A Seismic Design Method for Subsea Pipelines Against Earthquake Fault Movement

As there are no specific guidelines on design of subsea pipelines crossing active seismic faults, methods for land buried pipelines have been applied to. Taking the large seismic fault movement into account, this paper proposes improved methods for seismic designs of subsea pipelines by comprehensively investigating the real constraining of soil on the pipelines, the interaction processes of soil with the pipeline, the plastic slippage of the soil, and the elastic-plastic properties of the pipeline materials. New formulas are given to calculate the length of transition section and its total elongation. These formulas are more reasonable in mechanism, and more practical for seismic design of subsea pipelines crossing active faults.

Predicting scour beneath subsea pipelines from existing small free span depths under steady currents

An equation was developed to predict current-induced scour beneath subsea pipelines in areas with small span depths,S.Current equations for scour prediction are only applicable to partially buried pipelines.The existence of small span depths(i.e.S/D<0.3)are of concern because the capacity for scour is higher at smaller span depths.Furthermore,it is impractical to perform rectification works,such as installing grout bags,under a pipeline with a small S/D.Full-scale two-dimensional computational fluid dynamics(CFD)simulations were performed using the Reynolds-averaged Navier-Stokes approach and the Shear stress transport k-ωturbulence model.To predict the occurrence of scour,the computed maximum bed shear stress beneath the pipe was converted to the dimensionless Shields parameter,and compared with the critical Shields parameter based on the mean sediment grain size.The numerical setup was verified,and a good agreement was found between model-scale CFD data and experimental data.Field data were obtained to determine the mean grain size,far field current velocity and to measure the span depths along the surveyed pipe length.A trend line equation was fitted to the full-scale CFD data,whereby the maximum Shields parameter beneath the pipe can be calculated based on the undisturbed Shields parameter and S/D.

Development of current-induced scour beneath elevated subsea pipelines

When scour occurs beneath a subsea pipeline and develops to a certain extent,the pipeline may experience vortex-induced vibrations,through which there can be a potential accumulation of fatigue damage.However,when a pipeline is laid on an uneven seabed,certain sections may have an elevation with respect to the far-field seabed,e o,at which the development of scour would vary.This work focused on predicting the development of the scour depth beneath subsea pipelines with an elevation under steady flow conditions.A range of pipe elevation-to-diameter ratios(i.e.0≤e o/D≤0.5)have been considered for laboratory experiments conducted in a sediment flume.The corresponding equilibrium scour depths and scour time scales were obtained;experimental data from published literature have been collected and added to the present study to produce a more complete analysis database.The correlation between existing empirical equations for predicting the time scale and the experimental data was assessed,resulting in a new set of constants.A new manner of converting the scour time scale into a non-dimensional form was found to aid the empirical equations in attaining a better correlation to the experimental data.Subsequently,a new empirical equation has also been proposed in this work,which accounts for the influence of e o/D on the non-dimensional scour time scale.It was found to have the best overall correlation with the experimental data.Finally,full-scale predictions of the seabed gaps and time scales were made for the Tasmanian Gas Pipeline(TGP).

Cost consequence-based reliability analysis of bursting and buckling failure modes in subsea pipelines

Risk assessment is a fundamental activity to evaluate the integrity of pipeline systems during their life cycle,which allows pipeline operators to focus attention on integrity management activities in order to prevent and mitigate failure and maintain the design safety level.Risks for materials and contractor’s quality are a major source of concern about the integrity of pipelines and have a remarkable influence on the optimized balance between capital expenditure(CAPEX)and operation expenses(OPEX)in risk-based integrity management of pipeline.While other sources of risks to pipelines have been investigated thoroughly,the impacts of construction and material quality have not been well studied yet.This paper addresses the influence of construction accuracy/quality and corresponding uncertainties on the failure of submarine pipelines in the presence of corrosion.Corrosion-bursting and corrosion-buckling failure modes(which are addressed as ultimate limit states of subsea pipeline in DNV-OS-F101)are investigated in a reliability analysis to predict critical failure year of the pipe.Sensitivity analysis is also implemented to estimate the importance of parameters on failure probability using Monte Carlo Simulation(MCS).The results show a high sensitivity of failure probability to construction accuracy,which defines contractor quality.

Dynamics of risers for earthquake resistant designs

It is well known that no criterion about seismic design for risers is available, and relevant research has not been reported. A comprehensive study of riser dynamics during earthquakes is performed in this paper. A dynamic model for seismic analysis of risers is developed in accordance with the working environment of the risers and the influence of inertia force of the pipelines. The dynamic equations for the developed model are derived and resolved on the basis of the energy theory of beams. Numerical simulation for an engineering project in the Bohai Oil Field, China shows that the fundamental frequency of the riser plays the major role in the seismic responses, and for platforms in shallow water in Bohai Bay, the risers demonstrate a much lower stress response due to prominent differences between the riser frequency and the earthquake wave frequency. The presented model and its corresponding method for seismic analysis are practical and important for riser design resistant to earthquake waves.

An experimental study on the internal corrosion of a subsea multiphase pipeline

Based on the actual operational parameters of a subsea multiphase pipeline,an experimental study on the internal corrosion of a subsea multiphase pipeline was conducted in a dynamic,hightemperature autoclave,which had a similar environment to an actual field environment,using the partial pressure of CO2(PCO2),velocity of the corrosion medium,temperature,corrosion time,and corrosion inhibitor as variables.The results show that CO2 resulted in severe localized corrosion and that the corrosion rate increased as the PCO2 and velocity increased;the corrosion rate first increased and then decreased with increasing temperature.The corrosion rate peaked at approximately 65
C and then decreased continuously afterwards;the corrosion rate decreased as the duration of the experimental period increased.Under the operational conditions of the selected subsea pipeline,localized corrosion caused by CO2 was still the primary corrosion risk.Several types of corrosion inhibitors could inhibit the occurrence of localized corrosion for a short time period;however,most corrosion inhibitors could not completely inhibit localized corrosion.