In-Situ Measurement of Pipe-Soil Interaction in Deep Water

Abstract

Abstract The paper describes the evolution of a new deep water in-situ testing system that is designed to measure the seabed soil-pipe interaction forces in the vertical, axial and lateral directions in very soft clay encountered in the deep water environment. The forces and displacements on the pipe are measured using an instrumented 8-inch diameter section of polypropylene-coated steel pipe mounted within a deployment frame. In addition to the measurement of forces in three dimensions, changes in pore water pressure are measured at discrete points along the underside of the pipe section. The system is suitable for deployment from geotechnical and construction support vessels with a 20 tonne A-frame. The paper describes the development of the equipment from the initial specification through field trials to readiness for deepwater deployment. It also discusses how the data from in-situ measurements will complement and enhance the results from existing methods. For unburied high temperature, high pressure pipeline systems, correctly predicting axial " walking?? and controlling lateral buckling is extremely sensitive to the selection of the pipe-soil interaction parameters. Very soft clay soils in deep water are difficult to sample and characterise especially within the first half metre of the seabed where most deepwater pipelines interact with the soil. Several authors have developed analytical models to characterise pipe-soil interaction, but there remains a large degree of uncertainty in the soil behaviour. These models have been developed using numerical methods and model tests on soils reconstituted in the laboratory and in the centrifuge. These techniques by their nature do not properly account for in-situ conditions, particularly the soil structure which is destroyed during sampling and reconsolidation prior to laboratory tests. The paper describes how in-situ measurements could fill this knowledge gap and discusses the relative merits and limitations of each technique. Results from the equipment are load-displacement curves in the three directions, coupled with pore-water pressure measurements, enabling the effective stress state around the pipe to be understood. The data also provide information on the rate of consolidation of the soil under the installed weight of the pipe and the build-up of and interaction with soil berms created as the pipe displaces laterally. Comparisons are made between the in-situ test results, numerical models and 1g and centrifuge model tests to demonstrate how these measurements can be used to complement existing techniques. Subtle changes in the pipe-soil interaction parameters can make several tens of million dollars difference to subsea hardware CAPEX and OPEX. There have been some well-publicised failures of such flowlines and equally probably many over-designed systems. This subject is one case of soil-structure interaction where it is not possible to use a marginally conservative estimate of interface resistance and apply it to each mode of pipeline movement - i.e. what is conservative in the axial sense may not be conservative in the lateral sense. The authors believe that in-situ measurements will reduce this uncertainty and improve the reliability of unburied deep water pipelines. Introduction The assessment of pipe-soil interaction of High Pressure High Temperature (HPHT) and other deepwater pipeline systems in deep water is a highly complex subject. One of the fundamental issues with the problem is the low contact effective stresses (<10kPa) between the unburied pipe and soil. It is a poorly understood subject area within soil mechanics as most civil engineering applications concern stress levels considerably higher. This is compounded by the uncertainties on pipeline embedment due to the dynamic effects during pipeline installation, the variability in loading rate during start-up and shut-down and the interaction between lateral and axial pipeline movement.