| dc.description.abstract | Gassco is the operator of the world’s largest transportation network of natural gas located on the Norwegian Continental Shelf. Their daily operation includes a number of assets such as offshore installations, processing plants, and gas terminals spread all over Europe. The strategy of Gassco is to have a safe, reliable, and cost efficient operation, that manage to ensure that the end user in Europe receives gas with the right volume and quality at the agreed time. Hence, operational regularity has a high focus within the organization. In order to maximize the regularity, the whole operational chain must be considered, from the gas entering the gas transportation network at an offshore facility, until the end users receives it after being processed at a processing plant. Gassco’s current portfolio includes a mix of old and new facilities, where the first installation was built in 1967 and the latest project started operating in 2016. As the age of the facilities cover a wide span of 50 years, the need for maintenance vary significantly and is only expected to increase in the future based on residual lifetime assessments.
The system contributing to gas deliverance consist of several critical equipment whereas the emergency shutdown (ESD) system is one of them. The ESD system plays an essential part for the transportation, especially in terms of stimulating operational safety. During transportation must the gas pass through around approximately one hundred ESD systems, whereas some are classified as more safety- and production critical than others. Failure of one of the ESD systems and its associated emergency shutdown valve (ESV) can result into catastrophes such as loss of lives. Because of the risk associated with an ESD system that is not capable of performing its intended safety function, there exist both internal and external requirements the system must comply with. Based on these requirements is Gassco obligated to perform proof tests of their systems, which successively must be reported to the Petroleum Safety Authority (PSA) to prove compliance to the legal requirements. From a holistic point of view, have fortunately less than a handful of Gassco’s historical unplanned shutdowns over the last years been caused by failure of ESD systems. Their historical data of operational experience shows that the common root cause failure of this kind of systems are a combination of ageing, rapid decompression, and tear and wear governed by number of cycles.
End users in Europe will be dependent on Norwegian gas for a long time, with a perspective per today up to 2050. Typical design life-span perspective of projects Gassco is conducting can be in a range of 40 – 50 years (e.g. offshore pipelines), which is not expected to cover the entire time period end users will be dependent on gas deliverance. This means that current and future ESD systems are expected to operate many decades, with large associated cash flows from gas sales and tariff income along with operational and maintenance cost. Hence, it will be of great benefit to enable condition monitoring (CM) of such critical systems that can provide indications of degradation mechanisms, which successively improves the systems’ technical integrity and avoids high investment cost (CAPEX) contributed by unnecessary replacements of components that is included in the ESD system.
The main challenge is to identify interactions between common degradation mechanisms causing failure of the ESD systems and possible techniques within condition monitoring that enables detecting the respective symptoms before they develop to failure. Consecutively, how this will affect the technical integrity of the system and the need for testing it in context of the reliability-measurement of the safety integrity level (SIL). Therefore, the purpose of this thesis is to develop a model of optimizing test frequencies and ensuring compliance of SIL requirement of the ESD systems based on condition monitoring. The research work is based on case study at Gassco where several interviews, quantitative analysis of the Gassco’s historical database and field investigations were conducted within the study.
The results show that condition monitoring of the ESD systems can be enhanced by trending specific parameters e.g. valve closing time and several health monitoring techniques. Moreover, the result show a potential link between parameter and health monitoring, and SIL and test frequency requirements.
The developed model could provide an effective framework of how condition monitoring can enhance the capability to ensure the compliance of SIL requirement of the ESD systems and to optimize the test frequencies. The model suggests that valve operators can start with failure analysis in terms of a FMEA to determine the systems failure modes and degradation mechanisms. The second step is to determine the operational performance based on historical proof testing data, and classify the occurrence of failures. The third step is to explore the potential monitoring techniques and trending indicators applicable in order to enable scrutinizing symptoms of the causes identified in the FMEA, which allows for moving failures that were traditionally classified as dangerous undetected (DU) without CM to dangerous detected (DD) failures with CM. The final and fourth step is to update the average probability of failure on demand based on the credibility given the technology of CM. Successively, can an optimized test frequency that still complies with the associated SIL requirement be identified, based on the new estimated average probability of failure on demand.
The FMEA and performance analyses of the studied vales clearly illustrate that fail to close on demand are the most critical failure mode and a phenomenon which can be named as “sudden-jump” in the closing rate of the valves. This physical phenomenon which is observed in the testing data of closing time of the valves might be an effective indicator of the valve’s condition to be monitored over the time.
Seven condition monitoring techniques were determined based on the Gassco’s industrial needs and explored with respect to the studied value system in order to evaluate their effectiveness in monitoring the ESD system’s condition (performance and health). However, the monitoring techniques based on the valve closing time (within proof testing) was selected to be the most effective and reliable concept at the moment as it was hard to ensure the effectiveness of the other explored monitoring techniques.
The developed model, understanding the valve operational behavior and the potential monitoring techniques shall be enhanced and validated in the future with real application. Once the developed monitoring technique show its capability to detect symptoms of failure as early as possible, which enables moving failures classified as DU to DD, can a new average probability of failure on demand be estimated. Successively, can the developed model be validated in terms of how much it could help in optimizing current test frequency. | nb_NO |
