Turbine rehabilitation at the 112 MW Ohakuri Hydro Power Station in New Zealand was owner Mighty River Power’s most complex program of works of any it had previously performed.
By Glen Twining
Ohakuri Hydro Power Station in the North Island of New Zealand is the second of nine hydropower stations on the Waikato River owned and operated by Mighty River Power Limited. The Ohakuri plant contains four 28 MW Francis turbines designed and built by Dominion Engineering. The turbines were commissioned in 1961 and were operated through to 2011 without any major rehabilitation work. All four machines have had new turbine equipment installed with the fourth and final machine completed in April 2014. The main project objectives of modernizing the turbines and increasing performance have been achieved.
Project Development Process
With the Ohakuri Turbine Rehabilitation project involving the replacement of existing turbine equipment, the project scope was significantly bigger and more complex than the refurbishment focused projects completed in the past. This meant that more effort was put into collecting requirements and completing project planning. With the modern equipment providing improved turbine performance, the company also needed to determine how the rehabilitated machines should perform to best fit into the company’s generation portfolio. With these constraints in mind, the project was broken down in two project phases as described below.
Phase One: Feasibility and Planning
Phase One of the project provided approval of a budget for one year to carry out detailed analysis of how the rehabilitated turbines should perform, outline a procurement management plan, and plan procurements for major equipment required. This included tendering, evaluation and negotiation of an equipment supply contract for replacement turbine runners and associated equipment. With pricing for the major equipment and with historical pricing for past refurbishment work on site, a detailed budget with higher certainty was developed.
In addition to budget certainty, because the company completed the tender process for the turbine equipment required, the performance improvement was also known with certainty through performance guarantees. This information was used to update the project business case and seek approval for Phase Two.
|The turbine rehabiliation project took care of vaitation damage on the previous units.|
Phase Two: Design, Manufacture, Delivery and Site Works
Phase Two of the project – covering equipment supply and installation works for all four machines – was approved as a whole. It could also be broken into two phases to offer a progressive approach to development of the project budget. This was not needed in this case, as Mighty River Power had a good history of costs for disassembly and refurbishment of turbine components from past maintenance work. After the design of new equipment and any lower level design packages were complete, detailed scoping of the installation of new equipment and refurbishment of components to be reused was completed.
Benefits of the Approach
The two-phase approach to the Ohakuri turbine rehabilitation project allowed the following benefits:
– Provided Mighty River Power the time to properly understand the turbine performance requirements for a rehabilitated turbine. This was key given the changing nature of the generation fuel types in New Zealand with the recent addition of more geothermal power stations and wind farms. New Zealand has a large percentage of hydro generation that traditionally contributed to base-load generation. Adding base-load geothermal generation is changing the role of hydro power stations toward covering variability when wind turbines are not operating.
– Allowed greater certainty regarding the budget once the full project was approved;
– Allowed greater certainty of the project benefits as a result of running the equipment supply tender process in Phase One. The certainty of project budget and project benefits was an important step to fit in with the project governance requirements; and
– Provided a progressive approach so that work packages could be completed to the extent required to give the right inputs for the next phase. This step-by-step process also helped ensure that from a project management perspective there was greater certainty in being able to deliver the project on time, within budget and with the agreed level of quality.
The right approach for delivering a project depends on many factors, such as the technical knowledge available for determining the project requirements and supporting the project execution. The performing organization’s governance requirements will also be a significant determination in the selected approach. The maturity of the organization’s project delivery processes will have significant influence on the model selected and hence the degree of success achieved. All of these factors need to be considered to select the best delivery approach for a project.
Turbine rehabilitation Work
The Ohakuri turbines were typical of designs of the 1950s with regard to quality of the construction and turbine performance. The original runners were cast steel and the blades suffered from significant cavitation damage on the suction side near the outlet. The damage was repaired by welding every three to four years, which resulted in changing blade profile over time, further degrading the turbine performance. Compared to a modern design, the scroll case diameter is small and the turbine has a small wicket gate pitch circle diameter.
Rehabilitation of the Ohakuri turbines involved a substantial amount of work that was a step above the scope and complexity of work typically performed by Mighty River Power during a turbine refurbishment. An overview of the scope items and challenges faced are discussed below, specifically covering the more unique packages of work, such as fixed stay vane and draft tube modification.
From Mighty River Power’s observations of turbine rehabilitation projects similar to the one at Ohakuri, it appears it is the exception rather than the rule to modify significant parts of the turbine other than the runner and wicket gates. This is largely due to the cost of this additional work and the associated benefits. The scope of the Ohakuri turbine rehabilitation project was determined by: minimum requirements to reduce plant breakdown risk, performance gains achievable by replacing existing turbine components, the cost of the base work, and any incremental improvements.
The standard turbine rehabilitation package was to replace the runners and refurbish the remaining components to as-new condition. The objectives were to improve the performance of the turbines to better meet present and future operating conditions.
Originally it was decided to refurbish the existing wicket gates, as there was no performance benefit from replacing them. However this left a project risk of the wicket gates taking longer and costing more to refurbish. For the remaining three rehabilitations, new wicket gates were installed to provide a solution that will last as long as the new turbine runners and significantly reduced the project duration and cost risk.
Fixed Stay Vane Modification
As part of the rehabilitation work, modification of the fixed stay vanes was carried out. The existing stay vanes do have some form of profile to enhance turbine performance, but they are still very rectangular in shape. The design was carried out by the turbine equipment supplier, and the extension pieces were delivered with the new turbine runners.
The weld procedure for fixing the extension pieces to the stay vanes was developed with the input of the equipment supply contractor and Mighty River Power.
Before this work took place, the biggest risk identified by Mighty River Power was significant distortion of the stay ring due to heat applied during the welding process. Steps taken to minimize this problem include detailed planning of the weld procedure and the sequence for how the extensions were welded in place, as well as establishing an acceptable limit for movement.
With a good planning focus for this package of work and the use of personnel with the right skill and experience, the work was completed without problems.
Modifications of the fixed stay vanes may provide a performance benefit depending on the current turbine configuration and how it may link in with other proposed turbine modifications. The simple nature of the modification for Ohakuri made it beneficial to implement. Had the original stay vanes required substantial re-profiling in addition to adding new pieces, the cost would have been significantly higher, thus eroding the economic benefits. Additionally, the more complex the modification, the greater the risk of distortion and potentially introducing or accelerating any end-of-life failure modes into existing equipment. The decision to modify stay vanes needs to be evaluated on a project-by-project basis.
Draft tube modifications
Mighty River Power also decided to proceed with draft tube modifications; the turbine designer produced the new draft tube profile to be constructed on site. Mighty River Power contracted a consulting company, Opus International Consultants, to complete the design of the modifications to meet the new profile, produce the necessary drawings for the new profile and provide construction supervision at key stages of the construction.
Different construction methods were considered. These included raising the floor and thickening the sides of the draft tube or lowering the soffit and thickening the sides of the draft tube. Each option could be achieved by placing permanent steel liners in the draft tube and filling the cavity behind or by traditional concrete construction using removable formwork. The option chosen was to raise the floor with concrete using removable formwork, as this was determined to be the fastest and also the lowest cost method.
One of the biggest challenges faced on this project was health and safety. In Mighty River Power’s past turbine refurbishments, typically there was no major civil construction work, which meant the available civil contractors for the Ohakuri project had little knowledge of the health and safety requirements and work management systems in the electricity industry. Compared to the mechanical and electrical contractors used more frequently, this lack of knowledge presented a new risk that had to be managed while the draft tube work took place.
It was important with the draft tube modification to plan the sequence of activities and what time during the machine outage the draft tube work could be completed, considering that there would be turbine work taking place in parallel and physically above the draft tube work area. Additionally, given the use of CO2 as a generator fire suppressant, there was the additional health and safety risk of being able to quickly evacuate the draft tube area in the event of a CO2 release from an adjacent unit. The work was scheduled to start as soon as the turbine had been disassembled and the components taken away for refurbishment and to be completed in time to not delay reassembly of the turbine. Because of the high alkaline nature of concrete, water used in the construction process and for washing out the concrete truck mixer bowls had to be captured. No waste water could be released into the Waikato River because of the impact it would have on fish and other organisms. While this was not the most difficult part of the work, it added to the overall complexity and required planning to prevent any discharge of water and pre-determined follow-up actions to implement in the event of a spill.
Pouring of the concrete to thicken the sides and raise the floor presented a challenge because of the approximately 70 meter distance of Unit 4 from the station unloading bay and access door and the absence of a suitable area outside the powerhouse to place concrete trucks. A hard piping system was set up from the station unloading bay, along a balcony on the outside of the station, back inside the powerhouse at Unit 4, then vertically down from the generator floor to the draft tube. Because of the approximately 100 meter pipe run, slugs of concrete formed in the bends when deliveries were delayed. Careful planning of concrete arrival on site was required to avoid disassembly of the pipework for cleaning.
Self-compacting concrete was selected for the work to best ensure there were no voids behind the formwork. However, because of the vertical distance the concrete had to be moved, there was some risk of aggregate becoming more concentrated due to gravity. This problem is not new to pouring concrete and just needs to be managed to suit the specific application. Before and after the draft tube modification, Mighty River Power carried out a three-dimensional survey scan of the draft tube to compare to original drawings and then as a quality control step of the modifications. The pre-modification survey was only carried out at the beginning of the work. However, it is recommended that this be done well in advance of the actual work to minimize risk of unplanned work being required.
Although draft tube modifications may be uncommon in turbines of similar head and size as Ohakuri, the design and construction of the modification was very achievable. Due to the nature of the work and fitting in with an overall outage schedule, thorough planning is required to ensure that the works can be implemented according to the budget and schedule, ensure that the final dimensions required are achieved and most importantly that the work is completed safely.
Glen Twining is a Program Manager for Mighty River Power Limited in New Zealand.