Kevin Ewans

Observations of the directional spectrum of fetch-limited waves

Abstract Estimates of the wave directional spectrum of fetch-limited sea states are made from measurements made with a heave–pitch–roll buoy at the Maui location off the west coast of New Zealand. The fetch-limited sea states have significant wave heights between 0.5 and 4.5 m and are observed during persistent southeast wind events, which have a well-defined fetch of 200 km. The integrated properties of the estimated angular spreading distributions are in general agreement with those observed in previous studies. However, the angular distributions estimated for the Maui location are bimodal at frequencies greater than the spectral peak frequency. This result for deep water ocean waves is in contrast to the generally accepted unimodal angular distribution for wind seas, but it supports recently reported measurements of the angular distribution of fetch-limited waves in Lake George, Australia. Parametric relationships that describe the characteristics of the bimodal distributions are derived, and the impor...

Worldwide Measurements of Directional Wave Spreading

Abstract The directional spreading of waves is important for both theoretical and practical reasons. Enough measurements have now been made to draw conclusions about the behavior of wave spreading at sites in different climatic regimes. The measurements presented here of the directional spreading of fetch-limited waves agree in general with those of M. A. Donelan et al., but additional evidence is also found to support the conclusion of I. R. Young et al. that the spreading function is bimodal at high frequencies. The spreading factor ϕ is defined to be the square root of the in-line variance ratio defined by R. E. Haring and J. C. Heideman. This spreading factor gives an integrated measure of the degree of directional spreading in the wave spectrum and predicts the reduction in the in-line particle velocities under waves due to direction spreading. The value of ϕ is 1 for unidirectional waves and 0.707 for omnidirectional waves. For fetch-limited conditions, ϕ is essentially constant at 0.906. Results fr...

Estimation of Wind-Sea and Swell Components in a Bimodal Sea State

Methods for separating the spectral components and describing bimodal wave spectra are evaluated with reference to wave spectra from directional wave measurements made at the Maui location off the west coast of New Zealand. Two methods involve partitioning bimodal wave spectra into wind-sea and swell components and then fitting a spectral function to each component, while the third assigns an average spectral shape based on the integrated spectral parameters. The partitioning methods involve separating the wave spectrum into two frequency bands: a low-frequency peak, the swell component, and a high-frequency peak, the wind-sea. One partitioning method uses only the frequency spectrum while the other analyzes the complete frequency-direction spectrum. Comparison of the spectral descriptions and derived parameters against the measured counterparts provides insight into the accuracy of the different approaches to describing actual bimodal sea states.

Modeling the Seasonality of Extreme Waves in the Gulf of Mexico

Statistics of storm peaks over threshold depend typically on a number of covariates including location, season, and storm direction. Here, a nonhomogeneous Poisson model is adopted to characterize storm peak events with respect to season for two Gulf of Mexico locations. The behavior of storm peak significant wave height over threshold is characterized using a generalized Pareto model, the parameters of which vary smoothly with season using a Fourier form. The rate of occurrence of storm peaks is also modeled using a Poisson model with rate varying with season. A seasonally varying extreme value threshold is estimated independently. The degree of smoothness of extreme value shape and scale and the Poisson rate with season are regulated by roughness-penalized maximum likelihood; the optimal value of roughness is selected by cross validation. Despite the fact that only the peak significant wave height event for each storm is used for modeling, the influence of the whole period of a storm on design extremes for any seasonal interval is modeled using the concept of storm dissipation, providing a consistent means to estimate design criteria for arbitrary seasonal intervals. The characteristics of the 100 year storm peak significant wave height, estimated using the seasonal model, are examined and compared with those estimated ignoring seasonality.

Uncertainties in Extreme Wave Height Estimates for Hurricane-Dominated Regions

Inherent uncertainties in estimation of extreme wave heights in hurricane-dominated regions are explored using data from the GOMOS Gulf of Mexico hindcast for 1900-2005. In particular, the effect of combining correlated values from a neighborhood of 72 grid locations on extreme wave height estimation is quantified. We show that, based on small data samples, extreme wave heights are underestimated and site averaging usually improves estimates. We present a bootstrapping approach to evaluate uncertainty in extreme wave height estimates. We also argue in favor of modeling supplementary indicators for extreme wave characteristics, such as a high percentile (95%) of the distribution of 100-year significant wave height, in addition to its most probable value, especially for environments where the distribution of 100-year significant wave height is strongly skewed.

Field Measurements of Rogue Water Waves

AbstractThis paper concerns the collation, quality control, and analysis of single-point field measurements from fixed sensors mounted on offshore platforms. In total, the quality-controlled database contains 122 million individual waves, of which 3649 are rogue waves. Geographically, the majority of the field measurements were recorded in the North Sea, with supplementary data from the Gulf of Mexico, the South China Sea, and the North West shelf of Australia. The significant wave height ranged from 0.12 to 15.4 m, the peak period ranged from 1 to 24.7 s, the maximum crest height was 18.5 m, and the maximum recorded wave height was 25.5 m. This paper will describe the offshore installations, instrumentation, and the strict quality control procedure employed to ensure a reliable dataset. An examination of sea state parameters, environmental conditions, and local characteristics is performed to gain an insight into the behavior of rogue waves. Evidence is provided to demonstrate that rogue waves are not go...

On the Estimation of Ocean Engineering Design Contours

Understanding extreme ocean environments and their interaction with fixed and floating structures is critical for offshore and coastal design. Design contours are useful to describe the joint behavior of environmental, structural loading, and response variables. We compare different forms of design contours, using theory and simulation, and present a new method for joint estimation of contours of constant exceedance probability for a general set of variables. The method is based on a conditional extremes model from the statistics literature, motivated by asymptotic considerations. We simulate under the conditional extremes model to estimate contours of constant exceedance probability. We also use the estimated conditional extremes model to estimate other forms of design contours, including those based on the first-order reliability method (FORM), without needing to specify the functional forms of conditional dependence between variables. We demonstrate the application of new method in estimation of contours of constant exceedance probability using measured and hindcast data from the Northern North Sea, the Gulf of Mexico, and the North West Shelf of Australia, and quantify their uncertainties using a bootstrap analysis.

NEW INSIGHTS IN EXTREME CREST HEIGHT DISTRIBUTIONS (A SUMMARY OF THE 'CREST' JIP)

The objective of the CresT JIP was ‘to develop models for realistic extreme waves and a design methodology for the loading and response of floating platforms’. Within this objective the central question was: ‘What is the highest (most critical) wave crest that will be encountered by my platform in its lifetime?’ Based on the presented results for long and short-crested numerical, field and basin results in the paper, it can be concluded that the statistics of long-crested waves are different than those of short-crested waves. But also short-crested waves show a trend to reach crest heights above second order. This is in line with visual observations of the physics involved: crests are sharper than predicted by second order, waves are asymmetric (fronts are steeper) and waves are breaking. Although the development of extreme waves within short-crested sea states still needs further investigation (including the counteracting effect of breaking), at the end of the CresT project the following procedure for taking into account extreme waves in platform design is recommended: 1. For the wave height distribution, use the Forristall distribution (Forristall, 1978). 2. For the crest height distribution, use 2nd order distribution as basis. 3. Both the basin and field measurements show crest heights higher than predicted by second order theory for steeper sea states. It is therefore recommended to apply a correction to the second order distribution based on the basin results. 4. Account for the sampling variability at the tail of the distribution (and resulting remaining possibility of higher crests than given by the corrected second order distribution) in the reliability analysis. 5. Consider the fact that the maximum crest height under a complete platform deck can be considerably higher than the maximum crest at a single point.Copyright

Directional Spreading in Ocean Swell

Directional wave spectra derived from a data set measured off the west coast of New Zealand are used to investigate the directional spreading within swell. The location where the measurements were made is particularly useful for the study as a more or less constant swell component originating from the Southern Ocean is observed in the sea states. The spectra are partitioned into wind-sea and swell components, and estimates of the directional spreading of the swell component is made. A function for the directional distribution of the swell is proposed. INTRODUCTION The wave directional distribution is an important quantity in wave forecasting and in the design and operation of offshore engineering facilities. Considerable effort has focused on the directional distribution of active wind-seas, resulting in significant improvement in the understanding of directionality during wave growth. In most regions around the world, the extreme sea states for which offshore facilities must be engineered are also associated with active wind-seas. However, there are locations, such as offshore West Africa, and offshore operations for which swell is equally if not more important. For example, swell governs the wave design criteria offshore Nigeria, and the persistent swell from the Southern Ocean has an important influence on the workability of vessels off the west coast of New Zealand. But the directionality of swell, and particularly the spreading in swell, has received far less attention than the wind-sea component. The paper reports an evaluation of a wave directional data set, recorded off the west coast of New Zealand. The spectra are partitioned into wind-sea and swell components, characteristics of the swell spreading are established, and a swell spreading function is developed. 1 K. C. Ewans Metocean Engineer, Offshore Technology, Upstream Sector, Shell Global Solutions International B.V., P.O. Box 60, 2280 AB Rijswijk, The Netherlands, k.ewans@siep.shell.com Ewans 1 THE LOCATION The wave data were recorded with a Datawell WAVEC buoy moored adjacent to the Maui A platform, in 110m of water, approximately 30 km off the west coast of the North Island of New Zealand. The location is dominated by a more or less constant swell component that arrives from the southwest. These components arrive through a narrow window defined by a sector with bearings 219° and 249° at the Maui location. These two bearings coincide with the northwestern tip of the South Island of New Zealand and the southern tip of Tasmania respectively. The window defined by this sector has great circle paths that extend all the way to Madagascar, some 11,000 km away. The measurement location and the Southern Ocean swell window are given in Figure 1. 249o 219o 45° S 30° E

West Africa Swell Spectral Shapes

Wave spectra measured at sites off West Africa are dominated by the constant presence of one or several swell wave systems. The West Africa Swell Project (WASP JIP) was carried out to propose and assess parametric models for the shap es of the swell components. Bias, variability, and dispersionof estimates are affected by the length/stationarity comprom ise of the record lengths and the window-tapering used to reduce th eir variability. In particular, shapes with sharp angles are st rongly smoothed, for instance a triangular peak would appear round and reduced by 15 to 25% with rectangular or Tuckey windowing. Models that consider each wave system individually, an d an arbitrary number of those, were preferred to global ones. Pa rtitioning of directional spectra is thus a prerequisite, an d needs to be tuned taking account some prior knowledge of the swell characteristics. Triangular, log-normal, Gaussian and Gl ennJonswap shapes were considered. Sampling variability make s it difficult to distinguish between those shapes as far as swell s are concerned. The models also indicate that the width of the spe ctrum in frequency should be inversely proportional to the pe ak frequency. Directional spreading width shows a similar tre nd.