presentation - NGI

Water Depth (m)
The Frontier
3500
3000
2500
2000
1500
1000
500
0
1940 1950 1960 1970 1980 1990 2000 2010
Year

Water Depth (m)
Air Speed (km/hr)
The Frontier
3500
3000
4000
3500
2500
3000
2000
1500
1000
500
2500
2000
1500
1000
500
0
0
1940 1950 1960 1970 1980 1990 2000 2010
Year

The Frontier
Image Courtesy of Jeanjean

The Frontier

The Frontier

Risk and Reliability
on the Frontier
of Offshore Geotechnics
Bob Gilbert, The University of Texas at Austin
Don Murff, Consultant
Ed Clukey, BP America
6

Achieving
Appropriate Risk
Requires Balancing
Risk and
Conservatism

Probability of Failure with C or More Cost in Lifetime
Target Risk
1.E+00
1.E-01
1.E-02
Not Tolerable
1.E-03
1.E-04
Tolerable
1.E-05
1.E-06
10 100 1,000 10,000 100,000
Cost, C ($1,000,000 U.S.)

Riser – Installation Load
Ballast Box
Driven Pile

Probability of Failure with C or More Cost in Lifetime
1.E+00
1.E-01
Not Tolerable
1.E-02
1.E-03
1.E-04
Tolerable
1.E-05
1.E-06
Riser Tower Foundation
10 100 1,000 10,000 100,000
Cost, C ($1,000,000 U.S.)

Required Weight of Ballast (% of Design Load)
80%
70%
Proposed Design
60%
50%
40%
30%
20%
0.0001 Probability of Overload
0.001 Probability of Overload
10%
0%
0 20 40 60 80 100
Set-Up Time (days)

Riser Bundle –
Sustained Load
Suction
Caisson

Design Check
Contractor Proposed
Uplift Capacity
Load Factor
Resistance Factor
Tension Load
W caisson
Versus
Uplift Capacity
Load Factor
Resistance Factor
Tension Load
W caisson

Probability of Failure with C or More Cost in Lifetime
1.E+00
1.E-01
1.E-02
Not Tolerable
Riser Bundle
Foundation
1.E-03
1.E-04
Tolerable
1.E-05
1.E-06
10 100 1,000 10,000 100,000
Cost, C ($1,000,000 U.S.)

Managing Risk
Requires
Understanding
Loads as well as
Capacities

Annual Probability of Exceedance
10
1
0.1
0.01
0.001
0.0001
0.00001
0.5 0.6 0.7 0.8 0.9 1
Actual Load/Design Load

Gulf of Mexico – Mudslide Prone Area

Mechanism for Wave-Induced Mudslides

Mudslide scars after Hurricane Ivan
3 km
From Thomson et al. 2005 OTC 17738

Pipeline Failure from MP-151 after Hurricane Ivan
From Coyne et al. 2005, OTC 17734

Hurricane Paths
Delta

Hurricane Ivan vs. Hurricane Katrina
Hurricane Ivan
Hurricane Katrina

Factor of Safety
Ivan vs. Katrina at MC 20
1.2
14 sec.
Hurricane Ivan, Period = 16.1 sec.
1.1
Peak Spectral Period = 14 sec.
Hurricane Katrina, Period = 15.2 sec.
15 sec.
Stable
1.0
17 sec.
16 sec.
Ivan
Katrina Katrina
0.9
Unstable
0.8
15 50 15 18 60 18 21 70 21 24 80 24
27 90 27
30 100
Maximum Maximum Wave Wave Height (ft) (m)

Main Pass Oil Gathering Pipeline Damage
Hurricane Ivan
MPOG
Platform
Leg
Pipeline Damage
Photos from Wicklund (2005)

Bottom Currents in Hurricane Ike

Developing
Effective
Geotechnical
Designs Requires
Understanding
Their Role in the
Systems They
Support

Image Courtesy of
Sandstrom et al.

Probability of Failure in Design Life
1.0E-02
1.0E-03
1.0E-04
1.0E-05
Rope&
Chains
Anchor
Total
Rope&
Chains
Total
Rope&
Chains
Total
1.0E-06
Anchor
1.0E-07
1.0E-08
1,000
(Semi-Taut)
2,000
(Taut)
Anchor
3,000
(Taut)
Water Depth (m)

Structural Systems
Grout
Member
Doubler
Repair
Legend
True North
5 deg
Platform
North
(+)10’ Elv.
(-)20’ Elv.
(-)51’ Elv.
B
1
2
3
4
A

Base Shear
Foundation System
Capacity Interaction Curve
Shear
Failure
Unstable
Combined
Failure
Unstable
Unstable
Stable
Stable
Stable
Overturning
Failure
Overturning Moment

Structural System

Structural System

Structural System

Tripod Failure in Hurricane Ike
Leg C
Leg A
Leg B

Base Shear (kips)
Pile Flexibility
2500
2000
Peak Side
Shear
Mobilized on
All Piles
1500
1000
Residual Side
Shear Mobilized
on All Piles
500
0
Maximum
Load in Ike
0.0E+00 1.0E+05 2.0E+05 3.0E+05 4.0E+05 5.0E+05
Overturning Moment (ft-kips)
Maximum
Capacity
Mobilized on
All Piles

Base Shear (kN)
Base Shear (kN)
Pile System Robustness
3-Pile System Failed in
Overturning
6-Pile System Survived
Potential Shear Failure
12000
*Foundation system capacity includes the effect of the conductor.
30000
* Foundation system capacity includes the effect of the conductors.
25000
8000
20000
15000
4000
Sensitivity
Possible
to +/-30%
range of
Change
foundation
in
system
Lateral
capacity* due to increasing or decreasing the
& Axial Soil Capacity for Single Pile
axial and lateral capacity of one pile by 30 %
0
0.0E+00 2.0E+05 4.0E+05 6.0E+05
Overturning Moment (kN-m)
10000
5000
Possible range of foundation system
Sensitivity to +/-30% Change in Lateral
capacity* due to increasing or decreasing the
& Axial axial Soil and lateral Capacity capacity for of one Single pile by Pile 30 %
0
0.0E+00 5.0E+05 1.0E+06 1.5E+06
Overturning Moment (kN-m)

Maximizing Value
of Information
Requires
Considering How It
Might Affect
Decisions

Value of Reliability Approach
Less Data
Acceptable
Reliability
Cost of
Foundation
Unacceptable
Reliability
Conventional
More Data
Cost of Investigation

Mature Field
N
Boring (pre 1982)
Boring (post 1982)
20 km
Geophysics
Geophysics + Boring

500
1000
1500
2000
2500
3000
3500
4000
0
Expected Pile Capacity (MN)
Geo-Statistical Model
34
Older Boring
Modern Boring
33
32
31
30
29
Unconditional Mean
28
27
26
x Coordinate (m)
1-m Diameter, 100-m Long Pile
0
4000
3500
3000
2500
2000
1500
1000
500
y Coordinate (m)

500
1000
1500
2000
2500
3000
3500
4000
0
c.o.v. in Pile Capacity
Geo-Statistical Model
0.080
Unconditional c.o.v.
0.060
0.040
0.020
Older Boring
Modern Boring
0.000
x Coordinate (m)
0
500
4000
3500
3000
2500
2000
1500
1000
y Coordinate (m)
1-m Diameter, 100-m Long Pile

Partial FS due to Spatial Variability
Value of Information
2.00
1.75
Marine Clay over
Alluvial Sand and
1.50
Platform
Site
1.25
Marine
Alluvial Sand
and Clay
1.00
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Coefficient of Variation due to Spatial Variability

Communicating Uncertainty
Boring A
Platform
Location
Boring B
Normally
Consolidated
Marine Clay
100 m
Over
Consolidated
Clay Crusts
Buried
Alluvial
Channels
300 m
200 m

Uncertainty Multiples

Tension Leg Platform

Probability Density Function
Tendon Load
Pile Capacity
Pile Design
Method B
Pile Design Pile Design
Method A Method C
0 1 2 3 4 5 6
Failure Region
Force/Design Load

Design Pile Capacity

Lower-Bound Pile Capacity

Probability Mass Function (PMF)
Probability Density Function (PDF)
Lower Bound Capacity
PDF
PMF
Lower
Bound
Design or
Median
Pile Capacity

Probability of Foundation Failure in Design Life
1.E-02
Estimated Lower Bound of Pile Capacity (% of Median)
0% 10% 20% 30% 40% 50% 60%
1.E-03
Target
1.E-04
1.E-05

Probability of Foundation Failure in Design Life
1.E-02
Estimated Lower Bound of Pile Capacity (% of Median)
0% 10% 20% 30% 40% 50% 60%
1.E-03
Installation
Target
1.E-04
Re-Tap
1.E-05

Summary
Risks
Costs

Probability of Failure with C or More Cost in Lifetime
Target Risk
1.E+00
1.E-01
1.E-02
Not Tolerable
1.E-03
1.E-04
Tolerable
1.E-05
1.E-06
10 100 1,000 10,000 100,000
Cost, C ($1,000,000 U.S.)

Offshore Oil
Industry
Hurricane Protection
System
$30 Billion Damage
100% Evacuation
0 Fatalities
$30 Billion Damage
80% Evacuation
1,600 Fatalities