Date post: | 28-Jan-2016 |
Category: |
Documents |
Upload: | aldous-chandler |
View: | 215 times |
Download: | 0 times |
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• + + +