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The top ends of marine risers are usually hinged under the floating platform, and the vortex-induced vibration due to ocean currents often leads to the potential fatigue damage. The non-intrusive optical measurement (high-speed camera) was employed to monitor the displacements of top-hinged-and-bottom-fixed flexible risers arranged in a circulating water flume. The experimental results indicate that, both the mode order of the riser excited in 3 directions and the dominant frequency increase gradually with the reduced velocity. The maximum out-of-plane root-mean-square vibration amplitude has a first downward and then upward trend in the mode transition cases. The spatial distribution of energy transfer between the fluid and the riser varies with the direction, resulting in the asynchronous mode transition. One of the in-plane vibration frequency coincides with the out-of-plane dominant one, which, if dominating the in-plane response, will make a strong coupling mode, or else a weak coupling mode. [ABSTRACT FROM AUTHOR]

ObjectiveThe viv-FOAM-SJTU solver is used to carry out the numerical simulation of the vortex induced vibrations of a riser under different top oscillation excitation amplitudes. MethodsThe flow field is computed by the strip theory and Reynolds-averaged Navier-Stokes (RANS) method, while the Bernoulli-Euler bending beam theory and finite element method are employed to model the structure of the riser. ResultsThe simulation results show that the cross-flow vibrations transform from multi-mode to single mode with the decrease in oscillation amplitude. At the same time, the inline vibrations maintain single mode, but the amplitude of the mode vibration decreases. ConclusionWith the variation in top oscillation excitation amplitudes, the washing out of the riser by shed vortices affects the characteristics of vortex induced vibrations.

[Objectives] As oil and gas exploration enters the field of ultra-deep water,once the outer structure of the flexible riser is damaged,the inner carcass will carry huge external hydrostatic pressure. As one of the geometric imperfections,the gap between the carcass and pressure amour may cause a significant reduction in the collapse strength of flexible risers,which will lead to the so-called wet collapse.[Methods] Aiming at this problem,this paper presents a spring-supported arch model for estimating the collapse strength of a subsea flexible riser with a layer gap,and the wet collapse process of the carcass is divided into pre-contact and post-contact phases. In the pre-contact phase,the instability behavior of the inner carcass is analyzed using the stability theory of single ring structure. When the interlayer gap is closed,the carcass begins to be supported by the pressure armor,and the structure enters the post-contact phase,here the outside pressure amour is considered as springs which support the detached portion of the inner carcass. At the same time,the finite element collapse model is employed to verify the reliability of this spring-supported arch model for a set of gap widths.[Results] The predictions of the proposed models agree very well with the numerical results,and the collapse pressure is very sensitive to the stiffness of the pressure amour and layer gap width. When the initial gap width between layers increases from 0 to 0.5 mm,the collapse pressure of the flexible riser will decrease by about 18%。If the tightly fitted carcass is supported by a stiffer pressure amour,the collapse strength will be greatly improved.[Conclusions] For a flexible riser with a harder pressure amour and a smaller layer gap,the proposed analytical model can effectively predict its hydrostatic crushing strength.

The microstructure， mechanical properties and hydrogen-induced cracking resistance of high strength steel used for flexible riser after different heat treatments were studied by means of optical microscope(OM)， scanning electron microscope(SEM)， transmission electron microscope(TEM) and electron back-scattered diffraction(EBSD). The results show that compared with the traditional quenching and tempering treatment， multi-stage heat treatment can refine the microstructure， promote the dispersion of fine precipitates and increase the density of grain boundary so that the strength of the tested steel can be improved greatly. Moreover， since multi-stage heat treatment increases the number of hydrogen traps， reduces the size of precipitates and improves the distribution of hydrogen traps and the hydrogen solubility， the hydrogen-induced cracking resistance of the tested steel is also improved. [ABSTRACT FROM AUTHOR]

This paper focuses on the efficient arrangement plan of buoyancy modules, which plan is used to secure the safe operation and structural stability of a marine riser. The marine riser is connected between a vessel and seabed devices. The movement of the vessel and the seabed devices are affected by the motion of the riser. The riser of a deep-seabed integrated mining system exerts a strong influence on the healthy transfer of minerals. So, buoyancy modules must be equipped to compensate for the problem which is the structure stability and the dynamic motion. Installation locations and quantities of the buoyancy modules are determined by real sea experiments. But this is not easy to do because in real sea experimental conditions the cost is expensive as well as being, time-consuming and dangerous. Therefore, the locations and quantities should be determined by numerical simulation. This method is called simulation-based design. The dynamic analysis models of the riser and the buoyancy modules are built into the commercial software of DAFUL. [ABSTRACT FROM AUTHOR]