OMC International (OMC) is considered the world authority on mooring analysis and design. Extensive experience and research have lead to the development of the Simulation Package for the Motion of Ships (SPMS), a numerical model for determining vessel motions in actual met-ocean conditions.
The SPMS numerical model has been used to analyse motions, line tensions and fender forces of bulk carriers, oil tankers, container vessels, frigates, submarines, Ro-Ro vessels and tugs at a wide variety of berths and deepwater moorings throughout the world. These studies have included spread moorings, swamp moorings, fendered jetties, single point moorings and Cavotec MoorMaster™ units. The model has also been applied for the analysis of the relative motions of vessels tethered for transhipment.
The SPMS numerical model includes the capability to model the slow drift oscillations of moored vessels due to long waves (periods greater than 30 seconds) and hydrodynamic interaction effects caused by passing ships, as well as the wave frequency oscillations caused by sea and swell waves. Long period waves can be the most important driving force on moored vessels exposed to ocean swell or passing ships because they can excite large amplitude, low frequency oscillations of the moored vessel in the horizontal modes of surge, sway and yaw. These oscillations occur at frequencies similar to the corresponding natural frequencies of the ship/line/fender system, leading to possible resonance amplification of the moored vessel motions, breakage of mooring lines and damage to berth and fenders - unlike wave frequency motions which are relatively highly damped, low frequency motions are only lightly damped, hence the great danger of resonance amplification arising from any significant long wave excitation.
In addition to full six degrees-of-freedom modelling of dynamic motions, the SPMS model includes the capability of modelling quasi-static loadings due to wind, current and tug forces. The effects of fender friction forces and nonlinear mooring line and fender load-response characteristics are included in all time domain simulations.
Major calibration and validation exercises for the SPMS moored ship model have been completed during 1998/99 for two vessels at Port Taranaki on the west coast of New Zealand, during 2005 for two vessels at Geraldton and in 2006 for three vessels passing a moored vessel at Port Hedland, Western Australia.
These exercises involved DGPS measurement of moored ship motions, direct measurement of line tensions by strain gauges and collection of short and long wave data at the berth. An important outcome of these studies was the full-scale calibration and validation of the low frequency damping forces generated by the SPMS model.
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