実機配管における減肉速度の確率特性調査方法検討と信頼性評価手法への適用

Investigation of probability characteristic about thinning speed in actual plants and application for reliability analysis

栗原 朋之*; 戒田 拓洋*; 岡島 智史 

Kurihara, Tomoyuki*; Kaida, Takuyo*; Okajima, Satoshi

プラントの圧力設備の分野において、供用適性評価(Fitness-For-Service、以下FFSと略す)技術の検討が行われ、欧米を中心に規格化されている。FFS減肉評価では、減肉部の詳細な肉厚測定結果から対象設備の肉厚形状を想定し、その形状に対する強度の評価を行い、適性が判断されている。この際、供用中の減肉の進行については将来腐食代として、推定減肉形状に反映される。このため、供用中の減肉の進行の正確な予測、すなわち正確な減肉速度の把握が本評価において必要不可欠である。本検討では実プラントの検査データをもとに、定量的な供用適性評価を行うための手法を開発、検証することを目的とした。このため、実際のプラントの配管の肉厚測定データを活用し、統計解析を行うことで減肉速度の確率特性を求め、信頼性評価を実施した。

In Fitness-For-Service (FFS) assessment of pressure equipment, high reliability is needed in petroleum and petrochemical plant. Future Corrosion Allowance (FCA) is one of the most important factors in the metal loss assessment of FFS because it decides predicted value of the thickness during service periods. For the evaluation of FCA, the accurate corrosion rate from actual thinning data is needed and it is necessary to make a consideration of their variability of the thinning phenomena. In this study, the measured thickness data of the oil piping in the actual plant are collected and statistically analyzed as corrosion rate data which are classified by the operation condition. By the analysis, the probabilistic properties of corrosion rate are clarified in this piping system. Especially it is cleared that the corrosion rate of the air injection part is higher than that of the no air injection part. And the reliabilities of the protection of the failure by pressure and their change are quantitatively evaluated from the probabilistic properties of corrosion rate. From this case study, the procedure of FCA evaluation of FFS assessment was established and it's clear that the statistical analysis by the feature of thinning phenomena is important for accurate evaluation of corrosion data.