At the first Subtech Conference in 1983, I chaired a seminar on manned submersibles To a question on the length of the future of manned submersibles, Frank Busby replied, ‘About ten minutes’ Four years later that prediction remains stubbornly unfulfilled, and companies, agencies and authorities still purchase manned submersibles

It is, however, fair to state in the case of offshore work, that competing technologies outsell manned submersibles by about ten to one

At one time it could be supposed that this was due to the reduced risk in using ROVs instead of manned submersibles, but the continued use of saturation diving, with far higher physiological risks, surely indicates that this is not the case It is most probable that economic considerations in the overall sense are paramount, including costs of surface platforms, safety back-ups, limited-duration dives, and battery-charging times

CHANGING PATTERNS OF USAGE

Since the late 1970s, changes in the use of manned submersibles have occurred, as the trends indicate below(available in full paper)

These figures indicate a reducing usage of manned autonomous submersibles, a steady usage of ROV microsubs, which may be used in an ROV-only mode much of the time, and an increase in cost-effective alternatives to ROVs such as the patented Intelligent Tow Fish (OSEL Group 1984) and guidewire-deployed manipulators

NEW WORK AREAS
Subsea completions

The rapid fluctuations in oil prices have concentrated the attitudes of oil companies to consider overall value for money very much more than previously, and to disregard technical ‘snobbery’ In 1980 no self-respecting underwater engineer would consider designing a subsea completion to include diver intervention Today, the most cost-effective system is chosen without high-technology bias, and many of the prophets of diverless technology have been converted to a less messianic, more unbiased approach, or have departed from the business altogether

There is a great degree of difficulty inherent in predicting the optimum technology for any particular field This is due to the very high sensitivity to parameters such as reservoir geology, predicted price of product, water depth, location, adjacent facilities, etc For instance, it is generally accepted that diver intervention is limited to depths shallower than 300 m, deeper than 300 m, alternative technologies must be used

It is also accepted that for the most complex field structures some form of atmospheric manifold will be used, requiring manned intervention using atmospheric, mobile, tethered bells or personnel lock-out submersibles These may be deployed from surface platforms or have sufficient endurance to depart from their home base using on-board, long-endurance power sources

The type of surface platform used to support subsea field production has a major impact on field economics The most sophisticated field-support vessels (FSVs) may cost four to six times the day rate of the older-generation, simple vessels A subsea completion design using the lowest-cost, surface support-platform will reduce field-maintenance costs by 50% or more, even if a sophisticated FSV is needed for one or two interventions per year

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