The Wave command enables the user to model the effect of ocean waves impacting a partially submerged piping system.
The Wave command uses static analysis to model hydrodynamic effects. A given static analysis computes the response of the structure to a static load representing the force distribution at a given instant of the wave loading cycle (defined by the phase angle, q). Thus, a series of these instantaneous static analyses can be used to trace out the system response over the entire wave cycle.
Notes:
AutoPIPE does not print any calculated wave results. Internally, the wave results are converted as loads on the piping.
AutoPIPE modeling suspended piping from from vessel, platform, or buoyancy modules should be limited to vertical riser as a snaking catenary shape as seen below cannot be modeled in AutoPIPE.
In AutoPIPE buoyancy weight is applied in GR case. It has both longitudinal and transverse components. When the pipe is vertical, difference in pressure cap forces is the same as buoyancy load and the transverse buoyancy is zero. When the pipe is horizontal, the cap forces from external hydrostatic load adds longitudinal stress and associated movements in addition to non zero transverse (vertical) buoyancy load.
Waves forces are applied at bend nodes (near, far and if present a mid point).
Wave loading beyond water depth:
When a user defined current profile (depth - 0 to 115 ft , for example) does not cover the entire range of submerged pipe (Water elevation to Depth: 0 to 270 ft ) AutoPIPE extrapolates the current velocity as explained below.
In the above instance, for depths between 115-270 ft, index is set to 5 and the following equations are used
V = VCUR_index+DV
Where,
- DV = (DCUR_index-DFP) ((VCUR_(index-1)-VCUR_index )) / ((DCUR_index-DCUR_(index-1)) )…..(i.e. difference in depth x slope)
- DCUR= current depth from table (DCUR4 = 75 ft, for example)
- VCUR = current velocity from table (VCUR5 = 1500 ft/s, for example)
- DFP = depth of the point below 115 ft
- Index = index of the table.
When DFP > DCUR5, AutoPIPE sets index = 5
This set of equations creates a linearly varying velocity profile for depths below 115 ft:
If decreasing velocity was defined between index 4 & 5 i.e. depth 75 & 115 ft, then this profile would've been linearly decreasing below 115 ft. Note that it may not necessarily reach 0 at depth of ocean bed. The affects wave force calculation has not yet been investigated to see any affects.
- Checked the Stream Function Wave water velocities at the following net link:: http://www.coastal.udel.edu/faculty/rad/streamless.html
- Suggest performing an complete review of the wave by adding multiple wave loading with same data but assigning different phase angles U1-U5 = 0,45,90,135,180.
- The forces on the pipe are the sum of drag (velocity) and Inertia(acceleration). The chart below is a manually calculated with these values using Airy Wave.
Note: The maximum force due to drag occurred at 0 deg phase angle, while maximum force due to inertia occurred around 90+/- deg phase angle. For this wave, with the given parameters, total max force on the pipe, sum both curves, occurred at approximately 90+/- deg phase angle.
- Hydrodynamic factors are only applied to physical points in the model. They are not interpolated for soil and mass points. Only imposed displacements are interpolated and applied to mass points.
Questions and Answers:
- Add one drag and inertia coefficient per wave
- Number of nodes needed on the pipeline to simulate the hydrodynamic forces same as defining the CD and CM in the wave load interface
- Another way of define current lift coefficient other than using "Hydrodynamic data"
- Sample models for incorporating wave loading with imposed displacements
- Define location where wave phase angle = 0 starts
- Input marine growth density
- Enter the Sea surface, Sea-bed floor, and current profile per depth
- Does Buoyancy affect piping above water elevations an below Wave Crest elevation?
- If an insulation thickness is provided for a member, does AutoPIPE adjust the effective diameter for wind/wave force calculations?
Answer: Yes.