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Nesjavellir Geothermal Power Plant

Region: South Iceland
Coordinates: 64.1082246° N 21.2592959° W

Exploration and Planning started already in 1947 and was continued for two years in this 50 km² High Temperature Area, one of at least 27 such resources, which are directly connected with the most volcanic regions of the country. A few experimental boreholes were drilled for the evaluation of the exploitable power and the chemical composition of the steam.

After a rather long intermission exploration and research continued with shorter intervals from 1965 to 1986. The construction of the Geothermal Power station was commenced in 1987 and continued incessantly until the cornerstone was laid on May the 13th 1990. It was officially started on September 29th the same year.

The Hengill Area is amongst the largest High Temperature Areas of the country. The geothermal activity is closely connected to three active volcanic systems in the region. The source of the natural- and artificial (boreholes) thermal activity in and in the vicinity of the horticultural village Hveragerdi to the south of the mountain range is an extinct central volcano, called ‘The Grenjadalur System’. North of it is still another one, called ‘The Hromundartindur System’, which was last active about 10.000 years ago. The westernmost of the three systems, The Hengill System, has erupted several times during Holocene and every now and then we are reminded of the natural forces at large by tremors and earthquakes.

The Nesjahraun Lava field was created during a fissure eruption of the Kyrdalur Fissure near the Power station 2000 years ago and at the same time the largest island of the lake came into being.

Results of extended research have shown that the precipitation in the mountains north of the geothermal areas percolates deep down into the earth and then flows through subterranean faults and fissures to lower regions. On its way the water is heated by the hot rock and resurfaces as boiling water or steam through fissures extending to the surface or can be reached by boreholes where there is no obvious thermal activity on the surface. According to the scientists, the hot water travels at a depth of only 3 km on the average, but in places it comes as close as 1 km to the surface.

Because of extended recent volcanic activity in the area, the rock strata are relatively young. The uppermost 500 m consist of hyaloclastites but beyond that are differently thick layers of basaltic lavas. Dykes become more frequent at greater depths and at 1400 – 1600 m they are prevailing. Water veins are common at the margins of the dykes, especially if they are acidulous.

The geothermal gradient of the ‘Kyrdalssprunga Volcanic Fissure’ is the greatest in the area. At sea level the temperature is 100°C and at 2000 m it exceeds 350°C.

Since 1972 the boreholes drilled have been fitted for future exploitation. The results have been extremely good. The average hole yields up to 60 mw. of exploitable power on the average, sufficient for the central heating of a habitation of 7500 people.

All together 23 holes have been sunk, thereof 13 exploitable. Four of them were connected at the first stage. Their total output equals 140 mw. At the end of the second stage additional 100 mw. will be exploited, of which 50 were pipelined to the power station in 1991. The remaining 50 MW will be activated this year (1994) bringing the total output of the station up to at least 200 mw. The power station is designed for a maximum output of 400 mw. in the year 2010 and according to estimates the present area can be exploited economically with an average 300 mw. output for at least 30 years.

Simultaneous exploitation of the excess steam for electrical production is ideal. Then the steam is pipelined through turbines. Some of the holes deliver almost clean steam for that purpose and are estimated to suffice for up to 80 mw. The pumps of the geothermal power station use 14 mw. of electricity at it’s maximum output (400 mw.).

A mixture of water and steam is pipelined from the boreholes to the station through separators. The steam continues to the heaters and the turbines. Exhaust chimneys are used for the excess steam. In the heaters the steam condenses and the boiling water is pumped to condense heaters where it heats up cold ground water. The condense water cools of to about 20°C

The separated water from the bore holes containing various minerals, which are deposited on the inner surface of the heater. To remove these deposits, steel balls are placed in the current to pulverize them continuously.

The cold ground water is supplied through boreholes in the lava field close to the lake and pumped to containers by the station. From there it continues to the separator heaters or condense- and steam heaters, where it is heated to 85°C or 90°C.

The cold water is saturated with oxygen, which causes corrosion after heating. To eliminate the oxygen and other gasses the water is boiled at a lower pressure level. This process cools the water to 82-85°C. Eventually a minute quantity of acid steam is added to the water to eliminate the remnants of oxygen and reduce the ph of the water to prevent deposits in the pipes. This tiny quantity of sulphuric acid also prevents oxygen to reaccumulate in the supply tanks of the capital and lends the funny smell to the water when we turn on the tabs at home.

The natural discharge from the Nesjavellir Thermal Area has for centuries been carried to the lake. The increased discharge from the power station might affect the organic life. Therefore constant research and observations are carried out to spot any changes immediately. Experiments with pumping the discharge back into the ground are also continued, as it has become a known fact, that the circulation of the water can be overexploited if we are not careful.

The main buildings of the power station are at an elevation of 177 m above mean sea level. From there the hot water is pumped through a pipeline, 90 cm in diameter in the beginning but narrowing to 80 cm for most of the 27,2 km distance to the capital. The pumps have only to be used to conquer the uphill gradient through the mountains to the 406 m level and after that it free flows downhill to Reykjavík. The pipeline carries a maximum of1870 l of 100°C hot water per second. It is thoroughly insulated and the heat loss never exceeds 2°C, the greater the quantity of water running through, the less the heat loss. Snow never melts on the pipeline in winter, which goes to show how well it is insulated.

The steel pipeline to the surge tank has a diameter of 900 mm and a wall thickness of 12 mm. It is designed to carry water up to 96°C hot with a maximum transport capacity of 1870 litres per second. From the surge tank the water flows by gravity to storage tanks on Reynisvatnsheidi and Grafarholt from where it is distributed to Reykjavik and the nearby communities. Those tanks are at an elevation of 150 m above sea level. The steel pipeline from the surge tanks has a diameter of 800 mm and a wall thickness of 8 to 10 mm, depending on the maximum inside pressure. The steel pipe is laid mostly above ground and rests on concrete pillars. It is insulated with rock wool and covered with plastic and aluminium sheets. Five kilometres of the pipe is an underground-preinsulated pipeline with polyurethane insulation and about 17 mm thick polyethylene plastic cover. From Nesjavellir to Grafarholt the transmission pipe measures about 27 km in length.

For design, material, and manufacturing specifications the AD-Merkblâtter and DOM standards were used. Testing requirements were chosen according to the same Merkblâtter, but for pressure testing of the steam collecting lines the ANSI B 31,1 standard was used. Due to the mountain slope the elevation difference between the highest and lowest wellheads is 140 metres. By using steam for pressure test, this elevation difference had not to be taken into account, when choosing the pipe wall thickness. Quality control is based on different control levels, which includes approval of welders and weld inspectors, and welding procedures.

All welders had to have a welding certificate according to DIN 8560, R 2-m to be accepted at the site. All welds were marked by initials to keep good record of each welder. For the steam supply and the piping inside the powerhouse, the quality standard for the welding and testing were based on the German AD-Merkblâtter HP series. Contractors were required to keep record of all welds tested and to hand over these reports to the inspectors at the end of each job. In the steam supply system welding requirements were according to the DIN standard 8563, part 3, quality class BS. The wellheads are prefabricated and welded in a workshop. Each wellhead is 100% X-rayed and visually inspected on both the outside and as much as possible on the inside. The welding method mostly used is manual metal arc welding.

Steam collecting pipelines have a diameter range from DN 250 to DN 1000. The pipes are mostly spiral welded and came in random lengths of about 12 metres. The steel quality was St-37.0 according to DIN 1626, with certificates according to DIN 50049, 3.1.B. All welding seams of the pipes were 100% inspected by ultrasonic testing during the manufacturing process and the pipes were pressure tested. All pipes on site were welded with manual metal arc welding method. 25% of each weld were ultrasonic inspected. Some radiographs were taken to compare the result to the ultrasonic inspection. The pipelines had to be laid through lava fields and down steep hills with an inclination up to 45°. Overall welding quality was very high, only small defects were found, mostly porosity, slag and misalignment in large diameters. Most of the cold water pipes are ductile iron with sockets, which do not require welding.

Inside the powerhouse, most of the pipes are made of stainless steel with a diameter range from DN 100 to DN 800. These stainless steel pipes are thin walled and have longitudinal welds. The manufacturing process test requirement is similar to the steam collecting pipes. The most used welding method was TIG welding. For few weldings, where the back gas shielding was difficult to achieve, manual metal arc welding was used. The welding quality control was 25% X-Ray supported by dye penetrant testing. Only few minor defects were detected.

The original transmission pipe design was based on spiral-welded pipes according to steel quality St 37-3, DIN 17100. Pipes made from steel quality St 52-3 were offered at the same price as those made of steel St 37-3. The St 52-3 pipe material was chosen and due to the stronger steel, a lower welding factor was required, allowing a reduced welding control. The total length (27 km) of the transmission line was divided into three sections. Erection time was chosen two years to enable Icelandic firms to carry out the work. Three Icelandic companies built the pipeline and a forth company made and erected the concrete fundaments. The welding method was MMA. The welds were carried out inside tents, which was obligatory, because of windy weather and often heavy rain. Most of the welders preheated the joints to 50-100°C before welding. The welding rods were kept in heating boxes on site. The welding quality required was according to the DIN standard 8563/3, quality class BS. All welds were inspected visually from the outside and the inside. Some times the inspectors requested minor repairs, mostly due to undercuts or incomplete penetration.

After the visual inspection the inspectors chose the welds to be X-Rayed. Due to the close visual inspection on the outside and the inside, an X-Ray inspection amount of only about 2% of the total weld length was required, unless there was a quality problem with a new welder. The result from the X-Ray inspection was very good. Only minor defects were found, like porosity. Ultrasonic testing was also performed at random places. The result was also very good. Each new welder was under strict inspection during the first weeks. That way the inspectors noticed very quickly if a welder had problems with fulfilling the quality requirements. Two or three welders gave up welding after a short time even though they had passed the welders test. At the end of the project a weld report was published. A part of this project was to dismantle two storage tanks at Oskjuhlid in Reykjavik and to erect them again at Reynisvatnsheidi. These storage tanks were built in the year 1968 and all parts were visually inspected. Some plates had to be rejected due to deep pitting corrosion and the material was replaced by a new on. The inspectors performed the same quality control and inspection as on the transmission pipe.

This project is one of the biggest welding and welding quality control projects in Iceland to this date. The welding quality control was a part of the task of the construction supervision team. This brought the extension of the quality control directly to the Owners decision. The co-operation between the welding quality control and the welders was good and no claims were blamed on the execution of the quality control. Due to this close co-operation between all involved parties, quality control was achieved at minimum cost. The plant and transmission line have been in operation since 1990. No weld failures have occurred showing so far that the weld quality is satisfying.

Reykjavik:
1928-30 14 boreholes inLaugarnes.
1942-62 17 boreholes in the capital.
1958 25 boreholes in the capital.
1967> 13 holes sunk in Ellidaar Valley; 8 exploited in 1993
1993 10 holes exploited in the capital.

Mosfellssveit:+
1933-55 77 boreholes sunk.
1970 > 39 boreholes sunk.
1993 34 boreholes exploited.

Photo Credit: By Hansueli Krapf [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

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