Introduction

1. Purpose of program

It is well known that ocean currents may cause vortex induced vibrations (VIV) on slender marine structures. The processes that governs VIV has been studied as a part of the DEEPER project, and an effort has been made to improve the physical understanding of VIV for long risers, cables and pipelines in sheared current. The main result of this work is the computer program VIVANA, which has been maintained and developed continuously since the DEEPER project was finished.

The purpose of VIVANA is to calculate the response of a slender beam structure in a current, excited by vortex shedding. Cross-flow and in-line response can be calculated separately. VIVANA is not a complete analysis program in the sense that it is a stand-alone program that can be applied without any support from other programs. VIVANA requires that a description of the structure and its static shape is available on a specific file format. In its standard implementation, VIVANA is linked to the analysis program system RIFLEX (see RIFLEX Theory Manual). RIFLEX can handle a large variety of slender marine structures such as tensioned and flexible risers, anchor lines, umbilicals, tendons, pipelines during installation and free spanning pipelines. Such structures may hence also be analysed by VIVANA. Some of the structure types that VIVANA is able to handle are seen in Figure 1.

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Figure 1. Examples of slender structures that can be analysed by VIVANA.

2. Program documentation

The program documentation comprises of:

  • General description, see references.

  • User Manual containing description of input and output (this document).

  • Theory Manual containing a description of mathematical models used in the program, see references.

  • Acceptance manual containing input data for test cases and results from analyses, see references.

3. Structure of VIVANA

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Figure 2. Components of the VIVANA program system.

A complete VIV analysis consists of:

  • An initial RIFLEX analysis using the INPMOD (system modelling and environment data) and STAMOD (nonlinear static analysis) modules.

  • Calculation of vortex induced vibrations in VIVANA. The analysis method is described in the VIVANA Theory Manual.

  • From the computed response amplitudes and frequencies VIVANA can calculate fatigue damage and drag force amplification.

  • An updated static analysis may be carried out by a second use of STAMOD where magnified drag coefficients are introduced. This type of analysis will require a special input file for RIFLEX STAMOD, which is described in the RIFLEX User Manual.

Note that RIFLEX can handle complex structural systems in three dimensions and arbitrary current profiles, including variation of current direction with water depth. The present VIVANA version is, however, limited to handle structures that consist of one main line only (no branching) and that in its initial configuration is found in one plane. The current profile must be unidirectional and attach the structure in its main plane or perpendicular to this plane.

4. Explanation of files used

When running the RIFLEX and VIVANA modules, different kinds of files are needed. The files can be divided into the following categories:

  1. Symbolic input/output files (i.e. readable ASCII files).

  2. Binary files for internal communication between RIFLEX and VIVANA modules.

  3. Files for export of results for post processing.

An overview of files used is given in Figure 3. The user will need to specify input files for the INPMOD, STAMOD and VIVANA modules.

The internal file communication is organized via run command procedures and therefore hidden from the user. Description of the file name conventions used in the standard run command procedure supplied with a VIVANA installation is given in Running VIVANA.

4.1. Symbolic input/output files

Each analysis module needs a symbolic data file to read input data from (extension .inp’) and one symbolic file to print out major results (extension .res’). These files are denoted:

  • xxxxxx.inp - symbolic input file to module xxxxxx

  • xxxxxx.res - symbolic result file from module xxxxxx

xxxxxx here means either INPMOD, STAMOD or VIVANA, see Figure 3. The INPMOD and STAMOD input files are described in the RIFLEX User Guide, while the VIVANA input file is described in Input to VIVANA.

4.2. Files for internal communication between modules

Files for internal communication are binary, direct access data files in either SAM-DMS format (extension .sam) or in FFILE format (extension .ffi).

A short description of files used will be given in the following:

  • ifninp.sam - storage of all data given as input to the INPMOD module. System data read by STAMOD for generation of finite element model.

  • ifnsys.sam - contains system finite element model generated by STAMOD.

  • ifndmp.sam - temporary storage of all system data. To be used in possible restart analysis in STAMOD.

  • ifnsta.ffi - storage of results from static analysis.

  • infnviv.ffi - storage of results from VIV evaluation and response calculations.

Results from VIVANA are available on two files, the <prefix>_VIVANA.RES file and the <prefix>_VIVANA.mpf file. The first file contains echo of input and key information, while the other can be handled by MATRIXPLOT for production of plots. Note that <prefix> in the file names represents a user specified prefix that is also used as prefix for input files for all RIFLEX modules and VIVANA.

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Figure 3. File system for communication between modules.

5. Applied units and physical constants

In the input specification to the RIFLEX module INPMOD the user is allowed to select mass and force units. This is done by specifying the gravitational constant as the ratio of force to mass units. In order to allow inconsistent units, e.g. \(\mathrm {kN}\), \(\mathrm {kg}\), \(\mathrm {m}\), \(\mathrm {s}\), the acceleration in terms of force to mass will be different than acceleration in terms of the ratio length to squared time. A constant is therefore introduced as a specification of the difference, see the RIFLEX User Guide for further details.

The unit specification made by the user in INPMOD also applies to the results from the VIVANA module.