To elucidate the influence of initial water content on the undrained cyclic shear behaviour of marine soft clay, reconstituted specimens with initial water contents of 1.4wL, 1.8wL and 2.2wL were prepared using the slurry-consolidation method, and a series of undrained monotonic triaxial compression tests and cyclic triaxial tests were performed. The results show that, as the initial water content increases, the water content and void ratio of the consolidated specimens increase, whereas the monotonic and cyclic shear strengths decrease. During cyclic loading, excess pore-water pressure accumulates more rapidly, the effective mean principal stress decreases more markedly, and the double-amplitude axial strain enters the rapid-growth stage earlier. When the cyclic stress amplitude is normalised by the corresponding monotonic shear strength, i.e., qcyc/qf, the cyclic strength data for specimens prepared at different initial water contents converge to a unified curve. This indicates that the influence of initial water content on cyclic strength can be characterised through the corresponding monotonic shear strength under the tested effective confining pressure and loading conditions. In addition, analysis of the evolution of the cyclic secant modulus (E) using a stiffness degradation model shows that the initial water content mainly affects the shape of the E curve.
Safety issues during design and installation of flexible offshore pipelines should be addressed. However, existing models neglecting axial-bending coupling or treating bending stiffness locally may over predict the stress of pipeline, leading to overdesign and resource waste. To address these issues, a nonlinear finite element model that accounts for the axial-bending coupling effect is proposed, and is utilized to examine the mechanical behaviors of flexible offshore pipelines during laying process. A systematic comparison is conducted between the proposed model and the classical catenary and boundary layer models to demonstrate the necessity of globally incorporating bending stiffness and axial-bending coupling, and its effectiveness and correctness is further validated against OrcaFlex software. Parametric studies examine the effects of seabed strength, vessel tension, water depth, and current velocity and direction. The results indicate that increasing water depth raises free span and damage risk of pipeline. Higher soil strength increases stress in the touchdown zone. Appropriate vessel tension reduces overall stress of pipeline. Ocean current loads impair pipeline safety more severely under upstream conditions than under downstream conditions. Changes in current direction induce simultaneous axial and lateral forces. This investigation provides a valuable guidance and theoretical basis for offshore pipeline design and installation.
