Elzbieta M. Bitner-Gregersen

Joint distributions for significant wave height and wave zero-up-crossing period

Several types of joint distribution function for significant wave height and zero-upcrossing period are compared with reference to measured wave data from the Norwegian Continental Shelf. The comparison is based on the utility of the distribution functions for predictions of extreme response of offshore structures. Logarithmic contour plots and contour plots of normalised deviations between the data and the fitted distribution are used in the comparison. A joint distribution combining a marginal 3-parameter Weibull distribution for significant wave height with a conditional log-normal distribution for zero-up-crossing period is recommended on the basis of this investigation.

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.

Uncertainties in data for the offshore environment

Abstract Uncertainty sources and expected accuracy of wind, wave and current measurements used as an input for offshore structure reliability calculations are presented. The accuracy of measured processes are specified by the bias, precision, and the statistical distribution of the individual observations. The sampling variability constitutes an important part of the random errors. Examples show that the data uncertainties may be of some significance for engineering applications.

Joint Probabilistic Description for Combined Seas

An extension of joint omnidirectional probabilistic model including combined seas and possibility of environmental parameters approaching from different direction is presented. The following parameters are considered: mean wind speed, wind direction, significant wave height (sea and swell), spectral peak period (sea and swell), wind sea direction, swell direction, current speed and current direction. Two approaches for wind sea and swell modelling are proposed. The joint description is developed for three locations at the Norwegian Continental Shelf and for the West Shetland. Uncertainties related to it are discussed. The model is meant to be applied in reliability analysis of offshore structures, in particularly FPSO units, for which a proper account of directional effects and combined seas is important.Copyright

UNCERTAINTIES OF LOAD CHARACTERISTICS AND FATIGUE DAMAGE OF SHIP STRUCTURES

Abstract The uncertainty in ship response characteristics and fatigue damage accumulation arising from the use of Global Wave Statistics data is assessed, and compared with the variation in loads and fatigue between ocean areas. The Global Wave Statistics data are fitted with a joint distribution combining a marginal three-parameter Weibull distribution for significant wave height with a conditional log-normal distribution for zero-crossing wave period. It is shown that the joint environmental model, developed originally for the North Sea and the Norwegian Sea, gives a satisfactory fit for global wave conditions. Some uncertainties related to the application of the joint environmental model in ship response analyses are indicated and compared with the effect of wave climate. Results are presented for 1-year and 20-year extreme wave-induced bending response and for accumulated fatigue damage within the frame-work of the Palmgren-Miner linear damage formulation.

Second-Order Theory and Setup in Surface Gravity Waves: A Comparison with Experimental Data

Abstract The second-order, three-dimensional, finite-depth wave theory is here used to investigate the statistical properties of the surface elevation and wave crests of field data from Lake George, Australia. A direct comparison of experimental and numerical data shows that, as long as the nonlinearity is small, the second-order model describes the statistical properties of field data very accurately. By low-pass filtering the Lake George time series, there is evidence that some energetic wave groups are accompanied by a setup instead of a setdown. A numerical study of the coupling coefficient of the second-order model reveals that such an experimental result is consistent with the second-order theory, provided directional spreading is included in the wave spectrum. In particular, the coupling coefficient of the second-order difference contribution predicts a setup as a result of the interaction of two waves with the same frequency but with different directions. This result is also confirmed by numerical...

Local properties of sea waves derived from a wave record

Abstract It is demonstrated that for linear deep sea waves with small directional scattering the particle motion at the sea surface and energy transmission may be retrieved from a wave record by means of the Hilbert transform. A physical interpretation of the envelope of the two-dimensional deep sea waves as well as a new method for wave group analysis is presented.

Ship and Offshore Structure Design in Climate Change Perspective

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COST EFFECTIVENESS OF HULL GIRDER SAFETY

The paper presents a cost effectiveness assessment of the safety in a design code for oil tankers. The marginal cost to safety improvements is based on code calibration studies for different target reliabilities. This allows basing the actual target reliability indices on risk acceptance criteria derived from cost effectiveness of the marginal change in scantling requirements. This approach is in agreement with the criteria defined in IMO submissions and used in the ongoing IMO coordinated Formal Safety Assessment studies on bulk carriers. The documentation that cost effectiveness criteria may be applied has previously been submitted to IMO. It is concluded that the method works quite well in the examples that are presented, and that the current codes are in close agreement with decision criteria used for other risk control options. As probabilities calculated by structural reliability methods are notional, it is also advantageous to use marginal costs to safety improvement instead of absolute numbers of probabilities as acceptance criterion. It is indicated that a cost effectiveness criterion may replace the current practice of basing target reliabilities on calibration against previous best practices. Although the basic safety philosophy is changed radically, the study does not indicate that the change in criteria would result in much change in design. The advantage of using the suggested approach is the consistency with ongoing FSA development at IMO.Copyright

Quality Control Issues in Estimating Wave Climate From Hindcast and Satellite Data

Accuracy of an estimated design wave condition such as the 100-year significant wave height is important information for risk assessment in design of marine structures. Assessment of accuracy of design conditions is already not simple when they have been derived from in-situ wave measurements at the project site. When using wave hindcast data this task is even less straightforward due to often a lack of sufficiently reliable measurement data near the project site for comparison. Different hindcasts may give significant discrepancies in prediction of extremes. Also, the quality of the hindcast may vary over time. In practice, we often do not much more than illustrating the quality of the data, for example by scatter plots of hindcast data versus measurements far away from the project site. In order to assess the quality of a design wave height from a hindcast systematically, errors in both the local hindcast data and in the extrapolation from these data need to be addressed. The paper discusses the overall idea and some building blocks for such an approach, while realising that there will not be one simple recipe applicable in all situations. One of the issues is the selection and use of observational data in quality control. Satellite wave data are an attractive data source for this purpose. For illustration, data from the seas around Norway are used.Copyright