On structural design of energy efficient small high-speed craft

Marine Structures 24 (2011) 43–59

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On structural design of energy efficient small high-speed craft I. Stenius*, A. Rosén**, J. Kuttenkeuler KTH Centre for Naval Architecture, Teknikringen 8, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 March 2010 Received in revised form 17 November 2010 Accepted 4 January 2011

This paper presents an integrated design procedure for determination of structural arrangement and scantlings for the complete structure of small high-speed craft. The purpose of the procedure is to serve as a tool in the preliminary design stage where it enables generation of weight minimized designs with very limited effort. The design procedure is applied in a material concept study for a high-speed patrol craft. The various concepts include single skin and sandwich composites, aluminum and steel. It is demonstrated that the mass of the aluminum hull structure can be reduced from the original 11.7 tonnes to 9.6 tonnes through application of the presented design procedure. The most weight efficient material concept is a carbon-fiber foam-cored sandwich with a structural mass of 4.8 tonnes, which is about 50% less than the refined aluminum version. Through simple hydromechanic analysis, potential for fuel and CO2 emission reductions of 8% for the refined aluminum version and 27% for the carbon-fiber sandwich version in relation to the original craft are indicated. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: High-speed craft Integrated design procedure Scantlings Weight minimization Composites Material concepts

1. Introduction With the increasing focus on the environmental consequences of the burning of fossil fuels and the expected increase in the oil price, energy efficiency becomes more and more of an issue also for smaller high-speed craft. The higher the speed the stronger is the relation between craft displacement and resistance. Minimization of the structural weight hence has a large impact on the energy efficiency for * Corresponding author.

** Corresponding author. Tel.: þ46 (0)70 258 02 10; fax: þ46 (0)8 207865. E-mail addresses: [email protected] (I. Stenius), [email protected] (A. Rosén), [email protected] (J. Kuttenkeuler). 0951-8339/$ – see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marstruc.2011.01.001

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Nomenclature

r s s E G HB HD HS ID m morg mstruct mprop mfuel MPR PD SFC tbstr tmin tsstr tsti

density (kg/m3) minimum of tensile or compressive strength (MPa) in-plane shear strength (MPa) Young’s modulus (MPa) shear modulus (MPa) hull bottom hull deck hull side internal deck (e.g. accomodation deck) displacement of craft (tonnes) displacement of original aluminum craft (tonnes) mass of hull structure (tonnes) mass of propulsion system (tonnes) mass of fuel (tonnes) propulsion system mass-power ratio (kg/kW) power required at design speed (kW) specific fuel consumption (g/kWh) thickness regarding bending strength requirements (mm) thickness regarding minimum requirements (mm) thickness regarding shear strength requirements (mm) thickness regarding stiffness requirements (mm)

high-speed craft. Lower structural weight gives lower displacement and thereby lower resistance, power requirements, fuel consumption, operational cost, and environmental impact. Lower structural weight alternatively gives room for more payload or enables higher speed with the same power installment, which also implies increased energy efficiency. Lower structural weight and displacement also gives other environmental benefits such as decreased material consumption and decreased wake wash. This paper considers two of the most crucial aspects regarding minimization of the structural weight for high-speed craft:  selection of material concept, and  determination of the structural arrangement and scantlings including appropriate application of the chosen material concept. The selection of material concept for a particular high-speed craft is ruled by several different aspects such as building cost, maintenance, craft weight, yard skills, ship owner traditions, etc. [1–3]. This paper however strictly considers material selection from a craft weight and operational energy efficiency point of view. As stated in Ref. [4] not many examples of thorough comparison between different material concepts in high-speed craft design are found in the literature. In Table 1 some of the available studies, Refs. [2, 5–9], are summarized. From this it is clearly seen that steel is out of the question if weight minimization has high priority. However, these studies do not give an equally clear picture regarding which of aluminum and different composite concepts that is most weigh efficient. In some cases, the composite craft versions are about 50% lighter than aluminum, while in others the composite craft version is reported to be up to 20% heavier than the corresponding aluminum version. Naturally this is partly a consequence of different composite concepts being used, still even for similar concepts a rather large difference in the resulting masses is reported. As for the choice of material concept, the determination of structural arrangement and scantlings for a particular high-speed craft is also ruled by many different aspects, e.g. general arrangement,

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Table 1 Normalized structural mass for high-speed craft with different material concepts. Craft

Length (m)

Patrol Ferry Ferry Catamaran Crew Boat HSC Passenger In general

52 130 100 120 43 60 24 –

Normalized mass

Reference

Steel

Alu.

Comp.

1 1.8 – 2.3 1.6 2 – 2

– 1 1 1 1 1 1 1

0.5 1 0.68 – 1.2 – 0.48 0.5

[5] [6] [7] [2] [2] [2] [8] [9]

production, designers skills and yards traditions. However, to minimize the craft weight, and also to enable just comparison between different material concepts, it is crucial that the chosen material concept is efficiently applied. For a complex framework, such as a high-speed craft hull, finding the best structural arrangement, beam cross-sectional geometries, material properties and scantlings is a complex task. It is further non-linear and mathematically non-convex limiting the prospects of applying gradient-based optimization algorithms. There exists a number of methods addressing rational integrated design and multi-objective optimization of larger ships such as Refs. [7,10–13]. To overcome the challenges in ship structure optimization, large complex problems are for example decomposed into a sequence of more simple readily solvable linearized convex problems [11], or include using various methods based on genetic algorithms, such as Klanac and Jelovica [13] illustrating optimization of a fast ferry, and Sobey et al. [14] in the optimization of local composite panel fields. For this study, a rule based integrated design procedure for the determination of structural arrangement and scantlings for the complete structure of small high-speed craft was developed. The purpose of the procedure is to serve as a tool in the preliminary design stage where it with very limited effort enables generation of weight minimized designs satisfying classification rules. In comparison to the methods mentioned above (e.g. Rigo, Hughes, .) the method presented here is less advanced, e.g. in terms of optimization and integration with other software systems such as CAD, FEM, etc. The strengths are instead the automated and rationalized rule-based design of small high-speed craft structures, and the presentation of the design results of each individual structural member in a graphical format enabling fast feedback of design deficiencies. In this paper the procedure is applied in the evaluation of several different material concepts through design simulations of a patrol craft. The material concepts studied include various kinds of single skin and sandwich composite concepts, aluminum and steel. The different concepts are evaluated in terms of structural weight, total displacement, power requirement, fuel consumption and emissions. 2. Design procedure The procedure that is presented and applied here is based on the strength in combining an efficient computational tool and the knowledge and engineering experience of the designer. The procedure is outlined in Fig. 1 and described in the following. The design tool, which the design procedure is built around, enables structured parameterization of the entire hull in such a way that the engineering calculations for stress, strain and deflection in a simple manner are interlinked with design constraints and rules. The parameterization is done with respect to the craft general arrangement and geometry, and includes structural arrangement, structural hierarchy, material properties, and beam cross-section descriptions (second step in Fig. 1). The structural hierarchy is defined through discrete load carrying members in terms of primary and secondary longitudinal and transversal structural stiffeners (e.g. girders are primary longitudinal stiffeners and floors are primary transversal stiffeners while stringers are secondary longitudinal stiffeners). In reality the structural hierarchy is obviously not discrete, but to enable determination of the scantlings based on simple rule formulas a discrete hierarchy has to be defined. The input arguments to the design tool basically consist of four main blocks of data: (1) main particulars and geometry description, (2) structural arrangement and hierarchy,

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Fig. 1. Outline of the design procedure.

(3) material properties, and (4) beam cross-section descriptions. The entire craft is compartment-wise described in an input text file, where a compartment typically is bounded by watertight bulkheads. The structural arrangement is defined for each compartment sequentially but constraints can be interlinked between different compartments, for example by making beam cross sections continuous between different compartments. Based on this detailed definition of the hull structure, scantling requirements are automatically calculated for each individual structural member (third step in Fig. 1). In the present version of the design tool scantling requirements according to the DNV [15] classification rules are used, but in principle an arbitrary set of criteria could be implemented. The parameterization of the structure makes it easy for the designer to, based on evaluation of the resulting scantling requirements, modify the structural arrangement and beam cross-sectional properties iteratively to find a structure with minimum weight. Minimum weight is related to maximum structural efficiency which in turn is related to maximum utilization of each individual structural member. The utilization of each structural member can therefore be used as a measure in the iterative modifications of the structural arrangement toward minimum weight (fourth step in Fig. 1). The utilization of one particular member is here schematically discussed regarding different levels of utilization. Note that the following numbers only are examples given here for illustrative purposes. The thickness requirements in a first iteration step can be pictured as in Fig. 2 (a). In this case the minimum requirement (tmin) is ruling and the utilization regarding this

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Fig. 2. Schematic illustration of scantling requirements and structural utilization.

requirement is therefore 100%. The other requirements are distinctly lower, meaning that the utilization regarding these aspects is poor: 50% regarding stiffness (tsti), 30% regarding bending strength (tbstr), and only 25% regarding shear strength (tsstr). If the member for example is a plate field in a stiffened panel, a modification for increased utilization regarding strength and stiffness could be to increase the plate field area by increasing the stiffener spacing. When new requirements are calculated after such modification the picture could be as in Fig. 2(b). Here the minimum requirement (tmin) is still the same and ruling and the corresponding utilization is still 100%. However, as a result of the modification the loading of the member has increased, which is equivalent to the utilization regarding strength and stiffness being increased. As seen the utilization regarding stiffness (tsti) has increased from 50 to 60%, regarding bending strength (tbstr) from 30 to 75%, and regarding shear strength (tsstr) from 25 to 40%. At the same time the weight of the structure has decreased because the plate thickness is the same (tmin) while the number of stiffeners has decreased. If the structure is further iterated in order to identify additional beneficial modifications and the structural arrangement could be updated accordingly the ultimate structure would be one with 100% utilization as pictured in Fig. 2(c). This is however practically impossible to reach. However, for an individual member it should be possible to reach 100% utilization in relation to at least two requirements as the sizing and structural arrangement can be balanced to mach strength or stiffness against the minimum criteria. In reality, there is typically both geometrical and loading variations across a panel field resulting in variations in the structural requirements and thereby also in the structural utilization, as for example seen in Fig. 3 (b). The maximum possible utilization also depends on the material concept used. In the present version of the design tool the modifications are determined manually based on designer judgment, however an automized procedure would be possible. 3. Design example As a design example, a part of the bottom of an aluminum hull structure will here be used. The studied part is pictured in Fig. 3(a) which also shows the initial tentative structural arrangement including a longitudinal girder, four transversals, and six longitudinals. The structural hierarchy and beam cross-sectional properties are also tentatively defined. The structural hierarchy chosen here implies that the plating fields are carried by the longitudinals which are carried by the transversals which in turn are carried by the girder. This results in eight transversal segments, thirty longitudinal segments, and forty plate segments which are all treated as individual members. Based on these definitions the design tool automatically determines design loads and the corresponding scantling requirements for each individual member. In Fig. 3(b) the scantling requirements for the different members in Fig. 3 have been boiled down to thickness requirements regarding minimum criteria (tmin), bending strength (tbstr), shear strength (tsstr), and stiffness (tsti). From top and down the graphs refer to the plating fields, the girder, the transversals, and the longitudinals. As seen the strength and stiffness requirements vary for the same type of members. This is due to the different locations of the members

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t (mm)

t (mm)

10

t (mm)

1 0.5 14

10

20

30

15

16

17

1 1.5 0 0.5 y

40 Girders

5 0

1 Transversals

5 0 0 10

x

a

0 0 10

10

t (mm)

z

1.5

Platings

5

2

4

6

8 Longitudinals

5 0 0

5

10

15

20

25

30

b

Fig. 3. (a): Tentative structural arrangement for the hull bottom in the design example. Only port side of the symmetric hull is shown with x-axis pointing forward and y-axis to port. (b): Scantling requirements for the hull bottom in terms of minimum requirements ( – tmin), bending strength (B – tbstr), shear strenght (þ – tsstr), and stiffness ( – tsti).

resulting in different design loads regarding slamming. For the plating fields there are neither stiffness nor shear strength criteria for metallic materials in the DNV [15] rules. These criteria are hence set to zero. Nor for beam elements DNV stipulates any stiffness criteria. However, here a complementary rule of thumb deflection criterion according to [16] is applied for all beams. Other criteria, e.g. regarding buckling, could easily be added. For the plating fields, which according to Fig. 3(b) are ruled by the minimum criteria, increased utilization and decreased weight can as in the schematic description above be achieved by increasing the plate field area by removing stiffeners. For the girder, which according to Fig. 3(b) is ruled by bending strength, increasing the web height should be beneficial, since strength increases to the power of two and weight increases linearly for an increased web-height. For the longitudinals on the other hand, decreasing the web-height should be beneficial since these are governed by the minimum criteria. For the transversals the utilization regarding bending strength and stiffness is very low and can be increased by decreasing the flange width. After a few iterations following this line of arguments the structural arrangement in Fig. 4 (a) is reached. The corresponding scantling requirements are shown in Fig. 4(b). As seen the minimum criteria and the other criteria now match more closely which means that the structural utilization has increased. If the structures in Figs. 3 and 4 would be designed with scantlings ruled by the individual member within each member category (platings, longitudinals, transversals, girder) with the strictest scantling requirements, the mass would be 270 kg for the tentative structure in Fig. 3 and 248 kg for the modified structure in Fig. 4. Hence, through this systematic procedure a reduction in structural mass of about 8% is achieved. Designing and building a structure in this manner, with the scantlings for all members within each member category ruled by the single member with most strict requirements, makes the structure easy and cheap to build. This will however be to the cost of weight efficiency, and thereby also to the cost of higher fuel consumption, operational cost and environmental impact. The other extreme would be to utilize the detailed definition of the hull structure and the scantling requirements for each individual member, and design and build with individual scantlings for every single member. This would give a very weight efficient structure but would of course not be feasible neither from a production point of view nor from a building cost point of view. The overall most efficient structure is somewhere in between, and a strength with the here presented and applied design procedure is that the detailed definition of the structure and requirements down to each individual member, in combination with methods for life cycle cost analysis (LCCA) and life cycle assessment (LCA), enables structured adaptation of the scantlings toward the over all most efficient design (fifth and sixth step in Fig. 1).

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t (mm)

10

t (mm)

0 0 10

5

10

15

16 17

a

x

0

0.5

1 y

1.5

t (mm)

15

b

30 Girders

Transversals

5 0 0 10

14

25

1

10

0.5

20

5 0

1 t (mm)

z

1.5

Platings

5

2

4

6

8 Longitudinals

5 0 0

5

10

15

20

Fig. 4. The refined configuration and scantling requirements in the studied design example. (b):  – tmin, B – tbstr, þ – tsstr,  – tsti.

4. Material concept study The design procedure described in the previous section is here applied in a material concept study for a 24 m high-speed patrol craft. Main particulars of the craft are given in Table 2. The craft was originally designed and built in aluminum according to the DNV rules for High Speed and Light Craft. The here presented study is also in accordance with the DNV rules [15]. The studied material concepts are presented in Table 3 and include sandwich and single-skin glass-fiber and carbon-fiber concepts, as well as aluminum and steel. For each of the different concepts there are a variety of different material qualities that can be used which affect the outcome of the design. In this comparison the aim is to use standard type materials. The aluminum is a standard marine quality (NV-5083), and the steel is a highstrength quality (NV40). For composites there is a challenge due to the vast range of material properties available and their dependence of matrix, fiber lay-ups, manufacturing processes etc. In this study the properties of the carbon-fiber and glass-fiber laminates are based on in-house experience of typical properties for craft in service. The primary load carrying structure is here considered, including bottom, side, strength deck, internal decks and bulkheads, while superstructure, doors and hatches are omitted. The hull girder is divided into 6 compartments separated by the watertight bulkheads marked in Fig. 5. The structure in front of compartment 6 is not treated.

Table 2 Original main particulars of the 24 m high-speed patrol craft studied in this paper. Length over all Length design water line Breadth over all Breadth at chine Draft design water line Displacement design water line Light displacement Hull structure mass Deadrize at center of gravity Vertical center of gravity Longitudinal centre of gravity Total power installed Design acceleration Service area restriction Class notation

23.85 20.05 5.1 4.34 0.97 48.2 38.2 11.7 20 1.77 7.63 1380 2.05 R3 1A1 R3 HSLC

m m m m m tonnes tonnes tonnes deg m m kW g – –

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I. Stenius et al. / Marine Structures 24 (2011) 43–59 Table 3 Material concepts. 1 2 3 4 5 6

Sandwich: Sandwich: Single skin: Single skin: Standard marine aluminum: High strength steel:

Carbon/Vinylester/Divinycell E-glass/Vinylester/Divinycell Carbon/Vinylester E-glass/Vinylester NV-5083 NV-40

For all material concepts the design starting points are given by the craft general arrangement, and a tentative layout of the main girders, transversals and longitudinals according to Figs. 5 and 6 and Table 4. The longitudinals are however omitted for the sandwich versions. The bottom girders are positioned to support the propulsion system in compartments 1 and 2. The mass of the three engines is distributed over a total of 6 longitudinal engine bed girders in compartment 2 and the mass of the three water jets is distributed over the 6 girders in compartment 1. The deck girder configuration comprises two girders placed in line with hatch coamings and the sides of the superstructure throughout all compartments. Based on this tentative layout all concepts are subsequently refined in order to minimize the structural mass as described in the previous section. For all iterations, strength, stiffness and minimum criteria are considered with respect to local loads. Because the craft length is much less than 50 m the scantlings given by the local design are assumed to fulfill also the global requirements in accordance with DNV [15]. In the following the design procedure for an aluminum concept is treated in more detail, while the other concepts are presented more briefly, however following the same procedure as for the aluminum concept. 4.1. Aluminum The aluminum version is built up of T-profile girders, web-frames and longitudinals, with material properties according to Table 5. The identification of critical parts and subsequent modifications constitutes a quite straightforward process and it is beyond the scope of this paper to present all details here. However, some main aspects and conclusions are discussed. For example, for the bottom layout, the results of the first iteration show that the bottom girders are ruled by minimum criteria where the margin to strength and stiffness is rather large. This indicates that the bottom girders are not utilized efficiently regarding the carrying of lateral loads. According to the discussions concerning beneficial modifications the design would hence be improved by decreasing the web-height and thereby increasing the utilization of the bottom girder. On the other hand, the selected web-height of the bottom girder may be beneficial or required for other reasons. First, preserving continuous load paths requires that web heights are not drastically changed between compartments, secondly supporting the inner bottom may be beneficial for the overall weight of the compartment. Hence, even though a comparison of the different criteria for an individual structural member suggests one modification other aspects may be equally or more important and as a consequence suggest another modification or no modification at all.

#1

#2

#3

#4

#5

#6

Fig. 5. Bottom structural layout showing compartments, bulkheads, and primary bottom girder configuration. The numbering refers to the different compartments.

I. Stenius et al. / Marine Structures 24 (2011) 43–59

#1

#5

#4

#3

#2

51

#6

Fig. 6. Deck structural layout showing compartments, bulkheads, and the primary deck girder configuration. The numbering refers to the different compartments.

The solution to the problem with the bottom girder above is simply to change the structural hierarchy. That is, instead of designing the floors to carry the bottom girder loads, the bottom girder should in this case carry the floors. The result is that, at the same time as smooth load paths are preserved and support is provided for the inner bottom, the bottom girder could be much more efficiently utilized. This may be a trivial example, but through the iterations these aspects are the most difficult parts in the refinement procedure, and bring up questions about to what point the design should be refined and which other aspects are relevant for the structural member at hand. It should however be noted that the designs are not optimal in strict optimization terms, and that a certain level of subjectivity is applied regarding how close to the limits each member should be driven and in the interpretation of the load distribution paths. These questions and considerations are here treated to the best of the authors’ knowledge and experience in order to generate realistic designs with minimum weight. An example of the resulting masses from the iterations of the aluminum concept is given for compartment 3 in Fig. 7. It can be seen that the mass of the compartment is reduced about 23%, from 4.45 tonnes to 3.4 tonnes. The major weight reduction is obtained during the first iterations, which naturally reflects the quality of the initial estimate. A minima is found at the fourth iteration, where after the weight is slightly increased again. This increase in weight corresponds to scantling adaptation, e.g. corrections in stiffener alignments in order to obtain continuous load paths through the entire craft. The resulting refined structural arrangement for the craft is shown as a visualization of the, for calculation purposes, idealized structure in Fig. 8. The hull mass is 8.7 tonnes. Comparing with the tentative layout according to Table 4, the bottom girder height is unaltered in order to support the internal deck. The weather deck girder height is however increased to 300 mm, while the flange width is kept at 120 mm. The web-frame spacing is also kept at 1.2 m, while the web height is changed in the bottom to 210 mm and in the side to 90 mm. The web-frame flange width is maintained at 120 mm. The target spacing for the longitudinals is kept at 250 mm in the bottom while changed to 450 mm for the side and 350 mm for the decks. The web height and flange width for the longitudinals are for the bottom structure increased to 70 mm, and in sides, and decks decreased to 40 mm. Table 4 Initial tentative values used in the design of the aluminum, single-skin, and steel versions of compartment 3. HB is hull bottom, HS is hull side, HD is hull deck, and ID is internal deck.

girder web-height girder flange-width web-frame spacing web-frame web-height web-frame flange-width longitudinal spacing longitudinal web-height longitudinal flange-width a

unit

HB

HS

HD

ID

(mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm)

666a 120 1200 120 120 250 60 60

– – 1200 120 120 250 60 60

120 120 1200 120 120 250 60 60

– – 1200 120 120 250 60 60

the height of the bottom girders is selected in order to support the internal deck.

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Table 5 Material properties for the aluminum concept.

NV-5083 a b

E

G

sa

sa

r

fweldb

(GPa)

(GPa)

(MPa)

(MPa)

(kg/m3)

–

70

26

205

79

2700

0.6

yield strength. strength reduction factor used in accordance with the DNV (2008) rules.

4.2. Sandwich FRP The carbon-fiber and glass-fiber sandwich concepts are here considered. The material properties are given in Table 6. For the carbon-fiber version a quasi-isotropic laminate (L1) is used in the skins of the hull (both single-skin and sandwich) and in the web of the longitudinal girders. Laminate L2 is mainly used in the webs for the transverse members and laminate L3 is used in the flanges of both longitudinals and transversals. For the glass-fiber version a quasi-isotropic laminate is used throughout the structure. It is realized that this favors the carbon-fiber version which is more optimally utilized with different laminate lay-ups and a higher volume fraction. This should however to some extent reflect how the different materials typically are utilized. Glass-fiber is a cheaper choice for which less effort is put on optimization, while if carbon-fiber is used typically more effort is put into optimization, e.g. through laminate tailoring. To simplify the calculations the laminate thicknesses are assumed to be continuously scalable with preserved material properties. All sandwich panels are subject to a simple local core thickness optimization procedure in the sense that each panel is individually optimized. The objective is to minimize the panel weight. In the optimization all material properties are kept constant. The only variables are the core thickness and face thicknesses. The core thickness is simply incrementally increased from the critical shear-strength thickness of the core, and for each increment the face thicknesses satisfying the strength and stiffness criteria are calculated. The iteration is continued as long as an incremental increase in core thickness gives a lower panel weight than in the previous step. Through the iterations it is found that a Divinycell H130 core used in bottom, side, decks and bulkheads results in the lowest weight (the minimum allowed core density in bottom is 130 kg/m3 according to DNV). H200 results in a structure about 18% heavier than with H130. A configuration with H130 core in the bottom and H80 core elsewhere was also analyzed. This however resulted in a local skin buckling critical design

Fig. 7. Hull masses of compartment 3 through the iterative refinement of the aluminum concept.

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53

Fig. 8. Visualization of the refined layout for the aluminum concept. Note that this is a graphical representation of how the structural members are idealized for calculation purposes.

where the H80 core was used. The lighter core (Divinycell H80) is used as filler material in the tophatprofiles, except for the girders in the engine room where H200 is used since they carry large localized loads. The resulting refined structural arrangement for the carbon-fiber sandwich concept is shown in Fig. 9, for which the total hull mass is 4.4 tonnes. For the sandwich panels, the core thicknesses yielding the lowest weight, mainly varies between 15 and 20 mm depending on panel size and location. A 20 mm core is therefore chosen for the entire craft, except in internal decks where a 24 mm core is chosen. The corresponding laminate thicknesses are 1.5 mm for the outer skins and 1 mm for the inner skins in bottom and at the transom. For the sides, decks and internal decks the skin thicknesses are about 1 mm. These thicknesses comply with the DNV minimum requirements but might from production and robustness points of view have to be increased. Generally, the laminate thicknesses in transversal members are about 2–3 mm and in girders 4–6 mm. The glass–fiber version of the sandwich concept is generated with a starting point for the design based on the final version of the carbon-fiber sandwich concept (Fig. 9). Through the refinement the only changes to the design where increased web-heights generally for all structural members and increased laminate thicknesses, hence resulting in very similar layout as for the carbon-fiber version. For the glass-fiber sandwich version the hull mass is 6.9 tonnes. Table 6 Material properties of the fiber composite sandwich and single-skin laminates. All carbon-fiber lay-ups are based on non-crimp carbon-fiber T700 and Vinylester. Laminate 1 (L1) is a [25%0 , 25%90 , 25%  45 ] lay-up, laminate 2 (L2) is a [10%0 , 10%90 , 40%  45 ] lay-up, and laminate 3 (L3) is a [72%0 , 14%90 , 7%  45 ] lay-up. The E-glass laminate is a quasi-isotropic [25%0 , 25% 90 , 25%  45 ] lay-up. Lay-up

Carbon L1 Carbon L2 Carbon L3 E-glass PVC H80 PVC H130 a b

E11

G12

s11a

s12a

r

yfb 3

(MPa)

(MPa)

(MPa)

(MPa)

(kg/m )

–

38000 23000 77000 15800 90 175

14000 21000 6000 6000 27 50

300 184 616 247 1.4 3.0

115 168 48 95 1.15 2.2

1476 1462 1472 1865 80 130

0.6 0.6 0.6 0.5 – –

ultimate strength. volume fraction of fiber in laminate.

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I. Stenius et al. / Marine Structures 24 (2011) 43–59

Fig. 9. Visualization of the refined layout for the carbon-fiber sandwich concept. Note that this is a graphical representation of how the structural members are idealized for calculation purposes.

4.3. Single-skin FRP As for the sandwich concepts both a carbon-fiber and a glass-fiber single-skin version is studied. The material properties for both carbon and glass are given in Table 6. In the DNV rules for single-skin carbon-fiber concepts there is no minimum reinforcement criteria, only glass is considered. Minimum reinforcement criteria are however given for carbon-fiber and glass-fiber for sandwich panels. In this paper it is assumed that the ratio in required minimum reinforcement between glass and carbon can be used for the single-skin concepts similarly as for the sandwich panels according to DNV [15]. For the single-skin concept, bulkheads and internal decks are sandwich constructions where H130 core is used in the internal decks and bulkheads, while the lighter H80 core is used as filler in the tophat profiles as for the sandwich concept. For the carbon-fiber version, the resulting refined structural arrangement is seen in Fig. 10. The total hull structural mass is 4.6 tonnes, i.e. quite close to the carbonfiber sandwich concept. The corresponding structural mass for the glass-fiber version of the single-skin concept is 7.7 tonnes. 4.4. Steel A high-strength steel design with material properties as presented in Table 7 is evaluated. As for the other concepts the starting point for the design of the steel concept is based on the girder arrangements in Figs. 5 and 6 and the tentative arrangement in in Table 4. All cross-sections have T-profiles. After iterations, the hull mass for the final version of the steel concept is 16.8 tonnes. The structural arrangement is very similar to the aluminum concept as illustrated in Fig. 8. In the steel concept the web-frame spacing is slightly increased compared to the aluminum concept, while cross-sectional dimensions and thicknesses are decreased. 5. Evaluation The resulting hull masses for all concepts are presented in Table 8. Comparing the masses in Table 8 it can be seen that: 1. Steel gives a structural mass which is about twice the mass of the aluminum structure.

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55

Fig. 10. Visualization of the refined layout for the carbon-fiber single-skin concept. Note that this is a graphical representation of how the structural members are idealized for calculation purposes.

2. The lightest structures (4.4 tonnes/4.6 tonnes) are obtained with the carbon-fiber sandwich and single-skin concepts, which are about 50% lighter than the refined aluminum design developed in this study. For the single-skin concept the structure is however much more complex (compare Figs. 9 and 10). 3. For the concepts with a less significant difference in the structural masses a more detailed design is needed to conclude which concept ultimately will be the most weight efficient. For the original craft the hull structure mass is 11.7 tonnes (Table 2). To relate the results in the present study to the original craft, an estimated mass penalty of 10% is added to the here derived structural designs to account for the superstructure and other parts which are not included in the design simulations. This results in a total hull mass of 9.6 tonnes for the here presented aluminum version and 4.8 tonnes for the lightest carbon-fiber sandwich version. Comparing the structural mass of the aluminum version with the mass for the original craft, 9.6 tonnes compared to 11.7 tonnes, shows a significant weight saving potential in applying the here presented design procedure. To evaluate the relations between structural mass, total craft displacement, fuel consumption, CO2 emissions, and the craft operational profile, a simple hydrodynamic analysis is performed. When determining the displacement for the different craft versions, all masses from the original craft are kept constant except for the structural masses, which are determined above, and the masses of the propulsion systems and the fuel, which are determined in the following. The displacements m (tonnes) of the different craft versions are determined in relation to the mass of the original craft according to org

org

org

m ¼ morg  mstruct  mprop  mfuel þ mstruct þ mprop þ mfuel ;

(1)

Table 7 Material properties for the steel concept. Data from DNV (2008).

NV40 a b

E

G

sa

sa

r

fweldb

(GPa)

(GPa)

(MPa)

(MPa)

(kg/m3)

–

210

81

390

150

7800

1.43

yield strength. strength reduction factor used in accordance with the DNV (2008) rules.

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Table 8 Structural weights of the 24 m high-speed patrol craft, based on the different material concepts studied in this paper. Material concepts

Struct. mass (tonnes)

Struct. mass (normalized)

Sandwich: Carbon/Vinylester/Divinycell Sandwich: E-glass/Vinylester/Divinycell Single-skin: Carbon/Vinylester Single-skin: E-glass/Vinylester Standard marine aluminum: NV-5083 High strength steel: NV-40

4.4 6.9 4.6 7.7 8.7 16.7

0.51 0.79 0.53 0.89 1.00 1.92

where superscript org refers to the original craft, and subscripts struct, prop and fuel refers to the mass of the structure, propulsion system, and fuel respectively. Insulation is a critical aspect when comparing different material concepts. The required amount of fire insulation can generally be considered to be larger for composite craft while the required amount of temperature and noise insulation is larger for aluminum craft. However, according to previous studies (e.g. [2,8]) this sums up to approximately the same amount of insulation for composite craft as for aluminum craft. The 2.6 tonnes insulation mass for the original craft is therefore kept constant for the different versions. Based on a simple hydrodynamic analysis according to Ref. [17], the top speed in calm water for the original craft at light displacement 38 tonnes is 33 knots with the originally installed power 1380 kW. By repeating this analysis for different craft displacements between 27 and 40 tonnes it can be concluded that, within this interval, the required installed power PD (kW) to maintain 33 knots in calm water is approximately linearly proportional to the craft displacement (tonnes) according to

PD ¼ 35m þ 45

(2)

for the particular craft studied. This is used to determine the installed power for the different craft versions. The corresponding propulsion system mass mprop (tonnes) is calculated as

mprop ¼ MPR$103 $PD;

(3)

where MPR (kg/kW) is the propulsion system mass-power ratio. Here MPR ¼ 6.6 is used based on the propulsion system mass 9 tonnes for the original craft. The fuel capacities for the different versions mfuel (tonnes) are calculated as

mfuel ¼ SFC$106 $PD$t:

(4)

where SFC ¼ 215 (g/kWh) is the specific fuel consumption based on data for relevant diesel engines, and t ¼ 11.5 is the operational range for the original craft at top speed, i.e. at 1380 kW power out take, with the original fuel capacity 3.4 tonnes. Because the mass of the propulsion systems and fuel depends on the installed power, which depends on the craft displacement, which in turn depends on the mass of the propulsion systems and fuel, these have to be determined iteratively. The results for three versions of the craft are summarized in Table 9. Compared to the original aluminum version, the refined aluminum version derived in this study gives a reduction in the total craft displacement of 3 tonnes (i.e. 8%) while the reduction for the carbon-fiber sandwich version is 10 tonnes (i.e. 27%). According to this simple hydrodynamic analysis the power requirement for a certain speed for this craft is linearly proportional to the craft displacement, (2), and also the relation between fuel Table 9 Evaluation of the derived refined aluminum version (Alnew) and the carbon-fiber sandwich version (CRPsw) in relation to the original craft (Alorg).

acg mstruct mprop mfuel m PD

[g] [tonnes] [tonnes] [tonnes] [tonnes] [kW]

Alorg

Alnew

CRPsw

2 11.7 9 3.4 38 1380

2 8.7 7.9 3 33.6 1220

2 4.4 6.5 2.5 27.4 1000

I. Stenius et al. / Marine Structures 24 (2011) 43–59

57

Table 10 Evaluation of the derived aluminum version (Alnew) and the carbon-fiber sandwich version (CRPsw) in relation to the original craft (Alorg).

mfuel mCO2

[tonnes/year] [tonnes/year]

Alorg

Alnew

CRPsw

170 440

156 405

125 325

hull mass (tonnes)

20

15

10

5

0

CRP(sw)

GRP(sw)

CRP(ss)

GRP(ss)

Alu(new)

Steel

Fig. 11. Structural weights of the 24 m high-speed patrol craft, based on the different material concepts studied in this paper (sw sandwich and ss single-skin).

consumption and power is linear, (4). Further, the ratio between CO2 emission and fuel consumption is around 2.6. Hence, the reductions in fuel consumption and CO2 emission will be equivalent to the reduction in total craft displacement, i.e. 8% for the refined aluminum version and 27% for the carbonfiber sandwich version compared to the original aluminum version. To see what this means in tonnes, the yearly fuel consumption mfuel (tonnes/year) is calculated as

mfuel ¼ SFC$106 $PD$

X

Qi ti :

(5)

i

where the sum expresses the craft operational profile in terms of operational times ti at certain power out-takes Qi. Here the operational profile is formulated according to the following: 1000 operation per year out of which t1 300 is at 100% power (Q1 1), t2 500 is at 50% power (Q2 0.5), and t3 200 is at 10% power (Q3 0.1), corresponding to around 30, 20 and 5 knots respectively. The resulting yearly fuel consumption and CO2 emission are given in Table 10. As seen a reduction of up to 45 tonnes of fuel and 115 tonnes of CO2 can be achieved per year for every single craft for the carbon-fiber sandwich version compared to the original aluminum version. 6. Conclusions The presented design procedure is based on rather simple principles. Still it offers a rational approach for designing weight minimized hull structures for smaller high-speed craft with very limited effort, taking into account stipulated design criteria as well as subjective designer input. The design procedure should however be further developed e.g. to include grillage analysis methods to better account for the real structural hierarchy, and optimization schemes such as those presented in Refs. [11,13]. Further, the detailed parameterization of the complete hull structure in the design procedure should also be coupled to databases for cost of material, labor, maintenance, energy consumption and emissions for material production and recycling, etc. This, in combination with hydromechanic analysis such as presented in the paper, will give efficient tools for life-cycle cost analysis (LCCA) and life-cycle assessment (LCA). The procedure is applied in a number of design simulations with different material concepts for the same craft. It is demonstrated how the mass of the aluminum hull structure for a patrol craft can be reduced from the original 11.7 tonnes to 9.6 tonnes through application of the presented design procedure. The most weight efficient material concept in this study is shown to be a carbon-fiber foamcored sandwich which gives a structural mass of 4.8 tonnes. The results, as presented in Table 8 and Fig. 11, in terms of mass relation between different material concepts show that the carbon-fiber

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I. Stenius et al. / Marine Structures 24 (2011) 43–59

sandwich concept is about 50% lighter than the aluminum concept while a steel version is twice as heavy. The results further show that the glass-fiber sandwich concept is about 20% lighter than a standard aluminum concept. These relations are in agreement with results presented in Refs. [7–9], where Holm and Olofsson compare aluminum and steel with carbon-fiber sandwich concepts while Hughes compares aluminum with a glass-fiber sandwich concept (Table 1). Through simple hydromechanic analysis it is further demonstrated how lower structural mass gives lower power requirements for the same operation, and thereby lower mass of the propulsion system and the fuel supplies. Comparing with the original craft studied the reduction of the craft displacement, yearly fuel consumption, and CO2 emission is 8% for the here derived refined aluminum version and 27% for the carbon-fiber foam-cored sandwich version. The essence of the paper is thus that there is a large potential for weight reduction and energy efficiency improvement for smaller high-speed craft in the selection of material concept and through application of integrated rational design procedures. 7. Future work The study presented in this paper is part of a larger project in collaboration with the Swedish Defense Materiel Administration (FMV) aiming at overall improvement of the design methods for highspeed craft including more efficient application of composite materials and introduction of novel material concepts. There are various reasons why the weight efficiency and thereby energy efficiency of many high-speed craft is relatively low. The list below highlights some of the limitations in the presently applied design procedures: 1. deficient formulation of the craft operational profile in relation to the intended use of the craft, 2. large uncertainties in how the craft is actually going to be operated in relation to the formulated operational profile, 3. deficiencies in the prediction of design loads, 4. irrelevant formulation of design criteria in relation to loading and response mechanisms, material properties and failure modes, 5. inefficient choice of hull structure material concept, 6. inefficient use of the chosen material concept, 7. little effort put on optimization of the structural arrangement, 8. simplified methods used to derive the scantlings, 9. much focus on the production of the craft and minimization of the production cost and limited or no focus on the life-cycle cost and effects, 10. little effort put on minimization of the weight of furniture and installed systems, 11. application of high factors of safety to compensate for all involved uncertainties and simplifications. This paper is briefly addressing points 5 to 9, and the list above can be seen as a road map for future work. Acknowledgments This research has been financially supported by the Swedish Defense Materiel Administration (FMV) and the USA Office of Naval Research (Grant no. N00014-07-1-0344). References [1] Hellbratt SE, Mäkinen KE. Design and production of grp-sandwich vessels. In: 5th international conference on marine applications of composite materials, MACM-5, Florida, USA; 1994. [2] Sielski RA. Review of structural design of aluminum ships and craft. In: SNAME maritime technology conference & expo and ship production symposium, Ft. Lauderdale, Florida, USA; 2007. [3] Torrez JB. Light-Weight materials selection for high-speed Naval craft. Msc thesis. Massachusetts, USA: Massachusetts Institute of Technology; 2007. [4] Schleicher DM, Swanhart CJ. Structural design of the 100 knot yacht. In: 7th International Conference on fast Sea Transportation, Ischia Porto, Italy; 2003.

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