1. Specification of section properties

This data group is mandatory. Property types such as excitation zone definition, added mass as a function of non-dimensional frequency, excitation coefficient as a function of non-dimensional frequency and response, damping and Strouhal number may be specified by the user.

For the various property types; e.g. added mass, excitation, Strouhal number; an arbitrary number of data sets may be defined in the input file. Within each of these property types the data sets are given unique numbers. The cross section properties for each segment are then defined by referring to these numbers. Appendix A: Typical VIVANA Input File shows an example where several property types are given.

1.1. Data group identifier, one input line

SECTion PROPerty SPECification
NSEGP

  • NSEGP: integer: Number of segments for which properties are given. Must be equal to the number of segments in the model specified in INPMOD

1.2. Specification of cross-section property for each segment, NSEGP input lines.

ISEGP IEXCZO IADDMA1 IEXCIT1 IDAMPG ISTROU IADDMA2 IEXCIT2

  • ISEGP: integer, default: 0: Segment number (global).

  • IEXCZO: integer, default: 0: Cross section property specification for excitation zone limits. Refers to IPRONO in Specification of section properties.

  • IADDMA1: integer, default: 0: Cross section property specification for cross-flow (IRSTYP = 1 or IRSTYP = 3) or in-line (IRSTYP = 2) added mass as a function of non-dimensional frequency. Refers to IPRONO in Specification of section properties.

  • IEXCIT1: integer, default: 0: Cross section property specification for cross-flow (IRSTYP = 1 or IRSTYP = 3) or in-line (IRSTYP = 2) excitation coefficient as a function of non-dimensional frequency. Refers to IPRONO in Specification of section properties.

  • IDAMPG: integer, default: 0: Cross section property specification for hydrodynamic damping. Refers to IPRONO in Specification of section properties.

    • The program has two possible ways to calculate the damping, see Specification of section properties in this manual :

      1. The damping terms \(\mathrm {R_{sw}}\), \(\mathrm {R_{lv}}\) and \(\mathrm {R_{hv}}\) will be used directly, ref. VIVANA Theory Description, Section The Damping Model, Hydrodynamic damping outside the excitation zone.

      2. The damping is calculated using the excitation curves defined for that section. A still water damping coefficient must be given as input.

  • ISTROU: integer, default: 0: Cross section property specification for Strouhal number. Refers to IPRONO in Specification of section properties.

  • IADDMA2: integer, default: 0: Cross section property specification for in-line added mass as a function of non-dimensional frequency for the case of combined cross-flow and in-line (IRSTYP = 3). Refers to IPRONO in Specification of section properties. Note that the response frequency iterations will adjust the in-line added mass so that the in-line response frequency is twice that of the cross-flow response frequency. IADDMA2 must be zero for pure cross-flow (IRSTYP = 1) or pure in-line (IRSTYP = 2).

  • IEXCIT2: integer, default: 0: Cross section property specification for in-line excitation coefficient as a function of non-dimensional frequency for the case of combined cross-flow and in-line (IRSTYP = 3). Refers to IPRONO in Specification of section properties. IEXCIT2 must be zero for pure cross-flow (IRSTYP = 1) or pure in-line (IRSTYP = 2).

The cross-section properties referenced in this data group must be defined using any of the following PROPERTY - data groups.

The default value of zero for IEXCZO, IADDMA1/2, IEXCIT1/2, IDAMPG and ISTROU tells the program to use the default models given in Table 1.

Table 1. Default cross-section properties, used when no property set is referenced for a segment.
Cross-flow VIV (IRSTYP = 1) In-line VIV (IRSTYP = 2)

Excitation zone limits

\(\hat{f}_{\mathrm {min}}\):0.125 \(\hat{f}_{\mathrm {max}}\):0.3

\(\hat{f}_{\mathrm {min}}\):0.2 \(\hat{f}_{\mathrm {max}}\):0.9

Added mass as function of \(\mathrm {\hat{f}}\)

See the figure with the VIVANA CF added mass model in the Theory Manual, Section Calculation of Response Frequencies, Added mass, CF response.

See the figure with the VIVANA IL added mass model in the Theory Manual, Section Calculation of Response Frequencies, Added mass, pure IL response.

Excitation coefficients as function of \(\mathrm {\hat{f}}\)

A modified version of Gopalkrishnan’s coefficients will be used, ref. the Theory Manual. Section Excitation Force Model, Default excitation coefficient model, CF response.

A modified version of Aronsen's coefficients will be used, ref. the Theory Manual. Section Excitation Force Model, Default excitation coefficient model, pure IL response.

Hydrodynamic damping

The damping terms \(\mathrm {R_{sw}}\), \(\mathrm {R_{lv}}\) and \(\mathrm {R_{hv}}_{}\) will be used directly, ref. the Theory Manual, Section The Damping Model, Hydrodynamic damping outside the excitation zone.

Same as for Cross-flow VIV.

Strouhal number definition

The program calculates the Strouhal number as a function of the Reynolds number for each element in the model, see the Theory Manual, Section Dimensionless Parameters, Strouhal number.

Same as for Cross-flow VIV.
A constant added mass of 1.0 is recommended for analyses with response frequencies corresponding to high modes; e.g. above mode 15. The parameters IMOD_CURVE and IMOD_CONST in Specification of section properties may be used to specify a smooth transition between frequency-dependent added mass and the constant still-water added mass.
A specified transition will also be applied to the default added mass model, see Illustration of transition between frequency dependent and constant still-water added mass.

1.3. Excitation zone properties

1.3.1. Specification of excitation zone range.

PROPerty EXCItation ZONE

1.3.2. Number of excitation zones specified

NEXZON

  • NEXZON: integer: Number of excitation zones specified in this datagroup.

1.3.3. Specification of excitation zone limits for various sections of the structure, NEXZON input lines.

IPRONO CPRPID FHMIN FHMAX

  • IPRONO: integer: Cross-section property specification number for property type Excitation zone properties.

  • CPRPID: character(8): Text to identify the cross-section property

  • FHMIN: real, default: 0.125: Minimum value of \(\mathrm {\hat{f}}\) in the excitation zone.

  • FHMAX: real, default: 0.2: Maximum value of \(\mathrm {\hat{f}}\) in the excitation zone.

It is not allowed to specify a broader excitation range for a cross-section than the range used for specification of the associated excitation coefficient curves.

1.4. Added mass properties

1.4.1. Specification of added mass as a function of non-dimensionalfrequency.

PROPerty ADDED MASS

1.4.2. Number of added mass curves and added mass transition

NADCUR ITRANS_LOW ITRANS_UP

  • NADCUR: integer >= 0: Number of added mass curves to be defined by the user.

  • ITRANS_LOW: integer >= 0, default: 1000000: Last mode number in the active VIV direction for which the full frequency-dependent added mass curves will be used.

  • ITRANS_UP: integer >= 0, default: 1000000: First mode number in the active VIV direction for which the constant still-water added mass will be used. ITRANS_UP >= ITRANS_LOW

A smooth transition between the frequency dependent added mass and the constant still-water added mass will be applied between ITRANS_LOW and ITRANS_UP, see Figure 1. The specified transition will also be applied to the default added mass model.

constant added mass is recommended for high modes.
um added mass transition
Figure 1. Illustration of transition between frequency dependent and constant still-water added mass

1.4.3. Specification of added mass as a function of non-dimensional frequency, repeated NADCUR times

One input line

IPRONO CPRPID NAMPT

  • IPRONO: integer: Cross-section property specification number for property type Added mass properties.

  • CPRPID: character(8): Text to identify the cross-section property

  • NAMPT: integer >= 1: Number of points in \(\mathrm {C_a}\,\hat{f}\) curve.

Constant added mass can be given either by using NAMPT=1 or by using an constant added mass curve NAMPT>=2.

if IRSTYP=3 (Combined cross-flow and in-line VIV) and constant added mass curve with NAMPT=1 is used for all sections, no IL frequency iteration is performed. The IL frequency is twice the CF frequency.

Constant added mass coefficient, NAMPT=1 input lines

ADDMCO

  • ADDMCO: real: Added mass coefficient

Added mass coefficient as a function of non-dimensional frequency, NAMPT>=2 input lines

FHAT ADDMCO

  • FHAT: real: Non-dimensional frequency

  • ADDMCO: real: Added mass coefficient

1.5. Excitation coefficient properties

Two different ways of specifying excitation coefficients are available in VIVANA. The excitation coefficient is used in the response analysis for calculating the excitation force on the cylinder. Calculation of excitation force is irrespective of how the excitation coefficients are specified by the user.

Excitation force is calculated for elements within the excitation zone. The excitation zone is defined by the excitation frequency bandwidth. The excitation frequency bandwidth is by default [0.125, 0.3] for cross-flow VIV, but it is possible for the user to define his/her own excitation frequency range for any part of the model, see Specification of section properties. We would normally recommend the user to set the frequency bandwidth equal to the frequency range for which excitation coefficients are provided. Elements outside of the excitation zone will add damping to the system, see the Theory Manual, Section Calculation of Response Frequencies, Added mass, CF response for details.

1.5.1. Specification of excitation coefficient as a function of non-dimensional frequency and response amplitude.

PROPerty EXCITATION COEFFICIENT

1.5.2. Number of excitation coefficient data sets to be specified

NLCT01 NLCT02

  • NLCT01: integer, default: 0: Number of excitation coefficient data sets to be specified on the same format as the default curves (Type 1).

  • NLCT02: integer, default: 0: Number of excitation coefficient data sets to be specified on the table format (Type 2)

1.5.3. Specification of default format excitation coefficient data (Type 1), 2xNLCT01 input lines

One input line

IPRONO CPRPID NFT01

  • IPRONO: integer: Cross-section property specification number for property type Excitation coefficient property.

  • CPRPID: character(8): Text to identify the cross-section property

  • NFT01: integer >= 2: Number of points in \(\mathrm {C_e(}\hat{f}\mathrm {,\frac{A}{D})}\) curve specification.

Excitation coefficient as a function of non-dimensional frequency and response amplitude, NFT01 input lines

FHAT ACL0 ACLMAX CLMAX CLA0

  • FHAT: real: Non-dimensional frequency

  • ACL0: real: \(\mathrm {\frac{A}{D}}\) ratio for when \(\mathrm {C_e}\) = 0

  • ACLMAX: real: \(\mathrm {\frac{A}{D}}\) ratio for when \(\mathrm {C_e}\) = \(\mathrm {C_{emax}}\)

  • CLMAX: real: Maximum excitation coefficient

  • CLA0: real: Excitation coefficient for \(\mathrm {\frac{A}{D}}\) = 0

The definition of these four parameters is shown in a figure in the Theory Manual, Section Excitation Force Model, Default excitation coefficient model, CF.

1.5.4. Specification of table format excitation coefficient data set (Type 2). The input data described in the following is to be specified for NLCT02 excitation coefficient data sets

IPRONO CPRPID NFT02

  • IPRONO: integer: Cross-section property specification number for property type Excitation coefficient property.

  • CPRPID: character(8): Text to identify the cross-section property

  • NFT02: integer >= 2: Number of non-dimensional frequencies for which an excitation coefficient curve is specified for this data set.

Specification of table format excitation coefficient curves (Type 2). The input data described in the following is to be specified for NFT02 excitation coefficient curves.

FHTYP2 NPOINT

  • FHTYP2: real: Non-dimensional frequency.

  • NPOINT: integer >= 2: Number of data points for FHTYP2

Excitation coefficient as a function of \(\mathrm {\frac{A}{D}}\) for FHTYP02, NPOINT inputlines

AD CL

  • AD: real: Amplitude to diameter ratio.

  • CL: real: Excitation coefficient

AD and FHTYP2 must be given in either increasing or decreasing order.

It is crucial for the analysis that the data covers to complete \(\mathrm {\frac{A}{D}}\) -ratio range. That means that the minimum \(\mathrm {\frac{A}{D}}\) -ratio should be zero and the maximum value approximately 2.

It is recommended that all extrapolation be done outside of VIVANA.

If the excitation curves are used to model sections covered with VIV suppression devices, one should note that:

  • The theoretical model can cover cases with up to approximately 75% coverage of VIV suppression devices. For these cases the bare riser controls the VIV behaviour (frequency, mode etc.)

  • For larger coverages the straked riser takes control of the VIV behaviour. Model tests indicate a different physical behaviour compared to a bare riser. The behaviour seems to be dependent on pitch and height of the strakes. The responding frequencies and modes are generally lower.

1.6. Damping factor properties

1.6.1. Specification of damping factors

PROPerty DAMPing FACTors

1.6.2. Number of damping factors specified

NDPFAC01 NDPFAC02

  • NDPFAC01: integer: Number of damping factor sets for various sections of the structure relative to Venugopal.

  • NDPFAC02: integer, default: 0: Number of damping factor sets for various sections of the structure using user defined excitation curves in table format.

1.6.3. Specification of damping factor sets for various sections of the structure, NDPFAC01 input lines.

IPRONO CPRPID FSTILL FLOWV FHIGHV

  • IPRONO: integer: Cross-section property specification number for property type Damping factor properties.

  • CPRPID: character(8): Text to identify the cross-section property

  • FSTILL: real, default: 1: Factor Venugopal still water damping contribution, \(\mathrm {R_{s\alpha }}\).

  • FLOWV: real, default: 1: Factor Venugopal low velocity region.

  • FHIGHV: real, default: 1: Factor Venugopal high velocity region.

1.6.4. Specification of damping factor for various sections of the structure, NDPFAC02 input lines.

IPRONO CPRPID FSTILL

  • IPRONO: integer: Cross-section property specification number for property type Damping factor properties.

  • CPRPID: character(8): Text to identify the cross-section property

  • FSTILL: real, default: 1: Still water damping factor

If a section has damping defined as NDPFAC02, the section must have excitation coefficients given on table format.

The still water damping coefficient is calculated using the empirical formulae

\(\mathrm {c_{sw}=\frac{\omega \pi \rho D^2}{2}(1+(\frac{A}{D})^2)F_{still}}\)

The value of \(\mathrm {F_{still}}\) must be found using curve fitting of the data points \(\mathrm {c_{sw}}\) as a function of the \(\mathrm {\frac{A_{}}{D}}\)-ratio. These data points can be found from still water decay tests.

1.7. Strouhal number properties

PROPerty STROuhal SPECification

The user may allow the program to calculate local Strouhal number for an element as a function of the Reynolds number. The program then interpolates in a user defined \(\mathrm {S_t(Re)}\) curve if given, or in a default \(\mathrm {S_t(Re)}\) curve.

Alternatively, the user may specify a constant Strouhal number for all the elements in a segment. This is done by specifying STROU. STROU will then be used as the Strouhal number for all the elements in the segment.

NSTRSP

  • NSTRSP: integer: Number of Strouhal number sets specified in this data group.

1.7.1. Specification of Strouhal number sets, NSTRSP data sets.

IPRONO CPRPID NPUDSC STROU

  • IPRONO: integer: Cross-section property specification number for property type Strouhal number properties.

  • CPRPID: character(8): Text to identify the cross-section property

  • NPUDSC: integer, default: 0: Number of points in user defined Strouhal-Reynolds number curve.

    • NPUDSC < 0: Default curve will be used

    • NPUDSC = 0: Constant Strouhal number

    • NPUDSC > 0: User defined curve will be used

  • STROU: real, default: 0.19: Fixed Strouhal number

    • Dummy for NPUDSC \(\mathrm {\neq }\) 0

1.7.2. User defined curve giving the relationship between Strouhal number and Reynolds number, NPUDSC input lines.

REYNUM STRNUM

  • REYNUM: real: Reynolds number

  • STRNUM: real: Strouhal number

Reynolds numbers must be given in increasing order.

2. Structural damping specification

This data group allows additional material and slip damping to be specified for some or all segments in the system. This structural damping is read from separate files and is applied in addition to the VIV response analysis RELDAM. The structural damping is given as a function of the response curvature and is therefore updated during the response iterations.

At present, Newton - Raphson response iterations must be used, i.e. IFRIT = 2: in VIV response analysis.

Table 2. The damping input options in VIVANA. Relative damping, RELDAM, is applied to the total stiffness matrix. The additional damping from file is limited to the bending deformation only.
Stiffness matrix

Damping input

Material and geometric

Material

Deformation

RELDAM

X

axial, bending and torsion

additional damping from file

X

bending

X

bending

2.1. Data group identifier, one input line

STRUctural DAMPing SPECification

2.2. Number of specifications, one input line

NSPEC

  • NSPEC: integer, default: 0: Number of segments with detailed structural damping specification

2.3. Detailed structural damping specification, NSPEC input lines

IGSEG  CHFILE

  • IGSEG: intger > 0, default: 0: Global segment number

  • CHFILE: character(256): File with detailed structural damping specification, See the Appendix C: Structural Damping File Description. The same file may be specified for multiple segments.

Detailed debug information about the applied structural damping may be found in the file <prefix>_eledam.asc.