The PIPESTAB Pipe-Soil Interaction Project developed a model that predicts the soil resistance to lateral motions of untrenched pipelines. The model improves upon the typical coulomb friction estimation by including soil strength information in the resistance prediction. An extensive data base containing results from monotonic and cyclic tests of full scale pipe sections on five distinct soil conditions forms the foundation for the model.
Pipe-soil resistance is far more complex than simple friction. Test data convincingly shows a marked dependence of lateral resistance on pipe penetration and soil strength. The improved empirical model captures this dependence by estimating the lateral resistance using two terms, a sliding resistance component plus a soil passive resistance component.
The empirical model developed incorporates all significant trends observed in the test results and is suitable for use in pipeline dynamic response predictions. The testing program significantly extends the industry's data base, especially in the areas of cyclic loading and correlation of soil parameters to measured resistance.
Lateral soil resistance is one of the fundamental components in offshore pipeline on-bottom stability design for wave and current forces. Present industry practice is to estimate the soil resistance with a coulomb friction model which expresses the lateral resistance as the product of the effective submerged pipeline vertical force (submerged pipe weight minus hydrodynamic lift force) and a soil friction coefficient that only depends on soil type, typically O. S for clay and 0.8 for sand.
The PIPESTAB Pipe-Soil Interaction Project improves the prediction of soil resistance opposing lateral motions of offshore pipelines by including soil strength information and loading history effects. The project contained an extensive full scale laboratory test program of pipe-soil interaction on five realistic offshore soil conditions:
Loose silty fine sand
Loose medium/coarse sand
Dense medium/coarse sand
Soft clay
Stiff clay
The laboratory tests included both monotonic and cyclic lateral load tests. The empirical model formulated from the test data captures all of the trends observed during the experiments. Specifically, the model incorporates a non-zero lateral soil resistance observed at times when the hydrodynamic lift force approaches the submerged pipe weight. The model predicts a higher resistance for a given pipe on soft clay than for the same pipe on stiff clay due to embedment differences. In contrast, the coulomb friction force approaches zero lateral soil resistance when the lift force approaches the pipe weight and does not generally predict different resistance for varying soil strengths.
The Pipe-Soil Interaction Project was one part of the overall PIPESTAB Project which investigated pipeline on-bottom stability under storm loadings. The project was sponsored by Statoil and Esso Norge with technical assistance from Exxon Production Research Company. The work on pipe-soil interaction was performed by the Norwegian Hydrotechnical Laboratory, NHL, (affiliated with SINTEF) in Trondheim, Norway.
To conduct the necessary experiments on full scale pipe sections, 1.0m and 0.5m (39 in. and 20 in.) O.D., a special purpose test facility was constructed at NHL,. The test facility permitted any arbitrary force or displacement controlled time history to be applied to the pipe section on a typical offshore soil while recording the reaction forces and displacements.
