Soft soil deformation and erosion in riser touchdown zone: insights from T-bar penetration tests

Soft soil deformation and erosion induced by riser motion strongly influence seabed trench development near the touchdown zone, with consequences for pipeline fatigue damage. Although previous studies highlight the influence of riser motion and clay softening, the detailed trenching mechanisms remain insufficiently understood. To address these gaps, this study directly visualizes trench development using camera-based monitoring, identifies the sequential mechanisms of trench formation, and reveals the critical contribution of soil cutting beneath a moving riser. In this paper, a series of T-bar penetration tests were conducted to investigate clay deformation and erosion representative of the riser touchdown zone. Undrained shear strength, cyclic amplitude and initial position were varied, and deformation and erosion were recorded in real time. The potential mechanisms of trench formation arising from pipeline–seabed interaction were then synthesised. Results indicate four deformation regimes, classified by penetration depth and motion amplitude, progressing from negligible flowback at shallow penetration to pronounced flowback, cutting and adhesion as depth increases, each strongly affecting erosion. Clay erosion occurs through two primary mechanisms: water-replacement erosion and moving-structure erosion. In the former, high local velocities generated as the T-bar approaches the clay surface entrain fines and strip surficial material, whereas in the latter, cutting produces adherent clay that is subsequently ablated by relative water–structure motion. • Conducted T-bar penetrometer tests to investigate the clay deformation and erosion near the riser touchdown zone. • Evaluated effects of different clay shear strengths, cyclic amplitudes, and initial positions on the clay deformation and erosion. • Revealed potential mechanisms of trench formation driven by interaction between a moving pipeline and the seabed.