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THEORETICAL SUPPORT FOR PROFESSIONAL SHIP HANDLING
Max J. van Hilten (Maritime Pilots' Institute Netherlands, The Netherlands)
Abstract: In order to provide masters and pilots with a tool to theoretically support professional ship handling, the author, on behalf of the Maritime Pilots' Institute Netherlands (MPIN) and the Royal Netherlands Naval College (RNLNC), is developing a menu-driven computer program. This paper will give a brief overview of the whole package and will go into more detail about two modules of the computer program: 'wind loads on ships and complex structures' and 'the rate of turn required for geographically fixed turns during current or tidal stream'. On both subjects the importance of the information for manoeuvring in daily practice will be dealt with. The same goes for implementation in the so-called, brand new, portable pilot unit (PPU).
1. INTRODUCTION
A pilot's job includes a number of activities with regard to e.g. determination of position(navigation), positioning(manoeuvring), traffic related problems(colregs), communications and human resource management. With respect to all of these activities experience has to be gained and thorough training is required. The increasing complexity and size of vessels as well as the increasing complexity of traffic situations lead to extra demands. At the same time a large turnover and a threatening shortage of pilots puts increasing pressure on the capacity to meet these demands: experience has to be gained in a relatively short period of time. For these reasons the use of modern aids like computers, on a broader scale than before, becomes increasingly important. Not only for training purposes, also in daily practice. Two parallel developments were initiated by MPIN: the introduction of the so-called portable pilot unit (PPU) and computer programs for support of professional ship handling ('Pilot guide'). In this paper the outlines of two of the modules of the computer program will be described: wind loads on ships and complex structures, and the rate of turn required for geographically fixed turns during current or tidal stream. The portable pilot unit will also be looked at: it will be described in general terms.
2. 'PILOT GUIDE'
'Pilot guide' is the name of a computer program dealing with a number of items regarding manoeuvring; it is intended to be a tool for decision support. The calculation methods are based on either existing theory, or theoretical approaches developed, and in some cases published, by the Institutes mentioned above. Up till now the following modules are implemented or planned for implementation:
- Wind loads on ships and complex structures,
- Current loads on ships,
- Estimate of drift speed caused by cross wind,
- Approach of an anchorage in restricted water during current,
- Required length of anchor cable,
- Stopping distances,
- Current triangle,
- Turning circles (e.g. estimate of increase diameter caused by decrease of under keel clearance),
- Rate of turn planning in bends, water track,
- Rate of turn planning in bends, ground track,(geographically fixed turns),
Supplementary to the basic modules regarding maneuvering, other items will be added. e.g.:
- Datum transformations,
- Items of regional importance (e.g. data or calculations regarding tidal windows).
2.1 Requirements
Wherever possible the program should meet the following requirements:
- Easy to use in daily practice,
- Not time consuming,
- Accurate,
- Reliable,
- Based on sound theory,
- Possibility to use it for training and educational purposes.
Easy to use in daily practice and not time consuming; From daily practice and from the regular refreshment training of Dutch pilots we know that these requirements are of great importance. Pilots are absolutely willing to use theory and to make calculations provided it is not too complicated and it doesn't take too much time. On closer consideration this is not surprising since complicated calculations may lead to errors and time is often not available, especially in cases of calamities.
Accurate and reliable; A high level of accuracy can easily be achieved when a computer is used for the calculations. However the strength of a chain is determined by the weakest link and therefore it is important that the points of departure, e.g. calculation method and input data, are also correct. When they are, a high level of reliability is reached. It is not always easy to determine if the calculation method and data are correct since these methods are usually based on theoretical models and data are often based on model tests (as opposed to full scale measurements). Consequently the results of the calculations may differ from reality. This is not a problem as long as the users are warned for imperfections and if they are aware of this. The latter can be achieved by help pages embedded in the program and by information provided during training and education. In the context of reliability attention was also paid to the quality 'foolproof'.
Based on sound theory; Who is able to determine whether or not a theoretical model merits the assessment 'sound'? Nevertheless one theoretical approach has to be chosen in case more are available. An example of this problem is the choice we had to make for the development of the program for wind loads on ships. This will be explained in one of the following paragraphs.
Possibility to use it for training and educational purposes; We expect that the use of the same program for daily practice and for education purposes will lead to the advantage of integration of continuous professional education and daily practice. Therefore it is a precondition that help pages included in the program not only provide information regarding the use of the program, but also regarding the theoretical backgrounds. In combination with a reader, these help pages should provide sufficient information to enable the user to study the subject matter.
2.2 Structure of the program
The original version of 'Pilot guide' is a menu-driven program in Matlab environment. As a result of that the user is being led through the program which offers, depending on the module chosen, different levels of accuracy. Obviously the level of accuracy is closely connected with the time required to walk through the program. Buttons labeled 'previous' and 'help' offer the possibility to return to a previous page or to show information regarding the subject and / or the use of the program. An example of a page of the program is shown in figure 1: the forces being exerted on a passenger vessel. (Clicking on the shape of the vessel in the middle of the screen will show the contours and the data of the vessel concerned).
Fig. 1 |
Example of one of the pages of the module 'wind loads' of the computer program. |
3. MODULE 'WIND LOADS ON SHIPS AND COMPLEX STRUCTURES'
3.1 Introduction
This module was the first one to be developed. The initial reasons for a study into wind loads on ships and complex structures were:
- Wind has given rise to dangerous situations during maneuvers with barges or ships carrying high and / or complex structures several times in the past, despite the fact that usually both master and pilot make at least rough calculations as to the wind loads;
- A new extension of the port of Rotterdam should be accessible for container vessels up to 385 [m] length. A very important issue for these vessels is the calculation of wind loads. This was a reason for MPIN to extend the study to wind loads on these large vessels as well;
- Another reason was the substantial difference in the results of calculations used by a major shipping company and the approach used by the Netherlands' pilots;
Obviously the results of the study are incorporated in the relevant module of the computer program 'Pilot guide'. In the next sub paragraphs the outlines of the theoretical backgrounds will be discussed first, and then the implementation in the computer program will be dealt with.
3.2 Basic formula for wind loads
Often the basic formula for dynamic pressure is used for wind loads. The formula for loads in lateral direction may serve as an example of this approach:
Ywind = 1/2 CY ρ V2 AL (1)
Where:
Ywind = lateral wind load [N],
CY = lateral shape coefficient [-],
ρ = air mass density [kg.m-3],
V = relative wind velocity [m.s-1],
AL =dry lateral area [m2].
Variants on this formula are those using an effective dynamic pressure determined by integration with respect to height, or a division into separate components for dynamic pressure and the pressure drop on the leeward side.
3.3 Air mass density
The air mass density of dry air is a function of the atmospheric pressure, the air temperature, the height and the rate of change with the height of both the atmospheric pressure and the temperature. The humidity factor will not be addressed in this paper since this subject is very complex and beyond the original scope of the assignment. Using the standard atmosphere as a starting point, the effect of all of the other factors mentioned above is included in one single formula [1],[2]:
Where
z = height (variable) [m],
ρ(z) = height dependent air mass density [kg.m-3] ,
P0 = atmospheric pressure at sea level [Pa],
R = gas constant (for dry air) [m2s-2K-1],
T0 = air temperature at sea level [K],
a = rate of change of vertical temperature for standard atmosphere [K.m-1],
g0 = gravity constant at Mean Sea Level [m.s-2].
This formula is used in the computer program where it is included in a process of numerical integration with respect to height. Using a realistic temperature range and atmospheric pressure range as a starting point, the possible differences of temperature have a far greater influence on the air mass density than the possible differences of atmospheric pressure.
Fig.2 |
Comparison of vertical wind profiles for wind velocity 20.7 [m.s-1] at height l0[m].
□=power law profile, α =0.2; ο= logarithmic profile, z0=0.2; |
3.4 Wind velocity
Two important phenomena which have to be taken into account when defining the wind velocity are the vertical wind profile and the differences of maximum mean wind velocities for different averaging time intervals. The latter is often taken into account by using a gust factor.
Vertical wind profile; In a natural environment the wind velocity usually varies with the height. Therefore a vertical wind profile has to be defined. Meteorologists [3] usually lay down this profile by means of a logarithmic function:
Where:
V = wind velocity [in.s-1],
h = height [m],
Z0 = roughness length [m].
The roughness length is a measure for the roughness of the surrounding terrain. In this way the boundary layer effect is taken into account. Contrary to the meteorologists most technicians (e.g. [41, [5], [6], [7]) use a power law to define the vertical wind profile:
Vh1 =Vh2 (h1/h2)α (4)
Where:
α=height exponent［-].
Figure 2 shows a comparison of wind velocity curves above rough terrain for a power law profile using a height exponent = 0.2, and a logarithmic profile using a roughness length = 0.2. (The curves are not exactly the same but at least there appears to be a certain similarity). It is obvious that, as a result of variations of wind velocity with the height, the height of the wind sensor has to be taken into account when observing wind velocities! In weather forecasts the expected wind velocity is usually given for a height of 10 [m].
Gust factor (1); An important issue regarding the wind velocity is the phenomenon gust factor (GF), a factor to be applied to average wind velocities for longer periods of time to find the maximum average wind velocity for shorter periods of time. A simple approach is multiplication of a given mean wind velocity by a constant factor. Factors (corresponding with several shorter averaging time intervals) for the multiplication of a 1-hour average are mentioned by the Permanent International Association of Navigation Congresses (PIANC) [8]. An example is the factor 1.28 to find the 1-minute mean value. Using this approach, substitution of AL by L * dz (L=length), and taking into account the variation of both the air mass density and wind velocity with the height, formula (1) changes into:
Where:
L=length [m],
hl=lower limit of height interval [m],
hu=upper limit of height interval [m],
GF=gust factor [-],
V(z)=height dependent wind velocity [m.s-1].
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