System Specification
1. General Principles and System Description Terms
This section will give an introduction to the terminology and principles
for system modelling in RIFLEX
.
The system definition starts with definition of the topology and proceeds in increasing detail to the line and component descriptions. It is possible to specify a system with general topology (Arbitrary Riser System, AR), but several alternatives are also available for simplified input of commonly used configurations with well defined standard topologies (e.g. standard system SA, SB, SD, SC). The line and component specifications will, in most cases, be independent of the system topology (see the figure `System definition INPMOD' below). There are, however, a few component types available for the arbitrary system that can not be used in standard systems, see Section 1.5.
1.1. System Topology Description
The system topology is in general described in terms of branching points and terminal points. These points are denoted supernodes. Supernodes are connected by simple lines. This means that the system topology is uniquely determined by the connectivity between a number of defined supernodes and lines.
A general supernode/line connectivity can be specified for arbitrary systems while a restricted system specific connectivity is available for standard systems.
1.2. Boundary Condition Modelling at Supernodes
Supernodes are classified as free, fixed or prescribed depending on
their boundary condition modelling. A supernode is denoted free if all
degrees of freedom are free (i.e. nodal position and rotations are
unknown prior to the analysis). In modelling of standard systems it is
further distinguished between free branchings (TSNBRA
) and free ends
(TSNFRE
) to ease system topology description.
Supernodes of type fixed are used for modelling supports at fixed structures, seafloor connection, etc. A supernode is denoted fixed if one or several degrees of freedom (dof’s) are fixed. For arbitrary systems it is possible to specify status code free/fixed for all degrees of freedom for each supernode of type fixed (i.e. status code specifications for global x, y, z translations and rotations). For standard systems, all dof’s of fixed supernodes are assumed fixed (rotation free support can still be specified using the "connector" component).
Prescribed supernodes are normally used for modelling supports with forced (prescribed) dynamic motions (e.g. connections to floating support vessels).
The interpretations/specifications are similar to fixed supernodes.
1.3. Specification of Supernode Positions for Stressfree and Final Configurations
The basis for calculation of structural forces and deformations in finite element analysis is a stressfree reference configuration defining the state of no structural forces/deformations. A stressfree configuration for structural parts with bending stiffness and no initial deformations will always be a straight line.
The stressfree configuration for arbitrary systems is therefore
specified on system level by specification of stressfree positions
(global x, y and zcoordinates) of all supernodes. Stressfree position
of intermediate FEM
nodes are then computed by the program assuming a
straight line configuration between stressfree supernode positions. The
stressfree configurations for standard systems are automatically
generated by the program, see Section 2 for a description.
Output of generated stressfree configurations are optionally available
in STAMOD
.
Final static position of relevant dof’s for fixed and prescribed supernodes are specified as a part of the system description.
1.4. Line and Segment Description
A line is a linear structural element between two supernodes which is identified by a line type number. This means that a line type can be referred to several times in the system topology description, which is convenient for modelling of systems with several identical lines (e.g. anchor systems).
1.4.1. The line is specified in terms of:

Sequence of segments with homogeneous cross sectional properties. Cross sectional component type, length and number of elements to be used for finite element discretization are specified for each segment (see `System definition terms' (below)).

Nodal components for modeling of clump weights, buoys, swivels and hinges etc. can be specified at segment intersections.

Fluid component (
FLUID
) for description of possible internal fluid flow.

SUPERNODE
: Branching points or nodes with specified boundary conditions. 
LINE
: Suspended structure between two supernodes. 
SEGMENT
: (Part of) line with uniform cross section properties and element length. 
ELEMENT
: Finite element unit.
1.5. Component Description
The components represents the elementary description of the mechanical properties. A component is identified by a numerical identifier called component type number.
The components available in present RIFLEX
version are:

Cross sectional components

Pipe cross section (
CRS0
) 
Axisymmetric cross section (
CRS1
) 
Bisymmetric cross section (
CRS2
) 
Cross section for advanced modelling of floating, partly submerged structures, bisymmetric (
CRS5
). Only available for "arbitrary" systems. 
General nonsymmetric cross section (
CRS7
)

Cross sectional stiffness properties are specified in terms of axial and, optionally, bending and torsional stiffness. Elements specified with axial stiffness only are represented by 3D bar elements. Elements with specified bending and torsional stiffness are represented by 3D beam elements. Linear or nonlinear stiffness specifications can be applied for all cross sectional types.
Additional data that must be specified for all cross sectional types are external and internal area, mass and hydrodynamical coefficients.
A special component denoted external wrapping (EXT1
) is also available
for modelling additional distributed weight or buoyancy.

Nodal components

Body (
BODY
) for modelling of clump weight, submerged buoys etc. 
Ball joint connector (
CONB
) for modelling of swivels, hinges etc.

Mass, volume and hydrodynamical coefficients must be specified for both component types.

Special components

Rollers for description of elastic contact forces between lines.

Tensioner component for modelling of tensioner mechanisms.

1.6. Element Mesh Generation
The element mesh is computed automatically based on the topology, line
and component description. Constant element lengths are applied within
segments. Connections between lines, segments and elements specified as
input and nodal/element numbers used in the finite element analysis are
available as output from STAMOD
.
2. Standard Systems
2.1. Classification
In order to simplify the system topology definition for commonly used configurations, a selection of standard systems are provided:

SA  Seafloor to surface vessel. One point seafloor contact. The Steep Wave, Steep S and Jumper flexible riser configurations are special cases of the SA system.

SB  Seafloor to surface vessel. Seafloor tangent and/or additional seafloor attachment point. The Lazy Wave and Lazy S flexible riser configurations are special cases of the SB system. The SB system is also convenient for modelling of anchorlines with seafloor contact at lower end.

SC  Free lower end. Riser during installation etc.

SD  Free upper end. Buoyed riser, loading system, etc.
The stressfree configurations are automatically generated for all
standard systems. Definition of system topologies and stressfree
configurations are further discussed in the remaining sections of this
chapter (SA
Seafloor to Surface Vessel, OnePoint Seafloor Contact
to SD
Free Upper End).
2.1.1. Global coordinate systems
The xy plane of the global coordinate system is placed at the sea surface with the zaxis pointing upwards.
The following conventions are in addition adopted for the standard riser systems:

Boundary conditions, i.e. terminal point coordinates are specified in xz plane

xcoordinate at lower end is zero for SA, SB and SD systems

xcoordinate at upper end is zero for SC systems
The global coordinate systems for all standard systems are shown in
figures presented in the remaining sections of this chapter (SA
Seafloor to Surface Vessel, OnePoint Seafloor Contact to SD
Free
Upper End).
2.1.2. Special analysis features
An important feature available for standard systems is simplified static analysis based on catenary analysis. It is also possible to use the catenary solution as starting point for the static finite element analysis or to apply conventional finite element analysis starting from stressfree position.
For further details, see Static Catenary Analysis
and Static Finite
Element Analysis
in the Theory Manual.
2.2. "SA" Seafloor to Surface Vessel, OnePoint Seafloor Contact
2.2.1. System topology
The riser is suspended between two defined points. The lower end is fixed while upper end is connected to the support vessel. The only type of branching elements are slender buoyancy or weight elements suspended in onepoint attachment. Only one branch is accepted per branch node. The branches are thus assumed to be vertical in a zero current condition.
2.3. "SB" Seafloor to Surface Vessel, Seafloor Tangent
2.3.1. System topology
Compared with the previous systems this system includes additional features:

Seafloor tangent boundary condition

Buoyancy guide at one point
The seafloor contact is modelled by bilinear stiffness. The stiffness is discretized and implemented as springs at the nodal points that may touch the seafloor.
2.3.2. Stressfree configuration
The stressfree configuration is placed horizontally. The vertical position is placed above the seafloor to avoid possible seafloor contact at the first steps in the incremental loading sequence applied in the static finite element analysis. Possible branches are assumed vertical.
2.4. "SC" Free Lower End, Suspended from Surface Vessel
2.5. "SD" Free Upper End
2.5.1. System topology
Single line system connected to seafloor at lower end and with free upper end.
2.5.2. Stressfree configuration
The stressfree configuration is assumed vertical with lower end in final position (e.g. at seafloor).
With a free upper end the configuration is governed by hydrodynamic forces in the horizontal direction. If the buoyancy element is surfacepiercing, it is assumed that it is a long, slender, spartype buoy.