The use of High-pressure Vessel in eco-friendly Natural Gas Vehicles (NGV) is
technologically feasible nowadays. Common applications of High-pressure Vessel are to carry
Compressed Natural Gas (CNG), hydrogen for fuel-cell vehicle, and high-compression air in the
new air-car technology. High-pressure Vessel is subjected to extreme compression-decompression
cycles that could cause fatigue failure. Therefore, vessel shall be inspected regularly to detect if
there is crack inside. The objective of this paper is to optimize the inspection interval of CNG Highpressure Vessel by means of Probabilistic Fracture Mechanics Analysis. Vessel is made of highalloy steel and assumed to have distributed elliptical cracks. Three length-to-depth crack ratios
(a/c), i.e. 3, 8, and 15, are simulated.

Pembahasan – pahala_b

Probabilistic Fracture Mechanics Analysis for Optimization of
High-pressure Vessel Inspection

By : Indera Sadikina, Djoko Suhartob, Bangkit Meliana, Kemal Supelli, Abdul Arya
Mechanical Engineering Department, Institut Teknologi Bandung, Indonesia
aisadikin@gmail.com, bds@labsurya.ms.itb.ac.id

Keywords: Pressure Vessel, Inspection, Crack Growth, Fracture Mechanics, Finite Element
Analysis, Probabilistic Analysis, Guided Direct Simulation.

Abstract. The use of High-pressure Vessel in eco-friendly Natural Gas Vehicles (NGV) is
technologically feasible nowadays. Common applications of High-pressure Vessel are to carry
Compressed Natural Gas (CNG), hydrogen for fuel-cell vehicle, and high-compression air in the
new air-car technology. High-pressure Vessel is subjected to extreme compression-decompression
cycles that could cause fatigue failure. Therefore, vessel shall be inspected regularly to detect if
there is crack inside. The objective of this paper is to optimize the inspection interval of CNG Highpressure
Vessel by means of Probabilistic Fracture Mechanics Analysis. Vessel is made of highalloy
steel and assumed to have distributed elliptical cracks. Three length-to-depth crack ratios
(a/c), i.e. 3, 8, and 15, are simulated. Crack is assumed to propagate in fixed ratio. Stress Intensity
Factors at each crack tip are calculated by Finite Element Analysis and Crack Closure Technique.
Fatigue crack growth is simulated by Cycle-by-Cycle Integration Technique. The Fracture
Mechanics Analysis is then expanded to probabilistic analysis by considering stochastic nature of
analysis parameters. Probability of failure is computed by Guided Direct Simulation Method using
software which is specially written for this project [1]. Based on simulation result, High-pressure
Vessel is recommended to be inspected every 3 years.

Introduction

Compressed Natural Gas (CNG) is environmentally clean alternative to gasoline and diesel fuel. It
is made by compressing methane (CH4) extracted from natural gas. In Natural Gas Vehicles (NGV),
CNG is carried inside High-pressure Vessel. Pressure ranges from 205 to 275 bars. Due to high
compression-decompression cycles during usage, vessel is vulnerable to fatigue failure if there is
crack in the vessel material. To prevent catastrophic failure, vessel shall be inspected regularly to
see if there is any crack inside.
Inspection costs money and downtime to the NGV. Thus, it is very important to determine
optimum and cost-effective yet safe inspection interval by means of Probabilistic Fracture
Mechanics Analysis. The analysis is carried out using Progrow software which was developed by
authors [1]. Previously, the same method was used to optimize the inspection interval of sub-sea
pipelines based on data provided by Tronskar [2]. Compared to the previous work, the stochastic
input parameters in this High-pressure Vessel case are fewer and simpler.
It is revealed that crack inside the studied vessel is the result of corrosion due to water vapour
contained in the natural gas. If the gas is dried before it is compressed into the vessel, crack and
fatigue failure problem are solved. But according to Theory of Planning Scenario [3, 4], water
content is defined as the primary shaping factor of the problem. The fact is no fool-proof scheme
can guarantee the natural gas will be completely free of water vapour.

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