Dynamic Positioning in Single-Point Moorings
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ABSTRACT This paper deals with the concept of and a system for complete control of the slowly varying bow motion of a bow-moored ship using dynamic positioning. System performance is shown by simulated case studies with an offshore 1oading tanker. Significant improvements of motion behaviour and mooring force performance are demonstrated. A complete positioning system is presently runn1ng successfully onboard a 125000 dwt tanker in the North Sea. Some preliminary results are shown. INTRODUCTION Mooring of large ships at offshore terminals are done by single-point mooring. The ship is connected to the terminal through a single mooring line, or hawser, which allows H to position itself according to the preventing weather and sea conditions, like a weather-vane, and thereby to remain moored with a minimum mooring force when the direction of forces changes. A major problem faced by single-p01nt mooring In hostel areas is due to the motion of the ship and the terminal. The ship tends to oscillate slowly relative to its mooring point with a following oscillatory behaviour of the mooring stresses, and with the effects of wave following motions superimposed. When the mooring force exceeds a prescribed value, loading must be stopped and mooring abandoned. The low frequency ship motionstems from unstable ship mooring configuration and/or dynamic force excitations from wind and wavesl. From practical experience a certain amount of astern thrust is found to have a favorable effect on the slow motion, but it is questionable as a remedy for reducing peak hawser tension. Dynamic positioning (dp) has been applied with great success to freely moving ships and moored vessels for more than ten years. While keeping the ship In a fixed position is typical to most dp system applications, the desirable objective in single-point mooring is to keep the distance between the mooring terminal and the ship's bow as nearly constant as possible, while maintaining the overall weather-vane capability. In this paper a review of the essential slow motion behaviour of a bow moored tanker 1s presented with emphasis on stability and mode dynam1cs. The des1gn and the theoretical performance of a dp-system to meet the single-point mooring positioning objective Is presented and, finally, results from full scale tests with a 125000 dwt tanker during offshore loading are shown. ANALYSIS OF SLOW MOTIONS IN SINGLE-POINT MOORING Only static (elastic) properties of the mooring terminal affects the slow motion behaviour, while the high frequency (wave following) propert1es are reflected in the mooring force. Hence, the terminal is left out in the following analysis. Governing equations The governing equations describing the slow motion of a bow moored ship In wind, waves and current are reasonably well establ1shed. A detailed description is presented In1. Applying some symmetry assumptions, the equations of motion In moored ship coordinates.Keywords:
Mooring
Dynamic positioning
Tension (geology)
Position (finance)
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Rudder
Dynamic positioning
Position (finance)
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Transportation of crude oil in arctic areas via shuttle tankers can prove to be a viable alternative to a pipeline system. The shuttle tanker system will require offshore terminals equipped with suitable loading/mooring facilities. Year round berthing and loading of large displacement vessels in unprotected arctic offshore areas is a feasible yet untried concept. The principal design variables for mooring/loading systems for offshore arctic applications are discussed. The underlying considerations for the design of these facilities are highlighted and a number of mooring/loading options are presented. Methods to determine the mooring loads and the behavior of the moored vessel are also described.
Mooring
Offshore construction
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Dynamic positioning
Model Predictive Control
Position (finance)
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Due to the ever increasing size of tankers and the inadequate facilities in the United States to accommodate these large vessels, a case is presented for the construction of offshore terminals utilizing the single buoy mooring system. Background information on the increase in size of tankers since 1961 is given, and a comparison of the single buoy mooring system to the offshore island terminal concept is also included. Reasons are given for preference of the single buoy mooring system over the offshore island, and several varied uses of the single buoy mooring system are also included in the discussion.
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Based on the pseudo inverse algorithm with the weights,a rational and effective method of solving thrust allocation of ship is proposed.Because thrust allocation of ship dynamic positioning system causes fuel consumption of propulsion system,badly worn of thrusters and thrust error and so on.The whole thrust optimization allocation goal is to minimize energy consumption of the propulsion system,taking into account the rate of change of the angle of azimuth thrusters,prohibited area of thrust of thrusters,saturation and restriction of thrust and so on.Static solution gives thrust directions based on minimum energy.The direction angle of azimuth thrusters can slowly change,in considering banning angle cases.Dynamic solutions provides dynamic ability.After the combination,it become a kind of thrust allocation algorithm for the purpose of optimizing energy.The simulation results show that the combination of dynamic and static algorithms consumes less energy and the error is smaller than pure static algorithm.There is more extensive application prospect.
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This contribution presents a thrust allocation module for combined Dynamic Positioning and active Roll Damping of ships using solely its main propulsion system. To achieve the given thrust demands a multi-step optimization approach which considers the different spectral properties of the two regimes is proposed. Limitations of the thrusters, such as maximum thrust or rate constraints, are considered. The algorithm is tested on a set of artificial demands, which demonstrate the operating principle. A second setup shows the interaction of the allocation and the ship within a closed-loop Dynamic Positioning simulation. The results are promising and show that combined positioning and roll stabilization is possible with a single marine actuator system.
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Offshore mooring buoys are an inexpensive vessel storage solution. Unfortunately, offshore mooring buoys require substantially more water area than more expensive slips and piers. In some harbors, mooring anchorages have reached storage capacity and congest boat movement within harbors. The congestion in Newport Harbor, California triggered resident Marshall Duffield to propose replacing mooring anchorages with offshore mooring docks. These offshore docks enable vessels to be safely moored at a much higher density than mooring anchorages. By mooring vessels at a higher density, harbors can either reduce the water area dedicated to offshore vessel storage or they can increase mooring capacity without increasing the offshore mooring area. A case study of Newport Harbor demonstrates the use of mooring anchorages and the space savings potential of replacing the mooring anchorages with offshore mooring docks.
Mooring
DOCK
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Dynamic positioning
Position (finance)
Thrust vectoring
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Dynamic positioning
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