*VIV

Data format:

A single line with a maximum of 13 values

dvrmin dvrmax nbmode epsint BF CF sCF nmoviv stre nbptviv COR cv IGR

There is a possibility to read the eigenfrequencies, modes and curvatures from a the database of a previously computed case by writing: *VIV, file=PATH\file.dtbm

If BF = CF = -1, then SN curve is defined by different segments.

Each segment is defined by one line :

Scuti Bfi CFi

Status :

Required for VIV analysis

Purpose :

Define the parameter for the VIV analysis based on the DeepVIV modal approach. A modal analysis is also performed or the modes are read from a database.

Restriction :

Only one structure has to be defined in the analysis. A static analysis has to be performed first.

Details :

Parameter	Description
DVRMIN, DVRMAX	define the zone of excitation of the mode: a mode is excited if its frequency f is such that: 1/St - dvrmin < U/fd < dvrmax + 1/St, where d is the diameter and U the velocity of the current perpendicular to the structure at the location under consideration
NBMODE	To require a maximum of NBMODE excited. By default, enter 0 to let the program find all the excited modes
EPSINT	Internal parameter to converge on the amplitude. Enter 0 by default. If there is some convergence problem, enter a value smaller than 0.05
BF, CF, SCF	Parameters used in the fatigue analysis. The fatigue is based on a SN curve. The stress range due to the vibration is computed and the fatigue life is computed with a Milner sum:\(\frac{1}{fatigue~~life} = \sum_{modes}\frac{n_i}{N_i}\) For a given mode i , with frequency fi, and a bending stress range due to vibrations of Dsi, we have:\(\frac{n_i}{N_i} = \frac{T_{year}}{CF}f_i(\Delta\sigma_i)^{BF}\) The fatigue life is given in year, Tyear, being the number of seconds in a year. The coefficients BF and CF are obtained with a stress range in Pa. For SCF, 0 can be entered then the code considers that SCF=1. Different SCF may be defined for different part of the studied riser. To do so, the keyword *SCF must be used. The SCF defined here is used as default value. BF and CF need to be entered in order to obtain a meaningful results for the fatigue analysis. In case of several segments for the SN curve, set BF and CF to 1 and define the different segments with the following lines.
0 BF0 CF0	The stress are expressed in Pa.
DScut1 BF1 CF1	Below DScut1, BF0 and CF0 are used.
DScut2 BF2 CF2	Above DScut2, BF2 and CF2 are used.
NMOVIV	Enter 0 by default. The code will determine the number of modes to be computed. If at the end of the computation, there is a message to require more modes, a higher number of modes has to be entered by the user.
STRE	Strouhal number. If 0 is entered the code computes the Reynolds number and defines the Strouhal number
NBPTVIV	The number of equally spaced points used for the VIV analysis is selected by the code if 0 is entered. The user can modify that value.
COR	Coefficient for the lift force. If the user enter 1, the lift coefficient used is the the one defined is the function f __ Cl r. If 0 is entered, a correlation length as defined by Ottesen Hansen is taken into account. Otherwise, the lift coefficient is multiplied by COR. The correction length is given by: \(l_c = 3d + 35d \frac{AID}{0.5 - AID}~~~if~~~ AID < 0.5\) \(l_c = \infty~~~otherwise\)
CV	Defined the hydrodynamic damping. The hydrodynamic damping is based on the Morison formulation. Therefore CV is the drag coefficient in the crossflow direction. By default, CV=1.2.
IGR	group number to select the part of the model on which the VIV analysis is perfomed. By default, IGR = 0 to take the whole model. Of course, IGR must have been previously defined with RISER, MOORLINE or *GROUP

Examples:

Example 1 (single slope S-N curve):

*VIV,file= ..\ModesVIVcur1\ModesVIVcur1.dtbm

0.6 0.6 10 0.05 3.0 1.556e30 1.25 2 0 0 0 0 0

In this case, the VIV calculation is based on two modes only. The S-N curve is defined through a single segment with BF = 3, CF = 1.556E30. A stress concentration factor SCF = 1.25 is taken into account for all elements. The other parameters are set to their default value.

Example 2 (double slope S-N curve with endurance limit):

*VIV,file=..\ModesVIVcur1\ModesVIVcur1.dtbm
0.6 0.6 10 0.05 -1 -1 1.25 2 0 0 0 0 0
0.0    0.0    4.036e45
2.380e7    5.0    4.036e45
8.340e7    3.0    5.807e29

For the following curve D of the DNV :

No failure 0 \(\leq\) DS \(\leq\) 23.8 MPa
logCF = 15.606 and BF = 5.0 for 23.8 MPa \(\leq\) DS \(\leq\) 83.4 MPa
logCF = 11.764 and BF = 3.0 for 83.4 MPa \(\leq\) DS \(\leq\) 387.3 Mpa

Example 3 (double slope S-N curve with no endurance limit):

*VIV, file = ..\MODAL_3LPP_720_FAR\MODAL_3LPP_720_FAR.dtbm
1 1 0 0.1 -1 -1 0. 50 0.2 0 1 0 1000
0 5 3.77e+044
5.19e+007 3 1.4e+029