Anticipation and Mitigation of Geohazards for Subsea Power Cables (and Pipelines)

Advanced Subsea Power Cable for the Oil and Gas Industry Brussels Paris Newcastle London Hamburg Milan
Anticipation and Mitigation of
Geohazards for Subsea Power Cables
(and Pipelines)
Matt Owen
Senior Project Geophysicist
Cathie Associates London – September 2015

Historical Examples
First order:
1929 Grand banks slump –28 breaks in
transatlantic cables.
1979 Nice – debris flow during airport
construction.
2005 - 08 Gulf of Mexico – 100s of
pipelines.
2011 Japan – five communications
cables.
Second order:
Effect of mobility in exposing cables to anthropogenic hazards.

Deep water developments
Deep water Nile Delta
Santos Basin
Congo Basin
Deep water Gulf of Mexico
West of Shetland
Rigzone

Deep water developments
Deep water Nile Delta
Santos Basin
Congo Basin
Deep water Gulf of Mexico
West of Shetland
Rigzone
Deep water, or otherwise
remote developments may
involve connections (power
or oil and gas transport) over
potentially hazardous terrain.

1. What do we mean by geohazard?
2. Implications for cabling
3. A Case study
4. Quantifying risk
5. Mitigating risk
6. A methodology
Wilson et al.
(2004)
USGS

What is a Geohazard?
A hazard is something that poses a risk to something that matters to you.
Geohazards are derived from the earth system include a wide range of
processes.

What is a Geohazard?
A hazard is something that poses a risk to something that matters to you.
Geohazards are derived from the earth system include a wide range of
processes.
The hazards a project faces are determined by:
1. The type of project
2. The environment
3. The stage of the project life cycle

Geohazard Continuum
Frequent, but relatively mild events.
Rare, extremely damaging events.
Consequence/ severity
1 2 3 4 5
Frequency
5
4
3 ALARP
2
1
Risk Matrix

Geohazard Continuum
Frequent, but relatively mild events.
Rare, extremely damaging events.
Consequence/ severity
1 2 3 4 5
Frequency
5
4
3 ALARP
2
1
Wave and surge induced bottom
currents [e.g. scour and mobile
bedforms]
Rare, extremely damaging events
[e.g. large magnitude earthquake,
submarine landslides, tsunami]. Risk Matrix

Geohazards in Relation to
Cables
Different geohazards will affect different locations along
a cable route (absolute and frequency).
An individual geohazard event may have different levels
of severity along different locations along the cable
route.
Different regions have different primary hazards (e.g.
North Sea vs. eastern Mediterranean).

How to Identify
(Anticipate) the Relevant
Geohazards
Understand the setting
• Desktop study
• Geological/ MetOcean overview
• Preliminary surveys
Understand the project
• Finalise route
• Cable engineering
• Life span
Understand the hazards
• Specialist studies and surveys
Hsu et al. (2008)

Costs? Consequences?
Cost of repair
• Dependant of type of damage
• Dependant on location (e.g. remote, deep water costs are higher)
Cost of lost sales
• Down time (potentially weeks to months)
Financial penalties
Loss of reputation
• Share price

Case study – high
risk area
HVDC cable 100s km length
High levels of seismicity and
volcanism
Identified submarine and
terrestrial landslides

Determining Risk
Route zonation and potential for hazard avoidance based on hazards and
constraints.
Understanding of specific geohazards.
Engineering solutions when necessary – with prioritisation.

Determining Risk
Route zonation and potential for hazard avoidance based on hazards and
constraints.
Understanding of specific geohazards.
Engineering solutions when necessary – with prioritisation.
Seismic Risk
• Ground accelerations
• Displacement and co-seismic deformation
How severe? How frequent? Should we mitigate?
Balance between anthropogenic hazard and geohazard. e.g. don’t
bury the cable….

Return
period
(years)
PGA –
deterministic at
fault (% gravity)
Deterministic max
magnitude at fault
(Location A)
Max fault
displacement
(Location A)
25 <43 6.2-6.5 <1 m
100 <53 7.1 <2 m
300 NO DATA
Understanding individual faults
• Direction and distance of
displacement
• Frequency of rupture
• Shallow soil conditions
• Alignment in relation to route

A Workflow

Probabilistic
Assessment?
How much information do you have?
Remote sites or new regions this may be minimal.
Probabilistic assessments require larger sample populations.

Probabilistic
Assessment?
How much information do you have?
Remote sites or new regions this may be minimal.
Probabilistic assessments require larger sample populations.
Novel approaches may be required:
• Bayesian statistics
• Fuzzy Logic?

Case study – North Sea
Balance between Geohazard
and anthropogenic risk
Fishing and anchoring
provide greatest risk,
however…

Case study – North Sea
Balance between Geohazard
and anthropogenic risk
Fishing and anchoring
provide greatest risk,
however…
Mobile sediments may
increase this risk by
exposing the cable.
Conditions may present risk
to installation.
What is the most cost
effective solution?
Dredging? Re-routing?
Intervention?

Mitigation Methods
Re-route
Can hazard be avoided?
Increased cost of cable
Burial
Works for some, not others
Intervention
Mattressing
Cable engineering
Armouring
Thermal protection
Slack

Mitigation and Cost
Benefit
What is the potential cost to the project from damage caused by the
geohazard?
Financial
Other
What is the likelihood of this occurring?
What mitigation measures are available?
Effectiveness?
Associated cost?
Technical feasibility?

Mitigation and Cost
Benefit
What is the potential cost to the project from damage caused by the
geohazard?
Financial
Other
What is the likelihood of this occurring?
What mitigation measures are available?
Effectiveness?
Associated cost?
Technical feasibility?
A commercial decision.

A Methodology
Understand the site/route
Engineering/ Routing requirements
Environment (Metocean, geology)
Potential installation methods and constraints
Potential hazards
Factors influencing frequency/ severity
Desktop study
Route and Specialist survey(s)?
Probabilistic/ Bayesian assessment of frequency/ severity
Cost benefit analysis
Determine most appropriate mitigation measure(s)
Assess risk from mitigated hazards

A Methodology
Understand the site/route
Engineering/ Routing requirements
Environment (Metocean, geology)
Potential installation methods and constraints
Potential hazards
Factors influencing frequency/ severity
Desktop study
Route and Specialist survey(s)?
Probabilistic/ Bayesian assessment of frequency/ severity
Cost benefit analysis
Determine most appropriate mitigation measure(s)
Assess risk from mitigated hazards
Expert
knowledge
and
experience

Conclusions
1. Understand the project
• CAPEX
• Engineering and environment
• Life span, stakeholders etc
2. Understand the global hazard context
• Tectonic setting
• Historical events
• Analogues
3. Understand the local hazard context
• Specialist surveys and probabilistic assessment
• Variation along route
4. Cost benefit analysis to perform appropriate mitigation
• What is the consequence of doing nothing?
• What is the cost to reduce risk to ALARP?

Anticipation and Mitigation of Geohazards for Subsea Power Cables (and Pipelines)

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