1. Seafloor contact

The seafloor contact properties are relevant for riser systems with tubular cross sections, which are partly resting on the bottom. This may be the case for SB and AR systems.

1.1. Data group identifier, one input line

NEW COMPonent SEAFloor contact

1.2. Component type identifier and type

CMPTYP-ID CHSFCT

  • CMPTYP-ID: character(8): Component identifier

  • CHSFCT: character(4): Seafloor contact component type

    • = SPRI: Original RIFLEX seafloor springs normal to the seafloor and separate axial and lateral spring-friction contact in the seafloor plane.

    • = SOIL: Consolidated riser-soil interaction model

1.3. Original RIFLEX seafloor spring contact

The following three lines of input must be given if CHSFCT = SPRI

Seafloor normal contact parameters
STFBOT DAMBOT

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

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

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.

Seafloor in-plane contact parameters, two input lines
STFAXI FRIAXI DAMAXI

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

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

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

STFLAT FRILAT DAMLAT ILTOR

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

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

  • 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.

Contact in the seafloor plane 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.

1.4. Consolidated riser-soil seafloor contact

The following four lines of input must be given if CHSFCT = SOIL

The external contact radius R_EXTCNT must be positive for the segments that have consolidated riser-soil seafloor contact.

Seafloor soil properties
W A1 A2 V G

  • W: real > 0: Soil submerged weight \(\mathrm {[F/L^3]}\)

  • A1: real > 0: Soil shear strength at seabed \(\mathrm {[F/L^2]}\)

  • A2: real: Soil shear strength vertical gradient \(\mathrm {[F/L^3]}\)

  • V: real > 0: Soil Poisson ratio \(\mathrm {[1]}\)

  • G: real: Soil G-modulus/compressive strength \(\mathrm {[F/L^2]}\)

Consolidated riser-soil seafloor contact options
F ALPHA BETA KBC KT

  • F: real, default: 0.88: Relationship between dynamic stiffness and G-modulus \(\mathrm {[1]}\)

  • ALPHA: real, default: 1.0: Control parameter for suction release displacement \(\mathrm {[1]}\)

  • BETA: real, default: 1.0: Scaling factor for peak soil suction relative to peak soil compression \(\mathrm {[1]}\)

  • KBC: real, default: 0.05: Mobilization displacement for soil bearing capacity as fraction of pipe soil contact width \(\mathrm {[1]}\)

  • KT: real, default: 0.08: Mobilization displacement for max soil suction as fraction of pipe soil contact width \(\mathrm {[1]}\)

In-plane contact parameters, two input lines
STFAXI FRIAXI DAMAXI

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

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

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

STFLAT FRILAT DAMLAT

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

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

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

Contact in the seafloor plane 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.