Various types of models can be used in ship manoeuvring simulation in harbours and fairways. The most important distinction is physical (scale) models and mathematical (numerical) models.
Physical models can be used in two ways, viz. (1) steered by a human being, either on board of the ship or via a camera on board and remote control by the pilot on land, and (2) by using an autopilot system, where the ship follows a predetermined path and the autopilot system generates the rudder and engine orders to the ship.
Mathematical models can be incorporated in a ship manoeuvring simulator where real pilot/helmsman's orders are entered into the model and where the modelled ship reacts to these orders. Similarly to physical models with an autopilot system, mathematical models can also be provided with an autopilot system; in this case also the ship has to follow a predetermined line, and the autopilot generates orders to the ship in order to follow this line. The difference between runs made in man-steered models on the one hand, and autopilot models, on the other, is that runs made with autopilots are, in principle, completely repeatable in the same conditions whereas man-steered models contain an important stochastic element and are therefore, in principle, unrepeatable.
An important point in physical scale models is the fact that they are not only subjected to length scale, but, due to this, also to time scale (being the root of the length scale for Froude models). This means that steering of a physical model by a human being is subject to an important time-scale factor (the word "important" is used as in most cases this time scale will be 5 or more, given a length scale of 25 or more). This means an important disadvantage of using such models in simulation processes. Physical models with autopilot systems do not suffer from this disadvantage. It should be borne in mind, however, that the use of autopilot systems gives limited information only and a physical model is considered quite costly for such limited information.
The advantage of physical models is that there is less doubt, compared to numerical models, about the validity of the model. Physical models are, therefore, still useful in situations where insufficient mathematical knowledge is available, or where physical processes can be modelled only by very complicated and extensive mathematical descriptions, making the mathematical model slow and sometimes unreliable. Examples of this are complicated bank suction situations and the passing or taking-over of other vessels with strong interaction effects.
Physical ship manoeuvring models can sometimes be combined with hydraulic models in harbour design (current models, wave penetration models), and then their use can be more obvious. As numerical models become more and more common also in hydraulics, the combination of a physical hydraulic model and a physical ship manoeuvring model cannot very often be realised anymore.
Mathematical models are nowadays quite common in ship manoeuvring. They are used in ship manoeuvring simulators and fast-time mathematical models. The most important advantage of a ship manoeuvring simulator over a scalemodel is that it allows real-time simulation, not only since the problem of time-scale is avoided, but also since it allows the representation of the ship, the bridge and the environment on real-world scale.
The mathematical models describing the physical process of ship manoeuvring are, depending on the type and extent of the model, considered adequate for not too complicated situatiohs. Most mathematical models are based on physical model tests to determine the various coefficients in the model. The model which is most used is the Abkowitz model; the number of coefficients in this model is not fixed. Due to this, there will be a difference in accuracy between various models. The importance of this depends on the problem which is modelled.
Equality between numerical models and prototypes can be shown only by comparison of results in well-known and not too complicated environmental conditions. The latter is mostly only true in deep water, and the comparison is limited to turning-circle, zig-zag and spiral tests and acceleration and deceleration tests. Most mathematical models are able to present proper comparisons in this respect. The behaviour of ships in shallow water is never checked in the real world due to the risks. Therefore, numerical models have to be checked. in such condition by comparison with scale model tests. These comparisons give sufficient evidence of the reliability of numerical models.
The quality of ship manoeuvring simulation models is not limited to the quality of the mathematical model describing the ship's motions and path alone. An important point is also the modelling of controls, like rudder, engine and tugs. This modelling is not limited to physics alone, but also to adequate modelling of the real behaviour of rudder, engine, propeller and tugs in terms of reaction characteristics.
For ship manoeuvring simulators, there is also the similarity between simulator outfit and the real ship. This has regard to the presence of a bridge, the quality of the outside view, the presence of sound and possible vibrations on the bridge, short-periodic motions due to waves, communication aspects, etc. Most important in this respect are the presence of a real ship's bridge and the outside view. The relevant aspects in the outside view are: a sufficiently large view angle, a realistic view regarding motions (especially rate of turn), a realistic quality of the view with not too much details, and a picture which is as good or as bad as reality.
Attention in ship manoeuvring simulation studies is generally focussed on the validity of the mathematical ship manoeuvring model. Although this is a very important aspect, other aspects deserve also attention. This applies to a proper problem formulation, the experimental design method, the choice of subjects (pilots and/or masters), and the method of data analysis and drawing conclusions from the investigation. Moreover, there is still the aspect that it is often not necessary to have a ship in the simulator, which is exactly similar to one specific prototype vessel. In reality ships differ also and pilots are used to this.
There are many different simulator designs varying from simple micro-simulators to very sophisticated simulators. It is the working group's conviction that all of them serve a certain purpose and can be used in one way or another in the design process of a harbour or fairway.
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The following chapters of the report cover all stages of the design of a revetment. The report, however, is restricted to flexible revetments. Revetments are generally defined as :
"any composition of natural or artificial materials, deliberately placed in a marine environment to protect from degradation any artificial or natural body of erodible material".
The restriction to flexibility excludes all rigid systems such as monolithic structures and vertical seawalls, but still includes all possible additional flexible protection systems, which may be necessary to compensate for adverse effects associated with rigid structures in the marine environment. The most common examples of the latter are bottom and scour protection systems.
The Working Group has acknowledged the close correlation between hydraulic and geotechnic loadings, thus the need for integrated design procedures for a flexible revetment.
The revetment is intentionally treated as a system with the following constituents as subsystems or elements: a cover layer, one or more sublayers, and a core or base. Various functions may be assigned to sublayers, turning them into filter layers or cushion layers (Chapters 2 and 3).
Further, the fact that most revetments are part of a larger scheme or a coastal defence system may require a design philosophy that fits into a wider project or management policy. Therefore, risk assessment, environmental assessment, construction and future management of structures are typical aspects, besides the mere technical design, that are treated.
The contents of the final report are summarized below.
In Chapter 2, "Systems and Materials", an overview is given of the various materials that may be used. Their functions are also treated and it is shown how they can be combined in the revetment system as a whole, in a variety of environmental conditions.
"Design Philosophy" is treated in Chapter 3, where it is described how the functions of the revetment can be identified and defined to be used as criteria for the selection of alternatives. New means aimed at rational optimization of the design are provided by probabilistic design methods, which are shortly outlined while reference is given inter alia to PIANC reports (illustration of which is given in Section 5.8). A practical list of elements for quality systems is provided, applying to revetment design.
In Chapter 4, "Design Conditions", methods to determine the environmental boundary conditions are treated. "Hydraulic boundary conditions" (Section 4.2) concentrate on wave and wave-related parameters and under Section 4.3 the parameters related to the subsoil and other granular materials are addressed. Other boundary conditions having usually a much larger time scale and considerable impact are described under "Coastal Morphology" (Section 4.1) and in Section 4.4 dealing with climatic and other conditions.
In Chapter 5, "Design Procedures", a variety of design data, considerations and boundary conditions reappear, to be used in specific descriptions of interactions or responses between boundary conditions and the revetment system "External and Ice Loadings", Section 5.3.1 and "Internal Loading" in Section 5.3.2).
Further, design considerations, boundary conditions and interactions are combined into procedures, which the designer can fit into his specific problem.
Chapter 6 begins (in 6.1) with practical design considerations focusing on specifications, construction aspects, inspection and maintenance, and environmental and climatic conditions. In Section 6.2 "Economic Considerations" the basic principles to assess investment and management costs are outlined. The selection of alternatives based on these considerations is elaborated with an example. Section 6.3 basically consists of a "Designer's Checklist".
The report contains a glossary of terms and notation. In the two first appendices, practical examples of the design and performance of existing structures are presented in the form of a case study and an inventory of experiences. The third appendix includes recommendations for future work.
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Working Group 22 was set up by Permanent Technical Committee II (PTC 11) in March 1991.
Good progress was made until 1993 when, due to the economic recession,there was no available time to enable the work to be progressed. Drafting of the report was recommenced in January 1995.
The report gives practical guidelines on design methods, in a simplified form for everyday use in a commercial office. The results are based on reasonable approximations, bearing in mind that only rarely can a berth be designed for a specific ship with known characteristics.
The guidelines for design are based on the published work of many researchers and, wherever possible, sources have been referenced throughout the text. The Working Group apologises for any inadvertent omissions.
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