Experimental research of flow rate and diffusion behavior of nature gas leakage underwater

Highlights

• Experimental investigation of nature gas underwater leakage in a wind tunnel.

• Plume dynamics and orifice behavior was observed by a high-speed camera.

• Fitting correlation was established to evaluate the leakage flow rate and plume diameter underwater.

Abstract

In order to assess the potential risk of pipeline underwater leakage, a self-designed experimental setup is carried out to study the gas release rate and dispersion behavior in different release scenarios. A transparent organic glass tank with dimension of 1 m × 0.5 m × 0.5 m (height × width × length) was placed in a wind tunnel. The release pipeline made by stainless-steel with diameter of 25 mm were used to simulate for variation release depth. The different size and shape of leakage orifices in 1 mm, 3 mm, 5 mm in round and 3.5 × 2 mm, 7 × 1 mm in rectangle were designed for comparison. The medium of methane gas was released from the controllable cylinder. The variation parameters of flow rate and pressure were measured by a flow meter and pressure gauge respectively. A high speed camera was employed to recorded the phenomenology of dispersion characteristics and breakup process for a wide range of orifice size in the time-resolved images. The dynamic plume diameter on water surface was measured by a Vernier caliper placed above the water tank. The considered factors including orifice size, leakage pressure and water depth effect on gas flow rate and dispersion behavior was quantitative investigated. The fitting correlation between the gas flow rate and variation parameters can provide fundamental information for evaluation the hazard consequences of gas release in engineering application.

Introduction

With rapidly development of explosion and drilling in offshore, submarine pipelines have been widely used in transportation of the offshore oil and gas. However, the increased amount of the under water devices and pipelines also enhanced the potential risks and hazards for safety management and production. Numbers factors of hazard sources include anchoring, falling objects and corrosion in the actual scenario operation of pipelines, which usually lead to potential failure and accidental release (Olsen and Skjetne, 2015; Li et al., 2018). The pipeline gas leakage underwater will pose a serious threat to the integrity of pipeline assets, the life safety of operators and the ecological environment of the seabed. The harsh natural conditions and the marine construction environment put forward more greater requirements for dealing with catastrophic accidents (Van der Geer and AuthorAnonymous, 2010; Yapa et al., 2012; Li et al., 2016).

Numerous researches have investigated on the flow behavior and breakup process near the orifice of a submerged gas jet release. Both methods of the experimental observation on the phenomenon of breakup dynamics and theoretical calculation for mechanism analysis are mainly research measures (Zheng et al., 2003; Premathilake et al., 2016a,b; Solsvik et al., 2016). The theoretical analysis mainly included the integral method, which combines some semi-empirical correlation of experimental data to establish the basic equations of gas-liquid in two-phase flow (Hissong et al., 2014; Olsen et al., 2017). The scale of the current experiments are mainly included: small-scale for laboratory test (water depth < 2 m), medium-scale test (7–50 m offshore) and field trial (>50 m). Koopman and Cederwall. (1982) was conducted an experimental of methane leakage in Burro, and analyzed the effects of different leakage conditions on plume characteristic and flow rate. As early as 2001, a field experiment named “Deepspill” was carried out in Norwegian waters, which provided data support for the initial formulation of prevention strategy information. But limited to the safety issues and high cost, it was difficult to implement another large-scale field experiments (Johansen et al., 2003). J. Rensen and Roig (2001) conducted the dynamics process of the unstable structure in two-dimensional bubble plumes underwater. The results shown the dominant periodic motion was recorded even for both of the different gas flow rates. Martı́nez-Bazán et al. (2002) experimentally studied the dynamic behavior of the bubble size from gas injection to the liquid fluid. Brandvik et al., (2013) established the lab-scale facility (dimension with 6 m in height and 3 m in width) to reveal the relationship between the droplet size versus nozzle diameters under various release conditions. Also the size distribution of the droplets from submerged blowouts in deep water has strong impact on the fate of the leakage fluid. Weiland and Vlachos. (2013) have performed experiments of gas jet release underwater and the length of jet penetration dominated by inertia, but these experiments are based on safety air for substitute and limited in confined scenarios. Premathilake et al., (2016a,b) established a model of prediction the pollutant transport for underwater oil and gas spills based on Lagrange integral method. Lee and Park [2017) presented the evolution process of plume trajectory underwater using high-speed camera technology, and proposed a convective velocity model of bubble wake vortices. Wang et al., (2018) observed and quantified the flow behavior from laminar to turbulent and predict the gas bubble size based on the accidental release of submarine pipelines. The results developed the models of prediction the fate and transport of gas motion and bubble breakup during subsea accidents. From those studies, available experimental data and jet behavior are necessary to assess the effectiveness of risk prediction and verify the previous physics or mathematical based numerical models. Also the methane gas is more compressible in a large pressure drops compared with the safety gas like air and nitrogen. Hence, it is important to observe the behavior of natural gas submerged release and investigate the scale law for prediction the leakage flow rate in engineering significance.

This work experimentally investigated on the flow behavior and breakup process of natural gas submerged release from a wide range of orifice diameter for quiescent conditions in a wind tunnel. The variation parameters of different orifice size and shape, leakage pressure and water depth were carried out to observe the bubble plume rising process and diffusion behavior on water surface, and measure the leakage flow rate for a scale law. The quantification relationship between the plume diameter and leakage flow rate in various factors has also been estimated.