Pipe-Soil Interaction Behavior for Pipelines Under Large Displacements on Clay Soils - A Model for Lateral Residual Friction Factor

Abstract

Abstract This paper addresses the behavior of pipelines partially buried on clay soils under large displacements, which is one of the principal concerns in the design of HP-HT subsea pipelines with lateral buckling. Subsea pipelines are being required to operate at higher pressure and temperature (HP-HT) in recent projects developed in deepwater. Many of these projects adopt a design philosophy of controlled buckles by limiting feed-in displacement at the buckle locations. DNV OS-F101 contains a strain based design approach that has been frequently adopted by designers. The use of the strain based design approach implies in a careful evaluation of soil resistance during a lateral buckling process, because a small increase in this parameter can produce a significant increase in the strains at the buckle apex. In the recent years PETROBRAS has developed several projects in deepwater areas of the Santos and Campos Basins, offshore Brazil, where soft clay is predominant. This has motivated the development of an extensive experimental test program for pipe-soil interaction, aimed at increasing the design reliability of subsea pipelines with lateral buckling. The model presented is based on full scale tests developed at the Institute of Technological Research (IPT) installations over the last three years. The experimental results were used to develop a model for the lateral residual friction factor based on dimensionless groups that govern the problem. The aim of this model is to improve knowledge obtained by the industry in recent years, mainly by the JIP SAFEBUCK I and II programs. Furthermore, the full scale tests produced results for the breakout friction factor that were compared with a predicted pipe response based on models currently adopted in the industry. Moreover, some "very light?? to "heavy" cyclic behavior highlights are presented and discussed. The model developed for the residual friction factor provides an important improvement especially for "heavy" pipes, where a gap was verified in deepwater pipeline design. A classification for "very light" to "very heavy" pipes is proposed, based on the experimental results and soil bearing capacity expressions from geotechnics. This classification is a very useful tool to understand pipe-soil behavior for large displacements. Introduction Partially buried subsea pipelines and subject to large displacements present a complex interaction between the pipe movements and soil resistance that is outside of traditional geotechnical behavior. HP-HT pipelines can be susceptible to lateral buckling developing displacements of several diameters at the buckle location. Pipe-soil interaction is a key issue in the design of HP-HT pipelines, so controlled buckling is being adopted in the design process as a cost effective solution for deepwater pipelines. Soil resistance behavior of partially buried pipes for small displacements in clay soils has historically been the main focus, due to research conducted for stability analysis. Models developed by the PIPESTAB project[1] and the American Gas Association (AGA)/Pipeline Research Committee[2] are examples of research for pipe-soil interaction in the range of small displacements. Later, Verley-Lund[3] developed a model for breakout soil resistance based on experimental results and dimensional analysis extensively used by the industry. More recently JIP SAFEBUCK[5] presented an expression for Breakout soil resistance with the same format of proposed by Verley-Lund, but with different calibrated parameters. Pipe-soil interaction research in the range of large displacements is much more recent and limited to the SAFEBUCK JIP[5,6] that presented a model for residual soil resistance based on experimental data. Subsea pipelines in clay soils frequently present "heavy" pipe behavior, which exhibit significant differences from "light" pipes. A gap in the SAFEBUCK model for residual resistance was found for the "heavy" pipe case, and this has motivated PETROBRAS to develop a large scale test program for pipes in clay soils in the range of large displacements.