Open Roadstead Anchorage Ahead of a Deepening Low
MV Nordic Trader, a 22,000 dwt general cargo ship, is anchored in an open roadstead off a West African port awaiting a berth. Anchoring depth is 24 m, holding ground fine sand over clay, with 9 shackles in the water, one anchor down and engines on 30 minutes' notice in a SW 20 knot wind. The berth is expected in three to four days. The latest forecast: a tropical low at 8N 12W is expected to deepen to 985 hPa and pass 60 nm to the south-west of the anchorage in about 36 hours, bringing SW winds of 45-55 knots veering to W, a swell of 6-7 m and a duration of about 30 hours. The anchorage lies 4.5 nm off a low sandy coast with no natural shelter. The nearest sheltered anchorage is 45 nm to the north-east inside a river estuary with soft mud holding; it is six hours' steaming away and the port authority requires four hours' notice. There are 14,000 t of bagged cement in the holds and two of four holds have been partly worked. The crew has been at anchor for five days and the second mate is on his first contract.
1. Assess the ground tackle and the anchor's holding capability against the forecast.
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Scope. 9 shackles is 9 x 27.5 = 247.5 m of chain, but scope is measured to the hawse, not the waterline: 24 m depth plus about 8 m bow height gives 32 m, so 247.5/32 is roughly 7.7:1. That is fine in fine weather and marginal at 45-55 knots. With a 6-7 m swell the bow rises and falls through that range; at the top of the pitch effective depth is near 38 m, scope falls to about 6.5:1, and the chain will snub and jerk the anchor out of the ground in cycles.
Holding. A 22,000 dwt ship carries an anchor of roughly 3 t, with holding power in sand of perhaps four to six times its weight, so 12-18 t, plus chain catenary and friction. Wind load at 55 knots (28 m/s) on about 800 m2 of above-water area with Cd near 1.2 is 0.5 x 1.225 x 28^2 x 800 x 1.2, roughly 460 kN or 47 t, before yawing, current and wave forces. The anchor will drag.
Aggravating factors. The wind veers SW to W, so the ship swings through a wide arc and loads the cable from a new direction, which breaks the anchor out; sand over clay is progressively disturbed in a heavy swell; the anchorage is only 4.5 nm off a lee shore, so a one-mile drag already leaves no margin; and five days on station with an inexperienced OOW degrades the watchkeeping that would detect dragging early.
2. What is your decision - stay, shift to the estuary, or put to sea - and what is the reasoning?
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The decision is not to ride this out at anchor. Test it three ways.
Can the ground tackle hold? No. Wind load alone is about double the anchor's likely holding capacity, and the swell will snub it out. Is there sea room if it drags? No - 4.5 nm to a lee shore, and a drift of only 0.2 knots over 30 hours covers 6 nm. Can you leave later? No - weighing anchor and manoeuvring out of an open roadstead in 50 knots and 7 m seas is far more dangerous than doing it now in 20 knots.
Of the two exits, the estuary 45 nm NE is preferable to putting to sea, provided the entrance depth over the bar, the tide and the availability of an anchoring position are confirmed with the harbour master before weighing. Soft mud gives better holding, the entrance gives real shelter from W and SW, and six hours' steaming leaves ample margin inside the 36-hour window. Putting to sea is the fallback if the estuary is refused or the entrance depth is inadequate: in that case plan to stay well clear and on the safe side of the forecast track, with deep water to the south-west and room to heave to with the wind on the bow.
Execute now: give the port authority notice, secure the partly worked holds - bagged cement must be levelled and hatch covers secured or it will shift - make the engine ready for immediate manoeuvring, secure the deck and the anchor for sea, brief the bridge team, and report the decision and the reasoning to the company.
3. If, for any reason, the ship had to remain at anchor, what preparation and mitigation would you put in place, and what human-element issues would you manage?
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Ground tackle. Veer to 11 or 12 shackles to raise scope towards 9-10:1, keep the windlass brake set and a man on the brake in the heavy phase, and use the engine to ease the snubbing loads when the chain comes up slack. Prepare the second anchor but do not automatically let go - two cables in a veering wind and a swell can foul.
Watchkeeping and detection. Run a continuous anchor watch with position fixing at short intervals, a dragging alarm set on the GNSS, a hand-bearing on two transits, and an echo-sounder watch; agree a clearly understood dragging drill with engine, cable and VHF actions. Call the master early, not late.
Machinery and readiness. Engine on immediate notice rather than 30 minutes, steering gear tested, both anchors ready if the plan requires, and the windlass, brake and chain stopper checked. Have the intended escape course to deep water laid off and briefed.
Human element. Five days on station, a second mate on his first contract and a forecast like this is a fatigue and supervision problem before it is a seamanship problem. Pair the second mate with an experienced officer, split the night into shorter watches, ensure meals and rest are protected, and brief the whole bridge team on the plan and the abandoning-anchorage criteria in writing. Language and confirmation loops matter more in a stressed, multinational crew: have each officer repeat back the dragging drill and the escape course. If any of this cannot be assured, that is a further argument for leaving before the weather arrives.