Design Methods– Prof. Stein Ove Erikstad

Selecting Design Alternatives

TMR4115

Prof. Stein Ove Erikstad

Reality

Decision Models

Design Methods– Prof. Stein Ove Erikstad

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Selection problems:Example

• You work as a design engineer, and you are given the task of designing a high speed vessel for a particular route. Several alternative vessel concepts are relevant, catamaran, SES, SWATH, foil-cat.

• How can you decide which one is the best conceptual solution?

Design Methods– Prof. Stein Ove Erikstad

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Selection problems:Example 2

• You work in the transport division of Statoil. You have just received more than 100 offers for shuttle tankers for 10 years contracts between Statfjord and Mongstad.

• The offers contain vessels with different cargo capacity, service speed, quality, age and price. How can you – rationally and efficiently – select the best offer?

Design Methods– Prof. Stein Ove Erikstad

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Related to the design process

Problem statement

Alternative 1

Alternative 2

Alternative 3

Alternative 4

Alternative 5

Alternative 6

Alternative 7

Generationof solution

descriptions

Basic/detaileddesign

Designingselected solution

Alternative 1

Alternative 3

Alternative 4

Alternative 7

Removinginfeasiblesolutions

Alternative 4

Selecting”optim al” solution

Design Methods– Prof. Stein Ove Erikstad

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Classification of selection problems

1-dimensional

No uncertainty

1-dimensional

Uncertainty

No uncertainty Uncertainty

Multi-dimensional

Multi-dimensional

1

42

3

Design Methods– Prof. Stein Ove Erikstad

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Your task is to select the main machinery configuration for the vessel , and you have five different alternatives before you.

1. Your objective is lowest weight. Only one criteria that can be determined with no uncertainty

2. Your objective is the best combination of cost, weight and specific fuel consumption – i.e. multiple criteria which can be determined with (a reasonable degree of) certainty

3. You want the solution that will give you the lowest lifecycle maintenance cost, i.e. one criteria with an uncertain value

4. You want the solution with is preferable taking price, weight, maintenance cost and lifetime into consideration – i.e.multiple criterie with (at least some) uncertain values

Examples

Design Methods– Prof. Stein Ove Erikstad

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Core concepts and notation

• Decision problem:– We must select a conceptual solution for a high speed

passenger vessel

• Decision alternatives:– A - SES

– B - Catamaran

– C - FoilCat

• Attributes:– X1 - Max. service speed

– X2 - Motion behaviour

– X3 - Total annual cost

• Attribute values:– xA - (42, good, 8) (knots, – , mEUR)

– xB - (34, medium, 5)

– xC - (48, excellent, 10)

Design Methods– Prof. Stein Ove Erikstad

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Objective and objective hierarchy

Preferable features

1. Complete – cover all relevant aspects

2. Operational – measurable and relevant criteria

3. Decomposable – splitting/grouping possible

4. Non-redundant – avoid counting some features twice

5. As small as possible

Design Methods– Prof. Stein Ove Erikstad

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Objective hierarchy -example

Speed at max power

Endurance at 27 knots

Speed & endurance Logistics/R&M

No. of tanks

No. of vehicles

Max tank condition

No. of helos

No. of vehicles

Max helo condition

Cargo capacity Safety

Pierside

In-stream

Cargo capability

Mission capability

Design Methods– Prof. Stein Ove Erikstad

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Objective hierarchyHigh speed vessel example

Max lifecycle profit

Max income Min LC cost

Max pass capacity

Max “cargo factor”

Min invest-ment

Min oper. cost

# seats

# trips/yr

Comfort

Transport-ation time

Security

Mainte-nance

Fuel cost

Other

Design Methods– Prof. Stein Ove Erikstad

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Pareto

Dominans:B is dominated by AD is dominated by CG is not dominated

Pareto-optimal set:The set of non-dominated solution form a Pareto-optimal set

x 1 - Hastighet

x2

- S

ikk

erh

et

A

B

C

D

EF

G

x 1 - Hastighet

x2

- S

ikk

erh

et

A

B

C

D

EF

G

Design Methods– Prof. Stein Ove Erikstad

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The form of the value function

xi

xi xi

xi

vi(xi)

vi(xi)vi(xi)

vi(xi)

Design Methods– Prof. Stein Ove Erikstad

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One-dimensional value function v i(x i)

1. Determine scale

2. Normalise

3. Determine form of function

Design Methods– Prof. Stein Ove Erikstad

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Example:

3. Determine form of function

2. Normalize value function

vv

(27) = 0.0

vv

(33) = 1.0

1. Determine scale

Lower limit: xv

min = 27 kn

Upper limit: xv

max = 33 kn

0.0

1.0

27 33

Design Methods– Prof. Stein Ove Erikstad

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The Assumption of an additiv value function

Examples of dependencies between the attributes:

X1 – max speedX2 – motion behaviourX3 - total annual cost

where

i.e.

we are willing to accept high annual cost for a vessel with a high service speed IF the motion characteristics are good.

If, on the contrary, the motion characteristics are bad, we are not able to exploit the speed potential, and thus not willing to pay much for a high max service speed.

Design Methods– Prof. Stein Ove Erikstad

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Practical approaches to selection problems

1) Determine an objective hierarchy with corresponding attributes

2) Determine weights for each attribute, such that Σ λ i = 1

3) Determine upper and lower limits for each attribute, and give these the values 1.0 and 0.0, respectively. Choose the form of the value function – default is linear.

4) For each attribute: Evaluate the different alternatives, use the value function to determine the value

5) Calculate v(x) for each alternative. Choose the alternative with the highest value.

i

Design Methods– Prof. Stein Ove Erikstad

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Decisions under uncertainty

• Types of uncertainty: None, known, full.

• Risk concepts (Risk = Probability * Consequence). Risk aversion/preference.

• Rationality: We ACT with limited rationality (e.g. St. Petersburg paradox, Lotto, etc.), or not very CONSEQUENT (e.g. Lotto vs. bonds, stocks). Risk premium.

• Utility structures under uncertainty: Minmax -expected value

• Problem s by such “calculations”: M oral – model related

Design Methods– Prof. Stein Ove Erikstad

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Warning

• The main purpose here has been to discuss a RATIONAL PLATFORM for decision-making –N O T to give a “recipe” for how such decision SHOULD be made

• The most important result from this process is not a “definite” answer – but as a documentation of the process leading to the decision