General Line Data Specification

General line data enables the user to model simple as well as complex line systems. Loads due to gravity, buoyancy, current and seafloor contact are accounted for. Several line systems may be modelled. Each line system may consist of one or several lines connected to one or several bodies and/or to earth.

Each line system is described in a finite element formulation. The quasi-static system responses are calculated at each time step during time domain analysis.

1. Line system

A line system is identified by a unique line-system identifier. The line system topology is in general described in terms of branching points and terminal points. These points are denoted line nodes. Line nodes are connected by simple lines. This means that the line system topology is uniquely determined by the connectivity between a number of defined line nodes and lines.

2. Line node

Line nodes are classified as free, fixed, or prescribed, depending on their boundary condition specification. Line nodes of type fixed are used for modelling termination at fixed structures, seafloor connection, etc. Prescribed line nodes are used for modelling supports with prescribed motions, i.e. connection to bodies.

A line node is identified by a unique line-node-identifier. This means that a line node may be referred several times in a system topology description, and also referred to by several line systems.

3. Line and segment

A line is a structural element between two line nodes. Its composition is described in a line type specification. The line type is specified in terms of a sequence of segments and nodal components. A segment has homogeneous cross section properties. For each segment a cross section type and a number of elements to be used for the finite element discretization, are specified, see the figure below. Nodal components (clump weights and buoys) may be specified at segment intersections.

The line is identified by a unique line-identifier. The line type is identified by a unique line-type-identifier. This means that a line type may be referred several times in the topology description of one or several line systems.

4. Cross section and nodal component

The cross section and the nodal component represent the elementary description of the mechanical properties. Cross section properties are given by axial stiffness, weight (in air and submerged) and drag coefficients (transverse and longitudinal). The vertical force and drag coefficient of nodal components may be depth dependent.

A cross section is identified by a unique cross-section-identifier, and a nodal component by a nodal-component-identifier. Thus a cross section type or a nodal component type may be referred several times in the description of line types.

app a image037
Figure 1. System definition terms

1 input line

GENEral LINE DATA

If the data shall be exported to HLA, 1 identification input line.

HLA EXPOrt

If HLA EXPOrt, HLA name of general line system.

CHGHLA

  • CHGHLA: character(120): Character string

In case of more than one line system, the following data groups have to be repeated: - Line System Definition - Topology specification - Seafloor support condition (optional)

5. Line system definition

1 input line

LINE SYSTem DEFInition

System identification, 1 input line

LINE-SYSTEM-ID

  • LINE-SYSTEM-ID: character(8): Line system identifier

Topology specification

1 input line

LINE TOPOlogy DATA

Topology, as many input lines as necessary (1 input line per simple line in the system)

LINE-ID  LINE-TYPE-ID  LINE-NODE-ID-1  LINE-NODE-ID-2

  • LINE-ID: character(8): Line identifier

  • LINE-TYPE-ID: character(8): Line type identifier

  • LINE-NODE-ID 1: character(8): Line node 1 (from end 1) identifier

  • LINE-NODE-ID 2: character(8): Line node 2 (to end 2) identifier

Seafloor support condition (optional)

The bottom is given as one non-curved plane that may be oriented inclined to the water surface. The contact forces between the bottom and a line is given by vertical bilinear springs.

1 input line

BOTTom CONTact DATA

1 input line

XB  YB  ZB  XN  YN  ZN  BOTSTIF  ZBLOAD

  • XB: real: X coordinates of point on seafloor

  • YB: real: Y coordinates of point on seafloor

  • ZB: real: Z coordinates of point on seafloor

  • XN: real: X components of the normal vector at the seafloor point

  • YN: real: Y components of the normal vector at the seafloor point

  • ZN: real: Z components of the normal vector at the seafloor point

  • BOTSTIF: real: Spring stiffness (normal to seafloor) \(\mathrm {[F/L]}\)

  • ZBLOAD: real: Minimum distance from seafloor plane for distributed element load formulation, i.e. elements attached to a node with a distance less than ZBLOAD from bottom plane will be given a lumped load formulation, see the figure below.

load formulations
Figure 2. Load formulations

Advanced analysis option (optional)

1 input line

ADVAnced ANALysis OPTIon

1 input line

LRELV  MET_S  MAX_S  MIN_S  MET_D  MAX_D  MIN_D  TOLINC  TOLNOR  MAXIT

  • LRELV: integer, default: 0: Control parameter for relative velocity

    • Dummy in present version

    • = 0: Drag force caused by current action only

    • = 1: Drag force caused by relative velocity between current velocity and structural velocity

  • MET_S: integer, default: 1: Static analysis (STAMOD)

    • Incrementation control parameter

    • = 1: Constant incrementation

    • = 2: Variable incrementation

  • MAX_S: integer, default: 100:

    • if MET_S = 1: Number of incrementation steps

    • if MET_S = 2: Maximum incrementation steps

  • MIN_S: integer, default: 5:

    • if MET_S = 1: Dummy

    • if MET_S = 2: Minimum incrementation steps

  • MET_D: integer, default: 2: Dynamic analysis (DYNMOD)

    • Incrementation control parameter; dynamic analysis

    • = 1: Constant incrementation

    • = 2: Variable incrementation

  • MAX_D: integer, default: 2:

    • if MET_D = 1: Number of incrementation steps

    • if MET_D = 2: Maximum incrementation steps

  • MIN_D: integer, default: 1:

    • if MET_D = 1: Dummy

    • if MET_D = 2: Minimum incrementation steps

  • TOLINC: real, default: 10\(\mathrm {^{-3}}\): Displacement norm for termination of global equilibrium iteration; Used during incrementation

  • TOLNOR: real, default: 10\(\mathrm {^{-4}}\): Displacement norm for termination of global equilibrium iteration; Used for last incrementation step

  • MAXIT: integer, default: 100: Maximum number of iterations

6. Boundary conditions and coordinates for line nodes

Data group identifier, 1 input line

LINE NODE DEFInition

Boundary condition and coordinates, 2 input lines per line node.

Line node identifier and type

 LINE-NODE-ID NODE-TYPE

  • LINE-NODE-ID: character(8): line node identifier

  • NODE-TYPE: character: type of boundary

    • = FIXEd (Earth-fixed)

    • = FREE

    • = BODY (Attached to body component)

IF NODE-TYPE = FIXED or FREE: Node coordinates and reference system

REF-SYSTEM  X  Y  Z  BODY-ID

  • REF-SYSTEM: character: reference system

    • = LOCAL

    • = GLOBAL

  • X: real: Initial coordinates for fixed and free line nodes

  • Y: real: Initial coordinates for fixed and free line nodes

  • Z: real: Initial coordinates for fixed and free line nodes

  • BODY-ID: character(8):

    • REF-SYSTEM = LOCAL: Reference to body ID for local reference system

    • REF-SYSTEM = GLOBAL: Dummy

IF NODE-TYPE = BODY: Body component reference

BODY-COMP-ID

  • BODY-COMP-ID: character(8): Reference to body component ID for which the line node is attached.

7. Line type definition

A line type is labeled with a unique identifier. The line is described by segments and nodal components in a sequence from line node 1 to line node 2, ref data group Topology specification. The line must consist of minimum one segment. Nodal components (buoys/clump weights) may be inserted at segment ends.

Data group identifier, 1 input line

LINE TYPE DEFInition

Line type identifier, 1 input line

LINE-TYPE-ID

Line part specification. Number of input lines as many as necessary

LINE-PART-TYPE  CROSS-ID / NODAL-COMP-ID  SLENGTH  NELSEG

  • LINE-PART-TYPE: character(8):

    • = SEGMENT

    • = NODAL . LINE-PART-TYPE = SEGMENT

  • CROSS-ID: character(8): Cross section identifier

  • SLENGTH: real: Segment length \(\mathrm {[L]}\)

  • NELSEG: integer: Number of elements for FEM analysis . LINE-PART-TYPE = NODAL

  • NODAL-COMP-ID: character(8): Nodal component identifier

8. Cross section description

The cross section properties data below has the same definition as the corresponding line characteristics data given in section Catenary anchor lines.

Data group identifier, 1 input line

CROSs SECTion DEFInition

Cross section identifier and properties, 1 input line

CROSS-ID  DIAMETER  EMOD  EMFACT  UWIA  WATFAC  CDN  CDT

  • CROSS-ID: character(8): Cross section identifier

  • DIAMETER: real: Cross section diameter used for axial stiffness and hydrodynamic forces \(\mathrm {[L]}\)

  • EMOD: real: Modulus of elasticity \(\mathrm {[F/L^2]}\)

  • EMFACT: real: Factor of elasticity

  • UWIA: real: Unit weight in air \(\mathrm {[F/L]}\)

  • WATFAC: real: The ratio between weight in water to weight in air

  • CDN: real: Transverse drag coefficient

  • CDL: real: Longitudinal drag coefficient

The axial stiffness is calculated by:

\[EA=EMOD\times EMFACT\times DIAMETER^2\times \pi /4\]

Drag force per unit length by:

  • \(\mathrm{F^D_N = 0.5 \times RHOW \times CDN \times DIAMETER \times V^2_N} \quad \quad\) Normal direction

  • \(\mathrm{F^D_I = 0.5 \times RHOW \times CDL \times DIAMETER \times V^2_L} \quad \quad\) Longitudinal direction

9. Nodal component description

Data group identifier, 1 input line

NODAl COMPonent DEFInition

Identifier and number of points in force and drag coefficient table, 1 input line

NODAL-COMP-ID  NFZ

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

  • NFZ: integer >= 1: Number of points in the force/drag force coefficient versus vertical position table

Vertical position and corresponding vertical load and drag force coefficient, NFZ input lines.

This feature is meant for modelling e.g. buoyancy modules that may float to the surface.

 Z(i) Fz(i) CDFz(i)

  • Z: real: Vertical position (global reference system). Dummy if NFZ=1 \(\mathrm {[L]}\)

  • Fz: real: Corresponding vertical force \(\mathrm {[F]}\)

  • CDFz: real: Corresponding drag force coefficient \(\mathrm {[FT^2/L^2]}\)

Interpolation within table. Constant values outside the table