## 1 Introduction
[30] implemented the two-way coupling of fluid-structure interaction (FSI) to investigate the resultant effect from slug flow into the structural domain by varying the volume fraction and mixture velocity in a subsea jumper for gas-low viscous liquid. In addition, they developed a structured methodology to determine the likelihood of failure and predict the reduction of the designed life of a piping system due to slug flow behaviour. [PERSON] and [PERSON] [31] and [PERSON] et al. [32] analysed the effect of hydrodynamic slug forces on structural behaviour by changing the volumetric gas-liquid fractions and flow rate. They found that slug velocity exerts a significant effect on the vibration response of piping systems. [PERSON] et al. [33] developed a one-way coupled FSI model to establish a prediction model that correlates slug frequency and induced stresses by using air-water mixture flowing in a horizontal Plexiglass piping. Furthermore, [PERSON] et al. [19] also extended his air-water slug flow investigation and studied the effect of slug liquid length and slug translational velocity on the mechanical stresses in a Plexiglas pipe. [PERSON] et al. [1] analytically explored the dynamic responses of gas-liquid two-phase slug flow in a simply supported piping system by varying the piping materials and translational velocities. They concluded that for a high gas superficial velocity, the dynamic response manifests as a periodic-like motion. Recently, several researchers utilised various intrusive and nonintrusive methods to investigate the flow pattern and identify the two-phase flow characteristics [1; 2; 3; 4; 5]. [PERSON] et al. [34] used electrical capacitance tomography (ECT) to measure liquid holdup and utilised the high-speed camera images (HSCI) to investigate multiphase flow patterns. [PERSON] et al. [35] combined the typical electrical capacitance tomography (ECT) with the convolutional neural network (CNN) algorithm and predicted the two-phase flow patterns and 'gas-liquid' flow rates. [PERSON] et al. [36] measured the void fraction and identified the annular regime by utilising gamma radiation attenuation and artificial intelligence. Furthermore, [PERSON] et al. [37] experimentally determined the flow pattern and predicted the oil-water-gas volume fraction by using sodium iodide crystal detectors, X-ray tube combined with group method of data handling (GMDH) neural network. [PERSON] et al. [38] used ultrasonic phased array technology to identify the 'air-water' vertical flow regimes in multiphase flow. In sum, most previous studies investigated the effect of varying the operating conditions of slug flow on the induced stresses in pipes whilst keeping the liquid physical properties unchanged. As mentioned previously, slug flow occurs frequently in oil and gas systems and is known to cause serious damage to pipelines. Therefore, the effect of changing the crude oil grade on slug characteristics and the resulting induced stresses should be investigated in detail. In this study, we aimed to investigate numerically the effect of varying crude oil grades on slug characteristics and the structural responses through the implementation of a partitioned one-way coupled FSI technique of a natural gas-crude oil two-phase flow in a horizontal carbon steel (ASTM SA106 Grade B) piping. The developedCFD and structural models were validated against the experimental results for air-water two-phase flow of [PERSON] [19].