The success and efficiency of deepwater subsea construction, repair and maintenance depends heavily on the capabilities of the remotely operated vehicles (ROVs) used to support these tasks. As work activities have moved into deeper waters, vehicle support requirements for heavy-duty tasks have become increasingly demanding. As a result, ROV manufacturers have developed more powerful vehicles to support the bollard thrust and tool power required for deepwater tasks.
Typical work-class ROV systems provide maximum power levels ranging from 100 to 200 horsepower. While these vehicles produce impressive thrust in either vertical or horizontal directions, inefficiencies in the power system designs limit peak system performance to these two points on their thrust curves. When the vehicles perform tasks that require thrust in both vertical and horizontal directions, overall system limitations become apparent: the hydraulic system does not adjust to varying demands to efficiently use available power.
This paper describes the design and development of a vari-able-pressure power delivery and propulsion system that sig-nificantly increases overall system efficiency to maximize use of available power.
Deepwater construction poses many challenges. As water depth increases, surface vessels must become larger to support the equipment needed to reach the seabed. Remotely operated vehicles (ROVs) have become more necessary than ever, and face much greater demands. The simplest way to date to in-crease ROV functionality has been to design vehicles with larger onboard power systems that provide more available thrust to support a larger variety of tasks. However, it has be-come increasingly important to maximize power efficiency rather than simply adding to it, since the second-order effects of increasing system power exact a price in deck space and vessel size.
Subsea equipment manufacturers have made hardware changes in recent years for more effective equipment handling and completion of required tasks using ROVs. However, many hardware items have still not been optimized for ROV intervention, placing heavy demands on ROVs during installation and intervention activities. This paper discusses the design of a variable-pressure power delivery and propulsion system that significantly improves an ROV's ability to efficiently accomplish required tasks.
The benefits of increased ROV power lie in the vehicle's per-formance of useful work such as lifting heavy objects, pushing large equipment items into position, and acting as a supply for high-powered tooling. Increases in useful output result in a more capable machine, but also carry many system costs.
Two direct consequences of increased input power on an ROV system are increased electrical current capacity requirements for the umbilical/tether system and increased motor, pump, and thruster sizes. Secondary system changes are then re-quired to support these primary size/capacity increases. More copper in the umbilical requires more steel armor on the cable because weight of the conductors is entirely parasitic. In-creased weights of the hydraulic power unit (HPU) and thruster require additional flotation material to maintain the vehicle's neutral trim.