'Targetless' sensors for DP

June News - Published in Seaways - The Nautical Institute

Dave Sanderson

A modern Dynamic Positioning (DP) equipped vessel is reliant on its position reference sensors to enable the advanced functionality that computer control brings. These reference sensors fall broadly into two categories: global references, and local references. Global references are systems such as GNSS, which rely on a set of globally visible satellites or other known global reference to provide a position. Local reference systems provide a range and bearing to a local cooperative target. These might be laser or microwave Radar-based local position reference sensors.

Conceptually, there is little difference between local position reference systems, traditional celestial navigation, and GNSS based systems, other than the actual range to the reference point. Practically, however, both the stars and GNSS satellites are fixed. They are very hard to disrupt or disturb – issues with GNSS jamming notwithstanding. Local reference points are more prone to accidental disruption. In the local reference system the reference point, or target, is often overlooked as a point of failure, even though the system is reliant on it to function. A typical example can be seen in platform supply operations. Many platforms are equipped with laser targets which allow the supply vessels to line up with the appropriate spot on the platform and then maintain position using their DP systems. An ideal operation will allow the vessel to approach the platform using multiple different local reference sensors to ensure safety, thorough redundancy, and speed of operation. If the platform does not have appropriate targets installed, then the vessel will typically approach under manual control and hand over to the platform crew a target for them to temporarily install – for example, a prism cluster that will then act as a laser target. The vessel will standoff while the platform crew install the target, and then re-approach under DP control once the target is in place. This practice exposes weaknesses in the safety chain:

  • The initial approach to hand over a suitable reference is not as safe and simple as it could be. 
  • The vessel crew has to rely on the platform crew to position the target appropriately. 
Another weakness with dedicated targets, even when they are permanently in position, is that the system complexity is distributed across multiple separated devices:
  • Radar responders are active pieces of electronics, with power supplies and associated potential failures.
  • Prism clusters, although a totally passive piece of equipment, can fail due to damage sustained in the handover process. 
  • Retro reflective tape targets often suffer from ageing related issues, such as UV damage, abrasion, and being made from poor quality retro-reflective tape.
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