Third generation rigs are the most common, predominately the Aker H3 design. These rigs were more rugged than previous designs, capable of operating in the harsher environments of the North Sea and the mid-depths of the Gulf of Mexico.
As the result of the rapid development of 3rd generation rigs, few (13) 4rth generation rigs were constructed. This small group was noted for their increased VDL's (4500-7000 tonnes) and capabilities in the harsh environments off eastern Canada & western Europe.
The early to mid-90's were characterized by modernization programs, particularly on third generation rigs. Rigs were made capable of drilling in deeper environments, up to 3050 meters.
Upgrading older rigs is limited by the original design, with contraints on variables such as displacement and deck space. Fifth generation rigs, such as the RBS-8D, are characterized by higher displacements and more capable equipment such as big-bore drill pipes and dynamic positioning systems.
Sixth generation rigs are capable of operating more autonomously in harsh conditions, such as extremely deep water (~3,000m+) and sub-Arctic zones.