Seabed trenches, as a geological hazard, adversely influence the safety of adjacent anchors. Seabed trenches predominantly appear in the Gulf of Guinea, where the seabed soils exhibit some features, e.g., high plasticity, high water content and low shear strength. However, the marine engineering geology where seabed trenches appear is not well understood, as well as the trenching process related to soil erosion. In this paper, the information about seabed trenches was summarized, and marine engineering geology where seabed trenches appear was analyzed in detail. It was found that the marine clay with high plasticity index, high organic content, higher sensitivity, low effective unit weight and shear strength is easier to form seabed trenches. Then, penetration tests of a chain-bar penetrometer were conducted to investigate the soil deformation and erosion near the surface. Experimental observations reveal that the normalized soil resistances from both penetrometers exhibited similar trends, despite different soil deformation mechanisms. In the cyclic tests, the soil resistance was degraded significantly in the first 20 cycles, and water flow induced by chain motion eroded the soil particles near the chain links. This study provides insights into the marine engineering geology and development process of seabed trenches.

Steel catenary risers (SCRs) provide a cost-effective solution for deepwater oil and gas production. However, SCRs are susceptible to potential fatigue failure due to the cyclic motions of floating platforms. Previous studies on the physical modelling of cyclic SCR-seabed interactions have primarily focused on either the continuous cyclic motion of an SCR or a single rest period between two SCR motion packets. However, our understanding of the development of seabed trenches and excess pore pressure and their effects on SCR fatigue during multiple episodes of SCR motion and soil reconsolidation remains limited. This study presents a newly developed model container capable of modelling three-dimensional SCR motions including heave, surge, sway, and vortex-induced vibration in a geotechnical centrifuge. A centrifuge test is conducted to investigate the vertical cyclic SCR-seabed interaction, considering five vertical cyclic motion packets with intervening periods of reconsolidation. The results indicate that ignoring the effects of reconsolidation leads to an overestimation of the fatigue life of an SCR. In this test, the SCR fatigue life is reduced by 18%-23% after five episodic SCR motion packets and intervening reconsolidation.

Soil deformation and erosion induced by anchor chain motion has a significant influence on the formation of trenches into the seabed near the touchdown area. However, the soil deformation and erosion are not well understood in the present trench formation, which limits the wide application in the prediction method. This paper presents a series of monotonic and cycle T-bar penetration tests to investigate the soil deformation and erosion phenomena at the seabed. Different soil undrained shear strengths were adopted, and a camera was used to record the soil deformation in real time. During the monotonic and the first penetration in the cyclic 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.