As recently as 10 years ago, deep water for drilling rigs was 600 feet and beyond. Really deep water, 1000'+, was on the horizon but reserved for dynamically positioned ships. Only 2-3 years ago some experts still proclaimed that moored operations beyond 2000' were not possible or realistic. Today, a number of semi's moor routinely in 2000-3000' WD; the new Trendsetter class semi, "Zane Barnes," now operates in the Gulf of Mexico in up to 4000' WD, while D.P. drill ships are working beyond 700Q'. Both types are projected to go well beyond this with present day equipment. Although distinctly different, technologies for deep water mooring and dynamic positioning have come of age. They have proven reliable, effective, and economical. The question no longer is whether one could moor at all or should use D.P., but which is the more economical and expedient way to operate. Today, moored drilling operations are realistic in up to 6000' WD and may possibly exceed this with current state-of-the art technologies. Economic considerations will dictate where the transition to D.P. occurs. A hybrid, i.e., a semi that has deep water mooring as well as D.P. capability, may eventually evolve if market conditions demand drilling beyond 6000' WD. Design aspects of riser systems and sub-sea equipment will dominate and outweigh stationkeeping concerns.
Reading & Bates has been a leader in deepwater mooring technology from the start. Until the "Zane Barnes" was built, the RB semi's "M. G. Hulme, Jr." and "Sovereign Explorer" have held water depth records in the North Sea. The key to success in deepwater mooring is the result of many factors, namely:
Reliable design, analysis, selection, and testing of the mooring tackle, i.e., anchor, chain, and wire rope.
Innovative design and selection of the proper machinery to store, deploy, retrieve, and tension the mooring lines.
Design, selection, and use of a reliable and proven anchor handling system.
Employment of a sufficiently powerful and adequately outfitted anchor handling vessel managed by an experienced crew.
Selection and training of knowledgeable and experienced marine rig operating crews.
Reliable and expedient engineering support during construction, start-up, and operations.
Design effort for the "Zane Barnes" got under way early in 1~84. The emphasis was on new technology, not simply scaling up existing designs. Many new ideas took hold and bore fruit. Today, one can simply state that most goals were achieved, and the "Zane Barnes" is a resounding success.
Primary emphasis in the initial concept formulation for a new class semisubmersib1e was put on much greater deck variable load, superior motion, very deep water capability, efficient and safe operating characteristics.
Previously, maximum water depth was approximately 1500–2500' for moored semi's, and about 6000' for D.P. drill ships. Market projections in 1984 were for 3000' WD in the near term, and 5000' in the long term.
