Ship concept design
Unlike a market economy in which new design is a response to a perceived market need, new ship design is typically in response to an expressed need coming from such sources as the advent of new technologies, changes in world politics, new strategies and, lessons learned from previous ship development. This need is identified by Naval planners in the form of a design brief, which will be the basis for subsequent development. This brief will be taken up by designers, who will explore ways in which the design brief can be met, and will eventually develop the most promising of these into detailed instructions for manufacture. In this effort, they will be assisted by design analysts, who use analysis and simulation techniques to test the fitness for purpose of the design proposals, and development engineers who carry out experimental work on the test rigs and on prototypes to make detailed refinements of the design. This group will be supported by research engineers, who carry out experimental or theoretical work to fill in gaps in understanding of materials, processes, or techniques.
The ship design spiral process focuses on what the ship needs to carry. Typically ship machinery is dictated and the owner has decided to buy a certain type of engine or the Navy has a certain machinery system which plays a very important parameter in terms of cost and complexity. From there, the shape of the hull form and displacement are considered, because the ship has to float. “TGIF, to a naval architect does not mean Friday, it means floats,” Day added. Then the next step, in ship design, is powering. Existing machinery and fuel conservation must be considered with waves and stability. With the advent of computer-aided design and concurrent design, many of these tasks are completed simultaneously.
The design of a naval warship is one of the most complex tasks with which an engineer can be involved. The process involves to the synthesis of an enormous variety of intellectual inputs to develop an efficient and effective weapons platform, capable of operating in an increasingly hostile environment.
The genesis of a naval warship design lies in the desired functions and performance of the vessel against a perceived threat. The naval design function commences with the analysis of this requirement with the purpose of developing a variety of conceptual solutions, ranging from the conservative to the abstract and encompassing the latest technological advances and developmental research. These studies include :
- number and mix of platforms;
- inter-relationship with existing defence equipment, equipments and facilities;
- weapon types and combinations;
- alternative hull form configurations and sizes.
The development of the required concepts is based on extrapolation from existing designs, design databases, appropriate design algorithms-design software programs, reasoned intuition and estimation-all of which are assisted by computer aided mathematical modelling techniques. These conceptual solutions will be developed in sufficient detail to enable the initial evaluation of the following aspects :
- operational capability;
- technological risk/engineering complexity;
- cost (acquisition and through-life).
The information derived from this conceptual analysis will identify those solutions worthy of further investigation, and their relative merits. Concepts showing the greatest promise will be studied and developed in greater detail-specifically in those areas of high technical risk or offering cost reduction. These design studies would include trade-off studies in :
- individual components of the weapon system and system configuration;
- optimisation of the hull form with respect to powering, seakeeping and stability;
- ship signature reduction methodologies;
- propulsion machinery alternatives; and
- manning/automation.
At the completion of this phase of the development process, a single preferred design configuration will have been selected, with associated systems and major equipments having been determined and high risk aspects reduced to an acceptable level. A baseline general arrangement drawing will be available, together with outlines of all major compartments and spaces. These include :
- topside arrangement including antenna arrangement, combat system equipment locations and weapon firing arcs;
- main and auxiliary machinery space arrangements;
- command and control compartments including the bridge;
- weapons compartments; and
- flight deck and hanger layout.
The performance characteristics, both for the ship as a whole and for each individual component system, will be available and consequently an accurate cost estimate will be derived.
The next stage of the process is aimed at developing a complete engineering definition of the ship to a level of detail sufficient that design for construction can commence immediately upon the award of a contract to the shipbuilder. The primary deliverable of this phase is the ship specification, which contains details of:
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all compartment arrangements;
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system specifications including performance of all equipments and associated integration details, design and construction standards, shock specification, availability, reliability and maintainability;
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full engineering analysis including weapon performance characteristic modelling, ship structural analysis, noise and vibration analysis, hydrodynamic assessment including model testing, weight and stability assessment;
A new warship must undergo many tests and trials before acceptance into service. Test and trials measure the performance of a warship with respect to the requirements at the time the ship is to enter service. They also prove that the shipbuilder is delivering the warship to the contracted level of performance. Tests and trials are a check against specifications and design requirements for individual equipment and systems that make up the warship, and the overall, integrated performance of the warship. Rigorous checking is required because slight deviations from the specification for one system may affect the performance of other systems.
The range of trials covers the operation of individual equipments through to the operation of the systems of which those equipments are a part, and ultimately the operation of the entire warship. Due to the long gestation period between the initial definition of performance requirements and the time that a ship is finally delivered, there may be a difference between the contracted requirements and the contemporary operational requirements. In this case the shortfall must be made good at the earliest opportunity.
By its nature, shipboard repair work is complex and demanding. The compact arrangement of machinery and systems aboard ship, the sophistication of systems installed in naval ships, and the Navy’s absolute requirement for reliable operation, create a unique repair environment that demands special expertise and capability. Naval ships are designed and built with a high degree of interaction among components and systems. Repairs or modifications to a single system or component may have widespread effects on the operation of many other systems or components that are physically remote from the one being repaired. A thorough understanding of these effects and the ability to manage shipboard work as an integrated package are absolutely essential. Successful accomplishment of ship repair work requires careful coordination of a work force possessing a wide mix of skills and trades.
Aspects of technology can be subdivided into those that increase warfighting capability, those that provide the ability to avoid enemy damage or increase the probability of surviving such damage, and technologies that reduce the cost of vessels and hence result in funds being available for other capabilities.
The most significant development in radar has been the introduction of active phased array multifunction radars. The main benefits are increased detection performance, greatly improved reliability, and better electronic counter-countermeasure performance. Other areas of development are low probability of intercept radars for covert operations, passive infra-red search and track systems to complement radar coverage, and a move to unmanned aerial vehicles for surveillance and over-the-horizon targeting.
The trend in anti-ship missiles is towards increased speed and reduced signature. In response, defensive missiles are becoming faster and more autonomous. Gun-based close-in weapon systems (CIWS) have smarter prediction algorithms and increased rates of fire. It is possible that laser-based CIWS will become a reality within ten years. There is a move towards active off-board decoys to counter anti-ship missiles.
The main development trends are towards more integration (of command and control, communications, command support systems), wider bandwidths (optical fibre databases) and distributed processing. Multi-sensor data fusion systems are becoming more sophisticated and comprehensive. However, there is a question mark over the development of systems to integrate, manage and present to the operators the ever-increasing amount of operational data available.
In order to avoid detection by enemy sensors and to maximise the probability of own-ship decoys seducing hostile weapons, great effort is being made to control and minimise the various ship signatures produced by a warship. Radar signature is being reduced through appropriate shaping of all warship external surfaces and masking of any deck reflectors, and the use of radar absorbent materials and paints. Infra-red signature is being reduced through cooling exhaust plumes, masking high temperature areas through internal arrangement, and concealing exhaust plumes between hulls of advanced hull form design vessels. Underwater noise signature is being reduced through more accurate hydrodynamic design of hulls and propellers, and the use of air masking techniques; and the application of self noise monitoring techniques will soon allow for the identification of internal noise shorts or machinery imbalance. Magnetic signature is being reduced through the application of degaussing techniques and consideration of non-metallic materials for construction.
Should a warship suffer damage in action it is desirable that it retain some warfighting capability, or at least allow sufficient time for the crew to disembark safely. Hence considerable technology is applied to ensuring its survivability through :
- ensuring sufficient post-damage stability and watertight integrity;
- minimising the weapon impact through ballistic protection, shock protection, use of separation and design of redundant systems;
- maintaining structural integrity through ultimate strength techniques and use of box girder structures;
- selecting appropriate materials of construction and outfit, providing damage control strategies, and designing autonomous and semi-autonomous safety systems.
Considerable efforts are made to improve the seaworthiness aspects of the warship platform mainly in the areas of speed, seakeeping, dynamic stability, fuel economy and signature management.
Ship design followed a pattern of mimicking the previous design for many, many years. Hull forms in the late 19th Century were direct descendants of the previous hull forms, all at full-scale ship size.
A turning point of the hull form development for hydrodynamics was the David Taylor Model Basin Model 632, the parent form of the Taylor Standard Series. The three parameters (displacement length ratio, beam draft ratio and prismatic coefficient) drives most resistance characteristics. Admiral Taylor tested these characteristics and developed a series of contours of resistance-per-ton for various parameter values and speeds. The characteristics have been digitized and re-analyzed for a different extrapolation method, but they are still used today for assessing hull form shape.
Taylor also investigated the bulbous bow configuration, a remnant from the ram bow configurations of early steam warships. The bulb generates a wave system that interacts with the wave system of the hull so that the resistance at a particular Froude number (or a particular speed) will be reduced. Modern experiments complement calculations of wave resistance reduction due to bulbous bows.
Prediction of drag on a ship hull is always a challenging task for a naval architect. At the start of the design process, hull forms are developed given certain requirements. One of the major design tasks is to estimate the powering performance so that propulsion requirements can be determined. Early estimates of resistance and power are often based on simple empirical formulas derived from data for similar ships. As the design process proceeds, a more reliable approach becomes necessary to predict resistance; scale-model testing has been generally adopted for this purpose.
A long and heavy displacement ship tends to be more expensive. For example, the nuclear cruiser California Class (CGN-36) displaced 10,600 tons, carried one gun and two missile-launchers, and was considered a rather large ship for the weaponry onboard. In today’s environment one moves towards shorter hull forms. For example, DDG-51 is a shorter hull than DD-963, but with equal seakeeping performance
As the size of the ship increases, its volume and displacement increases faster than the surface area drag. A 9,000-ton destroyer requires 100,000 horsepower to cruise at 30 knots, but a 90,000-ton carrier needs only 280,000 horsepower to sustain the same speed. A ten-fold increase in displacement requires only a three-fold increase in propulsion power.
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