The 7.2-MW expansion of the 6-MW Maggotty Falls hydropower project began in 2012 and was completed in 2014. It was the first new hydro facility implemented in Jamaica in the past 30 years.
By Alston Watson and S. Mark Allen
According to the Jamaican Ministry of Science Technology Energy & Mining, “Jamaica’s energy system is highly dependent on imported fossil fuels; petroleum imports account for over 90% of electricity production. This oil import dependency comes at a high cost. Oil import costs peaked with the global oil price spike in 2008, reaching US$2.7 billion. Import costs remain high and can be expected to increase further as oil prices rise in the future. Electricity prices for Jamaica’s people have also shot up dramatically in recent years, reaching a high of US$0.42 per kWh. Consequently, Jamaica is charting a new path to energy security based on domestic renewable energy sources in order to build an energy system that is socially, economically and environmentally sustainable.”
Jamaica Public Service Company Ltd. (JPS) is a privatized electrical utility majority owned by subsidiaries of Marubeni Corp. of Japan and Korea East-West Power Co. In 2008, JPS requested MWH (now part of Stantec) to evaluate several potential sites to augment the utility’s existing hydropower 24.72-MW capacity from eight JPS facilities in an effort to reduce the dependency on expensive imported fuels.
MWH completed its feasibility study in June 2008 and concluded it would be viable to double the capacity of the nearly 50-year-old 6-MW Maggoty Falls facility (see Figure 1) on the Black River in the parish of St. Elizabeth, in western Jamaica. This facility was chosen above the other options under evaluation based on least cost consideration. Expansion would involve replacing the woodstave and steel portions of the existing penstock, which were leaking and prone to failure, using a larger-diameter penstock and the addition of two turbine-generator units located in a new powerhouse constructed alongside the existing powerhouse. JPS maintained the existing powerhouse over the years, which included the upgrade of the electrical and control system. The existing powerhouse operated with a high level of reliability over the years and will remain as part of the hydro fleet for years to come.
The following provides a brief history of the project, some of the challenges encountered in executing the work and how these challenges were met to achieve a successful project outcome.
The existing Maggotty Falls hydro facility included a diversion weir with an intake located on the left bank. The project expansion called for modifying the existing headworks, constructing a new pipeline, modifying the existing surge tank and constructing a new powerhouse. JPS expressed its preference that the pipeline be replaced with a fiberglass-reinforced-plastic (GRP) pipe because of its minimal maintenance requirements. Basic and detailed design of the facilities would be performed by an engineering, procurement and construction (EPC) contractor.
As the first significant hydropower project development in Jamaica in decades, JPS encountered not only the technical challenges inherent in a major energy project but significant scrutiny on the part of the press and the public. JPS also had stringent environmental standards to adhere to during the construction phase of the work.
The existing approximately 11 m long by 5 m wide intake, located just upstream of the weir on the left bank, was sized for the old hydro development and would not be adequate to provide sufficient flow for the combination of the old and the new hydropower plants. Also, the right bank rises steeply, which would have made an extension of the existing intake challenging, requiring extensive excavation and slope stabilization.
To address this problem, MWH planned installation of a new intake similar to the existing one but just downstream of the weir, along with removal of a small portion of the weir to make room for the new intake, which is about 16 m long and 6 m wide. To maintain the weir (spillway) capacity, a section of new weir was required near the face of the new intake.
|Kier Construction Ltd., project EPC contractor, largely adopted this conceptual plan for the Maggotty Fall expansion.|
As with many small hydro developments, the initial (capital) cost presented a challenge.
To enhance the cost-effectiveness of the project, MWH suggested the following measures involving reuse of existing components:
- Use of a Wye connection to link the two intakes and utilize a common section of concrete-enclosed water conductor from about station 0+25 (25 m downstream of the intake) to 0+68, a length of about 43 m. This would mean additional minor hydraulic losses but would remove the need for significant demolition, foundation work, and pipe-laying in this area. In the final design, the EPC contractor elected to do the additional demolition and install new pipe in this area;
- Reuse of the 21.4-m-high by 4.6-m-diameter surge tank, taking care to ensure that by carrying out hydraulic transient analysis using both the old and new turbine unit characteristics, the existing structure would be adequate to address upsurge and downsurge while keeping stresses in the pipeline within allowable levels. This approach was confirmed by computer modeling during final design and adopted for the permanent works;
- Use of an existing, short (109 m long) 1.5-m-wide by 2.3-m-high tunnel where the water conductor penetrates a ridge; and
- Use of a widened tailrace channel, rather than requiring the construction of an entirely new tailrace. This was achievable by careful location of the new powerhouse.
Negotiations with EPC contractor
After selection, JPS requested support for negotiations with the bidder, Kier Construction, to find a technical basis to achieve the best cost. MWH and JPS had two rounds of meetings with Kier and its engineer, Mott MacDonald Ltd., for this purpose. Ultimately, an EPC price of about US$26.8 million was agreed upon for the turnkey effort. The negotiated price was 18% lower than the bid price. JPS still perceived this figure as being on the high side, but this perception was mitigated substantially by the following factors:
- The ultimate addition to plant capacity, once final hydraulic design and field construction had occurred, was higher than the conservative 6.3 MW indicated in the feasibility study; and
- The penstock supplying the old powerhouse would have required replacement within 12 months in order to avoid outages (i.e., the major 2009 failure) taking the existing unit out of service for months.
The new GRP penstock provides water for both the old and new powerhouses, and so the project in effect results in a reliable overall capacity of 13.2 MW.
The EPC contract commencement began on January 13, 2012.
During construction, Kier Construction, elected to use precast saddle supports for the new penstock. These precast saddle supports were manufactured off site under controlled conditions to enhance quality and meet a challenging two-year start-to-substantial completion schedule. The alternative would be to cast support in situ, which is more time consuming and subject to weather conditions.
Demolition of the left side of the existing weir and construction of the new intake and weir extension revealed the weir construction was not solid concrete. To address this problem, additional concrete and dowelling were required at the point of connection.
Basic design of the project was largely completed and the purchase order for the GRP penstock was placed in August 2012. Also at the end of August, the order was placed for the turbines and generators from overall electromechanical supplier Andritz.
Delivery of the GRP pipe sections (manufactured by Flowtite) by supplier OTEK started in late November 2012. Penstock installation was completed in August 2013. Installation of mechanical and electrical equipment was completed in December 2013, testing and commissioning occurred in January 2014, and a Taking Over Certificate was issued on Jan. 29.
Thus, the project went from a preliminary design to a completed, functional hydropower system in about two years. Rapid, but thorough review of the EPC contractor’s technical submittals by JPC’s engineer formed a key part of the successful completion of this fast-tracked project. During testing, the units have performed slightly better than their targets, and the system continues to perform well.
|Prior to its expansion, this was the existing Maggotty Falls weir on the Black River in the parish of St. Elizabeth in western Jamaica.|
Accomplishments at the Maggotty Falls expansion project include the following:
- Replaced a leaking and failure-prone water conductor (woodstave pipe) with a modern GRP pipeline;
- Utilized existing project components such as the river diversion weir, a short tunnel, and the switchyard to reduce project cost and construction duration.
- Required careful integration of the control and emergency bypass systems for the existing hydropower plant, to mitigate pressure surges and protect the penstock.
- Was executed at a total cost of about US$35.8 million, which is about US$593,000 below project budget; and
- The expansion project was constructed on time and without any major safety incident.
This article has presented a few of the many issues that were addressed in order to achieve the success of the Maggotty Falls hydropower expansion project.
Maggotty Falls and the 3-MW Munro wind farm are projects recently undertaken by JPS, which are intended to provide renewable, competitively-priced electricity to its customers while reducing its dependence on expensive imported oil. These projects are important components in the quest to provide Jamaica with reliable electricity, to promote its economic growth and to thereby enhance the prospects for social stability and peaceful development.
Alston Watson, PE, is manager of expansion projects for Jamaica Public Service Co. S. Mark Allen, PE, was project manager for planning and owner’s engineering services for the Maggotty Falls expansion and is a vice president at MWH (now part of Stantec).
In this audio file at http://bit.ly/2ru67l8, Alston Watson and S. Mark Allen briefly share additonal details about some of the challenges they successfully overcame during the Maggotty Falls expansion project.