Pipe-Soil Interaction Models in Deepwater Very Soft Clays

Abstract

Abstract This paper presents a simplified plastic model for estimating the horizontal resistance of a pipe in a pre-formed trench of highly plastic very soft clay, representative of the touchdown zone of an SCR. This model has been based on the results of eight tests performed in a West African clay with a shear strength increasing linearly with depth. The tests formed part of a larger testing programme of twenty two pipe-soil interaction tests which were performed at pipe motions of 5 mm/s (‘slow’ test) and 100 mm/s (‘fast’ test) to verify and extend the applicability of the expressions proposed by the CARISIMA and STRIDE JIPs for both horizontal and vertical pipe-soil interaction in relation to steel catenary riser (SCR) design for highly plastic very soft clay. From the simplified model it is possible to approximate the horizontal pull force during pipe-trench interaction for shallow and deep pipe embedments in the ‘slow’ tests. For pipe motions of 100 mm/s the lateral force was found to increase by 50% to 100% compared to the ‘slow’ tests. This was attributed to extra suction behind the moving pipe and more effective heaving of the soil ahead of the pipe, in both cases due to the suppression of cracks in the clay. The model consists of separate expressions for base shear, the initial passive resistance of the soil trench wall and the increased passive resistance accompanying the creation of soil heave ahead of the pipe. It provides a simple basis for assessing the interaction forces that should be considered in the design of SCRs when the lateral resistance of the riser against the seabed trench is of concern. Introduction The CARISIMA and STRIDE JIPs involved pipe-soil interaction tests that have led to the development of lateral and vertical pullout resistance expressions through a series of laboratory tests using relatively low plasticity clays. Oliphant et al (2006) have reported a further twenty-two small-scale pipe-soil interaction model tests that were conducted in a highly plastic very soft West African clay. This work was carried out at Cambridge University, on behalf of Technip. These results were presented and compared with the mathematical expressions developed from the CARISIMA and STRIDE JIPs, and those expressions were recalibrated where appropriate. The main finding from this work was that the direct application of the horizontal and vertical resistance expressions developed during the STRIDE and CARISIMA JIPs to highly plastic clays was found to be inappropriate. The primary focus of this paper is to:present and discuss the results of the horizontal pull tests previously reported by Oliphant et al (2006), exploring the influence of trench depth and speed of pipe movement;compare the results with known plasticity solutions for surface and deep foundations; anddevelop a simple plastic behaviour model defining the horizontal resistance of a pipe in a pre-formed trench of highly plastic very soft clay. It is known that the dynamic motion of an SCR during operating conditions leads to the creation of a trench within the touchdown zone. These trenches can be several diameters deep and wide (Bridge & Howells 2007). During severe loading conditions the SCR can be swept against the wall of this trench, leading to high lateral reaction forces, which should be considered in design.