Mooring Loads Under Storm Surge for Small Craft: A Design Bound for Harbour Moorings When the Water Rises

Storm surge defeats harbour moorings in two distinct ways: a floating pontoon rises with the water until it lifts off the tops of its guide piles, and a vessel tied to fixed bollards rises above them until its lines, first slack and then suddenly taut, are stretched by the vessel's own buoyancy. This paper computes both failure levels for one representative small-craft berth — a 10.2 m, 6,500 kg vessel on 16 mm polyester lines (breaking load 58 kN) rigged with 0.29 m of slack to fixed bollards, alongside a floating pontoon guided by piles whose tops stand 2.40 m above reference water level — and takes the lower level as the arrangement's surge limit, which is the bound this paper originates. The pontoon disengages from its piles at a surge of 1.90 m, at which point the mooring lines are still carrying essentially no load. The lines alone would reach their design limit of 50% of breaking load at 3.54 m of surge in calm conditions and at 3.44 m under a coincident 25 m/s wind (3.29 m at 40 m/s); line breaking load is reached at 4.24 m. Tension onset is deceptively late: the lines remain slack until the surge reaches 2.70 m, then load rises steeply, from 9.55 kN at 3.0 m to 47.81 kN at 4.0 m. Because the two failure levels are independent, money spent on only one side buys nothing beyond the other: longer lines ($408) or snubbers ($532) alone gain exactly zero, a 1.5 m pile extension ($2,480) raises the limit to 3.40 m, and only the combination of taller piles with line upgrades reaches 4.66-4.90 m, at $8,008-$8,540 per berth. The model is static and quasi-static; wave dynamics, surge currents and debris impact are excluded, and that exclusion matters — the bound is a still-water ceiling on capability, not a storm survival prediction.