Advantages Of Aluminium As A Shipbuilding Material Engineering Essay

The use of aluminium in any vessel involves a radical set of production methods compared to traditional shipbuilding processes. Hence, the methods used for the construction of aluminium vessels are a key point in the feasibility study. The welding of aluminium is vunerable to hot cracking and can only just be done using certain processes. It is important to employ the right welding methods to serve different purposes. As aluminium technology matures over the years, new production methods such as aluminium extrusions were introduced in a bid to save lots of time and which has also proven to be economical.

The use of aluminium in naval shipbuilding exists in two forms; first you have the aluminium-steel ship, where in the case, the superstructure is manufactured out of aluminium designed for topside weight saving, and the hull made from steel. Then there is the all-aluminium ship, with the purpose of achieving a significant overall decrease in weight. It is important to understand that though both forms have their advantages, there are design conditions that must be addressed related to the utilization of aluminium in naval vessel.

Background

1. 2. 1 Advantages of aluminium as a shipbuilding material

Aluminium's most important characteristic is its light-weight. When in conjunction with an acceptable tensile strength, it has grown to become the decision of material for many naval ships in the world. In a study by Wade (1996), as it pertains to naval shipbuilding, mission capability is the most heavily evaluated standards of this program. Speed can be an increasingly important parameter under mission capability due to the shift in the maritime strategy of the world's navies from blue-water functions that include traditional Anti-Submarine Warfare, Anti-Air Warfare and Surface Action to littoral businesses concentrating on surveillance, mine-clearing, counter-terrorism and support for landing operations.

Ship Structure Committee (2012) suggests that there are numerous design parameters that can be optimized for a better performance, where structural weight is one particular parameter that provides the most out of cost efficiency. According to Lamb and Beavers (2010), a decrease in weight relates right to the reduction in material costs and operating costs throughout the service life while decrease in the energy demand provides higher fuel efficiency, higher speed, longer range and additional tonnage capacity. Also, aluminium gives additional benefits in the form of maintenance cost benefits, where less painting is required.

Brown (1999) mentioned that corrosion protection proposed by aluminium is just about 100 times slower than structural steel. The wonderful corrosion-resistance of aluminium owes its trait to the thin layer of aluminium oxide that forms immediately when the metal is exposed to air, protecting it from external elements. The use of lightweight material like aluminium can also lead to stealth improvement (International Ship Structure Committee, 2012).

In a timespan of just over ten years, aluminium broadband vessels have evolved from 30m long vessels that carried passengers and operated in littoral waters, to 120m long vessels which could carry both passengers and vehicles which operated on view waters.

Ship Structure Committee (2012) gave a synopsis of the chance of aluminium in naval shipbuilding. Aluminium is a growingly popular metal in the marine industry, typically the naval shipbuilding industry because of the wide range of physical and mechanical properties that may be created through the alloying process. Aluminium can be alloyed with chromium, copper, magnesium, manganese, scandium, silicon, silver, tin, titanium, zinc and zirconium. This wide selection of alloying produces different grades of metal each with different properties.

Promising properties includes decrease in stress corrosion susceptibility, increasing of toughness, strength and hardness, bettering of strength without a reduction in ductility, good weldability, upsurge in tensile strength, elimination of hot cracking in welds, reduction in electrical conductivity and reduction in quench-sensitivity.

However, the discrepancy of the material property and behaviour of aluminium was found to alter with different sources (Sielski, 2007). The dissimilarities come therefore of different standards used for deciding yield strength. Some recent tests were done utilizing a 50-mm gage length that measures only weld metal and heat-affected zone, and other tests use a 250-mm gage length sample which includes the bottom metal. Shown in the following is one such exemplory case of aluminium's yield strength discrepancy.

Table (1), extracted from (Sielski, 2007).

1. 2. 2 Disadvantages of aluminium as a shipbuilding material

Like any other material, aluminium also offers its drawbacks. Both most important properties of your material are perhaps its yield strength and modulus of elasticity, a structure will be designed with considerations around the two properties to ensure that it is in a position to withstand a given load without exceeding certain permissible deflections and stress level, where in fact the stress level is equal to the yield stress divided by a factor of safety.

Albeit aluminium alloy has a high strength-to-weight ratio, it is to be noted that for each and every strong aluminium alloy in conditions of yield strength, there's a better structural steel available. In terms of Modulus of Elasticity, which is the way of measuring stiffness of a material, aluminium and steel measures at 69 GPa and 200 GPa respectively. Since aluminium's stiffness is merely one third of steel, it'll be deformed 3 x more easily than steel if put under high strain. Therefore the use of aluminium alloy is generally only limited to vessels of up to 130 meters in length (Ship Structure Committee, 2008); the longer the vessel the more stiffening is necessary, until a spot of impracticability. The figure below illustrates the undefined yield strength of any aluminium alloy as compared to mild steel. It is important to note that for aluminium, normally 0. 2% strain limit or proof stress is employed for design purposes.

Yield Strength of Steel and Aluminium

Figure (1), graph extracted from (http://aluminium. matter. org. uk/content/html/eng/default. asp?catid=217&pageid=2144417131)

Another consideration is the low melting point of aluminium. Like a naval vessel is going to be put through on-board fire if it comes under attack, the increased loss of mechanical properties of aluminium when temperature exceeds 200C (Ferraris, 2005) is unfavourable. Some classification societies and navies do not permit the use of aluminium for structural applications. While DnV, ABS and RINA permit the use of light alloy and AA5xxx series, Lloyd's register will not.

Brown (1999) noted that the price of aluminium is roughly five times the price of steel. Though it might be feasible to replace structural steel with aluminium alloy because of the latter's weight-saving and corrosion resistance properties, but it could definitely not be economical.

Table (2)

Current Methods

To determine the applicability of aluminium in naval vessels, it is important to check out the current aluminium technology available. The manufacturing and production process for aluminium is relatively new. Aluminium welding like the FSW process was invented just two decades ago at the Welding Institute in the UK. For aluminium usages to be simple for large scale production of naval vessels, then the overall productivity must be improved. Such may be accomplished through the application of aluminium extrusion and FSW as these methods offer significant cost savings (Collette et al. , 2008). The existing studies on the reliability of aluminium stiffened panels can also give a clear idea of some of the impacts of aluminium usage.

1. 3. 1 Aluminium extrusions

Adding to the features of using aluminium is its ability to be extruded. Extrusion makes it possible for complex design of stiffeners to be produced which can, if used appropriately; reduce the aftereffect of stresses experienced in the mid-ship region due to hull girder bending. Collette et al. (2008) researched on the ultimate strength and optimization of aluminium extrusions. Extrusion allows a designer to replace conventional welded plates or stiffeners with extruded profiles of varying thicknesses and it can be applied to decks and side shells, places with large amount of area for an increase in weight savings. This technique effectively reduces the number of welds to be performed and also reduces the complexity of the overall design of the structure.

The study examined three different kinds of extruded stiffeners, the conventional 'T' type, the sandwich type and the hat type for use up to speed a high-speed vessel. The performance of all three types was found to be similar, and the study figured the panel should be selected based on considerations rather than which includes the best strength to weight ratio. Such considerations can include cost, ease of construction and material fatigue. Inside the figure below, the joining of conventional plate to the stiffener requires welding while for the extruded panel, both the plate and stiffener is extruded as an individual unit.

Sectional view of Aluminium Extrusion panels

Figure (2)

1. 3. 2 MIG and FSW welding

To consider the feasibility of using aluminium in shipbuilding, it's important to check out aluminium's weldability. Metal-Inert-Gas (MIG) welding, a subtype of Gas-Metal-Arc-Welding (GMAW) is the initial form of welding for aluminium plates. Within the 1950-60s, further developments gave more versatility which resulted in an extremely used commercial process nowadays.

Until recently, a fresh and better method of aluminium welding is invented, namely the Friction-Stir-Welding (FSW). FSW is a fresh concept of welding where in fact the metal is not melted for the joining process so the mechanical properties remain unaltered whenever you can. The join between the two plates is then softened for the metal to fuse using mechanical pressure.

Kulekci (2010) notes that the FSW increases tensile, impact, and fatigue strength of the welded joint when compared with MIG process. Less hardness change and a narrower heat-affected zone should be expected in the welded material as less heat is produced from the FSW process. Higher heat intensity from the MIG process may damage the mechanical properties of aluminium. Through the use of FSW, production rate and quality increase and production costs will decrease.

Friction-Stir Welding Process

Figure (3), pictures from (http://www. fpe. co. uk/processes/friction-stir-welding)

1. 3. 3 Heat-Affected Zone

Mahoney et al. (1998) researched on the FSW process induced Heat-affected zone (HAZ) of the 7075 T-651 aluminium alloy. Some tensile tests both longitudinal and transverse to the weld produced results that showed the weakest region reaches the lower temperature location within 7 to 8mm from the edge of the weld area. As the average weldable aluminium alloy displays a 30 to 60% reduction in yield and ultimate strength, the loss in ultimate strength of FSW aluminium alloy is only around 25% and the yield strength at the HAZ is about 45% significantly less than the base metal.

1. 3. 4 Strength and reliability in aluminium stiffened panels

Benson, Downes and Dow (2009) note that as aluminium alloy is an established structural material in the shipbuilding industry for broadband crafts and naval vessels, the analysis for large high speed craft operating in ocean environments have since developed rigorous methodologies for the analysis of ultimate strength in the hull girder.

The fast upsurge in capacity and size of aluminium vessels has led to the demand in new engineering tools and answers to effectively analyse the structural performance of the vessels. One of it could be the analysis on the best and fatigue strength of aluminium stiffened panels. The ultimate and fatigue strength of the panels can be predicted by using the Reliability method, which involves firstly using limit state equations to determine when the structural member has failed. Secondly, to look for the average value and the assortment of random variables distribution in the limit state equation. Then the final step is to estimate the likelihood of a failure.

Collette (2005) researched that in the Stress-life or S-N fatigue approach, the fatigue life of an material depends upon applying continuously a varying load of frequent amplitude until a crack is observed. However the key drawback is that it's not able to give feedback on the seriousness or how big is the crack. That is where the Initial-propagation of I-P method became more useful. The main difference between both is that whenever the crack starts to form in the material, I-P method can estimate the growth utilizing a fracture mechanics model.

1. 3. 5 All-aluminium naval ship

With all the existing technologies and methods designed for aluminium shipbuilding, aluminium has the potential to replace steel in the foreseeable future as the main ship construction material. Lamb and Beavers (2010) studied on the importance of an all-aluminium naval ship. It proposes two types of aluminium frigate, one with a lower draft, the other an aluminium exact carbon copy of a steel frigate, with identical draft and similar in weight. Aluminium ship with a lower life expectancy draft makes it possible for for a reduction of block coefficient, thereby reducing resistance and increasing speed. Which has a finer hull, less power is necessary for propulsion, in turn cutting costs during operation.

The authors continued to conduct an analysis of steel and aluminium equivalent naval vessel design concentrating on the acquisition and ownership costs. The findings showed that an aluminium ship can be constructed with just 7. 5% of the cost of an equivalent steel ship even though 50% more labour hours are required for construction of the aluminium ship. The authors highlighted that this is possible because of the overwhelming benefits of aluminium's significantly lighter weight. Aluminium ship was also found to have operational and ownership cost advantages. Furthermore, advancement of aluminium technology in manufacturing process and design methods has closed the gap between steel and aluminium acquisition costs which in some cases, shipyards are producing aluminium structures more cost effectively than equivalent steel structures.

1. 3. 6 Hull-superstructure interaction

One of the critical indicators to consider when making a naval vessel is its hull-superstructure interaction. With the aid of structural analysis software MAESTRO, Hughes and Jeom (2010) determined that Hull-Superstructure Interaction is an extremely complex study that can only just be visualised effectively through 3D finite element model, rather than an inadequate beam theory. The vertical center of gravity for any naval vessel is crucial, it is therefore important to keep carefully the center of gravity only possible, either by reducing the scale or by using a lighter material in the superstructure.

Another important thing to note of Hull-Superstructure Interaction is the superstructure continuity with the ship side. A superstructure will participate substantially in hull girder bending in vertical continuation with the ship sides if the superstructure is long and continuous. It'll undergo the same bending radius as the hull. In the event the superstructure rises from the same plane as that of the ship sides, then your bending will be maximal. To exclude the superstructure from any hull girder bending, you'll be able to achieve this task through offsetting it from the medial side sides. If superstructure is not based on the ship sides, because of the overall flexibility of the deck beams, the sides of the superstructure can be subjected to a much bigger radius of curvature. Regarding such design, then an intermediate transverse bulkhead must be included in amid-ship for the purpose of terminating excessive cyclic deflections and stresses in the deck structure.

The above are specially critical as a design consideration in relation to naval vessels. To help expand complicate matters; in a naval vessel, the amid-ship portion is used for RAS operation, or Replenishment At Sea. RAS operations are extremely difficult manoeuvres to execute; and it must take place in amid-ship due to the heavier pitching motions of the vessel at both ends. In addition to that, RAS operations also require a sizable open deck area on both sides of the vessel. This implies a decrease in the scale, or width of the superstructure in amid-ship, precisely the area which encounters the greatest hull girder bending.

1. 3. 7 Fatigue-induced cracking

In the situation of an aluminium superstructure, the fatigue experienced will be sustained than for an equivalent steel superstructure (Grabovac et al. , 1999). The cases of similar Royal Australian Navy FFG-7 class frigates which experienced fatigue-induced cracking in the aluminium superstructure were caused by a combo of applied cyclic stresses and stress concentration interacting with an area of material weakness.

This problem of fatigue-induced cracking has surfaced in virtually all ships of the class. The vessel has a continuous aluminium superstructure welded atop a steel hull, which is prone to a large amount of hull-girder bending (Hughes and Jeom, 2010). This further reflects about how the Hull-Superstructure Interaction make a difference a vessel. Regarding their study, composite material is then chosen for fixing of the cracked region by adhesive bonding, which proved to be working later on with subsequent series of assessments.

Methodology

Lamb and Beavers (2010) introduced three types of ship for their study, the baseline steel, the aluminium reduced draft and the aluminium reduced block coefficient ship for comparison. The aluminium reduced block coefficient gets the same draft as the baseline steel ship but its block coefficient is a lot lower than the other aluminium ship.

The authors designed a 10m long mid-ship section of a naval vessel and then derived the scantlings using the ABS BROADBAND Naval Craft Rules. The scantlings include steel, aluminium and aluminium extrusion. Subsequently, bending moment and stress calculations were performed and the results shown were significantly less than the design stress of 23. 5 t/cm† for steel and 12. 4 t/cm† for aluminium. Reasons for the huge differences received that almost all of the plating is dependant on allowable minimum thickness rather than that derived from the formulas.

1. 4. 1 Linear Stress Analysis

The structural study in the present paper will adopt the Linear Stress Analysis method. Like the work of Lamb and Beavers (2010), the material behaviour in this study is only going to be considered in the elastic range. Within the Linear Stress Analysis, the strain is assumed to be directly proportional to the strain and the structural deformations are proportional to the strain. Shown below is the stress-strain graph of the material, where in fact the limit of proportionality is the limit of the Linear Stress Analysis. Considerations will never be designed for the behaviour following the limit of proportionality. Where (C) is the proof stress of the material.

Stress-strain graph

Figure (4), graph extracted from (http://www. sr. bham. ac. uk/xmm/structures3. html)

In the situation of Non-linear Stress Analysis, problems are solved by applying the load slowly, and then take account of the deflection with each increment. Stresses will be updated with each increment before full load is applied. A more complex Non-linear FEA requires iterations for equilibrium with each increment; hence it is just a computationally expensive approach.

1. 5 Discussion

Aluminium use in naval shipbuilding has been increasing steadily through the years as shown in the literature above. Commercial and merchant aluminium vessels were constructed with different purposes and intentions at heart, some built for an increase in speed, some for additional capacity and some simply for costs saving. In the case of naval ships however, they share more similarities. Common objectives would be an increase in speed and payload, if not for a decrease in draft. Hence, it is important to understand what different aluminium alloys may offer for different specific function of the ship.

Aluminium alloy as mentioned earlier was found to involve some discrepancies among various authorities; this may be because of the poor definition of aluminium's yield strength due to the nature of its properties. Nonetheless, aluminium's yield strength will be studied as 0. 2% of its strain limit.

Designing an all-aluminium vessel of any 130m in length can be an inherently complex task, and the strongest design, most up-to-date methods of production and manufacturing must be adopted to lessen the risk of an structural failure. Due to the nature of the method used in this paper, where a standard steel hull will be replaced by an aluminium equivalent, it is important to consider the use of aluminium extrusion as a far more effective way to boost the stiffness of the hull to ensure no deformation takes place prematurely. As fatigue-induced stresses was entirely on almost all of the FFG-7 class frigates, there is a need to review the hull-superstructure interaction of the vessel and understand the consequences if applied on an all-aluminium vessel.

Among days gone by studies, few have made comparison between a steel ship and an aluminium ship. One notable work is from Lamb and Beavers (2010), which based their calculations on the hypothesised frigate. This present paper differentiates from that in a manner that it talks about the differences between the two materials if used on a preexisting vessel. For your ship that already has an optimum hull form, and re-designing it might be irrelevant due to specific mission capabilities, it will be beneficial to adopt this process. Another way to check out it is that one navies might would prefer to build an aluminium exact carbon copy of a steel Off-the-Shelf (OTS) ship like the FFG-7 which has shown to be a cheap and seaworthy ship, than to totally redesign an entire vessel. The design process of a new naval vessel may take up to several years.

Apart from that, days gone by research of the all-aluminium ship with its cost and feasibility study was found to be outdated and non-applicable for this year. Today's research provides an up to date costs comparison between steel and aluminium, including costs incurred in the welding process, and with the additional consideration for aluminium extrusions. Also critical would be the consideration of the various ways of welding.

It is important to note that there are limitations through this feasibility study. The Linear Stress Analysis method adopted is merely accurate to a certain extent and Non-linear Stress Analysis method should be adopted for just about any future work in this topic. Also, there are considerations which will not be covered in this feasibility study. Factors such as the lack of infrastructure for aluminium naval shipbuilding in conditions of aluminium workshops and offer of aluminium panels will never be considered. Limitations may also include the insufficient skilled employees and expertise in aluminium manufacturing and ship production.

1. 6 Aims and Objectives

In short, the purpose of this paper is to review the feasibility of using aluminium as a naval shipbuilding material. To achieve that, it's important to include the common shipbuilding material, steel, for comparison. This paper aims to give a clearer comparison, in terms of designs, methods used, costs incurred and production time of both ships.

The US Navy FFG-7 class frigate will be used as a base ship. Designed in the mid-1970s by Bath Iron Works and partner Gibbs & Cox, FFG-7 frigate is intended to serve as an inexpensive escort ship. Its portion of operations includes protecting merchant convoys, replenish groups, landing forces, submarines and carrier battle groups; also performing anti-submarine warfare or surface action. The frigate has a steel hull with an aluminium superstructure designed for weight saving.

The overall bending stress characteristics in the mid-ship section of the frigate will be presented through load, buoyancy, shear force and bending moment calculations; one with steel hull and the other, a hypothesised aluminium hull of the same dimensions. Essentially, constraining the dimensions of the hull for an aluminium equivalent will cause an increase in plate thickness because of the reduction in the section moduli of aluminium. An alternative solution could be the increase in the number of stiffeners to be utilized and ultimately, the final design of the aluminium equivalent mid-ship section should include both options for a section modulus increment. The results produced should show that an aluminium hull would be sufficient in conditions of section modulus to keep carefully the maximum bending stress values under the design stress of the frigate, at 131. 75 N/mm† (Ship Structure Committee, 2002).

All calculations in today's structural analysis depends upon the linear elastic region of the materials only. Through the analysis on a mid-ship section, it can provide a concept of the strain characteristics of the whole vessel as the utmost bending moment will usually takes place for the reason that region. Finite Element Analysis software MAESTRO will be used to model a mid-ship portion of the naval vessel and present a better understanding of the structural stresses functioning on the aluminium hull.

The paper will go on further to present the costs relating to the two ships, in conditions of acquisition, productivity and ownership of the vessels with regards to the current steel and aluminium prices. The results from the study will be analysed and discussed, from then on the conclusion will be drawn accordingly.

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