2.019 Design of Ocean Systems
Lecture 2
February 11, 2011
Typical Offshore Structures
Platforms:
– Fixed platform
– FPSO
– SPAR
– Semi-Submersible
– TLP (Tension Leg
Platform)
Key Components:
– Platform on
surface (with
production facility,
gas/oil storage)
– Risers
– Mooring
– Subsea system
– Pipelines or
transport tankers
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Semi‐Submersible Platform
Photo courtesy of Erik Christensen, CC-BY.
Tension‐Leg Platform (TLP)
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Gravity‐Based Platform
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FPSO (Floating, Production, Storage, Offloading) ƒ Movable
ƒ Versatile for use in smaller reservoirs
ƒ Relatively cheap in cost
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– Topside
– Hull
– Mooring/riser system
Courtesy of Linde AG. Used with permission.
Design of FPSO vs. Tankers/Ships Tankers/ships:
FPSO:
ƒStorage
ƒStability
ƒSpeed
ƒMotion (seakeeping)
ƒCost
ƒStorage
ƒStability
ƒNo Speed requirement
ƒMotion (seakeeping)
– Limited riser top-end displacement
– Production
– Offloading
– Equipment installation
ƒFreeboard
– Green water
– Bow impact
ƒMooring/risers
– Drift motion
– slowly-varying motion
ƒReliability
Basic Design Requirements of FPSO
ƒ Requirement for a turret mooring to allow the FPSO to weathervane and
minimize environmental loads on the mooring system
ƒ Selection of suitable hull size and form with good motion characteristics
ƒ Freeboard
ƒ Production facilities design to minimize motion downtime
ƒ Hull size to provide adequate buffer storage to minimize shuttle tanker
offloading downtime
ƒ Hull structure design (strength and fatigue)
ƒ Human safety
Hull Size and Form
Major requirements:
ƒ Crude oil storage
ƒ Deck space
ƒ Sea-Keeping performance
ƒ Availability of tanker hulls for conversion
Typically,
ƒ Weight of lightship (i.e. steel): 13-16% of displacement
ƒ Weight of topside: 7-8% of displacement
ƒ Cargo deadweight (i.e. oil storage): ~ 75% of displacement
ƒ Weight of risers/moorings: depending on type and number of riser/mooring lines
In general,
ƒ Hull steel weight: Length/depth ratio of hull and hull size
ƒ Deck space: Length X breadth
ƒ Mooring loads: Breadth/length
ƒ Stability: Breadth/depth
ƒ Sea-keeping (primarily pitch): Length
ƒ Green water on deck: Freeboard
ƒ Bottom slamming forward: Ballast capacity
ƒ Bow wave impact: Bow fineness
Structure Configuration
Double sides /bottom:
ƒMinimize risk of oil spilling due to collision
ƒSingle or double bottoms are OK for FPSO
ƒ2m width of wing tank for trading tankers
Bulkhead spacing:
ƒMinimize free surface effects due to partial-filled tanks
ƒLimits on maximum tank size to keep oil spillage below a certain volume
in the event of tank rupture
ƒStructure support of production/utility system
ƒMore tanks to allow safe inspection and maintenance while not
interrupting operation
Typically, the cargo tank region is subdivided by a centre-line bulked and
four or more transverse bulkheads.
Ballast System
The capacity and location of ballast tanks should be designed
to ensure that with crude oil storage tanks empty or part-fill, the
FPSO draft and trim meets requirements on:
ƒ stability
ƒ maximum trim for production equipment operation
ƒ minimum draft forward to prevent bottom slamming.
Turret Mooring/Riser System
Internal Turret:
ƒSmaller riser head displacement
due to pitch motion
ƒEasy for maintenance
ƒLarge deck space
ƒLonger ship length
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External Turret:
ƒNo effect on ship hull length and
deck area
ƒInconvenient for maintenance
ƒLarger riser head displacement
due to pitch motion
Courtesy of Linde AG. Used with permission.
Estimate of Heave and Pitch Natural Periods
For a barge, length L, width B, draft T
Heave:
Tnh = 2π
s
s
m + ma
CB T
= 2π
(1 + 0.4B/T )
K
Cw g
CB = ∇/(LBT ): block coefficient; Cw = S/(LB): waterplane coefficient
Pitch:
Tnp = 2π
q
2)
m(R2 +Ra
ρg ∇ H
≈ 2π
r
12T
L2
R2 + B
4
g
H: metacentric height in longitudinal direction
R: radius of gyration
Ra : radius of gyration of pitch added mass
Stability Requirements
Intact stability
Damage stability
Hull Structure and Deflection
Shear requirement
Sagging/hogging moment requirements
Deflection requirement
Hydrostatic Pressure and Forces
z x
Wetted Surface S
dS
Normal Vector n
Image by MIT OpenCourseWare.
Fz
Hydrostatic Pressure: p=pa − ρgz
RR
Force: F~ = − S ρgz~ndS
RR
~=−
ρgz(~x × ~n)dS
Moment: M
S
dV olume
Z Z Z
ydV olume
=
ρg
=
ρg × submerged volume
Fx
=
Fy
=
0
Mx
=
ρg
My
Z Z Z
=
0
ρg
Z Z ZV
V
Mz
=
0
xdV olume
Hydrostatic Stability — Fully Submerged Body
Waterline
Force: ρg Volume
B
Center of buoyancy
G
Weight
Image by MIT OpenCourseWare.
Moment Equilibrium: G and B are in a vertical line
Restoring moment Mr = −ρg∇ × GB sin θ under a rotation θ
Stable: G is under B (i.e. GB > 0)
Unstable: G is above B (i.e. GB < 0 )
Hydrostatic Restoring Moment — Surface Piercing Body
No change in
submerged volume
Total restoring moments:
Upward force from
submerged wedge
Mrx
=
=
B
G
=
Downward force
from emerged wedge
(−ρgIt − GBρg∇)θx
It
−ρg∇( + GB)θx
∇
−ρg∇ × GM × θx
Image by MIT OpenCourseWare.
For equal displacement volume, the waterline
contribution to restoring moments:
Mrx = −ρgIt θx
Mry = −ρgIl θy
(heel)
GM =
(trim)
Moments of inertia of waterplane area:
It =
RR
Aw
y 2 dA ,
Center
R R of
flotation (COF):
Aw
xdA = 0 ,
Il =
RR
RR
Aw
Aw
Metacentric height for transverse
rotation:
x2 dA
ydA = 0
It
∇
+ GB
• Stable if GM > 0
• Compared to the submerged
bodies, the presence of free
surface increases the restoring
moment.
Righting Arm
Heel angle
Metacentre
Centre of gravity
Centre of
buoyancy
before heel
Righting arm GZ:
GZ = −
GZ: righting arm
Centre of
buoyancy
after heel
Image by MIT OpenCourseWare.
righting moment
= GM × sin θx ≈ GM θx
displacement
GZ Curve
Righting arm (m)
GM at 1 Radian Heel
2.0
1.0
0.0
20
40
60
80
Heel angle (deg)
Image by MIT OpenCourseWare.
A floating structure is STABLE for heel and trim respectively when:
GMt > 0(heel)
GMl > 0(trim)
Wind Overturning Moment
P
Image by MIT OpenCourseWare.
Fwind =
1
2
ρ
C
C
V
A
air
s
H
10
2
Mwind =
Fwind × h
Cs: shape coefficient, Cs=1.0 for large flat surface
CH: height coefficient, CH=1.0 for FPSO
V10: wind velocity at 10m above sea surface
A: Project area of the exposed surfaces in the vertical or the heeled condition
h: vertical distance between center of wind force and center of resistance (by
mooring lines, etc)
Evaluation of Large Angle Hydrostatic Stability
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Wind heeling arm = (wind moment) / displacement
Safety Criterion:
Mmax: maximum righting arm
Mw: wind moment
Mmax ≥ Mw
Dynamic Stability Criterion:
Area B
Righting arm
Downflooding angle
Area A
Second
intercept
Heel angle
Image by MIT OpenCourseWare.
Erighting−moment ≥ 1.3(or1.4)Ewind
Eright−moment = Area B
Ewind = Area A Hydrostatic Stability Analysis of Ships and FPSOs
Intact stability analysis:
Ballasted
Righting arm
• Compute GZ curves for various loading conditions
• Compute heeling moments by wind, etc
• Compare righting lever, GZ, to heeling lever
6
4
Loaded
2
Slack
Damaged stability analysis:
• Both wind loading and a degree of flooding are assumed • Flooding due to waterline damage and/or flooding due to burst ballast
• Evaluate the stability as in intact stability analysis
• Typical rule requirements: 50 knot wind and side or bottom damage
0
0
20
40
60
80
Heel angle
Image by MIT OpenCourseWare.
GZ curve for a FPSO
ABS MODU (Mobile Offshore Drilling Unit) Stability Regulations
Intact Stability:
• Design wind speed: 100 knots
• The angle of inclination should be no greater than 25 degree
• Dynamic stability criterion:
Erighting−moment ≥ 1.3(or1.4)Ewind
Damage Stability:
• Design wind speed: 50 knots
• The angle of inclination should be no greater than 25 degree
• Horizontal penetration should be at least 1.5m • Longitudinal damage extent should be (1/3) L2/3 or 14.5m, whichever is less
• Damaged compartments are completely filled
• Downflooding angle must be larger than the first intersection of the righting moment and heeling moment Minimum extent of watertight
integrity provided.
Righting moment (>
_ Overturning moment, θD >
_ θ1)
First Intercept
Moment
Angle of
downflooding
Overturning moment
θ2
θ1
θD
Inclination about
the critical axis
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2.019 Design of Ocean Systems
Spring 2011
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