reducing the response of fixed offshore jacket platform using a damper

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2.  Fixed type offshore structures are very stiff( rigid) in
nature .
 It has tendency to attract more forces
 But the forces are cyclic in nature with high magnitude.
 Response to wave loads is relatively less but if at all
damage caused it will be very high.
 Expensive structural system
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3.  Safety, serviceability and fatigue life of fixed offshore
platforms.
 complex failure modes considered in addition to usual
design requirements.
 special design considerations that are costly and time
consuming to implement.
 Hydrodynamic Buoyant Mass Damper (HBMD) is
introduced as an alternative
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4.  employs both buoyancy and added mass in conjunction
with the inertia force of its vibrating mass
 consists of a fully submerged
vertical short closed cylinder.
 motion is restricted by a set
of springs that attach it to the
platform.
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6.  Linear wave theory (Airy’s theory)
 Stokes fifth order nonlinear wave theory
 Find velocity and acceleration fields given by Skjelbreia
 Morison equation to find wave force is,
 Added mass and drag coefficients are measured from Reynolds
number and Kulegan Carpenter number.
 In this paper Ca = 1.6 and Cd =0.65 based on API
recommendations
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F(t) = ρw V ü + Ca ρw V ( v̈ - ü ) + 1/2 Cd ρw A ( v̇ -u̇ ) 1 v̇-u̇ 1

7.  Nonlinearities
- Material
- Geometrical
- Hydrodynamic damping
 Newton Rapson fixed load step method
 Linear acceleration method
 Rayleigh’s damping method
assume damping ratio r = r = 3%
[C]= [M]+ [K]
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𝑚∆ 𝑢𝑖 + 𝑐∆ 𝑢𝑖 + 𝑘𝑖∆𝑢𝑖 = ∆𝑃𝑖

8.  OJSAP (Offshore Jacket Structural Analysis Program )
 It is based on
stokes 5th order wave theory
non linear dynamic equilibrium
load increment method
 a)Modelling of jacket
- four legged type platform
- depth of water,(d) = 120 m
- density of material,  = 7849 kg/m3
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9. - young’s modulus, E = 2*1011 n/m2
- origin is fixed at centre of platform
and at SWL
- fixed to sea bed using pile strubs
- Nodal masses each of 200 tonnes
at +11m on top of each leg.
- Remove bracings
- Leg Do = 1.5 m ; t = 2 cm
- Bracings Do = 1.0 m ; t = 2 cm
- modelled as two node beam element9/30/2017 9

10.  b) modelling of HBMD :
- rigid closed cylinder fixed at -63.5 m
- outer diameter, D = 10 m
- thickness, t = 50 cm
- connected with shear releases in wave direction and springs
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11.  For different hydro dynamic conditions and for different spring
stiffness values.
 Each model defined by a-b-c
 Example: model 1-5-1
1 shows wave parameters model 1
5-1 shows spring stiffness ks = 5*100+1*20 = 520 kN/m
Model no Wave height (m) Wave period (sec)
1 4 6
2 6 7
3 8 8.5
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12.  Displacement of deck in only x- direction is considered for response
study
 Reduction in structural response is more influenced by
HBMD’s buoyancy force,
HBMD’s mass damper.
 To examine the performance of HBMD and arrive at optimal stiffness
values for the spring connectors,
i) Maximum displacement criterion (R1)
ii) Total displacement criterion (R2)
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16.  COMSOL metaphysics software
 HBMD modelled as a moving boundary with its velocity extracted
from analysis.
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18.  Model 3-2-2 is analysed again by considering hydrodynamic
effects of HBMD
 New R1 and R2 factors are 0.577 and 0.409
 Indicating that eddies formations can be beneficial to HBMD’s
performance
 Shape and exact position of HBMD are important factors.
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19.  A jacket type fixed platform is analysed for three types of wave
loading along with HBMD.
 HBMD reduced the response to wave loading by doveloping
reversal forces
 Buoyancy force and inertia force are the main reversal forces
 Eddy formation in the proximity of hbmd can reduce the response
 Suggesting wave harvesting devices instead of springs in
controlling reposnse of platform
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20.  M. Moharrami, M. Tootkaboni, 2014 Reducing response of offshore
platforms to wave loads using hydrodynamic buoyant mass dampers,
University of Massachusetts Dartmouth, North Dartmouth, MA
02747-2300, USA.
 Chakrabarti SK. 1987 Hydrodynamics of offshore structures,
Computational mechanics.
 Clough RW, Penzien J. 1975 Dynamics of structures, McGraw-Hill.
 API RP 2A, 2000. Recommended practice for planning, designing
and constructing fixed offshore platforms. Washington: API
Publishing Services.
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