Backcountry Inspections and Fitness-for-Service Assessments of Vintage Hydropower Penstocks

Editor’s Note: While IJ primarily features articles from the oil and gas sector, the following case history from the electric utility industry offers valuable insights worth sharing with our readership.

Introduction

Regular equipment inspections are necessary to maintain safe and continued facility operation. With increased power demands in the modern era, power companies have begun implementing asset integrity management programs in the hopes of extending the life of their assets. However, conceptualizing the logistics of inspecting assets in the backcountry revealed novel obstacles and safety considerations. Namely, altitude sickness, aggressive animals, and rope access on dangerous terrain were just a few of these considerations. Further, the logistics of moving equipment, personnel, and other resources into and across mountainous terrain present additional challenges. Extensive pre-planning and constant communication during the inspection are necessary to achieve safety and efficiency.

This article discusses one such inspection to illustrate both the key planning considerations as well as the data typically gained from this type of inspection. Quest Integrity was approached to conduct fitness-for-service (FFS) assessments to ensure the safety and reliability of aging backcountry assets. Engineers reimagined traditional inspection methods to develop a bespoke backcountry inspection protocol and interval for 100-year-old penstocks located in heavily forested, mountainous terrain in the southern United States. A hydropower penstock is a large pipe that channels water under pressure from a reservoir to turbines in a hydroelectric power plant, enabling electricity generation. It must withstand high hydraulic forces and is typically made of steel. Over time, penstocks can suffer damage from corrosion, erosion, pressure surges (water hammer), structural stress, and fatigue, all of which can compromise performance and safety. This approach enabled the determination of the asset’s FFS and remaining life as part of an operator’s asset integrity management program.

Pre-Inspection Considerations and Analysis

Prior to mobilizing to the site, design drawings and loading calculations using ASCE 7-16 were used to conduct a finite element analysis (FEA) of the penstocks [1]. Stress results from the FEA were then used to pre-screen areas of the penstock. Regions with high stresses were ranked by severity and marked for further focused inspection. Areas typically highlighted by the FEA were near anchor blocks, bends, and using older construction methods. The FEA focused on the ultrasonic thickness (UT) and phased array ultrasonic testing (PAUT) inspection for any evidence of corrosion, cracking, or other potential issues. Some portions of the penstock were inaccessible due to terrain and underground sections, so PAUT and UT were focused to the areas of concern (i.e., bends, thickness changes, and appurtenances) identified in the FEA model. Results from those areas were assumed to be representative of the whole.

In addition to the focused PAUT and UT inspections, spot UT measurements were taken to check the thickness of the penstock outside of the focused inspection regions. A minimum of 15 measurements were taken in each discrete thickness section, regardless of the PAUT inspection locations, to provide statistically relevant data for the FFS analyses. The metallurgical inspection and replication locations were selected to assess both the upstream and downstream sides of each type of steel specified by the drawings. Given that many parts of the penstock were located on steep slopes or buried, replications were taken at a minimum near the top and bottom portions, near the dam and powerhouse, respectively.

The season and snowpack were also considered during the planning stage. This geographic location tends to receive considerable late fall to early spring snowfall, which would render inspection difficult if not impossible. Therefore, historical forecasts, nearby ski resort snowpack data, and long-term forecasts were consulted to inform the inspection timing. These and other safety concerns are detailed in the following section.

Backcountry Inspection

Typical power plant or process facility inspections consider confined space entry, working from heights, and chemical exposure. In industrial settings, working from heights and fall protection usually proceed on the assumption of anchor points and/or relatively flat places on which to stand and work, such as scaffolding or permanent stairs/manways. However, safely performing backcountry inspections requires innovative strategies. Steep slopes rarely afford a safe place to “set up,” so this work must be done before moving onto the slope. Non-permanent fall protection must be installed by skilled roped access technicians (Figure 1) who place and anchor ropes to allow travel on loose and/or steep slopes. Once in place, workers must maintain constant awareness of falling rocks or loose footing.