WAVE IMPACT ON SEMI-SUBMERSIBLE UNITS: A REVIEW OF DESIGN STANDARDS

On 15 February 1982, the damage sustained by the portholes on the ballast control room onboard the drilling rig Ocean Ranger during a storm kick started a chain of events that resulted in the loss of 84 lives and the rig. The event led to an update of MODU Code that govern the design of Mobile Offshore Drilling Units. Semi-submersible mobile offshore units have evolved in their design and utility since 1982. Most recently, on 30 December 2015, a fatality occurred onboard COSL Innovator when a storm hit the unit. The incident led to changes in the assessment of airgap and structural adequacy of existing designs operating in harsh environment such as North Sea. A comprehensive assessment of the consequences of wave impact on the deck structure of a semi-submersible is still required to understand the safety and reliability of such units subjected to similar incidents. This review paper is an attempt to list important design aspects of Semi-submersible mobile offshore units affected by wave impact on the deck and the current design requirements. Some of the important aspects that require consideration are stability, watertight integrity, local and global structural strength, stationkeeping, impact on safety critical equipment and control systems and safety of persons in living area subjected to wave impact.