ABSTRACT

Subsea pipelines and cables are vital components for transporting of gas and fluid in offshore oil industry and electricity in offshore wind industry. The pipe is exposed to complicate environmental loads induced by wave and current. One of the main challenges in design of the pipe is to ensure that the pipe will be able to remain at the desired position under such the environmental loads. The key factor which determines the pipe's capability to compensate the environmental loads is the resistance from the seabed. The present study investigates the influence of different pipe soil interaction models on the estimation of passive soil resistance as well as the initial penetration. Both sand and clay soil types from DNV-RP-F109, DNV-RP-F114 are considered in the study. A full on-bottom stability case study using different analytical pipe soil interaction models is presented to demonstrate the effect of initial penetration on the passive soil resistance and lateral displacement. Other factors affecting the on-bottom stability of the pipe including friction coefficient and undrained soil strength are also considered in the study.

INTRODUCTION

An appropriate design consideration for pipelines, umbilicals, power cables or flexibles is how to prevent them from moving excessively laterally once installed. In such a design, submerged weights are determined to be capable of withstanding hydrodynamic loads through friction and passive soil resistance. As an alternative, stability can be achieved through burial, trenching, continuous rock dumping and covering, or tension stabilization through intermittent interventions. As a result of hurricanes Katrina, Rita, and Ivan, the Gulf of Mexico has recently been experiencing the consequences of failed on-bottom stability design. As a result of pipeline failures, extensive lateral displacements (in the order of kilometers), collisions with subsea installations, and substantial production losses (Gagliano, 2007), several incidents occurred. In order to minimize burial costs and environmental impacts from seabed interventions, optimizing on-bottom stability design requires a minimum of additional weight or intervention.

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