Offshore engineering glossary

Added to the archive in 2026 · Reference note, not an original Bennett Offshore page

The restored Bennett pages were written for clients who already spoke the language: owners, rig managers, class surveyors. If you arrived from a Wikipedia footnote, some lines in the particulars tables will read like code. This glossary decodes the terms that appear most often on the archived pages, with a pointer to where each one shows up. Definitions follow the public reference articles they are drawn from, and the ORCA pages themselves where Bennett defined a term in its own words.

Platforms and vessels

Jack-up (self-elevating unit): A mobile platform with a buoyant hull and movable legs that can raise the hull above the sea while the legs stand on the seabed. Most drilling jack-ups have no propulsion of their own. See Jack-up rigs explained and the restored Jack-up Platforms page.
Liftboat: A self-propelled, self-elevating vessel with a large open deck and at least one crane, used to support offshore maintenance and construction. US liftboats fall under Title 46 of the Code of Federal Regulations. See Liftboats explained.
SEP (self-elevating platform): The name Bennett Offshore and the Dutch designer GustoMSC used for liftboat-type units. Other names for much the same vessel include MOU (Lloyd's Register), SESV, MPSEP and SEWOP. The restored menu groups the ORCA series under "SEP designs"; start at the overview.
MPSEP: Multi-purpose self-elevating platform, the term used by the operator Seafox and by Keppel. The ORCA 10000 page notes that the unit was originally called the KFELS MPSEP.
Semisubmersible: A floating unit that gets most of its buoyancy from ballasted pontoons below the surface, with columns supporting the deck. It has good stability and seakeeping, and a small waterplane area that makes it sensitive to weight changes. See Semi-submersible platforms explained and the restored Semisubmersibles page.
MODU: Mobile offshore drilling unit. The family covers jack-ups, submersibles, semisubmersibles and drillships. The IMO MODU Code is the international design and operating guideline for these units.
Spar: A floating production platform built around a large vertical buoyant cylinder, held by a spread mooring. Its deep draught keeps motions small and allows dry trees. Bennett's MinDOC was conceived as a multi-column alternative to the spar.
TLP (tension-leg platform): A floating platform held down by vertical tethers of high axial stiffness, which remove almost all vertical motion so wellheads can sit on deck.
FPSO: Floating production, storage and offloading unit: a vessel that processes hydrocarbons and stores oil until a tanker takes it away. A unit that only stores oil is an FSO.
WTIV: Wind turbine installation vessel, usually a self-propelled jack-up with a ship-shaped hull and a large crane. See Wind turbine installation vessels and the restored wind farm vessels page.
Accommodation platform (flotel): An offshore unit that provides living quarters, either temporarily for construction and maintenance crews or permanently bridge-linked to production platforms. Bennett's V-941 was drawn as an accommodation liftboat.

Legs, footings and jacking

Chord: One of the main vertical members of an open truss leg. Bracing ties the chords together, and each chord carries a toothed rack.
Rack and pinion: The jacking system used on most modern jack-ups. Motor-driven pinions in the hull climb racks fixed to the leg chords. Tubular legs more often use a pin-and-hole system.
Rack phase difference (RPD): The difference in elevation between the racks on the chords of one leg, caused by unequal chord loads when a footing sits on sloping or uneven ground. Excessive RPD wears teeth and guides and can crack welds or buckle braces.
Spudcan: The footing at the base of a jack-up leg, an inverted cone that spreads the load into the seabed and gives the leg resistance to lateral force. Designers check it for vertical, horizontal and torsional loads. The particulars tables give spudcan or pad area, for example 110.8 square metres per leg on the ORCA 10000.
Pad: The word the liftboat pages use for the footing at the base of each leg. The V-924 page lists pads 12 metres long, 8 metres wide and 1.55 metres deep.
Preload: Loading the legs beyond their working load, usually by pumping seawater into the hull, so the soil settles and any further penetration happens before the hull goes to working height.
Punch-through: A sudden, uncontrolled penetration of a leg into the seabed. In the 2019 Kristin Faye capsizing, divers found the port leg had penetrated about 40 feet in a punch-through.
Canhole: A depression in the seabed left by the leg of an earlier rig or liftboat. A footing landing on the edge of one can slide.
Air gap: The clearance between the underside of an elevated hull and the sea surface, chosen so waves, tide and current act on the legs and not on the hull.
Leg encircling crane (LEC): A crane built around one of the legs. The ORCA 3500-WIND lists a 600-tonne leg encircling main crane, and the V-924 lists two 200-tonne LEC units.

Weight, buoyancy and stability

Displacement: The weight of a vessel, found from the volume of water it displaces. Lightweight is the empty weight as designed. Load line displacement, as on the ORCA tables, corresponds to floating at the greatest draught allowed by the load line authority.
Draught (draft): The depth of the hull below the waterline, measured to the lowest part of the hull or another reference point. "Design draft" and "load line draft" on the restored pages are draughts at specified loading conditions.
Freeboard: The distance from the waterline to the upper deck where water could enter. For commercial vessels it is measured relative to the load line.
Variable load: The weight of equipment, consumables and deck cargo the unit can carry on top of its fixed weight. The ORCA pages define bare deck variable load as the variable load the hull can support with no superstructure fitted, meaning no quarters, cranes or helideck, and give a separate operating variable load for the afloat and elevated conditions.
Metacentric height (GM): The distance between a floating body's centre of gravity and its metacentre, a measure of initial stability. A larger GM means more resistance to heeling and a shorter, sharper roll.
Free surface effect: The loss of stability caused by liquid moving in a partly filled ("slack") tank as the vessel heels. A full tank is "pressed up". Operating manuals for floating units deal with it at length; see the restored marine operating manuals page.
Ballast tank: A compartment that holds water to control stability, trim, list, draught or hull bending. Semisubmersibles ballast down to work and deballast to transit; jack-ups use ballast water for preload.
Waterplane area: The area enclosed by the hull where it cuts the waterline. The Wikipedia article on semisubmersibles notes that their small waterplane area makes them sensitive to load changes, so onboard mass has to be trimmed to keep them stable.

Sea, motion and positioning

Six degrees of freedom: The motions of a vessel: surge, sway and heave along the three axes, and roll, pitch and yaw about them.
Sea state: The condition of the sea surface at a place and time, described by statistics such as wave height and period. The V-924 design speed is given "at sea state 3".
Significant wave height (Hs): Traditionally, the mean height of the highest third of the waves; today usually four times the standard deviation of surface elevation.
Seakeeping: How well a vessel suits the conditions it meets underway: its motions and how they affect its mission.
Azimuth thruster: A propeller in a pod that rotates to any horizontal angle, which makes a rudder unnecessary. Liftboats and WTIVs use them to manoeuvre and to position before jacking.
Dynamic positioning (DP, DP2): A computer system that holds a vessel's position and heading with its own thrusters, using position references and wind and motion sensors. Classification societies rate DP systems in equipment classes 1 to 3: class 1 has no redundancy, class 2 survives any single fault in an active component, and class 3 also survives fire or flood in one compartment. DP2 and DPS-2 on the restored pages refer to class 2. The Q4000 holds station on six azimuthing thrusters.

Class, rules and well work

Classification society: A non-governmental body that sets technical standards for ships and offshore structures, certifies new construction and surveys vessels in service. Owners need class to register and insure a vessel.
ABS: The American Bureau of Shipping, a classification society founded in 1862. ABS publishes its standards as the ABS Rules and Guides. The ORCA tables give their classification as "A1, SELF-ELEVATING UNIT, AMS, ACCU" preceded by a small Maltese cross symbol, reproduced on the restored pages as an image.
Well intervention and workover: Intervention covers any invasive work in a live well, such as wireline or coiled tubing operations. A workover is the larger job of pulling and replacing completion or production hardware to extend a well's life.
Coiled tubing: Continuous metal pipe, normally 1 to 3.25 inches in diameter, run into a well from a reel. Offshore, a coiled tubing spread takes more deck space than a wireline spread.
Plug and abandonment (P&A): Plugging a well and abandoning it at the end of its life. The ORCA series lists it among the roles the platforms could be fitted for.
Dry tree: A wellhead and valve assembly on the platform deck instead of on the seabed. The ATP Titan, designed on the MinDOC concept, was a dry tree drilling and production platform.

Missing a term? The jack-up primer explains many of them in context, and the history page places the projects in order.

Sources

Next in the archive: History of Bennett Offshore, 1997 to 2017 · Liftboats explained · Restored page: Innovations

Archive notes