The long-term settlement of subsea pipelines on a clayey seabed is crucial for the on-bottom stability of the pipelines, especially in deep waters. In this study, a poro-elasto-viscoplastic finite element analysis is performed for predicting long-term settlement of subsea pipelines by incorporating a rheological constitutive model. A method for identifying the creep-settlement (Sc) from the total-embedment (Sk) is proposed on the basis of the obtained linear relationship between the secondary consolidation coefficient (C alpha e) of the clayey soil and the total-embedment (Sk) of the pipe. The identifying method is validated with the existing theoretical solutions and experimental data. Parametric study is then performed to investigate the key influential parameters for long-term settlement of subsea pipeline. A non-dimensional parameter Gc is introduced to quantitatively characterize the soil rheology effect on pipeline settlement. The relationship between the proportion of creep-settlement in the total-embedment (Sc/Sk) and Gc is eventually established for identifying whether the proportion of creep-settlement in the total-embedment is remarkable.

The combined effect of impact load-corrosion causes great damage to offshore pipelines, which has not been sufficiently considered in structural analysis and safety design, raising potential risks. Addressing this point, the failure mechanism of pipelines with corrosion defects under transient impact loads is explored: first, a finite element analysis model is developed. Through a systematical verification, including the performance in describing the mechanical elastic-plastic behaviours of pipelines under impact loads, pipe-soil interactions, and structural failure behaviours influenced by corrosion defects, the model's fidelity is demonstrated. Then, structural responses corresponding to multiple influential factors are investigated, including the corrosion dimensions, pipe-soil interactions, internal pressures and pipeline geometrical parameters. Furthermore, surrogate models are derived to predict pipeline damage. Results show that: with the deterioration in corrosion defects, pipeline dent damage becomes severe, and multiple mechanical behaviours are triggered and coupled, including deformation compatibility, stress-strain concentrations, bending and bulging buckling, which consequently causes various deformation configurations of structures. The pipe-soil interaction can alleviate the impact damage, which is insensitive to soil strengths owing to the strain rate and strain softening effect. The dent damage on pressurized pipelines tends to be localized, but the stress is increased. Increasing wall thickness and pipeline diameter reduce the local dent damage with a gradually weakened effect, owing to the energy dissipation from global responses. With a coefficient of determination over 0.987, the surrogate model of multiple layer perceptron demonstrates promising feasibilities in damage prediction and safety assessment. The developed models and analysis results provide theoretical and technical reserve for pipeline safety design and maintenance.

Burial is an effective approach to offshore pipeline protection for impact loads. However, few studies address the influences of inherent soil spatial variabilities on failure behaviour of soil covers and pipelines, causing deviations. Therefore, a random field-large deformation finite element analysis framework is developed to explore the failure mechanisms of buried pipelines in spatially varying soils. The failure mode of soil cover is conformed to a local mode, where the failure path is insensitive to soil variability. The failure mechanism of pipelines depends on the competition mechanism between soil strengths and pipe-soil interactions, based on which two typical failure modes are summarized. Soil variability not only aggravates the impact damage but also stimulates the diversity of structural responses. Correlations between probabilistic damage degrees and multiple influential factors are discussed. Further, inspired by the principle of energy dissipation, an integrated quantitative risk assessment model is derived to reveal the failure risk evolution, where uncertainties from soil variabilities and structure-related factors are considered. The latter shows a significant influence, which may pose an additional failure probability of over 50 %. Different safety design approaches are compared, and spatial failure probability surfaces are configured for burial depth determination.

The shear behavior of the pipe-soil interface determines the frictional resistance of pipe jacking. In the interfacial direct shear tests of well-graded dense sand against steel pipe under both unlubricated and lubricated scenarios, the shear stress initially exhibits hardening followed by softening. The shear band forms in the hardening stage, and significant morphology of the shear band varies in the softening stage. Eventually, the shear band exhibits a bell-shaped distribution in the pattern of horizontal displacement influenced by boundary conditions and fabric anisotropy. Coarse particles exhibit greater displacement and more intense softening due to larger initial void ratios and rotational radius, while specimens with more fine particles possess smaller maximum vertical displacement away from the interface and larger critical interface friction angle. Increased normal stress restricts particle displacement, resulting in larger shear displacement at peak state, more severe particle breakage, reduced shear band thickness, and increased peak interface friction angle. The shear stress reaches the critical stage earlier with bentonite slurry (omega = 6 %) due to reduced dilatancy and particle breakage. When the slurry concentration exceeds 14 %, overall sliding of particle displacement occurs instead of the layered distribution with increased vertical particle movement and noticeable stress softening. Continuous accumulation of irreversible dilation might induce forward movement of overlying soil. Moreover, excessive slurry concentration increases hardening and interfacial friction coefficient.

Differential frost heaving can damage buried pipelines, with catastrophic outcomes. It is necessary to consider the interactions between the pipeline and soil as well as the stress characteristics of frost heaving. In this study, a mechanical behavior model of buried pipe suffering from frost-heaving force based on the Winkler elastic foundation beam theory is proposed. The concept of a frost heaving spring is proposed to replace the foundation spring of Winkler's theory. The frost heaving spring is a pre-compression spring that is dependent on the relationship between the frost-heaving force and frost heaving amount, the frost heaving state is similar to precompressed spring resilience. Since the pipeline is continuous, soil in the non-frost-heaving area is squeezed by the pipeline and generates a corresponding elastic response. Modeling mechanical behavior of buried pipe suffering from frost-heaving force based on a linear frost heaving spring assumption provided analytical solutions under two conditions. Results show that the modeled pipeline deformation and stress values conformed well to measured data of Huang Long and Caen test. The proposed model is mathematically simple and easy to apply to studies of mechanical behavior of buried pipe suffering from frost-heaving force.

The steel catenary riser (SCR) serves as a primary solution for deep-water oil and gas field development, but it encounters complex dynamics due to forced oscillations induced by wave-driven floater motions, especially at the touch-down zone (TDZ). Traditional pipe-soil models often fail to address these challenges, as they do not account for soil remoulding and the impact of irregular motion. This is particularly relevant in real sea states characterized by irregular waves, where the floater's movements have a significant impact on seabed trenching, thus complicating the dynamic responses of the SCR. To address these issues, this study integrates an innovative effective-stress-based pipe-soil interaction model into a global SCR analysis to explore its dynamic response and fatigue damage under irregular waves. The irregular movements of the floater, derived from response amplitude operator (RAO) data and wave spectra, are applied to the SCR's top end after a translation to the hang off location. This allows for dynamic simulations that consider the evolution of the seabed and the process of trenching. The study focuses on deriving the dynamic stresses experienced by the SCR at the TDZ and evaluating fatigue damage using the S-N curve method. It also examines the seabed interaction, including the evolution of trenching, changes in seabed stiffness, and soil resistance at various SCR locations. By considering real sea conditions, this study yields insights into trenching and seabed-SCR interactions, promising to enhance design methodologies and bolster offshore infrastructure performance and safety.