1. Standard system SB

Multiple seafloor contact points are allowed, with upper end connected to the support vessel. The frequently used lazy wave, lazy S and free hanging configurations are special cases of the SB system. The initial configuration of this system is two-dimensional in X-Z plane.

1.1. Topology

In addition to the branching feature of system SA it is also allowed to specify seafloor tangent and intermediate seafloor anchor point.

um ii fig18
Figure 1. Topology of system SB

  • Vertical branches with free ends are specified as for system SA

  • One anchor point can be specified (in addition to supernode 1)

  • Horizontal seafloor tangent can be specified

1.2. Data group identifier

SINGle RISEr SB

1.3. Definition of topology

NSNOD IBTANG

  • NSNOD: integer: Number of supernodes

  • IBTANG: integer, default: 0: Bottom tangent option

    • IBTANG=0: No seafloor contact

    • IBTANG = 1: Seafloor contact forces on all nodes that are below Z < ZBOT + R_EXTCNT. The modified 3D seafloor formulation is used. Friction contribution to torsional loading is possible.

    • IBTANG = -1: Equivalent with setting IBTANG=1.

    • IBTANG = -9: Seafloor contact forces on all nodes that are below Z < ZBOT using the original flat bottom formulation. If IBOT3D = 1, IBTANG is set to 1. This option is deprecated.

In the static CAT-analysis only contact in the vicinity of supernode 1 is included. Contact forces on all nodes that are below ZL are considered in static FEM analysis and dynamic analysis.

Note that flat bottom topology based on original FORTRAN code is planned to be removed and substituted by the general 3D seafloor contact formulation FORTRAN code. The old code will be kept for debugging purposes.

1.4. Line, line type and supernode connectivity

This data group defines the connectivity between lines and supernodes. If the line identifier is missing the line number implicitly defined by the order in which the lines are specified, will be used as the line identifier. References to line type IDs and supernode numbers are mandatory.

The lines must be specified in the order indicated in the figure `Topology of system SB' above. This means that the lines are given continuously from seafloor to upper riser end.

At each branching point the branching line(s) are specified before the next line in the main riser configuration. No ball joint components are accepted in branch lines.

NLIN (=NSNOD-1) input lines ~ LINE-ID LINTYP-ID ISNOD1 ISNOD2 ~ - LINE-ID: character(8): Line identifier - LINTYP-ID: character(8): Reference to line type identifier - ISNOD1: integer: Reference to supernode number at end 1 - ISNOD2: integer: Reference to supernode number at end 2

If only 1 alphanumeric string and 2 integers are specified, the string is interpreted as LINTYP-ID. ~ LINTYP-ID ISNOD1 ISNOD2 ~ The LINE-ID is taken as the line number as implicitly defined by the order in which the lines are given.

1.5. Boundary conditions

ZL XU ZU ALFL ALFU ZA XA

  • ZL: real: Z-coordinate of lower end \(\mathrm {[L]}\)

    • ZL will also be used as Z-coordinate of seafloor if IBTANG>0

  • XU: real > 0: X-coordinate of upper end \(\mathrm {[L]}\)

  • ZU: real: Z-coordinate of upper end \(\mathrm {[L]}\)

  • ALFL: real: Angle of lower end from vertical \(\mathrm {[deg]}\)

    • Dummy when seafloor contact is specified (i.e. IBTANG/=0)

  • ALFU: real: Angle of upper end from vertical \(\mathrm {[deg]}\)

  • ZA: real >= ZL: Z-coordinate of anchor point \(\mathrm {[L]}\)

    • Dummy if no anchor point is specified

  • XA: real: X-coordinate of anchor point \(\mathrm {[L]}\)

    • Required input when static FEM analysis is applied. The X-location of the anchor point is automatically computed so that the anchor line is vertical if the CAT or CATFEM analysis is used in STAMOD (i.e. XA is dummy). XA is also dummy when no anchor point is specified.

If the lower/upper end later in the specification is allowed to rotate freely around the Y-axis, ALFL/ALFU will be dummy.

1.6. Supernode types

NSNOD-2 input lines. ISNOD must be given in increasing order from 2 to NSNOD-2. Only to be given if NSNOD>2.

ISNOD ITYPSN

  • ISNOD: integer: Supernode no = 2,3,….., NSNOD-1

  • ITYPSN: character(6): Type of supernode

    • TSNFIX - Fixed

    • TSNBRA - Branch point

    • TSNFRE - Free end

Specification of supernodes: Supernodes at lower and upper end are not to be specified. The supernode number at lower end is automatically set to 1 and the supernode type is fixed (ITYPSN=TSNFIX). The supernode at upper end is automatically set to NSNOD and the supernode type is specified position (ITYPSN=TSNPOS) indicating the upper end is connected to the support vessel. A possible additional anchor point is defined by specification of a supernode of type TSNFIX. The additional anchor line must be connected to the first branching point along the main riser.

1.7. Seafloor support conditions

To be given only if IBTANG=1, -1 or -9.

STFBOT STFAXI STFLAT FRIAXI FRILAT DAMBOT DAMAXI DAMLAT ILTOR

  • STFBOT: real > 0: Seafloor stiffness normal to the seafloor \([\mathrm {F/L^2}]\)

  • STFAXI: real >= 0, default: 0: In-plane seafloor stiffness for friction in axial direction \([\mathrm {F/L^2}]\)

  • STFLAT: real >= 0, default: 0: In-plane seafloor stiffness for friction in lateral direction \([\mathrm {F/L^2}]\)

  • FRIAXI: real >= 0, default: 0: In-plane seafloor friction coefficient in axial direction [1]

  • FRILAT: real >= 0, default: 0: In-plane seafloor friction coefficient in lateral direction [1]

  • DAMBOT: real >= 0, default: 0: seafloor damping coefficient normal to the seafloor \([\mathrm {F\times T/L^2}]\)

    • Dummy for IBTANG=-9

  • DAMAXI: real >= 0, default: 0: In-plane seafloor damping coefficient in axial direction \([\mathrm {F\times T/L^2}]\)

    • Dummy for IBTANG=-9

  • DAMLAT: real >= 0, default: 0: In-plane seafloor damping coefficient in lateral direction \([\mathrm {F\times T/L^2}]\)

    • Dummy for IBTANG=-9

  • ILTOR: integer, default: 0: Option for applying lateral contact forces at the external contact radius, giving a torsional moment

    • = 0: Lateral loads are applied at the node *

    • = 1: Lateral loads are applied at the external contact radius if it is specified for the associated beam cross-section.

    • Dummy for IBTANG=-9

STFBOT is used for computing the spring stiffness normal to the seafloor, \(\mathrm {k_V}\) , for seafloor contact. \(\mathrm {k_V}\) = STFBOT \(\mathrm {\times L}\) where \(\mathrm {L}\) is the element length.

Horizontal contact with the seafloor is modelled independently in the axial and lateral directions. Contact is initially modelled with linear springs. Sliding will occur when an axial or lateral spring force reaches the friction force value. Springs will be reinstated if the line starts sliding in the opposite direction, or if the friction force increases and is greater than the spring force. The spring stiffness is calculated as \(\mathrm {k_s}\) = STFxxx \(\mathrm {\times L_h}\), where \(\mathrm {L_h}\) is the length of the element’s horizontal projection. The seafloor friction forces are calculated as F = FRIxxx \(\mathrm {\times F_{vert}}\) and are directed against the axial or lateral displacements.

1.8. Support vessel reference

Identification and location of support vessel.

IVES IDWFTR XG YG ZG DIRX

  • IVES: integer, default: 1: Vessel number (IVES = 1)

  • IDWTFR: character (6), default: 'NONE': Identifier for WF motion transfer function

  • IDWFTR = 'NONE' means no transfer function specified

  • XG: real: X position of vessel coordinate system referred in global system \(\mathrm {[L]}\)

  • YG: real: Y position of vessel coordinate system referred in global system \(\mathrm {[L]}\)

  • ZG: real: Z position of vessel coordinate system referred in global system \(\mathrm {[L]}\)

  • DIRX: real`: Direction of vessel X-axis.

Next data group is Line and segment specification.

um ii fig27
Figure 2. Vertical spring stiffness for seafloor contact