VM-AQWA-MECH-004

VM-AQWA-MECH-004
Hydrodynamic Diffraction Analysis of a Barge with a Circular Moonpool

Overview

References:

Fukuda, K. (1977). Behavior of water in vertical well with bottom opening of ship, and its effects on ship-motion. Journal of the Society of Naval Architects of Japan, (141), 107-122.

Vijith, P., Viswanathan, S., & Panneerselvam, R. (2014). Moonpool effects on a floating body. International Journal of Design and Manufacturing Technology, 5(3), 23-28.

Analysis Type(s): Aqwa Hydrodynamic Diffraction

Test Case

A rectangular barge with a circular moonpool has the following characteristics: The barge is 180 m long, 60 m wide, and has a depth of 11 m. There is a vertical circular moonpool with a radius of 9.1667 m, and its vertical axis passes through the center of the barge.

This test case calculates the natural frequencies of the water motion inside the moonpool and the barge's rigid body motions in the waves. The following results are compared to the reference (Vijith et. al., 2014): The natural frequency of the piston motion of the water inside the moonpool, the barge heave motion RAOs at wave direction of 0 degrees, and the moonpool free surface piston pressure mode RAOs at wave direction of 0 degrees.

Figure 214: Problem Sketch of the Barge Model with a Circular Moonpool

Problem Sketch of the Barge Model with a Circular Moonpool

Principal Dimensions of the Model

Length180.0 m
Breadth60.0 m
Depth11.0 m
Draft7.0 m
Moonpool Radius9.167 m
Displacement73752 m3
KG[a]5.0 m
Kxx[b]20.4 m
Kyy[c]48.6 m
Kzz[d]48.6 m

[a] Height of C.G. above base line

[b] Radius of gyration of rolling motion

[c] Radius of gyration of pitching motion

[d] Radius of gyration of yawing motion

Additional Parameters

  • Moonpool free surface damping factor: demp = 0.0 ~ 0.2

  • Piston mode generalized hydrostatic stiffness: 0.2605E+07 N/m

  • Mesh size: 2.0 m

Results Comparison

Natural Frequency of the Piston Motion of Water in Moonpool

The figure below, generated by Aqwa, shows the piston mode added mass. The natural frequency, that is, where the singularity of the solution occurs, is 0.836 rad/s as estimated by the Aqwa model.

Figure 215: Moonpool Piston Mode Added Mass

Moonpool Piston Mode Added Mass

Natural Frequency (rad/s)
Empricial (Vijith et al., 2014)0.817
WAMIT (Vijith et al., 2014)0.838
Aqwa0.836

Heave RAOs

The Heave RAOs at the wave direction 0.0° are compared between the results by WAMIT (Vijith et al., 2014) and the Aqwa model.

Figure 216: Heave RAOs at 0° by WAMIT (Vijith et al., 2014)

Heave RAOs at 0° by WAMIT (Vijith et al., 2014)

Figure 217: Heave RAOs at 0° by Aqwa

Heave RAOs at 0° by Aqwa

Moonpool Free Surface Piston Pressure Mode RAOs

The piston pressure mode RAOs at the wave direction 0.0° are compared between the results by WAMIT (Vijith et al., 2014) and the Aqwa model.

Figure 218: Piston Mode RAOs at 0° by WAMIT (Vijith et al., 2014)

Piston Mode RAOs at 0° by WAMIT (Vijith et al., 2014)

Figure 219: Piston Mode RAOs 0° by Aqwa

Piston Mode RAOs 0° by Aqwa