The calculation of axial resistance of a pipeline is a key component in lateral buckling design, upheaval buckling (UHB) analysis, axial walking assessment, pipeline anchoring and pull-in and retrieval analyses. The level of axial resistance will depend on the rate and duration of pipeline loading, pipeline displacement and on the relative roughness of the pipe-soil interface which is normally addressed by an interface friction factor. The former relates to the understanding that the soil response can be bounded by drained and undrained conditions. A method is described which allows the application of drained or undrained conditions to be estimated for pipelines in clay/slit soils. The main purpose of the paper is to present a method for estimating the axial soil resistance using a relative roughness parameter, as well as making recommendations on the displacements necessary to mobilize peak and residual drained and undrained resistances in sand and clays. Both untrenched and trenched pipelines with various types of backfill will be considered.
Understanding pipe-soil interaction behavior is important in the design of subsea pipelines for :
Lateral buckling design
Steel catenary riser (SCR) design
On-bottom stability analysis
The main design aspects involving pipe-soil interaction for each of these design are summariesed in Figure 1.
The lateral buckling solution is very sensitive to soil-pipe interaction. The SAFEBUCK JIP1 has been the focus of research and testing of deepwater very soft clay to improve the understanding of pipe-soil response. Sands are generally considered to be better understood than cohesive soils.
Guideline for pipe-soil interaction associated with SCR design have been developed from the findings of STRIDE JIP and CARISIMA JIP. The CARISMA soil-pipe interaction testing has allowed the development of mathematical expressions for vertical (compression and suction) and lateral resistances of risers. These expressions have been encoded in the finite element program RIFLES as part of a SCR design approach. The design approach is relevant to very soft clays with intermediate to high plasticity. However, in general, deepwater very soft clays are characterised with an extremely high plasticity. Oliphant et al2 has extended the original work by SCR design approach within Abaqus.
Two pipe-soil interaction models are generally used on-bottom stability analysis programs. The first and simplest is an equivalent 2D Coulomb friction model, while the second 3D analysis model adopts a more realistic non-linear lateral force displacement relationship.
In upheaval buckling (UHB) analysis the interaction between the pipeline and surrounding soil will mobilize uplift resistance generally in the vertical plane and axial resistance in the longitudinal plane. Technip has developed state-of-the art uplift resistance equations for a range of different backfill types, such as blocky clay and post-jetted very loose sand.
The development of accurate and realistic pipe-soil interaction models is critical to the safe and cost effective design of subsea pipeline. The scope of this paper is to make recommendations on calculating the axial resistance
Figure 1: Pipe-Soil interaction aspects of subsea pipeline design (available in full paper)