Lloyds List – 15Â August 2008
ALTHOUGH the phenomenon of parametric roll was first recognised many years ago, the increasing size of containerships and stack heights on deck, and the nature of modern, capacity-optimised designs has placed the issue back in the spotlight.
Parametric roll is of concern to various types of vessel, but its effects seem to have particularly worrying consequences for boxships.
Initiatives to help reduce the likelihood and onset of such motion have been spurred by cases involving large-scale damage to weatherdeck-stacked boxes and the loss of units overboard. ABS has been quick to address the situation through extensive research, the preparation of a design support guide, and the development of a class notation specific to parametric roll.
â€œParametric roll is one of those destructive phenomena that depend on a set of coincidences so finely balanced as to make the occurrence seem nearly impossible,â€ said ABS in a recent issue of its Activities newsletter. â€œThe shipâ€™s geometry has to have certain characteristics; its length has to be comparable to the wavelength of the sea conditions through which it is passing; and its speed must bear a certain relationship to both the wavelength and the vesselâ€™s natural rolling frequency,â€ said the society.
In what is believed to be an industry first, three new containerships operated by Hyundai Merchant Marine have been awarded the special, optional notation known as PARR C1. This has been conferred on the 4,700 teu Hyundai Forward and the 8,600 teu vessels Hyundai Faith and Hyundai Force, built by Hyundai Samho Heavy Industries.
The notation was issued against criteria contained in the ABS Guide for the assessment of parametric roll resonance in the design of container carriers, which provides analysis measures to determine if a particular vessel is vulnerable to parametric roll and the potential magnitude of the roll motions.
The society claims that this amounted to the first class criteria addressing the subject to have been firmly based on ship motion analysis supported by extensive simulations.
Research was jointly conducted by ABS and Hyundai Samho Heavy Industries.
By applying the criteria, designers can determine if a ship may be vulnerable to parametric roll in worst case scenarios.Â