DP Lightning Surge Protection

Introduction: I recently flagged a dynamic positioning (DP) design for not having lightning surge protection on the DGPSs & wind sensors, and they asked me if it was really required. While some people wouldn’t dream of DP sensors without them, they hadn’t planned for any, and rightly wanted to know if I was making it up. Let’s have a look.

Construction vs. Safe Operation: From a DP operation viewpoint, it’s a no-brainer – inexpensive protection to ensure safe operation, or a chance of a DP incident, lost time, client investigation, and rectification. For a DP operator, lightning surge protection has an excellent potential safety and cost payoff. It’s a small investment to avoid much larger trouble. However, a shipyard without this specified in their contract might look at it as an extra cost with no benefit. Is a DP vessel really required to have lightning surge protection on some systems? Wouldn’t the vendors have said something?

Vendors: Surely, the vendor would know. Sometimes the vendor salesmen do and sometimes they don’t. Vendors are often under considerable price pressure and their salesmen have been known to accidentally sell system configurations that don’t work according to their own engineers. By lucky happenstance, these mistakes typically make them more economically attractive than their competitors. In a hypothetical example, perhaps a DP company has changed over from mathematical averaging of motion reference unit (MRU) values over time to direct application of the information. These new systems require 3 MRUs for redundancy, while their competitors only need 2, so the salesmen might not notice the uncompetitive change and offer systems with only 2 MRUs to customers. In a similar way, surge protectors might forgotten in some places.

Rules: But are they really required? DNV ship rules 2023-07 Pt.6 Ch.3 say nothing directly about lightning surge protection. ABS Guide for DP Systems 2021 (latest) also says nothing. Lloyd’s Register ship rules 2016 (it’s been a while) Pt.7 Ch.6 also doesn’t directly mention lightning surge protection. Bureau Veritas ship rules 2022-07 Ch.11 Sec.5 also doesn’t talk about lightning or surge protection. IMO MSC. Circ. 645 & 1580 also don’t talk about the need for lightning surge protection.  

Should I Concede? It looks like I have nothing to stand on, but that is deceiving. All those rules require no single points of failure. But is a lightning strike a reasonable single point of failure? It could be argued that it comes from outside the vessel, but so does wind, wave, and current, and we require reasonable provision to be made to deal with them within limits. So what are the limits? The vessel is DP class 2 (DP2) and the difference between DP2 and DP3 is that DP3 considers unlikely failures that can cause loss of an area to fire and flood. For DP class 2, the threat must be more likely than loss of an area to fire and flood. That’s good, IMCA has collected almost 30 years of incident data and we can compare.  

IMCA DP Incidents involving Lightning:

98/37 – Took out both DGPSs and 2 DP computers

0108 – Took out both DGPSs

(2002) 0007 – Loss of DGPSs & 3 of 4 thrusters

0530 – Loss of 1 of 2 DGPSs

1112 – Loss of 2 of 4 DGPSs, 2 of 4 wind sensors, 2 of 3 gyros, & hydro-acoustic position reference system (HPR)

1217 – Loss of 1 gyro

1518 – Loss of gyro signals to HPRs

1529 – Loss of gyro signals to HPRs

1873 – DGPSs unreliable

1874 – Loss of both DGPSs

22025 – Loss of 2 wind sensors & 1 DGPS

Interpretation: There are a fair number of these incidents and lightning is obviously a significant failure mode that needs considered and protected against. It will be noted that the DGPSs usually provided the voltage path to other systems, such as the DP control system, gyros, or HPRs. The interaction of lightning and the DGPSs is a common DP incident. Lighting traveling through the DGPS system is a significant DP failure mode and needs to be considered in design, analysis, and operation. In other words, DP2 requires that DGPSs have proper surge protection. Unfortunately, the evidence does not support that argument for wind sensors, but it would be prudent and is found on many vessels.

What About DP3? For DP class 3, the probability requirements are much tighter and lightning surge protection shouldn’t even be a question. Install it and maintain it.

Guidelines: This probably argument is well and nice, but who has the time to figure this stuff out? IMCA & MTS. If only someone had done all the work and given some simple guidelines for no one to read. IMCA M103 Rev 5.1 2.20.7 says: "Antennas and external sensors such as anemometers should be provided with lightning arresters which are fitted to antenna leads and vulnerable connections." MTS expresses concern about DGPS & Wind Sensor lightning protection in its DP design guidelines (4.13, 18.2.9, 18.4.3) but appears to be most concerned about DGPSs. It is clearest in DGPS TechOp D-06 :

• "d) Lightning protection – potential to damage DP control system. 
• With ever increasing amounts of critical electronic equipment installed on vessels, lightning protection is more important than ever. A variety of Surge Protection Devices (SPD) can be used to protect hardware and personnel from direct and indirect lighting strikes. 
• A Lightning Protection Plan should be generated and reviewed by the project manager for new build vessels; the vessel manager is responsible for inspection and replacement. 
• DGNSS equipment usually shares the DP’s power supply which has its own surge protection, but it is impossible to prevent a direct lightning strike of high-up equipment like antennas and cabling so the risk of other equipment being damaged can be reduced by channelling any power surge to ground using SPDs. 
• SPDs should be properly grounded with the shortest-possible grounding cable and should be installed where they can be easily inspected and maintained, strictly according to the manufacturer’s instructions. 
• “Fine” SPDs provide very high voltage clamping with associated limitation on current. “coarse” SPDs can handle much more current at the expense of allowing higher transient voltage. A coarse SPD should be installed in the antenna cable outside the equipment room, with a fine SPD near the equipment rack but clear of other equipment."

This is all good advice, but it’s not rules.

Rules Pt.2: It's not rules, unless the rules point at the guidance and say “Do that.” The ABS Guide to DP Systems does this with IMCA M103 and the MTS DP Design Philosophy Guidelines. DNV-RP-E306 is the DNV adoption of the MTS DP design guidelines and it explicitly requires lightning protection in 2.14 (general consideration), 14.2.8 (DGPSs), 14.4.3 (wind sensors), and 14.5.2 (wind sensors). Why all the attention on wind sensors when the incidents were almost entirely DGPSs? One wind sensor goes to both the DP control system and the independent joystick system (IJS). You aren’t allowed to lose both at the same time. Bureau Veritas doesn’t reference MTS & IMCA, and my Lloyd’s Register rules are old.

Requirements: For ABS and DNV vessels, everything is reversed. They have to be able to defend why they didn’t install the recommended surge protection. That will not be defensible for DP2 DGPS systems, considering the DP incident history, or for DP3 DGPS or wind sensors, but is fine for DP1 & DP0. BV and LR DP2 vessels need to have DGPS lightning surge protection, as the failure mode has proven significant, and no significant single points of failure are allowed. They will need to cover both DGPSs and wind sensors for DP3, because the failure criteria are tougher, but DP1 & DP0 don’t need surge protection. Surge protection might be required on the wind sensor that supplies both the DP control system and the IJS on DP2 & DP1, but not DP0 where there is no IJS or it is not required to be independent.

Conclusion: I hope this clarifies the issue and makes sense. DGPS & wind sensor lightning surge protection is recommended. They are essentially required for DP3. DGPS surge protection is essentially required for DP2. The shared wind sensor needs surge protection for DP2.