Introduction

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:

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:

Dynamic analysis comprises:

The program is based on a nonlinear finite element formulation. The following key features are included:

The program documentation comprises:

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.

In running the 5 different RIFLEX modules different kinds of files are needed. The files can be divided into the following categories:

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.

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.