The prediction on in-line vortex-induced vibration of slender marine structures

The in-line （IL） vortex-induced vibration （VIV） that occurs frequently in ocean engineering may cause severe fatigue damage in slender marine structures. To the best knowledge of the authors, in existing literatures, there is no efficient analytical model for predicting pure IL VIV. In this paper, a wake oscillator model capable of analyzing the IL VIV of slender marine structures has been developed. Two different kinds of van der Pol equations are used to describe the near wake dynamics related to the fluctuating nature of symmetric vortex shedding in the first excitation region and alternate vortex shedding in the second one. Some comparisons are carried out between the present model results and experimental data. It is found that many phenomena observed in experiments could be reproduced by the present wake oscillator model.

Optimum Environmental Load Design Criterion for Marine Structures Based on Investment and Benefit Analysis

Marine structures operating in natural ocean environment are subjected to various stochastic loads. For design of the marine structures, the most important task is to determine environmental load design criterion. This paper presents a method to determine the optimum environmental load design criterion for marine structures. This method is based on the investment and benefit analysis and it can reach the design purpose of decreasing total costs during the service life of the structures and increasing economic benefits.

Waves of 3D Marine Structures Slamming at Different Initial Poses in Complex Wind-Wave-Flow Environments

Aimed at the hydrodynamic response for marine structures slamming into water, based on the mechanism analysis to the slamming process, and by combining 3D N-S equation and k-ε turbulent kinetic equation with structure fully 6DOF motion equation, a mathematical model for the wind-fluid-solid interaction is established in 3D marine structure slamming wave at free poses and wind-wave-flow complex environments. Compared with the results of physical model test, the numerical results from the slamming wave well correspond with the experimental results. Through the mathematical model, the wave-making issue of 3D marine structure at initial pose falls into water in different complex wind, wave and flow environments is investigated. The research results show that various kinds of natural factors and structure initial poses have different influence on the slamming wave, and there is an obvious rule in this process.

Dynamic Positioning Control of Surge−Pitch Coupled Motion for Small-Waterplane-Area Marine Structures

For general dynamic positioning systems,controllers are mainly based on the feedback of motions only in the horizontal plane.However,for marine structures with a small water plane area and low metacentric height,undesirable surge and pitch oscillations may be induced by the thruster actions.In this paper,three control laws are investigated to suppress the induced pitch motion by adding pitch rate,pitch angle or pitch acceleration into the feedback control loop.Extensive numerical simulations are conducted with a semi-submersible platform for each control law.The influences of additional terms on surge−pitch coupled motions are analyzed in both frequency and time domain.The mechanical constraints of the thrust allocation and the frequency characters of external forces are simultaneously considered.It is concluded that adding pitch angle or pitch acceleration into the feedback loop changes the natural frequency in pitch,and its performance is highly dependent on the frequency distribution of external forces,while adding pitch rate into the feedback loop is always effective in mitigating surge−pitch coupled motions.

Functionally Graded Material and Its Application to Marine Structures

Marine structures are exposed to harsh weather conditions,demanding special pre-requisites in design and functional perspectives.Under dynamic loads of larger magnitude,the material-centric design procedure alone is not feasible to ensure the safe disbursement of loads.The compliant offshore structures resist loads primarily by their geometric novelty,and hence their design is form-dominant and no more strength(material)dominant.Large displacements in the rigid body modes in the horizontal plane under lateral loads require their construction material to possess enough ductility to absorb this energy.Steel is one of the most competitive materials for marine structures as it offers good ductility,but corrosion in the marine environment is a major concern.It undergoes strength and functional degradations and therefore requires serious investigation.In the present study,functionally graded material(FGM)is proposed to substitute for steel in marine applications.The method of fabricating FGM and assessing its mechanical and durability properties are discussed.Results show that FGM possesses strength and durability properties at par with the conventionally used X52 steel for marine risers.The presented study will be a major initiative towards future research in exploring competent materials which will be strong and sustainable in the marine environment.

Research on Marine Structure Inspection Support System on Mobile Device Platform

Ship and offshore structure often work in very severe environment.Therefore routine inspection and maintenance are essential to keep ship structure and equipment in good state,and can decrease the possibility of sea pollution and safety problems.Besides that,the activity of inspection and maintenance is also directly connected with cost control and condition assessment.Current inspection is still mainly based on paper and pen,so it is rather time-consuming for transferring inspection data from paper to computer system,and more diffcult to make calculation based on the inspection records.In this paper,the concept of information integration model and inspection support system using mobile device is introduced.The structuralized data in the information integration model offer much convenience to data transferring.The inspection support system consists of one headquarter management subsystem and one inspector handling subsystem.A mechanism of data exchange between the two subsystems is developed based on standard inspection workflow.The headquarter subsystem works on computer,and the inspector subsystem is mobile application instead of paper and pen.Versatile tests and platform experiments are carried out to demonstrate and confirm the feasibility and validity.

THE COMPUTATION OF SEEPAGE FORCE ON MARINE STRUCTURES WITH FINITE ELEMENT METHOD

The wave-induced seepage force is investigated on marine structures resting on or buried in the seabed.The bed is modelled as a poroelastic medium containing a nearly saturated water.The governing equations are solved with Finite Element Method.For a pipeline buried in the seabed,agreement between the present numerical results and that of Cheng H.D.(1986)is quite satisfactory.

Effect of Silica Fume on Compressive Strength of Oil-Polluted Concrete in Different Marine Environments

In the present research, effect of silica fume as an additive and oil polluted sands as aggregates on compressive strength of concrete were investigated experimentally. The amount ofoil in the designed mixtures was assumed to be constant and equal to 2% of the sand weight. Silica fume accounting for 10%, 15% and 20% of the weight is added to the designed mixture. After preparation and curing, concrete specimens were placed into the three different conditions： fresh, brackish and saltwater environments （submerged in fresh water, alternation of exposed in air ＆ submerged in sea water and submerged in sea water）. The result of compressive strength tests shows that the compressive strength of the specimens consisting of silica fume increases significantly in comparison with the control specimens in all three environments. The compressive strength of the concrete with 15% silica fume content was about 30% to 50% higher than that of control specimens in all tested environments under the condition of using polluted aggregates in the designed mixture.

An Improved Formulation for the Ultimate Static Strength of Tubular Joints subjected to Combined Loads

-'The effect of interaction of loads on the ultimate static strength of tubular joints of offshore fixed platforms, is a practical problem. But there is still absence of rigorous theory to explain available experimental data and empirical criteria for the static strength of tubular joints. The idea of yield at hot spot of tubular joints is introduced in this paper. The interaction equations of plastic capacity for the tubular joints under combined loads (two and three different kinds) are derived. Thereafter the Yura's test data and empirical criteria of ultimate static strength for the tubular joints can be explained. The idea of classification of category of loads in accordance with experimental data and the present theory is suggested. Finally, the improved ultimate capacity equations for tubular joints are recommended. The physical significance of the coefficient of plastic reservation Qp is discussed.

Analysis on the applicability of VLFS for China

With the advantages on rapid construction,no limitation of extent,less influence on environment, the VLFS has been turned into the focus of coastal and offshore engineering studying.As the platform,VLFS was built as airport,huge oil house etc,and would play the role of ocean economics,politics and military.The restrictive condition of VLFS is strong wave by monsoon,seismic,serious bomb etc.The floating breakwater should be put forward to shelter VLFS.The analysis of wave condition including typhoon route,archipelago and water depth is performed in this paper.The advantage and disadvantage are compared between VLFS and the marine structure from land.

利用环境振动分析对受超大型船舶撞击后的码头进行结构监测的快速工具

Operational modal analysis is a non-destructive structural investigation that considers only the loads resulting from service conditions.This approach allows the measurement of vibrations on a given structure with no need to interrupt its use.The present work aims to develop a numerical model to represent the global structural behavior of a vessel breasting dolphin using a technique that is simple and cheap in order to obtain a fast answer about the stiffness of a pier after the collision of ships with capacity up to 400,000 t.To determine the modes of vibration,one accelerometer was installed on the breasting dolphin located on the pier and a frequency domain technic was conducted over recorded data to obtain modal parameters of the structure.In situ measurements were compared to data from a finite element model based on the original structural design in order to adapt the model to accurately represent the actual behavior of the system.This allowed a reliable structural analysis that accounted for existing structural damage and imperfections.The results of the experiment presented herein are the numerical characterization of the structure,along with the structural analysis to assess the degree of damage currently observed on the system.It is noted that the dolphin subjected to ship impacts presents a reduction in stiffness of approximately10%and its global damage level can be monitored from now after new accidents.

A LINEAR HYBRID MODEL OF MSE AND BEM FOR FLOATING STRUCTURES IN COASTAL ZONES

A linear hybrid model of Mild Slope Equation （MSE） and Boundary Element Method （BEM） is developed to study the wave propagation around floating structures in coastal zones. Both the wave refraction under the influence of topography and the wave diffraction by floating structures are considered. Hence, the model provides wave properties around the coastal floating structures of arbitrary shape but also the wave forces on and the hydrodynamic characteristics of the structures. Different approaches are compared to demonstrate the validity of the present hybrid model. Several numerical tests are carried out for the cases of pontoons under different circumstances. The results show that the influence of topography on the hydrodynamic characteristics of floating structures in coastal regions is important and must not be ignored in the most wave period range with practical interests.

Effect of tempering on carbides and hydrogen embrittlement in E690 high strength marine structural steel

The effect of tempering on carbides and hydrogen embrittlement in E690 high strength marine structural steel has been investigated.The steel was tempered at 600℃ for 1–3 h.Detailed characterization was carried out to characterize the microstructure,especially the dislocation density and grain size.The hydrogen permeation test and thermal desorption spectroscopy test were also implemented.The dislocation density decreases,the amount of carbide increases,and carbides(M_(23)C_(6) and MX)coarsen with the tempering time increasing.After tempered at 600℃ for 3 h,the diffusible hydrogen trapped by lattice and dislocation decreases while the non-diffusible hydrogen trapped by carbides increases,leading to the best hydrogen embrittlement resistance,although hydrogen diffuses rapidly due to the reduction of dislocation density.And the fracture mode changes from a combination of brittle cleavage and ductile dimpled fracture to fully ductile dimple fracture under hydrogen charging condition.Moreover,a phenomenon that hydrogen accelerates the dislocations movement of the steel during deformation was observed,which is related to the fact that hydrogen enhanced localized plasticity mechanism.

Numerical investigation into water entry problems of a flat plate with air pockets

Computational Fluid Dynamics(CFD)investigations into water entry problems of a rigid flat plate with air pockets were systematically conducted.The Volume of Fluid(VOF)model was utilised to capture localised slamming phenomena that occur during,and post-impact events.The model’s geometry was modified to include a pocket on the slamming impact surface to investigate the effect of air entrapment on the magnitude and distribution of slamming forces and pressures.A parametric study was conducted on the geometric parameters of the modelled pocket by altering its area,depth,and volume to exam-ine the response of slamming force and pressure loading under several impact velocities.The numerical results of slamming forces and pressures were in good agreement with experimental drop test measure-ments(with relative error of-6%and 7%for the magnitude of slamming force and pressure,respectively).The numerical results proved that the peak pressure is proportional to the magnitude of impact velocity squared(p maxαv^(2)).

Effect of three dimensional stress state on unstable fracture condition and crack opening level in a new crack growth model

A new model for the analysis of fatigue crack growth in the metal structures was proposed. This model shows a promising capability of explaining various fatigue phenomena. The new crack growth model is further completed by a continuous empirical formula for estimating the value of variable fracture toughness during crack propagation and a modified continuous equation for the crack tip stress/strain constraint factor used to calculate the stress intensity factor at the opening level. The prediction results are proved to agree well with the observed phenomena in test.