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*Floating Body

Purpose

To define a floating body and its associated properties.

Theory

Refer to Floating Body for further information on this feature.

Keyword Format

Block of lines consisting of floating body name followed by data specified in blocks, with each block beginning with a TYPE=line defining the properties of the floating body. The whole block is then repeated for different floating bodies.

Line to define floating body name:

FLOATING BODY=Floating Body Name

Block of data defining the floating body inertia:

TYPE=MASS

M11, I44, I55, I66

Block of data defining the floating body hydrostatic stiffness:

TYPE=STIFFNESS

K33, K44, K55, [K34], [K35], [K45], [K46], [K56]

Block of data defining the floating body geometric properties:

TYPE=GEOM

AL, AT, Lfore, Laft, T, B

Block of data defining nodes at critical locations on the floating body, along with its initial undisplaced orientation:

TYPE=NODE

Centre of Gravity, Body Orientation, Centre of Buoyancy, Centre of Wave Forces, Centre of Viscous Drag [, Centre of Added Mass]

Block of data defining the floating body reference and cut-off frequencies:

TYPE=FREQUENCY

Reference, Cut-off 

Block of data defining parameters which control floating body retardation functions:

TYPE=RETARDATION

Time Length, Time Step, Frequency Increments, Scale Disp., Scale Rot.

where:  

•M11, M22, M33        = Mass

•I44, I55, I66                = Roll, pitch and yaw inertia

•K33, K44, K55        = Heave, roll and pitch stiffness

•K34, K35                = Heave-roll and heave-pitch stiffness

•K45, K46                = Roll-pitch and roll-yaw stiffness

•K56                        = Yaw-pitch stiffness

•AL, AT                = Longitudinal and transverse areas exposed to wind action

•Lfore, Laft                = Length from centre of gravity to forward and aft perpendiculars

•T                        = Draft

•B                        = Beam

•Centre of Gravity        = CoG node (number or label). This is the point of application for the floating body structural mass.

•Body Orientation        = Initial/undisplaced heading of floating body (in degrees), measured anticlockwise from the global Y-axis.

•Centre of Buoyancy        = CoB node (number or label). This is the point of application for hydrostatic stiffness.

•Centre of Wave Forces = Primary force node (number or label). This is the point of application for first order forces (force RAOs, radiation damping), and second order forces (hydrodynamic, wind, current, drift).

•Centre of Viscous Drag = CoD node (number or label). This is the point of application for viscous drag forces.

•Centre of Added Mass = CoAM node (number or label). This is the point of application for the floating body added mass. CoAM node defaults to the wave force node if not explicitly specified.

•Scale Disp.                = Scale factor for output of displacements.

•Scale Rot.                = Scale factor for output of rotations.

The order in which the TYPE= statement appears is not significant, as long as the relevant data is given under each statement. The initial draft of the floating body should be at the level where the weight and buoyancy are in equilibrium. If you specify a node label rather than a node number, it must be enclosed in {} brackets.

Define

Table Input

Notes

(a)Any properties which you refer to for a particular floating body must be defined in other tables, such as Body - Hydrostatic Stiffness, Body - Geometry, etc.

Body - Frequency-Independent Inertia

Table Input

Body - Hydrostatic Stiffness

Table Input

Notes

(a) The coupling stiffness terms Heave-Roll, Heave-Pitch and Roll-Pitch are mutually symmetric, i.e. K34 = K43, K35 = K53, K45 = K54.

(b) The coupling stiffness terms Yaw-Roll (K46) and Yaw-Pitch (K56) are not mutually symmetric, i.e. K64 and K65 are always zero.

Body - Geometry

Table Input

Notes

(a)The two area inputs and the two length inputs are used together to compute the wind loading on the floating body.

(b)The two length inputs together with the Draft and Beam values are used to compute the current loading on the floating body.

(c)The initial draft of the floating body should be at the level where the weight and buoyancy of the floating body are in equilibrium (the body is floating). This is implicitly assumed by Flexcom, so that changes in the buoyancy loads, due to relative floating body/surface elevation changes, are accounted for by the buoyancy terms in the hydrostatic stiffness matrix.

Body - Nodal Locations

Table Input

Body - Frequency

Table Input

Notes

(a)The specified Reference Frequency is used in determining the added mass of the floating body. Refer to Wave Radiation Loads for further information on the convolution integral technique employed by Flexcom to model frequency dependent added mass and radiation damping terms in a time domain simulation. This input is relevant to time domain analyses only.

(b)The specified Cut-off Frequency is used to distinguish low and wave frequency regimes. Frequency-dependent added mass and damping apply in the high frequency regime, while low frequency added mass and damping, taken at the cut-off frequency, apply below the cut-off frequency value.

(c)If a Cut-off Frequency value is specified for a particular floating body, then added mass and damping must also be defined for that same floating body at the cut-off frequency in the Added Mass Cut-off and Radiation Damping Cut-off tables, respectively.

Body - Retardation

Table Input

Notes

(a)The retardation functions for a floating body are evaluated over a specified Time Length at regular intervals of Time Increment. If a time length is not specified, it defaults to 100s (or the total simulation time, if this is less than 100s). The time increment defaults to the analysis time step in the case of fixed time step analyses, and the minimum time step for variable time step analyses. At each time step, the value of each retardation function is computed by integrating the appropriate frequency dependent radiation damping coefficients with respect to frequency using the specified No. Of Frequency Increments. If a number of frequency increments is not specified, a default value of 200 is used by the program. The background and theory behind the time domain coupled analysis procedure is described in detail in Wave Radiation Loads.

(b)Flexcom provides a facility whereby the computed retardation functions may be examined and verified by the user. Specifically, the computed functions are echoed to an ACSII file, entitled ‘Ret_Fn_I.dat’, where I is an integer value indicating the number of the relevant floating body. The output displacement terms are scaled by the specified Displacement Factor, the rotation terms by the Rotation Factor, and the coupled displacement/rotation terms by an average of the specified factors. Both the displacement and rotation factors default to 1 if unspecified. The layout of the output file is as follows:

Time, R11, …, R22, …, R33, …, R44, …, R55, …, R66

where:

•Time is the time value in seconds

•R11 is the heave retardation function value

•R22 is the surge retardation function value

•R33 is the sway retardation function value

•R44 is the yaw retardation function value

•R55 is the roll retardation function value

•R66 is the pitch retardation function value

and the intermediate terms (e.g. R12, R13, R23 etc.) represent the coupled retardation function terms between various degrees of freedom at the relevant time.