Assessment - Prevention - Mitigation

Presented by James M. Strout

Why is scientific work in geohazard important - where does Geohazard fit in to oil business?

GEOHAZARDS, WHAT ARE THEY?

“Events caused by geological conditions or processes, which represent serious threats for human lives, property or the natural environment”

OnshoreVolcanism

Earthquakes

Slides/debris flows

Floods

Avalanches

OffshoreSlope instability

Earthquakes

Tsunamis

Shallow gas/hydrates

Diapirism

INTERNATIONAL CENTRE FOR GEOHAZARDSAssessment, prevention, mitigation and management

ICG vision:

Develop knowledge that can help save lives and reduce material and environmental damage.

To be, within 5 to 8 years, the world authority and the premier research group on geo-related natural hazards, with special emphasis on slide hazards, both on land and offshore.

HOST ORGANISATION

Norwegian Geotechnical Institute (NGI)

PARTNERS

University of Oslo (UiO)

NTNU

Geological Survey of Norway (NGU)

NORSAR

PARTNERS IN CENTRE OF EXCELLENCE

TsunamiTsunami

Offshore geohazards

Gas hydrates or free gas

Mud volcano

Overpressure

Debris flow

Diapirism Doming

Underground blowout

t

Retrogressive

sliding

Gas chimney

Wave generation

Earth-quake

t

Focus on underwater slope stability

• Field development on the continental slopes

• Enormous historic and paleo slides observed

• Large runout distances, retrogressive sliding upslope/laterally and tsunami generation may threaten 3rd parties in large areas

The Ormen lange field illustrates the importance of a geohazard study

Ormen Lange

Headwall 300 kmRun-out 800 kmVolume 5.600 km3

Area 34.000 km2

The Storegga Slide (8200 ybp)

Field development was contingent on the results of the geohazards study. It was necessary to: - understand the Storegga slide

- survey, sample, test and monitor to characterise site- develop failure mechanisms and models- evaluate the present day stability conditions

These studies resulted in the conclusion that the present day slopes were stable, and the site was safe for development.

• Site investigation (geophysical, geological & geotechnical)

• Assess in situ conditions and material properties

• Define relevant and critical geo-processes

• Assess interaction of processes

• Identify failure mechanisms

• Identify trigger mechanisms

Geohazards study – elements

• Overall geological understanding of site

• Assessment of probability of occurence

• Calculate/predict consequences

• Uncertainties:– Limited site investigations, measurement

and test data– Modelling of processes and mechanisms

Geohazards study – Assessment

Monitoring and measuring• Key parameters needed

– Seismic survey and metaocean data– Geological structures, history, sedimentation rates– Pore pressure and mechanical behaviour of the soil– Inclination/movement/settlement/subsidence– Gas releases or seepages– Vibrations/earthquakes– + + +

• Time dependent variable?– ’Snapshot’ measurement w/o time history– Monitoring w/ time history, e.g. to capture natural variations,

or effects caused by construction/production activity

• Timing: before, during and after field development

Closing comments

• Consequences of geohazard events can be very large, in terms of both project risk and 3rd party risk

• Thorough understanding of natural and human induced effects is needed in order to identify the failure scenarios relevant for field development

• Geohazard assessment require multi-discipline geoscience cooperation and understanding

Purpose of geohazards research

• improve our understanding of why geohazards happen.

• assess the risks posed by geohazards.

• prevent the risks when possible.

• mitigate and manage the risks when it is not possible to prevent them.

Thank your for your attention!

Overheads illustrating each element of a geohazard study

Geophysical investigationImproved imaging techniques

In situ conditions and material propertiesCorrelation of geological, geotechnical, and geophysical parameters

1.5 2 2.5 3D e n sity (g /ccm )

200

150

100

50

0

20 40 60 80Po ro sity (% )

1 1.5 2 2.5V e lo city (km /s)

40 80 120Ga m m a (AP I)

900

850

800

750

700

650 Sed.type

Age(ka)

SITE 22

Seafloor

INO2

INO3

INO4

INO6

60

-15

,M

ove

d b

y S

tore

gg

a S

lide

13

0-6

01

50-

13

02

00

-

15

0

TW T (m s) D epth (m )

INO5?

Sa

mpl

es

N orm al m arine and/or d ista l g lacia l m arine sed im ents;c lay w ith som e s ilt, sand and occasional grave l.G enera lly fine gra ined

D eposits m ost like ly of un its O 1 and O 2, bu t m oved and d is turbed by the S toregga S lide ,

G lacia l debris flow deposits and g lacia l m arine deposits. G nera lly qu ite coarse gra ined.

Defining critical geo-processes1D Basin model for Pressure-Temperature time history during

geological time Deposition rate

T=temperaturep=hydr. water pressureu=pore pressure=vertical soil stress’=eff. soil stress

dtdh

γ'tu

zu

c 2

2

v

z

Stress/pressure: p, u, ’

t

p u T

Sealevel change

h(t)

time

u ’

Contributing processes/interactionGas hydrate melting caused by climate change after deglaciation

Geothermal gradient 50C/km

0

500

1000

1500

0 10 20 30 40 50 60

Horizontal distance, km

De

pth

be

low

se

ale

ve

l, m

Sea bed

Potential zone of GH melting

Sea level LGM

BGHSZ after sea level rise

BGHSZ at LGM sea level at -130m m

BGHZ after intrusionof warm atlantic surface water

Shelf edge

Sea level today

BGHSZ at LGM sea level at -130m m

BGHSZ after sea level rise

BGHZ after intrusionof warm atlantic surface water

Potential zone of GH melting

Failure mechanismRetrogressive Sliding

• Development of material and mechanical models required for explanation of failure on low slope angles

• High excess pore pressure and/or strain softening (brittleness) required

• Local downslope failure (slumping) need to be triggered for initation of large slide

Triggering mechanisms Earthquake analysis

• 1D site response analysis of infinite slope• Material model for cyclic loading includes pore pressure

generation, cyclic shear strain, accumulated shear strain• Pore pressure redistribution and dissipation after

earthquake

0 2 4 6 8 10

0

100

200

300

400

500

600

700

Maximum Displacement, d (cms)

Dep

th b

elo

w m

udli

ne (

m) 0.30g

0.20g0.10g0.05g

0.01 0.10 1.00 10.00

0

100

200

300

400

500

600

700

Maximum Pore Pressure Ratioafter Seismic Event, u/s

vo (%)

Depth b

elo

w m

udlin

e (m

)

0.30g0.20g0.10g0.05g

Max. pore pressure ratio after event, %

Dep

th b

elom

mud

line,

m

Dep

th b

elom

mud

line,

m

Max. displacement, cm

0.30g0.20g0.

10g

0.0

5g

Overall geological understandingOrmen lange: the entire “geo-conditions” leading to instability

Evaluate consequencesTsunami modelling and prediction

Evaluating probabilities

• Variability/incompleteness of data• Modelling errors• Recurrence of triggering mechanisms• Presence of necessary conditions• + + +