5.2 Laser Relative References (Fanbeam/CyScan)
Key Takeaways
- Fanbeam and CyScan are laser-based relative PRS that measure range and bearing to reflective targets (prisms/reflectors) on a fixed structure or suitable vessel
- They provide high update rates and precise relative geometry for close-quarters work alongside platforms, rigs, and FPSOs
- Performance degrades in fog, heavy rain, glare, spray, and other optical obscuration; line-of-sight to targets is mandatory
- Reflector placement, target identification, and geometry quality are operator and setup responsibilities, not automatic guarantees
- Laser PRS are relative, not absolute geographic references: they fix the vessel to the target structure’s frame, not to WGS-84 by themselves
Laser Relative References in the DP Toolkit
When a DP vessel works close to a platform, semi-submersible, FPSO, or other structure, absolute satellite position is not always the most useful frame. The operational question is often: How far am I from that structure, and am I holding a safe offset? Laser relative position-reference systems — commercially known by product families such as Fanbeam and CyScan — answer that question by measuring range and bearing from the vessel to reflective targets mounted on the structure.
On the exam, recognise laser PRS by this phrase pattern: laser / light pulses / prisms / reflectors / range and bearing / relative to structure. Do not confuse them with RadaScan (microwave), taut wire (mechanical seabed weight), or DGNSS (satellite absolute).
How Laser PRS Work
A typical installation includes:
| Element | Role |
|---|---|
| Laser scanner / sensor head | Emits and receives laser energy; measures range and bearing to detected targets |
| Reflective targets (prisms, retro-reflectors) | Mounted on the structure; return strong signals for tracking |
| Processing unit | Identifies targets, filters noise, computes relative position of vessel CRP or antenna reference |
| DP interface | Provides continuous relative position (and often quality/weight) to the controller |
The system maintains track on one or more designated targets. From geometry (range + bearing, and known sensor mounting offsets and heading), it computes the vessel’s relative position with respect to the target structure. Update rates are typically high compared with some acoustic cycles, which helps the Kalman filter and operator displays respond quickly during close work.
High Update Rate and Close-Work Value
Why laser systems are popular for alongside / close approach tasks:
- Relative geometry matches the risk — collision and contact risk is relative to the structure, not to a distant datum.
- High update rate — frequent measurements support tight station-keeping envelopes near steel.
- Independence from seabed — no clump weight, no acoustic multipath from the bottom (though other optical issues apply).
- Independence from GNSS sky view — useful when satellite geometry is poor in the shadow of large topsides (though GNSS may still be online).
- Diversity — adds a different physics principle for voting when mixed with DGNSS and HPR.
| Strength | Exam phrasing |
|---|---|
| Precision near structure | Excellent for relative offsets alongside platforms/rigs |
| Fast updates | Supports responsive DP control during close work |
| Optical line-of-sight | Requires clear path to reflectors |
| Target-based | Quality depends on correct target selection and placement |
Optical Limitations: Fog, Rain, Glare, Spray
Laser light is optical. Anything that attenuates, scatters, or blinds the beam degrades tracking:
| Condition | Typical effect |
|---|---|
| Fog / mist | Reduced range; target loss; noisy or dropped PRS |
| Heavy rain | Attenuation and false returns |
| Sea spray / thruster wash spray | Intermittent loss when wash crosses the beam path |
| Sun glare / strong backlight | Reduced signal-to-noise; tracking difficulty |
| Snow / heavy atmospheric moisture | Similar to fog/rain degradation |
| Smoke / exhaust plume | Temporary optical blockage |
Operational implication: a laser that was excellent at noon in clear weather can become marginal or unusable in a squall. That is not a DP controller failure — it is environment vs optical PRS. The DPO’s job is to recognise the degradation, watch residuals and PRS status, and ensure other independent references (often microwave relative, DGNSS, or acoustics) remain online so voting can deselect the laser without starving the estimator.
Exam contrast to memorise: RadaScan / microwave generally copes better than laser with fog, rain, and spray because radar wavelengths are less affected by those optical obscurants. Both can still suffer from other issues (interference, target problems, geometry).
Reflector Placement and Target Management
Hardware on the vessel is only half the system. Target quality decides whether the laser is a precision tool or a random-walk generator.
Placement principles (conceptual for DPO exams):
- Reflectors should have clear line of sight from the vessel’s working positions and headings planned for the job.
- Targets should be mounted on a structurally stable part of the platform/rig — not on a swinging crane boom or temporary scaffolding that moves independently of the “fixed” worksite frame the DPO assumes.
- Multiple targets improve flexibility for different approach headings and can support better geometry, but the operator must select the correct target ID and not track a reflector on a supply boat or a wrong deck.
- Vertical and horizontal offsets from the intended worksite reference point must be understood: the laser measures to the prism, not automatically to the hose connection or well centre unless geometry is configured that way.
| Setup issue | Consequence |
|---|---|
| Wrong target selected | Vessel holds relative to wrong object → collision or excursion risk |
| Target on moving sub-structure | Apparent vessel motion when structure moves |
| Obstructed LOS after heading change | Sudden PRS loss mid-task |
| Dirty / damaged prism | Weak returns, dropouts |
| Only one laser + no peers | No voting diversity if laser fails optically |
Relative, Not Absolute
Critical exam distinction:
| Property | Laser PRS (Fanbeam/CyScan) |
|---|---|
| Absolute WGS-84 position? | No (not by itself) |
| Relative to structure? | Yes |
| Depends on satellites? | No for the laser measurement itself |
| Depends on line of sight to targets? | Yes |
| Useful for open-ocean absolute station-keeping with no structure? | No — needs targets |
If the structure itself is drifting, listing, or the targets are on a mobile unit, the laser will faithfully report relative geometry that may still be “good relative DP” while absolute earth-fixed position changes. For some tasks (hose connection to an FPSO) relative is what you want. For other tasks (hold a fixed geographic pipeline crossing) absolute DGNSS/HPR may be the mission frame. Know which frame the job needs.
Operator Practices During Laser Use
- Confirm target IDs and that the selected reflectors match the procedure / permit-to-work location.
- Verify LOS for planned headings and thruster wash directions (spray across the scanner path).
- Enable laser as one of several independent PRS — do not run critical Class 2/3 work on laser alone.
- Watch quality/weight and residuals when weather closes in; fog alarms are expected physics, not mysteries.
- After selecting a new target, allow filter settling; thruster activity may increase briefly.
- If laser is rejected by voting, investigate optical path and targets before forcing it back online.
Exam Scenarios
| Stem pattern | Answer theme |
|---|---|
| Fanbeam / CyScan named | Laser relative range/bearing to reflectors |
| Best for close platform work in clear air | Laser relative PRS |
| Fails in fog/rain vs microwave | Optical limitation of laser |
| Needs prisms on structure | Target/reflector placement |
| Provides geographic lat/long alone | False — relative, not absolute |
Bottom Line for Assessment
- Fanbeam/CyScan = laser relative PRS → range & bearing to reflectors on structure.
- High update rate supports tight relative station-keeping.
- Fog, rain, glare, spray are classic performance limiters.
- Target placement and selection are safety-critical.
- Relative ≠ absolute — do not call laser a satellite system or a taut wire.
Fanbeam and CyScan are best classified as:
Which environmental condition most characteristically degrades laser relative PRS performance compared with microwave relative systems?
Why must a DPO treat laser PRS as a relative rather than absolute geographic reference?
During close approach, the laser locks onto a reflector mounted on a swinging crane jib rather than on the fixed platform jacket. What is the main operational risk?