Classifies spare parts demand by occurrence frequency and volume stability to help shipowners build more informed forecasting, procurement, and inventory strategies.
SBC demand classification is also known as:
This classification addresses a core problem with spare parts: demand is discontinuous, and simple averages can be misleading.
Many spare parts see no demand for long periods, yet when they are needed, the impact on operations can be significant.
Researchers therefore use ADI (Average Demand Interval) and CV² (squared coefficient of variation of demand size) to identify demand patterns and help select appropriate forecasting methods, inventory levels, and procurement strategies.
Using ADI and CV², items can be grouped into four demand types:
This method is commonly applied to ship spare parts, maintenance materials, low-frequency consumables, and slow-moving inventory.
The goal is to support better choices for forecasting methods, inventory levels, and procurement strategies.
Represents how often, on average, a non-zero demand occurs.
Measures how much demand size fluctuates when demand does occur.
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This chart classifies spare parts demand into four types using ADI (Average Demand Interval) and CV² (demand size variation). Each blue dot represents one spare part; the red lines mark commonly used classification thresholds. The result helps shipowners choose appropriate forecasting methods, inventory levels, and procurement strategies.
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| Quadrant | Meaning |
|---|---|
Smooth demand |
Demand occurs frequently with stable quantities |
Erratic demand |
Demand occurs frequently but quantities vary widely |
Intermittent demand |
Demand occurs infrequently but quantities are relatively stable |
Lumpy demand |
Demand occurs infrequently and quantities are unstable |
Place the quadrant image at ../assets/images/sbc/03-quadrants.png, or update the src above to point to your file.
In practice and research, the following thresholds are commonly used to split demand into four types:
A higher ADI means demand occurs less often; a higher CV² means each demand event varies more in size. For ship spare parts, this helps determine whether conventional average-demand management is appropriate.
| Demand type | ADI | CV² | Meaning | Ship spare parts examples |
|---|---|---|---|---|
Smooth demandSmooth demand |
Low | Low | Demand occurs frequently with stable quantities. | Common filters, O-rings, cleaning consumables |
Erratic demandErratic demand |
Low | High | Demand occurs frequently but quantities vary widely. | Maintenance bolts, gaskets, oil seals |
Intermittent demandIntermittent demand |
High | Low | Demand occurs infrequently but quantities are relatively stable. | Specific sensors, solenoid valves, relays |
Lumpy demandLumpy demand |
High | High | Demand occurs infrequently and quantities are highly unstable. | Special main-engine spares, pump assemblies, critical spare kits |
This pattern resembles conventional inventory items. Moving averages, exponential smoothing, reorder points, or simple safety-stock logic often work well.
Typical shipboard items: daily consumables or scheduled maintenance supplies.
| Strategy | Description |
|---|---|
| Automatic replenishment | Generate purchase suggestions when stock falls below minimum |
| Batch procurement | Reduce unit cost and freight through bulk orders |
| Standard stock levels | Set standard min/max inventory per vessel |
| Supplier consolidation | Reduce fragmented suppliers to improve procurement efficiency |
These items are requested often, but quantities swing widely. Relying on average demand alone can underestimate peak requirements.
| Strategy | Description |
|---|---|
| Dynamic safety stock | Adjust safety stock by volatility, not averages alone |
| Flag exceptional demand | Separate major overhauls, incidents, and project demand from routine consumption |
| Fast-supply agreements | Establish quick quote and delivery mechanisms for frequently used but volatile items |
| Fleet-wide procurement | Pool orders across vessels when multiple ships use the same items |
| Periodic consumption review | Review quarterly or semi-annually for equipment aging or repeat failures |
When an item is rarely used but a stockout would cause downtime, delays, or class/PSC risk, maintain a minimum strategic safety stock.
| Strategy | Description |
|---|---|
| Minimum strategic stock | Keep a baseline quantity even when historical demand is low |
| Critical spare parts list | Have the technical department confirm criticality |
| Lead-time-based stocking | Longer lead times require more advance stocking |
| Verify supplier capability | Confirm availability with manufacturers/agents before an urgent need arises |
| Alternates and equivalents list | Reduce single-part-number supply risk |
| Fleet shared inventory | Hold high-value items at shore bases rather than on every vessel |
Examples: main-engine control sensors, generator protection relays, boiler control components, separator-specific modules, steering gear control parts, automation PCB boards.
These items combine infrequent occurrence with unstable quantities; statistical forecasting alone is often insufficient.
This is the category shipowners should scrutinize most closely.
Even when demand is rare, if a stockout could stop the vessel, disable the main engine or generator, affect steering gear, or create class or PSC exposure, manage by risk rather than forecast alone.
| Strategy | Description |
|---|---|
| Critical spare parts policy | Define which lumpy items require strategic stock |
| Minimum strategic safety stock | Retain baseline quantity even when historical demand is low |
| Shore-based central stock | Hold high-value items at company warehouses to support the fleet |
| Fleet shared inventory | Share high-value, low-frequency spares across same-type vessels/equipment |
| Consignment stock | Have suppliers hold inventory; pay only after use |
| Repair exchange program | Use exchange units to shorten downtime |
| Framework agreements | Agree price, lead time, and supply priority with manufacturers or agents |
| Alternate parts confirmation | Pre-confirm equivalent or interchangeable spares |
For lumpy critical spares, history may understate actual risk. Factor in criticality, lead time, off-hire exposure, and supply stability.
| Classification combination | Recommended procurement strategy |
|---|---|
| Smooth + low cost | Use automatic replenishment and batch procurement to reduce unit cost. |
| Smooth + high criticality | Use min–max inventory management and stable supply agreements with vendors. |
| Intermittent + low criticality | Buy to demand; avoid overstocking and obsolete inventory. |
| Intermittent + high criticality | Maintain minimum strategic safety stock to reduce operational risk from stockouts. |
| Lumpy + low criticality | Generally do not stock; keep only a small buffer when lead times are very long or supply is unstable. |
| Lumpy + high criticality | Establish a critical spare parts policy; evaluate shared inventory, consignment, or repair exchange. |
| High cost + long lead time | Use framework agreements or call-off contracts to balance lead time and cash flow. |
| Obsolescence or discontinuation risk | Plan last-time buys, confirm alternates, and manage phase-out. |
These values are often cited as fixed standards, but they are better understood as boundaries for choosing forecasting methods—not universal cut-offs for every industry.
In practice, do not base inventory strategy on SBC classification alone. Also consider:
Item price should not change how you classify demand, nor should it justify skipping a critical spare.
For shipowners, the first question is whether a spare is operationally, maintenance, or safety-critical; if it is required, plan how to obtain it.
Cost belongs in procurement optimization—onboard stock vs. shore pooling, consignment, framework agreements, or repair exchange—not in downgrading necessary spares.
SBC demand classification helps answer whether a spare part’s demand can be forecast reliably; the final inventory decision still requires criticality, lead time, off-hire risk, and supplier reliability.
These are Reay’s study notes on applying SBC demand classification to spare parts forecasting, procurement, and inventory management. If you spot an error, please contact me so the material can be improved: ReayHuang@gmail.com