| Product(s): | SACS |
| Version(s): | ALL |
| Area: | Seastate |
Community Question:
Hello Bentley, In one load condition, the wind loads are acting on various (vertical) areas of an offshore platform. Those areas can be found at various heights, which means that various wind pressures are to be used in the load condition, However, SACS only seems to accept ONE wind pressure per load condition. Is there an option to use VARIOUS wind pressures in one load condition? I need to include something like this
Answer:
One of the features of SACS is the easy way to input wind forces using the module SEASTATE, In SACS we only have to specify the wind velocity with the reference height of 10 meters (this is specified by API), this data is usually provided on the design basis or an environmental study. Using this parameter SACS calculates the wind press in each element depending on his position.
For now, SACS is able to calculate only the wind pressure for tubular members, since the projection area of these doesn't change independently if this has a rotation angle, but also is possible to add areas in which SACS calculates the adequate load according to the center of geometry set in the area.
Using only the wind velocity, reference height (10 meters by default), and Gust time period, SACS can calculate with precision the pressure and wind forces in the elevation in which the elements or areas are.
Example:
For this little example, we have added two projection areas in X direction with the same area but with different height
Area 1 A = 800ft^2 center of application x=-24ft y= 0ft and z=75.2ft
Area 2A= 800ft^2 center of application x=-24ft y= 0ft and z=31.5ft
Wind Parameters
Wind Velocity = 14.4 m/s (47.24 ft/s)
Reference Hight = default 10 meters (32.81 ft)
Period = 1 hour (3600 sec)
SACS Results
Below is shown the result in SACS
As can you see, SACS has calculated a force of 4.25 Kips, now, below will be shown the hand calculation to corroborate this force
Hand Calculation:
Wind Velocity/Pressure Calculation (as per API RP 2A 21st Edition)
The wind velocity at a particular elevation as per API RP 2A 21st ED., Section 2.3.2.b
INPUT
Wind Velocity For 1-Hour U0 = 14.4 m/s (47.24 ft//s)
Reference Elevation zR = 10 meter (32.81 ft/s)
Average Time Period t0 = 3600 sec
Gust Time Period t = 60 sec
Wind Pressure using 1 hour of reference
Wind velocity at a given elevation over one hour,
U(z) = U0 [ 1 + C * ln ( z / 10 ) ]
where
C = 0.0573 * sqrt ( 1 + 0.15 * U0 )
C = 0.10186
U(z) = 14.40 m/s
Wind pressure,
pz = ( w / 2g ) * ( Ut(z) )2
where, w = 0.01188 kN/m3
Hence, pz = 0.1256 kN/m2
Wind Pressure Considering Gust Effect of 1 minute
Gust factor, = 1 - 0.41 Iu ( z ) * ln ( t / t0 )
where
Iu(z) = turbulence intensity
Iu(z) = 0.06 x [ 1 + 0.043 x U0 ] ( z / 10 ) - 0.22
Iu(z) = 0.0972
Therefore, Gust factor, = 1.163
Wind velocity at a given elevation for a given gust period,
Ut(z) = U ( z ) * Gust factor
Ut(z) = 16.75 kN/m2
Wind pressure, pz = ( w / 2g ) * ( Ut(z) )2
where, w = 0.01188 kN/m3
Hence, pz = 0.1699 kN/m2
Following the previous calculations for each height, we can get the result of pressure and force on each elevation required
As can you see comparing the results, SACS gives us a force of 4.25 kips, and in our hand calculation we get 4.5 kips, with this we can corroborate that the generation of force using the Seastate module is correct.
In summary, you don't need to include different velocities or pressures, SACS only uses one velocity or pressure for reference for calculating the pressure and forces for the different positions of elements or areas
Below is presented the wind velocity and pressure profile considering a reference velocity of 14.4 and a height reference of 10 meters for each time gust recommended by API