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Kaasen K.E.,Norwegian Marine Technology Research Institute
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2015

The principle of dynamic positioning with thrusters is described and the problem of thrust allocation discussed. To allocate the demand for surge and sway forces and yaw moment from the DP controller to a greater number of thruster variables requires some extra mathematical conditions to make the problem uniquely solvable. Often the extra conditions are obtained by formulating a criterion for optimal thruster usage, typically an allocation that is power-minimal. Such problems usually cannot be solved without resorting to numerical search methods, which can be unattractive when the available time is scarce, which is the case when DP with allocation is used in model testing or when a large number of long stochastic simulations need to be carried out. For such cases, simple weighted least squares allocation with linear equality constraints is an attractive alternative. This method is very close to being power minimal provided the weights are chosen correctly. To handle cases of thruster saturation a method is proposed, which uses bias thrust and a certain matrix of nullspace vectors. A similar principle is proposed for presetting of favourable angles for azimuth thrusters to reduce the effect of the slow azimuth response.

Ormberg H.,Norwegian Marine Technology Research Institute | Bachynski E.E.,Japan Agency for Marine - Earth Science and Technology
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2012

This paper describes the extension of a well-proven state-of-the-Art simulation tool for global analysis of floating structures to also include offshore wind turbine applications, floating as well as bottom-fixed. All structural components of the wind turbine system are included in the finite element model of the complete system. The aerodynamic formulation is based on the blade element/momentum theory, including empirical tip loss and dynamic stall corrections and upwind tower shadow effects. Various wind field descriptions are included, covering simple steady uniform wind to fluctuating turbulent wind, with or without shear profile. A simple PI-control algorithm is used for regulation of the blade pitch angle, and the electrical torque is determined by a lookup table based on generator speed. The system response is calculated by nonlinear time domain analysis. This approach ensures dynamic equilibrium every time step and gives a proper time domain interaction between the blade dynamics, the mooring dynamics and the tower motions. The developed computer code provides a tool for efficient analysis of motions, support forces and power generation potential, as influenced by waves, wind, and current. Results from the developed code are presented and compared with results obtained from other simulation codes. This study also includes the sensitivity of results with regard to various modelling aspects. The last part of the paper presents a benchmark study against the codes of the Offshore Code Comparison Collaboration project. The floater motions, tower forces, and power generation, are presented and discussed. Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE).

Myrhaug D.,Norwegian University of Science and Technology | Fouques S.,Norwegian Marine Technology Research Institute
Coastal Engineering | Year: 2010

The paper provides a joint distribution of significant wave height and characteristic surf parameter. The characteristic surf parameter is given by the ratio between the slope of a beach or a structure and the square root of the characteristic wave steepness in deep water defined in terms of the significant wave height and the spectral peak period. The characteristic surf parameter is used to characterize surf zone processes and is relevant for e.g. wave run-up on beaches and coastal structures. The paper presents statistical properties of the wave parameters as well as an example of results corresponding to typical field conditions. © 2010 Elsevier B.V.

Perera L.P.,Norwegian Marine Technology Research Institute
IFAC-PapersOnLine | Year: 2015

A novel methodology to predict future vessel behavior in ship maneuvers is presented in this study. The method consists of estimating the respective vessel states by an extended Kalman filter with the measurements of vessel position, heading, yaw rate and acceleration values and calculating the future vessel position and orientation (i.e. heading) by a vector dot and cross product based algorithm, where the respective pivot point information is also incorporated. The proposed method is simulated and successful computational results on predicting future vessel behavior in ship maneuvers are also presented. Therefore, this method can be implemented in an integrated bridge system to improve the navigation safety in ship maneuvers. © 2015, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.

Wu M.,Norwegian Marine Technology Research Institute
Ocean Engineering | Year: 2014

Offshore renewable wind energy market is expected to expand dramatically in the next 5-10 years. Reduction of downtime is crucial to the competitiveness of this new sector. One important part of the research efforts is the evaluation of operational limits (weather limits) of different vessel/access concepts for transportation of maintenance personnel, equipment, and spare parts to the offshore wind turbines. This paper gives a brief description of possible types of service vessel and access system. It presents methodologies for numerical analysis of docking operation by an active motion compensated access device and a simple fender. The proposed frequency-domain approach to the analysis of docking operation with fender is new and highly efficient compared to time-domain simulation. The methods have been applied to two vessel/access concepts in this paper and they can be used in docking operability assessments for a variety of vessels that employ an active motion compensated device or a fender as access system to offshore wind turbines. © 2014 Elsevier Ltd. All rights reserved.

Norstad I.,Norwegian University of Science and Technology | Norstad I.,Norwegian Marine Technology Research Institute | Fagerholt K.,Norwegian University of Science and Technology | Laporte G.,HEC Montréal
Transportation Research Part C: Emerging Technologies | Year: 2011

Tramp shipping companies are committed to transport a set of contracted cargoes and try to derive additional revenue from carrying optional spot cargoes. Traditionally, models for ship routing and scheduling problems are based on fixed speed and a given fuel consumption rate for each ship. However, in real life a ship's speed is variable within an interval, and fuel consumption per time unit can be approximated by a cubic function of speed. Here we present the tramp ship routing and scheduling problem with speed optimization, where speed on each sailing leg is introduced as a decision variable. We present a multi-start local search heuristic to solve this problem. To evaluate each move in the local search we have to determine the optimal speed for each sailing leg of a given ship route. To do this we propose two different algorithms. Extensive computational results show that the solution method solves problems of realistic size and that taking speed into consideration in tramp ship routing and scheduling significantly improves the solutions. © 2010 Elsevier Ltd.

Kristiansen T.,Norwegian University of Science and Technology | Kristiansen T.,Norwegian Marine Technology Research Institute | Faltinsen O.M.,Norwegian University of Science and Technology
Journal of Fluids and Structures | Year: 2012

In this paper we propose and discuss a screen type of force model for the viscous hydrodynamic load on nets. The screen model assumes that the net is divided into a number of flat net panels, or screens. It may thus be applied to any kind of net geometry. In this paper we focus on circular net cages for fish farms. The net structure itself is modelled by an existing truss model. The net shape is solved for in a time-stepping procedure that involves solving a linear system of equations for the unknown tensions at each time step. We present comparisons to experiments with circular net cages in steady current, and discuss the sensitivity of the numerical results to a set of chosen parameters. Satisfactory agreement between experimental and numerical prediction of drag and lift as function of the solidity ratio of the net and the current velocity is documented. © 2012 Elsevier Ltd.

Kristiansen T.,Norwegian University of Science and Technology | Kristiansen T.,Norwegian Marine Technology Research Institute | Faltinsen O.M.,Norwegian University of Science and Technology
Applied Ocean Research | Year: 2012

In this study we present a numerical wavetank with a floating body based on a new domain-decomposition method. The method couples a Naviér-Stokes solver (CFD) with potential theory. The main feature is that the CFD domain is fully submerged in the fluid such that the free surface is computed in the potential domain. The thought is that potential theory is best at propagating waves, while the CFD incorporates flow separation e.g. at bilge keels. The presently implemented code is two-dimensional, but the method is directly applicable for three dimensions. The goal is to provide a methodology capable of being the basis for an engineering type of tool for analyzing gap resonance problems, such as moonpools and ship-by-ship operations. Focus is therefore put on computational speed. © 2011 Elsevier Ltd.

Korsvik J.E.,Norwegian University of Science and Technology | Korsvik J.E.,Norwegian Marine Technology Research Institute | Fagerholt K.,Norwegian University of Science and Technology | Laporte G.,HEC Montréal
Computers and Operations Research | Year: 2011

The purpose of this paper is to present and solve a new, important planning problem faced by many shipping companies dealing with the transport of bulk products. These shipping companies are committed to carrying some contract cargoes and will try to derive additional revenue from optional spot cargoes. In most of the literature on ship routing and scheduling problems a cargo cannot be transported by more than one ship. By introducing split loads this restriction is removed and each cargo can be transported by several ships. In this paper we propose a large neighbourhood search heuristic for the ship routing and scheduling problem with split loads. Computational results show that the heuristic provides good solutions to real-life instances within reasonable time. It is also shown that introducing split loads can yield significant improvements. © 2010 Elsevier Ltd. All rights reserved.

Utne I.B.,Sudan University of Science and Technology | Brurok T.,Norwegian Marine Technology Research Institute | Rodseth H.,Norwegian Marine Technology Research Institute
Journal of Loss Prevention in the Process Industries | Year: 2012

Improved condition monitoring of production equipment enhances process safety and the ability to prepare maintenance, to perform maintenance efficiently, and thus reduce downtime and associated costs. Currently, there is a somewhat limited focus on condition monitoring of static equipment, such as heat exchangers and separators. This is due to organizational barriers, available technology, and budget constraints at management level. The objective of this article is to present a three-step approach that supports the decision-maker in the selection of condition monitoring methods for production equipment. The approach is exemplified by a heat exchanger and the focus is on condition monitoring in the operational phase, including assessment of life cycle costs (LCC). © 2011 Elsevier Ltd.

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