Abstract

Increasingly, hydrocarbon discoveries in the North Sea are remote fields with limited reserves at high temperature and pressure. Reserves from these fields are often transported, in an untreated state, through highly insulated, small diameter, steel pipelines to existing infrastructure in the area.

The installation and operation of such pipelines raises a number of issues which must be addressed at the design stage. Small diameter, insulated pipelines are usually very light, which can cause problems during installation and are often susceptible to rapid upheaval buckling on start-up or cyclic ratchetting during extended operation.

A number of incidents of pipeline floatation and upheaval buckling have occurred in recent years in the North Sea1,2. The majority of incidents can be attributed to a lack of understanding of geotechnical conditions during installation and operation. The cost of remedial measures can be very significant.

Coflexip Stena Offshore have sponsored a number of geotechnical research programmes since 1995 in an attempt to mitigate risk during pipeline installation and operation.

Several breakthroughs have been made including the practical demonstration of pipeline flotation during backfilling; the validation of a mechanism for cyclic ratchetting in a full-scale laboratory experiment; confirmation that soft clay backfill can be effectively modeled as a frictional material when considering uplift resistance; and derivation of applicable axial friction factors for coated pipelines in sand.

Introduction

Small diameter, steel pipelines, linking remote subsea fields toexisting infrastructure, are increasingly common in the North Sea. These pipelines are often relatively light due to significant insulation systems and are required to transport hydrocarbons under very high temperature and pressure and are therefore susceptible to upheaval buckling.

The majority of small diameter pipelines in the North Sea are trenched to provide environmental stability and physical protection against fishing gear. Trenches are usually backfilled to provide restraint against upheaval buckling and to provide increased thermal insulation of the pipeline.

Floatation during backfilling has been proposed as a mechanism to explain vertical displacement of pipelines following installation1,2. Floatation during backfilling is a comparatively new phenomena which has become more prevalent as an increasing number of small diameter pipelines have been backfilled.

Upheaval buckling can occur if a pipeline is subjected to a compressive load when its thermal expansion is restrained. The compressive load can cause the pipeline to buckle when lateral and axial frictions are insufficient to prevent movement.

Floatation and upheaval buckling can result in loss of cover or even exposure of a pipeline at the seabed. In extreme cases, the over-stressing of the pipe itself, due to significant bending, may occur. It is therefore necessary to mitigate floatation during backfilling and upheaval buckling during operation at the design stage, to ensure that the pipeline system remains fit-for-purpose throughout its design life.

The two major geotechnical components that affect the prediction of upheaval buckling are the axial friction between the seabed soil and the pipeline and the uplift resistance of the backfill above the pipeline.

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