Steel catenary risers (SCRs) are an enabling technology for deepwater oil and gas production. Tools to analyse and design SCRs are available which show that the point where the riser first touches the soil, termed the touchdown point (TDP) is critical. However our understanding of fluid/riser/soil interaction is limited, hence the oil and gas industry has concerns regarding the levels of conservatism in SCR design, and margins of safety. The purpose of this study is to examine the interaction between a pipe (representing a section of the SCR), a clay seabed, and the surrounding seawater.
y seabed, and the surrounding seawater. This paper documents some of the results and observations from the full scale harbour test riser experiments which examined the 3D effect of fluid/riser/soil interaction around the TDP. The riser, a 110m (360-ft) long 0.1683m (6-5/8 inch) diameter pipe, was draped from an actuator on the harbour wall to an anchor point on the seabed. The top end of the pipe string was actuated using a programmable logic controller (PLC) to simulate the wave and vessel drift motions of a spar platform in 1000m (3,300-ft) water depth, both in-line and transverse to the SCR plane. The pipe was fully instrumented to provide tensions and bending moments along its length.
Observations from the harbour tests show that a trench forms around the TDP. Evidence collected shows that the trench created was tear-drop shaped, with a maximum width of 2.5 riser diameters and a maximum depth of 1.2 diameters. The trench was thought to be created from a combination of the applied vessel motions and fluid flow across the riser and the seabed, however the exact trenching mechanisms are unknown.
The work was conducted as part of the successful STRIDE JIP (Steel Risers in Deepwater Environments Joint Industry Project).
If you would like a copy of this technical paper.