DESIGN CRITERIA
Oleh:
Ir. Murdjito, MSc.Eng
Dosen Jurusan Teknik Kelautan
Fakultas Teknologi Kelautan
Institut Teknologi Sepuluh Nopember (ITS)
Surabaya
DESIGN LOADS & CONDITIONSReferences
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API-RP2A, "Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms-Working Stress Design",
American Petroleum Institute, Washington, D.C., 21st ed., 2000.
DET NORSKE VERITAS, Offshore standard: structural design of
offshore units (wsd method), APRIL 2002, DNV-OS-C201
BS6235, "Code of Practice for Fixed Offshore Structures", British
Standards Institution, London, 1982.
DOE-OG, "Offshore Installation: Guidance on Design and Construction",
U.K., Dept. of Energy, London 1985.
Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design and
Hydromechanics", Springer, London 1992.
Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing
Co., Houston, 1981.
Graff, W.J., "Introduction to Offshore Structures", Gulf Publishing Co.,
Houston, 1981.
DESIGN LOADS & CONDITIONS
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Dead Loads:
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Functional loads:
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Weight of the platform structure in air incl:weight of piles, grout, &
ballast
Weight of appurtenant structures permanently mounted on the platform
Hydrostatic forces acting on the structure below the water line incl:
external pressure & bouyancy
Operating Loads: Fluid, contents in piping and equipment
Live Loads: the loads imposed during its use and may change during a
mode of operation: static or dynamic functional loads arising form personnel,
helicopter, maintenance loads, etc.
Environmental Loads: arise from the action of wave, currents and
winds on the structure
Seismic loads: arise as result of the ground motion
Accidental Loads: arise as result of accident or abuse or exceptional
conditions: boat impact, dropped objects, etc
Consctructions Loads: resulting from fabrication, load out,
transportation & installation
Dynamic Loads: loads imposed due to response to an excitation of a
cyclic nature as wave, wind, earthquake, etc.
Design loads
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Loads criteria
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Permanent (dead) loads.
Operating (live) loads.
Environmental loads including earthquakes.
Construction - installation loads.
Accidental loads.
Environment criteria :
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US and Norwegian regulations:100 years
British rules : 50 years or greater
LOADING CONDITIONS
The environmental conditions combined with appropriate
dead and live loads
z Operational (Normal) Condition:
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Storm Condition
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100-year return period environmental loads
Allowable stresses: increased by 1/3
Seismic Condition
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1-year return period environmental loads
Allowable stresses max 1.0
Consider the effects of all gravity loads in combinations with
simulatanous and collinear of loads due to ground motion
Allowable stresses: increased by 70%
Accidental Loads
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Consider the effects of collision loads and due to dropped
objects
Allowable stresses: increased by 1/3
For local design of elements, a dynamic load factor of 2.0 shall
be used
DESIGN CODES
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API RP 2A WSD OR LRFD
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AISC
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Manual of Steel Construction, Allowable Stress
Design
AWS D1.1
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Recommended Practice Planning, designing and
Constructing of Fixed Offshore Platform
Structural Welding Code
API RP 2L
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Recommended Practice Planning, Designing and
Construction Heliport for Fixed Offshore Platform
Wind Loads
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on the portion of a platform above the
water level
z The wind velocity profile (API-RP2A )
Vh/VH = (h/H)1/n
1/n=1/13 to 1/7,
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depending on the sea state, the distance from land
and the averaging time interval.
approximately = 1/13 for gusts and 1/8 for
sustained winds in the open ocean.
Wind loads
= (1/2) ρ V2 Cs A
z ρ : the wind density (ρ ~ 1.225 Kg/m3)
z Cs : the shape coefficient
z Fw
Cs = 1,5 for beams and sides of buildings,
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= 0,5 for cylindrical sections
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= 1,0 for total projected area of platform.
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z Shielding
and solidity effects can be
accounted for
Wind loads
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combination with wave loads:
DNV and DOE-OG rules recommend the most unfavorable of the
following two loadings:
z 1-minute sustained wind speeds combined with extreme waves.
z 3-second gusts.
API-RP2A distinguishes between global and local wind load effects.
z first case: it gives guideline values of mean 1-hour average wind
speeds to be combined with extreme waves and current.
z second case: it gives values of extreme wind speeds to be used
without regard to waves.
Wind loads are generally taken as static. When the ratio of height to the
least horizontal dimension of the wind exposed object (or structure) > 5,
then this object (or structure) could be wind sensitive.
API-RP2A requires the dynamic effects of the wind to be taken into
account in this case and the flow induced cyclic wind loads due to vortex
shedding must be investigated.
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DESIGN LOADS – WAVE & CURRENT LOADS
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Two different analysis concepts are used:
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Design/ regular wave concept:
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a regular wave of given height and period is defined and the forces due to this
wave are calculated using a high-order wave theory.
Usually the 100-year wave is chosen.
No dynamic behavior of the structure is considered. This static analysis is
appropriate when the dominant wave periods are well above the period of the
structure.
This is the case of extreme storm waves acting on shallow water structures.
Statistical analysis:
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on the basis of a wave scatter diagram for the location of the structure.
Appropriate wave spectra are defined to perform the analysis in the frequency
domain and to generate random waves, if dynamic analyses for extreme
wave loadings are required for deepwater structures.
With statistical methods, the most probable maximum force during the lifetime
of the structure is calculated using linear wave theory.
The statistical approach has to be chosen to analyze the fatigue strength and
the dynamic behavior of the structure.
Wave Theories
•linear Airy theory,
•Stokes fifth-order theory
•solitary wave theory,
•cnoidal theory,
•Dean's stream function theory
•numerical theory by Chappelear.
WAVE THEORY
Wave Pattern
Wave Statistics
Wave Spectrum
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S (f,σ ) = S(f).D (f,σ )
z S(f): wave energy density spectrum
z D(f,σ): directional spreading function
z σ : the angle of the wave approach
direction
DESIGN LOADS – WAVE & CURRENT LOADS
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Represented by their static equivalent using
Morisson’s equation
For deep water: requires a load analysis involving the
dynamic action of the structure
For global structure: ignored lift forces, slam forces,
and axial Froude-Krylov forces
If D/L >0.2, use diffraction theory
Total base shear and overturning moment are
calculated for global structure forces
Local member stresses: due to local hydrodynamic
forces (incl. slam, lift, Froude-Krylov, buoyancy) and
loads transferred due to global fluid-dynamic force and
dynamic response of the structure
CD ≈0,6 to 1,2 and CM ≈ 1,3 to 2,0.
PROCEDURE FOR CALCULATION OF WAVE PLUS
CURRENT FORCES
WAVE DIRECTION
1
A
B
2
WAVE PARAMETER
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Wave Kinematic factor:
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Consider wave directional spreading or irregularity in wave
profile shape
Tropical storm: 0.85 – 0.95
Extra tropical storm: 0.95 – 1.0
Current Blockage Factor:
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Reducing current speed due to the presence of the structure
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Marine Growth: Increased in cross sectional area
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Drag and Inertia Coefficient, depend on:
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Reynold Number
K-C number
Roughness
Current/Wave velcity
Member Orientation
: R = Um D/ν
: K = 2 Um T2/D
: e = k/D
: r = V1/Vmo
CD, CM vs Re
CD, CM vs KC
CURRENT BLOCKAGE FACTOR
# of
legs
3
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6
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Heading
factor
All
0.90
End-on
0.80
Diagonal
0.85
Broadside
0.80
End-on
0.75
Diagonal
0.85
Broadside
0.80
End-on
0.70
Diagonal
0.85
Broadside
0.80
CONDUCTOR SHIELDING FACTOR
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upon the configuation of the
structure and the number of conductor
z To be applied to the drag and inertia
coefficient for conductor array
z Appropriate for:
Steady current with negligible waves
z Extreme waves with Umo Tapp/S > 5π
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DIAGRAM CONDUCTOR SHIELDING
FACTOR
Wave lift and slamming Loads
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addition to the forces given by
Morison's equation, the lift forces FD and
the slamming forces FS, typically
neglected in global response
computations, can be important for local
member design.
FL = (1/2) ρ CL Dv2
z FS = (1/2) ρ Cs Dv2
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z CL
≈1,3 CD.
z Cs ≈ π Æ For tubular members
Earthquakes
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Two levels of earthquake intensity:
z strength level (SLE)
z ductility level (DLE).
SLE: reasonable likelihood of not being exceeded during the
platform's life (mean recurrence interval ~ 200 - 500 years), the
structure is designed to respond elastically.
DLE: maximum credible earthquake at the site, the structure is
designed for inelastic response and to have adequate reserve
strength to avoid collapse.
API-RP2A recommends: X, Y, 0.5 Z
DNV rules: 0,7X, O,7 Y and 0,5 Z
The value of a max and often the spectral shapes are determined
by site specific seismological studies.
Ground acceleration
Design Spectra
Marine Growth
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Marine growth is accumulated on submerged
members.
Its main effect is to increase the wave forces on the
members by increasing not only exposed areas and
volumes, but also the drag coefficient due to higher
surface roughness.
It increases the unit mass of the member, resulting in
higher gravity loads and in lower member frequencies.
Depending upon geographic location, the thickness of
marine growth can reach 0,3m or more.
It is accounted for in design through appropriate
increases in the diameters and masses of the
submerged members.
Tides
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Tides affect the wave and current loads indirectly, i.e. through the
variation of the level of the sea surface.
The tides are classified as: (a) astronomical tides - caused essentially
from the gravitational pull of the moon and the sun and (b) storm surges caused by the combined action of wind and barometric pressure
differentials during a storm.
The combined effect of the two types of tide is called the storm tide.
The astronomical tide range depends on the geographic location and the
phase of the moon.
Storm surges depend upon the return period considered and their range
is on the order of 1,0 to 3,0m.
When designing a platform, extreme storm waves are superimposed on
the still water level
while for design considerations such as levels for boat landing places,
barge fenders, upper limits of marine growth, etc., the dailyvariations of
the astronomical tide are used.
Tides
Seafloor Movements
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Scour:
z Removal of seafloor soil caused by currents and
waves
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for foundations
z Design condition scour depth ~ 3 ft
Settlements
z Ground motion due to overstressing of foundation
elements
Subsidence
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Ground motion due to failure of seafloor slope