The Study on Ship Navigation Method based on Virtual Reality

Abstract. As to accurately simulate the ship navigation in marine environment, this paper developed a Ship Navigation Virtual Reality System by using numerical simulation technology. Through this method, all of the marine environment parameters have been simulated in detail. After that, the simulated datum of wind, current and wave are introduced into simulating the ship movement and motion in this marine environment. At the same time virtual reality technology are used to create the 3D sea scene including marine environment and ship motion. It makes the vision effect better. This research proves SNVRS is a useful tool to simulate the ocean environment. It has good performance and can be widely used in research of virtual reality.

Key words:ship; navigation; virtual reality

1.Introduction

Virtual reality (VR) is a new technology based on the main computer graphics techniques, the simulation technology and sensor technology, etc. This technology has been widely used for navigation training, evaluation hydraulic construction, battlefield environment simulation, etc. In the past few years, this technology has been introduced into ship navigation simulation. Through this method, Marine environment scene special place can be rebuilt and ship motion can are described in detail. Through these studies, much progress has been made in [1]. But no matter the environment and ship motion is actually scene with the case is not focused on. Of course, ship motions also not necessarily the case. Its development numerical prediction method is the environment characteristics of ship motion simulation results by motor equation.

2.THE CONSTRUCTION OF SHIP NAVIGATION VIRTUAL REALITY SYSTEM

The whole Ship Navigation Virtual Reality System (SNVRS) includes three parts as shown in Figure1: (1) the environment forecast module; (2) the ship maneuver forecast module; (3) the scene virtual reality module.

The environment forecast module is used to forecast the environment factors in the navigation area. For example, wind, fog, current, wave and rain can be calculated by this module. After that, the ship maneuver forecast module used these environment factors to calculate the ship movement and motion in this area based on the maneuver equation. The final step is to realize the virtual reality of the ship navigation. The whole scene, including environment, ship and wharf building, will be shown in three-dimension vision in ship virtual reality module.

3.THE ENVIRONMENT FORECAST MODULE

The Marine environment is a very complicated system, including the sea surface wind, current and high sea, and the sea surface temperature, wave high and so on. Some of them, such as the sea wind, current and wave directly influence the movement of the ship. How to get a realistic Marine environment is very important. Generally speaking, there are three ways to deal with the problem: geometry, fluid mechanics and statistics methods. Water power numerical simulation is a kind of method, has been widely applied in the weather and current simulation [3]. But because of the large computing cost, numerical simulation is rarely used in ship scene virtual reality. In this way, can get an away from findings in the field of environment, also make the ship navigation simulation more believable.

Figure.1 The Structure of the SNVRS

The Marine environment is such a place, where air, sea and wave of influence each other. The wind, and the waves generated current drive. On the contrary, the atmosphere can be influenced by ocean through the roughness length and the sea surface temperature. In order to improve the forecasting accuracy, these types of feedback mechanism should be a very good description. Many models developed world, but these are learning a theme. Model can simulate the air and sea atmosphere motion model is used to study the sea. How these single model and considering the interaction between them is a difficult problem. At the same time, always bring numerical model calculation problem, cannot be sure that the calculation efficiency. The whole environmental forecast module consists of three components: prediction of atmosphere, ocean wave prediction and predicting part of the parts. They influence each other.

Objective forecast module is predicting atmospheric air elements, such as the sea wind, rain, and visibility. Many methods can be used to do it. Estimates of the sea surface wind the calculation of a single radio wind depends on the wind, the weather chart fax surface. But now, the sea wind field numerical forecast main. These models are always limited-area, nonhydrostatic, terrain-following sigma-coordinate designed to simulate the mesoscale atmospheric circulation or prediction model.

In this paper, due to atmospheric fields covers place small, bird's nest grid technology has been applied in this part. The inner grid, this also is the fine grid, including port area. Outside of the grid, also is the thick grid, covers the inner grid. Outer grid is far outweigh the inner grid. Way of relaxation method is used to exchange between the nests of incomplete information dynamic and fine mesh thick grid. On a clear boundary grid model to establish a transition zone relax prediction variables. It ensures that the prediction variables of the rough model for transportation grid better, improve the stability of the model. The model's performance test, many cases have been tested. The result shows that, comparing with observation.

Current forecast part is the most important part of Environment forecast module and used to forecast the current, sea surface [4]. As to simulate these kinds of value in the limited area, the two-dimension shallow water equation as followed has been introduced. Also a numerical model has been developed. The Figure2 show the grid of simulated area Tianjin harbor. A rectangle grid is constructed with 320×200 horizontal grid points with horizontal resolution of 50m. Time step of current model is 5s. The entrance of breakwater has been defined as the only one open boundary.

Figure.2 The grid of harbor area

Wave forecast module is used to simulate the significant wave height, wave direction and wavelength. It is a key part to form the ocean scene. A fully third generation spectral numerical wave model SWAN (Simulating Wave Near shore) has been introduced in this paper. It is a third-generation wave model that computes random, short-crested wind-generated waves in coastal regions and inland waters.

4. SHIP MANEUVER FORECAST MODULE

As to simulate the ship maneuver, what should be done first is to construct a ship model. The ship model construction can be realized in 3DMAX software. Using this software, we can draw the three-dimension model of ship according to the ship design characters, such as ship length, width, draught, supper-structure, etc. Figure 3 shows a 3D oil tanker model. After that, the ASE file, output file of 3DMAX, will include the top value of three-dimension model, construction value of triangle nest, normal vector and vein picture. This kind file can be directly used in OPENGL.

Figure.3 The ship model of oil tanker

As to simulate the movement track of ship, sea surface wind and sea surface current are used in the equation (4) and (5).

where ,are the distances of ship movement in x and y direction respectively, is the ship speed,is the current speed,αandβare the angles of ship velocity and current velocity respectively.andare the excursion distances induced by wind in x and y direction respectively, which can be calculated by equation (6):

where K is coefficient (0.041), Ba is windward area of hull over waterline, Bw is windward area of hull under waterline, Vs is ship speed, Va is relative wind speed, T is the length of forecast time.

Comparing to the simulation of ship movement, simulation of ship motion is more complex. Ship pose in the ocean connects with not only hydrodynamic performance, but also the environment conditions. It can be simulated by ship maneuvering mathematical model [5] as follows.

where， are ship mass, mass moment of inertia, additional mass and additional mass moment of inertia respectively,r v u ,are ship longitudinal speed, transverse speed and rotation angular velocity； Y X,are the force in directionxandydirection respectively,

are force on hull, rudder and propeller respectively,Nis the moment on the hull.

In this paper, the force on the hull induced by environment factor can be defined as the following equation:

where,X ,Y are the force on hull in x and y direction respectively, are the force induced by wind, current and wave in x direction respectively,are the force induced by wind, current and wave in y direction respectively. These variables call can be calculated by corresponding function, such as force induced by wind can be describe by follow equation:

whereis the air density,is the hull projection area over waterline on x direction,is the hull projection area over waterline on y direction,are coefficients.

By combining the ship hydrodynamic parameters with forecasted environment elements, the 3D virtual sea scene with sky and wave can be constructed. Therefore, the 3D real time dynamic simulation of ship’s time-related motions (horizontal swing, horizontal sway and head-sway) in this scene can be realized.

5.VIRTUAL REALITY OF NAVIGATION ENVIRONMENT

After forecasting the marine environment and ship motion, the next step is to use the forecast results to create a 3D virtual sea scene. In this paper, OpenGL is selected to do it. As to make the sea scene realer, virtual reality technology has been introduced. There are so many simulation software, such as Vega, 3DMax, OpenGL, developed to do these work. Virtual reality of Navigation environment includes two aspects.

In spite that so many environment elements have been forecasted, such as wind, sea surface temperature, current, wave, rain, visibility, cloud and so on, only some of them, such as wave, current and visibility, are useful for virtual scene making.

This paper just describes how to draw the wave there. Firstly, it is important to get the data on entire eyeshot sea surface. Because environment forecast module can get the wave data only on net grid point, it should interpolate these data of grid points to entire eyeshot sea surface. This value is related to significant wave height, wave direction and wavelength.

Secondly, the sea surface color should be optimized. Because the values of wave height gradually change, sea surface color can also be considered gradual change. The sea surface colors of maximum and minimum wave height are defined beforehand and the sea surface color of any other wave height can be gotten by interpolation technology. Finally, it is important to set light effect on the wave surface. A typical illumination model in compute graphics is used in this paper.

Navigation building includes the wharfs, sea-routes, navigation mark, shoreline and so on. The subjects of navigation building can be draw from Electricity Navigation Chart (ENC). The traditional ENC is emphasizing particularly on 2D model designed displaying，especially to a lot of 3D information which can be visualized in 2D symbol or symbol or number under ECDIS. But the 2D model can not reflect such real sailing environment intuitively and rapidly as seabed under keel，island or mountain’s shape，and the visual environment under a ship’s keel Which constrained by her draught. But, at same time，there is much data in database of ENC, which can be displayed intuitively and real-time in 3Dmodel，for example 3D sounding data ， real-time water-level and tide information precaution from national marine department etc. It includes the DEM model of the seabed，dynamic simulative sea-surface，and shape or signal light model. Especially to the new building, the subjects can be gotten from DWG file of CAD. Figure 4 shows the vision effect of a dock.

Figure.4 The virtual reality of navigation building

6. CONCLUSION

In this paper, wave model has the same horizontal grid system as coarse grid of the atmosphere forecast part. The SWAN model includes the action balance equation, source term, boundary conditions and solution technique. The wave fields of different representative directions of the harbor area can be computed with SWAN model, and the wave parameters of the harbor can be gotten.

7. References

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