Monday November 19. 2007

Guðmundur Pétursson, Head of the Kárahnjúkar project management at Landvirkjun, was a speaker at the HYDRO 2007: New Approaches for a New Era.  

The conference was held in Granada, Spain in mid october 2007 and in his presentation Guðmundur gave a comprehensive account of the Kárahnjúkar project from the start and to this day where the end is in sight. The Kárahnjúkar project was the main topic in many presentations at the conference and the International Journal of Hydropower dams covered the project extensively in a special edition published at the conference.

A summary of Mr. Péturssons presentation follows:

The 700 MW Kárahnjúkar Hydroelectric Project in eastern Iceland is being essentially completed this fall 2007.  Construction started in spring of 2003 and all 6 generating units are due to go into full service by end of 2007.

 

The very tight construction schedule over a period of 4½ years has put many constraints on the project mainly due to unforeseen geological conditions at the site.

 

The main features of the project are the 200 m high concrete faced rock fill dam, being the highest in Europe of such kind, and the extremely long semi horizontal headrace tunnel of 40 km length in one stretch plus access and side tunnels, another 20 km.  The gross head is 600 meters and two parallel vertical steel penstocks of 450 m height, among the highest in the world, conduct the water to the 6 high head Francis turbines which are equipped with high efficiency splitter blade runners.

 

The Powerplant is primarily being constructed to supply electricity to a new Aluminium smelter being built by Alcoa of USA at a distance of approx. 50 km on the east coast.  Developer and Owner of the Kárahnjúkar Powerplant is Landsvirkjun, The National Power Company of Iceland.

 

The construction work is being carried out under more than 30 main Contracts by both local and international construction companies and manufacturers.  Design and site supervision is performed by a number of international and Icelandic engineering organizations.  Overall project management and co-ordination of works is executed by the Owner.

 

Fault treatment delayed the dam construction

 

After diversion of the river (Jökla) in December 2003 and subsequent excavation in the river canyon it became apparent that faults in the rockbed were crossing the dam foundation.  Special measures for fault treatment had to be undertaken and the massive concrete toe wall in the canyon had to be re-located and re-designed.  This delayed the dam construction by many months and concreting work on the toewall (80.000 m³) had to be carried out throughout the winter 2004/2005 at severe winter conditions.  In spite of this delay rockfilling of the dam (8,5 Mio m³), and concreting of the water sealing face slab (approx. 100.000 m²), could be concluded in time to allow start of reservoir water filling according to the original schedule in September 2006.  This was made possible by working through all winter 2005/2006 in harsh arctic climate on concreting of the face slab and finishing it and the dam rock filling up to the required minimum height of 590 m a.s.l. prior to start of reservoir filling.  The remainder of the concrete face slab was constructed after water filling had started and was finished by end of 2006.  The 7 m high concrete parapet wall on the top of the dam, as well as the spillway chute, were completed this summer (2007) as the water level rose in the reservoir. 

 

By modifying and accelerating construction procedures and sequences for the dam and related structures it was secured that the dam was completed and that the Powerplant would be capable of providing full power to the important customer during coming winter and spring, until the next summer flood and glacial melt will fill the completed reservoir again next summer.

 

Difficulties in the headrace tunnel

 

Another major obstacle along the project route were difficult geological conditions in the headrace tunnel.  The main streach of the 40 km long headrace tunnel was excavated by 3 tunnel boring machines (TBM´s) of 7,2-7,6 m diameter.  All 3 encountered difficulties and were slowed down or held up significantly by heavy ingress of water (TBM3), fractured rock and loose in-fills (TBM2) and by soft sedimentary layers (TBM1).

 

TBM3 was first stopped prematurely due to water ingress and slow pace and was turned around to drill towards TBM2 leaving the remaining tunnel section (approx. 1 km) to be drilled and blasted the conventional way.  By doing so 3-4 months were saved in construction time for the respective sections.

 

TBM2 got stuck twice in loose rock and fault zones with gravel infill and water ingress and was practically held up for 6-7 months, progressing only some 70 m during remedial and support works in the tunnel.

 

TBM1 was slowed down, due to soft layers of sedimentary material in the initial phase, but broke through first of the three after some 15 km of drilling on September 9^th 2006.  Last break through by TBM3 was on December 5^th 2006.

 

A joint effort of contractors, the engingeer and the owner

 

Because of these delays in tunnel excavation the finishing works inside the tunnel, i.e. rock support, surface treatment, concrete structures, cleaning out, etc. became most critical for the project completion and great efforts had to be made in order to speed up those works.

 

Apart from increasing the workforce up to above 700 men working inside the tunnels additional equipment was brought in.  Shotcrete equipment and concrete handling equipment, additional trains and railway system, etc.  Transport logistics and material handling gained crucial importance.  Modified designs to allow better constructability and acceleration of works were introduced among other things, such as incentive payments and increased working time.  Drop shafts were drilled from the surface (180-200 m above tunnel) to allow more efficient transport of concrete to the tunnel.  Additional access to the tunnels was also provided for by constructing a fourth adit, and by using the surge tunnel and the surge shaft for transportation of equipment and personnel to the work fronts.

 

By doing all of this by joint effort of Contractors, the Engineer and the Owner the delays in tunnel excavation could be mitigated considerably.

 

In addition to the above described also other works have had to be adjusted to the actual situation and circumstances.

 

The start-up date on time

 

Guðmundur Pétursson, Head of the Kárahnjúkar project management, andLok Home, President of the Robbins Company.

The underground powerhouse construction as well as manufacturing and installation of electro-mechanical equipment has basically been according to the original schedule.  Some minor delays have occurred in equipment manufacturing and in installation of the pressure shaft steel linings and hydro mechanical gate equipment which, however, did not affect the start up date.

 

In order to make start-up power available on time to the aluminium smelter it was decided to operate the first generating unit of the powerstation without water as synchro condenser.  This was made possible by installing appropriate additional electronic converter equipment for start up of unit no. 1 from the grid.  The generator is de-coupled from the turbine shaft, which is lowered slightly, and a protective cover installed on top of the draft tube in place of the draft tube cone.  With these arrangements for unit no. 1 the weak electrical network on the east coast is supported substantially, transmission capacity increased and voltage regulation with the generator provided for.  Thus this allowed start-up of the aluminium smelter on time in April 2007 from the national grid with an initial supply of up to 100 MW.  The two 220 kV transmission lines between powerplant and smelter with a link to the existing 132 kV national grid were commissioned in January/February 2007.

 

The testing and commissioning period for the remaining generating units was then to be shortened and accelerated considerably in order to compensate for the delay in tunnel works and water availability.  Two and two generating units were to be commissioned simultaneously on separate pressure shaft penstocks.  This of course called for precise planning and additional commissioning resources.  Due to further difficulties and delays in completing the headrace tunnel, however, the generating units no. 2-6 have been operationally tested with water and commissioned up to synchronization and part load operation at much reduced head water pressure and flow (450 m instead of 600 m water column).  The reason for this being, that it was possible to complete the lowest third (15 km) of the semi-horizontal headrace tunnel earlier (end of July 2007) and so enable water filling of that section considerably ahead of the remaining tunnel sections which are only being filled these days. 

 

This has saved 2-3 weeks of testing time for each of the generating units which can now be brought on the line and operated at full power within one week each after availability of full head and flow.  This methodology has brought about a substantial advancement of full power production from the Kárahnjúkar plant. 

 

An outstanding performance of the workers and project personell

 

The filling of the lowest third of the headrace tunnel was effected by inflow of leakage water into the tunnel and pumping, to fill that tunnel section as well as both vertical pressure shaft penstocks.  Filling of the remaining part started on October 13^th and will be completed on November 1^st.

 

In spite of the severe delays in tunnel completion the project is scheduled to be in full power operation by end of 2007, close to the original plans (October 2007), and all work shall be completed by end of 2008.  

 

The main bulk of work left for next year is in a second catchment and diversion area (Jökulsárveita/Hraunaveita), connected to the main headrace tunnel through a 10 km long and 7,2 m wide side tunnel presently being drilled by a TBM.  Further tunnels and two smaller earth fill dams are also being constructed in that area.

 

The construction workers and the project personnel have performed outstandingly under toughest environmental conditions at the site, rough weathers, dangerous working areas and remote location away from the families and deserve recognition for their great efforts.

 

Guðmundur Pétursson

Head of the Project Management

Landsvirkjun