CFD Simulation and Experimental Study on Coupled Motion Response of Ship with Tank in Beam Waves

Tank sloshing is widely present in many engineering fields, especially in the field of marine. Due to the trend of large-scale liquid cargo ships, it is of great significance to study the coupled motion response of ships with tanks in beam waves. In this study, the CFD (Computational Fluid Dynamics) method and experiments are used to study the response of a ship with/without a tank in beam waves. All the computations are performed by an in-house CFD solver, which is used to solve RANS (Reynold Average Navier-Stokes) equations coupled with six degrees-of-freedom solid-body motion equations. The Level Set Method is used to solve the free surface. Verification work on the grid number and time step size has been conducted. The simulation results agree with the experimental results well, which shows that the numerical method is accurate enough. In this paper, several different working conditions are set up, and the effects of the liquid height in the tank, the size of the tank and the wavelength ratio of the incident wave on the ship’s motion are studied. The results show the effect of tank sloshing on the ship’s motion in different working conditions.

Jump bifurcation phenomenon during varying wind speeds in floating offshore wind turbines

Floating Offshore Wind Turbines (FOWTs) are susceptible to an instability which has come to be called negative damping. Conventional land based wind turbine controllers when used with FOWTs may cause large amplitude platform pitch oscillations. Most controllers have since been improved to reduce motions due to this phenomenon. In this paper, the motions induced using one of the original controllers is studied. The current study is performed using the coupled time domain program FAST-SIMDYN that was developed in Marine Dynamics Laboratory (MDL) at Texas A&M University. It is capable of studying large amplitude motions of Floating Offshore Wind Turbines. FOWTs use various controller algorithms of operation based on the available wind speed depending on various power output objectives i.e., to either maximize or level out power absorption. It is observed that the transition region for controllers is often chaotic. So most studies focus on operations away from the transition region below and above the transition wind speeds. Here we study the transition region using the theoretical insight of non-linear motion response of structures. This study reveals the presence of a very interesting and potentially hazardous nonlinear phenomenon, bifurcation. This finding could help explain the chaotic motion response that is observed in the transition region of controllers. Understanding the nature and cause of bifurcation could prove very useful for future design of FOWT controllers.

Numerical Investigation on Flood Water Sloshing Influence on Intact and Damaged Stability of Ship

Sloshing affects the intact and damage stability of the ship, which causes variation in dynamic metacentric height (GM) under critical load conditions. The transient flooding soon after the ship damage is analyzed, with floodwater accumulation in large space and causing the ship to suffer huge heel angles. The ship motion and stability changes when sloshing becomes high in partially flooded compartments. Most of the previous researches focus on the motion response of ship alone, hence the variation of stability due to sloshing is to be more critically studied. In the present study, three critical damage locations are identified and flooding through these locations are analyzed using the volume of fluids method. The method focus on finding damage ship motion response, flood water dynamics, and coupled dynamics of both. This is studied using the numerical method FLOW3D. Motion and stability behaviour will be different for different damage locations; hence portside, starboard-side, and aft-end bottom damage cases are considered. The effect of compartment shape and damage location on motion response and stability of the damaged ship is highlighted.

Numerical simulation of the mooring capacity comparison of the subsea suspended manifold with two mooring schemes

The next-generation underwater production system (NUPS) is based on the suspension cluster manifold (SCM) as a new conceptual scheme. SCM mooring stability is essential for establishing NUPS. Therefore, comparing the SCM mooring stability in different mooring systems is vital for evaluating system adaptability. This paper detailed two mooring schemes designed for the SCM, including the steel catenary riser (SCR) mooring system and the new steep wave (NSWR) mooring system. OrcaFlex software was used to establish the mooring system model, analyzing the static motion response of the SCM under the current and fluid density. Furthermore, the mooring system adaptability in the cluster wellhead layout was also evaluated and compared. The results showed that the maximum offset of the SCM with the SCR mooring system was within 2 m under the current, while the deflection of the SCM with the NSWR mooring system was within 1.5° in extreme fluid densities. Furthermore, the SCM with the SCR mooring system displayed superior station-keeping capability in the current, while the NSWR mooring system exhibited better stability when transporting extreme fluid densities and was more adaptable in cluster wellhead layouts.

IDENTIFICATION OF INFLUENTIAL PARAMETERS IN A SHIP’S MOTION RESPONSES: A ROUTE TO MONITORING DYNAMIC STABILITY

Six degrees of freedom motion response tests of a Ro-Ro model have been carried out in irregular waves under intact conditions. A stationary model was tested in different sea states for following, astern quartering and beam seas. The investigation was limited to the effect of encountered frequency components and associated magnitude of energy of the ship’s motion responses. Analysis of heave, pitch and roll motions confirmed the vulnerability of the model to certain frequency ranges resulting in an adverse effect on the responses, and these were closely related to its natural frequencies. It was confirmed that the roll motion maintains its highest oscillation around the natural frequency in all sea conditions regardless of heading angles. However spectral analysis of the heave and pitch responses revealed the wave peak frequency. Roll is magnified when the peak frequency of wave approaches the natural roll frequency; therefore keeping them apart avoids a large motion response. It was concluded that peak frequency and associated magnitude are two important inherent characteristics of motion responses. Detection of influential parameters of encountered wave through heave and pitch responses could be utilised to limit a large ship’s motion at sea.

FULL-SCALE MOTIONS OF A LARGE HIGH-SPEED CATAMARAN: THE INFLUENCE OF WAVE ENVIRONMENT, SPEED AND RIDE CONTROL SYSTEM

To assess the behaviour of large high-speed catamarans in severe seas, extensive full-scale trials were conducted by the U.S. Navy on an INCAT Tasmania built vessel in the North Sea and North Atlantic region. Systematic testing was done for different speeds, sea states and ride control settings at different headings. Collected data has been used to characterise the ship’s motions and seakeeping performance with respect to wave environment, vessel speed and ride control system. Motion response amplitude operators were derived and compared with results from a two-dimensional Green function time-domain strip theory seakeeping prediction method. An increase of motion response with increasing vessel speed and a decrease with the vessel moving from head to beam seas was found. In higher sea states and headings ahead of beam seas an increasing influence of the centre bow on pitch motion damping was found. Significant motion RAO reduction was also found when the ride control system was active. Its effectiveness increased at higher speeds and contributed to heave and pitch motion RAO reduction. Predicted motion magnitudes with the time domain seakeeping code were consistent with the measured motion responses, but maximum heave was predicted at a rather higher frequency than was evident in the trials.

Experimental Study on Operation Performance of Two-Body Wave Energy Generator

The two-body oscillating type wave energy converter (WEC) is a hot research topic at present. A two-body device with damping disc was taken as the test model in this paper. The two bodies were connected by a hydraulic piston cylinder to realize the relative motion energy conversion. Physical experiments were carried out in a wave-making flume to study the operation performance. The effects of wave elements and load on the hydrodynamic characteristics and capture width ratio (CWR) of the model were analysed respectively. The results showed that wave frequency and external load were the main factors affecting the motion response and energy conversion of the device. With the increase of wave frequency and external load, the response amplitude operator (RAO) and the capture width ratio both increase first and then decrease. Wave height has little effect on system characteristics. There exists a best-matching wave period condition, and the optimal motion response and energy conversion are obtained.

A Distributed Virtual Reality System Based on Real-Time Dynamic Calculation and Multi-Person Collaborative Operation Applied to the Development Of Subsea Production Systems

To train inexperienced workers for the construction, production, and maintenance of subsea production systems, a virtual reality simulation platform was developed. The entire framework, software, and hardware platforms of the system were designed and introduced. A multi-person collaborative simulation was achieved based on the high-level architecture protocol. The real-time dynamic calculation software Vortex was used to add physical properties to each geometrical model and set collision detections and motion constraints so that the VR system can reflect the real motion response of the structures in real-time during virtual simulation. Visual simulation software Vega Prime and Vortex were integrated to realise the real-time rendering and drawing of virtual ocean engineering scenes. Thus, a virtual simulation system with large-scale complex scenes based on real-time dynamic calculation and multi-person collaborative operation was established. A typical ocean engineering case of subsea manifold installation was simulated using the virtual simulation system, and the detailed simulation flow was explained. A multibody dynamics system of the ship-cable-subsea manifold was established using Orcaflex to obtain the accurate motion response of the subsea manifold during lowering. In the virtual simulation process, the obtained hydrodynamic calculation results can provide an important guideline and reference to the operators. The developed simulation system is a suitable tool for training ocean engineering workers and realistically simulating ocean operation cases.