Semi-submersibles are floating structures used for drilling and, less commonly, for production. They are technically vessels, regulated under maritime law, and employ both maritime crew and drilling or production specialists.
How Semi-Submersibles Work
A semi-submersible floats at a fixed depth with its hull submerged below the wave action zone. The submerged hull provides stability in heavy seas by keeping the buoyancy centre below the level where wave forces are strongest. The platform deck sits on columns above the waterline.
For drilling operations, the rig is positioned over a well location. In shallow water it may be anchored. In deep water, dynamic positioning keeps it on location. The drilling operation proceeds through the moon pool, a well opening through the hull.
The DP system on a semi-submersible drillship keeps the rig precisely positioned over the wellhead during drilling operations. Losing position can damage the riser, the pipe connecting the rig to the wellbore. This is expensive and potentially dangerous. DP officers on drilling rigs operate to more critical tolerances than supply vessel DP work.
Maritime Roles on Semi-Submersibles
The marine department runs the vessel systems: dynamic positioning, ballast, propulsion where applicable, lifesaving equipment, navigation during transit.
Master: vessel command. On a working drilling semi-submersible the master manages marine operations and interfaces with the offshore installation manager who runs the overall platform operation. The relationship between the master and the OIM is clearly defined but requires ongoing coordination.
Chief Officer: marine department head. DP operations, ballast management, safety equipment, marine crew management.
Dynamic Positioning Officers: the DP watch requires dedicated qualified officers on rigs with active DP operations. This is distinct from the master and chief officer roles on larger rigs that have enough crew to man DP watches separately.
Chief Engineer: manages all vessel machinery. On a semi-submersible this includes the power plant, thrusters, ballast systems, drilling support systems and accommodation systems.
The Application Process
Semi-submersible marine department positions are filled through specific channels. Manning agencies with offshore or drilling rig specialisation are the primary route. The major drilling contractors, Transocean, Valaris, Diamond Offshore, Noble, manage their own marine departments and recruit through their own channels as well as through agencies.
Building DP experience on supply vessels before targeting semi-submersible positions is the standard pathway. DP officers who have accumulated substantial sea time on PSVs or AHTS vessels are the pool from which semi-submersible marine departments recruit.