| 1. |
- Alexandersson, Martin, et al.
(författare)
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Prediction of roll motion using fully nonlinear potential flow and ikeda’s method
- 2021
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - : International Society of Offshore and Polar Engineers. - 1098-6189 .- 1555-1792. - 9781880653821 ; , s. 1670-1679
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Konferensbidrag (refereegranskat)abstract
- Getting the best possible accuracy with the lowest possible computational cost is an important factor in the early design stage of ships. Potential flow-based analysis presents such a solution for seakeeping analyses. The accuracy of roll motion in potential flow is however not so good, due to the large influence from vicsous roll damping, which is missing in these calculations. This paper proposes a hybrid method, as a solution to this problem, where the viscous roll damping from Ikeda’s semi-empirical method is injected into an existing 3D unsteady fully nonlinear potential flow (FNPF) method. The hybrid method is investigated using roll decay tests with the KVLCC2 test case. This investigation shows that the accuracy of simulated roll motions is significantly improved and also shows good agreement with the corresponding roll decay model tests.
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| 2. |
- Baxevani, Anastassia, et al.
(författare)
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Modelling significant wave height in the North Atlantic
- 2003
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189. ; , s. 30-37
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Konferensbidrag (refereegranskat)abstract
- The surface of the ocean, and so such quantities as the significant wave height, can be thought of as a random surface in space which develops over time. In this paper, we explore certain types of random fields (in space and time) as models for the significant wave height and fit these models to data obtained from the TOPEX-Poseidon satellite. The data consist of observations along different one-dimensional tracks over time. It is assumed that, for the region of ocean considered and for a fixed time, the data can be considered stationary. Further-more, the shape of the data suggests that it is reasonable to use a lognormal distribution. As the covariance function may change over time, the model chosen is fitted to the data for each time separately. The data over space exhibit variation at different scales and hence the covariance function needs to reflect this property. Consequently, a mixture of Gaussian functions is assumed for the covariance function. To fit the model to the data, the theoretical variogram is fitted to the empirical variogram using weighted least squares. Stochastic models for the variation of the parameter values were investigated. The results of fitting these models are discussed and interpreted.
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| 3. |
- Baxevani, Anastassia, et al.
(författare)
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Relation between velocities and global maximum for Gaussian seas
- 2004
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189. ; , s. 47-54
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Konferensbidrag (refereegranskat)abstract
- We study the probability distribution of the height of global maximum for a Gaussian random field evolving in time. Particularly, we discuss the role of the wave kinematics. It is observed that taking into account time dynamics of spatial characteristics results in distributions different from those obtained for the static case. We show how different characteristics of the sea surface like significant wave height, wave period and wave length along different directions, affect the distribution of the height of high crests. We demonstrate the results by computing the derived distribution for different Gaussian seas. Copyright
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| 4. |
- Calcagni, Danilo, et al.
(författare)
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Comparative analysis of the hydrodynamic performance of untwisted and twisted rudders using a hybrid RANSE/BEM model
- 2014
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189 .- 1555-1792. - 9781880653913 ; , s. 674-682
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Konferensbidrag (refereegranskat)abstract
- Results of a computational study on the hydrodynamic perfor- mance of twisted and untwisted rudders for a single screw vessel are presented. Ship flow simulations are performed by using a hybrid RANSE/BEM model. Numerical predictions by RANSE and BEM solvers are validated through comparisons against ex- perimental data for the ship considered in the study. The com- parative analysis of untwisted and twisted rudders is carried out by fully–appended ship flow simulations at uniform ship speed and rudder angles between 0 and 20 degrees. Numerical results predict comparable manoeuvring capabilities for the alternative layouts, whereas the twisted design allows to mitigate negative pressure peaks observed on the untwisted rudder surface over a range of helm angles.
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| 5. |
- Daiyong, Zhang, 1991, et al.
(författare)
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Sensitive analysis of voyage optimization algorithms for ship voyage planning
- 2021
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189 .- 1555-1792. ; , s. 2772-2779
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Konferensbidrag (refereegranskat)abstract
- Voyage optimization systems search for a ship's potential sailing area by estimating ETA, fuel consumption, fatigue, etc., along sailing area. However, the reliability of a voyage optimization system may be affected by different uncertainties, such as weather forecast input, ship performance models, and parameter settings in optimization algorithms. This study investigates voyage planning uncertainties associated with parameter setting of four optimization algorithms. Half years’ full-scale measurement of a containership sailing in the North Atlantic is used to study the sensitivities of voyage optimization due to parameter settings of the algorithms. Important parameter settings are summarized from the sensitivity analysis.
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| 6. |
- Engsig-Karup, Allan P., et al.
(författare)
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Unstructured Spectral Element Model for Dispersive and Nonlinear Wave Propagation
- 2016
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189 .- 1555-1792. - 9781880653883 ; 2016-January
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Konferensbidrag (refereegranskat)abstract
- We introduce a new stabilized high-order and unstructured numericalmodel for modeling fully nonlinear and dispersive water waves. Themodel is based on a nodal spectral element method of arbitrary order inspace and a σ-transformed formulation due to Cai, Langtangen, Nielsenand Tveito (1998). In the present paper we use a single layer ofquadratic (in 2D) and prismatic (in 3D) elements. The model has beenstabilized through a combination of over-integration of the Galerkinprojections and a mild modal filter. We present numerical tests ofnonlinear waves serving as a proof-of-concept validation for this newhigh-order model. The model is shown to exhibit exponentialconvergence even for very steep waves and there is a good agreementto analytic and experimental data.
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| 7. |
- Gharraee, Behrad, 1984, et al.
(författare)
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Numerical Simulation of Cavitation on a Horizontal Axis Tidal Turbine
- 2016
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189 .- 1555-1792. ; 2016-January
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Konferensbidrag (refereegranskat)abstract
- For tidal turbines mounted on floating structures the possibility ofcavitation occurring on the blades is higher than for seabed mountedtidal turbines. In this study we present Reynolds-Averaged Navier-Stokes (RANS) solutions of the well-studied Southampton three bladedhorizontal axis tidal turbine (HATT). The numerical simulations werecarried out using the ReFRESCO viscous flow solver using three typesof simulations: (i) steady wetted flow; (ii) unsteady wetted flow and(iii) unsteady cavitating flow. The wetted flow simulations gave overallgood prediction of thrust and power coefficients over the entireexperimental range of tip speed ratios (TSRs), with the unsteadysolution providing the better result. Low numerical uncertainties wereobtained for medium to high TSRs and larger for low TSR values,where the flow is transitional and highly separated. The dynamiccavitation simulation was carried out for the case of a cavitationnumber of 0.63 at a TSR of 7.5. The simulations showed a goodagreement of the extent of the sheet cavity. However, the dynamics ofthe sheet cavities have not been fully captured and the power and thrustcoefficients are under predicted compared to the experiments. This ismost likely due to lack of mesh resolution outside the wetted flowboundary layer where the cavity dynamics occur, and due to highnumerical and experimental uncertainties for such a complex flow case.The simulations showed that existing methodology used for computingcavitation on marine propellers could be applied to HATTs, yieldingreliable results. Importantly, simulation of cavitation on HATTs couldbe used as input for noise and erosion predictions.
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| 8. |
- Han, Jiru, et al.
(författare)
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An Artificial Potential Field Model for the Risk Assessment of Icebreaker Escort Operations
- 2022
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189 .- 1555-1792. ; , s. 1295-1302
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Konferensbidrag (refereegranskat)abstract
- The icebreaker escort operations are typical navigation models in Arctic shipping. The current studies mainly focus on analyzing and quantifying the ship navigation risk for icebreaker escort operations concerning ship-to-ship collision avoidance. However, scenarios of ship stuck in sea ice are also interested and lacking study. To this end, we proposed a risk assessment model based on the artificial potential field theory, including two parts: a) the icebreaker risk field and b) the sea ice risk field. The icebreaker risk field describes the ship-to-ship collision risk, while the sea ice risk field describes the risk of ships stuck in sea ice. The proposed model can determine fleet members’ minimum and maximum safety distances to enhance the safety of Arctic fleet shipping.
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| 9. |
- Hildeman, Anders, 1984, et al.
(författare)
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An evaluation of conditional spatial predictions of significant wave height based on the nonstationary spde model
- 2020
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Ingår i: Proceedings of the International Offshore and Polar Engineering Conference. - 1098-6189 .- 1555-1792. ; 2020-October, s. 2176-2183
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Konferensbidrag (refereegranskat)abstract
- The sea state plays an important role in offshore-and marine operations. It affects both direct costs as well as risks for human and/or material loss. A better understanding of the present-, near-future-, and far-future sea states will increase efficiency and safety in shipping since it allow a ship to reroute to a safer and/or more cost effective route. In the offshore industry it allows for minimizing downtime and aids in planning the construction of new offshore sites. Due to the complex nature of the sea state, its spatial distribution over a large region of ocean should be modeled using a probabilistic model. In this way, uncertainties due to lack of information and/or computing power can be quantified and decisions can be taken based on both what is known and what is not known. We analyze such a spatial probabilistic model in order to assess its ability to predict the significant wave height in the whole north Atlantic based only on measurements on a small line path, i.e., conditional prediction. This work is relevant for several applications, for instance data assimilation, oceanographic forecasting, and routing of ships.
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| 10. |
- Kjellberg, Martin, 1983, et al.
(författare)
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A nested domains technique for a fully-nonlinear unsteady three-dimensional boundary element method for free-surface flows with forward speed
- 2011
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Ingår i: 21st International Offshore and Polar Engineering Conference, ISOPE-2011; Maui, HI; 19 June 2011 through 24 June 2011. - 1098-6189. - 9781880653968 ; , s. 673-679
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Konferensbidrag (refereegranskat)abstract
- This paper presents the work on a fully nonlinear body-exact unsteady three-dimensional boundary element method (BEM) for free-surface flows with forward speed based on potential flow theory. The BEM is coupled with free-surface markers updated in a semi-Lagrangian way. Time-dependent nonlinear boundary conditions are applied on the instantaneous position of the free-surface and the wetted surface of thebody and the boundary value problem (BVP) is solved at each time step in an earth-fixed reference frame, thus using a moving grid technique. A fourth-order Runge-Kutta scheme is used for the time-integration of the free-surface boundary conditions controlling the free-surface markers.The ongoing development of this method is done with the outlook of computing ship motions although this paper only discusses captive hull computations with forward speed in calm water for the purpose of validating the forward speed effects in a controlled manner, thus keeping the same mathematical model and approach developed for the wave-body interaction.
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