Exploration • Drilling • Field Services
Deep water MiniROVs and delivery systems
MiniROVs can deliver the same flight performance as larger ROVs without the higher cost of the equipment and commissioning. Cyril Poissonnet reports.
T
raditionally 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.
Te 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. Te 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. Te 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. Tough 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. Te 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.
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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. Te most common types of TMS are top-hats (receives the ROV from underneath
Fig. 1. Drag induced by a long tether.
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