Modelling Pipeline Behavior on Clay Soils During Storms
Authors: Hale J.R. , Morris D.V. , Yen T.S. , Dunlap W.A. 
Published: October 2008

This paper describes laboratory tests performed to better define minimum weight requirements for offshore pi pe1ines to resist 1atera1 motion under storm loading. Pipe-sediment interaction was studied using cyclic displacement-controlled lateral loading tests. These tests were conducted on a specially mounted section of pipe in a tank filled with soft sediment. A major objective was to verify an existing model l that predicts pipe embedment and the maximum horizontal resistance of the sediment as a function of loading history. The prediction model was found be to accurate for very soft sediment, but very conservative for stronger clay sediments. A modification to the model to better fit the test data is described. The results described herein have led to a better understanding of pipe/soil interaction during storm conditions.

INTRODUCTION

One of the major considerations in designing weight coating for an offshore pipeline resting on the seafloor, is to prevent excessive horizonta1 movement. Cyclic wave and current forces can result in small amplitude pipe motions which embed the pipe into the soil. A 1imited amount of test data is available on this subject from previous work (Brennodden, et. al., 1986), (Brennodden, 1987), (Morris, et. al., 1988). However, no data was available for clay soils with undrained shear strengths in the 30 to 200 psf range. Only very soft clay sediments (20 to 30 psf shear strength), and very stiff (approximately 5000 psf shear strength) had ever been tested. In the later tests on very stiff clay only frictional type soil resistance was observed. The pipe did not embed into the soil, and the tests provide no real history dependent data. From these tests all existing prediction models (Wagner, et. al., 1987), (Brennodden, et. al., 1989) are based. No data have been available for clay soils of different strengths.

Thi s test program endeavored to remedy this deficiency. High quality tests were conducted on near full scale model pipes, on clays with remolded undrained shear strengths from 1 to 8 kpa (22 to 160 psf). Results include pipe embedment as a function of load i ng history, and horizonta1 forces required to displace the pipe out of its' embedded position. These data were compared with a computer model widely used in industry, and the model was calibrated to better fit the new experimental data.

TEST EOUIPMENT

The experiments were conducted using a closed loop, servo-controlled hydraulic actuator. This was controlled by an MTS Model 458.20 servo-controller. The controller included load and displacement modules and a built-in wave function generator to supp1y transverse wave functions of desired wave shapes and frequencies.

The horizontal loading system consisted of two aluminum triangular side plates [1 meter (40 inches) wide, 0.6 m (25 inches) high and 0.01 m (3/8-inch) thick] rigidly connected on both ends of two 1.8-m (6-foot) long parallel I-beams. One set of linear ball bearings was mounted on the outside bottom edge of each side plate, to allow free horizontal movement of the loading system.