Sisi Nubi Area of Interest (SNB AOI) is an ongoing project to develop the marginal resources located in Sisi Nubi offshore field operated by Pertamina Hulu Mahakam (PHM). As the resources being developed are small and scattered, low cost solution platform design is required to develop these marginal resources. Therefore, design optimization is a must. Fit-for-purpose design concept is used as the basis to develop process and safety facilities in new SNB AOI platforms while maintaining the compliance with government regulations and international codes and standards. Various exercises were performed in order to ensure optimized process, safety and operating philosophy requirement. Lesson learnt and benchmarking from various established platforms design including PHM's minimalist offshore/swamp platform were performed. The previous designs are reviewed, adopted and SNB AOI platform design is adjusted as per operating philosophy requirement. Based on the available basis of design and operating parameters, fully rated topside facilities design is considered feasible to be applied for new SNB AOI platforms. With this design, relief system size (i.e. vent stack and vent knock-out drum) can be further optimized. In addition, closed drain drum and LP (low pressure) vent knock-out drum is also combined in one single vessel after careful safety verification. Other optimization is externalizing equipment that could be managed with Operating Expenditure (e.g. well offloading activity by using external facility). As for process safety design, preliminary reviews based on international codes and standards showed that the following designs are considered feasible: portable Gas Detector by operator during platform visit (in lieu of permanent Gas Detection system), Fusible Plug loop installation only (instead of redundant protection using Flame Detector system), non-provision of dry fire water mains, application of Passive Fire Protection (PFP) only after confirmation from fire risk study (risk-based approach), and considering not permanently manned installation, 1 (one) stair & 1 (one) ladder combination on each platform deck (instead of dual stairs). Overall, the proposed design maintains the safety of installation with minimum impacts to future platform operation. This optimization has contributed to reduction of dimension and weight of topside installation (i.e. 32% less compared to PHM's latest Offshore Minimalist Platform design) and also weight of the structure. Design of SNB AOI project could become a good reference for other Pertamina affiliate and other companies and could open a further way forward to economically develop the "marginal resources", especially in offshore area, by implementing the "fit for purpose" design concept.
Tunu is one of the biggest gas fields in Indonesia with 1400 km2 area in Mahakam Delta, East Kalimantan. This field has been producing since 1990 with cumulative production of more than 9.5 tcf and 190 mbbl condensate by the end of 2020 from over 1000 operating wells. Today, Tunu field contributes for approximately 40% of Mahakam production. After 30 years of production, Tunu production level is currently in declining phase, shown by its yearly production profile which exhibits a declining trend since 2008. Furthermore, Tunu well development project was considered marginally economical due to depleting reserve per well. Thus, an integrated study was conducted in order to reduce surface expenditure cost of Tunu pipeline based on current operating parameters. The study consisted of WHSIP history matching to determine new pipeline design pressure, evaluation of future wells production lifetime, and adjustment of pipeline corrosion allowance based on actual corrosion rate observed in Tunu field. Results show that most of future Tunu wells are predicted to have WHSIP below 200 barg and 1.5 to 3 years’ production lifetime. Corrosion rate in Tunu field as measured using corrosion coupon in piping with corrosion inhibitor injection is found to be less than 1 mm/20 years. Therefore, corrosion allowance for Tunu pipeline is optimized from 5 to 3 mm for 10-years design lifetime. For exceptional circumstances where actual well WHSIP > 200 barg, other method of producing the well will be implemented. Hence, by integrating recent subsurface behavior (WHSIP and well lifetime) with surface understanding (corrosion rate), it was then proposed new pipeline design for Tunu development. This study has generated USD 13 million cost saving for pipeline procurement in 2020. Moreover, implementation of the new pipeline design reduces 40% of pipeline unit cost for future pipeline procurement. This study has become the basis for future well development projects in Tunu field which significantly prolong Mahakam's production sustainability.
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