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 twodimensional in XZ 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.

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
1.3. 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 belowZ < ZBOT + R_EXTCNT
. The modified 3D seafloor formulation is used. Friction contribution to torsional loading is possible.

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 (=NSNOD1)
input lines
LINEID LINTYPID ISNOD1 ISNOD2

LINEID: character(8)
: Line identifier 
LINTYPID: 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 LINTYPID
.
LINTYPID ISNOD1 ISNOD2
The LINEID
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
: Zcoordinate of lower end \(\mathrm {[L]}\)
ZL will also be used as Zcoordinate of seafloor if
IBTANG>0


XU: real > 0
: Xcoordinate of upper end \(\mathrm {[L]}\) 
ZU: real
: Zcoordinate 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
: Zcoordinate of anchor point \(\mathrm {[L]}\)
Dummy if no anchor point is specified


XA: real
: Xcoordinate of anchor point \(\mathrm {[L]}\)
Required input when static
FEM
analysis is applied. The Xlocation of the anchor point is automatically computed so that the anchor line is vertical if theCAT
orCATFEM
analysis is used inSTAMOD
(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 Yaxis, ALFL
/ALFU
will be dummy.
1.6. Supernode types
NSNOD2
input lines. ISNOD
must be given in increasing order from 2
to NSNOD2
. Only to be given if NSNOD>2
.
ISNOD ITYPSN

ISNOD: integer
: Supernode no = 2,3,….., NSNOD1 
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.
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
: Inplane seafloor stiffness for friction in axial direction \([\mathrm {F/L^2}]\) 
STFLAT: real >= 0, default: 0
: Inplane seafloor stiffness for friction in lateral direction \([\mathrm {F/L^2}]\) 
FRIAXI: real >= 0, default: 0
: Inplane seafloor friction coefficient in axial direction [1] 
FRILAT: real >= 0, default: 0
: Inplane 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
: Inplane seafloor damping coefficient in axial direction \([\mathrm {F\times T/L^2}]\) 
DAMLAT: real >= 0, default: 0
: Inplane 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 crosssection.

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 \(\mathrm {[L]}\)
Confer Data Group E: Support Vessel Data. 


DIRX: real
: Direction of vessel Xaxis.
Next data group is Line and segment specification.