Huilong Ren

Experimental and Numerical Analysis of Hull Girder Vibrations and Bow Impact of a Large Ship Sailing in Waves

It is of great importance to evaluate the hull structural vibrations response of large ships in extreme seas. Studies of hydroelastic response of an ultra large ship have been conducted with comparative verification between experimental and numerical methods in order to estimate the wave loads response considering hull vibration and water impact. A segmented self-propelling model with steel backbone system was elaborately designed and the experiments were performed in a tank. Time domain numerical simulations of the ship were carried out by using three-dimensional nonlinear hydroelasticity theory. The results from the computational analyses have been correlated with those from model tests.

Experimental Investigation of Wave-Induced Ship Hydroelastic Vibrations by Large-Scale Model Measurement in Coastal Waves

Ship hydroelastic vibration is an issue involving mutual interactions among inertial, hydrodynamic, and elastic forces. The conventional laboratory tests for wave-induced hydroelastic vibrations of ships are performed in tank conditions. An alternative approach to the conventional laboratory basin measurement, proposed in this paper, is to perform tests by large-scale model measurement in real sea waves. In order to perform this kind of novel experimental measurement, a large-scale free running model and the experiment scheme are proposed and introduced. The proposed testing methodology is quite general and applicable to a wide range of ship hydrodynamic experimental research. The testing procedure is presented by illustrating a 5-hour voyage trial of the large-scale model carried out at Huludao harbor of China in August 2015. Hammer tests were performed to identify the natural frequencies of the ship model at the beginning of the tests. Then a series of tests under different sailing conditions were carried out to investigate the vibrational characteristics of the model. As a postvoyage analysis, load, pressure, acceleration, and motion responses of the model are studied with respect to different time durations based on the measured data.

Development of a Shipboard Remote Control and Telemetry Experimental System for Large-Scale Model’s Motions and Loads Measurement in Realistic Sea Waves

Wave-induced motion and load responses are important criteria for ship performance evaluation. Physical experiments have long been an indispensable tool in the predictions of ship’s navigation state, speed, motions, accelerations, sectional loads and wave impact pressure. Currently, majority of the experiments are conducted in laboratory tank environment, where the wave environments are different from the realistic sea waves. In this paper, a laboratory tank testing system for ship motions and loads measurement is reviewed and reported first. Then, a novel large-scale model measurement technique is developed based on the laboratory testing foundations to obtain accurate motion and load responses of ships in realistic sea conditions. For this purpose, a suite of advanced remote control and telemetry experimental system was developed in-house to allow for the implementation of large-scale model seakeeping measurement at sea. The experimental system includes a series of technique sensors, e.g., the Global Position System/Inertial Navigation System (GPS/INS) module, course top, optical fiber sensors, strain gauges, pressure sensors and accelerometers. The developed measurement system was tested by field experiments in coastal seas, which indicates that the proposed large-scale model testing scheme is capable and feasible. Meaningful data including ocean environment parameters, ship navigation state, motions and loads were obtained through the sea trial campaign.

Influence of Nonlinear Mooring Stiffness on Hydrodynamic Performance of Floating Bodies

Coupled dynamic analysis between floating marine structures and flexible members such as mooring lines and risers, is a challenging work in the ocean engineering field. Coupled analysis on mooring-buoy interactions has been paid more and more concern for recent years. For floating offshore structures at sea, the motions driven by environmental loads are inevitable. The movement of mooring lines occurs due to the excitation on the top by floating structures. Meanwhile the lines restrict the buoy’s motion by forces acting on the fareleads. Positioning is the main function of mooring system, its orientation effects can’t be ignored for floating structures such as semi-submersible, FPS, and TLP, especially when the buoy’s equilibrium position shifting to another place. Similar as hydrostatic restoring forces, mooring force related with the buoy’s displacement can be transformed into mooring stiffness and can be added in the differential equations of motion, which is calculated at its equilibrium point. For linear hydrodynamic analysis in frequency domain, any physical quantity should be linear or be linearized, however mooring stiffness is nonlinear in essence, so the tangent or differential stiffness is used. Steel chains are widely used in catenary mooring system. An explicit formulation of catenary mooring stiffness is derived in this article, which consists of coupled relations between horizontal and vertical mooring forces. The effects of changing stiffness due to the shift of equilibrium position on the buoy’s hydrodynamic performance are investigated.Copyright

Time domain Rankine-Green panel method for offshore structures

To solve the numerical divergence problem of the direct time domain Green function method for the motion simulation of floating bodies with large flare, a time domain hybrid Rankine-Green boundary element method is proposed. In this numerical method, the fluid domain is decomposed by an imaginary control surface, at which the continuous condition should be satisfied. Then the Rankine Green function is adopted in the inner domain. The transient free surface Green function is applied in the outer domain, which is used to find the relationship between the velocity potential and its normal derivative for the inner domain. Besides, the velocity potential at the mean free surface between body surface and control surface is directly solved by the integration scheme. The wave exciting force is computed through the convolution integration with wave elevation, by introducing the impulse response function. Additionally, the nonlinear Froude-Krylov force and hydrostatic force, which is computed under the instantaneous incident wave free surface, are taken into account by the direct pressure integration scheme. The corresponding numerical computer code is developed and first used to compute the hydrodynamic coefficients of the hemisphere, as well as the time history of a ship with large flare; good agreement is obtained with the analytical solutions as well as the available numerical results. Then the hydrodynamic properties of a FPSO are studied. The hydrodynamic coefficients agree well with the results computed by the frequency method; the influence of the time interval and the truncated time is investigated in detail.

Numerical analysis of the motion and load responses of a 400,000 DWT ore carrier in waves

The need to benefit from economy of scale has driven the design of larger and larger ships with increasing tonnage. The hull structures of large ships are more flexible and the natural frequencies of the hull girder can even fall within the range of wave encounter frequencies resulting in the resonance phenomenon termed springing. A numerical linear hydroelastic investigation of the wave induced motion and load responses of a 400,000 dead weight tonnage (DWT) ore carrier is carried out in this work using an in-house program: Linear Elastic Compass Wave Loads Calculation System (WALCS-LE). The full load condition at different ship speeds, wave lengths and incident wave headings in regular waves is investigated. Long term predictions are also made. It is imperative to account for springing loads and the probable extreme motions and loads at the design stage of large ships to ensure their endurance and safety at sea.

Approximation of Higher-Order Derivatives of the Frequency Domain Free Surface Green Function

The higher-order derivatives of the free-surface Green Function are critically important in three-dimensional frequency-domain boundary element methods using mixed dipole-source distribution. To improve the accuracy and efficiency of numerical schemes, the computing domain is divided into five areas. Derivatives in four areas are calculated analytically since the Green function is defined analytically. The 5th area is divided into a number of sub-areas in which truncated Double Chebyshev series are used to approximate the Green function. Unlike the usual way in which the derivatives of Green function are obtained by differentiating the series, we re-approximate the derivatives by new Chebyshev series with new coefficients. Numerical results show that the new series are more accurate, in particular, second order derivatives.Copyright

Fatigue Crack Propagation Rate Test of Q235 Steel in Ship Hull

A series of crack propagation rate tests of compact tension specimen for Q235 steel are conducted. Compliance method and pixel method are introduced to measure the crack length, and the comparison of the two methods is proposed. Incremental polynomial method and least square method are applied to fitting and regressing the test data. By statistical analysis, the statistical characteristics of crack propagation parameters C and m are obtained and the fatigue propagation rate described by Δ CMOD and ΔK is proposed. On the basis of probability fracture mechanics theory, the functions of crack propagation rate under different probability standards are presented.Copyright

The Fatigue Crack Propagation of the Ship Structures in Random Sea States

The traditional method for the fatigue strength assessment of ship structures is based on S-N curves and Miner linear cumulative damage rules. However, with the development of the ship mechanics, the fracture mechanics method has aroused people’s attention. Some researchers have begun to use the fracture mechanics method to perform the fatigue strength assessment of ship structures. A fracture mechanics based approach for the fatigue assessment of ship structures in random sea states is presented. First, the fatigue stress history of the ship structures in random sea states is simulated. Then, the stress intensity factor in random sea states is calculated through the weight function and the fatigue stress of the ship structures in random sea states. Finally, the crack growth is calculated using Pairs equation for each stress cycle throughout the fatigue stress history of the ship structures in random sea states.Copyright

Study on Structural Form Design of Trimaran Cross-Deck

As a new high performance shipform, the structural form of trimaran is special and the stress concentration of its cross-deck structure is serious. According to the Rules for Classification of Trimaran Ships developed by Lloyd’s Register, the global finite element model of a trimaran is built. Main factors such as Thickness of bulkhead and wet deck, transitional forms and strengthening forms, which affect the stresses at local details are compared and discussed. Then the best structural form of trimaran cross-deck is given. The result can offer the reference for the trimaran’s design and development.Copyright