P. Temarel

Hydroelasticity of ships: Recent advances and future trends:

Investigations into hydroelasticity of ships commenced in the 1970s. Since then the theory has been employed to predict the responses of a wide range of marine structures, such as mono- and multihulled ships, offshore structures, and VLFS. In recent years, with increasing market demands for new buildings of slender ocean going carriers and the continuously updated high-speed and unconventional multihulled designs, the maritime industry began to notice the advantage of assessing the usefulness and applicability of hydroelasticity in ship design. At first instance, the aim of this paper is to illustrate some of the applications of hydroelasticity theory to ships, with particular reference to recent and ongoing developments focusing on ship design applications and the effects of non-linearities and viscous flows. The paper also discusses the longer term potential use of weakly and fully non-linear fluid—structure interaction, as well as Navier—Stokes based fluid dynamic methods, for the improved modelling of ship dynamic response problems.

Comparison of experimental and numerical sloshing loads in partially filled tanks

Sloshing describes the movement of liquids inside partially filled tanks, generating dynamic loads on the tank structure. The resulting impact pressures are of great importance in assessing structural strength, and their correct evaluation still represents a challenge for the designer due to the high level of nonlinearities involved, with complex free surface deformations, violent impact phenomena and influence of air trapping. In the present paper, a set of two-dimensional cases, for which experimental results are available, is considered to assess the merits and shortcomings of different numerical methods for sloshing evaluation, namely two commercial RANS solvers (FLOW-3D and LS-DYNA), and two academic software (Smoothed Particle Hydrodynamics and RANS). Impact pressures at various critical locations and global moment induced by water motion in a partially filled rectangular tank, subject to a simple harmonic rolling motion, are evaluated and predictions are compared with experimental measurements.

Comparison of experimental and numerical loads on an impacting bow section

Impact pressures, due to slamming, are of particular importance in assessing local and global structural strength. Knowledge of pressure distribution and slamming forces, experimental or numerical, in the forefoot and aftbody of a ship can be used in the evaluation of global wave-induced loads, for example, whipping bending moment. Impact pressures and slamming forces for a bow section, impacting upright and at two angles of heel are presented in this article. The relevant impact loads are predicted using a range of two-dimensional potential and viscous flow methods and compared with available experimental measurements.

Influence of Viscous Effects on the Hydrodynamics of Ship-Like Sections Undergoing Symmetric and Anti-Symmetric Motions, Using RANS

The aim of this investigation is to assess the influence of viscous effects on the predicted hydrodynamic coefficients for a range of ship-like sections, such as rectangular, triangular, chine and bulbous. Hydrodynamic coefficients, of added mass or inertia and fluid damping, for two-dimensional sections harmonically heaving, swaying and rolling at the undisturbed free surface are obtained using the ANSYS-CFX11.0 RANS solver, for a range of frequencies of oscillation. All predictions are compared with available experimental measurements and other numerical predictions (potential flow and RANS). It is concluded from these comparisons that the proposed RANS approach can offer a better prediction for the hydrodynamic coefficients when viscous effects become significant, in particular for sway and roll motions. It is important that a reliable and systematic approach is adopted for the application of the unsteady free surface RANS methodology.

A comparative study of the dynamic behaviour of a fast patrol boat travelling in rough seas

This paper describes a comparative study between full scale measurements recorded on a fast patrol boat encountering heavy winter seas, south of the Isle of Wight and theoretical predictions accounting for the slamming behaviour of the flexible hull. A hydroelastic approach is adopted and satisfactory agreement between measured and predicted responses demonstrated.

On the limitations of two- and three-dimensional linear hydroelasticity analyses applied to a fast patrol boat

The aim of this paper is to study the symmetric (i.e. heave and pitch motions and distortions associated with vertical bending) wave-induced dynamic behaviour of a fast patrol boat using a unified hydroelasticity analysis. This includes two- and three-dimensional structural idealizations using beam and three-dimensional finite element modelling. The fluid—flexible structure interaction is carried out using three-dimensional potential flow analysis, for both structural idealizations, based on a pulsating source singularity distribution on the mean wetted surface. The calculations are carried out in regular waves for two forward speeds (Froude numbers Fn = 0.5 and 0.63) and three heading angles, i.e. 180 (head), 135, and 90 degrees. Results from full-scale trials are also presented in order to compare rigid body motion transfer functions with numerical predictions. There are large differences between numerically predicted and measured motions, as is to be expected for this fast hull form. The paper reports that the evaluation of the dynamic behaviour of the fast patrol boat, with small length to beam ratio, by means of the unified hydroelastic analysis, shows some inherent limitations of the beamlike approach for this particular type of vessel.

A validation study on mathematical models of speed and frequency dependence in seakeeping

Abstract An extensive theoretical validation exercise is presented into the speed and frequency dependent solutions associated with surface piercing vessels travelling in waves. The basis of the study lies in the formulation of the Greens function satisfying the traditionally posed linearized boundary value problem of an oscillating ship with forward speed and the development/implementation of appropriate numerical schemes of study for solution. Two widely different mathematical models in formulation and numerical algorithm, denoted as methods A and B, are discussed and employed to predict hydrodynamic coefficients, wave loads and responses of a Series 60 form and an NPL monohull. Only a selection of results are shown, but great care was taken in the overall investigation to verify and validate intermediate steps within the separate calculation procedures as well as to compare final predicted values. The extensive qualitative and quantitative agreement of comparable results from methods A and B provides a measure of confidence that the presented findings are solutions to the posed seakeeping problem. The influence of speed and frequency dependence within the context of this study are discussed as well as a preliminary study into their influence in the occurrence of irregular frequencies in the numerical schemes.

Hydroelastic responses of pontoon and semi-submersible types of very large floating structure in regular head waves

The symmetric dynamic behaviour of two types of Very Large Floating Structure (VLFS) is investigated. The structures are of pontoon (or mat like) and semi-submersible type and have the same beam, length and displacement. The responses for these stationary and free-floating structures in regular head waves are investigated using the three-dimensional hydroelasticity theory, applicable to structures with arbitrary shape. The “dry analysis” is carried out by discretising the structures using beam and shell finite elements, as appropriate. The solution of the fluid-structure interaction problem is achieved through a pulsating source distribution whereby the mean wetted surface of either structure is discretised using four-cornered panels. The symmetric dynamic characteristics of both structures are compared, both in vacuo (e.g. natural frequencies and mode shapes) and in water (e.g. generalised added mass and hydrodynamic damping). Predicted responses such as vertical deflections and direct stresses, in regular head waves, are also discussed and compared.Copyright

The hydrodynamics of ship-like sections in heave, sway, and roll motions predicted using an unsteady Reynolds-averaged Navier–Stokes method

The application of strip theory for predicting ship seakeeping response in waves relies on sectional added mass and viscous damping. A Reynolds averaged Navier—Stokes (RANS) approach is adopted to obtain two-dimensional hydrodynamic coefficients that include viscous and rotational flow effects as well as free surface wave generation. Calculations are made for circular and rectangular sections of cylinders swaying, heaving, and rolling in the presence of a free surface, for a range of frequencies. The predicted hydrodynamic coefficients are compared with available experimental and numerical results. The method is successfully validated for sway of a submerged circular cylinder. Good agreement is obtained for sway and heave of a rectangular cylinder at a free surface. For roll, the results are highly sensitive to the mesh applied due to the vortex shedding that occurs at the sharp corners, even at relatively low roll amplitudes. The appropriate density of mesh influences the convection of the vortex, the production of vorticity, and the position of the vortex core. These all have a large impact on fluid damping. A hybrid-based zonal mesh generation approach is proposed to maintain a compromise between necessary mesh refinement and the overall number of cells. Using this technique and a small fixed time step leads to improved predictions of the roll hydrodynamic coefficients, particularly for fluid damping.

Coupling between flexible ship and liquid sloshing using potential flow analysis

The purpose of this study is to investigate the influence of hull flexibility on the hydrodynamic forces and moments associated with liquid sloshing, as well as the dynamic characteristics = (e.g. resonance frequencies) of the whole system. For this purpose, symmetric and antisymmetric structural responses such as bending moments and torsional moment, etc. for an idealized LNG carrier in head, beam and quartering regular waves are studied with and without coupling effect from liquid sloshing.