THE FOURTH CHESAPEAKE POWER BOAT SYMPOSIUM ANNAPOLIS, MARYLAND, JUNE 2014
Composite High Speed Craft: Practical Design Approaches Albert Nazarov, Albatross Marine Design, Thailand
Figure 1 – Composite craft designed by AMD: a – H65 18.2m catamaran yacht (BakriCono Shipyard, Thailand); b – AT1500 15.4m passenger catamaran (AusThai Marine, Thailand); c – SM16 15.2m fast interceptor craft (SOLAS Marine, UAE/SriLanka)
1. INTRODUCTION For high-speed craft, small boats and other types of advanced marine vessels, there exists obvious trend towards replacement of traditional metals used in ship construction by contemporary composite materials (fig.1). By today, composite craft comprise about 90% of all small craft (i.e. craft with hull length LH≤24m), and composites are currently used for construction of ships up to 60…70m in length. Shift towards composites requires complete re-thinking of design approaches and specifics of composite engineering is has become essential area of naval architects’ expertise. For number of years, Albatross Marine Design (AMD) is working on design and construction of composite pleasure, commercial and special craft built in number of countries in Asia and Europe (fig.1). In present paper, some insights of composite craft engineering and review of potential problems are provided, based on experience of practicing boat designer.
2. PROS AND CONS OF COMPOSITE APPLICATION For small and high speed craft, composites provide definite advantages by reduction of weight of structures compared to aluminium hulls (fig.3), thus giving better fuel efficiency or higher operational speeds, easy service maintenance without corrosion, better use of interior volumes, excellent insulation. Composites are best suited for mass production and require less skilled labour, thus providing definite cost savings (fig.2). Expensive tooling is often referred as biggest of disadvantages of composites, though efficient methods of one-off construction exist – see section 9. For special applications, important features of composites are low radar signature and non-magnetic properties. Ability to produce nice shapes and image factor of being ‘high tech’ are extremely important for pleasure craft market. Major disadvantages of composites are related to structural fire protection, performance of sandwich structures at low temperatures and low resistance to abrasion. Nevertheless, fire insulation solutions exist and AMD has experience of designing composite boats with restricted ice class. Composite structures are not recyclable; the utilisation technologies have been developed but still they are not widely available.
The biggest problem with composites is conservative ‘lack of trust’ approach in some jurisdictions, causing discrimination of composite craft. Regulatory issues are also the limiting factor: composite technology is fast, the rules are quite slow to adapt and only some regulatory bodies practice prompt adaptation of their standards to changing technology. Actually most of problems with composite applications can be successfully solved once experience of designer and builder are applied.
Figure 2 – Comparative cost of structures in different materials, including materials, labour and overheads
Figure 3 – Mass of structure for different types/length of craft in composite and aluminium
Figure 4 - Mouldings assembling illustration for ASV1100 11m catamaran.
3. COMPOSITE DESIGN PHILOSOPHY Composite material is engineered parallel with the process of design of the craft’s structure; this allows distribution of reinforcements according to loads but also requires more detailed engineering analysis. On other side, this results very efficient structures in terms of weight, strength and cost. Structural design of composite craft often starts from its very first sketches. Thinking about mouldings, release angles, unification of parts and combining structural elements with accommodation mouldings and liners are just few considerations to mention here (fig.4). Often the designer if facing the task to re-use existing tooling for new craft, including such big items as hull and cabin moulds. The process of structural design in general is made in the course of the following steps: • • •
Prepare structural concept of craft based on mouldings; Define laminates based on experience; Select standards and proceed with calculations;
•
Adjust the structural concept and/or laminates to comply with chosen load cases.
For composite craft structure, hull material is produced directly during lamination of hull from raw materials: reinforcements (woven and miltiaxial fabrics, mats), resins and cores. This means that quality of construction process and quality of materials define the overall quality of the craft. Composite material is non-isotropic, excluding some low-tech solutions as chopped-strand mats (CSM) and bulking mats. Generally, structural design for small and high speed craft can be performed using following methods: • • • • • •
Rulebooks of Classification Societies - [13-15,19, etc.]; ISO standards, namely ISO12215-5 and -6 [4,5]; Direct calculations using basic engineering principles; Testing or trials; Using empirical data obtained from service history; Use of prototype craft.
Testing method is applied for smaller craft, i.e. drop test for craft with hull length LH<6 m by ISO12215-5 [4] or lifeboats. The latter two methods are less applied at present due to certification complexities, but they are used for some racing and novel types of craft.
Figure 5 – Diagram of structural calculations process for small and high sped composite craft.
Figure 6 – Comparison of tensile modulus E1 in direction 0°, defined using formulations from different Rules of classification societies with results of panel testing for QX800+LT600 E-glass laminate with polyester matrix, fiber content 56%.
There are few major approaches used for design of composite structures: • •
• •
Directive approach – the laminate is defined directly from standard using prescribed spacing of stiffeners; this method is not commonly in use now except some simplified or obsolete standards as RS Rules [15] for composite craft. Quasi-isotropic approach – thickness of laminate (or weight of reinforcement) and section modulus of stiffeners are calculated using available methods/formulas for isotropic materials. This method is used mainly for relatively simple and balanced symmetrical single-skin laminates consisting of CSM and woven rovings (WR). Sometimes this approach is also applied for global strength analysis where more sophisticated laminates are replaced by single-skin equivalent, with subsequent analysis of hull girder with customary tabular methods. Orthotropic approach – two-dimensional, based on laminate stack analysis where laminate is presented as number of plies with individual structural properties, also variable in different directions [5,13,14] (see table 3). Three-dimensional approach – based on combination of FEA study of 3D geometry with orthotropic approach for every elementary plate (see fig.14).
On fig.5 typical process of structural design of composite craft is presented; it should be noted that priority is given to local strength and vertical acceleration is used throughout all calculations. Levels of composite analysis vary from basic beam theory approach to sophisticated finite elements analysis (FEA) [1,17]. Unfortunately, up to date FEA software used for
analysis of composites are not in full agreement with structural criteria used by classification societies; but such analysis ‘a must’ for complicated composite structures in advanced materials. In AMD’s practice, combination of methods is used for every design, with application of ‘rulebook’ approach for local loads and FEA for global strength and elements analysis [9,12]. Table 1 – Sample of composite properties based on ISO12215-5 and presented in form suitable for FEA CSM – chopped strand mat; WR –woven roving, BX – bi-axial; MD-CP – multidirectional cross-plied fabirc
4. COMPOSITE MATERIAL PROPERTIES There are different approaches to definition of structural properties of composites; hereafter they are listed with decreasing complexity but increasing safety margin factors [4]: • • •
Method A - tested laminate - material properties are defined based on testing of sample panels– tensile, compressive, flexural, etc.; Method B - tested glass content - material properties are calculated using approximate formulas and based on measured glass content Ψ in sample defined by burn-out test or by infusion record; Method C - untested laminate - material properties are calculated using approximate formulas and glass content Ψ prescribed by standards and/or assumed from type of lamination process, type of material, etc.;
It should be noted that testing of material samples (usually 5 samples in each direction, for each laminate, etc.) to ISO527, etc. is quite costly and time-consuming, and would not be feasible for small craft built as custom or small series. In AMD’s designs, usually 10…20 different lamination schedules are used for different areas and structural members of each craft; this approach results light and efficient structures. Requirement of obligatory testing of samples imposed by some societies and expected usual hassles with convincing them to accept calculations of complex laminates by common engineering methods will often cause undesirable simplification of structure by designer/builder with use of only 1…2 types of laminates. Such approach is associated with increase of structural weight and shift towards low-tech solutions; unfortunately this is often seen due to known regulatory issues.
In formulations for structural properties used in methods B and C, two approaches are applied: • •
Structural properties are defined in form of empirical formulas deriving from published test data; Structural properties are defined using classic laminate theory (CLT) with or without coupling effects.
The first approach assumes that properties of materials such as tensile and compressive modulus E, shear modulus G, ultimate tensile and compressive stresses σu , ultimate shear stresses τu, etc. are presented in form:
E , G, σ u , τ u = f (Ψ , type of reinforcement, type of resin, etc.) Application of CLT requires consideration of each orthotropic layer in cross-plied reinforcements, and properties are assessed through composition of multiply matrix (Table 2) using methods described in [14]. Meanwhile, from results of testing and calculations (see fig.6), some practical conclusions can be made. In range of practical interest for woven and multiaxial material (ψ=45…60%), pre-calculated properties from different sources are similar. Formulations from RS Rules [15] give the highest value of E1, though they refer to WR type of material. Such difference can be explained by application of different confidence interval or testing methods. Application of CLT by BV (symmetrical laminate) and GL give exactly the same values of properties. Inclusion of coupling effect for asymmetrical laminates results slightly lower properties, though in GL this effect is neglected. In general, use of CLT seems to be a complication not really associated with increase of accuracy for most of real-life laminates. Approximate formulas are quite reliable and already provide some safety margins to results of tests. Given realistic budgetary and time restrictions of small composite craft design, testing of material samples can be omitted in most projects and calculated properties can be applied without any safety degradation, on condition of proper quality control during construction.
5. DESIGN CRITERIA Though most of the rules rely on stress (or strain) criterion plus allowable deformation, there are some differences in specific criteria such as minimum thickness requirements, buckling, etc. In our practice, following criteria are considered for FEA of composite structures [8,9]: • • • • •
Maximum stress criteria; Maximum strain criteria; Tsai-Wu criteria; Interlaminar shear stress criteria; Deformation criteria
For sandwich structures, of additional interest are core shear criteria and variety of skin compression and buckling criteria. General expression for permissible design stresses σd of composite structures is [1]:
σ d = σ m ke kd k s where σm – medium value of breaking stress, obtained from testing of samples; ke – external factors coefficient (temperature, water absorption, etc.), 0.8…0.95; kd – internal defects coefficient, 0.65…0.90; ks – coefficient of material properties variation, 0.65…0.90. In design of marine composite structures, ultimate strength σu is often taken for confidence interval of 90…95% from results of testing, depending on classification society. Thus, design stresses are almost unified for classification societies and comprise (0.3…0.33)σu for tensile and compression and 0.4τu for shear stresses, for short-time loads [12,17]. In ISO12215-5 standard [4] permissible design stresses are increased to 0.5σu и 0.6τu, but this difference is somehow compensated by 20% reduction of default structural properties of composites in ISO122155, in case if method C for untested laminates is used. Generally, increased design stresses can be accepted by classification societies if advanced analysis is used where all local concentrators of stress are modeled.
Figure 7 – Compact wave buoy WaveTector from IMAA (UK) and sample record of wave time history during sea trials
Figure 8 - Time history of vertical accelerations on L=7.1m catamaran, on head wave H1/3=0.34m, speed 30kts recorded at forward perpendicular (FP) and center of gravity (CG).
6. DESIGN ACCELERATIONS High speed craft with their often complicated geometry and operational profiles are subject to considerable global and local structural loads. For special craft, mission has to be performed often in extreme environments thus their structures should be carefully treated at design stage with concern given to structural optimization and weight. Design accelerations are the basis for definition of design loads for high speed craft [11,16]. This statement is also applicable to any small craft, except very slow displacement types. All design accelerations can be subdivided into: • •
Operational – mostly vertical accelerations on seaway az, but some rules also consider other directions; Emergency – accelerations experienced in case of collision accidents and defined by HSC Code [3] comprising of longitudinal, transverse and vertical components.
During recent years, AMD has conducted large number of sea trials of high speed craft with measurements of operational accelerations [9]. During sea trials, wave buoy (fig.7) is used to assess sea states. The buoy allows registration of wave profiles including height and period, and saving them in file for further processing. In design practice and in most rules and standards, evaluation of vertical accelerations on high-speed craft (including catamarans) is made using Savitsky-Brown formula [16] with declared accuracy of about 20…30%. Based on results of numerous measurements (fig.8), in practice of AMD modified version of Savitsky-Brown formula is used [9,11,12] providing good match of predicted data with results of full-scale measurements on catamarans and monohull craft. Acceleration so calculated should be converted to 1/100 occurrence level for use in structural design [16]. In practice of classification societies, acceleration at forward perpendicular is often taken as double of acceleration at CG,
i.e. aFP=2aCG. This is a kind of simplification and it has been shown during sea trials that this is not always correct for slower semi-planing craft (fig.9). As a summary of assumptions used for structural engineering, design envelope for the craft is provided to the operator (fig.10).
Figure 9 - Measurements and calculations of vertical accelerations on catamaran LWL=17.8m, at head wave H1/3=0.53m, at the centre of gravity (MID), and forward (FP) and aft (AP) perpendiculars.
Figure 10 – Design envelope for ASV1100 11m catamaran, showing limiting lines for structural compliance, passenger comfort limit and speed limit due to available power.
7. DESIGN LOADS Structural design of high speed craft is made considering the following groups of design loads [13-15]: • • • •
Local loads on bottom, sides, deck, cabins - estimated as for monohull boats; Local strength of tunnel – specific slamming loads for catamarans; Global strength: longitudinal (especially for hydrofoil craft) and transverse (for catamarans); Specific elements loads on elements such as engine base, masts, anchoring and towing equipment, etc.
Local loads typical for monohull craft can be defined using ISO12215-5 standard [4] (for craft with hull length LH<24m) or appropriate structural rules of classification societies such as LR or GL special service craft rules [14,13]. Comparison of design loads imposed by number of rules is presented in [7,12], where hull panels of the same size were taken and run through the calculations. In most of rules, formulas for bottom design pressure p have following configuration:
p=
∆ × az × ∏ ki A
This equation describes falling of craft with weight ∆ on reference hull surface area A with vertical acceleration of az. In this equation, Πki is the product of coefficients, accounting for panels/stiffener size, position, local deadrise angle, etc. As general practice of AMD, at early design stages structural analysis for local loads is performed using ISO12215-5 [4] standard that proved to be extremely efficient and easy to use, thus allowing answering the questions on weight of structure and dimensions of structural elements, specifying preliminary laminates, etc. At later stages, rules of classification societies (see table 3) and FEA (fig.14) are applied for final calculations. Longitudinal global strength is usually not an issue for conventional composite craft with LH<30…40m, exclusive of some long slender craft with unusual proportions and structural schemes [17]. For pleasure craft LH<24m, this analysis is prescribed in ISO12215-6 [5] where maximum longitudinal bending moment Mm is presented as:
M m = ∆LH (0.5 + 0.6a CG ) Conditions of global transverse strength are of special interest for catamarans [8,10,11]. Usually they are by default
satisfied for small craft (LH≤24m), but checks are often required for thin-skin sandwich hulls, it can be defining condition for some catamarans with lack of transverse stiffening in bridgedeck area. Most of classification societies use standardized design loads for transverse bending moment MB, transverse torsional connecting moment MT, transverse connecting shear force QT:
M B = kMB ∆BCL acg ; M T = k MT ∆LWL acg ; QT = kQT ∆acg where ∆ - is weight displacement of craft; BCL – distance between hulls centerlines; LWL – length of waterline; aCG – design value of acceleration at CG of craft; aCG=2…6 depending on type of craft. The coefficients in those formulas depend on design category and standards applied, and usually comprise kMB=kQT=1.25…2.5; kMT=0.63…1.25. For smaller catamarans or higher design categories, where one of demihulls is likely to become airborne during operation, the coefficients tend to be higher.
Figure 11 – Global design loads specific for catamarans
Figure 12 – Global design loads on hydrofoil craft
For hydrofoil craft, longitudinal strength when running on foils on seaway turns to be the critical condition. In absence of detailed weight distribution data, maximum longitudinal sagging moment is taken as:
Mm =
∆LS kaCG 8
where LS – distance between foils, k=1.15 – load distribution safety factor. Support reactions for forward PF and aft PA foils and shear forces QF, QA at foil locations are also calculated using acceleration value aCG:
PF =
∆ ∆ ; PA = ; Q F = aCG PF ; Q A = aCG PA x F − xCG xCG − x A 1+ 1+ xCG − x A x F − xCG
With given global bending moments and shear forces, the analysis of hull girder strength is similar to analysis of laminated stiffener by stack laminate method. More complicated cases such as torsion are to be processed through FEA study (see fig.14), but again, for most of typical monohull craft those global loading cases are not defining and can be omitted. In practice of AMD, loads on elements of high speed craft are usually taken as inertial loads from collision acceleration sdefined by HSC Code [3]. For most of small high speed craft, the design accelerations would comprise: • • • •
Longitudinal acceleration (forward) is taken equal to collision accelerations ax= gcoll=12g; Longitudinal acceleration (aft) ax=-2g; Transverse acceleration ax=2g; Vertical acceleration taken az=aCG at craft’s CG and correspondingly az=axi at element’s location, but not less than 2g.
Following loads are considered for FEM analysis of composite structures, such as engine base studies presented in [7,12]. Additional conditions can be added subject to specific requirements.
Number of tests has been performed by AMD by measuring of bottom impact pressures on radio-controlled models and full size craft, combined with record of vertical accelerations (fig.13). The measurements allow to verify the design loads prescribed in rulebooks and standards, more specific to different types of craft and also allow operational monitoring of loads. This work is still in progress due to known difficulties with extensive pressure measurements requiring large number of gauges, but preliminary results are promising.
Figure 13 – Combined time history of vertical accelerations and impact pressure on bottom of radio controlled model
Figure 14 - Sample of FEA analysis of hydrofoil composite craft structure – case of longitudinal bending, shown are the original model and safety factors for three-dimensional stress
8. STRUCTURAL FIRE PROTECTION Composites are combustible material that imposes some limitations on their use where the craft has to comply with IMO Codes and class requirements. This problem is specially pronounced for craft LH>24 and passenger craft. Modern approaches to structural fire protection derive from number of regulations: • •
•
•
ISO9094 [6] – no structural fire protection is required for small pleasure craft; it is deemed that adequate level of fire protection is provided by material itself and fire extinguishing system. MCA Small Commercial Vessel Code [18] – for craft of categories 0 and 1, or craft with over 16 persons on board fire test of bulkhead is required with 15 minutes of exposure. In practice, this level of protection can be reached by heavy WR lamination and fire retardant resins. Similar requirements are foreseen in IMO Small Commercial Vessel Code for Caribbean [2]; Det Norske Veritas Standard 2-21 [19] for craft LH<24м – fire protection of B-15 class or equivalent is required for craft LH>15m. Similar requirements are in Germanischer Lloyd’s Rules for boats and yachts below 24m [13] – required for craft with installed propulsion power 400kW and above. IMO HSC Code [3] applicable to passenger craft and cargo craft above 500RT making international voyages – structural fire protection of A0 to A60 class, depending on location and category of adjacent space. This protection is achieved by fire insulation material intended specially for composites, such as blankets and mats like FireMaster.
As it can be seen, for small craft little evacuation time allows to reduce required structural fire protection time and drop the protection level to a reasonable minimum. On other side, structural fire protection of composite small craft is somewhat grey area where the authorities sometimes come with unreasonable requests, due to lack of internationally accepted rules. Say, AMD has met the requirements to use steel plates as A0 class protection in engine room on boats LH<24m from some classification societies. Though such protection doesn’t make any practical sense as A0 bulkheads should be cooled from opposite side by fire hydrant that is not possible with composite bulkheads due to natural thermal insulation of composite bulkhead. It is evident that composite material suppliers should further work on type-approved off-shelf solutions available for the
designers and builders. Regulatory work is necessary, preferably at the level of IACS and IMO, on internationally accepted and realistic documents on structural fire protection, to avoid misinterpretations and over-regulation by authorities.
Temporary mould built of MDF and beginning of 16.7m boat hull lamination, hand lay-up
Vacuum infusion process for composite hull of 16m boat in permanent moulds
Construction of hull of 18m passenger catamaran using prefabricated sandwich panels
Strip planking of 18m sailing catamaran hull with foam core strips, with subsequent lamination
Figure 15 – Illustrations of construction methods used for AMD designs
9. COMPOSITE CONSTRUCTION METHODS AND APPLICATION OF MATERIALS In its designs AMD is using different construction methods that should be defined prior to structural design of craft (see fig.15). Not only the construction method affects fiber/resin content and type of core, but also structural arrangements and hull/superstructure shape restrictions. The most common method is hand lay-up or infusion in permanent tooling; the cost of tooling fiberglass is usually estimated as 2…3 times the cost of the moldings or 300…500USD per 1m2 of surface, in South East Asia. As an alternative option, temporary molds made of plywood or MDF can be applied, for the cost of 20-30% of permanent tooling; such molds allow up to 2-4 hulls with some repair. The molds are assembled of pre-cut panels extracted from computer model. Finishing quality of mold and molding surface can vary and fully depends on builder’s attention; the mold surface can be finished and as a result even pleasure boat hulls require very minor surface finish after demolding. There is also an experience of using vacuum bagging and infusion in temporary molds without any damage to the molds. Assembling in pre-fabricated panels and shells or strip-planking in foam strips are popular method for the one-off craft for ease and speed of construction. Such craft often come in form of kits or panels are made on site, also using vacuum bagging. The biggest disadvantages are extra weight and a lot of finishing and fairing labor involved. There are also some restrictions to sandwiches only and certain types of cores.
Selection of materials and technologies is reviewed below on number of samples. In table 4 study of sandwich panel designed to GL rules for particular project in advanced composites is provided, with variation of available core materials. Structural criteria specific to GL are used; each criteria is compared to its maximum permissible value in form of ratio. It is evident that best ‘balanced’ design results are obtained where all the criterions are close to 1.0; panels with much lower ratios are likely to be over designed. It should be noted that for light loaded panels, where weight reduction is not the primary target, honeycombs provide balanced design with lower cost factors. For weight sensitive applications as well as heavily loaded parts, or structures with fire resistance, it is advantageous to use foam cores. Table 4 – Results of calculations of sandwich panels with different core options A B C Laminate of skins (same for all options) ELT420 / CSM300 Panel size, mm 1400×4000 Type of core DIAB DIAB DIAB H60 H80 H130 Foam Foam Foam Density, kg/m3 60 80 130 Shear modulus, MPa 16 23 40 Shear strength, MPa 0.63 0.95 1.90 Thickness of core, mm 20 Design pressure, kN/m2 7.5 Results of structural criteria evaluation (ratio of acting value to maximum permissible value) - deformation 0.997 0.993 0.981 - strain 0.937 0.932 0.921 - shear stress in core 0.842 0.559 0.279 - skins buckling 0.674 0.531 0.367 Cost and mass Mass of 1m2, kg 4.896 5.296 6.296 Comparative cost, % 100 110 150
D
NidaPlast HP8 Honeycomb 95 12 0.60
0.999 0.938 0.884 0.664 5.596 70
10. CONCLUSION Use of composites for high-speed craft offers not only provides the advantages in achievable speed and weight reduction craft, but allows new functionality features. Design and construction of composite craft is an area still standing outside of traditional ship design; it is the area where specific composite experience matters. Combining design expertise with wide range of available technologies, experience of craft operation supported by leading edge research allows cost effective and highly competitive solutions. Global trend towards cost reduction for composite material components, expanding availability of qualified labour in Asia and decrease of manual operations in new lamination processes further encourage application of composites as materials for boats. New composite materials appear on the market every year and their application and service history in marine industry often starts from application on small pleasure boat structures. Known issues are related not only to build of trust to composites from boat operators side, but also from the side of boat designers as traditional naval architecture education does not fully cover high-tech composite craft design issues. Thus, the Author believes the review proposed in present paper would be helpful to colleague designers and students making first steps in composite engineering.
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Barbero E.J. Finite Element Analysis of Composite Materials. CRC Press, 2008. Code of Safety of Small Commercial Vessels (SCV Code for Caribbean countries) IMO, 2010. International Code of Safety for High Speed Craft (2000 HSC Code) – IMO, 2008 Edition ISO 12215–5:2008 Small craft - Hull construction and scantlings - Part 5: design pressures, design stresses, scantling determination. ISO 12215–6:2008 Small craft - Hull construction and scantlings - Part 5: structural arrangements and details. ISO 9094 Small craft - Fire protection.
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