Assessment of Significant Wave Height – Peak Period Distribution Considering the Wave Steepness Limit

2013 ◽  
Author(s):  
Michele Drago ◽  
Giancarlo Giovanetti ◽  
Claudia Pizzigalli
Keyword(s):  
Normal Distribution ◽  
Wave Height ◽  
Significant Wave Height ◽  
Wave Steepness ◽  
Peak Period ◽  
Near Surface ◽  
Significant Wave ◽  
Physical Limit ◽  
Log Normal ◽  
Log Normal Distribution

The physical limit of the significant wave steepness is generally exceeded when assessing the seastate climate and the extreme iso-probability contours, i.e. too short significant wave peak periods Tp are sometimes associated to a certain significant wave height Hs. The occurrence of not physically consistent Tp is clearly due to a fault in the generally made assumption of a log-normal distribution of the Tp, where the physical limit for the period would be Tp > 0, i.e. the existence limit of the log-normal distribution, which is well below the real physical limit for significant wave steepness. If this is not a problem for pipeline design, where stability and fatigue are dominated by longer peak periods associated at each significant wave height, loads overestimation could arise for near surface structures, e.g. riser, where the largest loads and fatigue, are caused by the shorter peak periods associated to a certain significant wave height. Hence, the possibility to define a Tp distribution which respects the physical lower bound of the limiting wave steepness has a significant relevance when dealing with design and installation of near surface structures. The present paper proposes a new methodology for the assessment of the Hs-Tp distribution which respects an a-priori defined wave steepness limit. This can be done basing on the definition of significant wave steepness Sp = 2πHs/gTp2 which, assessing a limiting steepness Sp, provide an physical lower bound Tplim for the peak period Defining a new variable Tp′ = Tp – Tplim and imposing that Tp′ follows a log-normal distribution, hence having a physical limit Tp′ > 0, is equivalent to assess a Tp distribution which respects the defined significant wave steepness limit Tp > Tplim. A test case compares results obtained with the ‘old’ and ‘new’ methodologies and shows the implication on the design loads. Moreover, another test case has been investigated to verify the performance and characteristics of the new methodology.

Bivariate Distributions of Significant Wave Height With Characteristic Wave Steepness and Characteristic Surf Parameter

2008 ◽  
Author(s):  
Dag Myrhaug ◽  
Se´bastien Fouques
Keyword(s):  
Wave Height ◽  
Deep Water ◽  
Significant Wave Height ◽  
Surf Zone ◽  
Bivariate Distribution ◽  
Wave Steepness ◽  
Peak Period ◽  
Characteristic Wave ◽  
Significant Wave ◽  
Run Up

The paper provides a bivariate distribution of significant wave height and characteristic wave steepness, as well as a bivariate distribution of significant wave height and characteristic surf parameter. The characteristic wave steepness in deep water is defined in terms of the significant wave height and the spectral peak period, and is relevant for e.g. the design of ships and marine structures. The characteristic surf parameter is given by the ratio between the slope of a beach or a structure and the square root of the characteristic wave steepness in deep water. The characteristic surf parameter is used to characterize surf zone processes and is relevant for e.g. wave run-up on beaches and coastal structures. The paper presents statistical properties of the wave parameters as well as examples of results typical for field conditions.

Derivation of Wave Height and Peak Period Statistics Accounting for Physical Limitations

2014 ◽  
Author(s):  
Cécile Melis ◽  
Guillaume Bonnaffoux
Keyword(s):  
Lower Bound ◽  
Normal Distribution ◽  
Wave Height ◽  
Joint Probability ◽  
Peak Period ◽  
Original Dataset ◽  
Extreme Response ◽  
Physical Limitations ◽  
Log Normal ◽  
Log Normal Distribution

When assessing the joint-probability of significant wave height and peak period, (Hs,Tp) measured over years at a given site, it is customary to fit a log-normal distribution to assess Tp dependence on Hs. The parameters of this distribution are then used either to compute N-year return period design curves in order to compute extreme response by means of short-term analysis, or response distributions, by means of response-based analysis. The main drawback of the Log-Normal distribution to represent the variability of Tp wrt. Hs is that its lower bound is zero, while physics tell us that wave steepness cannot be infinite, hence the lower bound, Tplim(Hs) should be greater than zero. If the distribution is kept unbounded, the resulting statistical fitting tends to predict occurrences of sea-states with (Hs,Tp) pairs having unphysical or unlikely steepness. This is particularly true in the range of 10–15s, where some ship-shaped units mooring systems responses are at their maximum. Attempts have been made in the past to introduce a lower bound to the log-normal distribution, for instance by Drago et al, [1], by shifting it by a predefined value of limit steepness. By doing so, some points of the original dataset had to be discarded as they were falling below the lower bound. An evolution of their methodology is proposed in this paper, which uses the points of the dataset in a relevant region which will be defined hereafter, and then uses this limit to shift the Log-Normal distribution. The obtained environmental contours are then compared against observed data to check which one fits most accurately the original set of measured (Hs,Tp) pairs.

scholarly journals Buoy Measurements of Wind–Wave Relations during Hurricane Matthew in 2016

2017 ◽  
Vol 47 (10) ◽  
pp. 2603-2609 ◽  
Author(s):  
S. A. Hsu ◽  
Yijun He ◽  
Hui Shen
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Wind Wave ◽  
Neutral Stability ◽  
Wave Steepness ◽  
Peak Period ◽  
Significant Wave ◽  
Hurricane Ivan ◽  
Wind Sea ◽  
Sea Storm

AbstractStudies suggested that neutral-stability wind speed at 10 m U10 ≥ 9 m s −1 and wave steepness Hs/Lp ≥ 0.020 can be taken as criteria for aerodynamically rough ocean surface and the onset of a wind sea, respectively; here, Hs is the significant wave height, and Lp is the peak wavelength. Based on these criteria, it is found that, for the growing wind seas when the wave steepness increases with time during Hurricane Matthew in 2016 before the arrival of its center, the dimensionless significant wave height and peak period is approximately linearly related, resulting in U10 = 35Hs/Tp; here, Tp is the dominant or peak wave period. This proposed wind–wave relation for aerodynamically rough flow over the wind seas is further verified under Hurricane Ivan and North Sea storm conditions. However, after the passage of Matthew’s center, when the wave steepness was nearly steady, a power-law relation between the dimensionless wave height and its period prevailed with its exponent equal to 1.86 and a very high correlation coefficient of 0.97.

Semi-Empirical Crest Distributions of Long-Crest Nonlinear Waves of Three-Hour Duration

2017 ◽  
Author(s):  
Zhenjia (Jerry) Huang ◽  
Qiuchen Guo
Keyword(s):  
Nonlinear Wave ◽  
Wave Height ◽  
Model Test ◽  
Significant Wave Height ◽  
Spectral Peak ◽  
Wave Crest ◽  
Empirical Formulas ◽  
Peak Period ◽  
Significant Wave ◽  
Semi Empirical

In wave basin model test of an offshore structure, waves that represent the given sea states have to be generated, qualified and accepted for the model test. For seakeeping and stationkeeping model tests, we normally accept waves in wave calibration tests if the significant wave height, spectral peak period and spectrum match the specified target values. However, for model tests where the responses depend highly on the local wave motions (wave elevation and kinematics) such as wave impact, green water impact on deck and air gap tests, additional qualification checks may be required. For instance, we may need to check wave crest probability distributions to avoid unrealistic wave crest in the test. To date, acceptance criteria of wave crest distribution calibration tests of large and steep waves of three-hour duration (full scale) have not been established. The purpose of the work presented in the paper is to provide a semi-empirical nonlinear wave crest distribution of three-hour duration for practical use, i.e. as an acceptance criterion for wave calibration tests. The semi-empirical formulas proposed in this paper were developed through regression analysis of a large number of fully nonlinear wave crest distributions. Wave time series from potential flow simulations, computational fluid dynamics (CFD) simulations and model test results were used to establish the probability distribution. The wave simulations were performed for three-hour duration assuming that they were long-crested. The sea states are assumed to be represented by JONSWAP spectrum, where a wide range of significant wave height, peak period, spectral peak parameter, and water depth were considered. Coefficients of the proposed semi-empirical formulas, comparisons among crest distributions from wave calibration tests, numerical simulations and the semi-empirical formulas are presented in this paper.

Development of Operational Limit Diagrams for Offshore Lifting Procedures

2015 ◽  
Author(s):  
Leonardo Roncetti ◽  
Fabrício Nogueira Corrêa ◽  
Carl Horst Albrecht ◽  
Breno Pinheiro Jacob
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Technological Development ◽  
Automatic Generation ◽  
Dynamic Responses ◽  
Offshore Platforms ◽  
Peak Period ◽  
Significant Wave ◽  
Wave Heights ◽  
Offshore Crane

Lifting operations with offshore cranes are fundamental for proper functioning of a platform. Despite the great technological development, offshore cranes load charts only consider the significant wave height as parameter of environmental load, neglecting wave period, which may lead to unsafe or overestimated lifting operations. This paper aims to develop a method to design offshore crane operational limit diagrams for lifting of personnel and usual loads, in function of significant wave height and wave peak period, using time domain dynamic analysis, for a crane installed on a floating unit. The lifting of personnel with crane to transfer between a floating unit and a support vessel is a very used option in offshore operations, and this is in many cases, the only alternative beyond the helicopter. Due to recent fatal accidents with lifting operations in offshore platforms, it is essential the study about this subject, contributing to the increase of safety. The sea states for analysis were chosen covering usual significant wave heights and peak periods limits for lifting operations. The methodology used the SITUA / Prosim software to obtain the dynamic responses of the personnel transfer basket lifting and container loads on a typical FPSO. Through program developed by the author, it was implemented the automatic generation of diagrams as a function of operational limits. It is concluded that using this methodology, it is possible to achieve greater efficiency in the design and execution of personnel and routine load lifting, increasing safety and a wider weather window available.

Comparison of Bivariate Models of the Distribution of Significant Wave Height and Peak Wave Period

2007 ◽  
Author(s):  
Catarina S. Soares ◽  
C. Guedes Soares
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Bivariate Normal Distribution ◽  
Wave Period ◽  
Peak Period ◽  
Data Set ◽  
Significant Wave ◽  
The North ◽  
Bivariate Models ◽  
The North Sea

This paper presents the results of a comparison of the fit of three bivariate models to a set of 14 years of significant wave height and peak wave period data from the North Sea. One of the methods defines the joint distribution from a marginal distribution of significant wave height and a set of distributions of peak period conditional on significant wave height. Other method applies the Plackett model to the data and the third one applies the Box-Cox transformation to the data in order to make it approximately normal and then fits a bivariate normal distribution to the transformed data set. It is shown that all methods provide a good fit but each one have its own strengths and weaknesses, being the choice dependent on the data available and applications in mind.

Effect of Uni- and Bi-Directional Coupling of Ocean-Met Interaction on Significant Wave Height and Local Wind

2017 ◽  
Author(s):  
Adil Rasheed ◽  
Jakob Kristoffer Süld ◽  
Mandar Tabib
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Surface Wind ◽  
Wave Model ◽  
Ocean Surface ◽  
Observation Data ◽  
Local Wind ◽  
Near Surface ◽  
Significant Wave ◽  
Bidirectional Coupling

Accurate prediction of near surface wind and wave height are important for many offshore activities like fishing, boating, surfing, installation and maintenance of marine structures. The current work investigates the use of different methodologies to make accurate predictions of significant wave height and local wind. The methodology consists of coupling an atmospheric code HARMONIE and a wave model WAM. Two different kinds of coupling methodologies: unidirectional and bidirectional coupling are tested. While in Unidirectional coupling only the effects of atmosphere on ocean surface are taken into account, in bidirectional coupling the effects of ocean surface on the atmosphere are also accounted for. The predicted values of wave height and local wind at 10m above the ocean surface using both the methodologies are compared against observation data. The results show that during windy conditions, a bidirectional coupling methodology has better prediction capability.

scholarly journals ON LONG-TERM STATISTICS FOR OCEAN AND COASTAL WAVES

1978 ◽  
Vol 1 (16) ◽  
pp. 2 ◽  
Author(s):  
Michel K. Ochi
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Contingency Table ◽  
Statistical Properties ◽  
Significant Wave ◽  
Coastal Waves ◽  
Normal Probability ◽  
Log Normal ◽  
Confidence Domain

This paper discusses the statistical properties of long-term ocean and coastal waves derived from analysis of available data. It was found from the results of the analysis that the statistical properties of wave height and period obey the bi-variate log-normal probability law. The method to determine the confidence domain for a specified confidence coefficient is presented so that reliable information in severe seas where data are always sparse can be obtained from a contingency table. Estimation of the extreme significant wave height expected in the long-term is also discussed.

Spectral Modeling of Sea States With Multiple Wave Systems

1992 ◽  
Vol 114 (4) ◽  
pp. 278-284 ◽  
Author(s):  
C. Guedes Soares ◽  
M. C. Nolasco
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Theoretical Models ◽  
Wave System ◽  
Spectral Parameters ◽  
Peak Period ◽  
Multiple Wave ◽  
Significant Wave ◽  
General Acceptance ◽  
Spectral Models

The spectral models of individual wave systems have one peak and are described by theoretical models that have gained general acceptance. This work deals with sea states with more than one wave system, leading to spectral models with two or more peaks. Use is made of spectra derived from measurements off the Portuguese Coast and data is provided as to their probability of occurrence as well as about the dependence of the spectral parameters on the significant wave height and peak period. It is shown that wind-dominated and swell-dominated two-peaked spectra tend to occur in different areas of the scatter diagram. The spectral parameters of the two-peaked spectra show little correlation with significant wave height and peak period.

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