The 2005 Hurricane Season was Possibly the Most Active Atlantic Hurricane Season on Record, with a total of 28 storms, 15 hurricanes and 7 major hurricanes. Four of the major storms — Dennis, Katrina, Rita and Wilma — struck the Gulf Coast, resulting in more than 2200 fatalities and total damages exceeding US$120 billion. Hurricane Katrina alone caused more than $80 billion in damage, the most expensive natural disaster in U.S. history.
I live in Gulfport, Mississippi, so my experience in damage assessment is both personal and professional. These extreme weather events have caused the public to question whether more can be done to prevent power outages or reduce restoration time. As a result, the public service commissions (PSCs) of several states initiated formal inquiries into the need to harden the systems.
Florida Power and Light (FP&L) reported that approximately 1% of distribution poles that experienced hurricane-force winds during Wilma failed, and that most failures were associated with fallen trees or wind-blown debris. FP&L installed 930 miles (1497 km) of overhead distribution conductor and 570 miles (917 km) of overhead service conductor, and made 1.1 million overhead splices, but replaced only 12,632 distribution poles as a result of the 2005 hurricanes. Both wood and non-wood transmission lines also suffered damage during Wilma, including a section of a major 500-kV steel transmission line. FP&L's forensic analysis showed that the poles in its system performed as expected.
STORM DAMAGE STATISTICS
In the Gulf Coast region, wood pole performance was initially questioned. Texas and Florida considered eliminating the use of wood transmission poles. Information provided to the Florida PSC indicated proportionate failures of wood and concrete distribution poles. Forensic studies of wood pole failures found few failures associated with groundline decay.
Mississippi Power reportedly sustained damage to about 65% of its T&D system, which consists of approximately 6000 miles (9656 km) of line. There were around 26,500 spans down, but they replaced only about 9000 poles out of their standing inventory of approximately 200,000 poles.
In September 2005, Rita struck western Louisiana and eastern Texas, causing extensive damage in the coastal areas. In a presentation filed with the Texas Public Utility Commission on Jan. 30, 2006, Entergy reported that 77% of its 372,891 Texas customers were without power for some period of time. Approximately 8870 distribution poles, or 2.8% of its standing distribution inventory, were replaced. Approximately 5.6% of its 142,358 distribution transformers were replaced. In addition, 981, or 2.8%, of the 34,600 wood transmission poles were replaced, and 26, or 2.8%, of the 940 steel-lattice transmission towers required replacement.
Field observations show that most of the distribution system failures are associated with secondary-damage effects such as fallen trees and that designing for extreme wind would have little effect on system performance. The Florida PSC initially considered requiring all distribution lines be designed to the NESC extreme wind loading criteria, even though, based on these field reports, the 2007 NESC retained the extreme wind exemption for structures 60 ft (18 m) and less in height.
Overhead systems can be made stronger by using stronger poles and other system components, reducing spans, increasing guying, increasing safety factors using present design loads or increasing extreme weather design loads to higher values. Reducing span length and significantly increasing the number of poles per mile is one way to meet the NESC extreme wind criteria. If secondary-damage effects are indeed responsible for most pole failures, then this approach could result in more pole failures rather than fewer, and the time to restore service could be longer rather than shorter.
The utilities in Florida indicated that requiring distribution systems to be designed to extreme wind criteria would double to quadruple the cost. Based on this feedback from utilities, the Florida PSC dropped its demand that all distribution lines be designed to the NESC extreme wind load criteria. Instead, the PSC has asked utilities to harden lines serving critical infrastructure such as hospitals, police stations and fuel terminals, and to complete forensic analyses in future storms to evaluate the effectiveness of the targeted hardening. In addition, Florida has mandated inspections at set frequencies for all lines and improved vegetation management for all lines.
Martin Rollins, a principal at H.M. Rollins Co. Inc. and a registered professional engineer, testified on behalf of the North American Wood Pole Council (www.woodpoles.org) before the Florida PSC during its storm-hardening debate. He is a member of ANSI O5.1, the committee responsible for wood pole standards, and provides technical assistance to the wood pole industry in various activities. firstname.lastname@example.org