1. Seafloor support conditions

Seafloor contact specification
IBTANG ZBOT IBOT3D

  • 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 = 3: Seafloor contact elements will be added according to the specification given in the data group SEAFLOOR CONTACT SPECIFICATION.

  • ZBOT: real: Z-coordinate of seafloor (ZBOT < 0). \(\mathrm {[L]}\)

    • Dummy variable if IBTANG = 0 or IBOT3D = 1.

  • IBOT3D: integer, default: 0: Code for 3D bottom

    • IBOT3D = 0: flat bottom at depth ZBOT

    • IBOT3D = 1: 3D topology, file to be specified in input to STAMOD

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.

Seafloor stiffness, friction and damping

The following input line must only be given if IBTANG=1

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 \([\mathrm {F\times T/L^2}]\)

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

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

  • 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.

STFBOT is used for computing the vertical spring stiffness, \(\mathrm {k_V}\) , 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 \(k_h=\mathrm {Stalks}\times L_h\), where \(\mathrm {L_h}\) is the length of the element’s horizontal projection. The seafloor friction forces are calculated as \(F=\mathrm {FRIxxx}\times F_{vert}\) and are directed against the axial or lateral displacements, where \(\mathrm {F_{vert}}\) is the vertical contact force between the pipe and the bottom.