Ove T. Gudmestad

Water wave kinematics

Water wave kinematics is a central field of study in ocean and coastal engineering. The wave forces on structures as well as sand erosion both on coastlines and in the ocean are to a large extent governed by the local distribution of velocities and accelerations of the water particles. Our knowledge of waves has generally been derived from measurements of the water surface elevations. The reason for this is that the surface elevations have been of primary interest and fairly cheap and reliable instruments have been developed for such measurements. The water wave kinematics has then been derived from the surface elevation information by various theories. However. the different theories for the calculation of water particle velocities and acceleration have turned out to give significant differences in the calculated responses of structures. In recent years new measurement techniques have made it possible to make accurate velocity measurements. Hence. the editors deemed it to be useful to bring together a group of experts working actively as researchers in the field of water wave kinematics. These experts included theoreticians as well as experimentalists on wave kinematics. It was also deemed useful to include experts on the response of structures to have their views from a structural engineering point of view on what information is really needed on water wave kinematics.

HYDRODYNAMIC COEFFICIENTS FOR CALCULATION OF HYDRODYNAMIC LOADS ON OFFSHORE TRUSS STRUCTURES

Abstract The current American Petroleum Institutes recipe [API RP 2A WSD, Recommended practice for planning, designing and constructing fixed offshore platforms, working stress design. API, USA, 1993.] for calculation of hydrodynamic loads on offshore truss structures is compared with the corresponding North Sea Design Practice, as given by the rules of Det Norske Veritas. Most emphasis is put on the hydrodynamic coefficients and the estimation of design current as these issues are identified to be particularly critical. Use of the updated API (1993) recommendations in which the drag coefficient for roughened cylinders is increased from a minimum of 0·6 (API 1991) to 1·05 (API 1993) and where current is included, could lead to a general increase in the estimated load level on slender offshore structures [Petrauskas, C., Heideman, J.C. & Berek, E.P., Extreme wave force calculation procedure for the 20th edition of API RP 2A. OTC paper 7153, In Proc. OTC 1993, Houston, Texas, 1993, pp. 201–211]. The main emphasis with regard to the impact of the new API recommendations, however, is that a consistent approach is provided to the calculation of 100-yr directional loads. This includes taking into account the effect of marine growth on force coefficients, modifying the wave kinematics for directional spreading, and considering current blockage effects, conductor shielding effects, and joint occurrence of wave height and current (i.e., using the associated current as being representative of the current that would lead to the 100-yr load). It is concluded that a consistent approach, such as that underlying the new API RP 2A (1993) recipe, is preferable to the current North Sea Design Practice [Det Norske Veritas, Environmental conditions and environmental loads. DNV classification notes 30.5, 1001.] in this field, and thus that the North Sea Design Practice should be updated. This relates in particular to selection of hydrodynamic coefficients. Measurement programmes to obtain full scale global force data simultatneously with wave and current data are furthermore recommended.

ENGINEERING APPROXIMATIONS TO NONLINEAR DEEPWATER WAVES

Recent measurements of wave kinematic indicate that the horizontal wave velocity is smaller at the crest and higher (more negative) in the trough than predicted by the Stokes higher order theories which are normally used in a deterministic design process. This has led to postulation of engineering methods for description of wave kinematics1,2 A methodology has been developed to establish second order corrections to the engineeringmethods. The purpose is to find a description of the wave kinematics which predicts measured behaviour with good degree of accuracy. The methodology has been applied to the engineering methods proposed by Wheeler1 and Chakrabarti.2 The second order Chakrabarti approximation (the ‘alternative approximation’) demonstrates good agreement with measured wave kinematics.

Measured and predicted deep water wave kinematics in regular and irregular seas

Abstract This paper summarizes recent efforts to predict measured deep water wave kinematics in regular and irregular seas. While the theoretical foundation for prediction of water wave kinematics is solid for regular waves in deep water, no theory presently exists which can consistently predict the kinematics for irregular waves, a problem of major importance for design of deep water offshore structures. A study of non-linear interaction between a long wave and a short wave riding on the long wave, however, holds considerable promise for bridging this gap between understanding regular wave kinematics and irregular wave kinematics. Recommendations regarding wave kinematics models for engineering purposes are given in the paper.

RAMS data collection under Arctic conditions

Reliability, availability, maintainability and supportability analysis is an important step in the design and operation of production processes and technology. Historical data such as time between failures and time to repairs play an important role in such analysis. The data must reflect the conditions that equipment has experienced during its operating time. To have a precise understanding of the conditions experienced, all influence factors on the failure and repair processes of a production facility in Arctic environment need to be identified and collected in the database. However, there is a lack of attention to collect the effect of influence factors in the reliability, availability, maintainability and supportability database. Hence, the aim of this paper is to discuss the challenges of the available methods of data collection and suggest a methodology for data collection considering the effect of environmental conditions. Application of the methodology will make the historical RAMS data of a system more applicable and useful for the design and operation of the system in different types of operational environments.

Comparison of the physical environment of some Arctic seas

Extensive experience on construction, deployment and operation of Arctic offshore structures has been accumulated by the western oil companies in the Beaufort Sea. The transfer of this experience to the Russian Arctic offshore can be facilitated by a comprehensive comparison of the environmental conditions of the Beaufort Sea and Russian Arctic seas such as the Barents, Pechora and Kara. The environmental factors of wind, waves, temperature, current and ice conditions for each sea are reviewed and compared.

A new approach for estimating irregular deep water wave kinematics

A new approach for estimating wave kinematics in deep water irregular waves is presented. The development of the approach has been based on results from measurements of regular water wave kinematics 1 and represents and extension of a regular wave theory developed by Gudmestad and Connor. 2 The approach is suggested to improve the description of irregular waves and thus the reliability of irregular wave time domain analysis of forces and response of deep water offshore structures as nonlinearities and dynamic effects in irregular sea states are treated in a consistent manner. An application is presented in a separate paper. 3 The suggested approach should be documented through careful measurements of irregular water wave kinematics.

Integrating QRA and SRA Methods Within a Bayesian Framework When Calculating Risk in Marine Operations: Two Examples

This paper concerns itself with the integration of QRA (quantitative risk analysis) SRA (structural reliability analysis) methods. For simplicity, we will use the term S instead of SRA methods in the paper. The Bayesian (subjective) approach seems to most appropriate framework for such integrated analyses. It may, however, not be to all what the Bayesian approach really means. There exists alternative Bayesia proaches, and the integration of SRA and QRA is very much dependent on what the is. The purpose of this paper is to present two marine operation examples, impleme two different Bayesian approaches: the ‘‘classical Bayesian approach’’ and the ‘‘f Bayesian approach.’’ Following the classical Bayesian approach, we estimate a objective risk, whereas in the fully Bayesian approach, risk is a way of expressing u tainty about future observable quantities. In both examples, one initial accidental eve investigated by using a fault tree and by integrating SRA into this fault tree. We conc that the most suitable framework for integrating SRA and QRA is to adopt the ‘‘ Bayesian approach.’’ @S0892-7219 ~00!00703-2#

Utilisation of principles from structural reliability in quantitative risk analysis: example from an offshore transport problem

Abstract The objective of this paper is to discuss the use of methods developed for calculation of the reliability of structures as a general tool for calculating probabilities within the context of quantitative risk analysis. By applying these methods the analyst is enabled to model the system, the uncertainties and the parameter correlations separately and systematically. This is ensured by flexible event and system modelling by logical combination of limit state functions and suitable uncertainty modelling by assigning marginal probability distributions and correlation measures. These properties might enable the analyst to include more knowledge in the analysis, compared to models traditionally applied in quantitative risk analysis. The subjectivistic theory of probability, which consistently allows subjective considerations to be included in the analysis, is adopted and provides the framework for the discussions. The paper concludes that methods of structural reliability represent a useful tool for calculating probabilities in a great number of situations in quantitative risk analysis. An example from an offshore towing operation sketches how these methods can open for stochastic modelling more in line with the characteristics of the actual system, compared to a typical event tree approach.

Stochastic characteristics of orbital velocities of random water waves

This paper presents the stochastic properties of orbital velocities of random water waves in intermediate water depth. Both the emergence effect and weak nonlinear effects are studied; the theoretical predictions are compared with measured kinematics and the deviations from linear theory are quantified. This study includes new ideas in fluid dynamics. An analytic formula for probability distribution for velocities modified by the emergence effect as well as by nonlinearities of the wave motion in intermediate water depth is developed. This probability function gives us the first statistical moment, the second statistical moment for modified velocities in an analytical form , and by numerical integration the third statistical moment for modified velocities. The theoretical formulae for the statistical moments for surface elevation and for velocities up to third order, with nonlinearities of the motion taken into account, for the case when the emergence effect can be neglected, i.e. below the surface layer, have been developed. This includes a generalized formula for free-surface elevation setdown and calculation of the asymmetry of the horizontal velocity, which is found to be negative in agreement with measurements of Anastasiou et al. (1982 b ). From the first statistical moment of the modified horizontal velocity, the mean flux between any two levels in the wave flume may be calculated. When the integration is carried out from the bottom up to + ∞, it leads in approximation to the formula for total mean flux found by Phillips (1960). This agreement with Phillips’ formula encourages one to interpret the positive mean value of horizontal velocities as a ‘real current’. This interpretation also provides a new understanding of the fluid dynamic implications of results presented by Tung (1975). Theoretical prediction of the measured kinematics has allowed a better estimation of the return flow in the wave flume, and in the vicinity of the mean water level currents in two different directions are noted. Firstly, the emergence effect gives rise to a current at the mean water level in the direction of the wave advance. Secondly, a flow in the opposite direction, interpreted as a return current in the wave flume, is noticed just below that level.