Hydrostatic Pressure |
 
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Hydrostatic Pressure |
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The hydrostatic pressure due to an internal fluid is computed using the following equation.
(1)
where:
•P is the total hydrostatic pressure
•ρ is the mass density of the internal fluid
•g is the acceleration due to gravity
•Δh is the hydrostatic pressure head. This is equal to the difference between the elevation of the point of interest (based on nodal position), and the total elevation of the relevant internal fluid (dictated by the Level Above Mudline entry under the *INTERNAL FLUID keyword).
•PU is an (optional) user-defined constant pressure above hydrostatic. This is defined via the Internal Pressure entry under the *INTERNAL FLUID keyword.
For element sets which do not experience slug flow, the hydrostatic pressure within each element is computed independently based on Equation (1) above.
While Equation (1) relates to internal pressure, a similar expression is used to compute external pressure due to seawater. In this case, ρ is the mass density of seawater (specified under *OCEAN), and Δh is the elevation difference between the point of interest and the still water line elevation (specified under *OCEAN). PU is not relevant in this case.
Slug Flow
For an element set which experiences slug flow at some time during a simulation (i.e. where the same element set is referenced under both the *INTERNAL FLUID and *SLUGS keywords), the hydrostatic pressure within each element is derived from (i) the hydrostatic pressure at a reference point and (ii) the connectivity of the element set. Specifically:
•The first node of the first element in the set is assumed to act as a reference point for the entire set.
•The hydrostatic pressure at the reference point PRP is computed using Equation (1).
•The hydrostatic pressure at the end of the first element, P1-END, is computed using Equation 2, where ρINT is the density of the fluid contained within the element, Δh is the difference in elevation between the element end points, and the remaining symbols are as defined previously.
(2)
•If the element is fully filled with internal fluid, ρINT is the density of the internal fluid as specified in the *INTERNAL FLUID keyword. If the element is fully filled with slug material, ρINT is the density of the slug material as specified in the *SLUGS keyword. Where the element contains a mixture of fluid and slug, ρINT represents an equivalent fluid density which is computed on an integration point-by-point basis, with each integration point assumed to govern a local section of the element surrounding the integration point. Refer to Slug Flow for further information on partial filling.
•The hydrostatic pressure at the start of the second element in the set is equal to the value at the end of the first element i.e. P2-START = P1-END.
•The hydrostatic pressure at the end of the second element is computed using Equation 2, and so on.
•As the hydrostatic pressure is computed incrementally using the connectivity of the element set, Flexcom mandates that any element set which experiences slug loading must form a continuous line of elements from start to finish.
Note: In earlier versions of Flexcom (up to and including Flexcom 8.10.2), the hydrostatic pressure term was computed solely on the basis of the internal fluid definition, unaffected by presence of any slug flow. This approach was overly simplistic and in some cases could led to incorrect buoyancy forces being modelled, particularly for relatively long slugs in non-horizontal elements.
Finally, internal fluid flow causes an increase in pipe wall tension, which is modelled as a dynamic pressure term in Flexcom.