In oil and gas fields, flow assurance issues are linked to liquid/gas two-phase flow, while flow regime is a problem occurring at select gas and liquid flow rate ranges. Slugging is a common undesirable flow pattern or regime occurring in offshore pipeline riser systems and horizontal wells. It’s better described as a global flow regime than a local flow patten. Slugging together with other unstable flow regimes lead to an avalanche of flow assurance problems, including low liquid production, pipeline fatigue, corrosion, and gas waste. In light of these problems, several studies have been conducted in a bid to better understand flow regime in pipeline-riser systems.
Throughout the many studies, researchers agree that global flow regime can be categorized into two basic forms, stable flow (ST), and an unstable flow (UST). They also agree on an intermediate flow regime, also known as irregular flow regime, characterized by alternate occurrence of stable and unstable flow regimes.
Severe slugging is a form of unstable flow regime. It’s most typical form ‘Severe Slugging 1’ (SS1), consists of five stages, namely: slug growth, liquid outflow, gas penetration, blowout, and liquid fallback. Both irregular and unstable flow regimes are undesirable and should be prevented from occurring to avoid severe outcomes.
Although designing a proper pipeline-riser system would curtail the risk, a potential key lies in the risk evaluation of undesirable flow regimes. Several commercial software applications have been developed to calculate two-phase flow in oil and gas pipelines, but unfortunately their simulation results based on a course mesh remain undependable to some extent. To this end, a mechanism study of global flow regime transition is highly desirable. The objective of the mechanism study would be to come up with transition criteria to predict the global flow regime. To accurately predict global flow regime, both upstream flow conditions and riser bottom conditions is urgently needed.
To address this challenge, researchers Dr. Suifeng Zou, Prof. Liejin Guo, and Dr. Tian Yao of Xi’an Jiaotong University, China, investigated the transition of undesirable flow regimes by analyzing upstream local flow behaviors and upstream to downstream flow development. Their investigation was based on the local flow measurements along the pipeline-rise system. Their research work is published in the journal, Chemical Engineering Science.
The research team studied the various mechanisms of undesirable flow regimes in various pipe sections, and came up with the corresponding criteria. They went ahead to validate the novel hybrid criteria through experimental results of different pipeline-riser systems. The researchers also discussed a potential extension of their study.
Going by the pressure drop signal the authors collected along the experimental loop, they were able to find the mechanisms of transition from stable flow regime to various unwanted flow regimes. This way, the authors developed and validated the corresponding criteria through the experiential data sourced from various experimental facilities. Comparing the novel criteria with existing models revealed that the criteria aim more common unwanted flow regimes and performs better.
The authors discovered that the transition mechanism from stable flow regime to irregular oscillation flow regime was the beginning of the long hydrodynamic slugs in the upstream horizontal section. The transition mechanism from stable flow regime to oscillation flow regime was the slug dissipation in the inclined section. The transition from stable flow regime to severe slugging was an outcome of slug dissipation as well. However, the authors took note of the extent of the blowout stage in the criterion. Results validated from the recorded flow parameters and results computed by the given boundary conditions agree with each other, and with the experimental results. This way it’s possible to extend the proposed mechanism to the design of pipeline-riser system.
As pointed out by CNOOC, one of the petroleum giants in China, the key issue and current challenge of slugging-related flow assurance lies in the prediction of multiphase flow in a field-scale pipeline-riser system through the experimental results obtained from a laboratorial test apparatus. The most distinguishing feature of the proposed criteria is that the length and the gas fraction of the upstream long horizontal section are absent. Hence, geometric similarity of the pipeline would be hopefully no longer a strict requirement for the experimental simulation of a field-scale pipeline-riser system through a laboratory-scale one.
Research Group of Flow Assurance for Deepwater Oil and Gas Fields
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University (XJTU), Xi’an, China
Aiming at the flow assurance problems induced by undesirable multiphase flow regimes in deepwater oil and gas fields, the research group focuses on not only fundamentals of multiphase flow, but also recognition, forecast and elimination of undesirable flow regimes in marine pipelines and risers. Supported by China ‘985’ Project, National Science Foundation Committee of China, and China National Science and Technology Major Project, the research group constructed three sets of pipeline-riser facility, with operating pressure range covering from atmospheric pressure to 30 MPa. The total length of the three pipeline-riser systems is ~150 m, ~380 m and ~1680 m, respectively. Flow instrumentation covers electrical capacitance tomography (ECT), electrical conductance probes, focused beam reflectance measurement (FBRM), rheometer, etc. Over the past 15 years, new regime transition theory, methods for fast recognition and forecast of undesirable flow regimes, and also automatic control of slugging flow based on upstream platform signal, have been put forward. Research achievements have been applied to the design or operation of offshore oil and gas pipelines, including those in Wenchang (China), Liuhua (China), and Egina (Nigeria) oil fields operated by CNOOC.
Suifeng Zou, Liejin Guo, and Tian Yao. Upstream-flow-based mechanisms for global flow regime transition of gas/liquid two-phase flow in pipeline-riser systems. Chemical Engineering Science, issue 240 (2021) 116542.