Marine Spare Parts Inventory: Think in Port Calls, Not Days

A missing part on board is not resolved by a call to the supplier. It is resolved by a port call, a freight forwarder, a ship's agent and a customs clearance. The dominant constraint is not the price of the part, it is the logistical window that allows it to be received on board.

Building an enormous stock is not the answer either: storage capacity on board is finite, every crate has a weight, and a store room does not get any bigger. Managing MRO inventory on a ship means arbitrating continuously between three constraints: the risk of the vessel being immobilised, the capital tied up, and the space available.

The method itself does not change: ABC classification, safety stock, reorder point. What changes is the unit of measure. Ashore, you reason in days of supplier lead time; at sea, you reason in port calls. This article translates each part of the method for a ship in service, with its mandatory spares, its ship-shore split, its dormant stock and its dry-dockings.

The lead time that matters is not the supplier's

Counting in port calls, not in days

Ashore, the supplier quotes three weeks and you size the minimum stock on three weeks of consumption. On board, that reasoning does not hold. The supplier may ship within forty-eight hours: if the vessel is at sea and the next port call able to receive the goods is seventeen days away, the real lead time is seventeen days. And if the agent in that port cannot clear customs in time, it becomes the lead time of the call after that.

A ship's reorder point must therefore be expressed as a number of port calls, not a number of days. That is the structural difference with shore-based inventory management, and it changes every calculation downstream. A part that needs two port calls to replenish does not carry the same alert threshold on a vessel making weekly coastal calls as on a deep-sea service calling once every three weeks. Same item, same price: two different reorder points.

What the lead time of a part actually covers

Obtaining a part on board breaks down into segments that have nothing to do with manufacturing:

• The supplier lead time, from order to goods ready
• The transport leg to the port of call, which depends on the mode chosen and the weight of the part
• The suitability of the call: not every call can receive freight, some are too short, others are at anchor
• Customs clearance and local formalities, whose duration varies widely from port to port
• Delivery alongside and actual receipt by the ship

A single one of those segments going wrong pushes the part to the next call. An onboard CMMS must therefore allow a lead time to be recorded in port calls and revised whenever the rotation changes, as our article on spare parts management at sea sets out in detail.

Classifying parts: ABC and dual criticality

The three classes applied to the ship's store

ABC classification remains the foundation: segment line items by economic weight so that management effort is concentrated where it does the most good.

Class A, the structural spares. A minority of line items carrying most of the value of the store. Turbocharger rotor, cylinder head, pump shaft, shaft generator: high unit cost, and an absence capable of immobilising the vessel. Individual tracking, regular review with the supplier, and safety stock calculated part by part.

Class B, the regularly consumed spares. Bearings, mechanical seals, filter elements, contactors. Their consumption is predictable because it is tied to the planned maintenance system. They should be driven by automatic reorder points, with no manual intervention as long as nothing unusual shows up.

Class C, the consumables. Fasteners, standard O-rings, welding electrodes, common cable: the majority of the lines in the store for a marginal share of its value. Generous batches, wide thresholds. Applying Class A control to them means spending chief engineer's time to protect a negligible financial risk.

Dual criticality: value and replenishment lead time

The classic mistake is to stop at value. A part can be economically insignificant and operationally critical because it is proprietary or single-source. A controller board costing a few hundred euros, available only from the original manufacturer with several months' wait, is not managed like a box of bolts. The classification must therefore cross two axes: value, and replenishment lead time expressed in port calls.

It is this quadrant that produces most of the avoidable immobilisations. A useful CMMS must let these items be flagged independently of their value, otherwise they slip under the radar of a purely accounting analysis. Setting the thresholds is the job of the inventory management module, which must accept both axes.

Mandatory spares: stock that escapes classification

Part of the ship's store follows no turnover logic at all. Certain items must be held on board under the regulations applicable to the vessel or the requirements of its classification society. They enter the store as an obligation, regardless of any probability of consumption.

Economic ABC classification does not apply to these items. A mandatory spare that has not moved in eight years is not dormant stock: it is a condition of keeping the certificate. Disposing of it because a report flags it as immobile exposes the vessel to a non-conformity.

The right approach is a separate category in the CMMS, with three rules.

• Automatic exclusion from turnover reports. These items must never appear among the candidates for disposal.
• Recording the source of the obligation. Each line carries the reference of the document that requires it, so the link is verifiable at an audit.
• Tracking physical validity. The obligation is for a part fit for use, not merely a part present: a mandatory spare that has expired does not meet the requirement.

The exact nature of the spares required, their quantity and any exemptions depend on the vessel and its area of operation. These points are confirmed with the flag State administration and the ship's classification society, never by analogy with another vessel in the fleet. The documentary logic behind that traceability is the one described in our article on the ISM Code and maintenance compliance.

Minimum stock, safety stock and reorder point

Minimum stock: covering the logistical window

Minimum stock is the quantity consumed between the order leaving and the goods being received alongside. The formula is the classic one, with a single substitution: supplier lead time becomes port call lead time.

Minimum stock = average consumption per port call × number of port calls needed to replenish

Take the main engine lube oil filters. The vessel uses six per call on average, scheduled oil changes included, and the supply chain takes two calls. The minimum stock is therefore twelve filters. Reasoning per call matches the real rhythm of operations and absorbs variations in speed and routing.

Safety stock: absorbing the call that is dropped

Minimum stock assumes everything goes to plan. In practice a call can be dropped for commercial reasons, a port skipped because of weather, a package held in customs, and a breakdown can consume in one week what was budgeted for a quarter. Safety stock absorbs those gaps. Its size should be proportionate to four factors:

• The consequence of running out: loss of redundancy, operational restriction, vessel immobilised
• The regularity of the rotation: a liner service allows a thinner cushion than tramping, where the next call is unknown
• The reliability of the logistics chain in the ports actually visited
• The variability of consumption, depending on whether the part is tied to a maintenance interval or to chance

For a Class A part whose absence immobilises the vessel, a safety stock equal to a full call of consumption is justified; for a consumable available everywhere, it can be cut to the minimum. What it must never be is a uniform percentage applied across the whole store: the surest way to over-invest in the trivial and under-protect the essential.

Reorder point: the trigger threshold

The reorder point, or alert level, automatically triggers the purchase request in the CMMS.

Reorder point = minimum stock + safety stock

On our filters: twelve units of minimum stock, six of safety stock, giving a reorder point of eighteen. As soon as the store drops to that level the purchase request goes out, and the delivery arrives while six filters remain on board.

The difference with a shore store appears the moment the alert fires. Ashore, you order and you wait. On board, the alert has to be matched against the schedule of calls: ordering a heavy part for a six-hour call at anchor makes no sense. That cross-check is what separates a marine CMMS from a generic inventory package.

The ship, shore and supplier split

What has to stay on board

The question is not only how much to hold, but where. A ship's store is not extensible, and every kilogram embarked is deducted from the vessel's useful capacity: weight and volume are decision criteria just as much as price. What justifies a place on board comes down to three cases:

• The part whose absence immobilises the vessel or degrades its safety, whatever its price
• The part consumed regularly under the planned maintenance system
• The part required by the applicable regulations or by the classification society

Everything else can legitimately be pushed ashore: a heavy, expensive and rarely used part has no place on board if it can be delivered at the next suitable call.

What belongs in the owner's shore store

The shore store does something the ship cannot: it pools. A Class A part common to several vessels of one series does not need to exist in as many copies as there are hulls. One unit ashore, shipped to the call of whichever vessel needs it, covers the whole series.

That shift depends on two conditions: an up-to-date view of every vessel's stock from head office, and a purchasing chain able to ship quickly to any port, which is a matter of the procurement function and the quality of the agency network. And there is a third option, too often forgotten: not holding stock at all, and negotiating an availability commitment from the supplier in the ports on the service.

Transfers between sister ships

This is the most profitable and the most neglected lever available to a multi-vessel owner. Two vessels of the same series share most of their parts list. When one needs an item the other holds in duplicate, a direct transfer almost always beats a new order: the part exists, it is the right one, and it has already been paid for.

In practice it is rarely done, for a simple reason: nobody knows what the next ship holds. Stocks are kept in files that do not talk to each other, and the chief engineer of the first vessel has no idea that the seal he has been waiting six weeks for is sitting in the store of the second.

This is first of all a tooling question: a single fleet repository where the stores of every vessel can be searched in one query, with real quantities and the date of the last movement. Then an explicit rule is needed: who decides the transfer, how the value is recharged between vessels, and who replenishes the donor ship. Without it, no chief engineer will agree to give anything up. The natural moment to organise these movements is when two vessels cross in the same port, which can be planned as soon as the rotations are known.

Clearing out dormant stock

A ship's store accumulates. Twenty years of service leave behind parts for equipment removed long ago, duplicate purchases made by two successive chief engineers, emergency orders that arrived after the repair, and leftovers from dry-dockings. Nobody disposes of them, because nobody has the mandate to decide that a part will never be used.

Identifying by absence of movement

The first task is mechanical: pull the list of items with no movement for several years, excluding mandatory spares from the outset, then cross it with the vessel's equipment register. A part whose original equipment was removed or replaced during a dry-docking has no reason to remain on board: it occupies space for a need that no longer exists.

Weighing book value against space occupied

Then comes the decision, and this is where things stall: an item that is expensive on the balance sheet is hard to write out of stock, because doing so records a loss. But book value is not value in use. A part that no longer matches any equipment on board is worth nothing.

Three criteria settle it. Can the part still serve on another vessel in the fleet, in which case it goes ashore or to a sister ship. Does it have a buy-back value with the supplier. Does it occupy space the ship needs for active stock. That last criterion is usually the decisive one and the least often stated: a metre of racking taken up by a useless part is not available for a critical spare.

Phantom stock: the expired critical part

The most dangerous case is not the useless part, it is the useful part that is no longer fit for service. Seals, elastomer components, membranes, batteries and certain chemicals age in the store even when they are never used. A packing set kept twelve years in a hot alleyway will not do its job on the day it is fitted.

An expired critical part is phantom stock: it appears on the register, it is reassuring, and it protects against nothing. Worse, it blocks the replacement order, because the system counts it as available and raises no alert.

The remedy is easy to state: age-sensitive items should carry a shelf-life date in the CMMS, and the system should alert as the date approaches, exactly as it alerts on a quantity threshold. Storage conditions matter too: an overheated or damp store shortens the life of everything in it.

The dry-docking kit: project stock

A dry-docking is not prepared out of the running operational stock. It is prepared months ahead, with a dedicated list built from the work specification: parts to be renewed, yard consumables, anodes, inspection gaskets, treatment products, and spares to anticipate the work discovered once the job opens up.

This project stock follows the opposite logic to the running store: it has no reorder point, no turnover and no safety stock, it has a date. The items must be at the yard at the right moment, neither too early, when they are in the way, nor too late, when they hold up the schedule and extend the time out of service.

Confusing the two is expensive in both directions. Raiding the dry-docking kit to keep the ship running empties the yard preparation without anyone noticing before the day of the strip-down. Conversely, tipping the leftovers back into the running store without sorting them feeds the dormant stock directly.

Good practice is to isolate the kit in the CMMS under a separate project reference, with its own list, its own reservations and its own goods-receipt tracking, then to deal explicitly with the leftovers at the end of the job: what goes into the running store, what goes ashore, what is scrapped. Our annual haul-out checklist sets out how the work specification, the supply chain and traceability fit together.

What the CMMS has to deliver

Linking work orders to consumption

Everything else depends on this. Each time a work order is closed, the parts consumed must be taken out of the store in the same movement. Without that link, average consumption figures are wrong, reorder points rest on invented data and the theoretical stock becomes unusable. On board, it means data entry that works in the engine room, without a permanent connection, in a few taps.

Triggering the order at the right moment

The system must watch the thresholds continuously and generate the purchase request without human intervention, but also match that alert to the schedule of calls. Anticipating the requirements of the planned maintenance system also allows orders to be grouped and cuts down on emergency shipments, always the most expensive.

Seeing the fleet, not just the ship

Finally, the CMMS must give the owner a consolidated view: every store in one repository, items with no movement, mandatory spares kept separate, age-sensitive parts approaching their shelf-life date, and duplicates between sister ships. That view is what turns a collection of independent stores into a single stock.

Conclusion

The method does not change when you move from the quay to the ship: ABC classification, safety stock and reorder point remain the right tools. What changes is the unit they are expressed in. The relevant lead time is not the supplier's but that of the next port call able to receive the goods, and the reorder point is counted in calls.

From there, four distinctly maritime pieces of work determine the quality of the store: keeping mandatory spares separate, since they escape turnover analysis; deciding the split between the ship, the shore store and the supplier, weighing volume and weight as much as price; clearing out dormant stock regularly, remembering that an expired critical part is phantom stock; and running the dry-docking kit as project stock. Across a multi-vessel fleet, transfers between sister ships remain the most accessible gain, provided there is a single fleet repository and a clear rule to govern them.

To look at these questions on your own fleet, get in touch with our team.