GTCC design forges ahead

GTCC design forges ahead

Power shortages in many parts of Asia can easily be relieved by short gestation projects such as power barges. A project in India is demonstrating just how quickly these can be turned around. It includes two combined cycle units, and the barges themselves have some innovative design features which will allow the developers to forge ahead into this global market.

Infrastructure development in India has been the main focus of the nation in the last six to seven years since the government announced a liberalized economic policy in 1991. Since then, the central and state governments have implemented various incentives and concessions to attract foreign investment, particularly in the power sector.

The power sector has been identified as one of the critical areas for rapid economic development. While a few large power projects have been under negotiation or implementation, the demand-supply gap in the power sector is still increasing. The large projects with long gestation periods alone cannot provide an immediate solution. There is therefore an urgency to set up power generating facilities with much shorter gestation periods.

This is particularly the case in the state of Andhra Pradesh, the largest state in Southern India. The state has not been able to achieve the desired rate of economic growth on account of crippling power cuts which have sometimes reached a peak of 60 per cent.

However, this is set to change in early 2000 with the commissioning of two power barges in the port of Kakinada, East Godavari District, Andhra Pradesh. Located near the village of Uppada, this 2 x 106 MW project will make a significant improvement to power supply in the region. Developed under a build-own-operate contract by EPS Oakwell Power, the two barges will be equipped with a combined cycle gas turbine unit, and will also include other innovative features such as crew accommodation and water desalination facilities.

Demanding times

The demand for electrical energy in Andhra Pradesh is increasing at a compounded annual growth rate of 7.5 per cent. Since 1986, power shortages and power cuts have become a regular feature in Andhra Pradesh, as well as other states, affecting not only existing industries but also limiting the expansion of new industries.

To breach the power shortage gap, a policy was announced by the Indian government`s Ministry of Power in February 1996 to encourage the development of barge mounted power plants by the private sector. The Ministry believes that such projects will make power available quickly while reducing the burden of inland transportation of fuel.

In response to this, the Andhra Pradesh State Electricity Board (APSEB) invited competitive bids for generating power in 13 locations. More than 40 companies participated in this global tender, and nine letters of intent were issued to six successful IPPs in July 1996. One of these was to Engineering Power Systems Oakwell Power (EPS Oakwell), for construction of a barge project in the port of Kakinada.

In designing the barge mounted power plant, EPS Oakwell, a joint venture company established for the development of this project, studied two main options. The two viable alternatives for the power plant were the use of diesel engines or gas turbines as the prime mover.

After careful analysis, the gas turbine arrangement was chosen. A combined cycle gas turbine configuration offers more power in a smaller package, resulting in less costly engineering and construction, and offers good reliability and low maintenance costs. The barges are designed for a 40-year life.

The gas turbines are also suitable for quick start with fast acceleration from cold start to full load. Two duel fuel fired combined cycle barge mounted power plants each comprising two gas turbine generators, two waste heat recovery steam generators and one steam turbine generator were therefore selected by EPS Oakwell.

A global market

EPS Oakwell is a partnership between Engineering Power Systems Group Inc. (EPS, 43.75 per cent), CMS Energy Corp. (43.75 per cent), and Oakwell Engineering Ltd. (12.5 per cent).

These companies believe that there is a significant global market for power barges. EPS is currently negotiating several projects world wide, including in India, Turkey, Bangladesh, China and Sri Lanka. CMS and Toronto-based EPS recently formed a joint venture specifically dedicated to developing and operating barges throughout the world. While EPS` expertise, through its subsidiaries, lies in designing and constructing barges, USA-based CMS has considerable IPP experience throughout the world, including in India.

EPS` subsidiaries include Atlantic Seaboard Industries Limited (ASIL), an offshore ship and marine fabrication and outfitting company operating from Port Aux Basques, Newfoundland, Canada. For implementation of this project EPS Oakwell Power engaged ASIL and SNC-Lavalin Engineering as the main EPC contractor.

Other EPS subsidiaries involved in the project include:

à‚-M&M Engineering Limited, which will carry out the electrical and mechanical installation and testing.

à‚-Merlin Engineering A/S, an offshore, marine, and environmental engineering design company, is responsible for the design engineering of the barges with assistance from Alstom. The barges will be constructed in Newfoundland, Canada.

Other companies that will be involved in the barge project include Alstom, the international power engineering company, which will provide the power generation equipment and will also operate and maintain the power plant. Oakwell Engineering will develop the site in India and will also consult on the overall project.

EPS Oakwell has signed a power purchase agreement (PPA) with APSEB for the sale of power. APSEB will purchase the total annual average generation of 742 848 MWh from the power plant. This is based on a net output of 106.3 MW from each barge. The plant availability will be 92 per cent.

APSEB`s payment for electricity under the PPA will be paid in Indian rupees and comprises two elements:

à‚-Fixed charge – the combination of two main elements: a foreign debt service charge in fixed $/kWh; and other fixed charges in Rs/kWh

à‚-Energy charge – the cost of fuel and petroleum consumables, calculated each month throughout the term of the PPA.

The initial term of the PPA is 15 years from the commercial operation date. At the expiry of the 15-year term, this agreement may be extended or APSEB may exercise the option to “buyout”.

Total financing for the project is $220 million, met through a combination of debt and equity (75:25). Loans totalling $165 million have been obtained from the Export Development Corporation of Canada and Coface. The repayment terms of the loans, including interest, are seven years. Equity of $55 million has been obtained from the development consortium headed by EPS.

Adding value

The power barges are designed for use in remote locations and so are designed to be self-sufficient stand-alone units with on-site fuel storage capabilities, crew accommodation and water desalination facilities. This allows them to be versatile and highly mobile.

The barges can accommodate a crew of five people and are equipped with a kitchen, mess and R&R room. The reverse osmosis desalination plant is capable of supplying 23 m3/h of water.

The barges are also equipped with a fuel oil treatment plant to ensure the quality of the fuel, as well as a complete unloading, storage and supply system for either LNG or fuel oil.

Fuel oil is unloaded into the barge storage tanks by unloading pumps. The storage capacity of these tanks is 9500 t. Space for diesel storage is also required for the black start diesel generator.

When operational, the power plants will have a high degree of automation and require a minimum of personnel. The plant is designed for continuous operation year-round. The only planned shutdowns are for the yearly inspection and maintenance periods of varying length, dependent on the requirements of the manufacturers of the main equipment.

The intended operating profile, with long periods of continuous operation, places rigid demands on the reliability of the equipment. Workshop facilities are therefore provided on-board and vital maintenance components will be kept as spare equipment in order to minimize the time for shutdown of the plant beyond planned audits or major shut downs.

Generating power

The project site is approximately 80 m by 200 m in the form of a dug-out lagoon 6 m in depth on the shore of Kakinada Bay. The barges will be secured alongside each other inside this lagoon.

Each combined cycle power plant will be mounted on a 121 m x 22 m barge in a standard design employed by Alstom. It consists of two 36 MW gas turbines and a 38 MW steam turbine with two heat recovery steam generators (HRSG) installed behind the gas turbines. The HRSGs produce high pressure (HP) and low pressure (LP) superheated steam to feed the condensing steam turbine.

When operational, the plant will have a gross generation capacity of 109.1 MW. The gas turbines will operate using fuel oil treated in the fuel oil treatment plant. The alternative fuel will be LNG. A two-gas turbine configuration was chosen for the plant using Alstom MS6541B industrial heavy duty gas turbines, each rated at 38.34 MW. NOx emissions requirements will be met by steam injection.

The gas turbine module consists of a 17-stage compressor and a 3-stage single shaft turbine, with the combustors mounted on the compressor discharge casing. The axial flow compressor develops a pressure ratio of 11.8:1. The accessory module mounted on a separate base frame houses the mechanical and control elements required for gas turbine operation. The system is suitable for operation in a monsoon area and in a saltwater environment.

The exhaust system discharges gases into the atmosphere either through the HRSG or a bypass stack. It comprises exhaust silencers, ducts, and dampers. The gas turbine drives the generator at 3000 r/min via a load gearbox.

The steam turbine, also supplied by Alstom, is an indoor type nominally rated at 38 MW. It has a multi-stage impulse design featuring an axial exhaust. The steam turbine is a single cylinder model of the disc and diaphragm type comprising a cast steel HP casing and a fabricated steel LP casing assembly internally reinforced by brace tubes. The two casings are vertically joined together by bolted flange connections. Both the HP and LP casings are split and bolted at the horizontal joint.

Each gas turbine exhausts to a dual pressure natural circulation HRSG in horizontal configuration located beside the gas turbine. The HRSGs and the steam turbine operate in sliding pressure mode in order to maintain the best efficiency in the steam cycle at all loads.

The condensate temperature of the HRSG is 134 degrees C, and the exhaust temperature of the flue gas approximately 135 degrees C. The velocity of hot exhaust gas through the waste heat boiler is approximately 30 m/s.

The HRSGs have been designed to be capable of following the gas turbine starting time cycle and produce steam flow in the quickest time and to withstand transient mechanical and thermal shocks due to rapid start-ups. HRSG control as well as the main control panels is located in the central control room. The steam temperature variation will be limited to 5 degrees C.

Each 11 kV generator is rated to deliver the power available at the gas turbine and steam turbine shaft under all operating ambient conditions at 50 Hz. The generators are provided with Class F insulation and with Class B temperature rise.

The total estimated plant auxiliary load is 4000 kVA. Based on the total auxiliary load requirement for the power plant, three 3 MVA capacity station transformers are sufficient to cater for the plant auxiliary loads.

The transmission system required for the evacuation of power from the power barges consists of a 220 kV, 3-phase, submarine cable from the step-up transformers on the barges, to the transmission tower onshore, and from there to the APSEB substation which is under construction. From there, power will enter the APSEB grid for distribution. Protective relaying and associated breakers and switchgear are provided on each side.

The instrumentation and control system is provided for centralized, automated and fail-safe control of the power plant to achieve maximum efficiency, better reliability and availability.

The C&I system is designed to relieve the operator, to a large extent, of decision-making at all levels of plant control including the highest level of apportioning the load to each gas turbine/HRSG set and steam turbine. The system will also be capable of surrendering any level or part of it to manual control with appropriate security precautions. At the same time, the manual control will provide backup enabling the operator to control and monitor the plant with a lower level of automation should the higher level fail or be unable to recognise and to respond to particular constraints.

Environmental factors

The water supply for the power plant, including process water, make-up water and potable water is supplied by the desalination plant. The desalination plant is capable of supplying all of the power plant requirements while having minimal impact on the environment.

The plant uses fuel oil as a primary fuel with a sulphur content of 0.5 per cent and LNG as secondary fuel with a near zero sulphur content. It has been guaranteed by Alstom that emissions will fully comply with the environmental guidelines of the Ministry of Environment and Forests as well as World Bank standards. There will be minimal particulate matter in the flue gas from either of the fuels. The typical values of NOx in the flue gas are guaranteed in the range 40 to 65 ppm on a 15 percent O2 volume dry basis.

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Figure 1. The barge project will be located at Kakinada, Andhra Pradesh

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Figure 2. The combined cycle power barges include crew accommodation, desalination and fuel storage facilities

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Figure 3. Participating companies

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