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.

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.

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.

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.

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.

Wave Impact Loads on the Bottom of Flat Decks

2021 ◽  
Author(s):  
Daniel de Oliveira Costa ◽  
Julia Araújo Perim ◽  
Bruno Guedes Camargo ◽  
Joel Sena Sales Junior ◽  
Antonio Carlos Fernandes ◽  
...  
Keyword(s):  
Scale Effects ◽  
Offshore Structures ◽  
Model Tests ◽  
Analytical Models ◽  
Navier Stokes ◽  
Impact Loads ◽  
Wave Impact ◽  
Wave Channel ◽  
Waves And Currents ◽  
The Impact

Abstract Slamming events due to wave impact on the underside of decks might lead to severe and potentially harmful local and/or global loads in offshore structures. The strong nonlinearities during the impact require a robust method for accessing the loads and hinder the use of analytical models. The use of computation fluid dynamics (CFD) is an interesting alternative to estimate the impact loads, but validation through experimental data is still essential. The present work focuses on a flat-bottomed model fixed over the mean free surface level submitted to regular incoming waves. The proposal is to reproduce previous studies through CFD and model tests in a different reduced scale to provide extra validation and to identify possible non-potential scale effects such as air compressibility. Numerical simulations are performed in both experiments’ scales. The numerical analysis is performed with a marine dedicated flow solver, FINE™/Marine from NUMECA, which features an unsteady Reynolds-averaged Navier-Stokes (URANS) solver and a finite volume method to build spatial discretization. The multiphase flow is represented through the Volume of Fluid (VOF) method for incompressible and nonmiscible fluids. The new model tests were performed at the wave channel of the Laboratory of Waves and Currents (LOC/COPPE – UFRJ), at the Federal University of Rio de Janeiro.

Optimum Ship Routing

1960 ◽  
Vol 13 (3) ◽  
pp. 253-272 ◽  
Author(s):  
George L. Hanssen ◽  
Richard W. James
Keyword(s):  
United States ◽  
Travel Time ◽  
Long Range ◽  
Wind Waves ◽  
The United States ◽  
Ship Routing ◽  
Waves And Currents

The paper describes the system developed and used by the United States Hydrographic Office for selecting the optimum track for transoceanic crossings by applying long-range predictions of wind, waves and currents to a knowledge of how the routed vessel reacts to these variables. Over a period of two years, over 1000 optimum ship routes were provided to one authority, with an average reduction in travel time of 14 hours.

scholarly journals THE DYNAMIC RESPONSE OF OFFSHORE STRUCTURES TO TIME-DEPENDENT FORCES

1964 ◽  
Vol 1 (9) ◽  
pp. 29
Author(s):  
William S. Gaither ◽  
David P. Billington
Keyword(s):  
Degrees Of Freedom ◽  
Wind Waves ◽  
Offshore Structures ◽  
Time Dependent ◽  
Wave Forces ◽  
Ocean Bottom ◽  
Single Wave ◽  
Single Degree Of Freedom ◽  
The Many ◽  
Floating Objects

This paper is addressed to the problem of structural behavior in an offshore environment, and the application of a more rigorous analysis for time-dependent forces than is currently used. Design of pile supported structures subjected to wave forces has, in the past, been treated in two parts; (1) a static analysis based on the loading of a single wave, and (2) a dynamic analysis which sought to determine the resonant frequency by assuming that the structure could be approximated as a single-degree-of-freedom system. (Ref. 4 and 6) The behavior of these structures would be better understood if the dynamic nature of the loading and the many degrees of freedom of the system were included. A structure which is built in the open ocean is subjected to periodic forces due to wind, waves, floating objects, and due occasionally to machinery mounted on the structure. To resist motion, the structure relies on the stiffness of the elements from which it is built and the restraints of the ocean bottom into which the supporting legs are driven.

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