A MODU (mobile offshore drilling unit) is a drilling rig that moves from well to well rather than staying with a field. Jackups, semisubmersibles, and drillships are all MODUs. It produces nothing. It drills, completes, and works over wells, then leaves. The defining hazard is well control, and it is the sharpest one in the industry. A well being drilled is an open path to a pressurized reservoir, held back by the weight of drilling mud and, when the mud loses, by the blowout preventer. Kick detection, the mud system, the mud gas separator, and the blowout preventer together form the barrier stack, and Macondo is what happens when it fails. In US waters MODUs sit with the Coast Guard, while BSEE covers fixed and floating production facilities, so the regulatory path differs from an FPU or an FPSO.
Functional safety relevance: Partly, and it is contested. Well control runs on its own drilling standards rather than on IEC 61511, but operators increasingly put SIL targets on blowout preventer control and emergency disconnect functions.
Key Points
- Drills, completes, and works over wells, then moves on. Jackups, semisubmersibles, and drillships are all MODUs.
- Well control is the defining hazard: kick detection, the mud system, the mud gas separator, and the blowout preventer form the barrier stack.
- The protection is episodic rather than continuous, tied to the drilling operation in progress rather than to a steady process, which is what separates it from production facility safety.
Example
The Deepwater Horizon was a MODU drilling the Macondo well in the Gulf of Mexico in 2010. A kick went undetected, the well flowed to surface, and the blowout preventer failed to seal it. Eleven crew died and the release ran for months. It remains the reference case for why the drilling barrier stack is treated as its own discipline rather than as an extension of process safety.
See Also: BSEE, WHP, subsea tieback
Cited Sources
- API Standard 53, Blowout Prevention Equipment Systems for Drilling Wells
- IMO Code for the Construction and Equipment of Mobile Offshore Drilling Units (MODU Code)