General information

MoorMotions is a time-domain code for calculating wave-induced motions of moored ships, together with mooring line and fender loads. It is developed by Dr Tim Gourlay at Perth Hydro and used for our ship hydrodynamics consulting.

Inputs

• First-order ship wave loads from WAMIT
• Second-order ship wave loads from WAMIT
• Ship impulse response functions (time-domain added mass and damping) from WAMIT
• Mooring line geometry
• Mooring line tension/extension curves
• Fender reaction/compression curves
• Fender friction coefficients
• Mooring line and fender energy dissipation ratios

Standard outputs

• Time-domain ship surge, sway and yaw
• Time-domain tension in all mooring lines
• Time-domain compression of all fenders
• Comparison of all outputs with PIANC/OCIMF criteria

Vertical motion outputs

Ship heave, pitch and roll are normally treated as linear and modelled in the frequency domain for greater efficiency. Nonlinear heave, pitch and roll can be modelled in MoorMotions along with surge, sway and yaw.

Method

MoorMotions uses a time-stepping method to calculate accelerations, velocities and displacements using a fourth-order Runge-Kutta solver. It is written in MATLAB.

Validation

Wave loads validation - Side-by-side ships

MoorMotions validation - Open berth

MoorMotions validation - Geraldton harbour

History

In 2015, full-scale trials were undertaken to measure wave-induced motions of inbound and outbound bulk carriers at the Port of Geraldton. The GNSS equipment was left recording while the ships were loading. After the motions in the channel were analyzed, a side project began to analyze the ship motions at the berth.

As it turned out, the ship motions at the berth were as interesting as the ship motions in the channel. The code MoorMotions was written to convert measured ship surge, sway and yaw into mooring line and fender loads.

The code was extended to take ship wave loads as input for open berths. For complex harbours, a coupled ship-and-harbour model was developed in WAMIT to provide wave loads. For side-by-side or tandem motions, the MoorMotions code was extended to take 12-DoF outputs from WAMIT.