Heavy lift cargo is a single unit too heavy for normal port equipment: transformers, reactors, plant modules, small craft or turbine components. In Indonesia, this kind of cargo often travels on multipurpose (MPP) vessels using their own cranes to reach smelters, power plants and infrastructure projects. Planning is different from general cargo because the ship itself moves with the load during the lift.
Ship cranes: SWL, outreach and hook load
A ship crane has no single capacity figure. Its SWL (safe working load) depends on outreach, the horizontal distance from the crane's slewing centre or the ship's centreline to the hook. The further the outreach, the lower the SWL. An illustrative table for an MPP with two 250 t cranes:
| Configuration (illustrative) | Outreach | SWL |
|---|---|---|
| Single crane | up to 18 m | 250 t |
| Single crane | 24 m | 180 t |
| Single crane | 32 m | 100 t |
| Two cranes in tandem | up to 16 m | 450 t |
These are examples only; use the load chart and certificates of your own ship. What you compare with the SWL is the hook load: cargo weight plus rigging (lifting beam, slings, shackles and, depending on the load chart, the hook block). Check two radii as well: the pick-up position on the quay and the landing position in the hold or on deck. The crane SWL calculator gives a quick first check.
Tandem lifts and load sharing
In a tandem lift, two cranes lift one item, usually through a lifting beam. The load on each hook depends on where the centre of gravity (CoG) sits between the hooks. Example: a 400 t hook load on a lifting beam with the hooks 8 m apart. The CoG is 0.5 m off the middle, so it is 3.5 m from hook A and 4.5 m from hook B.
- Hook A = 400 × 4.5 ÷ 8 = 225 t
- Hook B = 400 × 3.5 ÷ 8 = 175 t
Hook A carries more because it is closer to the CoG. If the tandem SWL per crane is below 225 t, the plan has to change. The combined CoG of cargo and rigging can be found with the combined CoG calculator; the moment method is explained in centre of gravity and weight distribution.
Stability with a suspended load
The moment the cargo leaves the quay, two things happen together:
- KG rises. The weight acts at the crane head, not at its physical CoG. The new KG is KG = (Δ·KG + w·h) ÷ (Δ + w), where h is the height of the crane head above the keel.
- A heeling moment appears. A load at outreach d from the centreline creates a moment w·d. For small angles, tan θ = w·d ÷ (Δ_new × GM_new).
GM shrinks at exactly the moment the heeling moment is largest. That combination is what makes heavy lifts risky. GM itself is explained in metacentric height (GM).
Worked example: heel during the lift and ballast compensation
Illustrative MPP: Δ = 12,000 t, KG = 7.00 m, KM = 9.60 m, so GM = 2.60 m (KM is taken as constant and free surface is ignored to keep the example simple). Hook load 220 t, crane head 30 m above the keel, outreach 18 m from the centreline towards the quay.
- New Δ = 12,220 t; with a TPC of 24 t/cm the ship sinks about 220 ÷ 24 ≈ 9 cm.
- New KG = (84,000 + 6,600) ÷ 12,220 = 7.41 m, so GM = 9.60 − 7.41 = 2.19 m.
- Heeling moment = 220 × 18 = 3,960 t·m.
- Uncompensated: tan θ = 3,960 ÷ (12,220 × 2.19) ≈ 0.148, θ ≈ 8.4°.
An 8.4° heel is well beyond the usual operating limit of a ship crane. Full compensation needs 3,960 t·m, for example by moving 220 t of ballast between anti-heeling tanks 9 m to port and 9 m to starboard of the centreline (an 18 m lever). In practice, part is moved before the lift (pre-heel) and the rest is pumped as the load moves.
| Stage (110 t pre-heel) | Δ (t) | GM (m) | Net moment (t·m) | Heel |
|---|---|---|---|---|
| Before lift | 12,000 | 2.60 | 1,980 away from quay | ≈ 3.6° away from quay |
| Load hanging at 18 m outreach | 12,220 | 2.19 | 1,980 towards quay | ≈ 4.2° towards quay |
| Slewed to centreline, ballast returned | 12,220 | 2.19 | 0 | upright |
| Cargo landed in hold (VCG 5.0 m) | 12,220 | 2.64 | 0 | upright |
The angles are not symmetrical because GM drops while the load hangs. Once the cargo is landed, KG falls to (84,000 + 1,100) ÷ 12,220 ≈ 6.96 m. Partly filled anti-heeling tanks also create a free surface effect that has to be included in the real calculation. The basic GM calculator is handy for a quick check.
Grillage and loads on the structure
Heavy lift cargo is rarely landed straight on a tank top or hatch cover. A grillage of steel beams or hardwood spreads the load into girders, floors and bulkheads and gives room to weld stoppers. Check:
- Uniform and point loads against the limits in the loading manual (see deck and tank top strength).
- That the cargo bears on the support points specified by the manufacturer (saddles, skids or pads).
- Hatch cover strength when cargo goes on deck, including the loads from the sea-fastening.
Sea-fastening, lifting points and CoG
Sea-fastening holds the cargo against roll, pitch and heave accelerations on the voyage, plus wind and sea loads on deck. It may use welded stoppers, chains, wire or a combination. Design forces come from the assessment method in the CSS Code, the ship's Cargo Securing Manual, or the marine warranty surveyor's criteria when the project is insured to a standard such as DNV's. Size it first with the sea-fastening calculator and the lashing calculator.
Lifting points (lugs, trunnions) must be certified and match the rigging plan. The cargo CoG must be known and marked on the unit; a wrong CoG makes the item tilt when lifted and shares sling loads in unexpected ways. Flat sling angles also increase the force in every sling.
Common mistakes
- Comparing only the cargo weight with the SWL, forgetting rigging and the lifting beam.
- Using the cargo's stowed KG for the lift stage. While hanging, the weight acts at the crane head.
- Ignoring the quay. A ship heeling towards the quay can crush fenders or bring the boom close to shore structures; check boom clearance from buildings and other cargo too.
- Forgetting tide and moorings. Changes in draft and heel tighten or slacken the mooring lines during the lift, so the crew must stand by to tend them.
- Ignoring wind. Bulky items such as modules or turbine blades catch the wind and swing; set a wind speed limit and use taglines.
Lift plan and coordination
Every heavy lift should have a written lift plan covering hook load, radius, slewing sequence, wind limits, ballast sequence, communication between crane operator, bridge and pump room, and stop criteria. STOWAGE.ID Vessel Stowage and the heavy lift stowage page help shore-side planners build scenarios and check the first numbers. It is decision support: the ship's class-approved loading computer, stability booklet and crane manual remain the official reference, and the master decides.