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Projects: Projects for Investigator
Reference Number NIA_NGET0103
Title Modelling the tape corrosion process for oil-filled underground cables
Status Completed
Energy Categories Other Power and Storage Technologies(Electricity transmission and distribution) 100%;
Research Types Applied Research and Development 100%
Science and Technology Fields PHYSICAL SCIENCES AND MATHEMATICS (Metallurgy and Materials) 50%;
ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 50%;
UKERC Cross Cutting Characterisation Not Cross-cutting 100%
Principal Investigator Project Contact
No email address given
National Grid Electricity Transmission
Award Type Network Innovation Allowance
Funding Source Ofgem
Start Date 01 October 2013
End Date 01 October 2016
Duration 36 months
Total Grant Value £637,000
Industrial Sectors Power
Region London
Programme Network Innovation Allowance
Investigators Principal Investigator Project Contact , National Grid Electricity Transmission (100.000%)
Web Site http://www.smarternetworks.org/project/NIA_NGET0103
Objectives Confirm the dominating mechanism leading to tape failure and consequent oil leak Develop a computational (cross-reading) method for calculating residual life before failure Identify input parameters required by the computational method from data of leak and non-leak case studies, history of oil pressure, temperature and temperature change environmental variables Quantify the risk of using nominal data and mechanism-based models in the life prediction Develop a database of input parameters for the computational model from both the research literature and the National Grid reports and database. Practical risk mitigation recommendations regarding to cable tape corrosion Quantitative understanding on the mechanisms of tape failure Innovative methodology and framework for cable condition assessment A deterioration model able to capture the dominant mechanisms of the tape corrosion problem. Database which provides key input for the model Sampling and decommissioning strategy which will enrich the database Risk mitigation recommendations based on understanding the dominant mechanisms of corrosion
Abstract A particular family of oil-filled transmission cables is vulnerable to failure of the tin bronze reinforcing tapes that provide mechanical support to the lead sheath. An oil leak detected in 1996 was found to have been due to the failure of the reinforcing tapes, causing significant reliability and environmental issues and resulting in the replacement of the cable. More cables have failed since then as a result of this deterioration mechanism. A series of asset management initiatives, including investigations and sampling, have been undertaken over the past 15 years in order to understand and prevent this problem. However it has been recently realised that the reinforcing tape failure may not be as severe as expected by our current understanding. It is in the consumers’ interest to avoid premature asset replacement. Due to the long lead time associated with planning cable replacement, accurate condition assessment and deterioration models are needed, as well as effective risk mitigation strategies. The conventional method of taking samples is impractical for underground cables because it is an invasive method which runs the risk of damaging the cable. More importantly sampling without a guiding strategy may not be able to reveal the worst corroded area. Therefore it is difficult to be confident about our current understanding in the cable’s conditions. A quantitative mathematical model together with reliable database is required to calculate the asset conditions and remaining life and to guide the sampling strategy. Such condition calculations and sampling strategy are extremely important to manage these assets effectively and to plan their replacement. Research The proposed research takes advantage of a "cross-reading" methodology that has been established and used for life prediction of engineering systems in other industrial areas. In this approach, the current conditions and remaining life are "cross-read" (calculated) from case studies of existing failure incidents. A pilot study by Leicester confirms National Grid’s previous studies that the reinforcing tapes failed by corrosion fatigue. The cross-reading will be based on a combination of empirical and mechanism-based models for corrosion fatigue. This approach is particularly useful in life prediction for engineering systems which haven’t yet experienced their full life cycle. The project will consist of three steps: 1) identifying an empirical model as the basis of cross-reading. For example, the crack initiation and crack propagation depend on local stress and corrosive damage (such as Paris’ law). The empirical parameters in those models are material and environmental specific. They can be back-calculated from existing cases studies using finite element models. This leads to step 2): developing a database for environmental correction - the empirical parameters will be modified according to the local chemical environment using mechanism-based models. The input parameters in the mechanism-based models are the electrochemical properties of the material and environment, which are adaptive to individual cables buried in different environments. Step 3) experimental studies to confirm the dominant mechanism for corrosion fatigue assumed by the mechanism-based model. These include SEM studies of the failed cables and corrosion experiments to mimic the chemical conditions at the crack tip. The biggest risk when calculating the remaining life is that different laying environments of the buried cables can lead to different underlying mechanisms of corrosion fatigue. National Grid’s limited number of failure cases and lack of data for the local environment are the major challenges. In this project, the lack of case studies will be compensated by using mechanism-based models and nominal data in the literature for corrosion fatigue. Using this approach, it is possible to calculate cable conditions and remaining life without further sampling. However further samplings would significantly improve the model accuracy. Balancing the cost of further sampling with the risk of using the current data is essential to National Grid’s decision making in assess management. This project will quantify the risk of using the nominal data and mechanism-based models. This is to be achieved by Monte Carlo simulations on the nominal data and studying the sensitivity of model prediction to the input parameters. The quantified risk, the identified key input parameters and the cross-reading model will then be combined to establish a strategy to determine where, when and how to sample. This project will also actively seek opportunities to take samples from cables that are de-commissioning, being repaired or maintained. However targeted sampling will only be carried out where it is useful and when the risk out balances the sampling cost. The developed sampling strategy will minimise the risk that unnecessary or wrong samples are taken from operating cables and enable National Grid to avoid decommissioning cables prematurely. This sampling strategy will be a key deliverable of the project.Note : Project Documents may be available via the ENA Smarter Networks Portal using the Website link above
Publications (none)
Final Report (none)
Added to Database 12/09/18