Øistein Hagen

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.

Bias and Uncertainty in the Estimation of Extreme Wave Heights and Crests

The accurate prediction of extreme wave heights and crests is important to the design of offshore structures. For example,

Threshold up-crossing by second order methods

Abstract The up-crossing of a scalar Gaussian process through various types of time-dependent thresholds is studied. Some approximate, analytical formulas yielding the expected number of up-crossings during a time interval [0, T ] is derived. An intercomparison study is performed, in which the Laplace asymptotic method, the parametric sensitivity factor method with FORM/SORM and the suggested approximate formulas, are compared.

Effect of Short-Crestedness on Extreme Wave Impact: A Summary of Findings From the Joint Industry Project “ShorTCresT”

Long-crested waves are typically used in the design of offshore structures. However, the corresponding statistics, kinematics and loading are significantly different in short-crested waves and up to date, there is no state-of-the-art methodology to apply short-crested models instead. The objective of the “ShortCresT” Joint Industry Project was to take into account short-crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver (empirical) design recommendations and methods. This paper gives an overview of the findings of ShorTCresT regarding wave crest and height distributions, a comparison of basin and field data, the role of wave breaking, the most realistic directional model, hindcast models as well as the related platform loading.Copyright

Second-Order Kinematics Underneath Irregular Waves

The present paper is concerned with the prediction of horizontal velocities underneath measured irregular wave surface elevations. The simple case of unidirectional waves in deep water is considered. The main challenge in calculating accurately the kinematics in the crest region is related to the treatment of the contribution from wave components with frequencies much higher than the frequencies near the spectral peak. When using linear or weakly nonlinear perturbation methods, the wave components are superimposed at the still water level and it is necessary to truncate the tail of the spectrum in order to calculate accurately the velocity in the crest region.In the present paper, results from three methods of calculating the crest kinematics are compared with the model test results of Skjelbreia et al. [1]:• The second-order model of Stansberg et al. [5] which truncates consistently the high frequency part of the spectrum.• The second-order model of Johannessen [13] which calculates the velocity directly at the instantaneous free surface.• The Wheeler [3] stretching method which stretches the linear velocity profile from the still water level to the instantaneous free surface.In addition to comparing the horizontal velocity profiles underneath the crest, time traces of horizontal velocity is compared at the free surface in the vicinity of a large crest. The latter comparison highlights the differences between the models and the challenge of accurate predictions close to top of crest. All three models show a reasonable agreement with model test results although it is clear that the first two methods are superior to the Wheeler method.© 2013 ASME

On Safety and Reliability for Platforms That are Unmanned During Severe Storms

The paper addresses safety and reliability issues for platforms where an unmanning strategy is premised.The standard NORSOK N-006 recommends how to deal with the specific aspects that engineers meet when assessing existing structures, including life extension. A possible mitigating measure for structures that do not meet today’s structural requirements for environmental loads is to unman the platform during storms. The basis for the unmanning criteria in this standard is that the safety for personnel on a platform that needs to be unmanned during storms is consistent with the safety for personnel on platforms that satisfy structural requirements for manned platforms.The prevailing metocean conditions at a North Sea location is modeled by a storm statistics approach. The capacity waves according to the codes checks are calculated for a jacket structure and the limiting metocean conditions that comply with the acceptance criterion are established. The expected frequency of unmanning events is determined, and the issue of forecast uncertainty discussed.The annual maximum wave height distribution for the location is compared with the corresponding distribution that applies when the platform is manned, i.e. for metocean conditions that do not trigger unmanning. The probability of failure for important limit states is calculated on condition that no unmanning is required, and for a platform that satisfies the requirements for manned platforms. The most likely realizations of sea state variables and extreme wave cycle are determined for the different cases.Parts of the structure may be loaded into the non-linear range and consequently the load-carrying resistance of the structure against future load cycles may be reduced. In such cases it is required to carry out a check of the cyclic capacity of the structure. The statistics for the second highest wave during storm conditions is investigated for unmanning scenarios and for a platform that satisfies the requirements for manned platforms.It is normally acknowledged that the structural failure probability associated with normal statistical variations is considerably less than the failures that are due to gross errors. The difference in risk due to gross errors between platforms that are operated as unmanned during storms compared to the gross error risk level for manned platforms is discussed.Copyright

Estimating Design Levels for Strongly Nonlinear Response

The present paper is concerned with the accurate estimation of wave induced design responses in model test campaigns where only a relatively low number of seastates can be simulated.Three response types are considered, namely an essentially linear response type such as the linear elevation of wave crests, a weakly nonlinear response type such as the second order crest elevation, and finally a discontinuous badly behaved response type such as wave in deck loading.For each of these response types, the 100 year and 10 000 year response levels are calculated for a Norwegian Sea environment. The fractiles at which these response levels occur at the governing seastate along the 100 year and 10 000 year contours, are also calculated.Having established these quantities, the efficiency of three methods is evaluated. Firstly an extreme value approach with a Gumbel assumption is considered and it is evaluated how many three hour simulations are required to obtain a good estimate of the relevant fractile levels in the governing seastate. Similarly, a peak over threshold analysis of maxima is considered and the number of seastates required is compared with the Gumbel results.Finally, the 100 year and 10 000 year response levels are estimated directly by employing an iterative procedure on the probability integral of the long term distribution of the responses together with a simple interpolation procedure. It is found that the latter procedure is efficient, particularly for strongly nonlinear response types.Copyright

Ensemble Forecasts Used for Extreme Wave Warning and Platform Unmanning Strategies

Surface waves often represent the most critical environmental parameter for offshore activities, and reliable forecasts are in many cases essential for safe operation.Currently, decisions on safety during extreme wave situations are generally taken based on deterministic wave forecasts, and uncertainty related to forecasts are acknowledged and accounted for via safety margins. However, unmanning criteria reflecting a consistent probabilistic model for the forecast uncertainty are generally lacking.In 1998 the European Centre for Medium-Range Weather Forecasts (ECMWF) introduced a new method for ocean wave forecasting based on the Ensemble Forecasting technique. In this paper we address how the ensemble prediction system (EPS) can be used to establish a probabilistic model applicable for platform unmanning criteria and procedures.Before ensemble forecasting techniques can be introduced operationally for this purpose it is important to investigate how the decision process can be improved with this method. By improvement we expect the probability forecasts to yield a higher hit rate and a lower false alarm rate.Extreme wave warnings are simulated based on a data series established for a 10-year period and the hit and false-alarm rate from deterministic and probabilistic forecasts are compared with measured data. The ensemble forecasts are used to:• establish a model for uncertainty of forecasts for bad weather• update safety margins used at present such that target safety is obtained if deterministic forecasts are used• discuss a revised scheme where the EPS forecasts are applied on an operational levelCopyright