Redeveloping Aberfeldie21 July 2009
Michael J Robertson of consulting engineers Knight Piésold discusses the technical aspects of a project to redevelop BC Hydro’s Aberfeldie hydroelectric project, including the optimization exercise, geotechnical investigations, hydraulic model studies of possible sediment excluders, removal of the old components and replacement with new
The Aberfeldie Hydroelectric Project on the Bull River in Southeastern British Columbia, Canada, is owned by the provincial power utility, BC Hydro. The original 5MW facility was constructed in 1922, with water for power generation diverted from the river with a small timber crib dam and transported via woodstave conduit and steel penstock to a powerhouse where it was then returned to the river. The original dam was replaced with a larger concrete gravity dam in 1953, constructed further downstream, between the original intake and the powerhouse.
BC Hydro decided in the early 2000’s that the facility had reached the end of its useful life (in particular the woodstave conduit and much of the electro-mechanical equipment) and initiated the redevelopment project. This started with an exercise to determine the economic optimum size for the facility, followed by geotechnical site investigations and detail design and construction of a completely new facility (with the single exception that the 1953 dam remains). In a departure from tradition, consulting engineering was outsourced by BC Hydro to a team headed by Knight Piésold Ltd (KPL) of Vancouver, assisted by subconsultants Amnis Engineering, Gygax Engineering Associates, Rutter-Hinz and JM Bean and Co. Ltd. One of the aims of the outsourcing was to familiarize BC Hydro with the work being done by private sector independent power producers in BC. Construction was done through conventional design-bid-build contracts: main (civil) works by contractor Western Versatile Construction Company and generating equipment by contractor VA Tech Hydro (now Andritz). Construction is presently essentially complete, with all three horizontal Francis turbines now in service. The official opening was held on Friday 12 June 2009.
Water Survey of Canada has maintained a flow measurement gauge on the Bull River immediately downstream of the point of discharge of the powerhouse tailrace, with a period of record in excess of 90 years. This provided a very good basis for reevaluating the hydro power potential of the project. Flows at the intake location were estimated by adjusting the basic flow record on the basis of drainage area (1530km2 at the gauge and 1528.5km2 at the intake), resulting in a long-term Mean Annual Discharge (MAD) estimate of 31.85m3/sec.
Not all the water in the river is available for power generation; a prescribed minimum must be left in the river at all times to protect its health, primarily the fish. These so-called Minimum Instream Flow Requirements (IFRs) vary from project to project and with time of year. They have been set for Aberfeldie as 0.25m3/sec for December to March, 0.5m3/sec for April, May, October and November and 2m3/sec for June to September. Table 1 is a summary of project details and Figure 1 a general arrangement plan of the project.
Prior to engaging KPL, BC Hydro had performed its own studies to determine the optimum capacity for the new development. KPL’s first task was to confirm BC Hydro’s conclusions. The optimisation exercise proceeded as follows:
• A decision was made to locate the new powerhouse adjacent to and downstream of the existing one rather than at a new site some 250m upstream.
• It was determined that the possible incorporation of a rubber dam on the existing Aberfeldie Dam spillway crest was not cost-effective (the extra head and energy did not justify the cost).
• For each of three chosen capacities (20, 25 and 30MW) the Low Pressure Conduit (LPC or low pressure section of the waterway) was first optimised to find the greatest benefit/cost of various pipe sizes and types, comparing the capital cost of the installed pipe (and bulk bench excavation where appropriate) with the value of the average annual energy generated (the net head at the turbines depends on the friction lost in the waterway).
• The High Pressure Penstock (HPP or high pressure section of the waterway) was then sized one diameter smaller than the optimized LPC, for each capacity.
• This was followed by the optimisation of the turbine-generator machine arrangement for each capacity, using the relevant optimised waterway. Combinations considered were two equal machines, three equal machines, four equal machines, two equal machines and one smaller machine, and two unequal machines. Horizontal axis Francis machines were assumed throughout. Comparison was on the basis of a Modified Unit Energy Cost (UEC), also incorporating estimates of the cost of the intake modifications, the powerhouse and other relevant data provided by BC Hydro. The UEC was ‘modified’ to reflect the fact that the value of energy generated differs from month to month.
• Modified UECs were compared over the range of capacity and machine options, for a range of discount rates.
The study concluded that the Aberfeldie Project could be redeveloped with a capacity anywhere between 20 and 25MW, using three equal sized horizontal axis Francis turbine-generators, without sacrificing any significant cost effectiveness. BC Hydro chose to redevelop on the basis of the 25MW option.
It was determined that with this optimised arrangement the project will produce an average of 105GWh of energy per year, corresponding to a plant capacity factor of approximately 48%.
Geotechnical investigations were undertaken in 2005 to provide foundation data for the design of the new facilities – modifications to the existing intakes at the Aberfeldie Dam, and construction of a new LPC, Surge Facility, HPP, Powerhouse and Switchyard. The site had also been investigated by KPL in 1990; the 2005 investigations incorporated those findings.
The 2005 investigations comprised:
• Three boreholes, four seismic cone penetration tests and one hand excavated test pit at the location of the proposed powerhouse. These holes were undertaken in order to establish ground and groundwater conditions for the geotechnical design.
• Three machine excavated test pits and one hand excavated test pit along the pipeline alignment in order to investigate further the nature and extent of organic soils identified in the fill platform in the 1990 investigation and to assess the downslope inclination of the rockhead surface.
• Three machine excavated test pits at the location of the proposed waterway headworks structures immediately downstream of the dam.
The investigations concluded that:
• At the site of the proposed powerhouse, the bedrock is overlain by a succession of alluvial and glacial deposits, which become thicker towards the Bull River. Subsequent seismic and differential settlement analyses of these superficial deposits confirmed the viability of a raft foundation solution for the proposed powerhouse. There was particular concern about possible seismic liquefaction of zones of cohesionless sand in the alluvium. Groundwater control during construction was identified as another significant issue, as was the possible impact of excavation works upon the stability of the adjacent natural soil slope.
• The ground conditions along the LPC alignment generally provide adequate founding material. However, there were sections identified where the existing woodstave pipeline had settled differentially due to uncompacted and/or saturated ground. The investigation also confirmed the presence of unsuitable founding material in the form of topsoil that was covered by fill placed during the original construction. All adverse foundation materials were removed and replaced with suitable fill. Significant volumes of rock were confirmed towards the end of the LPC, with some unstable sections in existing cuts.
• The rock in the face behind the powerhouse terrace, extending through the surge tank site at the top of the cliff and the end of the LPC, comprises moderately strong to strong sandstone and weak to moderately weak siltstone. The investigations revealed unstable sections of the natural rock slopes that would need to be (and later were) stabilized.
Water enters the waterway through intakes in the Aberfeldie Dam. The old scheme was fed through one intake, with a design flow of approximately 8m3/sec. Provision was also made during construction of the dam for a second intake, which was effectively left blanked off. These two intakes were modified so that each can take approximately 20m3/sec. Modifications included:
• a new trashrack on the face of the dam, together with an automatic raking machine;
• retention of the existing single set of stoplogs, supply of a second set and refurbishment of the slots into which they are designed to be installed;
• new head gates on both intakes, capable of remote and automatic hydraulic operation;
• removal of the existing steel pipe and replacement with a larger new one in the existing intake, and installation of an identical new pipe in the originally blanked-off intake; and
• a vortex suppression device upstream of the intakes and/or in the stoplog and gate slots, if this should prove desirable when tested during operation.
The modified intakes were included in a physical hydraulic model, as discussed in the following section.
Photos 1 and 2 above are general views of the dam and intake before and after the redevelopment and Figure 2 below is a schematic cross section after redevelopment.
The Aberfeldie Reservoir is filled with sediment and can no longer trap most of the sediment being transported by the Bull River. As the water abstracted for power generation will therefore contain some entrained sediment and this will subject the waterway and generating and other equipment to abrasion, an attempt was made to design a sediment removal facility at the head of the waterway. Knight Piésold has designed a number of run-of-river intakes for hydroelectric projects based on the opportunistic use of a lateral draw-off trough and submerged weir in the intake structure to pass the necessary IFR flows. We have found that the horizontal vortex induced by this design can result in 90% or more of the entrained sediment being discharged with the IFR flows rather than going on through the penstock, although it has to be said that the results are largely qualitative and the whole sediment delivery and transport process is difficult to predict and model with any quantitative accuracy.
It was originally anticipated that the Aberfeldie sediment exclusion structure would be provided immediately downstream of the dam and the modified intakes, on the bench of the existing pipeline. The structure was envisaged as a large open box, collecting the flow from the two inlet pipes, and passing it through a horizontal vortex sediment removal device and over a submerged weir before it enters the LPC. The discard flow from the structure would have provided the IFRs.
A physical hydraulic model was constructed to investigate this structure. It consisted of the existing dam, the proposed modifications to the existing intakes and the proposed sediment exclusion structure, to determine final hydraulic design details for these structures. The model showed that the sediment excluder was only partially successful in removing the sediment and that a significant proportion still went through to the turbines. BC Hydro therefore decided that it was not cost-effective to incorporate the excluder. The head of the waterway was therefore changed to a steel pipe wye, with offtakes in the bottom of both legs to provide the IFR flow return. The bottom offtakes will also trap and divert the larger sized portions of any sediment that is drawn into the intakes.
Sediment entrainment and abrasive equipment wear will be minimised through good operation (including, for example, closing down the turbines during large floods, when the flood waters are carrying high sediment concentrations). It was also noted, however, that the old turbine runners showed comparatively little abrasive erosion for their age, probably because of the nature of the sediment and the relatively low operating head on the turbines.
The site investigation included the excavation of a number of test pits at the proposed site of the sediment exclusion structure. All revealed solid rock within approximately 1.5m of the existing ground surface.
Waterway including surge facility
The waterway comprises the following main components:
• Headworks immediately below the dam where the two outlet pipes join in a wye at the head of the waterway – as discussed earlier.
• A Low Pressure Conduit (LPC) consisting of approximately 850m of buried 3.35m diameter Weholite HDPE pipe together with approximately 150m of surface-mounted 2.9m diameter steel pipe, the latter including two gulley crossings. Weholite is a structured-wall lightweight pipe manufactured as a speciality product by KWH Pipe, Canada. Photos 3 and 4 compare the original woodstave pipe with the new Weholite HDPE pipe. The old woodstave surge tank can also be seen in the background of Photo 3, and the new steel section of the LPC in Photo 6. Figure 3 shows a typical cross section of the buried LPC.
• A steel surge tank 15m high and 13m diameter at the junction of the LPC and HPP, to cater for transient pressures and surges during operation. The new surge tank can be seen in Photo 6.
• A 2.74m diameter continuously welded steel High Pressure Penstock (HPP) approximately 150m long down the cliff face immediately behind the powerhouse. The pipe is laid as straight as possible, on support pedestals, with appropriate thrust blocks and movement couplings. It terminates behind the powerhouse in a generating unit and emergency bypass distribution manifold. The old HPP can be seen in Photo 5 and the new one in Photo 6. Figure 4 shows a typical cross section of the HPP.
The new waterway follows as far as possible the alignment of the original, with the LPC on the same bench cut into the hillside and the HPP basically following the same path down the cliff. Exceptions are that the LPC had to be cut lower, to ensure that it remains safely under the hydraulic grade line, and all the bends but one in the HPP were removed.
The powerhouse has been designed to house three identical Francis horizontal axis turbine-generators procured through a ‘Water-to-Wire’ generating equipment contract, along with the associated Turbine Inlet Valves (TIVs) and other mechanical, electrical and hydraulic equipment. It also houses all the mechanical and electrical ‘Balance of Plant’ equipment. The machines are arranged ‘en echelon’ to optimize the use of space and the hydraulics of the penstock manifold. The building was set, in the vertical plane, primarily by the level of the tailrace weir (to exclude the annual flood in the Bull River) and the amount of submergence required by the turbines.
The substructure of the building is reinforced concrete, sized to suit the dimensions and forces supplied by the generating equipment contractor and the steel building superstructure, and to protect against flotation. It is founded on the insitu alluvial deposits that the site investigation showed to be more than adequate for the purpose. A movement joint is incorporated at the end of the HPP to allow for any differential settlement that might occur between the powerhouse and the HPP, which is founded on solid rock. The superstructure is a ‘pre-engineered’ insulated steel frame building complete with full service 65 tonne gantry crane supported by the main structural columns. The building includes control and washrooms and a service bay.
Photos 5 and 6 are external views of the old and new powerhouses, together with the HPPs. Photo 6 also shows the new surge tank and the end of the new LPC (the steel surface mounted section). Photos 7 and 8 are internal views of the old and new powerhouses. Figures 5 and 6 show the basic powerhouse plan and section respectively.
Emergency bypass facility
In the event of an electrical load rejection or any other event leading to a rapid closure of the turbines, there will be a lag time between the subsequent spill of water at the intake/dam and its arrival through the natural watercourse at the powerhouse tailrace. This could result in fish being stranded until the rejected flow arrives. A bypass facility is required to mitigate this possibility. The facility will open immediately when the turbines start to close and will then close (ramp down) slowly over a sufficient time period to allow the fish to escape into the main river channel. Ramping of the units will also be required when generation starts (ramping up). Ramp rates are in the order of two hours between full flow and zero.
The bypass has been designed to be a separate facility adjacent to the powerhouse as opposed to being incorporated inside the building. Bypass design capacity was established by BC Hydro, in consultation with Federal, Provincial and Local Agencies and other interested parties, as 50% of the maximum design flow, or approximately 20m3/sec. After much investigation of appropriate valves a wafer multi-port valve designed and manufactured by Stealth was selected, with backup/isolating butterfly valve and submerged diffuser. Water is finally discharged over a weir into the powerhouse tailrace.
‘Water-to-wire’ generating equipment
The initial optimization exercise indicated that the ‘Water-to-Wire’ (WTW) generating equipment should be three identical horizontal-axis Francis turbines direct-coupled to three identical horizontal-axis salient-pole synchronous generators, operating with a combined maximum design flow of 38.5m3/sec (12.83m3/sec each), and a gross head of approximately 84m. WTW tenderers were given a table of flows and net heads immediately upstream of the turbine inlet valves and asked to complete Performance Guarantees for their equipment based on the inlet conditions (i.e. guaranteed output MW for each flow). Since the project operates run-of-river, with no storage, there is effectively very little variation in dam water levels and subsequent heads across the turbines.
The WTW scope of supply is summarized as follows:
• Upstream pipe extension: a 2m long piece of steel pipe designed to be field fitted to the penstock manifold after installation of all WTW equipment.
• Turbine Inlet Valve (TIV): a flanged, open-lattice type, high performance butterfly valve, selected to match turbine inlet piping and minimise head loss. Capable of emergency shutoff against rated turbine flow.
• Francis turbine: horizontal-axis, overshot, overhung on generator shaft.
• Generator: horizontal-axis, salient pole, synchronous, closed circuit water cooled.
• Hydraulic Power Unit (HPU): one per generating unit.
• Generator switchgear/breaker: 13.8kV.
• Station service power supplies including transformer and auto-transfer switch, batteries and chargers, MCCs for equipment in supplier’s scope, etc.
• Control and protection panel, including PLC, I/O and I/O cabinets for unit control logic and synchronization, instrumentation monitoring and data logging, etc. Includes provision for communication with works at intake (water level sensor, isolating gate control, etc).
• Human-Machine Interface (HMI), consisting of PC-based annunciation and display screens for alarms, set points and control variables, data logging and trending.
• Installation, erection and commissioning of all equipment.
Electrical/mechanical balance of plant
The main civil contract included the supply and installation of all necessary mechanical and electrical equipment not included in the WTW Contract. So-called ‘Balance of Plant’ includes:
• Site services (service and potable water, septic and sewer system, compressed air, cooling water, dewatering systems, etc., as required).
• Building grounding system (up to individual equipment pigtails).
• Powerhouse AC power distribution to auxiliaries.
• Power and control cable tray layouts (including tray installation details).
• Interconnection wiring diagrams for items outside of the generating equipment contract.
• Building lighting layouts and details.
• Powerhouse crane.
• Heating, ventilation, and air conditioning as required.
• Powerhouse internal and external communications.
• Wiring/cabling to switchyard.
• Electrical and control services at the intake
Switchyard and interconnection
The scope for the switchyard design included all systems and equipment from the limit of the WTW Contract (the generator MV unit switchgear) through to the interconnection with the existing BC Hydro transmission line at the site.
The project has successfully achieved its overall objectives of delivering more generating capacity and energy to BC Hydro and the people of British Columbia and it has done so in a cost-effective and environmentally responsible manner. The old 5MW generating facility that had reached the end of its useful life has been completely replaced with a 25MW facility and the average annual energy production has gone from 34 to 105GWh. The redeveloped project is operating successfully.
Michael J Robertson, Knight Piésold Ltd., Vancouver, Canada, [email protected]