1. Specification of boundary conditions, stressfree configuration and static equilibrium configuration

Coordinates of all supernodes must be specified to define the initial stressfree configuration as well as the final static configuration. If the distance between supernodes in stressfree configuration do not correspond to the line length as specified in Type and location of contact point, NCNODE input lines, the length of the last segment in the line is adjusted, and a warning is written.

Boundary conditions and coordinates for supernodes with at least one fixed or prescribed degree of freedom

The following two or three input lines must be given in blocks for each of the NSNFIX supernodes.

Boundary conditions:

 SNOD_ID IPOS IX IY IZ IRX IRY IRZ CHCOO CHUPRO

  • SNOD_ID: character(8): Supernode identifier

  • IPOS: integer, default: 0: Boundary condition type

    • IPOS = 0: The supernode is not connected to a support vessel

    • IPOS = IVES: The supernode is connected to support vessel number IVES, 1 <= IVES <= NVES.

  • IX: integer, default: 1: Boundary condition code for translation in X-direction

    • IX = 0: Free

    • IX = 1: Fixed or prescribed

  • IY: integer, default: 1: Boundary condition code for translation in Y-direction (same interpretation as for IX)

  • IZ: integer, default: 1: Boundary condition code for translation in Z-direction (same interpretation as for IX)

  • IRX: integer, default: 1: Boundary condition code for rotation about X-axis (same interpretation as for IX)

  • IRY: integer, default: 1: Boundary condition code for rotation about Y-axis (same interpretation as for IX)

  • IRZ: integer, default: 1: Boundary condition code for rotation about Z-axis (same interpretation as for IX)

  • CHCOO: character(6): Identifier for boundary condition reference coordinate system

    • CHCOO = GLOBAL: Boundary conditions are referenced to global coordinate system.

    • CHCOO = SKEW-G: Boundary conditions are referenced to a skew coordinate system.

    • CHCOO = VESSEL: Boundary conditions are referenced to vessel coordinate system.

    • CHCOO = SKEW-V: Boundary conditions are referenced to a skew vessel coordinate system.

  • CHUPRO: character(3), default: NO: Computational parameter. Boundaries rotate with specified rotation

    • CHUPRO = YES

    • CHUPRO = NO

A supernode with prescribed motions during dynamic analysis must have IPOS>0.

Possible hinges at riser ends connected to fixed supports or to a support vessel may be modelled by either choosing the correct boundary condition code (see above) or using ball-joint connectors. Be careful not to use both these modelling options at the same time for a given super-node. This will lead to program abortion.

Note that if some of the translations are not prescribed, rotation-induced translations may cause drift-off if used in combination with global boundary conditions at a node attached to a vessel.

Coordinates for stress free and static equilibrium position:

X0 Y0 Z0 X1 Y1 Z1 ROT DIR

  • X0: real: Coordinates for stress free configuration specified so that the line between any two supernodes are straight and with zero tension. \([\mathrm {L}]\)

  • Y0: real: As for X0

  • Z0: real: As for X0

  • X1: real, default: X0:

  • Y1: real, default: Y0: Coordinates for static equilibrium position \([\mathrm {L}]\)

  • Z1: real, default: Z0

  • ROT: real, default: 0: Specified rotation of supernode from stress free position to static equilibrium position \(\mathrm {[deg]}\)

  • DIR: real, default: 0: Direction of axis for specified rotation \(\mathrm {[deg]}\)

ROT is the specified rotation in degrees from stress free position to equilibrium position and is analogous to ALFL/ALFU parameters used for the standard systems. The rotation ROT will be around the local YREF-axis as shown in the figure below. DIR is the rotation in degrees from global X-axis to XREF-axis. The local ZREF-axis is parallel to global Z-axis. DIR=0 signifies that the rotation ROT will be around the global Y-axis. If the line end is allowed to rotate freely around the local YREF-axis, ROT will be dummy. Free rotation around global Y-axis is obtained with IRY = 0 and DIR = 0.

Definition of rotation axis YREF versus global coordinate system, X, Y. The supernode is located in the origin

Definition of skew coordinate system One input line only if CHCOO = `SKEW-G' OR `SKEW-V'

XX XY XZ XP YP ZP

  • XX: real: Components of the skew X-axis referred to the global system. \([\mathrm {L}]\). See figure below.

  • XY: real: As for XX.

  • XZ: real: As for XX.

  • XP: real: Components of a reference vector from the supernode to a point in the skew XY-plane, referred to global system \([\mathrm {L}]\)

  • YP: real: As for XP

  • ZP: real: As for XP

The skew Z-axis is found by the cross product between the skew X-axis and the reference vector. The skew Y-axis is found by the cross product between the skew Z-axis and the skew X-axis

um ii fig37
Figure 1. Definition of skew boundary system.
Coordinates for completely free supernodes

This input group consists of NSNFRE input lines, where NSNFRE=NSNOD-NSNFIX gives the number of supernodes where all degrees of freedom are free. Skip this group if NSNFRE=0.

SNOD-ID X0 Y0 Z0

  • SNOD-ID: character(8): Supernode identifier

  • X0: real: Nodal coordinate in stress free configuration \([\mathrm {L}]\)

  • Y0: real: Nodal coordinate in stress free configuration \([\mathrm {L}]\)

  • Z0: real: Nodal coordinate in stress free configuration \([\mathrm {L}]\)