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Publication Title | Extreme waves Rogue waves Ship design Ship losses Sinking Risk

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10th International Symposium on Practical Design of Ships and Other Floating Structures Houston, Texas, United States of America

© 2007 American Bureau of Shipping

Abstract

Recent research has demonstrated that extreme waves, waves with crest to trough heights of 20 to 30 meters, occur more frequently than previously thought. Also, over the past several decades, a surprising number of large commercial vessels have been lost in incidents involving extreme waves. Many of the victims were bulk carriers. Current design criteria generally consider significant wave heights less than 11 meters (36 feet). Based on what is known today, this criterion is inadequate and consideration should be given to designing for significant wave heights of 20 meters (65 feet), meanwhile recognizing that waves 30 meters (98 feet) high are not out of the question. The dynamic force of wave impacts should also be included in the structural analysis of the vessel, hatch covers and other vulnerable areas (as opposed to relying on static or quasi-dynamic analyses).

Keywords

Extreme waves; Rogue waves; Ship design; Ship losses; Sinking; Risk.

Nomenclature

CSR, Common structural rules ft, foot, feet (0.305 m)

grt, Gross register ton

Hext, Extreme wave height, m HS, Significant wave height, m HTS, high strength steel

HY, high yield strength steel

IACS, International Association of Classification Societies

m, meter

N, Newton 2

Pa, Pascal (N/m )

psf, pounds force per square foot

psi, pounds force per square inch

SSC, Ship Structure committee

Introduction

Recent research by the European Community has demonstrated that extreme waves—waves with crest to trough heights of 20 to 30 meters—occur more frequently than previously thought (MaxWave Project, 2003). In addition, over the past several decades, a surprising number of large commercial vessels have been lost in incidents involving extreme waves. Many of the victims were bulk carriers that broke up so quickly that they sank before a distress message could be sent or the crew could be rescued.

There also have been a number of widely publicized events where passenger liners encountered large waves (20 meters or higher) that caused damage, injured passengers and crew members, but did not lead to loss of the vessel. This is not a new phenomenon; there are well-documented events dating back to at least the early 1940s.

These two facts, vessel losses combined with knowledge that waves larger than previously considered likely may be encountered, suggest that reviewing vessel design criteria may be necessary. (Smith, 2006).

Ocean Wave Environment

Marine weather forecasts report the significant wave height (HS), which is defined as the average of the highest one-third of the wave heights. A working definition for an extreme wave is one with a height greater than 2.3 times the significant wave height. In mathematical terms, this is:

Hext = 2.3 x HS (1)

Such waves are often referred to as rogue waves or freak waves, as their height lies at the extreme of what would be expected for a Rayleigh distribution of wave heights. Based on observations made by ship’s crews and on limited data from offshore platform measurements and satellite observations, these waves are asymmetrical and have unusually steep faces. They may be preceded or followed by a deep trough.

Extreme Waves and Ship Design

Craig B. Smith

Dockside Consultants, Inc. Balboa, California, USA

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