Loads due to vortex induced vibrations (VIV)

This load model is currently under development.

1. Description

The VIV load model calculates vortex-induced vibration loads during dynamic analysis. It is used when VIV response due to current loading needs to be accounted for directly in the time domain simulation.

The following VIV load options are available:

  • Cross-flow loads only

    • Cross-flow VIV loads are calculated based on the instantaneous relative flow velocity and vortex shedding frequency.

    • This is the standard supported option.

  • Combined cross-flow loads and in-line loads

    • Cross-flow and in-line VIV loads are calculated independently.

  • In-line VIV loads only

    • In-line VIV loads only. Under development.

The VIV load coefficients (force coefficient, natural shedding frequency, frequency range) are non-dimensional and specified as part of the cross section properties. The coefficients are typically determined based on experimental data, and depend on the flow conditions and structure type.

The VIV load coefficients are defined as follows:

  • CV: Vortex shedding force coefficient for the (instantaneous) cross-flow load term

  • FNULL: Natural cross-flow vortex shedding frequency

  • FMIN: Minimum cross-flow vortex shedding frequency

  • FMAX: Maximum cross-flow vortex shedding frequency.

  • CVIL: Vortex shedding force coefficient for the (instantaneous) in-line load term

  • FNULIL: Natural in-line vortex shedding frequency

  • FMINIL: Minimum in-line vortex shedding frequency

  • FMAXIL: Maximum in-line vortex shedding frequency.

FMAX > FMIN and FMAXIL > FMINIL are required for the VIV load model to be active. If the maximum frequency is not greater than the minimum frequency, the VIV load model will be inactive and no VIV loads will be calculated.
Table 1. Suggested VIV empirical parameters used for CHTVIV=CF, i.e. Cross flow only. CQY and CAY are non-dimensional drag force and added mass coefficients in normal direction.

Flow conditions

Structure type

Parameters

CQY

CAY

CV

FNULL

FMIN

FMAX

Constant current

Bare riser section

1.0

1.0

1.3

0.13

0.10

0.26

Buoyancy section (Lb/Lr=1/2)

Bare riser

0.9

1.0

1.2

0.18

0.10

0.22

Buoyancy element

0.3

1.0

0.08

0.10

0.05

0.15

Buoyancy section (Lb /Lr=1/1)

Bare riser

1.2

1.0

0.8

0.18

0.10

0.26

Buoyancy element

0.6

1.0

0.5

0.10

0.05

0.15

Vessel motion induced VIV

Bare riser & buoyancy section

1.2

1.0

0.8

0.216

0.10

0.26
Lb/Lr is the ratio between the length of the buoyancy element and the bare riser section.
Ratio between the length of the buoyancy element and the bare riser section

Ratio between the length of the buoyancy element and the bare riser section

Table 2. Suggested VIV empirical parameters used for CHTVIV=CFIL, i.e. combined Cross-flow and In-line flow. CQY and CAY are non-dimensional drag force and added mass coefficients in normal direction, Reynolds number =800-15000, see Table 5.6 in Farantos, C. (2022)

Flow conditions

Structure type

Parameters

CQY

CAY

CV

FNULL

FMIN

FMAX

CVIL

FNULIL

FMINIL

FMAXIL

Constant current

Bare section

1.2

1.0

0.85

0.144

0.08

0.208

0.75

0.36

0.10

0.9

2. Theory

For details, see Theory Manual - Loads due to Vortex-Induced Vibrations for the load formulation and synchronization model.