7/28/2019 Going Organic in Aircraft Carrier Design

1/324 MITE June/July 2013

The advent of the sail,

wooden hulled ships

designed to circum-

navigate the globe, the

first steam powered ships and the

transition from wood to steel are

all marked major turning points

in ship design. However, naval

engineers are finding themselveson the edge of a new revolution:

organic design.

Currently, the majority of

ship design processes are based

on a limited set of design data for

major structural design drivers.

Despite the tremendous techno-

logical advances, some ship com-

ponents are still designed using

techniques developed with the

first steel ocean liners in the early

20th century. As a result, ships

are complex, bulky and heavy.

These limitations, often solidifiedin the early stages of design, fre-

quently lead to increased material

and manufacture costs for the

final product.

Its easy to understand why

some of this inefficiency exists;

engineers are most concerned

with ensuring the ships safety.

Weight and long-term costs are

often secondary considerations

that occur late in the design

process. Design optimisation,

specifically around weight, is

sparingly performed in the shipdesign industry, with only a mini-

mum structural weight calcula-

tion being performed to

demonstrate concept feasibility.

This traditional approach

brings with it a number of issues,

including undesirable structural

arrangement constraints driven

by high-level design decisions fi-

nalised early on, higher likeli-

hood of costly iterative changes,

and sub-optimal weight and man-

ufacturing costs from poor design

parameters.Designers in other industries,

like automotive, generally do not

have these same issues and have

created a well-defined operational

profile from the frequent design

Taking a more natural approach to

design optimisation paves the way forlighter ships

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p Organic

design

optimisations

were used to

reduce the

weight of the

Queen Elizabeth

class aircraft

carrier

MITE June/July 2013 25

sation technology to mathemati-

cally and logically explore the op-

timal layout of material.

Employing this approach reduces

the number of design iterations,

subsequently reducing the design

cycle while ensuring an optimal

structural solution that meets de-

sign targets. Additionally, this re-

duced initial design timeframe

allows for exploration of more de-

sign starting points and enablesrapid trade-off studies.

Simulation and optimisation

technologies can be used in a va-

riety of ways with varying levels

of involvement and complexity to

create a balanced approach for

any project. This approach can be

achieved through rules-based ship

structure design calculations, fi-

nite element analysis (FEA) of

the structure or both. Fidelity is

maximised by allowing variables

to pass through both hand calcu-

lations and FEA at the same time.While FEA has been applied

in the past, its uses have not

reached the technologys full po-

tential; instead, it has been used

to merely validate and refine ex-

isting designs. Through simula-

tion-driven design, however, FEA

can be integrated at the very be-

ginning of the design process and

assist in design creation, placing

engineers one step closer to

lighter, more efficient and cost-ef-

fective designs.

Optimisation in actionThe potential of this technique

was realised when the British

Royal Navy was looking for a

new aircraft carrier. It turned to

cycles. Optimisation principles

applied by these industries help

improve the performance and

weight of products, while slashing

engineering and manufacturing

costs. The ship building industry

does not have this same benefit

due to low design turnover and

additional requirements particu-

larly when it comes to naval proj-

ects. These early design-decision

constraints make it difficult toadopt optimal solutions in initial

design stages and lead to higher

engineering costs in later stages.

The Missing LinkThis is where organic design prin-

ciples combined with the right

technology have the potential to

create a ship building design revo-

lution. Aerospace and automotive

engineers have been using opti-

misation algorithms for decades

to identify the design space, de-

sign constraints and the optimumvalues for design variables to sat-

isfy them. This technology is

readily available to the ship build-

ing industry; some cruise ship de-

signers have already begun using

increasingly sophisticated design

approaches, resulting in an aver-

age weight savings of 10% com-

pared to experienced engineers

using the traditional approach.

Traditional design is based on

prior knowledge, engineering ex-

perience and simple trial and

error, a costly and time-intensiveprocess that still may fail to iden-

tify optimal structures.

Enter simulation. Simulation-

driven design processes blend

structural simulation with optimi-

BAE Systems to help realise one

ofthe largest engineering proj-

ects currently being undertaken

in the UK: the Queen Elizabeth

Class (QEC) Aircraft Carrier. BAE

Systems partnered with Thales

UK., Babcock and the Ministry of

Defence to form the Aircraft Car-

rier Alliance (ACA), a partner-

ship that recognised the positive

track record of simulation-driven

design. To evaluate its potential

under the unique requirements

of naval design, ACA partnered

with Altair ProductDesign, a

global engineering firm with a

long history of applying optimi-

sation in the aerospace and auto-

motive industries, for a pilot

exercise on a section ofthe ves-

sels structure.

At the core of organic design,

like that used in the QEC project,

is optimisation, which can be sep-

arated into four key methods:

free-form or topology optimisa-

tion, free-size optimisation, size

optimisation, and shape optimisa-

tion.Free-form or topology optimi-

sation is focused on identifying

the areas ofa given design space

that are structurally important

and those that are structurally re-

dundant under specific design

loads, objectives and constraints.

This method is often used early

in the design process while maxi-

mum freedom of design is still al-

lowed. For ship building, topology

optimisation can identify the best

locations for bulkheads and

where openings within thosebulkheads can exist, ensuring

maximum stiffness, while reduc-

ing weightby eliminating redun-

dant material within the

structure.

Topology optimisation was

used on the QEC project to assess

the double-bottom structure,

which would be subjected to sig-

nificant loads from external hy-

drostatic pressures in addition to

the dynamic loads generated by

large equipment items within the

ship. Further complicating thedouble-bottom design was a Con-

fined Space Access and Escape

Arrangements Policy that re-

quired routing access openings

through the floors in the double

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ments, which is the only existing

alternative to traditional design

principles that also uses optimisa-

tion and simulation technologies.

While this existing fine-tuning im-

pacts plate thickness and stiffener

dimensions, it lacks the capability

to create the optimum starting

structural layout, nor does it ad-

dress the optimum shape of struc-

tural features.

For example, an opening in a

bulkhead may require thickening

of the surrounding plate in order

to meet design targets, based on

existing technologies. However,optimisation and simulation tech-

nologies engineered around or-

ganic design principles may

identify that the same resulting

structural integrity can be

achieved by adjusting the cut-in-

steel shape of the opening. A sim-

ple shape change such as this

saves designers from adding more

weight and steel to a bulkhead,

reducing fabrication and welding

time.

This was exactly what oc-

curred in the design of the QECdouble-bottom floor access open-

ings. Once topology optimisation

had identified the best locations,

size and shape optimisation were

employed to improve the stress

response of the structure and

minimise the steelwork mass re-

quired to meet the design targets.

Applying optimisation methods,

designers created a structure 9%

lighter than the baseline design,

while meeting all design targets.

Conversely, the baseline design

failed to meet stress targets. Opti-misation work by Altair and the

ACA went on to be used in the

QECs flight control module, stern

platform and transverse bulk-

heads.

bottom. Topology optimisation en-

abled designers to identify areas

of redundant structure that could

accept access openings without

compromising performance.

Similar to topology optimisa-

tion, free-size optimisation helps

to indicate areas of structural im-

portance and redundancy but acts

to vary the thickness of the exist-

ing structure, as opposed to physi-

cally removing or growing

structure.

Shape optimisation varies the

dimensions of structural fea-

tures, similar to size optimisa-

tion; however, it acts to change

the shape of a finite element

(FE) mesh used to describe a

structural feature, rather than

changing a dimensional value

within an equation or FE model.

This could take the form of a ra-

dius corner on an opening, for

example. Both shape and size op-

timisation have the potential to

be used on initial structures de-

fined by topology or free-size op-

timisation or may improveexisting concepts derived by

other means.

When employing these meth-

ods, designers have the freedom

to use size optimisation on both

FEA and non-FEA-based simula-

tion methods as it requires setting

dimensional values as variables.

However, topology, free-size and

shape optimisation are FEA-de-

pendent because of their reliance

on structural package spaces

being discretely divided with fi-

nite elements.

Lose weight, reduce stressAlthough optimisation can be

used at any stage in the design

process, using these methods

early in concept development

will ensure the most efficient dis-

tribution of material and loads.

Optimisation not only reduces

the structural weight of designs

but also minimises stress concen-

trations, a leading cause of reme-

dial work later in the design

process.The organic design principles

beginning to be used in the ship

building industry differ greatly

from the existing fine-tuning of

pre-conceived structural arrange-

Future potentialGrowth of optimisation-focused de-

sign within the maritime engineer-

ing profession has been slower

than its automotive and aerospace

counterparts, as the ship building

industry has traditionally not faced

the same demands for design im-

provement. That said, increasingly

stringent environmental regulation

coupled with rocketing bunker

prices is forcing owners and naval

architects to seek out efficiency

gains wherever they can.

In this respect, simulation of-

fers some of the same benefits tothe ship building industry as it

does to car makers and aircraft

manufacturers. Optimised ships

can reduce material costs by

identifying lighter, more efficient

structures, which may also re-

duce fabrication costs associated

with the complex structural de-

signs created by traditional de-

sign principles. All of this can

lead to fewer design cycles and

reduced remedial work.

The project that BAE Systems

undertook with Altair ProductDe-sign identified the shortcomings

and challenges associated with

traditional ship structural design

processes. It demonstrated that

simulation-driven design can pro-

vide benefits to ship structural de-

sign and manufacturing through

cost reduction, mass reduction,

and improved structural perform-

ance and efficiency. While organic

design adoption is beginning to

grow in ship building, future ef-

forts are underway to identify

how simulation-driven design canbe best applied to whole ship de-

sign and will provide future naval

architects with the tools and free-

dom to make better informed de-

sign decisions.

p Defining the

best structural

layout using

topology

optimisation

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26 MITE June/July 2013