semisubmersibla

Semisubmersibla, commonly known as semi-submersibles, are offshore vessels designed to achieve high stability and large deck capacity by partially submerging their hulls. They are used primarily as drilling rigs in deepwater environments and as heavy-lift transport vessels. The concept emerged in the 1960s and 1970s to access offshore fields in deeper waters.

Design and construction: A typical semisubmersibla hull consists of multiple vertical columns connected to submerged flotation pontoons. The deck sits high above the water and houses the derrick, cranes, and other equipment required for drilling or heavy lifting. Ballast tanks allow controlled submergence: pumping seawater into the pontoons lowers the hull, damping wave action and increasing stability. When mobility is needed, ballast can be adjusted to reduce draft and move the vessel. Many units are equipped with dynamic positioning systems to maintain position without anchors.

Operations and capabilities: In drilling mode, the hull is ballasted to the target draft and the drill floor is stabilized for operations. The submerged hull reduces motion from waves, enabling heavier risers and longer operations in challenging seas. In heavy-lift service, the large deck and submerged hull provide high load-bearing capacity and stability for deploying and recovering oversized modules.

Types and applications: The term covers column-stabilized semi-submersibles used primarily as drilling rigs and semi-submersible heavy-lift ships designed to transport topsides and other large offshore structures. Both rely on ballast-based stability, but differ in propulsion, DP capabilities, and deck configuration.

History: The semi-submersible concept developed to exploit deepwater fields and rough seas, with significant advances during the 1970s and 1980s that led to the modern fleet of offshore rigs and transport vessels.