Naval architects and engineers in structured organizations are frequently excluded from participating in the contracting and financing arrangements of vessel construction. This exclusion is most unfortunate and it is anticipated that this section will assist naval architects and engineers in contributing to these arrangements.
Far too often the approach to vessel selection and financing involves lawyers, accountants, financial planners, and ship construction people working independently of each other without the continuing interchange of ideas that is so essential during the planning stage.
The widest knowledge of any proposed vessel construction and the fullest participation in the mission aspects of new vessels provide the best climate for producing the most effective design and construction.
If continuing interaction of the interested parties is not feasible, the next best thing is to have those entering the field of vessel construction understand fully the various contributions of the lawyers, the accountants, the financial planners, and the operators to the design, and the documents and instruments developed over the years to ensure the continuum of events which must occur timely to produce the desired vessel within the desired time at an acceptable price. With the intellect considered to be the prerequisite of such understanding comes the patience to accept a less perfect alternative when it is more important to see the project move onward.
Accordingly, this section is proceeding on a format which is intended first as a narrative of a typical ship design genesis, its subsequent contracting and construction, and its delivery and operating inception. After this over-simplified case history approach, a more detailed discussion of the legal and financial aspects and impacts, including the documents usually involved in such transactions will be presented.
Where deemed appropriate, a sampling of alternative approaches will be included, all to show the reader that the kinds of agreements which can be made between a purchaser and a builder or between a financier and a purchaser/borrower are limited only by the law of the land and the ingenuity of the parties dealing in the matter - the assumption being in all cases that the objective is the best product at the least all-inclusive cost to the owner.
Also, where the narrative leads to identifiable problem areas, sufficient analysis will be outlined to permit understanding and insight into the course of these problems so that the naval architect or marine engineer might be better prepared to avoid controversial approaches in preparing ship construction documents for the clients or principals.
A significant number of naval architects, engineers and others are directly involved in United States shipbuilding and shipping business practices which differ in many respects from those in other shipbuilding countries. Although this chapter discusses international costing and contracting arrangements to some extent it is primarily concerned with U. S. practice.
To the extent considered necessary, reference is therefore frequently made to specific United States rules, organizations and operating procedures. At the same time, the general discussions apply equally to all international shipbuilding and shipping.
When dealing specifically with the United States government, those procurements made directly by the U.S. Navy and the U.S. Coast Guard are generally so identified; when the term government aid is used it generally means the government is not procuring the vessel but that the owner, buyer, or purchaser has arranged a construction differential subsidy (CDS) for the shipbuilder and/or obtained a government insured mortgage or other benefits available under the Merchant Marine Act.
In discussions of shipbuilding costing and contract arrangements, a number of terms are used that have specific connotations in this aspect of the shipbuilding process. The following definitions can be considered to apply.
Architect of Contract: A term borrowed from the legal profession to indicate the person or entity that authored the contract document.
Builder: In this text, builder, contractor, and shipyard are used synonymously. It is the entity that signs the construction contract and undertakes to physically build the vessel. The various forms are used as these terms are encountered in invitations, contracts and specifications.
Owner: This term is used to identify the buyer of a vessel to be constructed. In the parlance of MarAd contracts the term purchaser is usually substituted for buyer. Primarily, the intent is to name the party who selects the design and causes the initiation of the contract to build. It is recognized that in leveraged lease situations the owner of record of the constructed vessel may be someone or some group having only a financial interest. In such cases owner as used herein is the charterer.
Naval Architect: Anyone having decision authority over the design of the vessel to be constructed or reconstructed. ln-house naval architects are those on the wage payroll of the shipowner or entity contracting for a vessel. Outside or Contract naval architects are those persons whose business is the design and engineering of vessels, and who contract with owners or shipyards to perform their services for a fee.
Design Agent: A term used interchangeably with an outside or contract naval architect. It has come into use as shipyard designs have become prevalent. Shipyards are frequently design agents. They do employ naval architects, and those who work at design are usually in the engineering department or the planning department.
Reps: Abbreviation for representatives; as, for in¬stance, owner's reps are the inspectors and plan approvers working on-site during ship construction.
Lead Ship: The first vessel built to a new set of plans and specifications. It is not necessarily the first vessel delivered because under circumstances of the order being allotted to two or more yards, the first vessel in one of the other yards may be delivered first. This occurs because of better production methods, or because of unforeseen delays in the lead yard.
Following Ships: Ships built to the same plans and specifications whether in the same yard as the lead ship, or in other yards, are following ships.
Berth Term: This refers to dry cargo liner operations utilizing publicly issued schedules of port calls.
Cease and determine: A phrase used in Maritime contracts to indicate a full unconditional stop action plus an inventory of the financial position as of that moment.
On board ships equipped with machinery developing power for propulsion and auxiliary purposes, personnel are subjected to vibration. The vibration may cause annoyance, physiological damage to body organs, psychological disturbance of the crew or damage to shipboard equipment.
The effect of vibration upon equipment installed aboard ship has been one of the major factors resulting in premature failures of equipment which has previously proved satisfactory in land-based installations. The equipment supplier must consider the vibration aspect of the shipboard environment in the design and construction of marine hardware.
A magnitude of vibration which can do no harm to equipment or structure can, however, be a great nuisance to the crew. The degree of human perception to vibration in the frequency range of 30 to 4800 cycles per minute is a function of the amplitude of the vibration. Frequencies below 0.5 Hz may cause motion sickness.
While it is difficult to define precisely acceptable limits of intensities of vibration, two figures in this article identify vibration zones which may be used as a guide in determining general acceptability. Zone A defines that area within which a high probability of vibration difficulties exist; Zone С defines that area within which no vibration difficulties are anticipated; Zone В defines that area within which the subjective nature of vibration does not permit a reasonable assurance of acceptability.
The human ear has certain interesting characteristics. The normal hearing range for a young person extends from about 20 to 15,000 Hertz (Hz) (cycles per second), with the greatest sensitivity around 1,000 Hz. Aside from the noise attributes of loudness and annoyance, there is the factor of physical tolerance to noise, that is, the noise sound-pressure levels which the ear can stand without discomfort or damage.
The effect of noise on the human being with regard to hearing loss and communication has been studied and design criteria established through extensive habitability research in naval ship design. The effect of noise annoyance is, however, not as well defined. The wide range of noise levels' which various persons find disturbing makes this aspect of noise control more subjective and difficult to define. Factors which influence a person's reaction to noise include interest of the listener in the sound, whether the noise is unnecessary and could be avoided, the degree to which the listener can disregard the noise, the activity with which the noise interferes, the character of the listener.
For the more noisy spaces aboard ship, it has been determined that people can readily adjust to various environments and actually consider them normal, once they are conditioned to accepting them, provided the environment includes no hostile sounds. How well the naval architect can handle the problem of acoustical habitability depends largely upon how well he can control the magnitude of sound levels and how much he can shape the noise spectrum in any of the various ship's spaces.
The reduction of noise within certain spaces may produce counter-productive results. For example, staterooms normally receive a high degree of isolation from passageway noise, however, the resultant number and location of general alarm bells must be carefully reviewed to assure audibility within all staterooms. Also, the enclosing of machinery control stations has prompted some reaction from operating personnel that not hearing the machinery has degraded their effectiveness.
The major sources of noise generation aboard ship may be categorized into flow generated noise and mechanically generated noise. Each of these general categories contains several elements each of which must be considered by the designer to preclude objectionable noise conditions aboard an operational ship. Flow generated noise is produced by a fluid in motion. The fluid may be either a liquid or gas and may be either within the ship envelope or external to it.
Noise Generated by Ship Mooing Through the Water
Flow of water around the hull of a ship is almost completely turbulent, particularly in the bow and stern areas. The turbulent water flow path produces pressure fluctuations which tend to drive the hull plating into vibration. The resultant noise may be transmitted within the hull either as airborne noise or as structureborne noise. Discontinuities of the hull such as sonar domes, sea chests, and shaft struts function to increase the turbulence within the boundary layer thereby tending to be sources of external flow noise.
Propeller Generated Noise
Several different types of noise may be generated by the ship's propeller. The two types of propeller noise associated with fluid flow include cavitation and vortex shedding. When a ship's propeller is rotated at high speed cavities can form and collapse radiating a loud and continuous noise. Also vortices are shed from the trailing edges of the propeller blades. If the frequency of this shedding corresponds with a resonant frequency of the propeller blade the blade vibration will radiate a loud ringing noise.
Fluid Flow Noise
The noise sources within piping and duct systems are similar to those produced by the ship moving through the water and the propeller. The flow of fluid through a piping system may produce noise due to turbulence, cavitation and vortex shedding. In piping systems the noise may be intensified by the organ pipe effect of the pipe or duct. Restrictions or obstructions in the fluid flow path which increase the velocity are prime sources of cavitation and turbulence generated noise. Dampers and splitters within ventilation ducts may also produce noise due to vortex shedding.
Mechanically generated noise usually originates in rotating and reciprocating machinery. The sources of such noise may be the result of improper balancing, excessive tolerance between mating parts such as gears or the result of loose or worn parts. The characteristics of the machine and the noise produced may provide an indication of the problem area.
The acoustical aspects of controlling the ship's interior environment must be included at the beginning of the design process. In many cases, it is extremely difficult and expensive to correct a noise problem whereas the impact of precluding the problem at the early design stage would be minimal.
When addressing the acoustic or noise considerations both the noise source and the transmission path must be reviewed. Control of noise at the source may be accomplished by improved dynamic balance of rotating machinery, limiting velocity within fluid systems, avoiding turbulence within fluid systems, improved tolerance between mating parts, and application of suppression material to the noise source. In addition, the noise level within spaces such as staterooms may be controlled by locating them as remote as practical from spaces having a higher noise level. For example, it would not be prudent to locate a stateroom adjacent to a fan room.
Noise may be transmitted from the source to other areas via the structure as vibration, as airborne noise or as fluid-borne noise. Structureborne noise usually originates at machinery or equipment foundations. Transmission of noise from a machine to the supporting structure may be reduced by mounting the unit on resilient mounts or distributed isolation material.
Where such type mountings are used, isolation of the connecting piping or ductwork must also be accomplished. When resilient mounts are employed, care must be taken to assure the natural frequency of the mount does not coincide with the exciting frequency of the vibration source.
Airborne noise may be reduced by locating the offending unit within a space lined with sound absorbent material or, as in the case within the engine room, as remote as practical from the operating station.
Fluidborne noise may best be controlled by eliminating the source where practical. If a large pressure drop is induced by a single orifice in a piping system, consideration should be given to a multiple-step orifice thereby reducing the velocity through each step. If the source of the noise may not be eliminated as in the case of ventilation fans, absorbent material may be installed in the ductwork or, in some cases, the ductwork must be increased in mass to reduce the amplitude of response of the ductwork to the vibratory excitation.
With respect to naval vessels, acoustical considerations may extend well beyond the element of habitability due to the nature of the ship's function. The methods employed aboard submarines, for example, for controlling noise are several orders of magnitude more extensive than those normally encountered in merchant ship practice.
Economy in ship construction and improvements in the serviceability and service life of ship structures can be enhanced if several principles basic to welded construction are observed in the design process. These principles are derived both from service experience and from studies of the causes and prevention of structural failures in ships.
The mechanical toughness and corrosion properties of the base metals selected should resist excessive degradation from welding and forming practices. This precaution is particularly applicable to those materials whose properties have been enhanced by heat treatment or cold work. In addition, when materials of widely differing corrosion resistant characteristics are joined, possible adverse galvanic corrosion effects should be considered.
Examples of degradation which may be encountered are:
• Loss of toughness in the HAZ of some steels, particularly some higher strength steels, where weld procedures with excessively high heat input rates have been used.
• Loss of strength, ductility, and corrosion resistance in the HAZ of the heat treatable aluminum alloys.
• Accelerated corrosion attack on carbon steel located adjacent to an area overlayed with a stainless steel.
• Loss of ductility and toughness in materials subjected to excessive cold forming.
Points of high stress concentration such as may be introduced by a flaw or an abrupt change of geometry at an intersection have been identified as potential sources of brittle fracture initiation. Surveys have shown that they can be a primary source of fatigue cracking which produces many of the nuisance cracks. Such cracks can represent significant cost items in respect to their interruption of normal ship operations, and the time and effort required for their repair. Areas of high restraint where high weld residual stresses can develop should also be minimized.
The attainment of a sound weld joint and its proper inspection can only be achieved if appropriate clearances are provided. In considering this aspect, the designer should take into account the production weld process and inspection method. Access requirements for some automatic and semi-automatic weld processes may differ among each other as well as be different from shielded-metal arc welding. The extent to which adequate or inadequate access is provided could control the degree and facility to which an automated weld process could be considered which could in turn represent a significant production cost factor.
Direct overall design to minimize the probability of transverse fractures. In some cases this may involve the use of material of superior toughness or the judicious incorporation of designs of special geometry or redundant structure which would interrupt a transversely running crack.
Avoid Excessive Welding - In some cases overwelding may result in the imposition of excessively high welding stresses.
Through Thickness Loading
Since most conventional hull steels are not provided with minimum specified through thickness properties, they may exhibit weakness under such a loading condition. Where through thickness loading in a structure cannot be avoided by a design modification, special materials with enhanced through thickness properties should be considered.
Welding and Nondestructive Testing Symbols
Welding symbols are used to communicate a designer's and fabricator's requirements to those concerned with design, design review, and fabrication of a structure. While preliminary design may require few details of weld joints, the requirement for inclusion of more details increases as the development of plans progress through the preliminary design to contract design and working plan stages.
Detail design plans, when used in the shipyard, should contain complete details of the welds and any nondestructive tests that may be required. When plans form the basis of a contract, omission of any special requirements in respect to extent of penetration, finish, post weld nondestructive test examination etc. could lead to disputes between the purchaser and fabricator. When such details are omitted in final fabrication plans of the shipyard, such omission may allow for inadequately penetrated, finished or inspected welds.
As used herein, the term semi-automatic process means that the electrode is manipulated manually and all other welding parameters including rate of electrode feed are controlled automatically; automatic process means all parameters including electrode manipulation are automatic.
Shielded Metal Arc Welding
SMAW is a process where heat is produced by an electric arc between a covered metal electrode and the work. The arc melts the metal of the electrode and the spray or droplets formed transfer across the arc to coalesce as a molten pool before solidifying as weld deposit. The transfer mechanism involves a combination of complex phenomena of arc physics. The formulation of the cellulose or mineral base electrode covering assures that the covering will decompose or melt in the arc in an appropriate manner and rate, and accomplish the following:
• provide a gas or slag environment which shields the metal from the atmosphere during metal transfer and solidification; • establish a favorable electrical environment for arc stability;
• provide a slag covering for the deposited molten weld metal which refines the metal and may, in some cases, provide alloying additions; and,
• influence the fluidity of the molten weld metal which in turn influences the shape and contour of the deposited weld bead. Since the covering has a great influence on the transfer and natures of the resulting weld deposit, it is important that coverings be kept free of contaminants such as moisture or grease which could alter their characteristics.
Gas Metal Arc Welding
GMAW is an automatic or semi-automatic process in which a welding arc is formed between the work and bare electrode. The electrode is continuously fed from a spool which may weigh from 0.5 to 23 kg. An inert gas shields the arc and molten weld area from the atmosphere; such shielding is analogous in function to that of the covering in the SMAW welding-Carbon dioxide, argon, or helium or a combination of gases is used for shielding. When argon or helium are used for shielding the process is relatively expensive and is not generally used when more economical welding processes are applicable.
Gas Tungsten Arc Welding
GTAW is similar to the aforesaid Gas Metal Arc Process except that a tungsten electrode is substituted for the continuously fed filler metal electrode of the GMAW process; in GTAW the filler metal is provided by a weld rod which is fed so that its end is melted by the welding arc maintained between the tungsten electrode and base metal. Argon or helium is used as shielding gases.
Flux Cored Arc Welding
FCAW is similar to the GMAW process except that the inert gas shielding is replaced by a flux which is located in the core of the filler wire; the flux, when exposed to the welding arc, provides appropriate shielding and to some extent is analogous to the covering of the electrodes used in SMAW welding. Some variations use CO2 or CO2+ argon mixtures for auxiliary shielding. The process which is primarily used for steels, offers a means for achieving the economy of semi-automatic welding for many applications where the relatively slower but versatile SMAW process was previously used.
Submerged Arc Welding SAW
In this semi-automatic or automatic process an arc is maintained between a continuously fed spool and a work area. The welding zone is completely buried and shielded under a granular flux or melt provided from an independent feed tube. The flux or melt, when molten, maintains an electrical path of high current density which generates a great quantity of heat. The insulating characteristics of the flux concentrate the heat in the weld area and induce significant melting of base metal as well as welding electrode. Under such conditions, high welding speeds, high deposition rates, significant melting of base metal, and deep weld penetration can be achieved. SAW with two- or three-wire electrodes instead of a single wire provides even higher welding speeds and deposition rates.
Because of these features SAW is a frequently used automated welding process in steel ship construction. Common practice in making a sub-assembly of full penetration welds in a panel line is to submerge arc weld a sub-assembly of several plates from one side, turn the sub-assembly and complete each weld from the second side; welding from both sides is usually necessary to assure complete weld fusion. However, SAW welds with sound roots can be made from one side only, thereby eliminating the cost and time consumed in sub-assembly turning and re-welding from a second side. This form of the process designated as one-side welding requires close control of joint fit, plate waviness, and weld parameters. Additionally, a special backing or tape on the back side of the joint is usually necessary to contain the molten weld metal at the root so that it forms a sound weld deposit of satisfactory contour.
Electroslag, ES and Electrogas EG Welding
These are high deposition rate processes analogous to the SAW and GMAW processes respectively, except that the molten weld pool is contained within movable copper shoes at each side of the weld joint. A variation of the electroslag method, which uses a consumable guide tube instead of a permanent tube has been used for applications such as butt welding of underdeck longitudinals. Because of the exceptionally high deposition rates and large molten weld pools, arrangements are only available for vertical welding. In ES welding, a bar or strip is occasionally substituted for the one or more electrodes.
Exceptionally thick materials may be welded in a single pass and in the case of ES welding, materials in excess of 400 mm thick have been welded in a single pass. Appreciably higher speeds are attained as plate thickness decreases and as thicknesses approach 12 mm welding speeds of more than 10 times those shown are attained. Because of their relatively high heat input rates, ES and EG welding cause a greater degree of grain growth and other metallurgical changes in the weld heat affected zone, HAZ, than other processes. In some cases these may adversely affect HAZ properties, such as toughness, to the extent that use of the process must be restricted.
SW as used in shipbuilding, is an arc welding process wherein an arc is maintained between a stud or similar piece and the work, for a predetermined time so that both are properly heated. The stud is then brought to the work by spring pressure. A ceramic ferrule is sometimes used to provide partial shielding and some contour. The process is accomplished with an automated welding gun, power source, and control panel; the control panel regulates electrical parameters, welding arc time, arc distance, and the imposition of pressure between stud and work at the end of the welding cycle. The process is widely used in shipbuilding for attaching a wide variety of items such as studs, clips, hangers, and insulation pins to structural members.
Aluminum alloys used in marine construction are readily weldable with the inert-gas arc-welding processes; Gas Metal Arc, GMAW or MIG, and Gas Tungsten Arc, GTAW or TIG. The former process predominates because of its higher production speeds and greater economy. In welding aluminum, particular care should be taken to see that all surfaces in the way of welding are clean and free of contaminants, such as water stains, oxide films, and anodized layers; when present, they may be removed. Preheat is not generally needed except when welding exceptionally thick sections, under conditions of high restraint, when humidity is very high, or when temperatures are below 0°C. For the 5000 series alloys, prolonged preheating or exposure in the 65°C to 200°C range should be avoided, since it could sensitize the alloys to corrosion.
Welding Effect on Base Plate
Welding of the 5000 series alloys, where strength is usually derived from work hardening, produces a zone within approximately 13 mm to 25 mm of the weld where yield and tensile strength of base metal are reduced to values approximating annealed base plate properties This zone of reduced strength must be taken into account in design calculations. In the case of heat treatable alloys a greater degree of tensile and yield strength degradation occurs; in addition, ductility and corrosion resistance are also severely degraded.
Welding Dissimilar Metals
The possibility of adverse effects resulting from galvanic corrosion should he considered whenever dissimilar metals are joined. In some in stances special precautions, such as the application о coatings in the vicinity of the dissimilar metal joint may be indicated. The most common dissimilar metal combinations that are used in shipbuilding are stainless steel to carbon steel, and aluminum to carbon steel.
Stainless Steel to Carbon Steel
In welding stainless steel to carbon steel appropriate precautions should be taken to minimize deleterious effects associated with dilution о the stainless steel by the carbon steel base metal. Excessive dilution can produce crack-sensitive weld metal near the carbon steel interface. When stainless steels similar to the 18 percent chromium-8 percent nickel type commonly used in shipbuilding are joined to carbon steel, nickel-rich stainless filler metals such as type 309 or 310 are generally recommended for any stainless steel weld layers which come in contact with the carbon steel.
When butt welding stainless clad steels, the carbon steel side is usually welded first with the appropriate carbon steel filler metal; particular care must be exercised to prevent the carbon steel weld deposit from impinging on the stainless steel overlay. The second side is then welded with a nickel-rich stainless steel filler wire such as type 309 or 310. If the carbon steel layer is relatively thin, the entire weld may be made with the 309 or 310 filler metal. Similar procedures are usually used for welding other clad carbon steels; i.e., deposition of carbon steel filler metal on the cladding is avoided, and a filler metal that is compatible with the cladding and the underlying base metal is used.
Steel to Aluminum
When joining steel to aluminum consideration should be given to the possibility of galvanic or crevice corrosion. Galvanized and stainless steel fasteners have been used to join steel to aluminum by mechanical means using rivets or fasteners. Aluminum is not weldable to steel by conventional welding methods. To effect welded joints between aluminum and steel an inter-mediate composite plate material consisting of an aluminum and a steel layer is used. The bond between the aluminum and steel in the composite aluminum-steel plate, is made by special manufacturing processes such as explosion bonding.
Each plate side is then welded to similar material to form the configuration shown on the picture. This type of joint can be used for many purposes where an aluminum structure fastens to a steel structure. Familiar applications are the joint connecting the upper aluminum skirt plate and the lower steel skirt plate on some LNG carriers and the connection of aluminum deckhouses to steel decks. This type of connection has certain advantages in weather areas as regards corrosion compared to bolted or riveted connections.
A major change in ship construction occurred over 100 years ago when steel was introduced to replace iron and wood as a hull material. Subsequent important developments in materials and ship construction were the all-welded ship and the application of concepts of toughness for the prevention of the brittle hull fractures experienced with the all-welded steel ships of the 1940's.
Over the past thirty years, many new designs have been introduced such as container ships, liquefied gas carriers, high speed surface-effect ships, and mobile and fixed offshore structures. To meet requirements for such designs, high strength-to-weight ratio alloys and alloys intended for low temperature service have been introduced into shipbuilding. The complex networks of intersecting members in offshore structures require that consideration be given to material properties when the applied tensile loads are perpendicular to the plate surfaces. The increasing size of ships, such as the VLCC tankers, and the concern for economy, stimulated automation of fabrication processes.
The relatively simple concept of toughness developed to answer the brittle fracture problems in ordinary strength steel hulls required refinement and extensive development before it could be applied to the newer hull materials and structures. For some structures increased consideration had to be given to such factors as fatigue and corrosion. Accompanying all these changes, was an increased demand for the designer to provide for quality assurance by nondestructive methods. To meet the challenges presented by these new developments, the designer had to become familiar with metallurgy, welding engineering, nondestructive testing, and the materials sciences. An appreciation of the basic principles of these fields will provide more efficient and reliable hull designs through selection of appropriate materials, joining, and quality assurance requirements...
Aluminum Alloy Applications
Aluminum alloys find use where their special attributes such as low density and high strength to weight ratio, corrosion resistance in certain environments or retention of toughness at low temperatures are of value. Development of inert gas arc welding processes has facilitated the use of aluminum alloys for various ship structural applications. Aluminum alloys are frequently used in superstructures of large ships and for the entire hull structure of some ferries and small boats such as those serving the offshore industry.
The low density of aluminum alloys makes them particularly attractive for applications where high strength to weight ratios are of particular concern such as in surface effect and hydrofoil craft. Since aluminum alloys increase in strength and maintain toughness as temperature decreases, they have proven particularly suitable for cryogenic services such as containment of liquefied natural gas. Details of compositions, properties and methods of inspection are contained in American National Standards Institute documents...