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DNV Recommended Practice on
Modelling and Analysis of Marine
Operations
Underwater Technology Conference 2008
Bergen 4-5 June 2008
Arne Nestegård & Tormod Bøe
Det Norske Veritas
New DNV Recommended Practice
DNV-RP-H103
M d lli and
Modelling
dA
Analysis
l i off M
Marine
i O
Operations
ti
Supplement to DNV Rules for Planning
and Execution of Marine Operations
June 4th 2008
Slide 2
COSMAR - COSt Effective MARine Operations
„
Joint Industry Project with the
j
to:
objective
- Develop improved methods and
simulation procedures for
modelling and analysis of marine
operations.
„
Carried out by Det Norske Veritas in
cooperation with Marintek/Sintef
„
Partners:
- Implement new methods and
procedures in DNV
R
Recommended
d dP
Practice
ti (DNV(DNV
RP-H103) to become directly
accessible and useful for the
industry
industry.
-
June 4th 2008
StatoilHydro
Shell Technology
Petrobras
Acergy
gy
Technip
Slide 3
Background
„
Marine operations are involved at all major stages of an
offshore field with significant costs
- Transportation,
p
installation, maintenance, repair
p interventions,
decommisioning
„
Advanced, accurate and reliable numerical methods to
establish operational criteria will contribute to more optimized
and
d costt effective
ff ti marine
i operations
ti
- Prolonged profitable tail-production, increased exploitation of
petroleum resources
June 4th 2008
Slide 4
Background (cont)
„
Advanced modelling of physical effects are available, but the
use of such models in numerical simulations of marine
operations is not well established
established.
June 4th 2008
Slide 5
DNV RP-H103: Modelling and analysis of marine operations
CONTENTS
1 Introduction
2 General methods of analysis of marine operations
3 Lifting through wave zone - general
4 Lifting through wave zone – simplified methods
5 Deepwater lowering operations
6 Landing on seabed and retrieval
7 Towing operations
8 Weather criteria and availability analysis
June 4th 2008
Slide 6
General analysis methods for marine operations
„
Frequency domain analyses – use of RAOs
-
„
Time domain analyses
-
„
Available methods and tools. Accuracy of local flow description. Accuracy of
global load predictions
Statistics and extremes of marine operations
-
„
Different formulations. Equations of motion. Use and limitations. Multi-body
Multi body
simulations. Impact loads. Snap loads.
Computational Fluid Dynamics
-
„
Use and limitations/disadvantages
limitations/disadvantages, use of design waves
waves, irregular sea states
Assessment of extreme response under stationary / non-stationary
conditions. Statistics of snap loads and impact loads
Model tests of marine operations
-
Oscillation tests to find hydrodynamic coefficients. Wave tests. Tests of the
complete
l t set-up.
t
June 4th 2008
Slide 7
CFD Analysis of Protection Structure
Vertical force on
forward bucket
June 4th 2008
Slide 8
Lifting through wave zone - general
„
Hydrodynamic Loads and Load
Effects
- Quadratic and linear damping.
Dependence on KC-number. Inertia
forces due to moving object. Wave
excitation
it ti fforces.
„
Hydrodynamic coefficients and
empirical
p
data
- Added mass on typical structures
(subsea modules). Added mass and
damping for ventilated structures.
Drag coefficients.
„
Moonpool Operations
- Water kinematics.
kinematics Blocking effects on drag and added mass.
mass
June 4th 2008
Slide 9
Lifting through wave zone – simplified methods
The objective of the Simplified Method is to
give simple conservative estimates of the
forces acting on the object in order to verify
sufficient crane and rigging capacity.
Main assumptions:
„
the horizontal extent of the lifted object
j
is
small compared to the wave length
„
vertical motion of object
j
and water dominates
→ other motions can be disregarded
„
the vertical motion of the object is equal the
vertical crane tip motion
June 4th 2008
Slide 10
Simplified Method – Examples
Wave kinematics
The wave amplitude, wave particle
velocity and acceleration can be taken as:
„
„
„
ζ a = 0 .9 ⋅ H S
⎛ 2π
v w = ζ a ⋅ ⎜⎜
⎝ Tz
⎛ 2π
a w = ζ a ⋅ ⎜⎜
⎝ Tz
⎞
⎟⋅e
⎟
⎠
⎞
⎟
⎟
⎠
2
4π 2 d
−
T z2 g
−
⋅e
4π 2 d
T z2 g
June 4th 2008
Slide 11
Simplified Method – Hydrodynamic Forces
Slamming impact force
Slamming
g forces are short-term impulse
p
forces that acts when the structure hits the
water surface.
AS is the relevant slamming area on the
exposed structure part. Cs is slamming coeff.
The slamming velocity, vs :
v s = v c + v ct2 + v w2
„ vc = lowering speed
„ vct = vertical crane tip velocity
„ vw = vertical water particle velocity
at water surface
Fρ = ρ ⋅ δV ⋅ g
Varying
y g buoyancy
y
y force
Varying buoyancy, Fρ , is the change in
~
buoyancy due to the water surface elevation.
δV = Aw ⋅ ζ a 2 + ηct2
δV is the change in volume of displaced
water from still water surface to wave
crest or wave trough.
„ ζa = wave amplitude
„ ηct = crane tip motion
Fρ = ρ ⋅ δV ⋅ g
June 4th 2008
amplitude
„ Ãw = mean water line area in the
wave surface zone
Slide 12
Simplified Method – Hydrodynamic Forces
Drag force
Drag
g forces are flow resistance on submerged
g
part of the structure. The drag forces are
related to relative velocity between object and
water particles.
v r = vc +
„ vc = lowering/hoisting
l
i /h i ti speed
d
„ vct = vertical crane tip velocity
„ vw = vertical water particle velocity
The drag coefficient, CD, in oscillatory flow for
complex subsea structures may typically be
CD ≥ 2.5.
at water depth , d
„ Ap = horizontal projected area
R l ti velocity
Relative
l it are ffound
db
by :
Mass force
FM =
“Mass force” defined as a combination of
inertia force, Froude-Kriloff force and
diffraction force.
Crane tip acceleration and water particle
acceleration are assumed statistically
independent.
p
June 4th 2008
vct2 + v w2
[(M + A33)⋅ act]2 + [(ρV + A33)⋅ aw]2
M = mass of object in air
A33 = heave added mass of object
j
act = vertical crane tip acceleration
V = volume of displaced water relative to
the still water level
„ aw = vertical water particle acceleration
at water depth, d
„
„
„
„
Slide 13
Simplified Method – Hydrodynamic Force
The hydrodynamic force is a time dependent function of slamming impact
force, varying buoyancy, hydrodynamic mass forces and drag forces. In the
Simplified Method the forces may be combined as follows:
Fhyd = ( FD + Fslam )2 + ( FM − Fρ ) 2
„ The
structure may be divided
into main items and surfaces
contributing to the hydrodynamic
force
„ Mass
and drag forces
contributions are then
summarized :
FM =
∑F
Mi
i
FD =
∑F
Di
i
FMi and FDi are the individual
force contributions from each
main item
June 4th 2008
Slide 14
Simplified Method – Slack Slings
The Slack Sling Criterion.
„
Snap forces shall as far as possible
be avoided.
„
The following criterion should be
fulfilled in order to ensure that snap
loads are avoided:
Fhyd ≤ 0.9 ⋅ Fstatic − min
„ Fstatic-min
= weight
g before flooding,
g
including a weight reduction implied
by the weight inaccuracy factor.
June 4th 2008
Slide 15
Deep water lowering operations
„
Horizontal offset due to current
z
x
„
Dynamics of lifted object –
eigenperiods and vertical
resonance
U
ξ(z)
q
w
FD0
η
„
ξL
Modelling
M
d lli off h
heave
compensation
W0
L
m , EA
1
M + A33 + mL
3
T0 = 2π
EA / L
EA/
M
June 4th 2008
Slide 16
Landing on seabed and retrieval
„
Landing on seabed
-
Landing impact problem definition
Physical parameters
Numerical analysis procedure
Application to modules with and
without skirts
„
Skirt penetration resistance
„
Installation by suction
„
Levelling by application of suction
or overpressure
„
Retrieval of foundations
June 4th 2008
Slide 17
Towing operations
Submerged tow of objects
attached to vessel
Submerged tow of objects
attached to towed buoy
June 4th 2008
Slide 18
Submerged tow of long slender elements
towing direction
Z
wave direction
X
1200m
T wire
Top
i
Top assembly,
Bottom assembly,
net buoyancy 50 t
net buoyancy 30 t
bottom
wire
Top chain
Bottom chain
June 4th 2008
Slide 19
Surface tow
Maximum
M
i
towline
li tension
i
as function of seastate
7000
Axial Tension
n [kN]
6000
5000
L1
4000
L2
3000
L3
2000
L 1 < L2 < L3
1000
0
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
Hs [m]
June 4th 2008
Slide 20
Weather criteria and availability analysis
„
Environmental parameters relevant for marine operations
- Primary characteristics
- Weather routing
„
Accuracy
y of environmental data
- Instrumental data
- Numerically generated data
- Climatic uncertainty
„
Weather forecasting
- Weather restricted operations / uncertainties of weather forecasts
„
Persistence statistics
„
M it i off weather
Monitoring
th conditions
diti
and
d responses
June 4th 2008
Slide 21
Summary
„
A new Recommended Practice; ”DNV-RP-H103
”DNV RP H103 Modelling
and Analysis of Marine Operations” is scheduled for October
2008.
„
The new RP gives guidance on methods and simulation
procedures for modelling and analysis of marine operations.
June 4th 2008
Slide 22
June 4th 2008
Slide 23
June 4th 2008
Slide 24