Aluminium - Large Aluminium Extrusions in Marine Applications

Topics Covered

Background

Market Influences

Properties Of Large Extrusions

Joining Methods

Fast Catamaran Deck Design

Offshore Module Design

Background

Aluminium plate and extrusions are used extensively in the superstructures of ships where the designers wish to increase the above waterline size of the vessel without creating stability problems. In hovercraft and in the various types of surface skimming vessels, such as fast mulithulled catamarans, (figure 1), the weight advantage of aluminium has enabled marine architects to obtain more from the available power.

Figure 1. The use of large aluminium extrusions gives quality and cost benefits in fast multihulled catamarans.

On offshore oil platforms, aluminium has become the established material for helidecks and helideck support structures because of weight and through life maintenance advantages. For the same reasons it has found frequent use in stair towers and telescopic personnel bridges. Aluminium accommodation modules have been installed on the Snorre and on the Statfjord C platforms in the Norwegian sector of the North Sea. These modules have provided a range of benefits. An overall weight saving of the order of 40% compared to steel has been achieved in the case of the Snorre accommodation module. Cost advantages were obtained in the case of Statfjord C as a result of using only 60 tonne maximum load capacity platform crane for erection and assembly purposes.

Market Influences

In world ship building, certain types of vessels are increasing in popularity. The interest in cruise holidays has surged and whereas it was once simply a matter of converting former ocean liners, purpose built vessels are one of the fastest growing sectors of the industry. New fast ferries which can dramatically shorten journey times are entering service around the world.

The oil industry is seriously affected by the fall in world oil prices. If more marginal fields, for example some of the more difficult North Sea finds, are to be exploited then the costs of oil production hardware will have to be lowered. These market conditioned are pressurising designers, for a variety of technical reasons, to lower effective weight of structures, to cut construction costs and to reduce through life maintenance requirements.

If composite construction is adopted and very high strength fibres are used, fibre reinforced plastics can sometimes be an option to reduce weight, but problems can occur because of high material costs, high moulding costs and difficulties with fire ratings. Often the only feasible way of lowering weight is to adopt or change to aluminium.

Construction costs are very dependant on joining/assembly techniques. If joining can be reduced or made more simple by, for example, using the largest available extrusions or/and, where acceptable, using mechanical joints as opposed to welds, then construction times and hence costs can be lowered. The proven corrosion resistance of unprotected aluminium alloys in marine conditions, for example, the plate alloy AA5083 or the extrusion alloy AA6082, is well documented. This advantage over constructional steel has a considerable influence on through life maintenance costs.

Following the 1988 North Sea Piper Alpha oil and gas platform disaster, which claimed 167 lives, the new approach to safety has meant that accommodation modules are now installed on offshore structures as far away as possible from the more dangerous operations. This frequently means that the weight of the living quarters module is a factor which has a major influence on new build project costs.

Since the first offshore platforms were built, considerable advances have been made in the techniques for recovering ever higher proportions of hydrocarbons from the layered geological structures below the sea bed. These improved techniques have often meant that additional heavy pieces of equipment have had to be installed on the existing offshore facilities. Many of these ageing platforms are approaching their maximum designed topside weight. It is usually much cheaper to replace parts of an existing installation with new light weight modules than to install a completely new structure.

Properties Of Large Extrusions

The mechanical properties of extrusions are influenced by grain size. This in turn is largely determined by recrystalisation characteristics of the alloy, extrusion ratio, extrusion temperature and final heat treatment. The flow of material in the extrusion process causes a directionality of mechanical properties. Transverse proof stress and UTS are 85-90% of the longitudinal values.

One of the main advantages of the aluminium extrusion process is its ability to provide complex hollow shapes. Most hollow profiles are produced from die tooling which forms welds during the extrusion process. Judged by the criteria appropriate for the more familiar fusion welds, there would seem to be no problems with extrusion welds. Composition is constant, there is no filler metal and there is no liquid to solid phase change. Nevertheless, properties across the weld can differ from those of the parent metal because of differences in grain size and variations in the distribution of intermetallic phase particles.

The term extrusion weld covers two types of weld: seam welds formed when two streams of metal flow together in the die, and charge welds formed at the die ports between successive billets. Both types are solid state welds formed under deformation and pressure. From a correctly designed die it is very difficult to form a low quality seam weld. Quality problems from charge welds are unfortunately far more frequent if correct operating procedures at the press are not followed.

It is most important that the correct length of extruded material is scrapped at the start and end of each billet in order to ensure that the low property material is removed. Proportionally large billets are required for large extrusions to provide a sufficient length of material to allow the potentially defective front and back ends to be removed. This means, particularly for extrusions with high cross-sectional areas, that high extrusion pressures and not just large diameter press containers are essential.

Table 1 shows minimum property values for extruded AA6082 T6 material in the longitudinal and transverse directions and includes minimum transverse values taken across extrusion welds. The table also shows values of mechanical properties of AA6082 butt welds for comparison purposes.

Table 1. Mechanical properties of aluminium extrusions (minimum values)

Extruded AA6082 T6

Thickness range (mm)

 

-5

5-10

10-30

Longitudinal

0.2% Proof stress (MPa)
UTS (MPa)
Elongation A5(%)

260
310
10

260
310
10

260
310
10

Transverse no extrusion weld

0.2% Proof stress (MPa)
UTS (MPa)
Elongation A5(%)

245
290
8

245
290
8

235
280
6

Transverse with extrusion weld

0.2% Proof stress (MPa)
UTS (MPa)
Elongation A5(%)

245
290
5

245
290
8

230
260
3

Butt weld AA6182, filler rod AA5356/5183

Thickness range (mm)

 

 

-15

15-25

0.2% Proof stress (MPa)
UTS (MPa)
Elongation A5(%)

 

115
185
3-5

95
165
3-5

The longitudinal fatigue strength of AA6082 T6 after 107 cycles at stress ratio(R) = 0, is quoted typically as 130MPa. Fatigue tests made transverse to the extrusion direction give results of approximately 80% of this longitudinal value. Extensive testing of fusion welded flooring sections containing extrusion welds has shown that failures usually occur at the fusion welds or in the heat affected zone on either side of the weld seam.

Fatigue characteristics of samples taken transverse to the extrusion direction containing extrusion welds are similar to transverse values from the base material, always with the proviso that sufficient front end extrusion scrap has been removed to provide satisfactory extrusion weld quality. Large extrusions have better fatigue characteristics than similarly dimensioned assemblies of small extrusions fusion welded together.

Joining Methods

MIG and TIG welding have been in use for many years and have established themselves as reliable techniques when the correct procedures are employed. The various problems which can arise have also been studied in detail. Typical defects are shown in figure 2. The four fusion weld defects represented in the diagram affect different aspects of the mechanical properties of the base material. Whereas the local heating, over ageing and consequent softening of the heat affected zone on either side of the weld bead lowers proof stress and UTS, the micro and macro porosity and shrinkage defects can act as sites for fatigue initiation and as a result can lower fatigue properties.

Figure 2. Possible quality problems in fusion welds

In addition to problems caused by weld flaws, fatigue strength is affected by mechanical factors such as holes, threads and grooves and also by the positioning of flaw free welds. However, extrusion technology can be used to position fusion welds in non critical areas or to enlarge the section close to a weld in order to compensate for the loss in properties caused by the welding process.

The design rules for fatigue of aluminium structures are covered by a number of standards including British code BS8118 Structural use of aluminium and the European code ECCS - paper, Doc 68 European recommendations for aluminium alloy structures fatigue design. Of the two codes the British Standard is in general the more conservative. An efficient quality assurance system is needed to monitor and guarantee both performance of welding equipment and workmanship.

Often the most convenient and technically optimum way of joining two or more aluminium extrusions is to use a specially designed mechanical fixing arrangement. The combination of relatively few welds with a high proportion of mechanical joints has become standard for helidecks. In the latest designs for offshore accommodation modules, the outer skin is a welded structure and selected parts of the interior have been designed to incorporate mechanical joints with sealants between the individual flooring sections.

Fast Catamaran Deck Design

By making use of large extrusion technology simply to reduce the amount of welding, considerable quality and cost benefits can be obtained. Benefits from use of large extrusions in more complex parts of a structure than the deck are more difficult to quantify but nevertheless real. The advantage of being able to free more parts of the design from the potential difficulties created by the need to thoroughly inspect the fusion weld joining two standard extrusions can easily be appreciated.

Offshore Module Design

The Snorre accommodation module was built using more than 20 different profiles, some of which were relatively difficult hollow sections. The welded design needed some 780 tonnes of aluminium making it the largest all aluminium structure ever built. The total finished weight of the Snorre accommodation module was 2100 tonnes. The Statfjord `C' accommodation module was based on the same basic components as were used for Snorre.

It was considered that a design change should make possible a lower weight, lower cost module. The design change has involved reducing and simplifying the number and type of extruded sections and moving to a combination of welded and mechanical joints to lower construction costs. Since the new design requires relatively few profiles it is intended that these be held in stock to make virtual off the shelf delivery a possibility. This will make modules available in the very short delivery times, important for the offshore refurbishment market. The primary and secondary beams and ternary decking have been so designed to allow flexibility inside the module so that heavy items can be supported in the structure with relatively little design input.

 

 

Primary author: Robert Dean

Source: Abstracted from Materials World, vol. 3, pp. 65-67, 1995

 

For more information on Materials World please visit The Institute of Materials

 

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