Pipelines on mobile and
liquefiable seabeds:
How to convert LIMAS
results into design practice
Jesper Damgaard, HR Wallingford
Why ?
(Part 1/4)
Courtesy of David Osorio
Why ?
(Part 2/4)
Why?
(Part 3/4)
Conventional, Coulomb-friction stability concept
Waves
Current
FZ
Fx
Fµ
Seabed sediment
Ws
Why ?
(Part 4/4)
Possible situations on a mobile seabed
(a)
(b)
lee erosion
mobile
mobile
stabilised by
weight of pipe
mobile
stabilised
by sheltering
stable
+
(c)
mobile
erosion
mobile
For situation (b) and (c) the conventional
stability approach is inappropriate
Overview of presentation
• LIMAS research
– Aim of pipeline research programme
– Overview of pipeline experiments and numerical
modelling
– Main results
• Design guidance
– Existing guidance in codes & standards
– Soil investigations
– Suggested design practice
• Conclusions
LIMAS research
Aim of research
• To investigate the mechanism of liquefaction
and sediment mobility around a pipeline and
• To improve predictive capabilities and
design approaches for pipelines on mobile
seabeds.
Means of achieving the aims:
• Experiments (mainly) and
• Numerical modelling.
Pipeline Experiment:
ISVA, Denmark
Test flume
Pipeline Experiment:
ISVA, Denmark
Test setup
Pipeline Experiment:
ISVA, Denmark
Test set-up
Pipeline Experiment:
INPG, France
Pipeline Experiment: HRW and U. Cambridge, UK
HR Wallingford Wave Flume
Wave flume
Spending beach
All unit is in m
Wave Energy Absorbing
Beach (Shingle on 1:12
Slope)
Wave paddle
1.2
0.96
Screen
Screen
2
13
7.4
Screen
35
Soil test section
Array of pore pressure transducers
Pipeline
Ramp
1.5
False floor
1
Soil bed
1.6
All units is in m
False floor
1
0.3
Ramp
1.5
Pipeline Experiment: HRW and U. Cambridge, UK
Experiment
Participant
Soil type
d50
[mm]
Model
diameter
[m]
Model
density
[kg/m3]
Range
of H
Range
of T
[cm]
[s]
WP1 and
WP 5-2
ISVA
Silt
0.05
and
0.08
0.04 and
0.08
(0.02,
WP5)
WP1:Pipe
is fixed
WP5:
1.5-2.1
10-17
1.6
WP2
INPG + UPPA
Sand
(Hostun)
0.35
0.175,
0.200,
0.250 and
0.290
1887 2993
NA
2-8
WP5-1
HRW +UCam
Limestone
Silt
0.03
0.075
SG = 1.1
to 2.1
5 - 22
1.25
Comments
Water depth is 0.42 m
Water depth is 0.45 m
LIMAS research
Numerical modelling
HRW collaboration with Andrew Chan (Birmingham University) to
modify DIANA-SWANDYNE II for wave loading
LIMAS research
Main results
LIMAS research
Main results
MOVING
WATER
PIPE
MOVING
SOIL
STATIONARY SOIL
LIMAS research
Main results
LIMAS research
Main results
LIMAS research
Main results
LIMAS research
Main results
3
Pipeline
2.5
Far Field
2
p* = p / σ'z0
1.5
1
0.5
0
-0.5
-1
-1.5
-2
0
40
80
120
160
Time (s)
200
240
280
LIMAS research
Main results
Enhancement Ratio
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
0
0.1
0.2
0.3
D/d
0.4
0.5
LIMAS research
Main results
0
10
20
30
Time (s)
40
50
60
70
80
0.00
0.50
b
b/D
1.00
D
1.50
Pipe displacement
2.00
2.50
3.00
Liquefaction front
advancement
90
LIMAS research
Main results
0
200
400
600
Time (s)
800
1000 1200 1400 1600
0.00
0.50
1.00
b/D
1.50
2.00
2.50
3.00
3.50
4.00
b
D
LIMAS research
Main results
Specific gravity
1
1.1
1.2 1.3 1.4
1.5 1.6 1.7
1.8 1.9
2
2.1 2.2
0
0.5
1
1.5
D
b/D 2
2.5
3
3.5
4
b
Initial position for two
degree of movement
freedom
Final position for two
degree of movement
freedom
Initial position for one
degree of movement
freedom
Final position for one
degree of movement
freedom
Design
Design guidance
Guidance in existing codes
Code
BS 8010:part 3
(1993)
DnV, RP E305:
On-bottom
Stability of
Submarine
Pipelines (1988)
API - RP 1111
Guidance on liquefaction
•Seabed liquefaction should be considered when
assessing vertical stability
•Seismic action: The possibility of soil
liquefaction should be investigated
•Wave action: no such requirement
•However, under the description of liquefaction
it is stated that the causes include:
oWave action
oSeismic action
oTidal action
oRiver discharge
•For evaluation of liquefaction the code refers to
Gravesen and Fredsøe (1983)
•Buried pipelines should be checked for
sinking/flotation
•In soils which may liquefie: the specific weight
of the gas nor air filled pipe should be larger
than, or equal to, that of the soil
Wave-induced liquefaction not mentioned.
For earthquake-induced liquefaction: consider rerouting
Design guidance
Overview of stability design process
• Liquefaction assessment (at various levels)
• Prediction of magnitude and extent
• Risk management / mitigation
• Design
– Pipeline weight
– armouring
– Piled foundations
• O&M phase: monitoring
Design guidance
Two stage assessment of liquefaction risk
1.
‘Screen’ soil
•
Use Japanese guidelines based on PSD*
•
[Determine Cyclic Shear Stress Ratio compare with SPT]
If liquefaction hazard is significant, proceed:
2.
Perform more detailed soil investigations
•
In situ: PCPT (w. dissipation tests)
•
Lab tests [‘undisturbed’/remoulded]
•
Tri-axial
•
Cyclic tri-axial
•
Consolidation
Port & Harbour Research Institute, 1997.
Handbook on Liquefaction and Remediation
of Reclaimed Land, A.A. Balkema, Rotterdam, 1997
Design guidance
Prediction of magnitude and extent
• Predict liquefaction for all possible design
parameter combinations (it is not
immediately obvious which is worse)
– Parameter ranges or sensitivity study
– Monte Carlo simulation
• Range of models available –
see Dunn et al. 2004
• Multiply excess pore pressure with a factor
1.5 to 2 to account for pipe
• Assess depth of liquefied layer
Design guidance
Stability design (for unburied pipeline)
Assume that calculations show a situation where
• The depth of liquefied soil is large (i.e. > D)
• The lift force is negligible
• and the effective stress is negligible
In other words the soil near the mudline behaves
line a heavy fluid
Design guidance
Stability design (for unburied pipeline)
1. Calculate SGl.s. on the basis of ecrit
If ecrit is not available, use emax (non-cons.)
2. Estimate specific gravity of pipeline
corresponding to nominal embedment:
D
water
soil
SGpipe = ½ (1 + SGl.s.) for b = D/2
SGpipe = SGl.s.
for b = D
b
Design guidance
Stability design (for unburied pipeline)
sp
wave breaking
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
No liquefaction
Liquefaction
b/D
0.81
D
water
soil
b
0.64
0.39
0.24
0
0.5
1
1.5
2
2.5
3
3.5
KC
Conventional approach
Teh et al. (2003)
4
Conclusions
General
•
As part of LIMAS a number of successful
pipeline experiments have been conducted
•
The results can be used to develop more
rational pipeline design approaches
•
Ideally these improvements will find their
way into the pipeline codes / standards
Conclusions
•
Particular
The process of seabed liquefaction and
ensuing pipe movement is complicated.
It involves contraction of lower soil layers
and expulsion of pore water into an upper
layer that dilates
•
The presence of the pipe enhances the pore
pressure build-up
•
For a significant liquefaction probability the
design SG of the pipe can be derived from
the SG of the liquefied soil
•
More work is required in order to develop
practical methods to estimate the critical void
ratio for potentially liquefiable soils
Acknowledgement
Contributions from my LIMAS colleagues
–
Andrew Palmer, U. Cambridge
–
TC Teh , U. Cambridge
–
Scott Dunn, HR Wallingford
–
Mutlu Sumer, ISVA
–
Pierre Foray, INPG
–
David Bonjean, INPG
And from
–
David Osorio