Time-Dependent Pipe-Soil Resistance for Soft Clay

Abstract

ABSTRACT Diverless platform pull-in and curve lay-away of deep water offshore pipelines, is a cost effective concept for pipeline expansion control, provided technical feasibility can be ensured for the local seabed conditions. This concept is presently under consideration for NS Norske Shell's Troll Phase I development, where the seabed consists of soft clay. To establish a more reliable pipe-soil resistance model for analysis of lay-away expansion curves on soft clay, i.e. a total resistance force model instead of a traditional Coulomb friction model, full scale axial and lateral pipe-soil interaction tests are carried out. The consolidation effect on soil resistance to axial and lateral pipe movements was specifically investigated. The tests revealed a significant increase in peak soil resistance, both axially and laterally, with increasing time of consolidation. The complete set of test results forms a reliable basis for detailed parametric analysis of the curve lay-away concept. INTRODUCTION Thermal expansion of an offshore pipeline towards a platform riser is traditionally controlled by expansion spools, installed on the seabed to fit the riser/pipeline geometry and tied-in using hyperbaric welding. With increasing water depth this method becomes progressively less attractive due to the inherent need for manned subsea intervention. For the tie-in to the Troll Phase I platfonn (water depth 300 m) the so-called curve lay-away was identified as a promising alternative in combination with pull-in into a dry riser shaft via declivitated sealtubes. A general background for the Troll Phase I pipelines is available elsewhere [I]. Early works indicated that thermal expansion was partly prevented by high soil resistances on the seabed near Troll which contains soft clay. To verify the curve lay-away concept, further quantification of the soil resistance was therefore required. The study comprised two parts: An experimental part which is presented here, and an analytical part described elsewhere at this conference [2]. The descriptions of pipe-soil resistance developed in the first part have been implemented in the a finite element program for analysis of pipeline response in the second part of the study. The pipe-soil tests were run on soft clay, representative for Troll conditions, in test facilities and with testing procedures previously developed for testing of lateral soil resistance to pipeline motion [3]. The axial test pipe construction and the laboratory programme were completed in the period May to December 1990. TEST FACILITIES The test set-up comprised test pipes, mechanical rig, soil flume, computer system for test control and data acquisition and equipment for soil reconditioning and testing as described by Brennodden et al. [3, 4] and Wagner et al. [5] with a few modifications. Large steel pipe sections of 0.5m diameter and 1.5m length and with rough and smooth surface respectively were employed. Two novel pipe sections for measurement of axial soil resistance were developed. Figure 1 shows a sketch of the axial pipe set-up. The axial pipe set-up comprises two half steel cylinders, fit together by three steel rods.