Introduction:
The west Natuna Sea, Indonesia records a relatively high lightning activity, where the gas field is situated. The gas field once operational would bring gas to Singapore via 300km long subsea pipeline. Weather condition at this place where the CPP and WP is built is favorable for the development of lightning producing cumulonimbus clouds. The destructive power of lightning is so tremendous that apart from igniting fires, damaging electronic equipments, disrupting electrical power and telecommunication system, it may cause fatality either directly or indirectly. Hence, the gas field in west Natuna Sea needed a detailed review of the design, construction and installation of the lightning and earthing protection system to be deployed for it. A clear practical recommendation was also needed, considering the mid-way construction of the facility.
Facility Description:
The platform for drilling was built of steel with all major structures firmly welded to the deck. Earth bars were bonded to platform legs. The design company had considered the standards IEC 62305, IEC 1312-3, API RP 2003, BS 6651:1992, UL 1449, IEC 61024-1 and NFPA 780 for applicable rules and regulation in the design, installation and maintenance of the systems on-board and to minimize the effect of lightning strikes on humans and materials. The real challenge was to evaluate the then existing design of the project whose construction was mid-way and make feasible recommendations should there be any short coming.
Findings By PowerQ:
The standard considered was IEC 62305 which is for on-shore systems. It was recommended to refer IEC 61892.
Lightning Protection zone: All panels were listed as in inner zone, away from direct lightning strike but should also have listed the equipments in zones of direct lightning strike.
Assessment of external protection:
1. The lightning density had to be confirmed which according to LPATS, is≥60 strikes/ km2/ year.
2. IEC 61892 was used as basis for protection.
i. External Lightning Protection (LP) was not required as against IEC 62305, which recommends air termination system for WHP & CPP. IEC 61892 says that on well-bonded platform the panels and equipment system will not be exposed to direct lightning strike and the air terminals are optional/ additional measures.
ii. Instruments, equipments and panels not protected or installed in grounded or installed in grounded metallic structure and prone to direct stroke to be listed.
iii. SPD selection should ensure that temporary voltage variation due internal switching transients caused by starting motors, transformers, photocopiers, or other loads on and off shall not trigger SPD’s.
iv. The connection of earth bar to other earth bar and platform legs were not by welding which ensures better bonding. The earth connection to equipment and panels were not by compression gland.
Recommendations By PowerQ:
A prudent approach to risk management of the lightning hazard is by being proactive and to apply legitimate defences.
1. Harmonise various document to IEC 61892 standard for lightning and earthing protection.
2. External and Internal lightning Protection
· General
Lightning protection should be made in accordance with IEC 61892. CVT proposed was not recommended as per IEC, NFPA or other standards and also as the project was in construction stage. Platform should be well bonded.
· CPP
Air terminal was not recommended for the generator and compressor exhaust stack as per IEC 61892, which is applicable to offshore platforms. Platform shall be well bonded and structure shall be protected against lightning strike. It was recommended to bond stack and cladding of stack to support structure and main deck by welding.
· Similarly, air terminal for crane and flare stack on was not recommended. Considering that fixed part of crane was welded to structure, it was recommended that movable part of crane and rotating upper part of the crane to the platform shall be bonded. Also electrical and control panel for crane shall be protected with SPD.
· For the crane operator cabin, it was recommended to bond to structure. Air termination was not required as cabin it reportedly made of steel.
· For generator control panel which could not be installed with SPD for design reason, it was recommended that the panel not be installed on enclosure wall to avoid reduce secondary electromagnetic effect of lightning strike and to bond the enclosure. Any cable to this panel exposed to outdoor was recommended to be installed in cable trunking or tray covered with metal covers and shall be bonded.
· Considering criticality of the telecom antennae it was recommended to install air terminal for radio telecom and TV antenna installed on the LQ Roof as an additional protection measure. The air terminal may be bonded by down conductor to nearest welded structure on deck.
3. Equipotential Bonding
Poor bonding may result in voltage rise mismatches, which in turn may lead to “arcs and sparks” on sensitive low voltage equipment.Without effective bonding, any lightning protection system will not work therefore following measures shall be adhered to:
· Bonding shall be made locally in close proximity to the items, which require bonding.
· Normally bonding connections shall be made by bolted connections to steel pads (earth bosses) which shall be welded to the steel deck or structure.
· For equipment installed on deck to use stainless steel boss for earthling for corrosion protection. Every bonding connection to earth shall be of copper or other corrosion resistant material and shall be securely installed and protected against damage and galvanic corrosion.
· Lugs may be of copper or its alloy and shall preferably be of the serrated contact type and effectively locked. Connections shall be secured against becoming loose due to vibration. Earth cable shall be installed with slack.
· Advise to produce quality control document to ensure that the grounding system is bonded as per design.
4. Earthing
There is concern over earthing and lightning protection of crane, Generator and compressor panel and instruments for flare and flow metering and risk of direct strike and secondary effects of lightning currents.
· Flare boom, drilling rig or crane shall be bonded to the main structure. If satisfactory conductance through the structure is not achieved, additional earthing conductors shall be installed where necessary.
· Where possible isolate all wire-line communication services from remote ground with optical devices or isolation transformers.
· Minimum cross section in the local earthing shall be as per IEC and copper wire 6 mm2 due to mechanical strength.
5. The connection of earth bars and platform legs shall be by welding for better bonding. The earth connection to equipment and panels may be by compression gland. The main earth bars shall be solidly earthed via connections to a minimum of two earth bosses for each earth bar, cad welded to the main platform structure.
6. Surge Protection Devices
· A further rational approach was recommended for application of SPD. A coordination table shall be produced by the supplier of SPD.
· Installation practice and relevant code such as IEC62305-3 and 62305-4 to be followed for SPD installation.
· It was recommended that additional SPD may be applied to downstream boards if they support electronic load equipment and to confirm that the short circuit KA rating for SPD was coordinated with the panel kA rating.
· Install SPD with a Circuit breaker or fuse for isolation, protection to avoid damage to SPD.
· It was recommended to bond panels prone to direct strike of lightning by cad weld where SPD could not be installed and the cables to the panel to be installed in metal trunking or metal covered cable tray.
· The protection of electrical and electronic equipment located in equipment rooms should be a combination of the following:
· Bonding of all metallic components entering an inner zone from an outer zone
· To install of SPD on all cables entering an inner zone.
· Bonded enclosures to act as magnetic shield.
7. Personnel protection from Lightning Strike on platform- People must avoid being a part of the current circuit path. It was recommended that personnel protection from lightning and evacuation shall be covered under platform Operation and maintenance manuals and Environmental Health and Safety (EHS) procedures.
8. Cable Routing
· Routing outdoor cable as far as practicable, in covered areas or area along a metallic vertical structure where there is lesser risk of the cables.
· Cables for telecom antenna shall be installed in a metal covered cable tray or metal trunking.
9. Testing and Quality Control
· Detail out specifications for testing of bonding system. The earthing bonding inspection shall not rely on a visual means to confirm that a ground clamp and lead are in place for proper grounding or bonding, with confirmation via resistance meters.
· The flexible leads are subject to mechanical damage, wear, corrosion and general deterioration. Hence, recommended to be tested and inspected frequently.
· This quality control inspection should evaluate cleanliness and sharpness of the clamp points, stiffness of the clamp springs, evidence of broken strands in the cables, and solidity of cable attachments.
· A more thorough inspection to be made during commissioning and periodically during maintenance, using an intrinsically-safe ohmmeter to test ohmic resistance and continuity.
· Provide documentation for precautions during testing of panels with SPD.
10. Lightning strike can cause secondary effect such as interference to communication, and Audio video system. An effort shall be made to confirm Electromagnetic Compatibility (EMC) compatibility of systems such as Satellite communication, on site walkie-talkie system, CCTV system, Public address and telephone system.
11. WHP
The WHP was relatively at less risk due to smaller size and shorter structure than CPP.
The WHP was also metallic structure and was covered by crane. It was recommended to follow the same protection philosophy as CPP. It was recommended to identify any critical system such as ESD, PSD system installed on WHP.