Saeed Barzegar Valikchali, Mitchell Anderson, D. Molyneux, D. Steinke
The DP capability plot is a useful tool to show the limitations of a dynamic positioning system for ships or offshore structures under loading from wind, waves and ocean currents. At the preliminary design stage, it is desirable to use fast methods for calculating the forces and moments caused by the environment, preferably without the need for CFD simulations or model experiments. Empirical methods are available for predicting aerodynamic forces and moments, and hydrodynamic forces and moments from currents, but little is published for second order wave drift forces. Wave drift forces and moment calculations have been carried out using WAMIT, for a series of ship hulls from OSVs to VLCCs and the effects of wave direction and frequency on the Surge, Sway, and Yaw forces and moment have been studied. The presentation of the results allows the user to interpolate the resulting drift forces and moments as a function of wave direction for a given ship size. In terms of wave drift loads calculation, it is found that the very large vessels are dominant in the low frequency waves, while smaller size ships are in high frequencies. The wave frequency and direction in which maximum drift load occurs depends on the ship size.
{"title":"Estimating Second Order Wave Drift Forces and Moments for Calculating DP Capability Plots","authors":"Saeed Barzegar Valikchali, Mitchell Anderson, D. Molyneux, D. Steinke","doi":"10.1115/omae2019-96307","DOIUrl":"https://doi.org/10.1115/omae2019-96307","url":null,"abstract":"\u0000 The DP capability plot is a useful tool to show the limitations of a dynamic positioning system for ships or offshore structures under loading from wind, waves and ocean currents. At the preliminary design stage, it is desirable to use fast methods for calculating the forces and moments caused by the environment, preferably without the need for CFD simulations or model experiments. Empirical methods are available for predicting aerodynamic forces and moments, and hydrodynamic forces and moments from currents, but little is published for second order wave drift forces. Wave drift forces and moment calculations have been carried out using WAMIT, for a series of ship hulls from OSVs to VLCCs and the effects of wave direction and frequency on the Surge, Sway, and Yaw forces and moment have been studied. The presentation of the results allows the user to interpolate the resulting drift forces and moments as a function of wave direction for a given ship size. In terms of wave drift loads calculation, it is found that the very large vessels are dominant in the low frequency waves, while smaller size ships are in high frequencies. The wave frequency and direction in which maximum drift load occurs depends on the ship size.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"2011 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72790344","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jinlong Wang, Liao Lihui, F. Lim, Hui Zhang, Liangbin Xu, Leixiang Sheng, Jin Ruijia
Anti-typhoon drilling riser, a solution to overcome high time cost of offshore drilling riser emergency retrieval under the situation of imminent arrival of a typhoon, is to modify the existing drilling riser to make it disconnectable closer to the surface and leave the long riser string below (and subsea blowout preventer (BOP)) in a safe and freestanding mode to survive the typhoon. The freestanding riser is held up by a buoyancy can system. And during the normal drilling operation, the buoyancy can maintain neutral thus it has limited effect on the riser overall global performance. However, locally the buoyancy can system will have some effect on the riser system nearby. To study these effects, numerical analytical methodology and results of anti-typhoon drilling rise under connected mode and freestanding mode are proposed, and a model test of 1:21 scale factor is designed. Three configuration modes: freestanding mode, connected mode and disconnecting mode are simulated in the tank test. A series of load cases under various current and wave, buoyancy upthrust are conducted in the experimental test to evaluate the hydrodynamic and strength performance of the riser near buoyancy can. The numerical solution and model test design can be a significant basis of water tank test for anti-typhoon drilling riser and a valuable reference for deepwater drilling engineering.
{"title":"Numerical Solutions and Model Test Design for Anti-Typhoon Drilling Riser","authors":"Jinlong Wang, Liao Lihui, F. Lim, Hui Zhang, Liangbin Xu, Leixiang Sheng, Jin Ruijia","doi":"10.1115/omae2019-95196","DOIUrl":"https://doi.org/10.1115/omae2019-95196","url":null,"abstract":"\u0000 Anti-typhoon drilling riser, a solution to overcome high time cost of offshore drilling riser emergency retrieval under the situation of imminent arrival of a typhoon, is to modify the existing drilling riser to make it disconnectable closer to the surface and leave the long riser string below (and subsea blowout preventer (BOP)) in a safe and freestanding mode to survive the typhoon. The freestanding riser is held up by a buoyancy can system. And during the normal drilling operation, the buoyancy can maintain neutral thus it has limited effect on the riser overall global performance. However, locally the buoyancy can system will have some effect on the riser system nearby. To study these effects, numerical analytical methodology and results of anti-typhoon drilling rise under connected mode and freestanding mode are proposed, and a model test of 1:21 scale factor is designed. Three configuration modes: freestanding mode, connected mode and disconnecting mode are simulated in the tank test. A series of load cases under various current and wave, buoyancy upthrust are conducted in the experimental test to evaluate the hydrodynamic and strength performance of the riser near buoyancy can. The numerical solution and model test design can be a significant basis of water tank test for anti-typhoon drilling riser and a valuable reference for deepwater drilling engineering.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"2 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76906408","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A strongly coupled hydroelastic code based on OpenFOAM has been applied to evaluate dynamic responses of a bottom mounted monopile in regular waves at 3 different wave periods and 3 wave steepness. The code uses a free surface RANSE solver to determine accurately the hydrodynamic loads and a modal superposition approach to describe the dynamic structural responses. Numerical simulations include both rigid and elastic monopiles and the results are evaluated against publicly available model test experiments. The discussion includes a quality control of the numerically simulated incident wave fields, mesh dependency and the quality of the numerically predicted and experimentally measured third harmonic of the inline forces.
{"title":"Dynamic Response of Monopile Wind Turbine in Large Waves","authors":"S. Seng, C. Monroy, Š. Malenica","doi":"10.1115/omae2019-95288","DOIUrl":"https://doi.org/10.1115/omae2019-95288","url":null,"abstract":"\u0000 A strongly coupled hydroelastic code based on OpenFOAM has been applied to evaluate dynamic responses of a bottom mounted monopile in regular waves at 3 different wave periods and 3 wave steepness. The code uses a free surface RANSE solver to determine accurately the hydrodynamic loads and a modal superposition approach to describe the dynamic structural responses. Numerical simulations include both rigid and elastic monopiles and the results are evaluated against publicly available model test experiments. The discussion includes a quality control of the numerically simulated incident wave fields, mesh dependency and the quality of the numerically predicted and experimentally measured third harmonic of the inline forces.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89482420","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Min-guk Seo, Y. Ha, NamWoo Kim, B. Nam, Kangsu Lee
This study considers the wave impact loads on the semi-submersible structure. To evaluate wave impact loads on the semi-submersible structure, a series of experiments are conducted in a 2D wave flume. In the experimental test, the semi-submersible half model is used, and 11 uniaxial force sensors are installed in deck side, column side, and deck bottom. Wave probes are, also, attached in the test model to measure the relative wave elevation. To generate horizontal and bottom wave impact on the test model, focusing wave is applied. The test model is fixed without any motion during each test, while the trim angle of the test model is changed to examine the effect of trim angle on wave impact load. Through this, the characteristics of the wave impact force at each position were investigated.
{"title":"Experimental Evaluation of Wave Impact Loads on Semi-Submersible Structure According to Trim Angle","authors":"Min-guk Seo, Y. Ha, NamWoo Kim, B. Nam, Kangsu Lee","doi":"10.1115/omae2019-95406","DOIUrl":"https://doi.org/10.1115/omae2019-95406","url":null,"abstract":"\u0000 This study considers the wave impact loads on the semi-submersible structure. To evaluate wave impact loads on the semi-submersible structure, a series of experiments are conducted in a 2D wave flume. In the experimental test, the semi-submersible half model is used, and 11 uniaxial force sensors are installed in deck side, column side, and deck bottom. Wave probes are, also, attached in the test model to measure the relative wave elevation. To generate horizontal and bottom wave impact on the test model, focusing wave is applied. The test model is fixed without any motion during each test, while the trim angle of the test model is changed to examine the effect of trim angle on wave impact load. Through this, the characteristics of the wave impact force at each position were investigated.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"23 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89075533","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
For reduced-scale model tests of gravity-installed anchors (GIAs), it is of great significance to extrapolate the testing results to prototype. This highlights the necessity of investigation of similarity criteria. The present work aims to find the similarity criteria of three prioritized hydrodynamic characteristics including VT, HP, and Cd for GIAs during installation in water through CFD simulations. In the present study, free falling processes of different reduced-scale T98 anchor models and prototype anchor is simulated, from which VT, HP, and Cd are extracted and analyzed to get the fitting curves for these three characteristics over reduced-scale λ. Based on these curves, hydrodynamic characteristics for prototype and other reduced-scale model can be extrapolated from model testing results. And, the researching procedure in this paper sets an example and reference to study about similarity criteria for other hydrodynamic characteristics.
{"title":"CFD Analysis on Similarity Criteria of Hydrodynamic Characteristics for Gravity-Installed Anchors","authors":"Jiancai Gao, Haixiao Liu","doi":"10.1115/omae2019-95960","DOIUrl":"https://doi.org/10.1115/omae2019-95960","url":null,"abstract":"\u0000 For reduced-scale model tests of gravity-installed anchors (GIAs), it is of great significance to extrapolate the testing results to prototype. This highlights the necessity of investigation of similarity criteria. The present work aims to find the similarity criteria of three prioritized hydrodynamic characteristics including VT, HP, and Cd for GIAs during installation in water through CFD simulations. In the present study, free falling processes of different reduced-scale T98 anchor models and prototype anchor is simulated, from which VT, HP, and Cd are extracted and analyzed to get the fitting curves for these three characteristics over reduced-scale λ. Based on these curves, hydrodynamic characteristics for prototype and other reduced-scale model can be extrapolated from model testing results. And, the researching procedure in this paper sets an example and reference to study about similarity criteria for other hydrodynamic characteristics.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"29 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76419564","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The wave-induced responses in the bridge girder of long floating bridges supported by pontoons are often dominated by the vertical modes, coupled horizontal modes and rotational modes about the longitudinal axis of the bridge girder. Pontoons with and without bottom flanges have been seen in recent floating bridge designs. Viscous flow separation around the sharp edges of the pontoon or the bottom flange may have strong influences on the hydrodynamic performance of the pontoon in terms of wave excitation, added mass and damping effects. Morison-type wave and current loads are normally included empirically in the early design phases to account for the viscous effects that cannot be covered by a potential-flow solution alone. Empirical drag coefficients and perhaps a correction to the potential-flow added mass are the inputs to such numerical models, which represents a part of the modelling uncertainties. Previous sensitivity studies using different drag coefficients in the ongoing Bjørnafjord floating bridge project in Norway indicate an influence up to 15% on the maximum vertical bending moment around the weak axis of the bridge girder. This paper contributes to the understanding of viscous effects on the hydrodynamic characteristics, e.g. the added mass, damping and wave excitation loads, of a floating bridge pontoon with and without keel plate. This is achieved by exploring existing model tests for floating bridge pontoons, performing 2D Computational Fluid Dynamic (CFD) analysis for pontoon cross sections and numerical calibration in a simplified frequency-domain model with linearized drag loads. Scale effects are also investigated through CFD analyses in model and full scales.
{"title":"Numerical Investigation of Wave-Frequency Pontoon Responses of a Floating Bridge Based on Model Test Results","authors":"Yanlin Shao, X. Xiang, Jianyu Liu","doi":"10.1115/omae2019-96545","DOIUrl":"https://doi.org/10.1115/omae2019-96545","url":null,"abstract":"\u0000 The wave-induced responses in the bridge girder of long floating bridges supported by pontoons are often dominated by the vertical modes, coupled horizontal modes and rotational modes about the longitudinal axis of the bridge girder. Pontoons with and without bottom flanges have been seen in recent floating bridge designs. Viscous flow separation around the sharp edges of the pontoon or the bottom flange may have strong influences on the hydrodynamic performance of the pontoon in terms of wave excitation, added mass and damping effects. Morison-type wave and current loads are normally included empirically in the early design phases to account for the viscous effects that cannot be covered by a potential-flow solution alone. Empirical drag coefficients and perhaps a correction to the potential-flow added mass are the inputs to such numerical models, which represents a part of the modelling uncertainties. Previous sensitivity studies using different drag coefficients in the ongoing Bjørnafjord floating bridge project in Norway indicate an influence up to 15% on the maximum vertical bending moment around the weak axis of the bridge girder.\u0000 This paper contributes to the understanding of viscous effects on the hydrodynamic characteristics, e.g. the added mass, damping and wave excitation loads, of a floating bridge pontoon with and without keel plate. This is achieved by exploring existing model tests for floating bridge pontoons, performing 2D Computational Fluid Dynamic (CFD) analysis for pontoon cross sections and numerical calibration in a simplified frequency-domain model with linearized drag loads. Scale effects are also investigated through CFD analyses in model and full scales.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"29 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78187795","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Over the past years Heerema Marine Contractors (HMC) has developed the QUAD lift method enabling the lift of single piece objects up to 30,000mT. This development is driven by the demand from our clients to install or remove larger topsides, both in size and weight. Lifting with two (or more) vessels simultaneously has been done before. Unique is that the QUAD lift operation is performed on DP. Two vessels lifting the topsides from opposite sides, instead of lifting the topsides from one side, creates the possibility for increasing dimensions and layouts of the topsides. The connection of the two crane vessels to the lifted object means that all actions taken on one ship influences not only the lifted object but also the second crane vessel. This requires a clear communication plan and full understanding of the complete system such as the DP behaviour, stability, ballast operations and crane operations. During a QUAD lift operation the Dynamic Positioning (DP) system plays an important role. Therefore in the development process the DP-system was modified to ensure stability of the DP system throughout the entire operation. A systematic series of tests was performed to gain insight in the DP behaviour of both vessels with the goal to ensure a synchronous move with two vessels connected in a QUAD lift configuration. In 2018 a demonstration QUAD lift is performed using Balder and Thialf to show HMCs commitment to perform this kind of operations in the future. The QUAD lift demonstration lift was prepared and executed in the Heerema Simulation Center (HSC) with the offshore crew in preparation of the actual offshore execution. This paper will address the steps followed to prepare for the offshore execution, describing DP behaviour, vessel coupled dynamics and human factor aspects. Based on the offshore experience validation was performed on the numerical tools used in the preparation.
{"title":"QUAD Lift: Enabling Lifting of Larger Integrated Topsides","authors":"Ivan van Winsen, R. V. Dijk","doi":"10.1115/omae2019-95375","DOIUrl":"https://doi.org/10.1115/omae2019-95375","url":null,"abstract":"\u0000 Over the past years Heerema Marine Contractors (HMC) has developed the QUAD lift method enabling the lift of single piece objects up to 30,000mT. This development is driven by the demand from our clients to install or remove larger topsides, both in size and weight. Lifting with two (or more) vessels simultaneously has been done before. Unique is that the QUAD lift operation is performed on DP. Two vessels lifting the topsides from opposite sides, instead of lifting the topsides from one side, creates the possibility for increasing dimensions and layouts of the topsides.\u0000 The connection of the two crane vessels to the lifted object means that all actions taken on one ship influences not only the lifted object but also the second crane vessel. This requires a clear communication plan and full understanding of the complete system such as the DP behaviour, stability, ballast operations and crane operations. During a QUAD lift operation the Dynamic Positioning (DP) system plays an important role. Therefore in the development process the DP-system was modified to ensure stability of the DP system throughout the entire operation. A systematic series of tests was performed to gain insight in the DP behaviour of both vessels with the goal to ensure a synchronous move with two vessels connected in a QUAD lift configuration.\u0000 In 2018 a demonstration QUAD lift is performed using Balder and Thialf to show HMCs commitment to perform this kind of operations in the future. The QUAD lift demonstration lift was prepared and executed in the Heerema Simulation Center (HSC) with the offshore crew in preparation of the actual offshore execution.\u0000 This paper will address the steps followed to prepare for the offshore execution, describing DP behaviour, vessel coupled dynamics and human factor aspects. Based on the offshore experience validation was performed on the numerical tools used in the preparation.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"61 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80279634","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Soil cover, rock cover and a combination of soil and rock cover are common approaches to protecting pipelines, cables and umbilicals associated with offshore energy infrastructure. However, with the exception of a simplified rock cover case or a simplified sand cover case, there is limited published guidance with respect to this design consideration. Based on a bearing capacity approach, geotechnical numerical analysis techniques have been used to investigate the protection provided by a range uniform and mixed cover scenario. Effects related to rock placement in a slot-shaped trench have also been investigated. Relatively standard observations were noted with respect to the protection provided by uniform soil or rock cover cases, including noting the different behaviours associated with granular drained cover materials and cohesive undrained cover materials. Some significant observations were made concerning the mixed cover cases. Of particular note was the impact of very soft clay as a limitation on the protection provided by the rock material. The influence of adjacent soil conditions on the protection provided by a rock infilled slot-shaped trench was also noted from analysis results.
{"title":"Protection of Pipelines and Cables With a Combination of Soil and Rock Cover","authors":"D. Morrow, A. Small","doi":"10.1115/omae2019-95262","DOIUrl":"https://doi.org/10.1115/omae2019-95262","url":null,"abstract":"\u0000 Soil cover, rock cover and a combination of soil and rock cover are common approaches to protecting pipelines, cables and umbilicals associated with offshore energy infrastructure. However, with the exception of a simplified rock cover case or a simplified sand cover case, there is limited published guidance with respect to this design consideration.\u0000 Based on a bearing capacity approach, geotechnical numerical analysis techniques have been used to investigate the protection provided by a range uniform and mixed cover scenario. Effects related to rock placement in a slot-shaped trench have also been investigated.\u0000 Relatively standard observations were noted with respect to the protection provided by uniform soil or rock cover cases, including noting the different behaviours associated with granular drained cover materials and cohesive undrained cover materials. Some significant observations were made concerning the mixed cover cases. Of particular note was the impact of very soft clay as a limitation on the protection provided by the rock material. The influence of adjacent soil conditions on the protection provided by a rock infilled slot-shaped trench was also noted from analysis results.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"23 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76034171","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
To address the uncertainty in both the monitoring and the forecasting of the environmental conditions for offshore operations, DNV recommends the use of Alpha factors. These factors are defined for the environmental conditions in North Sea and the Norwegian Seas. Since no localised Alpha factors are available for regions outside of North Sea, these factors are conservatively applied everywhere in the world. In a region with benign weather characteristics such as the Mediterranean, this could be overly conservative for the selection of operational weather windows. A deterministic weather forecast is sufficient to assess weather risk for short term operations; nonetheless, uncertainties of longer term predicted weather should still need to be addressed. Shore float-in of umbilicals and cables requires a number of sequential activities with associated durations that build up to a total timeframe. This timeframe is beyond what can be accurately assessed using only deterministic weather forecasts. The operational activities are also very sensitive to tidal, current & sea-state variation; particularly associated with the use of personnel in the water like divers and waders for the removal of floats and the use of small crafts for control of the of the product alignment. The use of Ensemble Forecast which utilises probabilistic approach compared to the deterministic approach, addresses the uncertainty of long term weather forecasting and remove artificially added conservatism such as the Alpha factor. This forecast is derived from multiple simulations, each with minor variations of initial conditions and slightly modified weather models. Ensemble Forecast has been applied in offshore operation planning and execution in a project offshore Egypt in Mediterranean; particularly for a shore float-in of a main umbilical. It was used to define the main operational window, the start of the operation and weather risk beyond deterministic weather forecast. The use of the Ensemble Forecast for selection of overall weather window has been shown to optimise operability, reduce the risk of product being damaged, provide added confidence in the offshore operational safety for the personnel involved while minimising waiting on weather. The strategy of defining storm riding configuration for umbilical could enhanced overall operability. When the operational restrictions relating to limiting activities are anticipated to be surpassed but remains still within the storm riding sea-state, these activities could be safely halted and product held in place in stand-by mode until the weather conditions improved without affecting its integrity or overloading installation equipment.
{"title":"The Use of Ensemble Forecast in Defining Offshore Installation Operability: A Case Study on Umbilical Shore Float-In Operations","authors":"F. Tinoco, K. Ting, Kishor Chavan","doi":"10.1115/omae2019-96137","DOIUrl":"https://doi.org/10.1115/omae2019-96137","url":null,"abstract":"\u0000 To address the uncertainty in both the monitoring and the forecasting of the environmental conditions for offshore operations, DNV recommends the use of Alpha factors. These factors are defined for the environmental conditions in North Sea and the Norwegian Seas. Since no localised Alpha factors are available for regions outside of North Sea, these factors are conservatively applied everywhere in the world. In a region with benign weather characteristics such as the Mediterranean, this could be overly conservative for the selection of operational weather windows. A deterministic weather forecast is sufficient to assess weather risk for short term operations; nonetheless, uncertainties of longer term predicted weather should still need to be addressed.\u0000 Shore float-in of umbilicals and cables requires a number of sequential activities with associated durations that build up to a total timeframe. This timeframe is beyond what can be accurately assessed using only deterministic weather forecasts. The operational activities are also very sensitive to tidal, current & sea-state variation; particularly associated with the use of personnel in the water like divers and waders for the removal of floats and the use of small crafts for control of the of the product alignment.\u0000 The use of Ensemble Forecast which utilises probabilistic approach compared to the deterministic approach, addresses the uncertainty of long term weather forecasting and remove artificially added conservatism such as the Alpha factor. This forecast is derived from multiple simulations, each with minor variations of initial conditions and slightly modified weather models.\u0000 Ensemble Forecast has been applied in offshore operation planning and execution in a project offshore Egypt in Mediterranean; particularly for a shore float-in of a main umbilical. It was used to define the main operational window, the start of the operation and weather risk beyond deterministic weather forecast.\u0000 The use of the Ensemble Forecast for selection of overall weather window has been shown to optimise operability, reduce the risk of product being damaged, provide added confidence in the offshore operational safety for the personnel involved while minimising waiting on weather.\u0000 The strategy of defining storm riding configuration for umbilical could enhanced overall operability. When the operational restrictions relating to limiting activities are anticipated to be surpassed but remains still within the storm riding sea-state, these activities could be safely halted and product held in place in stand-by mode until the weather conditions improved without affecting its integrity or overloading installation equipment.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"84 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80456083","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Performing the structural analysis and its integrity evaluation is the ultimate goal of design. However, design value estimation based on load-based analysis is still used as a conventional procedure in the offshore industry. The conventional method can be overly conservative and unrealistic with inconsistent load conditions since external loads such as mooring/riser and higher order hull response is inconsistently considered based on simplified linear assumptions. To assess the reliable integrity of a floating offshore platform, the response-based analysis has been successfully applied. This paper presents a response-based time domain structural analysis of a floating offshore platform. Direct time domain structural analysis is applied by mapping of external environment loads on the floating platform at every instantaneous time interval. Accordingly, correct phase relationship between the various external loads and hull motion including nonlinear effects can be considered. For computational efficiency, present study uses a set of load components based on an efficiently selected basis function for hull motion and environment loadings. The stress time history is obtained directly by synthesizing the load components, and hence an actual time-domain structural response can be captured effectively. Thus, same structural analysis results can be used to evaluate both strength and fatigue criterion for a floating offshore structure. Present analysis method is successfully applied to the evaluation of extreme global strength for a conventional semisubmersible platform. Present time domain analysis result on the structure response is compared with conventional load-based analysis result.
{"title":"Response Based Time Domain Structural Analysis on Floating Offshore Platform","authors":"J. Kyoung, Sagar Samaria, Jang-Whan Kim, B. Duffy","doi":"10.1115/omae2019-96139","DOIUrl":"https://doi.org/10.1115/omae2019-96139","url":null,"abstract":"\u0000 Performing the structural analysis and its integrity evaluation is the ultimate goal of design. However, design value estimation based on load-based analysis is still used as a conventional procedure in the offshore industry. The conventional method can be overly conservative and unrealistic with inconsistent load conditions since external loads such as mooring/riser and higher order hull response is inconsistently considered based on simplified linear assumptions. To assess the reliable integrity of a floating offshore platform, the response-based analysis has been successfully applied.\u0000 This paper presents a response-based time domain structural analysis of a floating offshore platform. Direct time domain structural analysis is applied by mapping of external environment loads on the floating platform at every instantaneous time interval. Accordingly, correct phase relationship between the various external loads and hull motion including nonlinear effects can be considered. For computational efficiency, present study uses a set of load components based on an efficiently selected basis function for hull motion and environment loadings. The stress time history is obtained directly by synthesizing the load components, and hence an actual time-domain structural response can be captured effectively. Thus, same structural analysis results can be used to evaluate both strength and fatigue criterion for a floating offshore structure.\u0000 Present analysis method is successfully applied to the evaluation of extreme global strength for a conventional semisubmersible platform. Present time domain analysis result on the structure response is compared with conventional load-based analysis result.","PeriodicalId":23567,"journal":{"name":"Volume 1: Offshore Technology; Offshore Geotechnics","volume":"30 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84432348","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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