Theory Flashcards
Strategic Supply Chain Model ?
Production Capacity is a VARIABLE:
- Binary: building/closing of a production site/line
- Continuous: what capacity to install
Usually Multi-Site: consideration for transportation between production site/depots/customers
Product Model: use one Generic (tons, pounds, square meter)
Strategic Supply Chain questions ?
- Where should I build a production line ?
- Should I start serving clients in Asia ?
- How to structure my transportation/distribution network ?
Tactical Supply Chain Model ?
- Time: Multi-periods, days to months
- Given demand to be satisfied, and/or sales with maximum amount and price
- Production Capacity is given (including set-up/start-up/change-over costs
- Main decision is lot-size/production mix
- Possibly multi-level: some products are produced as input to others
Tactical Supply Chain questions ?
- What mix should I produce in each factory during next month ?
- Which contract should I use ?
- When should I plan the maintenance of my units ?
- What is the aggregate production plan to communicate to the plant manager ?
- What is the expected capacity usage/service level over the next months ?
- What is the expected lot size for the next month ?
Operational Supply Chain Model ?
- Time: Days to continuous
- Given demand to be satisfied
- Each production lines with its specificities is modelled
- Close to scheduling problem with:
- precedence constraints
- changeover time
- Product model: SKU Decision variable:
- machine assignment
- lot-size
- detailed production/transportation schedule
Operation Supply Chain questions ?
- What is the schedule for each production line at each moment for the next day ?
- In which order should the tasks be performed ?
- What is the precise routing of each vehicle in my transportation fleet ?
- And what should each individual vehicle carry ?
- What will be exactly the state of my stock at the end of the day ?
What is a graph ?
A graph is defined by:
- a set of Nodes (V)
- a set of Edges (E) with E included in V x V = a set of pairs (i, j) of nodes i, j included in V
What is a directed graph or digraph ?
A digraph is defined by:
- a set of Nodes (V)
- a set of Arcs (A) with A included in V x V = a set of ordered pairs (i, j) of nodes i, j included in A
What is the purpose of a graph ?
A graph is an abstraction used to model connections (edges, arcs) between agents (nodes).
A generic Strategic Supply Chain Model
SETS:
- S # set of suppliers
- P # set of production sites
- D # set of distribution center
- C # set of customers
VARIABLES:
- up, ud : quantity of goods annually handled by the corresponding site/center
- yp, yd : binary variable indicating if the corresponding site/center is open
- fsp, fpd, fdc : flow of goods between the corresponding entities
- wsp, wpd, wdc : corresponding binary variables indicating if the arc is open
PARAMETERS:
- Dc : demand at the customer site
CONSTRAINTS:
- Demand satisfaction: Σdfdc >= Dc for all c
- Flow Conservation & Capacity at Distribution and Production Site
How to estimate fixed costs ?
We have to compute the NPV of the investment.
How to extend to Multi-Item case ?
The typical case is when several product families share a common facility, therefore share the fixed cost.
For the model: just add an index set I and indices i for all variables and constraints (representing the different products).
How to extend to Multi-Periods case ?
Add set T for periods and associate index t to all variables & constraints.
Standard Capacitated dynamic lot-sizing model LS-C
- Discrete time horizon: set of periods indexed by t in (1..n)
- Demand Dt >= 0 for each period
- Production Capacity Ct for each period
- Production cost in period t is composed of two components
- a fixed cost ft
- a variable cost vt
- Holding costs ha only one variable component ht
- The objective is to decide the production level in each period in order to minimize production and holding costs while satisfying all customer demand on time
The model
Variables:
- xt >= 0: production in period t, with t in (1..n)
- st >= 0: stock at the end of period t, with t in (0..n-1)
- yt = {0, 1}: is one if there is a setup in period t (in this case production xt may be positive, but setup cost is incurred), and 0 if not

Lot-Sizing: network model

What if we can only produce in full batches?
We obtained this model by fixing the LS model with
xt = Ct* yt
► Just allow yt to be integer instead of binary.

What are the 3 types of capacity and how to model them ?
- C: varying capacity: capacity is different in each period
- CC: constant capacity: capacity is the same in each period
- U: Uncapacitated: capacity is so big that it is never binding
NB:
- lot-syzing problems with varying capacity are NP-Hard !
- lot-sizing problems with constant capacity or uncapacitated are polynomially solvable
- but uncapacitated problems are easier than capacitated ones

What is Backlogging ?
Service level is usually an important criterion/constraint in production environment, you want to:
- minimize cost, but still reaching a minimum service level
- otherwise it’s just less costly to produce nothing
Until now we’ve always assumed 100% service level (all demand is always satisfied on time. But sometimes, 99% service level might be acceptable and save a lot.
Therefore, backlogging is the satisfaction of the demand, but not necessarily on time, with some penalty.
The backlog at the end of period t is the amount of gods produced after t to satisfy some demand in t or before.
Lot-Sizing with Backlogging: Network model

What is the lead time?
In some applications, time periods are small so that if production of product i is started in period t, then it is only available for (own or external) consumption at time t + tlead.
If there is only one single item, if suffices to shift the demand earlier by t(lead) periods.
How to model Lead Time ?

What is Startup cost ?
Setup cost in incurred in each period where there is positive production. There is a start up cost in period t if there is no setup in period t-1 and there is a setup in period t.
What is a shutdown ?
There is a shutdown in period t if there is a setup in period t-1 and no setup in period t.
Suppose we have several products that share a common production line, how do we model the constraint ?
Joint Capacity Constraints
The most basic constraint is: sum xi,t <= C for all t.
The total quantity on the line is limited by a given capacity C.
This stands true as long as:
- x(i, t) represents items differing by colour only.
- sum xi,t <= Ct *for all t.
- For some reasons the production capacity can vary from one period to the next
- different number of shifts
- each period has different number of working days


