Effects of Waves and Currents on Extreme Loads on a Jacket

2015 ◽  
Author(s):  
Kjersti Bruserud ◽  
Sverre Haver
Keyword(s):  
Wind Waves ◽  
Offshore Structures ◽  
Load Estimation ◽  
Extreme Wave ◽  
Load Model ◽  
Extreme Loads ◽  
Extreme Load ◽  
Peak Over Threshold ◽  
Waves And Currents ◽  
Threshold Approach

In lack of simultaneous data of metocean parameters such as wind, waves and currents, Norwegian design regulations presently recommend a conservative combination of metocean parameters for estimation of characteristic metocean loads on offshore structures. A simplified parametric load model for a jacket, based on waves and currents, is assumed. Several approaches to load estimation are investigated and the following are considered; different averaging length of extreme currents, the effect of peak-over-threshold approach for estimation of extreme wave and currents compared to all-sea states approach and extreme load estimation directly from a load time series. When compared to the recommended approach, all other approaches yield a reduced estimated characteristic metocean load. The results are intended be illustrative and not suitable for use in design.

Effects of Waves and Currents on Extreme Loads on a Jacket

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Kjersti Bruserud ◽  
Sverre Haver
Keyword(s):  
Wind Waves ◽  
Offshore Structures ◽  
Load Estimation ◽  
Static Loads ◽  
Extreme Loads ◽  
Extreme Load ◽  
Peak Over Threshold ◽  
Waves And Currents ◽  
Design Calculations ◽  
Threshold Approach

In lack of simultaneous data of metocean parameters such as wind, waves, and currents, Norwegian design regulations presently recommend a conservative combination of metocean parameters for estimation of characteristic metocean loads on offshore structures. A simplified parametric load model for a jacket, based on waves and currents, is assumed. Several approaches to load estimation are investigated and the following are considered: different averaging length of extreme currents, the effect of peak-over-threshold approach for estimation of extreme wave and currents compared to all-sea states approach and extreme load estimation directly from a load time series. When compared to the recommended approach, all other approaches yield a reduced estimated characteristic metocean load. The purpose of this study is to indicate the possible conservatism in the Norwegian design regulations for estimation of quasi-static loads on a jacket. The results are intended to be illustrative and not suitable for use in specific design calculations.

Extreme Loads on a Jacket Based on Joint Metocean Data

2017 ◽  
Author(s):  
Kjersti Bruserud
Keyword(s):  
Wind Waves ◽  
Offshore Structures ◽  
Current Data ◽  
The Other ◽  
Load Current ◽  
Load Model ◽  
Extreme Loads ◽  
Extreme Load ◽  
Waves And Currents ◽  
Northern North Sea

In lack of simultaneous metocean data for wind, waves and currents, Norwegian design regulations recommend a combination of metocean parameters for estimation of extreme metocean loads on offshore structures assumed to be conservative. The possible conservatism in the design regulations and also the effect of currents in the estimation of extreme loads are considered. A simplified parametric load model for a jacket, based on waves and currents, is assumed. Both measured and hindcast wave data are combined with different measured current data into load time series and the extreme loads estimated. The extreme load according to the recommended approach is also estimated. This is done at four locations in the northern North Sea. When compared to the recommended approach, the other approaches yield a reduced estimated extreme metocean load. Current is found to have an effect on the total extreme load. The results are intended be illustrative and not suitable for use in design.

scholarly journals Loads on a Point-Absorber Wave Energy Converter in Regular and Focused Extreme Wave Events

2020 ◽  
Author(s):  
Eirini Katsidoniotaki ◽  
Edward Ransley ◽  
Scott Brown ◽  
Johannes Palm ◽  
Jens Engström ◽  
...  
Keyword(s):  
Wave Energy ◽  
Wave Train ◽  
Offshore Structures ◽  
Wave Energy Converter ◽  
Extreme Waves ◽  
Extreme Wave ◽  
Energy Converter ◽  
Point Absorber ◽  
Wave Energy Converters ◽  
Extreme Loads

Abstract Accurate modeling and prediction of extreme loads for survivability is of crucial importance if wave energy is to become commercially viable. The fundamental differences in scale and dynamics from traditional offshore structures, as well as the fact that wave energy has not converged around one or a few technologies, implies that it is still an open question how the extreme loads should be modeled. In recent years, several methods to model wave energy converters in extreme waves have been developed, but it is not yet clear how the different methods compare. The purpose of this work is the comparison of two widely used approaches when studying the response of a point-absorber wave energy converter in extreme waves, using the open-source CFD software OpenFOAM. The equivalent design-waves are generated both as equivalent regular waves and as focused waves defined using NewWave theory. Our results show that the different extreme wave modeling methods produce different dynamics and extreme forces acting on the system. It is concluded that for the investigation of point-absorber response in extreme wave conditions, the wave train dynamics and the motion history of the buoy are of high importance for the resulting buoy response and mooring forces.

Extreme Load-Response Mechanisms of a Tension Leg Platform due to Larger Wave Crests: Some Results of the ‘CresT’ JIP

2011 ◽  
Author(s):  
Janou Hennig ◽  
Jule Scharnke ◽  
Bas Buchner ◽  
Joris van den Berg
Keyword(s):  
Gulf Of Mexico ◽  
Extreme Event ◽  
Offshore Structures ◽  
Wave Load ◽  
Extreme Wave ◽  
Response Process ◽  
Load Response ◽  
Extreme Load ◽  
The Waves ◽  
Crest Height

For the design of ships and offshore structures the largest crest height which can be expected in their lifetime is of key importance. This was confirmed by several incidences e.g. in hurricanes in the Gulf of Mexico during the recent years. This is why MARIN started up the CresT JIP with a number of partners. The CresT JIP is now completed and some results of the extreme wave load and response mechanisms observed during model tests with a TLP will be presented in this paper. First an overview is given of the loading and response process during the most extreme event observed. As a next step the loading and response is related to the time and spatial characteristics of the waves, as it is not per definition the highest local crest or wave height that results in the most extreme dynamic response. Furthermore, the effect of different TLP design variations and short-crestedness will be discussed.

Optimal Design and Portfolio Risk Management for Groups of Structures

2004 ◽  
Author(s):  
Michael Havbro Faber ◽  
Marc A. Maes ◽  
Kazuyoshi Nishijima
Keyword(s):  
Risk Management ◽  
Optimal Design ◽  
Offshore Structures ◽  
Offshore Platforms ◽  
Wave Loads ◽  
Portfolio Risk ◽  
Extreme Loads ◽  
Extreme Load ◽  
Portfolio Risk Management

The present paper addresses the problem of optimal design of portfolios of fixed offshore structures. A new framework for design is developed where the effect of dependency in the performance of structures subject to common extreme load events is taken into account in the design by inclusion of the follow-up consequences resulting from the simultaneous failure of several structures in the portfolio. First the special aspects of optimal design subject to follow-up consequences are addressed from the perspective of structures portfolio risk management. Thereafter the problem of optimal design of groups of structures is defined with special considerations to the assessment of the relation between the design, the probability density function of the life cycle benefits and the number of structures considered (in a group). Using this model basis the optimum design of fixed steel offshore platforms where the capacity of the structures against extreme wave loads can be expressed as function of the Reserve Strength Ratio (RSR) is considered. Thereafter parametric studies are conducted to illustrate the significance of the number of structures considered in a group, the correlation between the extreme loads acting on the different structures and the significance of including the follow-up consequences into the design optimization problem.

A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures

2014 ◽  
Author(s):  
Xin Lu ◽  
Pankaj Kumar ◽  
Anand Bahuguni ◽  
Yanling Wu
Keyword(s):  
Real World ◽  
Offshore Structures ◽  
Air Gap ◽  
Irregular Waves ◽  
Linear Wave ◽  
Extreme Wave ◽  
Wave Impact ◽  
Loading Test ◽  
Wide Range ◽  
Wave Maker

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

scholarly journals EXTREME WAVE PRESSURES AND LOADS ON A PILE-SUPPORTED WHARF DECK - INFLUENCES OF AIR GAP AND WAVE DIRECTION

2018 ◽  
pp. 5
Author(s):  
Andrew Cornett
Keyword(s):  
Offshore Structures ◽  
Model Tests ◽  
Scale Model ◽  
Extreme Waves ◽  
Wave Direction ◽  
Coastal Structures ◽  
Extreme Wave ◽  
Wave Impact ◽  
The Past ◽  
Water Depths

Many deck-on-pile structures are located in shallow water depths at elevations low enough to be inundated by large waves during intense storms or tsunami. Many researchers have studied wave-in-deck loads over the past decade using a variety of theoretical, experimental, and numerical methods. Wave-in-deck loads on various pile supported coastal structures such as jetties, piers, wharves and bridges have been studied by Tirindelli et al. (2003), Cuomo et al. (2007, 2009), Murali et al. (2009), and Meng et al. (2010). All these authors analyzed data from scale model tests to investigate the pressures and loads on beam and deck elements subject to wave impact under various conditions. Wavein- deck loads on fixed offshore structures have been studied by Murray et al. (1997), Finnigan et al. (1997), Bea et al. (1999, 2001), Baarholm et al. (2004, 2009), and Raaij et al. (2007). These authors have studied both simplified and realistic deck structures using a mixture of theoretical analysis and model tests. Other researchers, including Kendon et al. (2010), Schellin et al. (2009), Lande et al. (2011) and Wemmenhove et al. (2011) have demonstrated that various CFD methods can be used to simulate the interaction of extreme waves with both simple and more realistic deck structures, and predict wave-in-deck pressures and loads.

Turkstra Profiles of North Sea Currents: Wider Than You’d Think

2011 ◽  
Author(s):  
Steven R. Winterstein ◽  
Sverre Haver ◽  
Alok K. Jha ◽  
Borge Kvingedal ◽  
Einar Nygaard
Keyword(s):  
North Sea ◽  
Deep Water ◽  
Current Speed ◽  
Marine Structures ◽  
Marginal Analysis ◽  
Water Currents ◽  
Extreme Loads ◽  
Peak Over Threshold ◽  
Direct Contrast ◽  
Sea Currents

To design marine structures in deep water, currents must be modelled accurately as a function of depth. These models often take the form of T-year profiles, which assume the T-year extreme current speed occurs simultaneously at each depth. To better reflect the spatial correlation in the current speeds versus depth, we have recently introduced Turkstra current profiles. These assign the T-year speed at one depth, and “associated” speeds expected to occur simultaneously at other depths. Two essentially decoupled steps are required: (1) marginal analysis to estimate T-year extremes, and (2) some type of regression to find associated values. The result is a set of current profiles, each of which coincides with the T-year profile at a single depth and is reduced elsewhere. Our previous work with Turkstra profiles suggested that, when applied in an unbiased fashion, they could produce unconservative estimates of extreme loads. This is in direct contrast to the findings of Statoil, whose similar (“CCA”) current profiles have generally been found to yield conservative load estimates. This paper addresses this contradiction. In the process, we find considerable differences can arise in precisely how one performs steps 1 and 2 above. The net finding is to favor methods that properly emphasize the upper tails of the data—e.g., using peak-over-threshold (“POT”) data, and regression based on class means—rather than standard analyses that weigh all data equally. By applying such tail-sensitive methods to our dataset, we find the unconservative trend in Turkstra profiles to essentially vanish. For our data, these tail-fit results yield profiles with both larger marginal extremes, and broader profiles surrounding these extremes—hence the title of this paper.

Downtime Analysis Techniques for Complex Offshore and Dredging Operations

2004 ◽  
Author(s):  
Remmelt J. van der Wal ◽  
Gerrit de Boer
Keyword(s):  
Wind Waves ◽  
Weather Forecast ◽  
Offshore Structures ◽  
Operational Mode ◽  
Offshore Structure ◽  
Hydrodynamic Models ◽  
Time Step ◽  
Made In

Offshore operations in open seas may be seriously affected by the weather. This can lead to a downtime during these operations. The question whether an offshore structure or dredger is able to operate in wind, waves and current is defined as “workability”. In recent decades improvements have been made in the hydrodynamic modelling of offshore structures and dredgers. However, the coupling of these hydrodynamic models with methods to analyse the actual workability for a given offshore operation is less developed. The present paper focuses on techniques to determine the workability (or downtime) in an accurate manner. Two different methods of determining the downtime are described in the paper. The first method is widely used in the industry: prediction of downtime on basis of wave scatter diagrams. The second method is less common but results in a much more reliable downtime estimate: determination of the ‘job duration’ on basis of scenario simulations. The analysis using wave scatter diagrams is simple: the downtime is expressed as a percentage of the time (occurrences) that a certain operation can not be carried out. This method can also be used for a combination of operations however using this approach does not take into account critical events. This can lead to a significant underprediction of the downtime. For the determination of the downtime on basis of scenario simulations long term seastate time records are used. By checking for each subsequent time step which operational mode is applicable and if this mode can be carried out the workability is determined. Past events and weather forecast are taken into account. The two different methods are compared and discussed for a simplified offloading operation from a Catenary Anchor Leg Mooring (CALM) buoy. The differences between the methods will be presented and recommendations for further applications are given.

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