In the period between 1981 and 1986 a comprehensive study of the hydrodynamic behaviour of remotely operated vehicles (ROVs) and their umbilicals was sponsored by the Offshore Supplies Office of the UK Department of Industry. This involved, as subcontractors, BHRA Fluid Engineering, the National Physical Laboratory, Cranfield Institute of Technology (CIT), and NMI Ltd (now part of British Maritime Technology Ltd (BMT)) Investigation of the hydrodynamics of the craft itself was entrusted to BMT, the work being carried out in two phases. Phase 1 was a largely experimental study of the hydrodynamics of a particular full-sue craft (Dand and Every, 1983) which produced, among other things, a mathematical model of the hydrodynamics of the craft in question This was used in a simulation model for craft dynamics developed by CIT to compute motions for comparison with measurement (Lewis, 1985)

This exercise was sufficiently successful to suggest that preliminary hydrodynamic design of an arbitrarily shaped ROV might be possible with the existing data, augmented as necessary by any new data that might prove to be necessary Accordingly Phase 2 of the BMT project was carried out to explore this possibility, and it became clear that a profitable approach would be to attempt to design and build a simple ROV, predict its performance, measure this on the actual craft, and compare with prediction.

This chapter briefly describes this attempt and the results obtained using, where appropriate, comparisons with hydrodynamic or aerodynamic aspects of design in other fields.

HYDRODYNAMIC DESIGN OF AN ROV
The ROV as a marine vehicle

As a marine vehicle, the ROV poses many interesting problems from the point of view of hydrodynamic design Some of these are familiar to the design of other vehicles, but some are unique.

Much of the operating time of an ROV is spent deeply submerged in an effectively infinite fluid, it can therefore be treated like an aircraft or a submarine However, at shallow submergence or on the surface, it is subject to free-surface effects, notably from wave action, and so can be treated more as a surface ship

It has to have the ability to move reasonably rapidly between operating sites and yet have good low-speed performance, this is analogous to the behaviour of a tug in "ship" parlance However, when at a work-site it may have to hover and behave like a helicopter from a control point of view.

Yet, whereas an ROV is like all of the above vehicles, it is almost unique in that, generally, it need not have one preferred direction of motion with a velocity vector which dominates all others, as in an aircraft for example, it can in principle have similar performance in all six degrees of freedom.

Finally, unlike most other vehicles, many ROVs are attached to a long umbilical providing power, control etc for which they must often pay a large hydrodynamic drag penalty.

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