Submarine Pipeline Support By Marine Sediments
Authors: Small Sam W. , Tamburello Russell D. , Piaseckyj Peter J. 
Published: December 2007

Although a more theoretically correct and mathematically elegant way of determining initial settlement of a submarine pipeline can be used, this approximate and pragmatic approach seems justified at this time. When methods of soil sampling and analysis improve and when ultimate bearing capacities of marine sediments are better understood, then the approach can easily be refined.

Introduction

When petroleum pipelines are laid under water, it is generally neither economical nor practical to prepare the bottom to receive the pipeline. The pipe must be laid on whatever soils are encountered. Because of this, the design engineer must evaluate soil-pipe interaction both during construction and after installation.

During construction by the lay barge method, initial settlement and lateral restraint due to soil friction and embedment must be evaluated. When using the bottom pull construction technique, longitudinal friction between the pipe and the soil must also be evaluated. When a pipeline is to be buried, the jetability of the soil, or in some cases its suitability for mechanical excavation, must be determined.

After installation, the positional stability of the pipeline is of major concern. Items to be evaluated pipeline is of major concern. Items to be evaluated depend upon whether the pipeline is buried or resting on the bottom. In the case of a buried pipeline, longitudinal restraint, long-term settlement and resistance to upward movement are important, whereas with the pipeline resting on the bottom, lateral restraint, longitudinal restraint and long-term settlement are of prime concern.

This paper deals only with the evaluation of initial settlement, and since initial settlement in sandy sods is not generally a problem, only initial settlement in cohesive marine soils is considered.

State of the Art

Quantitative evaluation of initial settlement of a submarine pipeline to be laid on a soft marine sediment is a difficult foundation problem. Textbooks provide a number of theories for calculating the ultimate bearing capacity of soils, but these theories were developed for use on land. And although some technical papers have been published that deal with the various papers have been published that deal with the various aspects of submarine pipeline foundation problems, the practicing engineer will be hard-put to find a comprehensive theoretical treatment of initial settlement of submarine pipelines, or any published works on experiments or experience in this area.

Ultimate Bearing Capacities Theories

When a submarine pipeline is placed on a soft marine sediment, it will settle until the ultimate bearing capacity of the sediment becomes great enough to support the weight of the pipeline. Therefore, quantitative evaluation of initial pipeline settlement depends on the ability to predict the ultimate bearing capacity of the soils for various amounts of settlement.

A classical problem in soil mechanics is the determination of the ultimate bearing capacity for a very long strip foundation. Eqs. 1 through 6 represent some of the solutions to this problem in current use.

Prandtl

(1) Qu = 5 .14 c

Fellenius

(2) Qu = 5 .52 c

JPT

P. 317