TITLE OF PAPER - Coventry University · Web viewThe additional mass for; vehicles, fittings, engine...

Marine Design, 3-4 September 2015, London, UK

A LIGHWEIGHTING METHODOLOGY OF A TRIMARAN HIGH SPEED CRUISE LOGISTICS FERRY (CLF)

C Bastien, L Malin, E Adams, P Eyres and J Venables, EBDIG-IRC, Coventry University, UK

SUMMARY

The proposed paper researches the opportunities for lightweighting of a high speed multi-purpose trimaran ferry, which currently complies with Lloyds Register regulations. The purpose of the study is to generate a lighter structure in order to improve fuel economy and reduce running costs of such vessels, as well as clarifying whether Lloyds Register rules over-engineer such ship design architectures. The methodology is using a newly generated computer algorithm of a wave which has been applied on the vessel, in sagging and hogging, on a full three dimensional finite element model of the ferry. Using these wave loadings, the study compares the level of over-engineering of the vessel and proposes a lightweighting process which will fulfil vessel fatigue loads present during its service. The research has found areas of the ship where mass could be reduced and still meet fatigue life of the material, leading to a total mass reduction of 96 tonnes.

1. INTRODUCTION

The Lloyd’s Register of Shipping is a maritime organisation, set up in the 1760s, which regulates the construction and classification of ships [1]. The Cruise Logistics Ferry (CLF) developed by McCartan et al. aims to revolutionise the way European sea motorways are used by acting as a replacement to coastal road transports and as passenger ferries; leading to reduced congestion on these roads and, therefore, fewer fossil fuels burned.

The current 130m CLF design follows the Lloyd’s Register, however, these regulations have changed little over the past 50 years meaning that many modern engineering concepts are not considered. To assess the over-engineering of the Lloyd’s Register the CLF is being optimised further – it is already being constructed out of aluminium – with aims of minimising mass.

Using Finite Element Analysis (FEA) the current ferry design shall be assessed based on the main loads that it will affect the ship in normal operation: hogging and sagging along the longitudinal and transverse planes, as well as torsion. These loads are simulated by applying a wave to the model, which is defined through pre-coded instructions using Hypermesh-TCL coding language.

Following on from the analysis of the CLF the main optimisation technique used is that of gauge optimisation which shall reduce mass by reducing the thickness of components based on the fatigue strength of aluminium, 110 MPa, whilst looking into utilising alternative materials in key areas. The methodology ensures that the vessel functionality is

not impaired through a reduction in storage space or through other means; also the vessel must maintain a double bottomed hull for health and safety purposes.

2. SCRIPTING

2.1. Wave propagationIn order to test the ship with the load cases described, an appropriate wave magnitude is used. As a static analysis, only the worst case is evaluated, which means using a wave of length equal to the ship [2]. There is different height to length ratios for different size ranges of wave because the wave centre sits lower, the higher the wave (Figure 1).

Figure 1 - Wave notation [3]

The study will use the results of previous work [2], where the wave can be approximated as a trochoidal wave built from a rolling radius, as illustrated in Figure 2.

© 2015: The Royal Institution of Naval Architects

Marine Design, 3-4 September 2014, Coventry, UK

Figure 2 - Trochoidal wave [2]

The waves listed in Table 1 apply only to certain range limits, with the chosen wave highlighted. The L/20 wave is a good prediction up to 90m, optimistic between 90m to 150m and increasingly unsatisfactory thereafter [2]. Other waves are taken to be fairly accurate predictions within their specified range.

Table 1 - Waves ranges [2]

WAVE (m) L/20 L/9 2.2L0.3 0.607L0.5

True radius 7.958 14.324 15.915 47.746Cycle radius 2.8 2.3 4.4 5.3Wavelength 50.0 90.0 100.0 300.0Wave height 5.556 4.5 8.758 10.514Wavelength

range 0 - 90 0 - 50 50 - 100 100 - 300

The resultant wave should always provide enough buoyancy to keep the ship in equilibrium, ignoring any acceleration of the ship up or down. This is demonstrated in Figure 3 which plots the weight of ship against distance across its length. A similar plot from [2], is superimposed over the top to demonstrate how severe the bending moment can be at the extreme wavelength and cycle position of wave.

Figure 3 - weight distribution vs buoyancy provided by water

The wave highlighted in Table 1 is represented in Figure 4, and has the axis’ proportional intentionally to demonstrate the gentle slope of the wave across its length. Note that there are two waves shown within the graph, since actual wavelength is 100m in this instance. This wave was chosen because of both the ship size and its range out at sea would not likely encounter waves within the region of 100-300 metres.

Figure 4 - Wave Model

Bad sea conditions are even more critical for high speed vessels since they would not be able to travel at their desired speed or worse, have to stay at port or seek shelter if such conditions exceed safety limits [4]. This gives further argument to the lower wavelength range of wave. Cancellation would occur when wave height exceeds 2.5 metres for a sailing period of over 2 hours [4]. Wave heights monitored on several ship routes around the Mediterranean rarely exceeded 2.5m on most routes, which means in realistic terms the ship considered would never be subjected to such a severe wave, however it does demonstrate the worst theoretical case for this ship. This theoretical wave needs to be implemented on a vessel compute model.

2.2. TCL Wave Pressure Formation CodeRunning a computer script to apply a series of pressures onto the hull surface representing a wave is considerably quicker than manually implementing each representative pressure. The computer programme applies a series of pressures onto 2D surfaces of a component or set of components in either a hogging or sagging wave, with the length of the object, defined as its length in the global x coordinate. This is because the maximum bending moments are when the length of the wave is the length of the object. The code also has the option of orientating the wave at 45 or 90 degrees to its normal position on the object to model longitudinal, transverse and torsional waves if desired, and also include options for draft height (Laden and Unladen). The wave algorithm flow chart is provided in Appendix A.

The load is changed through hogging and sagging by offsetting the wave by half a phase. The offset can be changed so that the peaks of the wave are applied to the ends of the boat (hogging) or so that the peak of the wave is applied at the centre of the boat (sagging). The specific offset value for hogging or sagging is applied to their respective variables and built into the pressure function so that the load case can be simply changed. Since the wave is defined in ‘free space’, the model is orientated to change the load case from longitudinal, transverse, or torsional by defining the corresponding variables at the start. Furthermore the wave can be transferred along the



relevant axis to simulate different severities of hogging and sagging.

3. ANALYSIS

3.1. FEA setup studyTo best set-up the analysis, it was determined that the stress concentrations around SPC constraints (Single Point Constraints) would affect the results too significantly so Inertia Relief was used instead [5].

3.2. FEA Model set upIn order to maximise available time and CPU power, the code and its implementation was refined. A large part of this was to minimise the number of elements the routine had to run through. This was done by using only the ship hull, and in most cases, only half, where the wave was symmetrical.

Figure 5 - Half Ship Meshed Model

The first stage of the setup was to delete all components from the half ship meshed model excluding the hull, thus leaving the outer hull components only. This increases load-case set-up efficiency as the number of elements was significantly reduced.

The appropriate wave scripts were applied to the elements on half the hull. The wave script parameters are customisable to give different wave types. All the different load cases are shown in Figures 10 to 12.

Figure 6 - Half Meshed Hull with wave

Figure 7 - Ship Hull with Longitudinal Sagging

Once the desired pressure load has been applied, all elements, surfaces and loads are reflected giving a full hull with a full wave. Additionally, all the elements and surfaces in the half ship without the hull are also reflected, giving a full boat (Figure 9).

Figure 8 - Ship Mesh without Hull

Figure 9 - Equivalenced Ship with two wave forces

The loaded hull is imported into the full ship model. All nodes are equivalenced (Figure 9) with a sliding tolerance starting at 0.01mm (and increasing with user control to ensure all nodes are equivalence) – attaching the separate half’s of the boat and the hull plates with the stiffeners are other relevant components. Element clean-up methods are performed on all the elements possible. Following an element quality check, elements which fail meshing quality criteria are deleted. The location of the waves



in the CAE models is depicted in Figure 10 to 12, following.

Figure 10 - Transverse hogging and sagging

Figure 11 - Longidudinal sagging and hogging

Figure 12 - Torsional sagging and hogging

3.2.1.Added MassesThe additional mass for; vehicles, fittings, engine components, fresh water and fuel are represented by 1D nodal masses. The following table (mass distribution) summarises the mass distribution across the decks for un-laden and laden models, and the figure below shows the decks.

Figure 18 - Ship Side Profile with Highlighted Decks

The 1D nodal masses are distributed evenly throughout the relevant decks in separate components, as listed in Appendix B.

3.2.2.Material Cards and Properties

Before the analysis can be run, materials and properties need to be created and applied to the elements. Materials are created using MAT1 which defines the material properties for linear, temperature-independent, isotropic materials [6]. The density, Young’s modulus and Poisson's ratio are defined for the specific grade for aluminium applied. For each component properties are created using PSHELL property card which is used for 2D

elements. The relevant thicknesses are defined for associated components.

3.2.3.Load Steps

Load steps for each load collector are created for analysis type: linear static. The appropriate load is selected for the corresponding load step. As no SPC’s are used in this analysis, the SPC box is left unticked.

3.2.4.Control cards

Control cards control the format and type of output data required/desired for the analysis. The following control cards were used;

CHECKEL is deactivated to allow analysis to continue despite negligible percentage of failed elements.

Optistruct PARAM: INREL is set to -2 to activate inertia relief for linear static analysis

GLOBAL OUTPUT REQUEST is set to format H3D to export output data (stress/ displacement/ strain) to an h3d file.

3.3. Analysis of Existing Design

3.3.1.Stress analysis

It is imperative to ensure that a good quality mesh is used to provide good quality results. Table 2 (on the following page) shows peaks stresses for each group of components for each load case.

Figure 13 - SS Transverse Frame: Torsional Hogging



Figure 14 - Centre Hull Stanchion: Torsional Hogging

A missing element is contributing to the stress peak in the transverse frame above (Figure 13), but since there are few missing elements the effect is less for this component. The greatest stress across the entire vessel is in the stanchions, which is unsurprising as they are the primary load bearing components. The stress exceeds the target stress of 110 MPa so they are strengthened through either increasing the thickness or by changing material. From the set of figures over the next pages the worst load cases were both longitudinal cases, as well as transverse hogging. The only drawback from the following figures is that the stress range is only relevant to each model shown (Appendix C).

3.3.2.Plane stress across Outer Hull and Decks

Stress along the outer-hull base and decks was analysed for peaks by plotting node paths across the length of the outer-hull, taken from the centre plane of the model. In some load-cases the node path was offset so that the most stressed elements were captured by the path. For each load-case the deck stresses were taken from the same plane as the outer-hull base. Node path or Distance along length of outer-hull/deck against von Mises Stress data can be plotted; highlighting the hull plates of interest. Areas of interest include any elements which exceed stresses of 110 MPa, as any stresses greater than this will have a considerable effect on the number of fatigue cycles. This section of the paper only looks at the stress from the laden ship (Appendix C).

3.3.2.1. Longitudinal

Figure 15 - Longitudinal Hogging: von Mises Stress vs Node path for Entity X Across Cut Plane

Substantial stress peaks across outer-hull plates which exceed the 110MPa threshold, peaking between 170-180MPa. As these stress values are significantly greater than the threshold and close to the yield (193MPa) the hull plates are a major consideration. Such areas were anticipated to be the highest stressed during longitudinal hogging as a result of the concentration of high pressure loads at the central area of the outer-hull. Double bottom, main garage and main deck exhibit stress peaks below the given threshold, hence no concerns for these areas during this load case.

Figure 16 - Longitudinal Sagging: von Mises Stress vs Node Path of Entity X Across Cut Plane

High stress peaks at the stern of the boat at the hull plates, again, anticipated due to the nature of the loadcase. Similar to longitudinal hogging, these Von-Mises values exhibited on the hull plates exceed the threshold significantly and therefore a critical consideration during longitudinal sagging. Associated decks show low stress peaks where values do not exceed 40MPa.



3.3.2.2. Transverse

Figure 17 - Transverse Hogging: von Mises Stress vs Node Path of Entity X Across Cut Plane

High stress concentrations across the central hull at the outer-plates where the majority of the pressure loads are applied during this loadcase. While the threshold is not exceeded in the hull plates and associated decks, Figure 17 stress peaks consistently exceeding 45MPa across the length of hull at the hull plates. It is suggested that the decks are marginally stressed with all Von- Mises values less than 10MPa.

Figure 18 - Transverse Sagging: von Mises Stress vs Node Path of Entity X Across Cut Plane

For transverse sagging, the most stressed areas were the hull plates on the side hull with negligible stresses through the central hull plane at the outer plates. Hence the plane for the node path was cut through the side hull section. Figure 18 emphasizes the effect of transverse sagging with relatively high stress peaks at the side hulls to a significant drop off where the side hulls end. Though stress at the side hulls do not surpass 35MPa. The deck stresses are insignificant in this load case and peaks do not exceed 3MPa.

3.3.2.3. Torsional

Figure 19 - Torsional Hogging: von Mises Stress vs Node Path of Entity X Across Cut Plane

Exceptionally high stress at outer-hull plates, exceeding the 110 MPa threshold set and exhibiting stresses close to the yield strength. Error: Referencesource not found19 for torsional hogging shows similar characteristics to longitudinal hogging with significantly high stress peaks through the centre hull plates and low stress values across the relevant decks.

Figure 20 - Torsional Sagging: von Mises Stress vs Node Path of Entity X Across Cut Plane

Figure 20 depicts the only load case to show the double bottom deck displaying a higher stress level than the outer hull plates. In can also be noticed that the stress peaks for the double bottom deck does not surpass the 110MPa threshold, along with the hull plates, main garage deck and main deck. Thus there are no major concerns regarding torsional sagging.

3.3.3.Conclusions from the initial stress analysis

The longitudinal hogging and sagging peaks just under 180 MPa and showed high stresses along the bulkhead points. Likewise torsional hogging shows similar magnitudes of stress, but otherwise the remaining graphs suggest stresses within acceptable range of the materials yield limit. The high stress peaks were concerning until reduced after a mesh quality improvement had been applied to the model.



Additionally the graphs (Figure 19 and Figure 20) show characteristics according to the associated load case; hogging stress peaks occur at the centre of the hull along the x-axis while sagging stress peaks occur at the stern of the ship. It was expected that there would be minor peaks at the bow of the ship for longitudinal sagging, however due to the combination of the nature of the wave and the height of the bow; there are insignificant stresses at this point.

3.3.4.Analysis summaryOne of the main points made from the initial analysis is the need for mesh improvement of the model to give sufficiently stable and reliable results. There is still a large amount of scope for further mesh improvement to the model in order to better predict its performance in waves. Due to the time required and CPU needed to run a potentially denser mesh additional justification may be in the form of further failure mode testing, i.e. dynamic analysis and hull breaching to determine the greater importance of bulkheads and grounding testing for the double bottom and forepeak for pounding.

The results on existing test methods do show scope for improvement as much of the components are well within their fatigue yield limit. Particular opportunities for light-weighting arise from areas of low stress but high volume of appearance. Such components are:

Stiffeners SS outer plating Deck plating Keel

The analysis revealed that the superstructure experiences smaller amounts of stress when compared to other areas of the ship, hence a reduction in its strength from weight reduction could be undertaken. There are various examples of superstructures mass being reduced through the use of alternative materials in industry; aluminium superstructures are fairly common in cruise vessels [7] and some Danish ships utilise Fibre Reinforced Plastic in the superstructure [8]. This shows that light weighting the superstructure is a viable technique.

Other areas are less suitable due to different failure modes, such as components designed for crash protection or maintaining buoyancy (i.e. bulkheads in the case of a hull breach). These are still evaluated but with a certain measure of precaution.

4. LIGHTWEIGHTING STRATEGY

4.1. Limitations and Opportunities for Lightweighting

There are several limitations to the ways in which the CLF design can be optimised. The research will focus upon the impact of lightweighting on the Lloyd’s Register structural requirements, and will therefore concentrate on the ship internal structure rather than its overall shape and design. The study will therefore be limited by maintaining:

Internal storage space as this is a key feature of the CLF’s functionality.

Hull materials are restricted due to welding on block building method.

Material cost and availability should take high priority with the volume needed for a ship.

Manufacturing complexity will impact cost and total assembly time.

Structure longevity should be considered as continual use for over 10 years is typical.

Maintenance should be considered when choosing materials and structure design.

Main source of strength within a trimaran is the cross deck, and hence must be maintained.

The opportunities for optimisation are:

Hybrid panels can be used to increase plate strength, and hence reduce the number of longitudinal beams in the structure [9].

The use of composite materials for internal components of the ship, taking into consideration the manufacturing of the component (e.g. Carbon fibre bulkheads).

Possible triangulation within the structure or scantling of the ship, replacing the standard longitudinal and transverse beams (within the double bottom).

Topology optimisation of the transverse structural plates of the ship [10].

Replacing the standard steel used in the structure with aluminium composite which could dramatically reduce the weight (applied to the superstructure).

Internal deck structure could be triangulated to provide the opportunity of reducing the number of scantlings in the structure.



The design specifications for this model state that all components shall be constructed out of aluminium, although this has been done without performing any improvements to the geometry, hence the failures in the initial analysis.

4.2. Gauge OptimisationThe chosen method of optimisation is through changing the thicknesses of the components. In order to gauge what the new thicknesses should be an Excel spreadsheet has been produced which quickly calculates the thickness of each component for a target stress value as well as the change in mass that results from this change in thickness.

The profile of each component is known and used to calculate a second moment of area, I, for each component; this is then used to find the bending stress in each component. Each component has a different type of profile so will require a different method of calculation, although each utilises the formula for a rectangular cross-section; where b is the breadth of the plate and d is the depth (see Equation 1). For the decking plates the equation is directly applied based on Figure 21, all other plates are fitted vertically so the profile rotates to follow this.

Equation 1 - Second Moment of Area of Rectangular Section

I=b . d3

12

Figure 21 – Rectangular Plate Cross Section

For the components with a T cross-section the web and flange are split into separate shapes and the second moment of area for each shape calculated separately using Equation 2, where y is the distance from the centroid of the web or flange to the centroid of the entire cross section. The total second moment of area is calculated by adding these values together. This technique is also used to find the second moment of area for components with an L cross-section.

Equation 2 - 2nd Moment of Area of Rectangular Section on multisectioned part

I=b . d3

12+ A . y

Figure 22 - T Cross section

Figure 23 - L Beam Cross Section

The second moment of area of the box cross-sections, which applies to the stanchions, is calculated using Equation 1. A value is calculated for both external and internal dimensions, the internal second moment of area being subtracted from the external one to find the total second moment of area for the component.

Figure 24 - Hollow Rectangular Section [11]

An issue in calculating the bending stress comes from the fact that a moment is required to calculate this, which is currently unknown (see Equation 3); the maximum stress on each component, for each load case, is known, however. By substituting this value for stress into the equation the bending moment can be calculated.

Equation 3 - Bending Stress

σ= M . yI

Using this known value for the bending moment in each component, assuming that as the forces remain constant this must also remain constant, Excel can be used to work out the calculation backwards and determine what the thickness of each component will be for a certain target stress. In this case a target value of 110 MPa has been used as this is the fatigue strength of aluminium at 100,000 cycles.

For the target stress of 110 MPa, the changes of thickness results in an overall decrease in mass of



over 200 tonnes compared to the original model, as listed in Table 2.

Table 2 - Improved thicknesses and Masses

ComponentChange in Thickness

(mm)

New Mass (Tonne)

Change in Mass (Tonne)

Keel -5.38 7.06 -4.98Keel Crash -12.42 0.07 -1.46Keelson -3.22 4.40 -2.09CH & SH Outer Plates -2.55 147.03 -50.25

SS Outer Plates -2.76 86.50 -33.05

CH Inner Plates -0.12 9.79 -0.31

CH & SH Bulkheads -7.35 9.50 -12.37

CH Bulkhead Girders

-5.06 1.42 -0.90

CH Primary Girders -7.35 3.09 -4.02

CH Secondary Girders

-4.57 11.60 -7.14

SS Primary Girders -4.16 22.01 -10.35

SS Secondary Girders

-0.42 3.32 -0.12

CH Crash Stiffeners -7.12 1.31 -4.97

CH & SH Bulkhead Stiffeners

-4.30 1.59 -2.52

CH Forepeak Stiffeners

-5.70 1.00 -2.46

CH & SH Primary Stiffeners

0.00 19.88 0.00

CH & SH Secondary Stiffeners

-0.02 61.56 -0.23

SS Stiffeners -0.58 34.82 -3.75

CH & SH Primary Transverse Frame

-0.38 132.31 -4.38

CH & SH Secondary

-0.11 18.72 -0.21

Transverse FrameSS Primary Transverse Frame

-5.58 39.96 -34.78

SS Secondary Transverse Frame

1.06 34.07 3.26

Double Bottom Plate

-3.15 14.99 -8.07

CH Decking Plates -2.20 68.46 -31.43

CH Vehicle Decking Plates

-4.80 11.74 -13.41

SH Upper Decking Plates

-3.29 2.67 -2.36

SH Lower Decking Plates

-5.45 0.91 -1.40

SS Decking Plates -0.41 62.32 -5.54

CH & SH Stanchions 2.61 2.34 0.80

SS Stanchions 2.49 3.03 1.01

Total Mass change (tonnes) -237.54

4.3. Gauge change justificationBefore reducing any gauge thicknesses in any of the components the new gauge calculations must be interpreted according to the load-cases applied. As a crash or pounding analysis has not been performed, reductions in gauge thickness for any components involving crash safety and pounding should be ignored. This will avoid compromise to passenger safety during these failure modes. This includes all forepeak stiffeners, collision bulkheads, crash stiffeners and crash keel.

The bulkheads contribution to the stiffness through the spine of the ship is significant. Furthermore bulkheads need withstand extreme hydrostatic pressure when outer hull plates have failed to maintain buoyancy by remaining water-tight. Therefore all gauge reductions suggested by the calculations are also ignored. The components which are to be strengthened include all the stanchions.



Table 3 summarises the final changes in gauge thickness for the selected components. A factor of safety 1.1 is applied to the new thicknesses and rounded to the nearest millimetre.

Table 3 - Gauge Thickness change for Selected Components

ComponentInitial

thickness (mm)

New thickness

(mm)

Estimated Change in

Mass (tonnes)

Keel 13 8 -4.63Keelson 10 7 -1.95CH Bulkhead Girders

13 8 -0.89

CH Primary Girders 13 6 -3.82


12 8 -6.25

SS Primary Girders 13 9 -9.96

SS Secondary Girders

12 12 0.00


7 4 -1.76

CH & SH Outer Plates 10 9 -19.70

Double Bottom Plate 9 6 -7.68

CH Decking Plates 7 5 -28.50


9 5 -11.20

SH Upper Decking Plates

7 5 -1.44

SH Lower Decking Plates

9 5 -1.03

CH & SH Stanchions 5 8 0.92

SS Stanchions 5 8 1.22

Total Change (tonnes) -96.70

The changes in gauge thickness will result in an estimated mass saving of 97 tonnes. Although this value will vary slightly as some failed elements are deleted, this is still a significant mass saving. Additionally this mass saving also includes the strengthening of the stanchions.

5. ANALYSIS OF OPTIMISED DESIGN

Gauge of the lightweighted components are listed in Appendix D

5.1. Plane stress across Outer Hull and DecksFor the longitudinal load cases, the stress peaks emulate their nature; longitudinal hogging peaks through the centre of the boat and falls towards the bow, while longitudinal sagging peaks at the stern of the ship and decreases towards the centre of the ship. No decking or hull plates exceed 110 MPa, longitudinal sagging peaks between 60-70 MPa at the main garage deck while longitudinal hogging peaks at the hull plates at just over 80 MPa.

Figure 25 - Longitudinal Hogging – Stress vonMises vs Node path entity X across cut plane for gauged model

Figure 26 - Longitudinal Sagging – Stress Von Mises vs Node path entity X across cut plane for gauged model

The transverse plane stress graphs again correspond to their associated load-case. Transverse hogging shows distributed stress peaks along the main hull whereas transverse sagging stress peaks at the side hull plating. Both load-cases are significantly below



the fatigue threshold and peaking at the side hull or main hull plating between 30-40 MPa.

Figure 27 - Transverse Hogging – Stress vonMises vs Node path entity X across cut plane for gauged model

Figure 28 - Transverse Sagging – Stress vonMises vs Node path entity X across cut plane for gauged model

Torsional hogging shows similar characteristics to longitudinal hogging; peeking through the centre of ship and decreasing stress at the decks higher above wave amplitude. The stress peaks at 75 MPa at the hull plating.

Torsional sagging is the only load-case where deck stress is observed higher than the stress on the hull plating. This is caused by the high concentration of hydrostatic pressure at the upper stern of the ship. The double bottom deck peaks between 55 MPa and 60 MPa with the next highest stress deck (main garage deck) peaking at just above 15 MPa.

Figure 29 - Torsional Hogging – Stress vonMises vs Node path entity X across cut plane for gauged model

Figure 30 - Torsional Sagging – Stress Von Mises vs Node path entity X across cut plane for gauged model

5.2. Analysis discussionGauge of the lightweighted components are listed inAppendix D at the beginning of this section shows the component stresses for the gauge optimised ship. Although anticipated due to the reduction in component gauge, there was an increased number of components which exceed the fatigue target 110 MPa.

Due to the gauging of the majority of the ships components to produce a significant reduction in weight, stress peaks exceeding the fatigue strength limit (discussed in the previous sections) occurred within several components which are identified in the aforementioned table.

The gauging of the decks and transverse frames within the hull caused a reduction in the stiffness of the ships structure. As a result of the CLF’s super structure spanning the length of the ship, greater strains occurred within the super structure. However the stress peaks occurring within several components within the super structure are due to deleted elements. These components are discussed in the previous analysis section.

From the plane stresses- as the deck height increases, the stresses generally decrease with the exception of torsional sagging. This was anticipated as the largest pressures are applied to the lowest points on the



outer hull plating, thus the stress wave dissipates as it travels up the height of the ship.

All the load cases show similar characteristics when comparing the original model to the gauged model. However for longitudinal hogging, longitudinal sagging, transverse hogging and torsional hogging, a significant reduction in stress values can be observed. Additionally, for the load cases torsional sagging and transverse sagging there are minor rises in the stress peaks. Regardless, all the load-case max stress values for the given decks are below the fatigue threshold of 110MPa.

The stress trend has slightly increased and appears to be more evenly distributed despite the overall stress reducing. There is an absence of the sharp stress peaks seen in the previous models graphs, which could again be due to better mesh quality or reduced thickness of

In conclusion gauging should only be used with extreme care as a method of weight reduction; but ideally should be combined with the use of shape optimisation and using composite materials to maintain the stiffness of the ship whilst also providing a major weight reduction.

6. CONCLUSION

A wave algorithm has been developed which allows for easy application of forces onto a boat model, of any size, for one of the main load cases. It can be easily altered for each load case as well as for different wave heights making it useful for a large number of situations.

The code has been used to analyse the stress distribution in the 120 m long Cruise Logistics Ferry allowing for the identification of areas where lightweighting could be achieved.

By reducing the gauge thicknesses for these areas, based on the bending stresses in each component, a total weight saving of 96 tonnes.

7. ACKOWLEGMENTS

The authors would like to thank Sean McCartan for providing information on the Lloyds Register, and especially Oliver Grimes, who acted as a project supervisor and advisor for ahis advice on TCL scripting.

REFERENCES

1. Rawson, K. J., Tupper, E. C. 2001 Basic Ship Theory. V.1, 5th ed. Oxford: Butterworth-Heinemann

2. Molland, A. F., 2011. A guide to ship design and construction. Southampton: Elsevier.

3. Office of Naval Research (ONR), 2009. Ocean in Motion: Waves - Characteristics. [Online] Available at: <www.onr.navy.mil/focus/ocean/motion/waves1.htm> [Accessed 2015].

4. Martínez de Osés & Castells, 2006. Wave height incidence on Mediterranean Short Sea Shipping routes. Barcelona: Technical University of Catalonia

5. Grimes, O., 2014 Tutor [interview].6. Altair, 2015. Hypermesh Desktop Reference

Guide, s.l.: Altair.7. Ferraris, S. & Volpone, L. M. 2005. ‘Aluminium

Alloys in Third Millennium Shipbuilding: Materials, Technologies, Perspectives.’ The Fifth International Forum on Aluminium Ships [online] available from <http://aluplanet.com/documenti/InfoAlluminio/AlGenFeb06Volpone.pdf> [Accessed 2015]

8. CFPA Europe, 2015. New Ship Materials Present Significant Fire Safety Challenges. [online] Available at: < http://cfpa-e.eu/new-shipbuilding-materials-present-significant-fire-safety-challenges/ > [Accessed 2015]

9. SSC, Ship Structure Committee - Feasibility conceptual design and optimization of a large composite hybrid hull, 2008. [online] Available at: < www.shipstructure.org/pdf/455.pdf>

10. Altair, 2014. Driving a Structurally Efficient Design of the Queen Elizabeth Class Aircraft Carrier [online] Available at: < www.altairproductdesign.co.uk/CaseStudy.aspx?id=7>

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http://www.onr.navy.mil/focus/ocean/motion/waves1.htm

http://www.onr.navy.mil/focus/ocean/motion/waves1.htm


APPENDIX A



APPENDIX B

DeckLoad Type Total

Distributed Loads

Structural

Outfitting

Engine Rooms

Fuel & Fresh Water

Ballast Weight

Payload

1.Inner Double Bottom D - - 298.92 267.10 - 566.02

1.Lateral inner Double Bottom D - 316.045 - - - 316.05

2.Lower Garage Deck D - - - - 37.5

0 37.50

3.Main Garage Deck D 228.9

8 - - - 78.75 307.73

4.Upper Garage Deck I D - - - - 117.

50 117.50

4.Upper Garage Deck II D - - - - 256.

25 256.25

5.Pax Deck 1 D - - - - 52.5 52.50

6.Pax Deck 2 D 101.96 - - - 52.5 154.46

7.Bridge Deck D - - - - 2.25 2.25

Total 961.29 330.94 316.05 298.92 267.098 597.

25 1810.26

Light Ship 1608.28

Ballasted Weight 2174.30

Full Load Weight Departure

2771.55

D: Weight of structure is put in the FEA using material properties for the different components and their thicknesses.

Mass Distribution across Decks



APPENDIX C

Component

Prof

ileT

hick

ness

(mm

) Loaded Stress (MPa)

Max

(MPa

)

FS R

atio

Lon

gitu

dina

l H

oggi

ng

Lon

gitu

dina

l Sa

ggin

g

Tra

nsve

rse

Hog

ging

Tra

nsve

rse

Sagg

ing

Tor

sion

al

Hog

ging

Tor

sion

al

Sagg

ing

Keel P 13 64.46 26.34 19.57 1.99 60.68 12.55 64.46 0.59Keel Crash P 13 3.65 4.91 3.00 0.15 2.99 4.28 4.91 0.04Keelson P 10 74.56 37.52 27.28 3.87 73.09 22.86 74.56 0.68CH & SH Outer Plates P 10 78.93 79.85 37.38 36.69 78.45 81.98 81.98 0.75SS Outer Plates P 10 79.59 45.70 21.65 8.86 78.42 23.50 79.59 0.72

CH Inner Plates P 4 94.42 106.60 47.86 16.51 81.90 66.66 106.60 0.97

CH & SH Bulkheads P 13 38.48 24.66 23.18 11.31 47.78 19.22 47.78 0.43CH Bulkhead Girders T 13 64.12 21.83 22.19 2.30 66.68 9.94 66.68 0.61CH Primary Girders T 13 43.90 44.45 27.77 5.12 47.28 20.91 47.28 0.43CH Secondary Girders T 12 65.06 41.74 41.05 11.94 67.53 19.17 67.53 0.61SS Primary Girders T 13 68.07 53.20 46.05 7.45 74.35 21.98 74.35 0.68

SS Secondary Girders T 12 101.12 60.06 11.93 5.90 106.1

0 33.44 106.10 0.96

CH Crash Stiffeners L 9 17.93 11.63 22.32 0.55 14.29 9.58 22.32 0.20CH & SH Bulkhead Stiffeners L 7 41.64 9.42 23.87 5.03 39.38 6.77 41.64 0.38

CH Forepeak Stiffeners L 8 30.89 11.66 15.13 0.71 25.21 7.08 30.89 0.28CH & SH Primary Stiffeners L 7 105.9

0 60.10 51.41 12.65 109.98 38.12 109.98 1.00

CH & SH Secondary Stiffeners L 6 83.29 105.0

0 49.31 13.41 109.57 59.68 109.57 1.00

SS Stiffeners L 6 81.60 66.34 33.77 11.74 98.85 36.31 98.85 0.90CH & SH Primary Transverse Frame T 12 88.73 53.86 39.97 22.35 106.4

0 52.41 106.40 0.97

CH & SH Secondary Transverse Frame T 10 108.7

3 41.75 41.00 19.68 103.60 18.67 108.73 0.99

SS Primary Transverse Frame T 12 54.99 33.16 18.44 10.86 58.24 20.08 58.24 0.53

SS Secondary Transverse Frame T 10 109.4

0 70.86 28.08 16.72 121.90 38.78 121.90 1.11

Double Bottom Plate P 9 45.58 24.25 20.14 4.68 46.48 13.47 46.48 0.42CH Decking Plates P 7 51.67 17.71 17.40 8.82 47.30 10.96 51.67 0.47CH Vehicle Decking Plates P 9 22.53 17.54 9.50 3.15 23.97 10.98 23.97 0.22

SH Upper Decking Plates P 7 15.07 15.01 30.97 12.25 15.91 12.30 30.97 0.28SH Lower Decking Plates P 9 2.58 17.11 3.16 4.78 10.05 14.76 17.11 0.16SS Decking Plates P 5 91.83 43.45 27.55 6.13 92.76 26.59 92.76 0.84

CH & SH Stanchions B 5 158.50 43.72 72.45 20.74 165.5

4 23.81 165.54 1.50

SS Stanchions B 5 162.90 50.82 89.08 9.23 151.0

0 19.54 162.90 1.48



Table 34 - stress analysis



Figure 21 - Stress Distribution Side View: Longitudinal Hogging

Figure 22 - Stress Distribution Side View: Longitudinal Sagging

Figure 23 - Stress Distribution Side View: Transverse Hogging

Figure 2431 - Stress Distribution Side View: Transverse Sagging

Figure 25 - Stress Distribution Side View: Torsional Hogging

Figure 26 - Stress Distribution Side View: Torsional Sagging

Figure 27 - Stress Distribution Bottom View: Longitudinal Hogging



Figure 28 - Stress Distribution Bottom View: Longitudinal Sagging

Figure 29 - Stress Distribution Bottom View: Transverse Hogging

Figure 32 - Stress Distribution Bottom View: Transverse Sagging

Figure 33 - Stress Distribution Bottom View: Torsional Hogging

Figure 34 - Stress Distribution Bottom View: Torsional Sagging



Figure 35 - Stress Distribution Rear View: Longitudinal Hogging

Figure 36 - Stress Distribution Rear View: Longitudinal Sagging

Figure 37 - Stress Distribution Rear View: Transverse Hogging

Figure 38 - Stress Distribution Rear View: Transverse Sagging

Figure 39 - Stress Distribution Rear View: Torsional Hogging

Figure 40 - Stress Distribution Rear View: Torsional Sagging



APPENDIX D

Component

Prof

ile

Thi

ckne

ss

(mm

)

Loaded Stress (MPa)

Max

(MPa

)

FS R

atio

Lon

gitu

dina

l H

oggi

ng

Lon

gitu

dina

l Sa

ggin

g

Tra

nsve

rse

Hog

ging

Tra

nsve

rse

Sagg

ing

Tor

sion

al

Hog

ging

Tor

sion

al

Sagg

ing

Keel P 13 78.52 29.92 26.30

25.43 73.80 14.30

78.52 0.71Keel Crash P 13 3.59 5.78 2.95 0.15 2.96 4.33 5.78 0.05Keelson P 10 93.09 48.88 36.2

94.45 91.08 30.3

893.09 0.85

CH & SH Outer Plates

P 10 83.07 94.28 43.95

44.52 80.95 90.68

94.28 0.86SS Outer Plates P 10 86.38 51.83 22.2

99.04 85.27 25.2

486.38 0.79

CH Inner Plates P 4 100.40

119.80

49.01

18.09 86.79 71.97

119.80

1.09CH & SH Bulkheads P 13 45.09 25.44 27.8

712.58 51.55 20.9

851.55 0.47

CH Bulkhead Girders T 13 76.72 26.62 28.31

2.85 78.91 12.05

78.91 0.72CH Primary Girders T 13 78.06 62.98 42.4

26.20 81.29 29.9

281.29 0.74


T 12 82.58 50.45 50.97

15.60 85.41 23.39

85.41 0.78SS Primary Girders T 13 99.85 54.85 75.8

59.68 115.60 25.8

5115.60

1.05SS Secondary Girders T 12 137.5

081.14 15.2

56.99 143.80 41.5

1143.80

1.31CH Crash Stiffeners L 9 19.67 13.77 23.1

10.61 16.15 10.9

923.11 0.21


L 7 49.36 12.65 27.38

6.45 47.24 9.29 49.36 0.45

CH Forepeak Stiffeners

L 8 32.44 12.31 16.12

0.75 26.41 7.45 32.44 0.29CH & SH Primary Stiffeners L 7 116.0

0 63.55 53.97 14.78 120.90 38.6

7120.90 1.10

CH & SH Secondary Stiffeners L 6 86.10 124.0

047.80 13.50 114.10 63.9

5124.00 1.13

SS Stiffeners L 6 88.98 69.55 35.20

12.51 106.90 38.11

106.90

0.97CH & SH Primary Transverse Frame T 12 88.43 53.20 37.8

8 28.42 108.50 51.90

108.50 0.99

CH & SH Secondary Transverse Frame T 10 121.3

0 46.18 42.24 19.69 109.30 28.4

4121.30 1.10

SS Primary Transverse Frame T 12 60.55 36.53 22.8

8 11.02 64.27 19.94 64.27 0.58

SS Secondary Transverse Frame T 10 130.8

0 83.55 31.82 17.30 144.48 43.3

7144.48 1.31

Double Bottom Plate P 9 75.99 42.83 46.05

9.15 67.57 18.62

75.99 0.69CH Decking Plates P 7 62.08 23.72 28.4

510.45 56.34 14.5

962.08 0.56


P 9 28.17 24.12 9.96 4.06 32.92 14.83

32.92 0.30SH Upper Decking Plates

P 7 21.25 21.39 3.54 14.59 21.28 17.62

21.39 0.19SH Lower Decking Plates

P 9 2.02 21.33 3.42 5.17 12.44 18.15

21.33 0.19SS Decking Plates P 5 90.08 42.31 28.0

46.62 90.54 25.9

790.54 0.82

CH & SH Stanchions B 5 141.00

36.09 61.20

18.31 147.10 19.10

147.10

1.34SS Stanchions B 5 118.7

035.79 61.8

371.72 117.20 16.8

3118.70

1.08Table 5 - Gauged component stresses


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