To cover future requirements for the offshore wind industry, which demands new innovative technology, a system approach to the transmission / export system has been investigated. Testing, modelling and manufacturing have been carried out to scale cable technology to higher voltage levels. This includes material testing, sample testing, and ultimately the manufacturing of full-scale cable system prototypes. Such prototypes comprise a multitude of different cable accessories, which together have been subjected to rigorous type-and pre-qualification tests per the present industry standards. Subsea HVAC transmission systems are well known, but exhibit different types of transmission losses, especially pronounced for high voltage and longer lengths. Such losses can to some extent be minimized through cable design and a tailored phase compensation system. A recently qualified 145 kV AC system is currently undergoing testing in order to scale this for higher voltages. Subsea HVDC cable systems exhibit far fewer transmission loss types but do require converter stations compromising complex electronics at both ends. Such converters are both costly, large, and add a link length independent loss penalty. A recently qualified 525 kV extruded HVDC system, now capable of operating at higher rated temperature, is a new enabler for large bulk power flow at 1.2 GW per single pole and beyond. Thereby, the technology has been scaled from dry 145 kV cable system to 245 kV AC or, from static 320 kV DC systems to a robust 525 kV DC offshore wind system. Based on successful test results, further prototypes are also to be manufactured in 2023. Floating offshore wind parks currently only exist at lower power levels and voltages. This paper presents how the required high power systems can be enabled using novel technology combined with technology proven for other use.
A two-sided marked push for subsea power transmission has taken place in the last 5 years, first initiated by the oil & gas industry's push for subsea electrification, then followed by a desire to reduce the CO2 footprint from oil & gas production throughelectrification of production facilities with power from onshore. More recently, the dramatic development in offshore wind and especially the floating wind market for clean renewable power results in the same need. Common for both areas is the need for transmission capability of the electric power at high voltage. Subsea cables for high voltage have existed since the early 20th century, but deep water and dynamic use of cables has, until recently, only been needed in niche areas. The know-how and experience from the niche areas combined with long-term operational experience for high voltage transmission cables has been a vital factor leading to the development of a qualified solution for deepwater and/or long-distance cabling system which also function in a dynamic situation. The latter is vital for larger offshore wind farms, as well as subsea development. Electrical insulation systems are grouped as wet or dry, where the latter traditionally has been used for voltage ratings above 52 kV. The traditional solution using lead as a water barrier is not suited for dynamic applications and this paper presents the process of qualifying a new industrial solution for dynamic high voltage cables suited for deep water and/or harsh weather for oil & gas or floating windfarms. A complete 145 kV dry design cable system has been qualified for dynamic and static use, with factory joints and repair joints making deepwater and/or long-distance transmission capacity possible. At the same time, qualification of a 72.5 kV wet design cable and factory joint has been completed. The qualified transmission system consists of a mixture of traditional cable design and a new novel water barrier system for subsea cables. Combining this development with the extended qualification of wet design supports increased power transmission capacity.
Traditional chemical treatment methods have considerable operation costs and represent a risk to the environment. Since 1987 Norwegian oil companies have been investigating alternative electrical heating methods for prevention of hydrate and wax plugs. A joint industry project ‘Concept Verification – Direct Heating of Oil & Gas Pipelines’ was initiated in 1996 and terminated in October 1999. During this work an electrical heating system was proved to be feasible on several fields in the North Sea. It will be installed on 7 flowlines of 13% Chromium (Crl3) with lengths between 6 km and 16 km. Electrical heating is used to maintain or raise the thermally insulated steel pipe temperature above the critical value for hydrate formation (typically 15–25 °C) or wax formation (typically 20–40°C). A single-phase power supply for the heating system is based on commercial components and connected to the platform power supply. The qualification work for the direct heating system has included full scale testing for single and parallel pipes, end termination at the template, bypass of a template and aspects concerning corrosion control. The rating of the system is dependent on the magnetic and electrical characteristics of the steel material. Such data is not commonly available. Measurements performed during the qualification program confirm that the magnetic characteristic may vary within a wide range for a specific steel quality and that mechanical stress and heat treatment can effect the magnetic characteristic. The difference in magnetic characteristic of individual Crl3 pipes results in variation of the pipe temperature and problems concerning differential pressure during melting. The problem can be handled by dividing the pipeline into a number of sections, each with a limited variation of the magnetic characteristic, thus keeping the temperature for the whole pipeline within acceptable limits. As a part of the pipe specification both electrical and magnetic characteristic should be available. These data can be determined by measuring arrangements in the production line of the mill. Measures to limit the variation of magnetic characteristic should be discussed.
Following the drop of oil prices during 2015 we have seen an increased attention from operators on using new technology, in particular active heating technologies. This paper presents an update to experience from early implementation with lessons learned to recent technology development of what today is the only field proven electric heating technology for rigid flowlines per today - Direct Electric Heating (DEH). It also discusses Electric Heat Tracing as promising technology undergoing qualification – but the main focus is on DEH. DEH had its technology qualification carried out in the 90-ties and early 2000's while the more recent years has focused on achieving long distance tie backs, qualification for deep water projects but also design alteration opening for cost reduction. Increased number of operators: During the first 10 years, all projects were in operation in the North Sea whereas during the last 8 years other operators has started using the technology, as well as applications in harsher environment and deeper waters. Deep water: Following internal testing to gain more information on material data of steel and copper, combined with an update in analysis technique – previous over-conservatism in design process can be reduced such that a more accurate analysis can be carried out. Consequently, cable designs which previously only were applicable for depths down to less than 800m WD can now be used at 1500m WD. Long distance step-outs: Wet design XLPE insulation to higher electrical gradients has been now successfully qualified, allowing DEH to be applied on longer distance tie-backs than before Technology improvements A 3-year R&D on high frequency DEH gives both technical advantages for interference with other subsea equipment, reduced AC corrosion as smaller conductors which leads to a significant cost reduction. With the above technical improvements alone or combined, and a better DEH system than that was qualified in year 2000 is now available to the market.
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