Research On The Response Of Deepwater Steel Lazy Wave Riser Under The Combined Action Of Slug Flow And External Current

With the development of economy,our country has an increasing demand for energy,and the exploration and drilling of oil and gas have extended from the sea to deep-water and ultra-deep-water.Although the development of deep-water oil and gas resources solves the country’s current energy needs,the environmental and safety risks of drilling or exploration in deep-water areas require reliable,robust,and economical riser configuration.Steel lazy wave risers are realized by adding buoyancy modules to steel catenary risers.The float section partially covers the buoyancy module outside the riser,providing damping and isolating the vessel motion from the touchdown point,which makes the steel lazy-wave riser more flexible than other riser configurations.In this paper,a dynamic model of deep-water steel lazy-wave riser system with internal flow and large displacement nonlinear effect is established.The riser system considers nonlinear boundary conditions,including a steady flow within the riser,which is modeled as a non-expandable tubular beam subjected to nonlinear hydrodynamic loads such as ocean currents or wave excitation.By combining the wake oscillator model with the approximate form of the nonlinear model derived in this paper,the internal flow vortex-induced vibration model is derived to investigate the effect of internal flow on vortex-induced vibration caused by external currents.Compared with the experimental results,the effectiveness of the proposed method is proved.The research content and results of this paper can be divided into the following aspects:1.The nonlinear large deformation beam theory is used to model the riser.The numerical solutions are performed in an incremental iterative manner,enabling efficient handling of riser-seabed interactions and avoiding the complexity of contact detection.At each increment,a new sequence equilibrium configuration is evaluated by approximating the total potential energy to a direct minimization of the Riemann sum,thus obtaining an algebraic system of nonlinear finite difference equations,which is further solved iteratively by Newton-Raphson technique.2.The general structure and characteristics of SLWR are analyzed.The different working conditions of steel lazy wave risers are discussed.Finally,the paper introduces how to select a reasonable parameter range and determine the optimal configuration.The method is verified by Orcaflex.Through static analysis,the effects of water depth,horizontal span,upper catenary length,extreme sea conditions and vessel motion on riser configuration,top tension,bending moment and shear force distribution are obtained.3.It is a complicated fluid-structure coupling problem to consider the vortex-indu ced vibration of a riser with external and multiphase internal flows.The vortex-induced vibration of SLWR with internal flow is compared with that of rigid catenary risers.When the length is similar,the influence of internal flow on steel lazy-wave risers is more significant than that on rigid catenary risers.On this basis,the vortex-induced vibration of SLWR under gas-liquid two-phase flow is calculated,and their effects on vortex-induced vibration of SLWR are discussed.The results show that the modal order increases with the increase of liquid volume flow,solid yield and gas volume mass.4.Slug flow is a common flow pattern in multiphase oil and gas riser system.The dynamic and structural response of the riser can be changed by time-varying slug flow forces.A method for evaluating the dynamic response of slug flow to deep-water SLWR is presented.The dynamic response of deep-water SLWR under the coupling of steady-state slug flow and external ocean current is calculated by global dynamic analysis method.According to the steady-state slug flow analysis results,the ideal regular slug flow is regarded as the time variation of internal fluid flow.Before the parametric study,the numerical model method was experimentally verified.The parametric study is to quantitatively determine the design conditions that are most affected by slug flow.The dynamic response of slug length and slug speed to SLWR is considered.The results show that the dynamic effect of slug flow on SLWR is significant.The key vibration characteristics of risers are numerically studied,such as the vibration induced by slug flow due to mass change and the displacement,amplitude and frequency changes caused by flow momentum.The effects of the length and movement velocity of the slug element on the vortex-induced vibration response are revealed.