Legs, spudcans and preload
When the National Transportation Safety Board looked into the loss of the liftboat Ram XVIII off Grand Isle in November 2018, it did not blame the crew, the hull or the jacking gear. It found that the industry had not given liftboat operators enough information about the seafloor they were standing on. The port leg had become unstable, and the investigators could not even say which of three things had happened under it: scour, a punch-through, or the collapse of an old canhole.
That finding sums up why the bottom end of a jack-up leg gets so much engineering attention. This note looks at the legs, the footings and the procedures that connect a self-elevating unit to the seabed. It complements Jack-up rigs explained, which covers the whole unit, and the restored jack-up primer and general consulting pages.
Two kinds of leg
The Bennett design pages show both families side by side. The ORCA 3500-WIND stands on tubular legs 100 metres long. The ORCA 4500-WIND, also on 100-metre legs, uses truss legs, and the ORCA 10000 has four triangular truss legs 106 metres long and 9.75 metres across.
A truss leg is a lattice: three or four vertical chords joined by a network of braces. Each chord carries a rack, and pinions in the jackhouse climb those racks. A tubular leg is a single large cylinder; the Wikipedia article on rack phase difference notes that units with tubular legs usually use a pin-and-hole jacking system instead of rack and pinion.
Spudcans and pads
The footing at the foot of each leg spreads the unit's weight into the soil. On drilling jack-ups it is usually a spudcan, an inverted cone that also gives the leg resistance against sideways force once it has pushed into the ground. Designers check a spudcan for vertical, horizontal and torsional loads. The Wikipedia article on spudcans cites a university report by Vlahos, Cassidy and Byrne on how spudcan footings behave in clay under combined cyclic loading.
The Bennett liftboat pages call their footings pads. The restored V-924 page lists pads 12 metres long, 8 metres wide and 1.55 metres deep on legs 100 metres long including the pads. The ORCA tables give spudcan area directly: 110.8 square metres per leg on the ORCA 10000.
Reading the seabed before you stand on it
Offshore geotechnical engineering differs from work on land in ways that matter here. Site investigation is expensive, so engineers often know less about the ground than they would like. Marine soils include materials that behave badly under load, such as calcareous sands and layers containing shallow gas. Structures carry large sideways loads from wind, waves and current compared with their weight, and cyclic loading can govern the design. The Wikipedia overview of the field cites a steel jacket off Australia whose piles had been designed for siliceous sand and turned out to stand in weaker calcareous sand; the remedial work was expensive.
For a jack-up or liftboat, the practical checks are a survey of the site and a review of what has stood there before. The survey looks for pipelines, debris and seabed features. The history check looks for canholes, the depressions left by the legs of earlier rigs and liftboats. A footing that lands on the edge of a canhole can slide into it and throw the leg out of line.
Preload, step by step
Preloading is the field test of the foundation. The liftboat routine described in the Wikipedia article on the type goes like this:
- Jack up only far enough to lift the hull clear of the tips of the significant waves.
- Fill the holds or preload tanks with seawater so the legs carry more than their working load.
- Hold that condition for several hours while the legs settle into the mud.
- Dump the water and jack up to working height.
Drilling jack-ups follow the same idea with their own ballast systems. The point of holding the hull low during preload is that if a leg does sink suddenly, the hull drops a short distance into the water instead of a long distance with the deck loaded.
Preload only tests the loads you apply. The NTSB report on the Kristin Faye, which capsized east of Venice, Louisiana, in September 2019, blamed preload procedures that did not account for shifting and loading the crane. The boat listed after workers extended one of its two bow cranes, and divers later found the port leg about 40 feet into the seabed.
Punch-through
A punch-through is a sudden, uncontrolled penetration of a leg into the seabed. In both the Ram XVIII and Kristin Faye cases investigators considered it, and in the second case divers confirmed it. The restored Jack-up Platforms page lists the related consulting work Bennett did for operators: detailed penetration analyses, SNAME 5-5A site assessments, punch-through recovery assistance, and leg and spudcan repair guidance, for locations from the Gulf of Mexico and the North Sea to Australia and India. Preload exists largely to bring on any punch-through while the hull is still close to the water.
Rack phase difference
The legs can also suffer from the ground in a slower way. When a spudcan rests on sloping ground, or slides as it passes through uneven layers, the chords of a truss leg carry unequal loads. Each rack shifts up or down by a small amount, the rack phase value, and the sum of those shifts is the rack phase difference. Too much of it adds stress between rack and pinion teeth, wears the racks against the leg guides and loads the welds between braces and chords. Past the allowable value, the damage list in the Wikipedia article includes cracked welds, buckled braces, sheared teeth and damaged jacking drives. The jack-up designer sets the RPD limit, and the crew watches it while jacking.
Bennett's own papers on the subject
The firm's engineers wrote about these problems for decades, and the restored technical papers page lists the titles; our note on how offshore engineers publish explains where they appeared. William T. Bennett, Jr. presented "Leg Loading and Unloading" at the first International Conference on the Jack-up Platform in London in 1985. Jacob H. Alford co-wrote papers on preloading independent-leg units at difficult seabed locations (1989) and on soil-structure interaction during preloading (Offshore Technology Conference, 1994). Three 2003 papers dealt with rack phase difference and with spudcan footprint interaction, and a 2004 paper described jack-up mobilisation in a complex soil environment using "Swiss cheese" mitigation. The same page notes that Bennett co-chaired the SNAME committee that drafted the original Recommended Practice for the Site-Specific Assessment of Jack-ups (T&R 5-5A).
Where this fits in the Bennett pages
The structural engineering page describes leg and hull strength work. The marine operating manuals page covers the written procedures, including preload, that a crew follows on location. The particulars tables on the ORCA pages give leg length, leg type and footing area, the three numbers that tell you most about how a design meets the ground. Terms used here are defined in the glossary.
Questions readers ask
What is a punch-through?
A sudden, uncontrolled penetration of a leg into the seabed, usually when a footing breaks through a strong soil layer into a weaker one below.
What is rack phase difference?
When the chords of a truss leg carry unequal loads, their racks shift slightly out of line. The sum of those shifts is the rack phase difference, and too much of it overstresses braces, welds and jacking gear.
What is a canhole?
A depression left in the seabed by the legs of an earlier rig or liftboat. A new footing landing on its edge can slide into it.
Sources
Next in the archive: The Ocean Ranger disaster · Studying naval architecture · Restored page: Structural engineering