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*Hydrodynamic Sets

Purpose

To assign hydrodynamic coefficients to element sets.

Theory

Refer to Hydrodynamic Loading for further information on this feature.

Keyword OverView

Hydrodynamic coefficients can be specified as being (i) constant throughout an analysis, or (ii) can vary as a function of Reynolds number Re, or (iii) can be defined for sets which may be contained in a moonpool. The actual coefficients are specified on two lines (for constant or moonpool values), on three or more lines (for values varying with Re), or four lines (for buoy structure properties). Blocks of lines may be repeated to allow a combination of specifications. If coefficients vary as a function of Reynolds number, an optional line may be included at the beginning of the keyword to indicate that the hydrodynamic forces are to be computed as a function of the instantaneous Reynolds number.

Morison's Equation Computation

Purpose

To specify whether element drag (used by default) or buoyancy diameters should be used during the computation of the added mass and inertia terms in Morison’s Equation.

Keyword Format

An additional DIAMETER= line at the beginning of the keyword.

[DIAMETER=Diameter]

Diameter may be either DRAG (the default) or BUOYANCY.

Table Input

Notes

(a)The program hydrodynamic force formulation is outlined in Hydrodynamic Loading. Traditionally, Flexcom has based the calculation of added mass and inertia loading on drag diameter, and this behaviour is retained as the default in order to maintain compatibility with earlier versions. However, strictly speaking, the added mass and inertia terms should be based on displaced volume (i.e. buoyancy diameter) as opposed to projected area (i.e. drag diameter), and the Buoyancy Diameter option facilitates this alternative approach.

Constant Hydrodynamic Coefficients

Purpose

To assign hydrodynamic coefficients to element sets where these coefficients are independent of Reynolds number.

Keyword Format

The two lines that make up the block for constant hydrodynamic properties, specifying either no drag lift or a linear drag lift coefficient:

SET=Set Name [, TYPE=CONSTANT]

Cd Normal [, Cd Tangential] [, Cm Normal] [, Ca Tangential] [, Ca Normal] [, Drag Lift or CURVE=Curve Name]

Drag Lift is specified for linear drag lift, whereas a Curve Name is specified for non-linear drag lift. If a curve name is specified, then it must be defined using *DRAG LIFT.

Table Input

Notes

(a)Refer to Constant Hydrodynamic Coefficients for further information on this feature.

Reynolds Dependent Hydrodynamic Coefficients

Purpose

To assign hydrodynamic coefficients to element sets where these coefficients are a function of Reynolds number (Re).

Keyword Format

The two lines that make up the block for coefficients which vary with Reynolds number:

SET=Set Name, TYPE=REYNOLDS

Re, Cd Normal [, Cd Tangential] [, Cm Normal] [, Ca Tangential] [, Ca Normal]

The second line can be repeated as many times as necessary to define properties for different Reynolds numbers.

Table Input

Notes

(a)Refer to Reynolds Number Dependent Coefficients for further information on this feature.

(b)Use as many lines as you need to completely define the hydrodynamic coefficients for a particular element set. Simply leave Column 1 blank for second and subsequent lines. For subsequent sets, put the set name in Column 1 and specify the coefficients in the same way.

(c)For values of Re intermediate to the values you specify, the hydrodynamic coefficients are calculated by linear interpolation.

Reynolds Number Computation

Purpose

To specify how the dependence of hydrodynamic coefficients on Reynolds number is to be calculated.

Keyword Format

The presence of the OPTION=INSTANTANEOUS line at the beginning of the keyword indicates that the hydrodynamic forces are to be computed as a function of the instantaneous Reynold’s number. If this line is omitted, the normal approach is adopted whereby Reynold’s number remains constant over each wave period.

[OPTION=INSTANTANEOUS]

 

Table Input

Notes

(a)Refer to Reynolds Number Dependent Coefficients for further information on this feature.

Moonpool Hydrodynamic Coefficients

Purpose

To assign hydrodynamic coefficients to elements of the structure that may be subjected to hydrodynamic loading within a vessel moonpool.

Keyword Format

The two lines that make up the block for moonpool hydrodynamic properties:

SET=Set Name, TYPE=MOONPOOL

Cd Normal [, Cd Tangential] [, Cm Normal] [, Ca Tangential] [, Ca Normal]

 

Table Input

Notes

(a)Refer to Moonpool Hydrodynamics for further information on this feature.

(b)Only constant hydrodynamic coefficients may be specified for moonpool hydrodynamic sets, i.e. hydrodynamic coefficients may not be specified as a function of Reynolds number.

Buoy Structure Hydrodynamic Coefficients

Purpose

To assign hydrodynamic properties to a set of elements that collectively model a subsea buoy.

Keyword Format

The four lines that make up the block for buoy structure hydrodynamic properties:

SET=Set Name, TYPE=BUOY [, AXIS=Axis Name]

X:CdAd, X:CmVin, X:CaVin

Y:CdAd, Y:CmVin, Y:CaVin

Z:CdAd, Z:CmVin, Z:CaVin

Axis Name can be any arbitrary local axis system defined under *LOCAL AXIS SYSTEM. If the axis system for the buoy is initially aligned with the global axis system, you can simply specify AXIS=GLOBAL and a local buoy axis system which is aligned with the global axis system will be created internally for you.

Table Input

Notes

(a)Refer to Subsea Buoys for further information on this feature.

(b)Note that all of the hydrodynamic properties for the elements of the subsea buoy set are specified here. It is incorrect to assign hydrodynamic properties to elements using Hydrodynamic – Constant or Hydrodynamic – Reynolds No., and then to also assign such properties using this Hydrodynamic – Buoy Structure facility.

(c)The Axis input gives you a choice of methods for specifying the Hydrodynamic – Buoy Structure coefficients. These can be specified in the Global axes or in an axis system which is Local to the buoy. If a local axis system referenced, it must be defined using the *LOCAL AXIS SYSTEM keyword. Coefficients specified in either axis system account for changes in the orientation of the buoy as it displaces and rotates in space.