1. Introduction
Development trends in outfitting and equipping ships aim at a reduction in weight and in assembly[ds_preview] efforts. A requirement stemming from this aim is the joining of different materials by means of alternative economic joining technologies, a task which can only partly be achieved by thermal joining procedures. The aim is to identify application areas for alternative joining methods and to introduce them into outfitting and equipping procedures.
Commonly used thermal joining procedures do have disadvantages when ship section outfitting level increases. If paint conservation is already enraged, assembling with welding technology does lead to a significant increase of efforts regarding preparation and reconditioning (see Fig. 1).
The reasons for researching different joining technologies besides welding are:
1. damages of preinstalled outfitting parts (e.g. control enclosures)
2. deformation caused by head treatment during welding process
3. paint damage caused by heat treatment during welding process
4. joining of dissimilar base materials
5. process optimisation
With conventional joining technologies those problems are not solvable. Therefore the definition of sufficient alternative joining technologies is necessary. The new technologies should be chosen regarding the mentioned criteria.
2. Solution
In several research projects with shipyards in north Germany the Fraunhofer AGP, Rostock, has reviewed different joining technologies in order to show economical and technological alternatives. In this paper three possibilities will be discussed form both points of view. The possibilities are presented below:
• blind rivet joining
• gluing
• framing channel system
2.1 Mechanical joining
The mechanical joining technology is a widely used joining technology especially in railway vehicle manufacturing and automotive industry. In Fig. 1 the blind riveting as such a typical mechanical joining technology is presented.
Blind rivets need a pilot punch hole in both joining parts. The static and dynamic load bearing capacity is comparable to screws. From a general point of view blind rivets do have the advantage of a one-sided placing process as well as a very fast placing time with a very small scatter range of load bearing parameters. Furthermore the fatigue strange of those kinds of fasteners is very high and comparable to screws. This kind of fastener has a security against self loosening.
From the shipbuilding point of view joining technologies with pilot hole operation are not usable for a wide range of outfitting applications.
2.2 Bonding
In outfitting processes bonding is not state of the art. Basically bonding could be divided into two different principles (see Fig. 3).
For outfitting processes flexible adhesives are especially capable. The load bearing capacity could be secured within a wide range of adherent thicknesses (depended to the chosen adhesive). Therefore the compensation of tolerances caused by production tolerances or heat influenced deformations is possible. The joining of different base material types has also to be mentioned.
There is insecurity about the complexity of assembling processes based on bonding. On the one hand this insecurity is based on necessary design of production processes. On the other hand the resistance of bonded joints under different environmental conditions is the major concern. Due to the need of overlap length bonding has to implemented in the design process of brackets for outfitting objects.
2.3 Framing channel system
Another possibility of optimizing outfitting processes is the usage of framing channel systems. The basic principle is the implementation of functional elements (framing channels) which can be used for bracket installation during outfitting process. The implementation of these framing channels in ship structure can be done as early as possible and before the application of paint and varnishes. Due to the degrees of freedom in at least two axes of those systems a high assembling flexibility is assured. The position of the framing system could also be integrated into CAD design. In this case the implementation of the system could be done at very early state of production assuring the necessary flexibility in assembling. Fig. 4 shows a framing channel system installed in a pipe tunnel (civil engineering) and two typical framing systems.
Cold rolled systems are already used in special shipbuilding. Those kinds of systems are used for outfitting systems that only occur static load bearing. A spread into standard shipbuilding has not taken place.
Hot rolled systems are currently not used in shipbuilding. Hot rolled systems are especially used (railway vehicles, civil engineering) if high static load bearing or fatigue load bearing capability combined with high assembling flexibility is relevant.
3. Application examples
In several research projects different application cases were taken into consideration. The mechanical joining technique was tested for assembling door frames. Gluing is investigated for assembling lash pots and thresholds. The framing channel system is investigated for assembling pipe supports.
3.1 Blind riveting (mechanical joining)
The potential of mechanical joining techniques will be discussed by the case of door frames in deck house outfitting. This application case was considered to have high potential from a technical and economical point of view. The conventional assembling method is described in Fig. 5.
This way of door way assembling implements a high amount of preparation and reconditioning work. On the left an intermit welded door frame is pictured. The door frame is usually already painted when delivered by the supplier. It is also possible to deliver the door frame in paint finish condition. If door frames are assembled by welding the paint will be damaged around the heat affected zone. Before assembling could start all pre-installed outfitting parts surrounding the joint area should be protected. In the case of welding an area of appr. Ø 2 m around the joint must be taken into consideration. To assure a sufficient corrosion protection it is in dependency to classification society’s necessary, to remove all heat affected paint parts. Afterwards the surface has to be blasted and the predefined coating system has to be applied. Therefore time for preparation and reconditioning joint area forms a significant part in assembling costs. Tab. 1 shows in summary advantages and disadvantages of alternative joining techniques.
A joining alternative is blind riveting as previously mentioned. Door frames could be generally delivered with pilot holes for assembling. This is already available for screw assembling. In Fig. 6 steps for preparation and assembling are presented.
To assure a sufficient load bearing the number of rivets was calculated in comparison to the conventional intermitted weld (fillet weld: 5 mm, length 30 mm). Fig. 7 shows the achievable results with a minimum surface damage. In this case three different blind rivet types have been used. The best optical results do have the counter sunk blind rivets. Those kind o fasteners could be painted. In the result a plain surface can be guaranteed.
In the case of outfitting processes in shipbuilding work equipment for field assembling is required. In Tab. 2 possible investment costs for one set of working equipment are summarized.
Those costs are just once necessary. They have to be paid for every single batch. The batch size has to be defined. The costs for consumables needed for the given example are listed in Tab. 3. For the presented example a calculation with 40 fasteners is chosen.
Beside the investment and consumable costs for optimization of production processes the working time is the major focus. Tab. 4 shows the working times and constituent parts.
The conventional assembling with ten fillet welds takes 7.75 min per welding bead. This value includes preparation, assembling (auxiliary process time, primary process time) and reconditioning. In summary the assembling time by intermitted welding for one door frame is 46 min. The investment costs for a welding machine are approximately 2,000 €.
Regarding the summarized working time and investment costs both assembling techniques are comparable. The consumable costs should be also comparable.
The major advantages of blind riveting are different amounts of the constituted parts of working time. Using intermitted welds, a great amount of working time is used for preparation (paint removal) and reconditioning (removals of weld sprats / painting). The heat affect of welding has also to be taken into consideration. Additional it has to be assured that damaged paintings are reconditioned with a high quality (exact lacquer layer). Using blind riveting the main part is the implementation of pilot hole into the ship structure. Paint damage could be extremely minimized.
3.2 Bonding
Around deck penetration and stairways coamings are used to prevent water (rain) and other liquids of flowing into lower decks. In Fig. 8 the conventional assembling technique by welding is described.
Coamings are welded on just one or both sides depending to the assembling condition. In this case linear heat affection could be considered. In the case of already painted surfaces paint removal is very intensive. Affected by this, deformation and paint damage is common when assembling coamings. The functional tasks of coamings are:
• tightness (liquids)
• corrosion prevention
• shock proof
Those functions could be assured by welding. But a welded coaming is over dimensioned regarding the named functional tasks. Therefore an alternative assembling technique only could lead to advantages, if paint doesn’t have to be removed. In the application case bonding will be investigated as an alternative assembling technique.
The important question to be answered is the adherent strength of typical paintings commonly used in shipbuilding. In shipbuilding all materials (steel) will generally be covered with shop primer. This is the precondition every assembling technique has to deal with. During outfitting process also intermediate layer as well as finish painted surfaces appear. The question in the case of bonding is, whether or not it is possible to bond directly on painted surfaces. Only if bonding on painted surfaces is possible, improvement could be achieved. In Fig. 9 the adhesive strength and the type of defect is shown for three typical painting systems.
Generally the failure type is a failure between shop primer and second lacquer layer. In dependents to the number of layers the strength will decreases significant. The load bearing capacity of typical flexible adhesive, which should be chosen for this kind of assembling, is app. 1 N/mm2 to 2 N/mm2. Those values are determined on painted surfaces (finish) during a concluded research project [Wanner et. al (2009)]. Therefore bonding on painted surfaces seems to be possible.
To implement a joint design suitable for bonding standard angle steel was chosen. The angle steel to assure a sufficient overlap length and is easy to supply. In Fig. 10 two coamings bonded on finished surfaces are presented.
For this application a one component and a two component adhesive has been used. The adhesive have been tested for chemical resistance and mechanical strength. For the one component adhesive an adhesive layer thickness of 4 mm was chosen. The two components adhesive was used with 2 mm adhesive layer thickness. The cure time for the one component system is two days. The cure time for the two component system is two hours. A component fixing is not required. The adhesive lay thickness could be assured with flexible spacer.
3.3 Framing channel system
The last application sample to be mentioned is the usage of frame channel systems instead of conventional equipment brackets. Followed a virtual control room on a RoRo-Vessel is considered. The discussed questions are
• Assembling flexibility
• Tolerances compensatory capability
• Required working time
• Resulting costs
In Fig. 11 a standard control room designed with conventional equipment brackets is shown. The major focus is lying on the supports needed for pipes with a small and a high diameter as well as for control boxes. Those three examples will be watched in detail.
Brackets for control boxes are made out of angle steel which is directly welded to the hull planking. To assure assembling flexibility hole pattern or elongated holes are used for the assembling of outfitting equipment on those brackets. The supports a commonly installed directly before assembling the equipment. Tolerance compensation is after bracket installation only restricted possible.
The pipes have to be installed are in accordance to DIN 86027. Assembling the pipes the brackets will be installed directly on profiles or on the hull planking using laminations (see Fig. 11 right side). The bracket length and position will be chosen directly in dependents to the pipe position. From this point of view the conventional system seems to be ideal. After installing the pipe brackets needed flexibility (support position) due to changes of pipe position is also not possible. For this application example eight angles, 22 small pipe brackets (DN16 – DN 50) and eight big pipe brackets (DN150 – DN 300) are needed.
Followed the conventional bracket system is replaced by frame channel system (hot rolled). The frame system could be assembled equal to the conventional system by welding it directly on the hull planking or profiles. In this case a full welded version and an intermitted weld are possible. Other possibilities are the usage of weld studs or bonding.
The bracket construction for control boxes is design as a framework (see Fig. 12, left side). Therefore four frame channels (two for on control box) are welded to the hull planking vertically. By using four horizontal frame channels, adjusted to the first frame channels by screws, the control boxes could be assembled with a maximum degree of flexibility. Fig. 12 shows the control room with a bracket system based on frame channels.
In comparison to the conventional bracket system the frame channel system allows due to its high flexibility a bracket assembling in a very early state of outfitting. Therefore an installation before finish paint is applied is possible. This will reduce reconditioning work significant.
Pipe brackets based on frame channel systems could also be welded directly to the hull plating. Due to the small available space between pipe and hull plating, the frame channel system could not always be implemented between. In those cases a console out of combined framing channels has to be used. One frame channel could also be used for more than one bracket. Tab. 5 compares the two assembling techniques.
For the considered assembling techniques (welded standard bracket; welded frame channel system; frame channel system assembled with weld studs) an economic comparison was carried out within a market survey, carried out by Fraunhofer AGP commissioned by »HALFEN Vertriebsgesellschaft mbH«. In Tab. 6 the calculated costs for the pre described example are listed. In this case the standard brackets are welded.
The costs per pipe meter are lying at app. 3.35 €. In Tab. 7 the costs for the full welded frame channel system are listed. The calculated costs per pipe meter are lying at 2.14 €. Compared to the conventional system the cost are significant lower. The full results of this market study with the scope of investigating technical and economical issues regarding typical application cases in outfitting processes are shown in Tab. 8. Here different possibilities of assembling the frame channel system are compared as mentioned before. The comparison shows significant possibilities of cost reduction if frame channel systems could be used from the technical point. The assembling version using weld studs decreases assembling time significantly and therefore costs.
Costs due to reconditioning work are not considered in this consideration. Reconditioning time will be reduced significant due to the possibility of assembling the frame channel system before finish paint is applied.
4. Conclusion
For outfitting processes in ship building time and cost saving alternative assembling method are available. As shown above alternative assembling techniques do have advantages compared to traditional assembling method, e. g. welding. From the technical point of view the described joining techniques do offer an equal load bearing capacity. The completion of additive constructive needs could be achieved by choosing capable fasteners or adhesives. Furthermore alternative assembling methods do offer a significant cost reduction potential which could be shown in chapter 4. Additional the higher flexibility regarding the time of assembling in the production process and the higher part of primary processing time could improve outfitting processes. The reduction of heat affected deformation and paint damage and therefore a higher production quality due to heat reduced joining techniques have also to be named.
Authors:
Dipl. Wirt.Ing. Normen Fuchs
nof@hro.ipa.fraunhofer.de
Prof. Dr.-Ing. Martin-Christoph Wanner
mcw@hro.ipa.fraunhofer.de
both Fraunhofer AGP, Rostock, Germany,
Dr.-Ing. Knuth-Michael Henkel
University Rostock, Faculty Mechanical Engineering and Shipbuilding, Professorship of Manufacturing Engineering
knuth.henkel@uni-rostock.de
Normen Fuchs, Martin-Christoph Wanner, Knuth-Michael Henkel
