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.

um ss fig3
Figure 1. System definition, INPMOD

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 z-coordinates) 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.

um ss fig4
Figure 2. System definition terms

  • 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:

  1. Cross sectional components

    • Pipe cross section (CRS0)

    • Axi-symmetric cross section (CRS1)

    • Bi-symmetric cross section (CRS2)

    • Cross section for advanced modelling of floating, partly submerged structures, bi-symmetric (CRS5). Only available for "arbitrary" systems.

    • General non-symmetric 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.

  1. 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.

  1. 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, One-Point Seafloor Contact to SD Free Upper End).

2.1.1. Global coordinate systems

The x-y plane of the global coordinate system is placed at the sea surface with the z-axis pointing upwards.

The following conventions are in addition adopted for the standard riser systems:

  • Boundary conditions, i.e. terminal point coordinates are specified in x-z plane

  • x-coordinate at lower end is zero for SA, SB and SD systems

  • x-coordinate 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, One-Point 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, One-Point Seafloor Contact

um ss fig5
Figure 3. Examples of configurations covered by SA

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 one-point attachment. Only one branch is accepted per branch node. The branches are thus assumed to be vertical in a zero current condition.

2.2.2. Stressfree configuration

The stressfree configuration is placed horizontally at seafloor, branches are assumed vertical.

um ss fig6
Figure 4. Example of stressfree configuration for SA system

2.3. "SB" Seafloor to Surface Vessel, Seafloor Tangent

um ss fig7
Figure 5. Examples of configurations covered by SB

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.

um ss fig8
Figure 6. Examples of stressfree configuration for SB systems

2.4. "SC" Free Lower End, Suspended from Surface Vessel

um ss fig9
Figure 7. Examples of configurations covered by SC

2.4.1. System topology

This group is characterized by a free lower end, all degrees of freedom being specified at the upper end. This configuration represents typical installation phases, but as indicated in the figure, towing configurations can be analyzed as well.

2.4.2. Stressfree configuration

The stressfree configuration is assumed vertical with vertical position of upper end equal to final position.

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).

um ss fig10
Figure 8. Examples of configurations covered by SD

With a free upper end the configuration is governed by hydrodynamic forces in the horizontal direction. If the buoyancy element is surface-piercing, it is assumed that it is a long, slender, spar-type buoy.