Soil deformation and erosion induced by riser motion has a significant influence on the formation of seabed trenches near the touchdown area, which affects the pipeline fatigue damage assessment. Previous studies emphasize the influence of bottom current and water velocity fields on the trench formation, but the specific process related to soil erosion is not well understood. In this paper, a series of monotonic and cycle T-bar penetration tests were conducted to investigate the seabed soil deformation and erosion phenomena near the riser touchdown area. Different soil shear strengths were adopted, and the soil deformation and erosion ware recorded in real time. During the monotonic tests, the soil was displaced by the T-bar. In the cyclic tests, the degradation of soil resistance mainly appears in the first 20 cycles, and the final resistance is originated from the water replacement in the trench. The soil erosion near the T-bar mainly appears in the cracks formed during the initial penetration. When the water in the trench is replaced by the T-bar, water flows and erodes fine particles through the cracks. The trenches are initially caused by the T-bar penetration, and then the soil along the trench wall and bottom are eroded away during cyclic loading to form stable trench profiles.

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