3D experiments investigating the interaction of a model SCR with the seabed

Steel catenary risers (SCRs) are used to transport hydrocarbon products between the seabed and floating production facilities, particularly in deep offshore environments. As developments move into deeper water the understanding of structural performance of the riser can become critical to operational longevity. SCRs can be prone to fatigue damage, especially in the region where the riser pipe reaches the seabed — known as the ‘touchdown zone’. The results of a fatigue assessment depend significantly on the assumed pipe–soil interaction conditions at the touchdown zone, which remains an area of uncertainty for designers.Typical experimental investigations into the problem focus on the two-dimensional elemental response of a short section of riser pipe with the soil in order to calibrate interaction models. This paper describes a different approach, where the three-dimensional response of the riser with the seabed is explored experimentally. The experimental equipment described represents the first such apparatus used to investigate 3D riser–soil interaction under controlled conditions in a laboratory. The model riser pipe was 7.65 m long and 110 mm in diameter and was loaded by both monotonic and cyclic motions via a computer-controlled actuation system. A range of instrumentation was used to assess the structural response of the model riser as well as trench formation and the development of excess water/pore pressures. In these experiments the pipe was placed on a bed of sand for benchmarking purposes although future experiments will explore the response in clay soils which are typically encountered in the locations where SCRs are used.Numerical analysis was used to determine an appropriate form for the distribution of soil reaction along the length of the pipe, in response to the uplift of the model pipe. Results from the numerical analysis displayed good agreement with the experimental data. A simple methodology is outlined for the back-calculation of the distribution of soil bearing stress beneath the model pipe. This provides a link between the 3D test results and the more typically conducted 2D tests, allowing the verification of pipe–soil interaction models derived from 2D experiments. A number of observations are drawn from the work regarding 3D riser response, including the effect of riser geometry and stiffness on soil reaction and vertical pipe–soil load paths and hydrodynamic ‘jetting’ induced trench evolution.