Offshore rig types at a glance
- MODUs (mobile)
- Jackup, semisubmersible, drillship, submersible/barge — moved rig to rig
- Fixed/compliant structures
- Fixed platform, compliant tower, TLP, spar — installed at one field
- Bottom-supported
- Jackup & submersible rest on the seabed (shallow water)
- Floaters
- Semisubmersible & drillship float (deep / ultra-deepwater)
"Offshore rig" covers two different things: mobile offshore drilling units (MODUs) that drill a well and then move on, and fixed or compliant production structures that are installed over a field and often carry their own drilling capability. The key engineering question for all of them is the same: in a given water depth, how does the structure stay in position against waves, wind, and current? The answer is what separates the types.
The mobile drilling units (MODUs)
MODUs are the rigs most people picture when they think "offshore." There are four:
- Jackup. A bottom-supported unit that floats to location on its hull, then jacks three or more legs down to the seabed and lifts the hull above the waves. Jackups work in shallow water, roughly up to 120–150 m. They're the most common offshore rig type. Full jackup guide →
- Semisubmersible. A floater that gets its buoyancy from large submerged pontoons. Ballasted down so most of the hull sits below the surface, it rides steadily through swell and is held by mooring lines or dynamic positioning. Semis work in deepwater.
- Drillship. A ship-shaped, self-propelled vessel that drills through a moonpool in its hull and holds station with dynamic positioning. Drillships reach the greatest water depths of any rig — ultra-deepwater beyond 10,000 ft — and carry large supplies for remote work.
- Submersible / drilling barge. A very shallow-water unit (swamps, rivers, marshes, shallow bays) that is floated into place and then ballasted down to rest on the bottom.
Bottom-supported or floating? That's the master split among MODUs. Jackups and submersibles physically rest on the seabed, which caps their water depth. Semisubmersibles and drillships float and never touch bottom — which is exactly what lets them reach deep and ultra-deepwater. For the deepwater pair head-to-head, see Semisubmersible vs. Drillship.
Fixed and compliant production structures
Once a field is developed, operators often install a permanent structure to drill the development wells and produce them. These aren't moved between fields; they're engineered for one location and a target water depth:
- Fixed platform. A rigid steel (jacket) or concrete structure fixed to the seabed, with the deck held well above the waves. Economic in shallow to moderate water depths.
- Compliant tower. A narrow, flexible tower designed to sway (comply) with wind and waves rather than resist them rigidly. It extends the fixed concept into deeper water, roughly the 1,000–2,000 ft range.
- Tension-leg platform (TLP). A floating platform held down by vertical steel tendons under high tension, anchored to the seabed. The tendons all but eliminate vertical motion, allowing dry trees on the deck. TLPs work into deep water, to roughly 5,000–7,000 ft.
- Spar. A large, deep vertical cylinder (hull) that floats upright, ballasted low for stability, and is moored to the seabed. Spars are stable platforms for very deep water — to roughly 10,000 ft.
Offshore rig types by water depth
The clearest way to compare offshore structures is by the water depth each is built to handle and how it holds position.
| Type | How it stays in place | Typical water depth |
|---|---|---|
| Submersible / barge | Rests on bottom | Very shallow (swamp, marsh, shallow bay) |
| Jackup | Legs jacked to seabed | Shallow — up to ~120–150 m |
| Fixed platform | Rigidly fixed to seabed | Shallow to moderate |
| Compliant tower | Flexes with waves; seabed-anchored | ~1,000–2,000 ft |
| Tension-leg platform (TLP) | Vertical tensioned tendons | To ~5,000–7,000 ft |
| Semisubmersible | Pontoon buoyancy; moored or DP | Deepwater |
| Spar | Deep cylinder hull; moored | To ~10,000 ft |
| Drillship | Dynamic positioning | Ultra-deepwater — >10,000 ft |
Ranges overlap in practice — a modern jackup, a high-spec semisubmersible, or a particular TLP design can push beyond the figures above. Treat the table as the typical operating envelope rather than a hard limit. The unifying logic is simple: the deeper the water, the more a rig has to float and the more it relies on tendons, mooring, or dynamic positioning rather than a leg on the seabed.
Common questions
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