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.