Sverre Haver

Wave climate off northern Norway

Abstract The wave climate off northern Norway is considered and the investigation is based on wave measurements made at Tromsoflaket by means of a waverider buoy during the years 1977–1981. Data quality of waverider measurements is briefly commented upon; however, more emphasis is given to an evaluation of the long-term representativity of the actual measuring period and to a procedure accounting approximately for a lack of representativity. The wave climate is presented in terms of a smoothed joint probability density function of the significant wave height, H s , and the spectral peak period, T p . Based on this distribution a consistent design curve in the H s , T p space is established.

On the joint distribution of heights and periods of sea waves

Abstract In the design of offshore structures, a consistent method of choosing the height and period for the adopted design wave may be crucial for a reliable design. With this objective, the joint probability density function for the height and period is considered in this paper. At first the adequacy of a simple theoretical model for the joint distribution under stationary conditions is investigated, using measurements achieved during storms in Northern North Sea. In a slightly modified form the model is found to be of a reasonable accuracy as far as the highest waves are of interest. Design curves regarding simultaneous values of heights and periods are estimated by means of this model and their possible impact for design is discussed.

Environmental Contour Lines for Design Purposes: Why and When?

Methods of prediction of structural loads corresponding to a required target annual exceedance probability are reviewed. Particular attention is given to utilization of environmental contour lines for such a purpose. This approach is based on using short term methods for predicting adequate estimates of the q-probability response. The environmental contour line approach is a very convenient approach if complicated structural problems are considered. For such problems one will often have to involve numerical time domain analyses or model tests to reveal the short term probabilistic structure of the response maxima, making a full long term response analysis impossible for most practical problems.© 2004 ASME

On some uncertainties related to the short term stochastic modelling of ocean waves

Abstract A short-term model for the representation of ocean waves is considered. The model is described and the associated uncertainties are briefly reviewed. Emphasis is placed on a discussion of the validity of the basic assumptions regarding stationarity and Gaussianity, together with an elaboration on uncertainties related to the shape of the spectral density function. The basis for the present investigation is 4586 time series for the sea surface elevation, representing a rather wide range of different sea states. Conclusions presented subsequently rest to some extent on the assumption that the actual time series reproduce the true time fluctuations of the surface with a sufficient accuracy. A verification of this assumption requires a very extensive investigation, and could not be included within the scope of this work.

Comparison of wave and current measurements to NORA10 and NoNoCur hindcast data in the northern North Sea

The objective of this study is to compare metocean design criteria for waves and currents based on measured and hindcast data and by that provide some insight in the expected differences. At the Norwegian Continental Shelf (NCS), the Norwegian Reanalysis Archive (NORA10) hindcast for wind and waves and the Northern North Sea Current Hindcast Study (NoNoCur) for currents are available. A comparison of NORA10 wave and NoNoCur current data to recent wave and current measurements during May 2011 to October 2015 at four locations in the northern North Sea has been done. For waves, significant wave height (Hs), spectral peak period (Tp), and wave direction are compared, and for currents, current speed (Cs) and direction at two water depths. Scatter and qq-plots of Hs, Tp, and Cs and directional roses, summary statistics, and time histories of Hs and Cs are provided. In addition, the extreme values of Hs are estimated, the conditional log-normal distribution for Tp given Hs is discussed, and the Hs-Tp contour lines are established. Good agreement between NORA10 and measured wave data in the northern North Sea is demonstrated. The NORA10 Hs is found to be slightly more conservative than the measured Hs. The NoNoCur data corresponds well to current measurements in the northern North Sea. However, the NoNoCur data does not correspond as good as the NORA10 data corresponds to measured data. Consequently, NORA10 can be recommended to be used for wave design criteria at NCS, while NoNoCur must be further developed and used with caution.

Requalification of an Unmanned Jacket Structure Using Reliability Methods

A reliability assessment of an existing jacket structure exposed to reservoir subsidence is considered in this paper. Due to reduced airgap the probability that a wave will hit the deck is increasing and this is crucial concerning the future safety for an existing structure. A consideration of the safety level as a function of subsidence is therefore established in order to identify when the safety level drops below an acceptable safety level and corrective actions must be taken. Reliability methods is used to describe the uncertainties involved in the problem and then used to make a cost optimal decision.

Measured and predicted dynamic behaviour of an offshore gravity platform

The paper discusses the dynamic characteristics and dynamic behaviour of the Gullfaks C platform as evaluated from full scale measurements and numerical predictions. Two issues are emphasized: identification of a dynamic model for the platform, and a discussion of the dynamic response. In the identification procedure both natural frequencies and mode shapes are used. These are estimated using a multichannel ARMA model. One important parameter is the stiffness of the soil. The identified value for this stiffness is compared with stiffness values estimated from displacement at mudline and calculated base shear. The identified model is furthermore compared with the design model, and the predicted response using the model and a theoretical load model, is compared with measured response. In the discussion of the measured dynamic response the paper features response composition, possible nonlinearities in the soil structure interaction and wave loading. Special attention is given to observed ringing response in the platform.

The Heidrun TLP: measured versus predicted response

Abstract The article presents comparisons between full scale measurements and numerical predictions for the Heidrun TLP. The main focus is on the horizontal motion response; however, some results are also given for tether tensions and accelerations. The relationship between measured and predicted response is investigated at two different levels: a general verification analysis is performed comparing long term measured response characteristics with reference curves established during the design phase. This is done to confirm original expectations regarding “average” platform behaviour. In order to achieve the desired objective, measured results collected during an extended period from September 1995 to January 1997 were used. In addition, a more detailed and quantitative investigation is presented for a smaller group of selected sea states. In this latter case the response behaviour is compared for individual storm situations with definite environmental characteristics (i.e. with specified significant wave height, peak period, wave heading, wind velocity, etc.). The overall conclusion is that computed and measured response are in fairly good agreement, with computed dynamics somewhat to the conservative side. A comprehensive discussion of results is offered throughout the article.

Uncertainties in Prediction of Wave Kinematics in Irregular Waves

Irregular water wave kinematics have recently been measured in the wave flume at Norwegian Hydrodynamics Laboratories (NHL) in Trondheim, using Laser Doppler Velocimetry (LDV) equipment (Skjelbreia et al., 1989 and 1991). The use of Wheeler stretching theory to estimate water wave kinematics in irregular waves is recommended to represent the best fit to the measurements. Reviewing available literature on water wave kinematics, Gudmestad (1992) arrives at the same conclusion.

An Experimental Investigation of Wave Impacts on the Deck of a Gravity Based Structure

The occurrence of wave impacts is a critical feature in the design and re-assessment of many offshore structures. With evidence of increasing storm severity, and with subsidence an important characteristic of some mature fields, the quantification of impact loads arising on both the columns and the underside of these structures remains a difficult but important issue. In contributing to this debate, this paper presents the results of a physical model study of a gravity based structure. This comprises an arrangement of storage caissons located on the sea bed, with four large diameter surface-piercing columns supporting the topside structure at a significant height above mean sea level. The purpose of the study was to provide new physical insights into the nature of the wave-structure and the local wave-wave interactions. This has been achieved by employing a large number of traditional wave gauges within the vicinity of the model structure and by complementing this data with two new measuring techniques in order to quantify the behaviour of the water surface in close proximity to the columns. These observations provide a clear understanding of how a large volume structure modifies the incident waves and why wave-structure and subsequent wave-wave interactions lead to a higher probability of wave impacts on the overlying deck. In particular, considerable attention was paid to the nature of the wave run-up on the front face of the columns. The results show that far from being a highly localised effect, involving a thin sheet of water, the run-up associated with a steep wave can involve significant volumes of water, traveling at very high velocities, leading to the occurrence of large impact pressures acting over substantial areas. Estimates of the run-up velocity are coupled with slamming coefficients to provide load predictions which are comparable to the measured wave-in-deck loads. Perhaps surprisingly, it is also shown that the highest and steepest waves do not always cause the largest impacts.Copyright