Manned diving involves a unique combination of occupational health and safety issues that require effective management and control at all levels. Use of diverless systems will negate those issues. Where the use of manned diving is unavoidable a remotely operated vehicle (ROV) will, in suitable conditions, provide a visual link for monitoring the safety of the diver.
DIVER/ROV INTERFACE
Some client requirements are that a small ‘eyeball’ type ROV be deployed when bell diving operations are being conducted from any diving sites. Where conditions permit it is also advantageous for one to be used during air diving. The function of the ROV is to monitor the safety of the diver when he is in the water, to provide the Diving Supervisor with anoverview of the diving operation, and to provide additional information on the underwater facilities such that the working time of the diver is optimised.
There is potential for an operational advantage using both a workclass ROV along with the diver, although due to weight and power considerations particular attention has to be paid to the controls to prevent interference and injury/damage.
ROV categories
In simple terms, ROVs can be divided into two categories, eyeball and workclass, as summarised below. With the correct preparation and tooling workclass ROVs are capable of diver replacement in a wide range of tasks. ROVs are not dextrous when compared with a human. They are capable of some ad-hoc tasks but by utilising pre-engineered interfaces on subsea equipment they can carry out a much wider range of tasks than might at first be thought possible.
The question is often not "can an ROV do this task" but "can we make an ROV deployed machine tool to do this task".
For example ROVs can carry a range of task specific tools to do virtually any task. Pre-engineering and the use of standard interfaces are the keys to cost effective ROV use.
ROV costs are typically much lower than for divers and often the work can be carried out by the rig ROV rather than requiring an expensive DSV, similarly if there is no rig on site an ROV support vessel can be used which is much less expensive than a DSV.
Limitations in their use are:
Water depth, typical limit at present is 10,000ft/3300m range, (beyond diver 200m range)
Dexterity less than for divers
Interfaces and tools have to match ROV capability
Restricted capability in the splash zone.
CLASSIFICATION OF ROVs
ROVs in common use are classified into two main types:
WORKCLASS ROVs
are large vehicles typically weighing typically 2Te and 2m cube dimension. They operate from high voltage supplies typically 1000 – 3000 volts, driving 100 HP systems. They have a power and payload capacity adequate to allow them to be adapted to various modes of operation, eg. Seabed survey, pipeline survey, construction support, structural inspection, drill rig support.
EYEBALL or Observation class ROVs
Are lower cost, easier mobilised, but a normal capability limited to visual inspection and monitoring, sometimes capable of simple tasks such as NDT. The systems are usually skid mounted with a dedicated crane or A-frame deployment system. Some have water ballasted deployment skids which don’t need welding down on deck, which is a positive advantage for production environments or over the floatation tanks on a jack-up rig. As well as CCTV options, the vehicles may have sufficient power/payload for small tooling packages. At the compact, low cost end of the eyeball class, are systems referred to as Mini, Flyball, or Suitcase ROVs which can be manhandled for mobilisation and deployment. The complete workable system could be transported by helicopter, manhandled down to a deployment site, and connected to a single phase power supply. They are low power and have negligible payload capacity for tools and therefore tend to be used for performing a “quick look” underwater.
DEEPWATER WORK
The benefits of ROV support on Deepwater Exploration and Appraisal rig work has already been realised West of Shetland where intervention by ROV saved significant non-productive time (NPT) for the rig in overcoming the requirement to turn around the BOP in deepwater or gasket changes etc. Deepwater development has been limited by economics and to date. The enabling technologies for the deeper water ROV remains the same apart from having to withstand the increased pressure, and the problems with longer underwater umbilicals and their associated heavier deck loadings, the main exposure is in system reliability deployment is a longer process, and contingency support is limited.
This activity for ROVs has developed on the strength of their success in the above drill rig support. The same workclass ROVs are used but their tooling suite is extended to perform extra jobs more cost effectively than previous systems employing divers/DSVs. Tooling for subsea production system intervention has conformed to an industry standard.
Typical tasks include:
· Seabed positioning and metrology
· Valve operations
· Jumper Connections
· Flowline connections
· Module and Choke component changeout
ROV Developements
Depth and diver costs have tended to drive development of ROV use and to date ROVs have been taking over an increasing number of tasks which had traditionally been carried out by diver. This trend will continue although for the foreseeable future divers will be required for some inspection and repair tasks which are difficult and expensive to provide ROV tooling for. Standardisation is an important theme for intervention. Designing seabed production related equipment with standard interfaces provides access to the standardised intervention tooling suites carried by ROVs. Such nterfaces can be adapted to a wide range of applications thereby extending the scope for assured ROV intervention. Although most seabedconstruction and commissioning is now routinely executed by ROV.
ROV Deepwater set-up
Procedures should highlight to ‘Soak’ test the ROV and TMS to various depths, monitoring the system hydraulic and electronic alarms together with close monitoring of the compensators.
This is for Deepwater. Lessons learnt from other deepwater ROV operations have highlighted the importance of having a higher standard of maintenance and cleanliness and also the main issues below:
- An extra compensator should be fitted to the TMS to allow for the extra compression of air retained in the sliprings and TMS hydraulics. Extra special care needs to be taken with the bleeding of the TMS systems. Monitoring of the TMS compensators either electronically or visually is vital.
- Same as 1 but for ROV Hydraulic systems
- Cable leakage is more susceptible in deepwater. All connectors MUST be spotlessly clean and properly fastened.
- Thoroughly test all equipment well before operational requirements as failure at depth affects critical operations to an even greater extent.
- Lessons learnt from previous operations must be implemented in any future operations.
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