Flowline and Riser: Soil Interaction in Plastic Clays
Authors: Dendani Hedi , Christophe Jaeck 
Published: October 2008

Flowlines in deep offshore areas are generally laid on the sea-bed under their own weight, without burial in a trench. They may experience lateral buckling and axial "walking" due to expansion, caused by their operating temperatures and pressures. Analysis of pipe-soil interaction is required to assess, control and limit buckling.

Fatigue analyses for riser are highly dependent on soil-riser interactions. The behaviour of the riser touch-down zone should be well investigated, in order to establish a realistic soil-riser interaction model.

This paper consists of three parts. The first part discusses the importance of a site-specific geotechnical characterisation of the very shallow formations. The second part is a summary of results of soil-pipe interaction experiments, conducted on re-consolidated soft clay recovered from deep-water offshore field. In the third part, simplified methods and practical calculations are proposed for flowline and soil-riser interactions.

INTRODUCTION

Flowlines in deep offshore areas are generally laid on the seabed surface under their own weight, without burial in a trench. When the internal pressure and temperature increase to operating conditions, flowlines tend to expand. Two different types of displacements may then occur: a/ axial displacement - commonly known as "pipe walking", b/ lateral displacement - displacement induced by pressure and temperature, which causes the pipeline to move laterally, and may induce lateral buckling.

These movements will result in stresses in the flowline itself, and at the connection spools and sub-sea structures. The continuous production cycles, of start-up and shut-down, result in cyclic loading and progressive movement of the flowline. This will cause cumulative fatigue damage to the flowline.

Extensive structural analyses should be performed, using both analytical and finite element analysis, in order to demonstrate that flowline movements will not lead to excessive stresses (exceeding yield, and maybe involving significant plasticity). In case these analyses show excessive stresses, the movements of the flowline should be controlled, in order to guarantee its integrity.

Realistic rules for the interaction between the soil and the pipeline should be integrated into the structural models. The objective of this paper is to propose simplified methods and practical calculations, to evaluate the axial and lateral pipeline resistance curves, which need to be integrated into the general pipeline models.

SOIL INVESTIGATION AND SOIL DESIGN PARAMETERS

Reliable information concerning soil layering and properties should be collected. The soil investigation program should include high-quality geophysical and geotechnical surveys. A geophysical survey allows detection of "geohazards", and provides a regional geological overview. The geotechnical survey should collect high-quality soil samples. In-situ geotechnical tests should be also performed (e.g. cone penetrometer tests, T-bar test, etc.). Particular attention should be paid to the investigation of very shallow strata (the first metre).