Einar Nygaard - Academia.edu (original) (raw)
Papers by Einar Nygaard
This CTD-report deals with CTD and water samples data obtained during the Russian Norwegian cruis... more This CTD-report deals with CTD and water samples data obtained during the Russian Norwegian cruise in the Kara Sea in August-October 1994. The Cruise was a cooperation between Arctic Antarctic Research Institute in Russia and Norsk Polarinstitutt in Norway. The cruise was divided in two parts. Part l of the cruise start ed in Archangels the 15th. of August and ended in Anderma the 19th. of September. Part 2 started in Anderma the 19th. of September and ended in Murmansk the I l th. of October. A Russian vessel named "Ivan Petrov" was hired for the expedition. Altogether 144 CTD-casts were made during the cruises.
Ocean Dynamics, 2018
This paper concerns full-scale wave measurements of individual wave heights and crest heights at ... more This paper concerns full-scale wave measurements of individual wave heights and crest heights at one location with water depth 190 m in the northern North Sea from 2004 up to date. The surface elevation was recorded by a Saab WaveRadar REX from one location in the northern North Sea. The measured individual wave heights and crest heights have been compared to well-known theoretical distributions. For wave heights, the Rayleigh distribution is found to form a conservative upper bound, whilst the Forristall distribution is considered to show a good accuracy, though somewhat conservative in the most severe sea states. For crest heights, the Rayleigh distribution is found to be nonconservative, whilst the Forristall distributions for long-and shortcrested sea both provide reasonable descriptions. Based on the present study, the Forristall distributions for individual wave height and crest heights in long-crested seas are considered to provide the most appropriate descriptions of these measured wave data.
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 2, 2002
A simplified reliability assessment is carried out for a generic jacket at 200m water depth. The ... more A simplified reliability assessment is carried out for a generic jacket at 200m water depth. The purpose is to indicate the sensitivity of the annual failure probability to the selected airgap and current design profile. Two example cases are considered. For one case the required airgap is defined by the 10−4 wave crest height, while for the other the required airgap is defined from the 10−2 wave crest height plus an uncertainty margin taken to be 10% of the crest height. For both cases, the required minimum design base shear capacities are determined both using the 10-year current profile (earlier practice at the Norwegian Continental Shelf) and the associated current profile (i.e. the current profile which when used in combination with the n-year wave height yields the n-year load). The investigation shown herein clearly demonstrates that the chosen airgap is a crucial parameter regarding the annual probability of structural failure. It is, furthermore, demonstrated that if a wave-deck impact is required in order to fail the structure (which will be the case for most jackets), the current modeling is not very important. However, if the structure is designed such that failure may occur for wave heights not reaching deck level (either due to a highly utilized design or a very generous initial airgap), the current modeling (both in terms of selected design profile and joint description of wave height and current speed) may be far more important.Copyright © 2002 by ASME
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, 2002
The accuracy of current modelling is critical when considering deepwater riser fatigue damage cau... more The accuracy of current modelling is critical when considering deepwater riser fatigue damage caused by vortex-induced vibrations (VIV). In the present study the use of empirical orthogonal functions (EOF) to extract the governing characteristics from huge amounts of current measurements has been assessed. The amplitudes of the time varying principal components (PC) have been organized into bins in scatter diagrams. The accuracy of this scatter diagram approach with different numbers of EOF modes involved has been evaluated in terms of riser VIV fatigue damage.
Volume 2: Structures, Safety and Reliability, 2009
Design of deep-water structures requires accurate models of currents versus water depth. Common m... more Design of deep-water structures requires accurate models of currents versus water depth. Common models are N-year profiles, which conservatively assume the N-year extreme current speed occurs simultaneously at each depth. To address this conservatism, we introduce Turkstra models of current profiles here. These yield a set of current profiles, each of which coincides with the N-year profile at a single depth and is reduced elsewhere. The degree of reduction is a direct function of the spatial correlation structure of the current process. Results are shown for a deep-water North Sea site, and compared with time-domain prediction of extreme loads for linear and drag load mechanisms. Extensions are suggested to combine these methods with procedures such as Empirical Orthogonal Functions, permitting the data to define the most economical set of basis vectors upon which the Turkstra logic is applied.
Volume 2: Structures, Safety and Reliability, 2011
To design marine structures in deep water, currents must be modelled accurately as a function of ... more 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 ...
This CTD-report deals with CTD and water samples data obtained during the Russian Norwegian cruis... more This CTD-report deals with CTD and water samples data obtained during the Russian Norwegian cruise in the Kara Sea in August-October 1994. The Cruise was a cooperation between Arctic Antarctic Research Institute in Russia and Norsk Polarinstitutt in Norway. The cruise was divided in two parts. Part l of the cruise start ed in Archangels the 15th. of August and ended in Anderma the 19th. of September. Part 2 started in Anderma the 19th. of September and ended in Murmansk the I l th. of October. A Russian vessel named "Ivan Petrov" was hired for the expedition. Altogether 144 CTD-casts were made during the cruises.
Ocean Dynamics, 2018
This paper concerns full-scale wave measurements of individual wave heights and crest heights at ... more This paper concerns full-scale wave measurements of individual wave heights and crest heights at one location with water depth 190 m in the northern North Sea from 2004 up to date. The surface elevation was recorded by a Saab WaveRadar REX from one location in the northern North Sea. The measured individual wave heights and crest heights have been compared to well-known theoretical distributions. For wave heights, the Rayleigh distribution is found to form a conservative upper bound, whilst the Forristall distribution is considered to show a good accuracy, though somewhat conservative in the most severe sea states. For crest heights, the Rayleigh distribution is found to be nonconservative, whilst the Forristall distributions for long-and shortcrested sea both provide reasonable descriptions. Based on the present study, the Forristall distributions for individual wave height and crest heights in long-crested seas are considered to provide the most appropriate descriptions of these measured wave data.
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 2, 2002
A simplified reliability assessment is carried out for a generic jacket at 200m water depth. The ... more A simplified reliability assessment is carried out for a generic jacket at 200m water depth. The purpose is to indicate the sensitivity of the annual failure probability to the selected airgap and current design profile. Two example cases are considered. For one case the required airgap is defined by the 10−4 wave crest height, while for the other the required airgap is defined from the 10−2 wave crest height plus an uncertainty margin taken to be 10% of the crest height. For both cases, the required minimum design base shear capacities are determined both using the 10-year current profile (earlier practice at the Norwegian Continental Shelf) and the associated current profile (i.e. the current profile which when used in combination with the n-year wave height yields the n-year load). The investigation shown herein clearly demonstrates that the chosen airgap is a crucial parameter regarding the annual probability of structural failure. It is, furthermore, demonstrated that if a wave-deck impact is required in order to fail the structure (which will be the case for most jackets), the current modeling is not very important. However, if the structure is designed such that failure may occur for wave heights not reaching deck level (either due to a highly utilized design or a very generous initial airgap), the current modeling (both in terms of selected design profile and joint description of wave height and current speed) may be far more important.Copyright © 2002 by ASME
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, 2002
The accuracy of current modelling is critical when considering deepwater riser fatigue damage cau... more The accuracy of current modelling is critical when considering deepwater riser fatigue damage caused by vortex-induced vibrations (VIV). In the present study the use of empirical orthogonal functions (EOF) to extract the governing characteristics from huge amounts of current measurements has been assessed. The amplitudes of the time varying principal components (PC) have been organized into bins in scatter diagrams. The accuracy of this scatter diagram approach with different numbers of EOF modes involved has been evaluated in terms of riser VIV fatigue damage.
Volume 2: Structures, Safety and Reliability, 2009
Design of deep-water structures requires accurate models of currents versus water depth. Common m... more Design of deep-water structures requires accurate models of currents versus water depth. Common models are N-year profiles, which conservatively assume the N-year extreme current speed occurs simultaneously at each depth. To address this conservatism, we introduce Turkstra models of current profiles here. These yield a set of current profiles, each of which coincides with the N-year profile at a single depth and is reduced elsewhere. The degree of reduction is a direct function of the spatial correlation structure of the current process. Results are shown for a deep-water North Sea site, and compared with time-domain prediction of extreme loads for linear and drag load mechanisms. Extensions are suggested to combine these methods with procedures such as Empirical Orthogonal Functions, permitting the data to define the most economical set of basis vectors upon which the Turkstra logic is applied.
Volume 2: Structures, Safety and Reliability, 2011
To design marine structures in deep water, currents must be modelled accurately as a function of ... more 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 ...