Remotely Operated Vehicles (ROVs) and associated tooling are being increasingly utilised in subsea installation and maintenance operations With the offshore industry developing into deeper waters, 3000 metres and beyond, questions are arising as to the suitability of ROVs This paper illustrates some of the current applications and capabilities of ROVs and highlights a number of the issues involved at 3000 metres and beyond
Remotely Operated Vehicles (ROVs) have now been with us for over twenty years. In that period, their reliability has increased dramatically and has resulted in ever-increasing usage in a variety of offshore applications ‘Diverless Technology’ is currently synonymous with ROVs or associated technologies and has gained ground in a number of important subsea areas and enabled work in deep water, beyond the point divers can currently venture This paper serves to highlight some of the important areas of ROV operations
ROV categorisation is now a complex subject Distinctions between classifications are being blurred in the drive to cut costs and look at using smaller vehicles and new technologies to do tasks previously performed by larger vehicles However, Table 1 attempts to summarise the situation and illustrates the basics of ROVs in terms of size, weight and capability
Each type of ROV has its place within the tasking chain whether it be a simple flying camera (‘Eyeball’ ROV') or a large vehicle performing a heavy duty task The trend is towards deeper water operation and has resulted in a typical ROV system now consisting of the same major components, but having differing sizes and weights The components are -
Tether Management System/Tether
Figure (1) illustrates this standard ROV ‘system’ Note that it is also common in shallow waters to remove the TMS/Tether and perform ‘live-boating’ operations using the ROV main umbilical only Suitability of a particular ROV system to carry out a task can be governed by a number of questions being satisfactorily covered, for example -
What tasks are required of the vehicle?
Where is it required to perform? (Depth? Current? etc)
What equipment/tooling will it need to cany? (Payload? Power? etc)
What performance is required of the vehicle? (Speed? Task time limits?)
Any size or weight restrictions for the Operation?
What are the financial constraints?
A major performance constraint, and particularly as equipment goes deeper, is the length of time to arrive on-site and return to surface. Typical winch deployment speeds are 30–40 metres per minute which at 1000m depth, means 25–30mns transit time For 3000 metre depth of operations, transit time will be of the order of 90 minutes - all of it basically dead working time In the event of a problem with the ROV, this could mean a minimum of 3 hours plus repair time before the ROV is back on site This is a very good reason for providing reliable equipment, as well as, obviously, increasing deployment speeds