Page 48: of Offshore Engineer Magazine (Oct/Nov 2013)

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charge into the air-gap cloud structure.

DAS reduces near-surface, electric-feld

The presence of the corona space strength to levels below which lightning charge in the gap smoothes the redis- streamer formation is likely (Fig. 2). tribution of voltage across the gap, so

With no streamers emanating from the

EPIC the initiation of the upward leader structure of concern, the lightning leader from the structure becomes impossible is more likely to connect to streamers under the infuence of only the electri- originating from either unprotected adja-

Fig. 3. Browns Ferry historical strike cal feld of a thundercloud.

cent structures (man-made or natural) or data shows reduced lightning after DAS

The even distribution of the total from any air terminals installed on these installation: top row is 3 years prior; corona current among the multitude of unprotected structures. bottom row is 3 years post-DAS.

The principle is consistent with the array’s splines increases on orders

Gauss’ electric feld divergence law, of lightning strikes around the off-gas of magnitude the total corona current which states (in one dimension) that stack in the three years before and after required for ignition of the streamer the vertical growth of the electric-fux DAS implementation. They compared fashes leading to the start of the density is proportional to the space- the number and location of lightning upward leader. Such high current can charge density. If this space-charge strikes within 500m, 3mi., 6mi., and be produced only by the electric feld density is positive near the surface 10mi. radius of the off-gas stack for of the fast-moving and not too distant (which is the typical case during a these periods (Fig. 3). downward-stepped leader.

negatively-charged leader approach), The weighted data for strikes The LPAS of a large diameter and then the electric feld increases to its showed that although lightning fre- the thousands of splines decreases the peak strength near the leader tip from a quency increased nearly 65% in the 3-, equivalent radius of attraction several surface value near the protected equip- 6-, and 10mi radii around the stack, in times compared with the lightning ment that is lower than if the space the three years after DAS implementa- rod of the same height. That results charge were not present. tion, an 80% reduction in lightning in a decrease of the total number of

As a result of this lower near-surface strikes was realized within 500m of the lightning strikes to a tall structure by feld strength, there is less likeli- stack. The result has been no light- an order of magnitude.

hood of streamer formation near the ning strikes to the off-gas stack since To date, Lighning Eliminators (LEC) protected equipment, and hence less installation. has installed over 3000 lightning likelihood of leader-streamer connec- protection solutions in more than 70 tion. The principal of operation rests countries and throughout the United

Recent work on the DAS being able to rapidly emit States, including recent work on

More recently, an analysis of the phys- ample space charge during the time of offshore platforms in Mexico, Egypt, ics behind charge transfer technology leader approach. Accordingly, the DAS Nigeria, Malaysia, and South Korea. and DAS was presented at the 2012 design attempts to maximize space- The company is currently experiencing

International Conference on Lightning charge emission, but without generat- the most growth outside of the United

Protection (ICLP). A paper titled ing streamers of its own. States, in countries like Qatar where

Lightning Protection of Tall Structures, companies are being proactive about in which the charge transfer system is the future dangers of lightning, includ- called an LPAS, concludes the following:

Browns Ferry example

A study was done at the Browns Ferry An array of thin metal splines with ing on and offshore facilities.

Nuclear Plant (BFN). BFN is an excel- sharpened tips evenly distributed over Lightning threats to the off- lent test case for this lightning protec- a large-radius, hemispherical surface shore community are very real. tion technology, since the science and (LPAS) can control the process of light- Thermodynamic processes generally methodology for protecting any facility ning discharge into the protected tall start near shore with the potential to is the same. structure by injecting the corona space evolve into open water events. Rising

In 1998, a DAS was temperatures worldwide are installed on the off-gas causing this increase in the stack, replacing a traditional frequency and severity of lightning protection system. lightning strikes.

Prior to DAS installation, lightning was repeatedly

Ofshore protection attracted to the off-gas stack In 2012, LEC produced and equipment on the stack lightning protection solu- and around its base was tions for nearly a dozen dif- routinely damaged. ferent companies’ offshore

As part of an internal platforms and rigs. Many of review process, BFN con- these included the DAS no-

Fig. 2. Production of positive space- sulted a database of light- strike protection system to charge around the DAS reduces near-surface, electric-feld strength, ning activity to determine prevent future costs associ- inhibiting lightning streamer formation.

the number and location ated with lightning strikes.

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