Mill Hydro Penstock Relining

Project  Summary

 Bancroft Contracting Corporation re-built approximately 400 linear feet of the Number 4 Penstock at the Mill Hydro hydroelectric facility in Millinocket, Maine for Brookfield Power of Gatineau, Quebec.  The Number 4 Penstock is one of six similar penstocks on the site, each feeding a generator/turbine pairing. Brookfield Power plans similar re-construction of several of the site’s remaining penstocks in coming years.

The Mill Hydro Penstock project required Bancroft Contracting to expose an existing 10-foot diameter underground steel penstock, cut two 25-foot openings in it, and then install a 9-foot diameter steel penstock inside it.  Bancroft installed the new penstock by lowering short sections through the access openings and pulling the new  penstock sections as far as 150 feet through the existing pipe.  The project demanded precise positioning of the new penstock within the old one.  Each section of new penstock material had to be exactly aligned  and welded out completely in multiple passes.   Finally, the void between the two penstocks had to be filled with flowable grout, the responsibility of subcontractor Knowles Industrial Services of Gorham, Maine.

 The project involved complex heavy rigging challenges, placement of over  a mile of full-penetration weld and pumping of approximately 215 cubic yards of flowable grout.

Cost of the project was approximately  $1.4 million.   Project Manager for Brookfield Power was Max Upton, engineering was by Devine, Tarbell & Associates. Installation of flowable grout was subcontracted to Knowles Industrial Services. Bancroft Contracting Corporation served as general contractor, represented on site by Kent Lewin, Project Superintendent.

Contribution to the Community

The Mill Hydro Penstock project represents a substantial investment in clean, renewable hydro-electric generation at this site by project owner Brookfield Power.  In today’s climate of wildly fluctuating fossil fuel prices and increasing concern over the environmental impact of generating electricity by burning fossil fuels,  the Mill Hydro Penstock project points in a different direction.  Brookfield’s continuing investment at the Mill Hydro site provides the community a proven source of environmentally responsible, cost-effective electrical generation.


Job Challenges & Innovation

The technical challenges inherent in the Mill Hydro Penstock project can hardly be overstated.  Each section of the new nine-foot diameter penstock material weighed up to 20,000 pounds and measured up to 20 feet in length.  It would have been challenging to assemble these pieces into a functioning penstock on their own and above ground.  However, Bancroft Contracting faced the challenge of assembling these sections underground and within the existing 10-foot pipe.  The Bancroft team responded to this challenge by creating a unique rail system and installing it in the bottom of the existing penstock.  The Bancroft crew then used the rails in conjunction a system of Teflon slides,  air-tuggers and hand-rigging to pull each successive section of the new penstock into place, in some cases over distances as long as  150 feet.


Challenges did not end with rough positioning of the new penstock sections.  Once pulled into position, each piece of the new penstock had to be aligned precisely with its mate and, further, positioned exactly in the center of the existing penstock.  Bancroft accomplished the precision alignment of the multi-thousand-pound penstock sections  using a system of screw-jacks that penetrated the walls of the new penstock and pushed off the walls of the existing pipe.


Once precise alignment of each new section was achieved, Bancroft welding crews faced the challenge of welding out the joints in materials up to 11/16-inch thick without access to the outside of the joint.  Each joint required over 28 linear feet of welded bead per pass and all joints required a minimum of seven passes in order to pass the rigorous ultrasound testing of each joint.  In all, Bancroft welders laid down 7,000 linear feet of welded bead.


As each successive section of the new penstock was placed, positioned and welded out, subcontractor Knowles Industrial Services  had responsibility for pumping flowable grout into the space between the old and new penstocks, completely filling the void and leaving the new penstock permanently anchored within the old.


Finally, as work progressed, all penetrations in the new penstock had to be patched and welded and all patches and joints had to be ground smooth, prepped, and painted.


One further aspect of the project offered a particular challenge.  The scope of work required replacing the scroll case elbow at the junction between the Number 4 penstock and turbine.  Bancroft crews floated this 7,000 pound 9-foot diameter long-radius elbow down Millinocket Stream and into the powerhouse through the tailrace of Unit 4.  Bancroft utilized floatation bags, a boat and a dive team to accomplish this tricky positioning.  Bancroft crews then built and utilized an overhead monorail to remove the old scroll case and install the new one.


Given the difficulty of access, the unprecedented rigging challenges, and the sheer amount of work that had to be accomplished within the confines of a nine-foot diameter pipe filled with obstructions, Bancroft site management and crew, owner Brookfield Power and subcontractor Knowles Industrial are to be commended for completing the project with no injury worse than a wrenched knee that resulted when a worker slipped and lost his footing within the wet pipe.




In addition to the environmental advantages inherent in hydro-electric generation as outlined above, Bancroft Contracting was able to construct the project with minimal impact to the adjacent watercourses by strictly isolating and minimizing introduction of construction pollutants into the water flow.  It is our belief that re-construction of the penstock had essentially no impact on the downstream Dolby Flowage.


Response to Client Needs


Original proposals from contractors exceeded Brookfield’s project budget.  Bancroft Contracting provided Brookfield with value engineering that  reduced the project’s cost.  Bancroft identified and delivered considerable savings in several areas:


Site De-watering


The Bancroft team was confident that an alternative to the site de-watering plan specified in the bid documents would save money.  Accordingly Bancroft offered to complete the de-watering scope on a Time & Materials, Not-to-Exceed basis.  This approach resulted in savings for Brookfield Power.


Fit-up of Penstock Sections


Meeting the original fit-up specifications for joining penstocks sections would have been challenging and costly.  Bancroft Contracting offered alternate pre-qualified weld details for various joint fit-up spaces.  This allowed Bancroft to  achieve the integrity required by the bid documents at lower cost to Brookfield Power.


Cleaning the Existing Penstock


The bid documents required use of a 10,000 psi pressure washer to clean the existing penstock. Using that equipment would have required a specialty contractor.  Bancroft Contracting was confident that a 5,000 psi washer would accomplish an acceptable cleaning using more labor man-hours but, ultimately, at less cost than the original spec.  Bancroft offered and Brookfield accepted a price-reduction for the alternate cleaning plan.


Construction Phase Savings


Through creativity, alternate approaches to the project, and great cooperation,  Brookfield and Bancroft together cut one month from the original construction schedule. This allowed Brookfield Power to begin generating electricity a month earlier than planned.


The original construction plan also called for shutting down an adjacent generating unit during some phases of construction because the draft tube of the adjacent unit was expected to interfere with removal of the scroll case elbow replaced as part of the work.  Bancroft’s unique overhead monorail system, however, allowed replacement of the elbow without removing the draft tube.  This enabled Brookfield Power to continue generating power with the adjacent unit, at least intermittently.


Bancroft and Brookfield also saved time by limiting the number of openings in the existing penstock to two.  Initially, project designers believed three openings in the existing line would be necessary.  However, in the course of construction, Bancroft and Brookfield recognized that offsets in the existing line were not sharp enough to prevent inserting the new penstock.  Bancroft accomplished the project with only two openings in the existing line.