Xinliang Tian

Experimental investigation on rogue waves and their impacts on a vertical cylinder using the Peregrine breather model

This paper presents an experimental investigation on the rogue waves and the corresponding wave forces acting on a vertical cylinder. The waves are modelled using the Peregrine breather solution of the third-order nonlinear Schrödinger equation. The experimental wave elevations are compared with the theoretical solutions, and the behaviours of wave energy distribution and evolution are calculated using the fast Fourier transformation and wavelet transform (WT) methods. The resulting wave forces acting on the vertical cylinder are compared with the numerical results based on potential flow calculations. Moreover, coherence analyses are conducted for the rogue waves and their impact forces acting on the cylinder based on the WT.

An experimental study on the inline wave force on a truncated vertical cylinder

ABSTRACT Cylindrical structures are widely used in offshore engineering, such as the hard tank of a spar platform, columns of a semi-submersible, jackets, drilling pipes, risers and so on. Interactions between the waves and offshore structures are of great importance for engineering applications. This paper focuses on the inline wave force acting on a truncated vertical circular cylinder. Experimental studies are carried out in the wave tank at Shanghai Jiao Tong University in China. The diameter of the cylinder (D) is 10 cm and five submergence depths (hs), i.e., hs = 0.5D, 1D, 2D, 3D, 4D, are considered, respectively. The inline force is found to be influenced by the submerged depth of the cylinder, wave steepness and scatter parameter. High-harmonic wave forces and the second-load cycle are also discussed in the paper.

Fourth-order split-step pseudo-spectral method for the modified nonlinear Schrödinger equation

Abstract In the field of water waves, the modified nonlinear Schrödinger (mNLS) equation which models the wave propagation in water is numerically solved by using the split-step pseudo-spectral method. In the present paper, the fourth-order split-step pseudo-spectral method is introduced with better numerical results. The proposed method is based on a split-step method which decomposes the original equation into two parts, a linear problem and a nonlinear problem. In order to demonstrate the high accuracy and capability of the newly proposed method, a simple problem of periodic waves is presented to compare the traditional first-order method with the fourth-order method in terms of the conservation error and computational cost. Meanwhile, another numerical experiment concerning the Peregrine breather solution of the nonlinear Schrödinger (NLS) equation is presented by using the fourth-order split-step pseudo-spectral method. It is found that the fourth-order scheme can provide higher conservative accuracy and is computationally more efficient compared with the first-order scheme.

A Numerical Study on the Nonlinear Effects in Focused Wave Modelling and Forces on a Semi-Submerged Horizontal Cylinder

ABSTRACT This paper concerns on the propagation of transient wave groups, focusing at one point in time and space to generate large waves. A numerical wave tank is developed based on the commercial software package FLUENT. Velocity inlet method is applied to reproduce the focused waves in this model, and the results are compared with linear and second-order theoretical results. The continuous wavelet transform is employed to analyse the time–frequency characteristics of the focused waves. Forces due to focused waves acting on a semi-submerged horizontal cylinder is investigated and compared with the regular wave sharing with identical wave crest height and trough-to-trough period. Secondary load cycle is observed which may cause “ringing”. This developed model is expected to serve as a basis for strongly nonlinear waves, such as freak waves.

Parametric study on the vortex-induced motions of semi-submersibles: Effect of rounded ratios of the column and pontoon

The vortex-induced motions (VIMs) of semi-submersibles have emerged as an important issue in offshore engineering, as they pose a threat to safe and reliable operations and severely affect the fatigue lives of risers and mooring systems. The VIM response depends on the shape of the submerged structure and thus is significantly influenced by the design parameters related to the columns and pontoons. Numerical simulations by the detached Eddy simulation method are validated by experimental data and then used for parametric analysis of the VIM performance of various semi-submersibles with different column rounded ratios (Rc/L) and pontoon rounded ratios (Rp/Lp). The results show that the transverse amplitudes of a semi-submersible with circular columns at a 0° current heading are twice as large as those at a 45° current heading. However, the semi-submersible with rounded square columns shows more significant transverse motions at a 45° current heading than at a 0° current heading. Furthermore, at the 45° cur...

Three-Dimensional Effects of the Flow Normal to a Flat Plate at a High Reynolds Number

Plate components are often found in offshore and marine structures, such as heave damping plates in spar platform and bilge keels in ships. Two-dimensional (2D) and three-dimensional (3D) numerical simulations are performed to investigate the 3D effects of the flow normal to a flat plate at a high Reynolds number (Re = 1:5×105, based on the height of the plate and the free stream velocity). The ratio of the plate thickness to the plate height is 0.02. The 2D simulations are carried out by solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k-ω Shear Stress Transport (SST) turbulence model, while the 3D simulations are carried out with the large-eddy simulation (LES) method. The hydrodynamic results (such as time-averaged drag coefficient, Strouhal number and mean recirculation length) are compared with the published experimental data. The near-wake flow structures are also discussed. The 3D simulation results are in good agreement with the published experimental data; however, the 2D simulations show a poor comparison with the experimental data. This shows that the 3D effects are important for the high Reynolds number flow normal to a flat plate.Copyright

The hydrodynamic performance of a tension leg platform with one-tendon failure

ABSTRACT As one of the most widely used types of deepwater offshore platforms, the Tension Leg Platform (TLP) has shown excellent motion performance in waves; however, a detailed evaluation of their performance under tendon failure condition is vital and necessary. In this study, numerical simulations were conducted in both time and frequency domains using the commercial software HydroD and OrcaFlex, respectively. The numerical results were validated by a series of wave basin model tests. The motions of the TLP, with one tendon failure, and loads on the effective tendons, were studied in dynamic analyses to evaluate the potential adverse impact of tendon failure. Moreover, sensitivity studies of TLP performance under tendon failure were also conducted with various quantities of hull ballast. The relation between the response of the TLP and quantities of ballast were established and some advices were provided about lowering the risks of the loss of TLP stability under one-tendon failure condition.

Experimental investigation on the hydrodynamic performance of a quay moored jackup

Abstract Offshore platforms under construction are normally moored along the quayside during the outfitting stage. The safety of the platforms must be guaranteed during the entire process of outfitting which may last for several months. In this study, the hydrodynamic performance of a quay moored jackup is investigated by using a set of wave basin model tests. In the seakeeping model tests, the dynamic responses of the system, including the six degrees of freedom motions of the jackup and the barge, tensions on the mooring lines and the collision forces on the fenders, were measured in various sea states, and statistical analyses were conducted. Meanwhile, the dynamic responses in current-and-wind-only sea states were analysed for comparison with those in the wave conditions to assess the influence of the wind and the current. It is observed that the offshore wind plays an important role in quay mooring systems and the mooring line tensions are found to exceed the strength of the lines in offshore wind conditions. The possible reasons are discussed in detail and suggestions are proposed for optimising the mooring system performance.

Influence of a Seabed Trench on a Taut Mooring Line

Taut mooring systems have become a prospective solution for the station keeping of offshore floating facilities in deep water. In such scenario, the associated anchor foundations may withstand substantial uplift loads. To maximize the holding capacity, it is common to attach the anchor chain to the suction caisson at 60 to 70 % of the embedded depth. However, new phenomenon has been observed, which may be a challenge to the integrity of mooring systems, such as the trenching of the seabed near the suction caissons of floating systems. It is unclear whether trenching will induce significant effect on the integrity of mooring systems. To evaluate the influence of the seabed trenching, where existing commercial codes have not covered, a numerical model has been developed in this study. Based on this developed model, numerical simulations have been conducted with and without considering the trenching effect at the seabed. It is found that the seabed trench has a limited influence on the tension at fairlead but has a significant influence on both the tension and the inclination angle at padeye.

Current Effects on the Generation and Evolution of the Peregrine Breather-Type Rogue Wave

Rogue wave is a kind of surface gravity waves with much larger wave heights than expected in normal sea state. Since this extreme sea event always occurred in the areas with strong currents, the wave-current interaction was considered to be one of the physical mechanics of the formation of the rogue wave. Some breather type solutions of the NLS equation have been considered as prototypes of rogue waves in ocean which usually appears from smooth initial condition only with a certain disturbance. In this paper, we have numerically studied evolutionary process of the Peregrine breather rogue wave based on the current modified fourth order nonlinear Schrodinger equation (the CmNLS equation). During the generation and evolution of the Peregrine breather rogue wave, the effects of the steady current was investigated by comparing with the results without current. The differences of the focusing position/time were observed due to the current influence.Copyright