Pipeline Stability on the Seabed

An Innovative Approach To Increase

Pipeline Stability on The Seabed

By:

Godwin Eton

PhD Student

School of Civil Engineering

Supervisors:

Prof. Barry Clarke and Dr Terry Cousens

February 2011

School of Civil EngineeringFACULTY OF ENGINEERING

Outline

Offshore pipelines

Behaviours of offshore pipelines

Current industry solutions and the challenges

Research in civil engineering, Leeds

Electro-kinetics principles in geotechnical engineering

Test conducted and some results

Conclusions


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Transportation of oil and gasUses

Diameter between 0.15m to 1.4m

Average of 0.3m for most field development

Size

Pipelines Larger diameter Flowlines Smaller diameter

Types

Offshore Pipelines

Petroleum field development using pipelines

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Flowlines are more unstable

on the seabed

To ease the flow of oil and gas and

prevent solidification of wax components

Pipeline tries to expand but is restricted by

interface friction between the pipe and the

soft seabed

Pipe is subjected to an axial compressive

load

When this load reaches some critical

value, the pipeline becomes unstable and

is displaced in various directions- vertical

(upheaval buckling) or lateral movements

(lateral buckling) or axial ( pipe walking)

These can jeopardise the structural

integrity of the pipeline and create a

potential hazard for in-service pipelines


Oil/gas now required

to operate at higher

temperature and

pressure

Behaviour of offshore pipelines

-The Problem

Upheaval buckling

Lateral buckling

This expansive behaviour of pipelines remains one of the major issues currently facing many

...large deep water subsea developments prompting need for research into pipe-soil interactions 4


Current Industry Solutions

The above are not only expensive but very difficult in deeper water

Most current and most cost effective

method is to lay the pipe in

Snake lay configuration (SLC)

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Dig a trench and bury the pipe Anchor the pipe to a fix

structureConcrete mattress/rock cover

SLC


Schematic representation of plan view of typical lateral buckling design

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SLC design allow the pipe to buckle laterally during operation but

depends on a very large extent on the strength of the soil supporting

the pipeline

Of all the design parameters, the soil response causes the greatest

uncertainty in the design due to the extreme dependency/sensitivity

of design solutions to axial and lateral resistance imposed by the soil

Increasing the soil resistance to pipe displacement is imperative to

enhance the effectiveness of snake laying approach


The Challenge

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None of the existing mitigation methods involved the modification of

ambient soil properties

Research is conducted here to study the deformation mechanism of

soil during pipe displacement as well as using Electrokinetic

processes to increase the strength of seabed soils which support the

offshore pipelines


Research in Civil Engineering, Leeds

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Electro-kinetic Principles of Soil Improvement

When a saturated fine soil

is exposed to direct electrical

current by means of

electrodes (anode (+) and

cathode (-), the cations in the

clay surface migrate to the

cathode and drag water with

them causing water flow

toward the cathode

Can result in increasing the

strength of the soil near the

anode

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Electro-kinetic Principles in Geotechnical Engineering

L.Casagrande, 1949 - Control pore water at excavation sites

Bjerrum et al, 1967 - Increase the shear strength of slopes and walls

V. Milligan 1995 - Improve the friction/bearing capacity of piles

Micic et al , 2001 - Increase the capacity of offshore foundations

EK not yet used in pipeline stability10


Test conducted- small-scale and full-scale

Moisture content

Undrained shear strength

Soil pH and electrical properties of the soil

Pull-out test to investigate the resistance of the treated soil to lateral,

axial and vertical displacement of a model pipe section partially

embedded in model soil

Results were compared with control tests and the difference assumed

to be due to EK treatment

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Small-scale test


Equipment Design and Methodology for this research

Before EK test After EK test

Test Setup for lateral, axial

and vertical pull-out tests

X-section of partially embedded

pipe section Model pipe section with electrodes in the small-scaled testing tanks

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Full-scaled test


Equipment Design cont.

Full-scaled testing tank No EKFull-scaled testing tank during EK tests

Pipe section with electrodes used for EK testElectrodes used for full-scaled tests 13

Results from the small-scaled

tests where up-scaled to fit the

large tank to investigate effect of

EK treatment in full-scale.

Test conducted in both axial

and lateral directions


Some results

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1

2


15

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Some results

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Lateral Pulling Test Results from iron

and Aluminium electrodes

Vertical Pulling Test Results from iron,

copper and Aluminium electrodes 7

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Changes in Cu between FE and AL

electrodes

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Some results effects of electrode materials

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Change in CU before and after EK treatment using Al electrodes109

EK process can be employed to increase the stability of subsea pipeline

Iron electrodes appear to be more effective for surface-laid pipes

Aluminium electrode might be better for buried pipes. Research ongoing

in this area

EK has potential applications of strengthening soft soil around subsea

pipelines and thus mitigate against axial walking and lateral buckling of

pipeline

EK Can be extended to stabilise buried pipelines, communication

cables e.g. fibre optic wires as well as other offshore foundations

This will not only reduce cost of installation but also reduce time to

commissioning of these structures


Conclusion

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Thank You For Listening

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Documents

Pipeline Stability on the Seabed