VM-AQWA-MECH-003

VM-AQWA-MECH-003
Motions of Adjacent Floating Structures in Oblique Waves

Overview

References:	Kodan, N. (1984). The motions of adjacent floating structures in oblique waves. Proceedings of theThird International Offshore Mechanics and Artic Engineering Symposium. New Orleans. 1(1), 206-213.
Analysis Type(s):	Rigid Body Dynamics

Test Case

Two parallel, slender structures– a barge and a boat– are in oblique waves. Model these objects in Ansys Aqwa and analyze the hydrodynamic interaction between them. Compare the results to those in the reference (Kodan, 1984).

Figure 206: Problem Sketch- A

Figure 207: Problem Sketch- B

Principal Dimensions of the Model

	Barge Model	Ship Model
Length	3.125 m	2.088 m
Breadth	0.600 m	0.369 m
Draft	0.113 m	0.131 m
Displacement	0.203 m³	0.081 m³
Water Plane Area	1.875 m²	0.685 m²
GMT^[a]	0.223 m	0.074 m
GML^[b]	7.184 m	2.523 m
KG^[c]	0.106 m	0.080 m
K_xx^[d]	0.118 m	0.097 m
K_yy^[e]	0.751 m	0.506 m
K_zz^[f]	0.760 m	0.480 m

^[a]Transverse Metacentric Height above Base Line

^[b]Longitudinal Metacentric Height above Base Line

^[c]Height of C.G. above Base Line

^[d]Radius of Gyration of Rolling Motion

^[e]Radius of Gyration of Pitching Motion

^[f]Radius of Gyration of Yawing Motion

Modeling Notes

In order for the Aqwa program to run properly, the Kodan's experimental models have been enlarged by a factor of a hundred. The new dimensions are shown in the table below.

	Aqwa Model Barge	Aqwa Model Ship
Length, L_S	312.5 m	208.7 m
Draft, d_R	11.3 m	13.10 m
Displacement	2.03e5 m³	8.1e4
Water plane area, A_WR	1.875e4 m²	6.85e3
GMT	22.8 m	7.7 m
GML	746.6 m	251.5 m
COG above base line	10.60 m	8.0 m
P_L	1.20 m	120.0 m

Additional Parameters

Lid Damping Factor: damp = 0.01
Gap for external lid: gap = 71.55 m

Figure 208: Aqwa Model

Additional Key parameters:

Central distance between the barge and ship model: P_L = 1.2 m
Forward speed of the ship model: Fr = 0
Wave direction: 𝛘=45 degrees and -45 degrees

Results Comparison

The figures below compare the results generated by Aqwa with the experimental results in the reference. As you will see, Aqwa's results track well with the experimental data. (Note: Experimental results of the wave exciting forces/moment and wave frequency have been dimensionalized.)

Dots in the graphs represent experimental data from the reference. Blue curves show the results when wave direction is 45 degrees, while the orange/yellow curves show the results for wave direction equalling -45 degrees.

Amplitudes of Wave Exciting Forces/Moment

Wave exciting force/moment (diffraction and F-K force) in sway, heave, roll direction (Global Y, Z, RX components) along the wave frequency

Figure 209: Force-Sway

Figure 210: Force-Heave

Figure 211: Moment Roll

RAO

RAO (Distance/Rotation) in sway and heave direction (Global Y, Z components) along wave frequency

Figure 212: RAO-Sway

Figure 213: RAO-Heave