Offshore Structures
•Drilling rigs: Exploration of oil and gas Stay in a place for a few months (Mobil or movable)
- Jack-up drilling rig- MODU (Mobil Offshore Drilling Unit)
• Production platforms: Production of oil and gas Stay in a place for at least a few years (usually 20 -30 years)
- Ground-base structure ( <500~800 m)- Floating Structures (> 800 m)
Fig. 3-2Example of jack-up drilling rig
Legs are retractable
Fig. 3-3A semi-submersibleDrilling Rig
•Mooring system
•or Dynamic positioning
DP -Dynamic Positioning
Fig. 3-5
Steel Jacket Platform in 20 – 200 m waters
< 500 m
Cannot be moved
OFFSHORE PLATFORM
Fig. 3-6
Concrete Gravity Structure
OFFSHORE STRUCTURES
OFFSHORE PRODUCTION & DRILLING
• AUGER TLP • OCEAN CLIPPER
OFFSHORE PLATFORM
• SPAR • FIXED JACKETED
DRILLING RIG & SPAR
Fig. 3-10 pp56New version Fig. 12 pp61
Articulated Tower
Fig. 3-11 pp56New Version Fig.13 pp62
Single Anchor Leg MooringSystem
Wave Forces on Offshore Structures
• Morrison Equations
• Diffraction/Radiation Theory* (Potential theory, neglect water viscosity)
• CFD (Computational Fluid Dynamics)*(Navier-Stokes Equations, considering water viscosity)
Keulegan-Carpenter Number (Non-dimensional)describing the relation between an oscillatory flow and a cylinder /
Peak (amplitude of) vlocity of the flow Period Diameter
m
m
K U T DUTDK
25 Particle movement is much greater than
5 255 Particle movement is smaller than
/ 0.2 Wavelength is much greater than / 0.2 Wavelength is not much greater or smaller
DK
K DD L DD L
than D
Morrison Equations & Modified ME
12
----- Drag Coefficient, ------ Volume ---- Added Mass Coefficient----- Projected area (normal to current)
Additional term due to votex induce (lateral) force12
in d A m V
d V
m
A
l L
dd C d C ddt
C dCd
d C
nn n
uF u u
F 2
0 0
cos(2 ),
, ------- Strouhal Number
----------- Lifting Coefficient
A
L
d f t
fDS S
C
n
n
u
u
Wave Forces on A Vertical CylinderVelocity & acceleration are a function of z & t Force (function of t)
2
0 0
02
12
, , 4 2
12
+4
(3 4)
in d A m V
A z V z
in in d zh h
m zh
dd C d C ddtDd Dd d D d R
d C D d
dD C ddt
nn n
n n
n
uF u u
F F u u
u
Wave Forces on A Vertical Cylinder
Wave Forces on a Horizontal CylinderVelocity & acceleration are a function of t only
2
0
2
12
, , 4 2
12 4
------ Length of the cylinder
in d A m V
A z V z
in inh
d m
dd C d C ddtDd Dd d D d R
d
dL C D D Cdt
L
nn n
nn n
uF u u
F F
uu u
Wave Forces on a Horizontal Cylinder
Example of Problem 3-1 pp73 (old v. pp64)
Computing the horizontal load on a vertical cylinder
Drag coefficient of a cylinder pp 72 & 75 (old v. pp63 & 64)
Added-Mass coefficient of a cylinder pp 72 (old v. pp64)
Wind & Current Forces
Steady & oscillatory portions
Steady current forces
2
1 Inline force2
1 cos(2 ), Transverse force2
in d
l L
C AU U
C AU f t
F
F
Wind Forces
0.113 0.125
10
10
1 , 2
Steady wind velocity
Fluctuated velocity (wind gustiness)Applied at the center of pressure
( )10
- wind velocity at 10 m above the sea
in a d
P
pP
C AU U U U U
U
UZ
ZU Z U
U
F
Forces on Pipeline Due to Wave & Currents
sin 0 (3.33) horizontal
cos 0 (3.34) vertical (3.35) Coeff. of friction
Minimum submerged weight (remain at sea bed)
(3.36)cos sin
d i f
n l
f n
d i l
F F F w
F F wF F
F F Fw
Drag force; inertia forceNormal force; lifting forceFriction force; Submerged weight
d i
n l
f
F FF FF w
lF
nFfF
dFiF
W
Free Body diagram of A Pipe under the impact of Wave & Currents