On the probability of occurrence of rogue waves

A number of extreme and rogue wave studies have been conducted theoretically, numerically, experimentally and based on field data in the last years, which have significantly advanced our knowledge of ocean waves. So far, however, consensus on the probability of occurrence of rogue waves has not been achieved. The present investigation is addressing this topic from the perspective of design needs. Probability of occurrence of extreme and rogue wave crests in deep water is here discussed based on higher order time simulations, experiments and hindcast data. Focus is given to occurrence of rogue waves in high sea states

Determination of Correlation for Extreme Metocean Variables

Metocean environmental load includes wind, wave and currents. Offshore structures are designed for two environmental load design conditions i.e. extreme and operational load conditions of environmental loads are evaluated. The ccorrelation between load variables using Joint probability distribution, Pearson correlation coefficient and Spearman’s rank correlation coefficients methods in Peninsular Malaysia (PM), Sabah and Sarawak are computed. Joint probability distribution method is considered as a reliable method among three different methods to determine the relationship between load variables. The PM has good correlation between the wind-wave and wave-current; Sabah has both strong relationships of wind-wave and wind-current with 50 year return period; Sarawak has good correlation between wind and current in both 50 years and 100 years return period. Since Sabah has good correlation between the associated load variables, no matter in 50 years or 100 years of return period of load combination. Thus, method 1 of ISO 19901-1, specimen provides guideline for metocean loading conditions, can be adopted for design for offshore structure in Sabah. However, due to weak correlations in PM and Sarawak, this method cannot be applied and method 2, which is current practice in offshore industry, should continueto be used

Freak Wave Events Within the Second Order Wave Model,”

ABSTRACT Recently significant interest has been paid to abnormal waves, often called rogue waves or freak waves. These waves represent operational risks to ship and offshore structures, and are likely to be responsible for a number of accidents. As shown by several authors, in 'the second order world' the freak waves are pretty rare events. The present study focuses on statistical properties of freak waves. The analyses are based on second order time domain simulations, short term distributions for crest statistics obtained from the literature, and long term field data. Time series of wave elevations are generated using the Pierson-Moskowitz, JONSWAP and two-peak Torsethaugen frequency spectrum for long-crested seas and deep water. Effects of combined seas (swell and wind sea) on wave statistics are discussed. Assuming 2 nd order wave theory, the short term and long term probability of occurrence of a freak wave is estimated. The difference between a "freak wave" and a "dangerous wave" is pointed out. Finally, 100 year and 10000 year crest events obtained by analysis procedures used in the offshore industry are discussed in relation to freak waves. INTRODUCTION Over the last decades several studies have been reported concerning unexpectedly large and/or steep waves. Such waves have been denoted freak waves or rogue waves, giant waves, abnormal waves, the "one from nowhere". These waves are expected to represent design and operational risks to ship and offshore structures. Although the existence of freak waves themselves has been generally not questioned, neither the occurrence of these waves nor their physical structure was well understood. Particularly the ongoing EU research project

DIRECTIONAL SPREADING IN TWO-PEAK SPECTRUM AT THE NORWEGIAN CONTINENTAL SHELF

ABSTRACT In practical applications, it is usually assumed that the wave spectrum is of a single mode form, and well modeled by a JONSWAP or Pierson-Moskowitz spectrum. This assumption is of a reasonable accuracy for severe sea states. However, moderated and low sea states are often of a combined nature, consisting of both wind-sea and swell and should be characterized by a double peak spectrum. The present paper discusses two-peak spectra observed in Norwegian waters, which are particularly affected by swell. The analysis is based on 5-year of directional data from Haltenbanken. A directional distribution for swell is suggested. Further, a procedure for including directional spreading in two-peak spectra is proposed. The method is illustrated for the two-peak Torsethaugen frequency spectrum INTRODUCTION The Pierson-Moskowitz and JONSWAP spectra are commonly used for engineering applications. Both spectra are of a single mode nature, characterizing pure wind-sea conditions. Several studies document that these spectra are of a reasonable accuracy for severe sea states. However, moderated and low sea states are often of a combined nature, consisting of both wind-sea and swell and should be characterized by a double peak spectrum

Ship and Offshore Structure Design in Climate Change Perspective

This book summarizes results of longstanding research and scientific contributions from many projects and relevant working groups. It collects and evaluates wind and wave climate projections under changing climate having design needs and marine safety in focus. Potential impact of projected climate change in met-ocean conditions on ships and offshore structures is discussed and illustrated by an example of the expected wave climate change on tanker design

A novel approach for navigational guidance of ships using onboard monitoring systems

A novel approach and conceptual ideas are outlined for risk-based navigational guidance of ships using decision support systems in combination with onboard, in-service monitoring systems. The guidance has as the main objective to advise on speed and/or course changes; in particular with focus on ship operations in rough weather. It is strived for to make use of a probabilistic framework considering the mathematical procedures that the guidance relies upon. The paper presents a novel concept which has the possibility to increase the reliability of the provided guidance, although information about on-site sea state parameters not necessarily is in complete agreement with the unknown and true wave parameters, nor may the hydrodynamical models of the vessel give a perfect quantitatively description of the vessel in waves. The paper includes an analysis of full-scale motion measurements and the proposed concept for navigational guidance gives promising results