Francis Griffin and Andrew Webb* report on how work is progressing at Gitaru hydro power plant in Kenya
Gitaru hydro power station is owned and operated by the Kenya Electricity Generating Company (KenGen). The power station is situated on the Tana river approximately 160km northeast of the Kenyan capital, Nairobi. Gitaru reservoir is relatively small; long-term storage is provided by the upstream Kamburu and Masinga reservoirs.
In 1978 the Gitaru underground powerhouse went into commercial operation with the installation of two 72.5MW vertical Francis turbo-generators. The civil designs and construction made provision for a third unit (No. 1) including the installation of the complete intake gates, penstock and draft tube and draft tube gates. Initially, it was expected that the third machine would have been installed within a few years of 1978. However, the demand growth in Kenya was such that schemes with a greater energy generating capacity were developed initially and it was not until 1996 that KenGen decided to proceed with the implementation of the third Gitaru machine.
By the mid 1990s electricity demand was outstripping supply within Kenya and it was clear that additional generating capacity had to be created quickly. A number of short, medium and long-term options were considered by KenGen. The Gitaru machines are amongst the largest generating units in the Kenyan system and whilst the third machine would provide little additional energy it would give peaking power and increase KenGen’s operational flexibility. Once the decision had been made KenGen acted quickly to secure this additional capacity. Funding for this high profile project was to come from within KenGen’s own resources and the contract was to be awarded by international competitive bidding.
In October 1996 KenGen awarded the engineering consultancy to Mott MacDonald and Knight Piésold of the UK. KenGen’s terms of reference emphasised the importance of implementing the project within the shortest possible time and every effort has been made to achieve this. First of all, it was necessary to review the original design proposals. These studies demonstrated that the rating of unit No. 1 should be increased and that the machine should be connected at 220kV instead of at 132kV like the existing machines.
The main elements of the installation were defined as:
•One vertical shaft Francis turbine/generator, operating under a rated head of 136m at 273rev/min capable of delivering a maximum continuous output of 80MW.
•One 95MVA 15/220kV generator transformer, dual OFAF/ONAN cooling.
•Miscellaneous auxiliary electrical equipment including 415V switchboards and batteries, 15kV XLPE cable, control and protection equipment.
•One single circuit 220kV transmission line 9km long between the existing Gitaru and Kamburu switchyards.
•Civil engineering works including turbine spiral embedment, inlet valve foundations, generator foundations and floor, switchyard foundations at Gitaru and Kamburu and miscellaneous other minor works.
As part of the design review Mott MacDonald considered the appropriate contract structure. It was apparent that the civil engineering works would pose the greatest difficulty due to their relatively low value and the fact that they would lie on the critical path. Three options were considered:
•The complete project let as a single turnkey contract.
•A turnkey contract offered for M&E works with a separate contract for civil works on a design and build basis.
•Separate M&E and civil works contracts with the civil design being completed by the engineer.
Turnkey packaging is nothing new in the hydro industry. A number of hydroelectric projects have been successfully implemented in this way. However, these have generally been for complete schemes and have been led by a civil contractor thereby reflecting the relative values of the civil and M&E works for most new schemes. This was not so for Gitaru No. 1, where most of the civil infrastructure already existed and the civil scope accounted for less than 10% of the contract value. It was therefore not possible to attract interest from civil contractors to lead the consortium.
It was decided that the complete project should be awarded as a turnkey contract and pre-qualification was restricted to turbine/generator manufacturers or joint ventures of turbine and generator manufacturers. It was expected that the civil elements would form a sub-contracted package of work. The importance of interfacing with the civil works was identified early on and it was clear that it would have to be managed closely. It was also clear that the major turbine/generator manufacturers had little experience with this type of contractual arrangement.
Tender documents were issued for international competitive bidding in mid April 1998, six months after the engineer had been engaged. The proposed project programme identified an implementation period of 25 months including all aspects of design, manufacture, erection and commissioning.
This was a short implementation period and several tenderers argued that it would be very difficult to achieve. Nonetheless, the tender adjudication process credited tenderers for guaranteeing an even earlier completion. The credit reflected the revenue benefit to KenGen for the additional generation. Contract documents also included performance and late completion penalties to protect the client’s interests.
Following a three-month tendering period and subsequent tender negotiations with the two lowest adjudicated tenderers, the contract for project implementation was awarded to the German consortium of Siemens and Voith in December 1997 for US$18.6M. The same consortium had installed the original machines 20 years ago and was able to guarantee implementation of the new unit within 22 months. Machine trial runs have been scheduled for mid October 1999.
Twenty-two months is recognised as being a short implementation time, so how will it be achieved? The Gitaru machines are by no means small. The generator housing outer diameter is 8m and the combined rotor/generator shaft weight is approximately 175t. Typical erection activities can take as long as 18 months to complete. After erection of the turbine spiral casing, the generator support structure is constructed. This is followed by erection of the internal parts of the generator and turbine before shaft coupling and commissioning.
Preceding this is a 12-15 month period for design, manufacture and transportation, which is obviously not a practical time scale for the Gitaru unit installation. In order to leave adequate time to manufacture and to achieve the contracted time for completion it will be necessary to reduce the erection time to 12-14 months and compress the design and manufacturing processes.
Turbine and generator erection activities must be carried out at the same time, while the impact of the civil works on the M&E erection must be minimised. The tender programme envisaged that the generator supporting columns and generator floor would be constructed before the start of spiral erection. It was acknowledged that this sequence would limit the availability of handling facilities for the spiral erection but it was anticipated that the spiral would be either partially or completely assembled on the maintenance bay then lowered into position through the inlet valve access. Temporary lifting points would also be required on the underside of the generator floor to assist during turbine erection. As soon as the generator floor achieved sufficient strength the generator assembly could commence with erection of the stator frame. Concrete embedment of the spiral and turbine erection could then continue at the same time.
Early contractual meetings focussed on agreeing this sequence of operations and selecting sub-contractors. It was also clear that close management of all activities was paramount to the success of the project. Design approvals, interfacing with existing systems and relevant Kenyan regulatory authorities, would all have to be dealt with in a timely and efficient manner.
In the early stages the contractor’s programme closely mirrored the engineer’s proposals and all seemed to be progressing smoothly. However in an attempt to reduce his costs the contractor modified the programme for the main civil works so that the generator floor will only be built after the spiral casing has been completed.
Site activities commenced in September 1998, some three months later than previously anticipated. Unfortunately progress has been further hampered by late completion of designs and difficulties with obtaining port authority clearances. Timely project completion is now rather less certain. However, all parties are trying hard to overcome the problems and building on past experiences the project momentum is now increasing. In December 1998 spiral-casing erection commenced and is expected to be complete by mid-January 1999. Spiral embedment and construction of the generator floor have become parallel operations that will start in February 1999. Erection of the turbine and generator is programmed to commence in May 1999.
Despite the difficulties, all parties remain firmly committed to meeting the contractual completion date in mid October 1999. It is expected be a very busy time at Gitaru over the coming ten months.