MiniROVs can deliver the same flight performance as larger ROVs without the higher cost of the equipment and commissioning. Cyril Poissonnet reports.
Traditionally focused on shallow water applications (50m to 100m), more and more operators are realising that MiniROVs work exceptionally well for light duty work in deeper water.
The increase in applications has driven the production of Observation Class ROVs up and the costs down. Today it is very common to see MiniROVs being used within the first 300m of water and some down to 1000m.
It has been recognised that MiniROVs can deliver the same flight performance as larger ROVs without the higher cost of the equipment and commissioning.
Peak ROV performance is achieved with short tethers. The deeper the MiniROV needs to operate, obviously, the longer the tether length needed. As the tether length increases, the vehicle becomes less manoeuvrable and less predictable, to the point where the drag of the tether becomes more than what the ROV can 'pull' or handle.
The drag on the tether is a function of: length, water velocity, cross section and texture. Water velocity on the tether is a function of the ROV speed as well as the water current in the environment.
In order to stay manoeuverable, the ROV needs to generate more thrust than the sum of the drag generated by its profile in the water plus the drag generated by the tether.
Though deployment from the side of a ship with a manual reel is effective for shallow water applications, clump weight systems or Tether Management Systems (TMS) are required in deeper water.
Clump Weight is a method to remove the effect of the tether drag from the MiniROV. The water flow along the tether generates drag that is eventually applied to the ROV.
In order to isolate the ROV from most of the tether drag, a clump weight is utilised to stabilise the section of the tether that is subjected to the most drag, providing a short section for the ROV to free swim. However, the clump weight method has some limitations. For example, the current takes the clump weight away from the intended target over the ground.
Deployment with clump weight is challenging and requires skilled operators. Also, the free swimming section is typically short, limiting the excursion capability of the ROV. Deployment of the MiniROV with clump weight is cumbersome and tether entanglement is common.
A TMS can be used to operate in currents and deeper waters without reducing the excursion capability. It offers a solution to many of the clump weight system issues and provides a space to install large and heavy power conditioning components, often necessary when using long cables needed to operate in deeper waters.
Work Class ROVs (WROVs) have used TMS for many years. The most common types of TMS are top-hats (receives the ROV from underneath the TMS frame) and garages (receives the ROV horizontally and provides protection around the ROV). Both types are equally effective with their own set of pros and cons.
A typical configuration consists of a TMS deployed from a strength armoured umbilical. A mechanism inside the TMS (usually an underwater winch) deploys the tether for the ROV excursion.
MiniROVs are often deployed manually due to their light weight; however, deep water operation requiring a TMS makes manual deployment nearly impossible. A Launch and Recovery System (LARS) needs to be added to the system. LARS consists of a support structure capable of moving the TMS and ROV arrangement from the deck to the water safely.
SeaBotix uses a 20ft shipping container to accommodate deep deployment of MiniROVs. It provides space for a motorised winch where the LARS is small enough to be integrated as part of the container.
Near the ceiling of the container, a twin boom system is deployed outside the container through double doors offering 3m of overboard reach. Mounted towards the end of the twin boom support structure is a hydraulically damped docking cone with two degrees of motion to help prevent umbilical wear when the TMS is deployed and the ship is pitching and rolling.
The winch system responsible for deploying the TMS can be outfitted with an active heave compensation module which has the ability to safely deploy and retrieve the ROV back to the TMS in higher sea states. The container also includes an AC controlled room for 2 operators.
The SeaBotix Containerized Delivery System (CDS) offers a fully integrated TMS, MiniROV & LARS package with small footprint for cost effective deep water applications.
For more information visit www.engineerlive.com/iog
Cyril Poissonnet is CDS Program Manager with SeaBotix Inc, San DiegoCA, USA. www.SeaBotix.com