(Published SP’s Military Year Book 2014-2015)
Naval ship design is unique in itself because of its complexity, long service life of the ships and the very few numbers that may be required. The warship is a homogeneous weapon system comprising of numerous complex sub systems integrated accurately within it, and it is required to carry out various missions across the oceans of the world in a hostile environment, for prolonged durations, in a service life spanning decades. During its life, it has to be capable of matching upgrades in technology by the adversaries as well as in the industry and have the capability to carry out essential repairs unassisted at sea. The crew has to be trained to perfection and the machines maintained to designed specifications for the ship to operate in the face of the enemy. The complete life cycle of a warship comprises of conceptual design phase, followed by system design, production phase, tests and evaluation, delivery, operational life (~35/40 years), periodic maintenance, modernization and final decommissioning.
The history of merchant and warship structural design is replete with instances of commonality and variance. In middle Ages merchant ships used to carry light guns and engage in warfare, whereas in the 16th century specially reinforced warships used to carry heavy guns, even though both the merchant ships and warships were made in the same shipyards. Technology was largely a common factor and the shipyards had no difficulty in switching from manufacturing of one to the other. During the late 18th century, warship technologies were used to construct armed merchant ships and in the early 19th century, some of the innovations by the British East Indies Company were adapted in warship construction by the admiralty. In the late 19th century, Royal navy heralded the age of structural design using engineering fundamentals and calculations. This led to many countries encouraging naval architecture studies for naval designers while the merchant marine lacked the same initiative. The naval design bureaus had capacities to carry out complex and voluminous calculations. During the following decades, this knowledge was efficiently applied to construction of the LNG carriers, ultra-large container ships and other specialized ships. Further developments in CAD tools, structural analyses methods, and probabilistic analysis have ensured a large degree of commonality in designs of naval and merchant ships.
Despite the convergence in structural designs, navies continued to refine the design of warships based on specific role and the experience gained during wars and extended operations in the cold war/post cold war era. However, scale modeling trials using many of the commercial software techniques replaced the full-scale sea trials, this led to development of special steels and specifications for reduction in battle damage of warships. Difference in operations and maintenance of merchant and naval ships also led to differences in constructional design specifications.
In many countries today the navies provide the performance criteria and the design and construction is carried out by commercial shipyards. The adaptation of commercial design and construction standards to incorporate traditional naval specifications is resulting in mutually beneficial and acceptable regimes.
Imperative Design Features Required for a Warship
The most important design feature that distinguishes a man of war from a merchant man is its ability to withstand weapon attacks and remain effective. Special design features, depending upon the role of the warship, achieve reduction of vulnerability and enhance its survivability. This is achieved by introduction of protective/ hardened structures, systems and structural arrangements, and stealth attributes. The warship under attack can receive hits from below (torpedoes and mines) and above waterline (missiles, rockets, bombs, shells and nuclear) with the resulting damage and effects which are very different in nature. A weapon can broadly cause damage by exploding at the point of contact, after penetration and a certain delay and by a standoff explosion. Generally, an underwater standoff explosion is much more damaging than a similar proximity detonation above surface. Similarly damage by penetration of an underwater weapon prior to exploding is lesser than the one from above the surface due to its much higher velocity. Since these lead to very important design considerations it is intended to discuss the differences in the blast damage (other than resultant fires) caused by underwater and above water weapons in the succeeding paragraphs.
Damage Characteristics of Underwater Weapons
Main weapons in this category are torpedoes and mines, which generally utilize the ships signature to attack and explode near the warship, causing extensive damage by shock and whipping.
Shock Wave is created by the detonation of the explosive under water, which converts the solid explosive in to a gaseous mixture with very high temperature and pressure. The disturbance travels in the surrounding water as a shock wave at nearly the speed of sound. Shock wave leads to rise of the pressure to its peak value and then decay like an exponential function. The velocity of this shock wave is about five times that of an air blast wave. The explosive gases form a bubble, which expands, and contracts resulting in a series of much weaker shock pulses as it migrates. Cavitation effects occurring from such explosions fall under the categories of local and bulk cavitation. Local cavitation is formed when the shock wave hits the target and bulk cavitation occurs when the shock wave encounters a free surface like the surface of water, thereafter it becomes a rarefaction wave and travels downward by relieving the pressure. The zone so created is the bulk cavitation zone. The structure of the warship in contact with the underwater explosion behaves similar to a ‘transient 3D fluid structure interaction’ problem. This needs to be predicted at the design stage itself.
Whipping is an important effect caused by underwater explosions; it is defined as the transient beam like low frequency response of the structure caused by external transient loading of the pulsating and migrating bubble formed as consequence of a non-contact under water explosion. The whipping effect is difficult to estimate.
Damage Characteristics of above water Weapons
A warship may be subjected to attack above water from a variety of weapons like missiles, rockets, gunfire, and bombs. These may explode before contact, on contact, or penetrate and then explode inside the ships structure. The cause of damage is a combination of blast, fire, and fragmentation. In an external blast the differences between nuclear and conventional explosives lies in the characteristic length of the blast wave and the lengths of the blast-loaded surface. The nuclear blast wave is about 100 times bigger than the conventional blast wave and causes effects along the length of the ship whereas the conventional shock wave is of the length of deck height/ frame spacing and causes local effects. The components of the ship’s systems essential for survival of the vessel need to be designed to meet the requirements of the role of the ship.
Internal Blast is caused by penetrating weapons and the blast waves are dependent upon the shape, size, weight, and position of the warhead in relation to the surrounding structure at the instance of explosion. The blast wave could be reflected multiple times from the compartment walls or equipment before it wanes. This is followed by the Quasi-static overpressure phase resulting from the heat and additional gas effects. The heat is a result of detonation, shock and after burn. The loading by the time dependent blast wave of the structure is similar to that in the quasi-static overpressure stage. It is also possible that due to rupture in the deck or due to doors, hatches and trunking, venting will result in reducing pressure in the affected compartment while causing damage in adjacent ones. There is also an attendant danger of further damage due to ignited unspent missile fuel. The impact of these effects needs to be calculated and factored in to the design.
Protection from damage due to fragments hitting the structure also needs to be factored in at design level; this is not a simple estimate as there are a variety of fragments, which may impinge upon the ship ranging from small caliber to large caliber and armour piercing munitions. This done by carrying out a detailed vulnerability analysis of compartments, after deciding which areas are likely to be subjected to fragmentation damage and up to what levels. Important areas are munition magazines, walls of important equipment and power lines.
The survivability of a warship is dependent upon three things namely, susceptibility, that is its ability to avoid detection; vulnerability, or the ability to withstand a hit and recoverability, the ability of the ship to restore and reconfigure essential systems after a hit. It needs to be noted that introduction of structural survivability measures should be such that after the ship is hit, its structure is able to sustain the designed operations of the ship. Structural strength is determined by carrying out a detailed analysis. Generally, there is a need to provide ship with enough residual strength so that its hull girder can survive and the ship can continue operating for about 100 hours after enduring one missile hit. Susceptibility involves reduction in a ship’s signatures. A warship generates various types of active and passive detectable signatures, active signatures, comprise emission of infrared, electromagnetic, acoustic waves etc, whereas passive signatures, consist of radar cross-section, acoustic target strength, static magnetic, and electric fields etc. Designers pay increasing attention at the design stage to reduce these signatures by use of shaped structures, different materials, and shielding.
Innovative Design Techniques
Systems engineering approach to warship design has shown a way ahead because it is fundamentally an interdisciplinary engineering management process covering all aspects of hardware, software and the human component. It caters for life cycle requirements, and economically beneficial integrated design.
Designing for survivability approach advocates, warship design of relatively smaller ships with much higher survivability and better weapons suit .It caters for a more dangerous battle environment, while reducing work force requirements.
Axiomatic design approach puts forth the argument that, currently the design process is an iterative process, in that, several individual attributes are first designed and then integrated, often leading to re-design and finally to a compromise solution. Axiomatic design consists of four domains namely, the customer domain, the functional domain, the physical domain and the process domain. The axiomatic process requires determining ‘what’ is required in each domain and then specifying ’how’ these requirements are satisfied in the successive domain.
Recent Trends in Structural Design
Advanced Hull Designs. In addition to operations at higher speed, there is a demand for novel hull designs to meet the requirements of design flexibility and better sea keeping. The designs are still evolving and may not be limited to hydrofoils, surface effect ships, trimarans, small water plane area twin hulls (SWATHs) etc, and this is evident from the arrival of unmanned systems in the naval arena with their own specific operational and maintenance requirements.
New Materials. Continuous search for newer and better materials by shipyards and designers to enable better maintainability, survivability and cost reductions. Composites, titanium coatings, and nano materials are heralding unprecedented changes in design.
Commercial Shipyards. Warships today are being increasingly constructed at commercial shipyards and not only at captive naval yards; also they are being contracted to yards in other countries. This is done to achieve synergies in technology, system integration, and cost.
Littoral Warfare. The ships today are being designed to operate not only in blue water engagement scenarios but also in the littoral warfare environment which involves attack by weapons of low technology and range but in large numbers, boat swarms, miniature submarines, shoulder fired missiles and rocket attacks are some examples. Further, the advent of littoral warfare has also led to more enhanced requirement of incorporating stealth features in design to reduce acoustic, radar, and infrared signatures of the warship. This in turn has led to incorporation of unique features, materials, and structural arrangements.
Quest for Speed. One of the biggest limitations at sea is the speed therefore structural designs for achieving higher and higher possible speeds with safe sea keeping are being continuously explored.
Multi mission and modular design. The requirement of a warship to carry out a variety of missions due to developments in unmanned vehicles for naval use, open software, and modular sub systems is leading to configurable design of warships. The construction of ships by modular methodology has already led to much higher efficiencies in building ships and reducing costs.
The main goals of modular construction of warships are threefold, firstly to enable mission flexibility and future upgradability for enhanced service life of the ship; secondly, to achieve synergies in procurement, integration, equipment and system testing, and parallel ship hull construction; and finally to enable reductions in life cycle costs and costly upgrades by simplifying complexities in future upgrades.
Fundamentally, modularization comprises of three classes of modules, namely construction, large-scale functional and small-scale functional modules based upon their sizes and utility. The construction module comprises large pre-out fitted sections, which are joined together. Detail work like wiring, piping, venting etc. is carried out before joining the sections. This part of modularization results in reduction in construction time as well as costs, however dismantling later is not envisaged, as it would involve considerable disassembly of subsystems. Large-scale functional modules comprise packaged units like mission units that carry out major functions and may be nearly as huge as a ships compartment. For e.g. the weapon module may include weapon launchers, armament handling systems, and magazines. The large-scale functional modules can be easily replaced for modernization or modifications. The small-scale functional modules are maintenance, repair friendly units, and are small as compared to other modules. These are modules, which can be assembled and disassembled, as these units form the ship’s support systems. This allows for exchange of systems and easy up gradation at a smaller level. Studies have indicated that savings in excess of 10% in acquisition costs are possible by modular construction. Two excellent examples of modular construction of ships where such savings have been achieved are the MEKOTM system at Blohm & Voss shipyard in Germany and South Korea’s commercial shipbuilding sector. The MEKOTM philosophy of containing an independent zone of the hull in a module provides a robust hull, good damage control, and flexible customizable design. Important services are run in three box beams, which are run at the keel, at the intersection of hull shell plating and on the main deck. The access is provided only along the main deck, which gives zonal integrity to the ship. These building blocks are supplied with their individual services of vertically designed systems with fully tested self-contained service modules. The methodology of vertically installing major modules and equipment by lowering them leads to saving of time during assembly as well as during overhaul, up gradation and repair. The South Korean Modular construction is very innovative in that it uses iGPS large volume metrology system, which overcomes the mega block integration misalignment problems. Dry dock is essential for integration of the mega blocks, vital tasks comprise, dimensioning, perpendicularity, level, and gaps between assembled parts. This mega block modular construction approach also requires a very skilled work force for its implementation.
Warship Building in India
The fundamental steps in warship building in India commence with the drafting of the Preliminary Staff Requirements (PSR). This is the result of deliberations between the Naval Staff and the naval designers, taking into account the needs of the Navy based on future threat perceptions and the availability of technologies and industrial capabilities. The PSR includes amongst others, role, armament, sensors, overall dimensions, speed and endurance. There after conceptual design work is undertaken; it includes sifting through various technical alternatives and selecting the most feasible one for the preliminary design. This has detailed schematics and calculations to provide the best design option as per the PSR. It is presented to Naval Staff highlighting areas of give and take with respect to the PSR. A desired preliminary design is arrived at, after detailed deliberations. The detailed design work follows thereafter. This involves detailed drawings, hydrodynamic modeling, modifications if required based on modeling studies, layout plans, detailing of specifications and commencement of dialogue with the building shipyard. The shipyard prepares for construction of the warship by making production drawings, procuring jigs, fixtures and equipment that may be required during production.
Modularisation in Indian Defense Shipyards
Currently the Defense Shipyards build ships by launching the hull in water after welding it and there after the shipyard’s artisans install machinery and equipment in highly cramped spaces. This has also contributed to inordinate delays in delivery of warships to the Navy as ships have taken nearly ten years to build. However, the major defense shipyards like MDL and GRSE are already in the process of modernizing by moving to modular ship building wherein 300-ton blocks are manufactured independently along with their equipment, electrical wiring, pipelines etc and then fitted to neighboring blocks precisely. It is expected that MDL’s modular shipyard costing Rs. 824 cr would soon be operational, and it is estimated that destroyers would be constructed in 72 months and frigates in 60 months. One of the areas defense shipyards need to study is outsourcing while retaining essential technical work force for critical defense related work. With the Indian industry maturing rapidly, many of the tasks like crew accommodation, painting, wiring, piping etc may be outsourced, however, with a mechanism to ensure that quality of work is ensured.
Conclusion Warship design is undergoing a change today forced by factors like economic slowdown, emergence of littoral threats, reduction in blue water engagements, development of powerful sensors and weapons as well as advent of unmanned vehicles on the horizon. The commonality of design and construction between merchant ships and naval ships is touching unprecedented levels whilst retaining unique battle environment requirements of a man of war. There may be a need to look into newer design methods like systems engineering design approach, designing for survivability and axiomatic design principles etc. rather than adhering to the telescopic iterative methodology in use in India. The Indian defense shipyards need to switch over to modular construction at all levels to ensure timely and cost effective deliveries.