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Celso P. Pesce
Professor of Mechanical Sciences
PhD in Ocean Engineering,
MSc Marine Hydrodynamics, Naval Architect
[email protected]
LMO - Offshore Mechanics Laboratory
Escola Politécnica
University of São Paulo
Brazil
Risers – Flexible Pipes and Umbilicals
Introduction and Global Analysis
Brasil – Japan Cooperative Courses
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Risers
junta
flexível
riser de aço
bóia
intermediária
poita
enrijecedores de
flexão
TLPFPSO
flutuadores
restritor de
curvatura
flexible joint
Steel catenary
riser
Bending
restrictors
Subsurface buoy Floaters
Bending stiffener
Lazy-wave riser
Weight anchor
Flexible pipe Flexible pipes
• Umbilical cables: control signals,
electrical power, fluid injection to the
submarine equipment at the well
head.
• Flexible pipes: conveying oil, gas,
from the well head to the production
floating system or to another storage
and offloading vessel after
processing.
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Hystoric and trends
• ‘70s – fixed platforms:
• ‘80s – Semi-submersible
platforms and TLP’s
• ‘90s: TLP’s, SPARS
• 2000s: FPSO’s, TLP’s, Mono
Column, Semi-subs
• Umbilicals and steel risers
(static)
• Umbilicals and flexible pipes
(dynamic)
• Umbilicals, flexible pipes and
steel catenary risers (dynamic)
• Umbilicals, mixed systems, riser
towers (dynamic)
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Catenary Risers
• Loading: environmental action:
• direct (current, waves) and
• indirect, driven by the floating
system.
• Mechanical failures: overloading,
fatigue, localized damage (impact),
collapse, corrosion, welding,
flexible joints, bending stiffeners,
connections, etc...
• Environmental action has a
stochastic nature.
TDP
“FAR”“NEAR”
terminação
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HYDRAULIC
CONTROL
ELECTRICAL
CONTROL CABLE
DYNAMIC ELECTRO-HYDRAULIC UMBILICAL
CHEMICAL
INJECTION
ELECTRICAL POWER
CABLE
OPTICAL FIBRE CABLE
DYNAMIC POWER-OPTICAL UMBILICAL
Typical Umbilical Cables
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Integrated Steel Tubed Umbilicals (STU)
Page 7
Typical Flexible Pipe
by F. Toni
Page 8
Typical failure modes
• Helical tendons rupture – under traction and internal pressure;
• Internal carcasses collapse – under external pressure, squeezing and
crushing;
• Wear and fatigue of metallic components;
• Helical armour layers instabilities (birdcaging and lateral buckling)
• Leakage of plolymeric layers due to aging, chemical atack, degradation;
• Extreme bending efforts, caused by flexural-torcional instabilities (loops)
during laying operations or during fabrication/storage;
• Thermal expansion and sudden variation of bending stiffness;
• Gas permeation in the anular region – corrosion.
• Creeping of polymeric layers;
• Hoses colapse; copper strands fatigue and kinking (umbilicals);
• Others....
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Typical failure modes
Minimum Bending radius Tests
Bending on a wheel
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Typical failure modes
4’’ flexible pipe:
pressure armor
and carcass
under crushing
tests
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Typical failure modes
Flexible pipe collapse modesexternal pressure, squeezing
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Typical failure modes
Corrosion due to annular gas
permeation
partial pressure with time in
annulus #3
CH4
H2S
Page 13
Typical failure modes
Instabilities of flexible pipes
birdcaging Lateral buckling
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Typical failure modes
Umbilical crushing
Page 15
Facts
• Long lengths and low
tensioning:
• High axial rigidity and low
bending stiffness:
• Geometrically nonlinear
boundary conditions:
• Dynamic perturbations:
➢ Global mechanics dominated by
geometric stiffness (tension);
➢ Dynamic equations numerically
rigid (several distinct frequencies
coexisting);
➢ Local analysis necessary at hot
spots (TDZ, Top, floaters)(;
➢ Global dynamics can be linearized
and end effects corrected a
posteriori.
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Global Analysis Approaches
• A complete design procedure deals with many (inter-related)
aspects of the dynamic response caused by FPU motions and
ocean currents, assessing their impact on ULS and FLS, as:
– first-order wave motions and slow-drift motions (wave and wind);
– VIV, wake-interference and clashing;
– dynamic instabilities;
– non-linear boundary conditions at:
• TDA;
• Hang-off.
• Design procedures rely on exhaustive mathematical modeling
and demand a huge computational effort.
• Even though, many (isolated) fundamentals aspects are not yet
fully understood.
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• There are at least three different time-scales in the catenary
riser dynamic structural problem:
– The first one is dominated by axial rigidity, giving rise to relatively
small periods of oscillation.
– The second one is related to the catenary or geometric rigidity.
– The third one is of a local nature and is due to the local flexural
rigidity effects.
• Such a diversity of time-scales can lead to serious limitations
concerning numerical integration methods by rendering dynamic
equations mathematically stiff.
• Even the starting problem, to determine the static configuration,
can pose serious numerical difficulties, as the flexural rigidity
effect is confined and dominant just inside small regions close to
the ends, the TDP (touch-down point) and the upper end-fitting
or close to other regions of high curvatures.
Global Analysis Approaches
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Conventional
• Numerical integration of the
nonlinear static equilibrium
equations;
• Nonlinear dynamic analysis
around the static equilibrium in
time domain;
Huge computational times.
Expedit
• Numerical integration of the
nonlinear static equilibrium
equations;
• Linear dynamic analysis around
the static equilibrium in
frequency domain;
• Local nonlinear correction
through boundary-layer
techniques at hot spots.
Global Analysis Approaches
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• Time Domain (TD) schemes are strongly recommended (API-
2RD ) for/as:
– comprehensive treatment of the fully nonlinear hydro-elastic
problem;
– extreme environmental conditions, large displacements, tension
coupling, nonlinear loading, foundation modelling;
– transient events: pull-in/pull-out/disconnecting operations, loss of
FPU station-keeping ability, mooring-system failures;
– a reference for equivalent frequency-domain analysis.
• Frequency Domain (FD) schemes are used for speeding-up
design procedures:
– TDA nonlinear boundary condition should be considered
consistently through asymptotic methods; Aranha et al (1997) and
Pesce and Martins (2004).
– ‘Equivalent’ linear spring modeling is not consistent and must be
consciously exercised vs TD simulations.
Global Analysis Approaches
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Global Analysis Approaches
• Numerous numerical methods have been discussed and
implemented in the last two decades; see, e.g., Leira and
Remseth (1985), Larsen (1992) …. Silveira and Martins (2003).
CDM HBM WTM NWM
Initial Procedures * * *** ***
Stability * *** *** ***
Accuracy ** ** *** ***
Flexibility * * ** ***
Speed *** *** * **
Numerical methods; a qualitative comparison.
(***: better evaluation); Silveira and Martins (2003)
CDM: Central Difference method
HBM: Houlbolt
WTM: Wilson-Theta
NWM: Newmark
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Computational CodesPOLIFLEX 2D and 3D
Dynamics in FD
Global Analysis Approaches
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Global Analysis Approaches
Computational CodesPOLIFLEX 2D and 3D
Dynamics in FD
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Global Analysis Approaches
Computational CodesPOLIFLEX 2D and 3D
Dynamics in FD
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Environmental loading
Direct
• Current:
– Drag
– VIV
• Waves:
– Mean drag;
– Dynamic loading
Indirect
• Motion imposed at top by FPU:
– In the wave frequency range;
– In low frequency range due to:
• waves;
• current;
• wind;
• DP systems.
Interactions
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Moored Semi-Submersible Production Platform
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Turret-mooring FPSO
Orcina
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Spread-mooring FPSO
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6 Azimuth Thrusters
2700kW
+459kN / -293kN
DP-FPSO
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Wave Spectrum
0 0.2 0.4 0.6 0.8 1 1.2 1.40
1
2
3
4
5
6
7
8
9
10
Frequência (rad/s)
Densid
ade E
spectt
ral (m
2/r
ad/s
)Pierson-Moskowitz
JONSWAP
TP=11,4s e HS=5,5m
−=
4
0
5
2
4
5exp.)(
gS O
4
0
2
216
5 SO H
g=
( ) )2/(exp
4
0
5
220
220
4
5exp.)(
−−
−=
gS O
( ) ln.287,01.16
5 4
0
2
2−= sO H
g
=
0
0
09,0
07,0
Page 30
Wind spectrum
10-2
10-1
100
0
5
10
15
20
25
30
35
40
Frequência (rad/s)
De
nsid
ad
e E
sp
ectr
al (
m2/s
) Occhi-ShinHarris
6
52
2862...1146)(
−
+=
VVCSV
Harris
Occhi-Shin
g
V
FVVS
2610).69750()(
−+=
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Wind speed sample
0 200 400 600 800 10000
5
10
15
20
25
30
Tempo (s)
Ve
locid
ad
e d
o V
ento
(m
/s)
0 200 400 600 800 10000
5
10
15
20
25
30
Tempo (s)V
elo
cid
ad
e d
o V
ento
(m
/s)
Harris Occhi-Shin
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Wind force and moment coefficients (OCIMF)
0 20 40 60 80 100 120 140 160 180-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Ângulo de Incidência (graus)
Cv
x
Condição: Cheio
Condição: Lastro
0 20 40 60 80 100 120 140 160 1800
0.2
0.4
0.6
0.8
1
Ângulo de Incidência (graus)
Cv
y
Condição: Cheio
Condição: Lastro
0 20 40 60 80 100 120 140 160 180-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
Ângulo de Incidência (graus)
Cv
n
Condição: Cheio
Condição: Lastro
lateral longitudinal
yaw
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FPU Motions due to waves
1000 1100 1200 1300 1400 1500 160099
100
101
102
103
Sw
ay
(m)
Baixa Freq.Total
1000 1100 1200 1300 1400 1500 160044
44.5
45
45.5
46
Ya
w (
gra
us)
1000 1100 1200 1300 1400 1500 1600-0.5
0
0.5
Ro
ll (g
raus)
Tempo (s)
First and second-order wave forces
VLCC 100% loaded; Sea state: Tp=11,4s e Hs=5,5m
Page 34
Low frequency motions caused by second order
forces due to waves
rmsjPOjPO
jrmsjPO
txtx
dRAOStx
),(866,1),(
),().(2),(
0max0
0
0
2
0
=
=
=
++=n
i
jijiijijPO RAOphasetRAOStx1
02
00 ))),((cos(),()(2),(
Page 35
Typical RAO - Surge
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Typical RAO - Sway
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Typical RAO - Heave
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Typical RAO - Roll
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Typical RAO - Pitch
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Typical RAO - Yaw
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Selection of Environmental Conditions
Facts
• Described by joint PDFs;
• Multi-directional seas;
• Huge number of combinations
and incidences.
Enormous computational
time, turning analysis
procedures cumbersome
Approach
• Selection of a certain number of
environmental combinations,
related to the particular station
keeping system (spread
mooring, Turret, DP);
• Local sea – wind correlation;
• Prelimanry analysis in
frequency domain
• Selected analysis cases in time
domain.
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Relation between Significant Wave Height and Wind Speed
Selection of Environmental Conditions
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Relation between peak period and significant wave height
Selection of Environmental Conditions
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Depth: 1200m
100 year Storm:
Hs=7.2m
Hmax=13.9m
Tp=14.2s
100 year Current
V=1,95m/s at surface level
profile according to table 4
310m
185m
20m
FPSOturret
Shell-BC10
Selection of Environmental Conditions
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Sea States
Preliminary analysis
Lazy-wave extreme conditions analysis
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Preliminary Study: 60 Cases
Case 1
Heading: 45o NV
N
EW
S
umbilical
Current
Sea
Swell
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Preliminary Study: 20 Cases
Case 2
Heading: 74 o NV
N
EW
S
umbilical
Current
Sea
Swell
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Preliminary Study: 20 Cases
Case 3
Heading: 100o NV
N
E
W
S
umbilical
Current
Sea
Swell
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Preliminary Study: 20 Cases
Case 4
Heading: 225o NV
N
EW
S
umbilical
Current
Sea
Swell
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Preliminary Study: 20 Cases
Case 5
Heading: 0 o NV
N
EW
S
umbilical
Current
Sea
Swell
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VIV
Gioria et al.
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VIV
• Fundamental findings and their impact in riser
dynamics:
– Reynolds number dependence;
– mass ratio dependence;
– effect of coupled stream and cross-wise
vibrations and bifurcations of shedding patterns;
– persistent vibration at high reduced velocities at
very low mass ratio.
• Still challenging riser dynamics:
– multi-modal (in and out-of-plane) simultaneous
excitation in sheared flow.
– curvature effects;
– stream and cross-wise sub-harmonic resonance;
– coupling of VIV with dynamics in other time-
scales;
– VSIV – VIV induced by FPU motions
– supressors and hydrodynamic loading.
Less important to
flexible pipes and
umbilical, due to high
structural damping
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Acknowledgements
TPN
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FLUID-STRUCTURE INTERACTION AND
OFFSHORE MECHANICS LABORATORY