CAPACITY AND TECHNOLOGY FOR THE SNØHVIT LNG PLANT CAPACITÉ ET TECHNOLOGIE DES INSTALLATIONS GNL DE SNØHVIT

Transcription

1 CAPACITY AND TECHNOLOGY FOR THE SNØHVIT LNG PLANT CAPACITÉ ET TECHNOLOGIE DES INSTALLATIONS GNL DE SNØHVIT R.S. Heiersted R. E. Jensen R. H. Pettersen S. Lillesund Den norske stats oljeselskap a.s. (Statoil) Stavanger, Norway ABSTRACT The Statoil paper will give an insight into the selection of capacity and technology for the Snøhvit LNG Plant development. The initial plant design was a single train of 3.4 million tonnes of LNG per annum capacity. Recent screening studies indicate that LNG train capacity contributes significantly to the economy of scale. A decision on the Snøhvit single train capacity will be balanced against specific risks related to the involved technology. The verification will particularly focus on technology exceeding present industrial practice when increasing train capacity into the range of mtpa of LNG production. RESUME L intervention de Statoil donnera un aperçu de la sélection effectuée, quant à la capacité et à la technologie, pour le développement des installations GNL de Snøhvit. Initiellement, ces installations sont un train simple d une capacité de 3,4 millions de tonnes de GNL par an. Les études de détail récentes indiquent que la capacité du train de GNL a une incidence significative sur l économie d échelle. Une décision portant sur la capacité du train simple de Snøhvit sera également évaluée en regard des risques spécifiques relatifs à la technologie utilisée. La vérification se penchera tout particulièrement sur la technologie dépassant la pratique industrielle actuelle, puisque la capacité du train devrait être augmentée pour passer à une production de GNL de l ordre de 4,0 5,0 mt/an. PS7-Sp.1

2 CAPACITY AND TECHNOLOGY FOR THE SNØHVIT LNG PLANT 1 THE SNØHVIT LNG CHAIN The gas reserves in the Barents Sea off the coast of Northern Norway were discovered in the early 1980s, and are located in three different fields, Askeladd, Albatross and Snøhvit. The development of all three fields will be part of the Snøhvit project. The fields are operated by Statoil. Other field owners are Norsk Hydro, TotalFinaElf, RWE-DEA, Amerada Hess and Svenska Petroleum. The Snøhvit project aims at cost efficiency in all aspects of the development, being in the fore front of technology and execution methods to obtain the lowest unit production costs. Regarding the chain capacity, the project has balanced risks related to the reserves, the offshore and onshore technology and market potential. The reserves will be commercialised through a grass root LNG chain of 4.3 million tonnes per annum capacity. The gas fields are located 160 kilometres offshore in 300 to 350 metres water depth. The total reserves are in excess of 300 billion standard cubic metres of gas and 20 million cubic metres of condensate. The field development will comprise a subsea production system and the well stream will be transported to the onshore receiving facilities in a multiphase transportation pipeline. The offshore system and multiphase pipeline is designed to obtain reliable operations under the given conditions. The LNG plant will be situated on the Melkøya Island in the vicinity of the city of Hammerfest. The plant development meets constraints, particularly on personnel air transport logistics and the fact that Norwegian labour regulations restrict personnel rotation options, all of which will have an impact on the investment costs. The LNG plant construction strategy is based on maximum prefabrication. The basic concept is to install a base load LNG process train and most of its utilities on a purpose built barge and ship it to site. Compared to other LNG plant executions, the Snøhvit project has changed the philosophy from on-site, stick-built solutions to yard prefabrication, placing focus on maximum work executed in fabrication yards. Acquiring shipping capacity has been a focus area. Given the planned LNG production capacity and the sales portfolio, the Snøhvit project will need four LNG carriers with a capacity of m 3 each. The shipping distances from the Snøhvit area to alternative markets balances with the current sales portfolio comprising southern ports of Continental Europe and terminals in USA. The Snøhvit project is aiming at increased commercial robustness by selling its production capacity to markets with different pricing mechanisms. 2 THE ENVIRONMENTAL EDGE OF SNØHVIT The design of the overall thermal efficiency of the LNG plant must meet economic criteria and environmental constraints. The life cycle cost robustness of the technology versus future purchase of quota according to the Kyoto Protocol and criteria related to Best Available Technology is relevant in a Norwegian context. The cogeneration of power and heat production must meet stringent requirements on CO 2 and NO x emissions. Therefore, the selection of energy optimised processes for the gas sweetening and gas liquefaction is important. PS7-Sp.2

3 As a significant measure in the environmental strategy of the project, the carbon dioxide will be separated from the well stream gas at the onshore pretreatment facilities and pumped for transport and deposition in an offshore structure. This concept resembles the underground storage of carbon dioxide already proven feasible by the Sleipner Field in the North Sea. 3 SCREENING PROCESS TECHNOLOGY FOR TRAIN SIZING In order to improve the economy of scale, by reducing unit cost of produced LNG and thus increase the net present value, the project performed a screening study on increasing the capacity of the LNG plant, involving the main engineering contractors, the technology licensors and the machinery vendors. The objectives of the screening were to select the optimum capacity increase and recommend the optimum technical solutions for the capacity increase based on high thermal efficiency, maximum economy of scale effect and lowest life cycle cost. This also included identification of the main technology qualification work for the recommended solution in order to reduce the risk with respect to increased capacity. The study was based on the initial concept of train capacity of 3.4 million tonnes LNG per annum. The contractors identified several potential schemes up to 150 % capacity. The short listed schemes were evaluated in more details taking into considerations given evaluation criteria such as life cycle cost, equipment size and duplication, availability, impact on barge size etc. These evaluations resulted in some recommended cases. Special consideration was given to equipment sizing for the carbon dioxide removal and liquefaction process design. The screening comprised several different driver options including industrial heavy duty gas turbines and aeroderivative machines. Both mechanical drive of refrigerant compressors as well as electrical motors as compressor drivers were considered. Both steam and hot oil were evaluated as waste heat recovery system. Relative investment (investment cost related to 100 % capacity) versus capacity, revealed a significant economy of scale effect by increasing the capacity up to 150 %. The scale up factor corresponded to an exponent of approximately The best potential of combining reduced unit cost with moderate technology and plant complexity is a single LNG train capacity around %. If capacity increases beyond 150 % this might trigger two LNG trains. Taking all relevant risks into consideration, the Snøhvit project decided an LNG production capacity in the single train of 4.3 mtpa. Subsequently after having set the capacity based on screening results, the project initiated a feasibility study. The scope focused on process selection with high thermal efficiency. The contractors were specifically asked to develop two driver configurations, one based on steam and one based on hot oil as waste heat recovery system. PS7-Sp.3

4 4 SCREENING DRIVER/COMPRESSOR CONFIGURATIONS Statoil provided a list of pre-qualified drivers subject to the screening study. A prerequisite is all gas turbines to be equipped with low-no x (DLE/DLN) combustors. The GE LM6000PD was not pre-qualified to be used in mechanical drive service on the basis of the present record of operation. The screening resulted in several alternative driver and compressor configurations, comprising both direct mechanical and indirect compressor drive by means of Gas Turbogenerators and electric VSD (Variable Speed Drive) motors. Combinations of gas turbines, helper motors, generators, steam turbines and electric VSD motors were selected to satisfy the power demands of the compressor drivers, process heat and electric power requirements of the LNG plant. To reach the most robust driver configuration design, the project is applying life cycle cost evaluations, taking the Norwegian offshore CO 2 tax regime and BAT (Best Available Technology) recommendations into the screening/selection criteria. Driver designs are checked versus fuel gas prices, reflecting upstream investments and carbon dioxide taxes of respectively NOK per tonne. Under this regime, only the most energy efficient designs will be competitive. Plant availability is another selection criteria, especially with regards to increased on-stream days versus investments. 5 AVAILABLE TECHNOLOGY LICENSED LNG PROCESSES In 1997 the Snøhvit project requested three contractors ( Kellogg, Bechtel and Linde ) to carry out conceptual designs for a baseload LNG plant located at Melkøya in Northern Norway. Kellogg selected the APCI propane pre-cooled process, C3/MCR Liquefaction Process, in their design. This is the far most utilised process for base load LNG plants, and have been utilised in virtually all base load LNG plants installed the last 20 years, with some few exceptions. Bechtel applied the Optimised Cascade Liquefaction Process based on Phillips technology. Linde based their design on a dual flow liquefaction process but proposed to change their design in eventual further stages of the project to a newly developed, proprietary Mixed Fluid Cascade Process, the MFC process. After evaluations of these three conceptual designs, the project decided to award an Extended Conceptual Engineering contract to Kellogg and Linde. The Bechel proposed technology was rejected for further studies, since it turned out that its overall energy efficiency was too low compared to the MFC process and the C3/MCR process, which virtually have the same efficiency. PS7-Sp.4

5 6 TECHNOLOGY QUALIFICATION The technology offered in the Extended Concept Engineering was qualified in accordance with Statoil Quality Control System. This technology qualification was based on a yearly LNG production capacity of 3.4 mtpa. The train capacity increase to 4.3 mtpa made it necessary to perform a new and more extensive technology qualification. The main purpose of technology evaluation is to get a firm basis for decision, where minimising risk is the main issue. A precondition for the use of any risk assessment techniques is that acceptance criteria is outlined. This naturally will shape the format of the evaluation, direct the focus to areas of importance and determine whether a risk is acceptable or not. Today no exact and broadly agreed set of criteria is available in the industry. For the purpose of this work the following acceptance criteria have been jointly drawn up by the licence partners in Snøhvit. These overall guidelines were used, when performing the evaluation of the technologies involved: When departing from known established technology, the stipulation of the acceptance criteria shall be based on the established technology, such as established, recognised standard. The new technology may be accepted, if the analysis demonstrate that specified characteristics such as risk contribution and unreliability do not increase in relation to the reference solution, the established and recognised standard. For rotating machinery, technical solutions are regarded as prototypes as long as machines in similar service have not successfully accumulated at least hours on one machine and additional, the total fleet has accumulated hours. If the above criteria are not fulfilled, and the proposed technical solution deviates from established technology, the gap shall be analysed. Quantities such as safety factors, experience in form of running time at similar service, testing and verification program etc. shall be used in the analysis. Together with any compensatory measures required, this shall form the basis for proofing that the risk level will be in line with the risk level for proven technology. 6.1 APCI liquefaction technology APCI has delivered technology to the majority of LNG base load plants. Virtually all of these plants have been based on their C3/MCR technology. However, for the increased single train capacity of the Snøhvit project, the Dual Mixed Refrigerant process was considered. The technology evaluation showed that either processes could be used, and that there are no identifiable advantages of DMR over C3/MCR for this specific case. APCI recommended to select the well proven C3/MCR technology. No technology stoppers were identified when assessing the technology selection. PS7-Sp.5

6 6.2 Linde liquefaction technology The MCP process proposed by Linde is in principle a cascade process, with the important difference that pure refrigerant cycles are replaced with mixed refrigerant cycles, and thereby improving efficiency and operational flexibility. Comparison work were performed between single flow, dual flow and mixed fluid cascade process options. Linde concluded that the MFC process was the most advantageous process. The following characteristics were found to apply to the MFC process: The process is new, and as a whole without any industrial references. However, theconcept is build up by well known elements. The liquefaction process utilizes a plate fin heat exchanger for pre-cooling, and two separate SWHE for liquefaction and sub-cooling. The size and complexity of the SWHE applied in the MFC process is considerably less when compared with today s dual flow LNG plants. The technology qualification has not revealed any technology stoppers in selection the Linde MFC process. The integrity and operability of the Linde SWHE has been proven by a test plant in Mossel Bay, South Africa. All critical elements of the liquefaction process have either references to plants in similar service, or have been qualified by extensive testing and verification calculations, based on sound engineering practice. 7 LARGE LNG STORAGE CONCEPT The potential advantages of adopting one 220,000 m 3 LNG tank over the two times 110,000 m 3 option became apparent when judged on an economic and project execution basis. These advantages included reduced labour and equipment requirements leading to a reduced effect on the local community and environment. In order to assess the viability of a 220,000 m 3 LNG storage tank, studies have been carried out based on a 9 % nickel inner tank and a pre-stressed concrete outer tank. Inner tank designs have been completed for both full height and partial hydro-test to identify the impact on shell thickness. Such large tank designs can be based on the rules of API 620 which requires a partial height hydrostatic test. If the tank were to be subjected to a full height hydrostatic test, then the design of the inner tank would exceed the API 620 limit. BS 7777 requires that the tanks are hydro tested to the maximum design product level and allows a maximum shell plate thickness of 30 mm. However, the code does allow thicknesses in excess of this figure with the purchaser's agreement. The new Eurocode under preparation will probably allow a maximum shell thickness for 9 % nickel steel of 50 mm and partial height hydrotesting of the inner tank. A benefit of the partial hydro test design for the 220,000 m 3 tank is a material saving of more than 700 tonnes of 9 % nickel steel. Further associated savings would be made on labour and consumables due to the reduction in weld volumes. Considering the extreme conditions at the Hammerfest LNG plant site in winter, it is essential to have a weatherproof outer tank by the end of the construction window to allow work to continue inside the tank. In this case the steel roof would be exposed to the PS7-Sp.6

7 elements on the top of the tank and would be subjected to wind and snow loads. The schedule requirement is to progress tank construction during the first summer to achieve a weather tight envelope, permitting work to proceed inside the tank during the winter months. The target is to complete the outer tank concrete roof, however in order to mitigate for weather delays to the concreting, the steel roof should be designed to accommodate the potential snow load in the event that the concreting operation is not complete. 8 CLOSING REMARKS In December 2000, the Snøhvit LNG project has passed an important milestone. The license partners have selected the main engineering contractor and the LNG technology licensor, and agreed to enter into the next phase of the project development, namely the front-end design and engineering for the LNG plant. Start-up of LNG production from the Snøhvit fields is scheduled for October PS7-Sp.7