The development of mobile offshore drilling rigs has steadily progressed since their inception, but the nature of the development has been more evolutionary than revolutionary. The advent of semisubmersible and dynamically-positioned rigs has had a significant effect on the capabilities of the equipment. However, the drilling systems have been adapted with only slight changes from those used onshore. As drilling progresses into the deeper waters of the continental margins, the use of conventional marine drilling systems will present an increasing number of problems.
One of the basic and most challenging problems in deep water operations is the marine riser, which is used to provide a conduit for well returns, to guide the drill pipe into the hole, and to serve as a support for control cables and choke and kill lines. In deep water, the riser requires supplementary buoyancy to achieve a better stress distribution and reduce the requirements for riser tensioners. This supplementary buoyancy substantially increases the unit cost of a riser joint while, at the same time, more riser is needed. Costs on the order of $1,500/foot or more are not expected to be uncommon for the longer risers.
Many alternatives to the use of the conventional marine riser system for deep-water drilling are already being investigated. These alternatives offer possible means of avoiding big, expensive, hard-to-replace risers and their associated large drilling vessels. Some of the riserless drilling alternatives given in the literature, including patents, are reviewed. An attempt is made to ascertain which alternatives seem to be most promising.