Under-Keel Clearance and the Capital Dredging Decision at a Coal Terminal
A dedicated coal terminal is served by a 6.2 km approach channel and a 1,100 m by 260 m turning basin. The berth pocket is charted at 12.4 m below Chart Datum and the largest regular caller draws 14.0 m. At the 8-knot transit speed squat is calculated at 0.8 m, and heel, wave-induced motion and reduced clearance in the swell-exposed outer reach add 0.5 m. The port under-keel-clearance policy requires a net UKC of 10% of draft plus a 0.15 m survey allowance. Siltation between the twice-yearly surveys reaches 0.4 m in the outer 2 km. Spring tidal range is 4.2 m and neap range 2.6 m, and a 1.5-knot cross-current restricts entry during the ebb. Capital dredging of the channel and berth to 14.5 m CD is priced at USD 28M and would take 14 months, during which the berth operates at 60% capacity. The terminal moves 3.2 Mt/yr under a 25-year concession with a net margin of about USD 8/t, and the concessionaire is liable for demurrage when vessels are delayed.
1. Calculate the required water depth and determine whether the vessel can enter today, and after dredging to 14.5 m CD, on spring and on neap tides.
Read the worked answer
Required depth stack (all in metres):
| Component | Value |
|---|---|
| Static draft | 14.00 |
| Squat at 8 knots | 0.80 |
| Heel, wave motion, outer-reach allowance | 0.50 |
| UKC = 10% of draft | 1.40 |
| Survey allowance | 0.15 |
| Siltation since survey | 0.40 |
| Required depth | 17.25 |
Today (charted 12.4 m):
- Spring: 12.40 + 4.20 = 16.60 m → 0.65 m short.
- Neap: 12.40 + 2.60 = 15.00 m → 2.25 m short.
The vessel cannot legally enter even on a spring tide. Because the terminal is clearly operating at 3.2 Mt/yr, the vessel must already be part-loaded or the policy is being applied with informal discretion - which is exactly the risk the port has been carrying.
After dredging to 14.5 m CD:
- Spring: 14.50 + 4.20 = 18.70 m → 1.45 m margin, comfortable.
- Neap: 14.50 + 2.60 = 17.10 m → 0.15 m short.
So a 14.5 m channel delivers access on all but the smallest neaps; the access window requires a tide of at least 17.25 − 14.50 = 2.75 m. Note also that the 0.4 m siltation allowance is a worst case immediately before survey and the 0.5 m motion allowance is concentrated in the outer 2 km, so the critical section is the outer reach, not the berth pocket. Those two facts are where the cheapest depth can be bought.
2. Evaluate the options - full capital dredging, localised deepening, operating on tidal windows, or part-loading - and recommend one.
Read the worked answer
Full dredging to 14.5 m. USD 28M capex plus 14 months at 60% capacity: lost throughput ≈ 3.2 × 0.40 × 14/12 ≈ 1.49 Mt, worth about USD 11.9M at the stated margin. Real cost ≈ USD 40M. For that money it removes all of the 0.65 m spring shortfall but leaves the 0.15 m neap gap.
The 0.15 m neap gap is worth very little. A capesize of this draft has a waterplane of roughly 11,500 m², so TPC ≈ 118 t/cm. Fifteen centimetres is about 1,800 t of cargo. At ~20 calls/yr, dead freight is ~36,000 t, about 1.1% of throughput, or USD 0.3M/yr, which capitalises at 8% over 25 years to roughly USD 3M. Buying that 0.15 m by uniform over-dredging is poor value.
Localised deepening. Deepen only the outer 2 km (where the wave-motion and siltation allowances concentrate) by a further 0.5–0.6 m, plus the berth pocket to 15.0 m. This is perhaps 25–30% of the full dredge volume, far shorter downtime, and it closes the neap gap.
Tidal windows and part-loading. Viable for the residual small neaps, and already effectively in use. Formalising it (real-time tide gauges, a documented decision rule, a published UKC policy for neap periods) converts an informal practice into a managed risk.
Recommendation.
- Dredge the channel and berth to 14.5 m CD as the base case, with the outer critical reach deepened to 15.1 m.
- Shorten the outage by dredging the outer reach first and sequencing berth pocket works in a shutdown window; investigate whether the 14-month estimate is driven by disposal-site distance or by plant availability.
- Adopt a real-time tide and UKC management system with a written policy signed by the harbour master, so that neap restrictions are a documented operating mode rather than a breach.
- Review the policy (not the depth) at two years with the operator's actual draft and motion data.
3. The works are to be re-measured. How should the dredging be specified, measured and paid for so that the authority does not pay twice for the same material?
Read the worked answer
The core risk. The 0.4 m siltation layer is material that was not there at the last survey. If payment is by hopper volume on a re-measurable basis, the authority pays full rate for dredging its own silt, year after year, and the contractor has no incentive to minimise overdredge or re-handling.
Specification and measurement measures.
- Define payment sections and the design profile precisely, referenced to Chart Datum, with an independent pre- and post-dredge multibeam survey by a surveyor appointed by the authority.
- State an allowable overdredge (commonly 0.3–0.5 m below design, with side slopes) and pay it at a reduced rate or not at all; pay nothing outside the defined prism.
- Pay on in-situ volume derived from survey difference, not hopper volume, or if hopper measurement is used, calibrate the dredger's hopper and dredge-depth monitoring and audit it.
- Insert a re-dredge clause: no payment for material removed from a section that the contractor has already been paid for if it returns within a defined period (say 12 months), unless caused by the authority's own works.
- Set a siltation allowance as a separate, clearly identified provisional sum with a stated rate, so the authority can see the cost of silt separately from the cost of the design prism.
- Specify disposal-site conditions, environmental windows and turbidity limits, and allocate the risk of contaminated or unexpected material to a defined provisional sum.
- Require the contractor to submit a dredging method statement with production rates and plant, so the 14-month programme and its 60% capacity assumption can be tested before award.