The Shetland Gas Plant (SGP) is a 500 MMSCFD capacity gas plant located at Sullom Voe on the Shetland Islands. It receives reservoir fluids, via twin 143km multiphase flowlines, directly from the Laggan-Tormore fields which are located 125km north-west of the Shetland Islands in approximately 600m water depth. Fluids arriving at the SGP are separated into gas and liquid phases. The gas is processed and then exported to St Fergus gas processing plant via the SIRGE and FUKA pipelines. The liquid phases are separated and the condensate is exported to the BP Sullom Voe facility for stabilisation and export by tanker. The aqueous phase (rich MEG) is regenerated at SGP to produce lean MEG for reinjection subsea, a by-product of the regeneration process is produced water. The produced water is then fed to the Effluent Water Treatment Plant (EWTP) for processing prior to being discharged to Yell Sound via a 3.75km pipeline. The effluent water treatment package was designed by SUEZ Eau Industrielle (patented design). The effluent water is required to meet strict discharge specifications as part of the operating consents, e.g. Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), MEG, Benzene, Total Suspended Solids (TSS), etc. The Effluent Water Treatment Plant (EWTP) consists of 3 stages of treatment: physical, chemical and biological. The physical treatment contains; – A Corrugated Plate Interceptor, which uses gravity to separate the free oil from the produced water; – A Stripping Column, which removes BTEX (Benzene, Toluene, Ethylbenzene, Xylenes) and volatile hydrocarbons entrained in the water via transfer to fuel gas. The chemical treatment contains; – A Dissolved Air Flotation Treatment Unit, to remove any residual free oil and TSS utilising flocculants and coagulants. The biological treatment contains; – A Biological Aerated Flooded Filter (BAFF) Unit, which is an aerobic biological filtration process whereby a biomass (bacteria) give biological degradation of soluble organics while simultaneously removing suspended solids via filtration. The biological process removes the remaining MEG and BTEX and has the ability to handle varying loads of COD and BOD. The EWTP has been in operation since the start up of the SGP in February 2016. The paper will discuss: – The initial challenges faced during start up and the first year of operation and how these were overcome; – Current operation of the process including ongoing challenges and areas of success.
The Shetland Gas Plant (SGP) is a large gas processing site built in a very fragile ecosystem (blanket bog) at Sullom Voe on the Shetland Islands. There are multiple environmental designations in the surrounding environment (RAMSAR, SSSI - Site of Special Scientific Interest, MPA, …). The site processes gas and liquid, the produced water effluent from these operations are then fed to the Effluent Water Treatment Plant (EWTP) for processing prior to being discharged to Yell Sound via a 3.75 km pipeline. There are numerous defined pathways and very sensitive receptors in the area which both the produced water and drainage water could directly impact in a short period of time. The regulatory regime is very intensive & wide ranging, this allied to the high societal expectations, especially socio-economic, meant that careful design and construction criteria, along with very reactive operational control methodologies, were required to manage the need for continuous monitoring and modifications. Best Available Technique (BAT) assessments were carried out to determine a predicted contaminant level; actual recorded levels were then compared to the predicted level. Over the course of a lengthy construction and commissioning period, it was found that the output contaminant balance was very complex and susceptible to large movements with small input changes; these were recognised as associated issues (multiple determinands and management of trigger levels). This required prolonged and intensive adjustment to the process to ensure that the regulator agreed Environmental Limit Values (ELV's) were met. This process had a multi analysis approach to allow for the many different variables which required to be controlled in tandem to meet the ELV's. Final environmental sign off from commissioning was achieved in July 2017 and even with the higher than anticipated usage and the legacy construction issues, the site continues to improve its performance and protect the receiving environment from process contaminant problems. It was found that engagement with the regulator at an early stage and regular/informed updates, good baseline assessments, a solid understanding of stakeholder management issues and initial and ongoing biodiversity identification allowed the project to manage this very sensitive development in a very fragile and reactive ecosystem.
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