Useful Indicators for Screening of Sea States for Wave Impacts on Fixed and Floating Platforms

2018 ◽  
Author(s):  
Tim Bunnik ◽  
Carl Trygve Stansberg ◽  
Csaba Pakozdi ◽  
Sebastien Fouques ◽  
Luke Somers
Keyword(s):  
Model Test ◽  
Screening Methods ◽  
Statistical Uncertainty ◽  
Probability Level ◽  
Critical Events ◽  
Wave Impact ◽  
Design Load ◽  
Design Loads ◽  
Design Value ◽  
The Impact

Design approaches for wave impact on marine structures in storm sea-states are being reconsidered due to events related to the safety of North Sea offshore structures, both fixed and floating (Valhall QP extended lifetime; COSL Innovator accident). There has been a strong research and tool development within the field during the last decade, both within model testing and numerical analysis, including CFD. However, there is still a lack of efficient methods and tools to properly analyze these phenomena and their probably of occurrence. One major aspect in this is to reduce the statistical uncertainties that are naturally arising in estimates of design loads related to extreme waves. In order to estimate the design loads it is common practice not to investigate all possible sea states (i.e. long-term analysis) but to investigate a few sea states and assume that the design value occurs at a prescribed probability level in the sea states with the same probability level (i.e. contour line approach). The estimate of the design value at that probability level is then based on results from a limited number of random realizations of these sea states. For linear or weakly nonlinear response types it is possible to estimate design loads accurately with a quite limited number of realizations. For strongly nonlinear/badly behaved problems however this is not possible due to the large variations in the tail of the distribution of the impact load, and many more realizations are required. This means that much more of these extreme, rare impact-related events should be collected to reduce the statistical uncertainty in the design load. This approach is restricted by time and costs and eventually one may have to accept an estimated design load with a large statistical uncertainty and account for the uncertainty with a higher safety margin. In this paper an improved methodology for estimating design loads related to extreme wave impacts will be presented. The methodology is based on screening many 3-hour realizations of the design sea states with simplified, fast but sufficiently accurate methods and to focus only on the potentially critical events with a model containing a more complete description of the physics. This can be either a model test or a non-linear impact simulation (i.e. CFD analysis). By doing this many more rare/critical events can be assessed, reducing the statistical uncertainty in the estimate of the design load. The main challenge is to find suitable screening methods, which may be different for different structures (fixed, floating, etc.). Several screening methods/wave impact indicators will be discussed, and their capability will be illustrated by analyzing existing model test data for fixed and floating structures, showing the correlation between indicator and actual impact events.

Column Slamming Loads on a TLP From Steep and Breaking Waves

2017 ◽  
Author(s):  
Tone M. Vestbøstad ◽  
Ole David Økland ◽  
Gunnar Lian ◽  
Terje Peder Stavang
Keyword(s):  
Model Test ◽  
Limit State ◽  
Breaking Waves ◽  
Complex Problem ◽  
Load Level ◽  
Special Focus ◽  
Ultimate Limit State ◽  
Wave Impact ◽  
Design Loads ◽  
Ultimate Limit

Previous model test campaigns of various large-volume platforms indicate that wave impact loads on vertical platform columns can become high in extreme sea states. Column slamming is a highly non-linear and complex problem and reliable estimation1 of Ultimate Limit State (ULS) and Accidental Limit State (ALS) design loads is a challenge. Previous measurements indicate ALS pressures of about 3 MPa acting on an area of typically 50m2 in North Sea and Norwegian Sea wave conditions. The corresponding ULS loads were in the range 1.5–2.0 MPa for the same impact area. Such high predictions for ULS and ALS impact pressures may be critical for both steel and concrete platforms, and accurate predictions of design loads is therefore crucial to establish the correct level of safety. A model test campaign dedicated to investigate column slamming has been performed on the Heidrun platform, a large concrete Tension Leg Platform (TLP). The column diameter is 31 m. The test campaign was performed in 2013 at Marintek (now Sintef Ocean), at a model scale of 1:55. The main objective of the test campaign was to estimate the characteristic slamming loads, defined as the q-annual extreme 3-hour slamming load level of 10−2 for the Ultimate Limit State (ULS) and 10−4 for the Accidental Limit State (ALS). To ascertain that the test campaign would result in reliable load estimates, a pre-study on column slamming was performed, involving a selected expert group with participants from several organizations. Review of previous work, identification of governing parameters for wave impact and assessment of model uncertainties and extreme value prediction of slamming loads was performed. It was concluded that two challenges were to be specifically addressed during the planning and execution of the test: 1) the localized nature and short duration of the slamming loads and 2) the large statistical variability of the slamming loads. To address the first challenge, special focus was given to the extent and quality of the instrumentation capturing the slamming loads. Comprehensive documentation of the instrumentation was also performed using hammer testing, structural analysis and drop tests. The second challenge was addressed with a carefully planned test strategy. The resulting model test campaign set a new standard for model testing of such loads, using over 80 slamming panels with a sampling frequency of 19.2 kHz, and over 300 sea state realizations. This paper presents the planning and execution of the model test campaign, including the instrumentation and model set-up, the test matrix, main challenges, findings and results.

On the Distribution of Wave Impact Loads on Offshore Structures

2017 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Øystein Lande ◽  
Øistein Hagen
Keyword(s):  
Model Test ◽  
Load Distribution ◽  
Impact Load ◽  
Peak Load ◽  
Offshore Structures ◽  
Impact Loads ◽  
Test Results ◽  
Wave Impact ◽  
The Impact ◽  
Crest Height

For offshore structures in harsh environments, horizontal wave impact loads should be taken into account in design. Shafts on GBS structures, and columns on semisubmersibles and TLPs are exposed to impact loads. Furthermore, if the crest height exceeds the available freeboard, the deck may also be exposed to wave impact loads. Horizontal loads due to waves impacting on the structure are difficult to quantify. The loads are highly intermittent, difficult to reproduce in model tests, have a very short duration and can be very large. It is difficult to calculate these loads accurately and the statistical challenges associated with estimating a value with a prescribed annual probability of occurrence are formidable. Although the accurate calculation of crest elevation in front of the structure is a significant challenge, industry has considerable experience in handling this problem and the analysis results are usually in good agreement with model test results. The present paper presents a statistical model for the distribution of horizontal slamming pressures conditional on the incident crest height upwave of the structure. The impact load distribution is found empirically from a large database of model test results where the wave impact load was measured simultaneously at a large number of panels together with the incident crest elevation. The model test was carried out on a circular surface piercing column using long simulations of longcrested, irregular waves with a variety of seastate parameters. By analyzing the physics of the process and using the measured crest elevation and the seastate parameters, the impact load distribution model is made seastate independent. The impact model separates the wave impact problem in three parts: – Given an incident crest in a specified seastate, calculate the probability of the crest giving a wave impact load above a threshold. – Given a wave impact event above a threshold, calculate the distribution of the resulting peak load. – Given a peak load, calculate the distribution of slamming pressures at one spatial location. The development of the statistical model is described and it is shown that the model is appropriate for fixed and floating structures and for wave impact with both columns and the deck box.

New Combined CFD and Model Testing Technique for Identification of Wave Impact Loads on a Semisubmersible

2017 ◽  
Author(s):  
Csaba Pakozdi ◽  
Anders Östman ◽  
Bjørn C. Abrahamsen ◽  
Ole D. Økland ◽  
Tone M. Vestbøstad ◽  
...  
Keyword(s):  
Time Series ◽  
Test Data ◽  
Model Test ◽  
Impact Force ◽  
Hydrodynamic Conditions ◽  
Wave Impact ◽  
Platform Motion ◽  
Matching Procedure ◽  
Wave Matching ◽  
The Impact

A procedure is presented describing how to estimate realistic loads using combined numerical and model test data. Measured platform motions are imposed on the structure during the CFD analysis. The combination of the wave matching procedure with the imposed measured platform motion gives a very good numerical reproduction of the observed extreme event. The numerical reproduction of model test events provide all necessary information on the hydrodynamic loads for further structure analysis. This represents an improvement in industry design applications. Imposing the measured motion from regular wave model test into CFD simulation is validated by comparison of relative wave height time series. This comparison shows a very good agreement between the measured and the simulated time series. Existing model test data from irregular model test and CFD generated numerical wave are compared. A wave matching procedure has been developed, which shows very promising results with respect to reproducing critical hydrodynamic conditions observed during the model tests. This paper presents a case study how CFD can be used to enhance model test data in an efficient way to provide the critical hydrodynamic conditions for structure analysis. Comparison of the measured free surface elevation of the calibrated waves with the time series of the numerical waves, as well as the measured and simulated relative wave probes time series and the slamming load time series show that the applied numerical wave events show similar physical conditions as those observed in the model test. The effect of the platform motion on the impact force is identified by comparison of the impact force time series of the simulation with and without platform motion against model test time series. The results demonstrate that the approach provides a clear improvement compared to numerical or model testing alone. The observed steep wave events are numerically reproduced in a simplified manner, instead of trying to reproduce measured events directly. This approach significantly reduces the computational time, as well as computational costs, to an industrially acceptable level. Traditional load estimation is not able to provide such reliable detailed local load history for structural design purpose at areas exposed to wave impacts. This new procedure, where CFD simulates realistic breaking waves with coupling to measured vessel motion, offers new possibilities for the design of structures subject to risk of wave impact loading.

scholarly journals Wave Front Steepness and Influence on Horizontal Deck Impact Loads

2020 ◽  
Vol 8 (5) ◽  
pp. 314
Author(s):  
Carl Trygve Stansberg
Keyword(s):  
Model Test ◽  
Incident Wave ◽  
Offshore Platforms ◽  
Rise Velocity ◽  
Wave Impact ◽  
Near Surface ◽  
Design Storm ◽  
Impact Phenomena ◽  
The Impact ◽  
Random Scatter

In design storm sea states, wave-in-deck forces need to be analysed for fixed and floating offshore platforms. Due to the complex physics of wave impact phenomena, numerical analyses should be complemented by model test data. With a large statistical variability, such experiments usually involve running many 3-h storm realisations. Efforts are being done to establish efficient procedures and still obtain improved statistical accuracy, by means of an initial simplified screening based on parameters derived from the incident wave record only. Here, we investigate the vertical rise velocity of the incident wave elevation at a fixed point in space, which indirectly measures both the local slope and the near-surface orbital velocity. A derived simple deck slamming model is also suggested, to support the check of the physical basis of the approach. Correlation against data from a GBS wave-in-deck model test is used for checking this model. The results show that, although there is a significant random scatter in the measured impact forces, especially in the local slamming forces but also in the global forces, there is a correlation to the rise velocity. Comparisons to the simple load model also show promising results when seen on background of the complex physics and random scatter of the impact problem.

Efficient Indicators for Screening of Random Waves for Wave Impacts on a Jacket Platform and a Fixed Offshore Wind Turbine

2019 ◽  
Author(s):  
Tim Bunnik ◽  
Jule Scharnke ◽  
Erik-Jan de Ridder
Keyword(s):  
North Sea ◽  
Large Diameter ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Probability Level ◽  
Wave Impact ◽  
The North ◽  
Statistical Variation ◽  
The North Sea ◽  
Design Loads

Abstract Renewed interest in wave impact assessment has risen for various reasons: • The low airgap of some existing Mobile Units in the North Sea • The COSL Innovator incident and related to this topic the new DNV-GL guidelines (OTG 13 and OTG 14). • the installation of many large-diameter monopile foundations for wind turbines in increasingly deep water in the North Sea. • The installation of many large-diameter wind turbines in increasingly deep water in the North Sea. • Seabed subsidence (and maybe water level rises due to global warming) and their effect on the decreasing airgap of fixed platforms. Wave impact assessment has been the subject of many recent studies and research projects, and there has been a strong knowledge and tool development during the last decade, both within model testing and numerical (CFD) analysis (Huang et.al (2017), de Ridder et.al, (2017), Vestbøstad et. al. (2017), Bunnik et.al. (2018)). However, there is still a lack of efficient methods and tools to properly analyze wave impacts and derive the statistical variation of these impacts in the sea states to which these structures are exposed during their lifetime. To reduce the statistical uncertainties that are naturally arising in estimates of design loads related to extreme waves, sufficient data must be gathered. In order to estimate the design loads it is common practice not to investigate all possible sea states (i.e. long-term analysis) but to investigate a few sea states and assume that the design value occurs at a prescribed probability level in the sea states with the same probability level (i.e. contour line approach). The estimate of the design value at that probability level is then based on results from a limited number of random realizations of these sea states. For linear or weakly nonlinear response types it is possible to estimate design loads accurately with a quite limited number of realizations. For strongly nonlinear problems however this is not possible due to the large statistical variation in the maximum observations, inherent to a random nonlinear process. Estimating accurately the tail of the load distribution requires many more realizations. This approach is restricted by time and costs and eventually one may have to accept an estimated design load with a large statistical uncertainty and account for the uncertainty with a higher safety margin. In this paper an improved methodology for estimating design loads related to extreme wave impacts will be presented. The methodology is based on screening many 3-hour realizations of the design sea states with simplified, fast but sufficiently accurate methods and to focus only on the potentially critical events with a model containing a more complete description of the physics. This can be either a model test or a non-linear impact simulation (i.e. CFD analysis). By doing this many more rare/critical events can be assessed, reducing the statistical uncertainty in the estimate of the design load. A screening method/wave impact indicator will be presented for a jacket platform and for a fixed offshore wind turbine. Existing model test data is used to show the correlation between indicator and actual impact events and to derive the efficiency of the impact indicators.

A Fully Nonlinear RANS-VOF Numerical Wavetank Applied in the Analysis of Green Water on FPSO in Waves

2014 ◽  
Author(s):  
Anders Östman ◽  
Csaba Pakozdi ◽  
Lucia Sileo ◽  
Carl-Trygve Stansberg ◽  
Daniel Fonseca de Carvalho e Silva
Keyword(s):  
Model Test ◽  
Parametric Design ◽  
Green Water ◽  
Test Results ◽  
Wave Impact ◽  
Measured Time ◽  
Time Histories ◽  
Practical Tool ◽  
The Impact ◽  
Impact Events

This paper presents numerical simulations of Green Water events and wave impact on a FPSO. The simulations are performed at model scale and the results are compared against experimental model test results. The commercial Star-CCM+ CFD software is used in the simulations. The incoming waves are modeled using 5th order Stokes theory, as implemented in the CFD software. Both fixed and free floating FPSO are considered. The moving FPSO are modeled using Chimera overset mesh technology. The vessels is free to move in heave and pitch at 180 (head sea), roll and heave at 270 (beam sea), while roll, pitch and heave is released at 225 (quartering sea). The computed water height on the deck and the relative wave height in vicinity the vessel are compared against model test results at several positions. Also the impact force on load cells blocks located at the deck of the vessel is computed and compared against model test results. The comparison of the time histories of the water elevation and load histories are in reasonable agreement with the measured time series. The number of grid cells range from 7M for the simulations at head sea, where flow is assumed to be symmetric, to 21M for the simulations at quartering sea. Total wall clock simulation time was about 10days for the most computationally demanding cases, which are the quartering sea simulations. This includes simulation of 12 wave periods with the ship fixed, and thereafter 8 wave periods of the free floating vessel. The computations show that CFD tools can be used as a research tool when studying the physics of green water and wave impact events. However, due to time CPU demanding simulations, this type of CFD analysis are not yet a practical tool for parametric design studies and deck structure optimizations. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.

scholarly journals Influence of the Spatial Pressure Distribution of Breaking Wave Loading on the Dynamic Response of Wolf Rock Lighthouse

2021 ◽  
Vol 9 (1) ◽  
pp. 55
Author(s):  
Darshana T. Dassanayake ◽  
Alessandro Antonini ◽  
Athanasios Pappas ◽  
Alison Raby ◽  
James Mark William Brownjohn ◽  
...  
Keyword(s):  
Dynamic Response ◽  
Pressure Distribution ◽  
Distinct Element ◽  
Breaking Waves ◽  
Breaking Wave ◽  
Wave Loading ◽  
Wave Impact ◽  
Pressure Distributions ◽  
Impact Area ◽  
The Impact

The survivability analysis of offshore rock lighthouses requires several assumptions of the pressure distribution due to the breaking wave loading (Raby et al. (2019), Antonini et al. (2019). Due to the peculiar bathymetries and topographies of rock pinnacles, there is no dedicated formula to properly quantify the loads induced by the breaking waves on offshore rock lighthouses. Wienke’s formula (Wienke and Oumeraci (2005) was used in this study to estimate the loads, even though it was not derived for breaking waves on offshore rock lighthouses, but rather for the breaking wave loading on offshore monopiles. However, a thorough sensitivity analysis of the effects of the assumed pressure distribution has never been performed. In this paper, by means of the Wolf Rock lighthouse distinct element model, we quantified the influence of the pressure distributions on the dynamic response of the lighthouse structure. Different pressure distributions were tested, while keeping the initial wave impact area and pressure integrated force unchanged, in order to quantify the effect of different pressure distribution patterns. The pressure distributions considered in this paper showed subtle differences in the overall dynamic structure responses; however, pressure distribution #3, based on published experimental data such as Tanimoto et al. (1986) and Zhou et al. (1991) gave the largest displacements. This scenario has a triangular pressure distribution with a peak at the centroid of the impact area, which then linearly decreases to zero at the top and bottom boundaries of the impact area. The azimuthal horizontal distribution was adopted from Wienke and Oumeraci’s work (2005). The main findings of this study will be of interest not only for the assessment of rock lighthouses but also for all the cylindrical structures built on rock pinnacles or rocky coastlines (with steep foreshore slopes) and exposed to harsh breaking wave loading.

Discipline and Sex Differences in Angle-specific Isokinetic Analysis in Elite Skiers

2019 ◽  
Vol 40 (05) ◽  
pp. 317-330 ◽  
Author(s):  
Marine Alhammoud ◽  
Baptiste Morel ◽  
Clint Hansen ◽  
Mathew Wilson ◽  
Regis Mecca ◽  
...  
Keyword(s):  
Statistical Parametric Mapping ◽  
Knee Extension ◽  
Current Data ◽  
Screening Methods ◽  
Full Extension ◽  
Isokinetic Testing ◽  
Parametric Mapping ◽  
National Team ◽  
Alpine Skiers ◽  
The Impact

AbstractStandard outcomes of traditional isokinetic testing do not detect differences between various muscle mechanical properties. This study i) explored a novel analysis throughout the range of motion based on statistical parametric mapping and ii) examined the impact of sex and discipline on hamstrings/quadriceps torque in elite alpine skiers. Twenty-eight national team skiers (14 females, 14 males; 14 technical, 14 speed) undertook an isokinetic evaluation of the knee flexors/extensors (range 30–90°, 0° representing full extension). There was no effect of sex (p=0.864, d=0.03) and discipline (p=0.360, d=0.17) on maximal hamstrings-to-quadriceps ratio and no effect of discipline on maximal torque (p>0.156, d≤0.25). Hamstrings torque and hamstrings-to-quadriceps ratio were lower in females than males toward knee extension only (p<0.05). Quadriceps torque was greater after 72° of knee flexion in technicians than downhill skiers (p<0.05). The current data showed that statistical parametric mapping analysis identified angle-specific differences that could not be evidenced when analyzing only maximal torques and reconstructed ratios. This may enhance screening methods to identify pathologic knee function or monitor rehabilitation programs, and inform sex- and discipline-specific training in alpine skiing.

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.

Hydrodynamic Сonditions of the Bakalskaya Spit Degradation (Western Crimea) over the Past 30 Years

2021 ◽  
Vol 37 (3) ◽  
Author(s):  
B. V. Divinsky ◽  
R. D. Kosyan ◽  
◽  
Keyword(s):  
Discriminant Analysis ◽  
Coastal Zone ◽  
Satellite Images ◽  
Wind Waves ◽  
Total Duration ◽  
Wave Model ◽  
Statistical Characteristics ◽  
Wave Impact ◽  
Annual Variability ◽  
The Impact

Purpose. The paper is aimed at studying the morphodynamic features of the Bakalskaya Spit evolution being influenced by the sea wind waves and swell, namely assessment of inter-annual variations in the alluvial (erosion) areas of the Bakalskaya Spit coastline, analysis of inter-annual variability of the wind wave parameters, determination of the surface wave characteristics (or a combination of a few ones) responsible for the processes of the bottom material erosion or accumulation in the coastal zone. Methods and Results. Based on the analysis of satellite images for 1984–2016, the areas of the bottom material accumulation or erosion of the Bakalskaya Spit coastline were determined. Application of the spectral wave model permitted to obtain time series of the main parameters of wind waves and swell (significant wave heights and propagation directions) in the Bakalskaya Spit coastal zone with the 1 hr time resolution for the period from 1984 to 2016. The characteristics of surface waves responsible for the coastline deformation were revealed using the discriminant analysis. Conclusions. Analysis of satellite images of the spit made it possible to distinguish three periods in the history of the Bakalskaya Spit evolution: 1985–1997, 1998–2007 and 2007–2016. The first period was characterized by relative stability. The strongest erosion took place in 1998; after that the alluvial and erosion cases alternated for 10 years weakly tending to general erosion that constituted the second period. The third one that began in 2007 can be defined as the period of spit degradation accompanied by the irreversible loss of beach material. The basic parameters conditioning hydrodynamics of the Bakalskaya Spit water area are: total duration of storms; average and maximum values of significant heights of wind waves and swell. Statistical characteristics of the wind waves’ parameters are of a fairly strong inter-annual variability. According to the average and maximum indices, the wind waves directed close to the normal to the coastline (WSW and WNW) are the most developed. The applied discriminant analysis permitted to draw a statistically reliable conclusion that the direction of the final (average annual) wave impact on the coastal zone, conditioning the processes of sand accumulation or erosion was set by the waves directed to NNW, at that the swell contribution was dominant. The impact degree is conditioned by strong storms with the directions close to the normal to the coastline, namely, the WSW ones

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