Waves in Water of Variable Depth—The Irrotational Green-Naghdi Model

Abstract: Summary and Conclusions The Irrotational Green-Naghdi equations are derived for finitewater-depth conditions, and tested numerically to show their self-convergence and accuracy. The linear and nonlinear dispersion ofthe permanent progressive wave solutions of the IGN equationsconverged to known exact solutions as the Level of the equationsincreases. The numerical solution of the IGN equations for un-steady problems, such as the run-up of solitary waves and nonlin-ear shoaling over a sloping beach, also converged. It appears thatthe IGN Level 3 equations can provide very accurate solutions formost engineering problems. The IGN Level 2 equations showedcomparable accuracy with the optimal WKGS equation set, whichis known to be the most accurate Boussinesq-type model.Once the accuracy of the IGN equations at each level is iden-tified, the level can be optimally chosen in future application ofthe model to various water-wave problems. Acknowledgment JWK and RCE have been supported by the ONR, Grant No.N000114-98-1-0800; the Program Manager was Dr. C. LinwoodVincent. JWK was also supported by Korea Science and Engi-neering Foundation for an initial stage of the present work. KJBwas supported by Grant No. R01-2000-00321 from the KoreaScience and Engineering Foundation. These grants are gratefullyacknowledged ~SOEST No. 6056!.

Fluid dynamic loading on curved riser pipes

Abstract: In order to gain a preliminary understanding of the fluid dynamics developed past a curved riser pipe, a numerical investigation into the flow past curved cylinders at a Reynolds number of 100 has been performed. To approximate the flow conditions on curved riser pipes, different velocity profiles and flow directions were applied and the corresponding results compared. In addition, the fluid dynamic loading and the wake structures for curved cylinder flows were investigated. The fully three-dimensional simulations were computed with a spectral/hp element method. The computational results were compared with experiments undertaken in the towing tank facility of the Department of Aeronautics of Imperial College.

Wave Trapping by Axisymmetric Concentric Cylinders

Abstract: It is both a pleasure and privilege to present a paper on the uniqueness of linearised water waves at this mini-symposium in honour of Professor John Wehausen whose classic review article Surface Waves (with E. V. Laitone) has done so much to influence workers in the field in the forty-two years since its publication. The question of the uniqueness of solutions to the linearised water wave equations was settled once and for all in a paper in the Journal of Fluid Mechanics by M. McIver (1996). She constructed a solution for the motion between a pair of fixed rigid surface-piercing cylinders in two dimensions which decayed at large distances from the cylinders. Soon after she was joined by P. McIver (1997) in producing an axisymmetric example in the form of a fixed rigid surface-piercing toroid of a special shape which supported an oscillatory motion in its interior fluid region whilst the motion in the exterior region decayed to zero. This wave trapping effect or non-uniqueness occurred for a particular relation between the wave frequency and the toroid geometry. In the present paper we show that such a phenomenon can occur for simple geometries also. In particular we show that wave trapping can occur in the annular region between two partially immersed vertical concentric circular cylindrical shells for particular values of radii and frequencies.© 2002 ASME

Multiple Stable/Unstable Equilibria of a Cylinder in the Wake of an Upstream Cylinder

Abstract: The static equilibrium position and its associated dynamic stability of a cylinder situated in the wake of an upstream cylinder is investigated in this paper Both the upstream and downstream cylinders are elastically mounted on springs to allow for streamwise and transverse displacements Due to the wake effect the downstream cylinder is subject to a lift force as well as a drag It is shown that under certain flow conditions there exist multiple stable and unstable equilibria for the downstream cylinder There also exist a critical flow velocity and once this velocity is exceeded no equilibrium positions of the downstream cylinder can be found, which suggests a likely occurrence of clashing between the two cylinders

Reliability-Based Structural Design of Ship-Type FPSO Units

Abstract: Design criteria based on LRFD format is proposed here for FPSO structural design. Thedesign criteria of FPSO structures accounts for load effects and their probabilistic dis-tributions, in which an ‘‘Environmental Severity Factor’’is introduced for considering thesite-specific conditions of hydrodynamic loading; and for limit states for three types offailure modes: primary, secondary and tertiary. The method presented in this paper can beapplied to develop structural design criteria for new-built FPSO and converted FPSOfrom oil tankers. @DOI: 10.1115/1.1554700#Keywords: Floating Production, Storage and Offloading Units (FPSO); Structural De-sign Criteria; LRFD Format

Improved Current Profile Criteria for Deepwater Riser Design

Abstract: New types of current profile criteria are described that are designed to take into account the vertical coherence of deepwater currents in offshore engineering. Reasoning is given why traditional types of current profile criteria can be inappropriate for deepwater locations. The paper describes current profile occurrence criteria that consist of a discrete number of characteristic profiles, each of which has an associated percentage frequency of occurrence. These criteria are appropriate for riser fatigue calculations (including Vortex Induced Vibration modelling) and operability assessments. Use of these criteria can reduce the level of over-conservatism associated with the traditional criteria, potentially leading to significant cost savings. Two possible methods of deriving these criteria are described, one of which uses Empirical Orthogonal Function (EOF) analysis to simplify the vertical structure of current profiles. The relative merits of each analysis technique are assessed.Copyright © 2002 by ASME

Parameter Identification of a Large Floating Body in Random Ocean Waves by Reverse MISO Method

Abstract: Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the Reverse Multiple Inputs-Single Output (R-MISO) method, to a single-degree-of-freedom system with linear and cubic nonlinear stiffnesses. The system mass is split into a frequency independent and a frequency dependent component and its damping is frequency dependent. This can serve as a model of a moored floating system with a dominant motion associated with the nonlinear stiffness. The wave diffraction force, the excitation to the system, is assumed known. This can either be calculated or obtained from experiments. For numerical illustration, the case of floating semi-ellipsoid is adopted with dominant sway motion. The motion as well as the loading are simulated with and without noise assuming PM spectrum and these results have been analyzed by the R-MISO method, yielding the frequency dependent added mass and radiation damping, linear as well as the nonlinear stiffness coefficients quite satisfactorily. @DOI: 10.1115/1.1493201#

Nonlinear Interaction of Submerged Cylinder With Free Surface

Abstract: The fully nonlinear problem on the unsteady water waves generated by submerged moving cylinder is considered. Using the analytic majorant method we prove local in time unique solvability of this problem. For the case when the dimensionless cylinder radius is small, the solution estimate obtained predicts rigorously dipole-like structure for the lowest order far field flow. The strength of dipole concentrated at the cylinder axis depends on the instantaneous wave form and fluid velocity at the free surface. Special case of the lifting accelerated cylinder starting from the rest is studied analytically in more detail.Copyright © 2002 by ASME

Numerical Computations for a Nonlinear Free Surface Problem in Shallow Water

Abstract: This paper describes a finite element method applied to a nonlinear free surface flow problem for a ship moving in three dimensions. The physical model is taken to simulate the towing tank experimental conditions. The exact nonlinear free-surface flow problem formulated by an initial/boundary value problem is replaced by an equivalent weak formulation. The same problem was considered earlier by Bai, et. al. [1] where some irregularities were observed in the downstream waves and a transom stern ship geometry could not be treated. In the present paper, specifically, three improvements are made from the earlier work. The first improvement is the introduction of the 5-point Chebyshev filtering scheme which eliminates the irregular and saw-toothed waves in the downstream. The second improvement is that now we can treat a transom stern ship geometry. The third improvement is the introduction of a new boundary condition to simulate a dry bottom behind a transom stern ship which is stretched from the free surface to the bottom at a high Froude number. Computations are made for two models. The first model is tested for the generation of the solitons in the upstream and smooth waves in the downstream. The second model is used to compute the generation of a dry bottom behind a transom stern which is one of highly nonlinear phenomena. The results of the first model show a good agreement with previous results for the generation of the solitons. The results of the second model also show a good agreement with the preliminary experimental observation for a dry-bottom, which will be reported in near future. The numerical simulation of the second model can be applied to the local flow behind a sail of a submarine in cruise, a sloshing problem in LNG tankers, and a dam breaking problem. Both computed models are assumed to be in shallow water for simplicity. However, the present numerical method can treat arbitrary water-depth and practical ship geometries.Copyright © 2002 by ASME

Limiting Forms of Internal Solitary Waves

Abstract: High sensitivity of nonlinear wave structures in the weakly stratified fluid with respect to small perturbations of density in the upstream flow was pointed out in the paper (Benney & Ko, 1978). In present paper the influence of fine structure of stratification on one of the limiting forms, namely plateau-shaped solitary waves is analyzed. It is demonstrated that new limiting forms of solitary waves are possible in the case of continuous stratification close to linear or exponential one.Copyright © 2002 by ASME