Introduction
1. Purpose of Program
RIFLEX
was developed as a tool for analysis of flexible marine riser
systems, but is as well suited for any type of slender structure, such
as mooring lines, umbilicals, and also for steel pipelines and
conventional risers.
These slender structures may be characterized by:

Small bending stiffness

Large deflection

Large upper end motion excitation

Nonlinear cross section properties

Complex cross section structure
Due to the complex cross sections typical found for flexible pipes, a
global cross section model is applied in RIFLEX
. This means that cross
section properties such as axial, bending, and torsional stiffness
must be specified as input. Furthermore, structural response is always
computed as global deformations and stress resultants (axial force,
moments). Hence, local strains and stresses in different cross section
layers and materials are not considered.
Nonlinear cross section behaviour is modelled by introducing nonlinear relations between global deformation parameters and stress resultant, i.e. curvature and moment; relative elongation and tension.
The program computes static and dynamic characteristics of the structure.
Static analysis comprises:

Equilibrium configuration

Parameter variations of tension or position parameters, current velocity and direction
Dynamic analysis comprises:

Eigenvalue analysis, natural frequencies and mode shapes

Response to harmonic motion and wave excitation

Response to irregular wave and motion excitation
The program is based on a nonlinear finite element formulation. The following key features are included:

Flexible modelling of simple as well as complex systems

Nonlinear time domain simulation of riser motions and forces

Nonlinear cross section properties

Generalized Morison type of load model Simplified analysis options:

Static analyses, catenary approximations Linearized time domain simulation

Frequency domain analysis

2. Program Documentation
The program documentation comprises:

Theory Manual containing a description of mathematical models used in the program

User’s Manual containing description of input and output

Release Notes for each release of the program
3. Structure of Computer Program
The program system consists of four programs or modules communicating via the file system as shown in the figure below.
A complete dynamic analysis must include a run of all modules. However, an efficient data base system simplifies the work during a complete study by storing input data and intermediate results. (I.e. problem description, static configurations, wave induced vessel motions and water particle velocities and accelerations).
Each module will be further detailed in the following.
3.1. INPMOD module
The INPMOD
module reads most input data and organizes a data base for
use during subsequent analyses. Once the INPMOD
module has been run,
several analyses can be performed by the other modules without rerun of
INPMOD
.
3.2. STAMOD module
The STAMOD
module performs several types of static analyses. The
results may be used directly in parameter studies etc., and are also
used to define the initial configuration for a succeeding dynamic
analysis. Element mesh, stressfree configuration and key data for finite
element analysis are also generated by STAMOD
based on system data
given as input to INPMOD
.
3.3. DYNMOD module
The DYNMOD
module carries out time domain dynamic analyses based on
the final static configuration, environment data and data to define
motions applied as forced displacements in the analysis. It is possible
to perform several dynamic analyses without rerun of INPMOD
and
STAMOD
. Response time series are stored on file for further
postprocessing by OUTMOD
. In addition to dynamic response, natural
frequencies and modeshapes can be calculated.
4. Explanation of Files Used
In running the 5 different RIFLEX
modules different kinds of files are
needed. The files can be divided into the following categories:

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

Binary files for internal communication between
RIFLEX
modules 
Files for export of results for postprocessing
An overview of files used is given in the figure
Figure 2. A RIFLEX
user will
only need to specify input files for the INPMOD
, STAMOD
, DYNMOD
and OUTMOD
modules.
The internal file communication is organized via run command procedures
and therefore hidden for the user. The file names and extensions may be
adapted to the computers operating system and the actual run command
procedures used. Description of the file name conventions used in the
standard run command procedure supplied with a RIFLEX
installation is
given in How to Run the Program.
Below is a brief description of the files used.
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 modulexxxxxx

xxxxxx.res
: symbolic result file from modulexxxxxx
xxxxxx
here means either INPMOD
, STAMOD
, DYNMOD
or OUTMOD
, see
Figure 2.
Description of data needed in the input files are described in Chapters
59 of the User Manual.
4.2. Files for internal communication between modules
Files for internal communication are binary, direct access data files in
either SAMDMS
format (extension .sam
) or in FFILE
format
(extension .ffi
). See RIFLEX
maintenance manual for further file
format description.
A short description of the files used:

ifninp.sam
: storage of all data given as input to theINPMOD
module. System data read bySTAMOD
for generation of finite element model, wave and transfer function data read byDYNMOD

ifnsys.sam
: contains system finite element model generated bySTAMOD

ifndmp.sam
: temporary storage of all system data. To be used in possible restart analysis inSTAMOD

ifnsta.ffi
: storage of results from static analysis 
ifndyn.ffi
: storage of results from dynamic analysis 
ifnirr.ffi
: storage of wave kinematics data for irregular dynamic analysis 
ifnplo.ffi
: storage of plot data generated byOUTMOD
4.3. Files for communication with external programs
The following files can optionally be applied to export results from
RIFLEX
for postprocessing by other programs:

startimes.ts
: Export of response time series fromOUTMOD
for postprocessing by the general purpose statistical analysis programSTARTIMES
. File format is standardSTARTIMES
format (binary, direct access file) 
ifrdyn.raf
: File for communication with general purpose program for advanced graphical presentation
5. Applied Units and Physical Constants
Throughout the theory description a consistent set of units is used.
In the program input the user is allowed to select mass as well as force
units. This implies that the user also has to specify the gravitational
constant as the ratio of force to mass units
\(\mathrm {[F/M]}\). In order to allow inconsistent units,
e.g. \(\mathrm {kN}\), \(\mathrm {kg}\),
\(\mathrm {m}\), \(\mathrm {s}\), the acceleration
in terms of \(\mathrm {F/M}\) ratio will be different from
acceleration in terms of the ratio length to squared time
\(\mathrm {[L/T^2]}\). A constant GCONS
is therefore
introduced as a specification of the difference GCONS
\(\mathrm {=\frac{F/M}{L/T^2}}\). In the example case,
GCONS = 0.001
.
Physical quantity  Symbol  Units, SI  Modified SI \(\mathrm {(kN)}\) 

Basic 

Time 
\(\mathrm {T}\) 
\(\mathrm {s}\) 
\(\mathrm {s}\) 
Length 
\(\mathrm {L}\) 
\(\mathrm {m}\) 
\(\mathrm {m}\) 
Mass 
\(\mathrm {M}\) 
\(\mathrm {kg}\) 
\(\mathrm {kg}\) 
Force 
\(\mathrm {F=ML/T^2}\) 
\(\mathrm {N}\) 
\(\mathrm {kN}\) 
Derived 

Pressure, stress 
\(\mathrm {P=F/L^2}\) 
\(\mathrm {N/m^2}\) 
\(\mathrm {kN/m^2}\) 
Velocity 
\(\mathrm {V=L/T}\) 
\(\mathrm {m/s}\) 
\(\mathrm {m/s}\) 
Physical constants 

Acceleration of gravity 

\(9.81\,\mathrm {N/kg}\) 
\(0.00981\,\mathrm {kN/kg}\) 
Acceleration of gravity 

\(9.81\,\mathrm {m/s^2}\) 
\(9.81\,\mathrm {m/s^2}\) 
Consistency of units 



Density of sea water 

\(\mathrm {1025\,kg/m^3}\) 
\(\mathrm {1025\,kg/m^3}\) 