Elsevier

Ocean Engineering

Volume 239, 1 November 2021, 109716
Ocean Engineering

Explosion risk-based water spray mitigation analysis of ultra-deep-water semi-submersible platforms

https://doi.org/10.1016/j.oceaneng.2021.109716Get rights and content

Highlights

  • Explosion risk mitigation analysis of water spray for ultra-deep-water semi-submersible platform is conducted.

  • Large number of gas explosion simulations is carried out by using FLACS codes.

  • Effect of water spray configuration on exceedance frequency is analyzed.

  • Machine learning-based approach is proposed to determine the optimal water spray configuration.

Abstract

Water spray is an economical and environment-friendly alternative to mitigate gas explosion. This study conducts explosion risk analysis (ERA)-based water spray mitigation analysis of ultra-deep-water semi-submersible platform. 5000 gas explosion simulations with and without water spray are conducted by using FLACS codes. The outcome demonstrates that water spray with droplet size 700 μm and water application rate (WAR) 26 L/m2 min is most effective to mitigate explosion scenarios. The droplet size is the significant factor to stimulate the mitigation effect of water spray under middle and small exceedance frequencies, e.g., 1E-05 and 1E-06. WAR becomes dominate to awaken the water spray's negative effect under large exceedance frequency, e.g., 1E-04. A precise and efficient approach of BRANN integrated with artificial bee colony optimization algorithm is used to determine the optimal water spray configuration given the desirable exceedance frequency and the expected design accidental load that the ultra-deep-water semi-submersible platform structures could resist.

Introduction

In the oil and gas exploration and production industry, ultra-deep-water is defined as greater than 5000 feet, where large amount of remaining unconventional oil and gas reserves exist. Exploration and production of such unconventional resources contributes to alleviating the global energy production and consumption conflict (Li et al., 2020a). Semi-submersible platform is one of the widely used systems for ultra-deep-water oil and gas exploration and production, which however may face the potential gas blowout risk due to the extreme marine environment and geological conditions such as typhoon and shallow gas reservoirs (Meng et al., 2018; Shi et al., 2018a). The gas blowouts once being ignited can induce the fire and explosion accident causing severe casualty and property losses (Li et al., 2020a, 2020b; Yang et al., 2019). In April 2010, rapid gas release from the shallow gas reservoir caused the gas blowout and the subsequent explosion disaster, resulting in the loss of 11 lives and tremendous environmental damage (Dadashzadeh et al., 2013).

In order to mitigate the gas explosion disaster, safety design and analysis of semi-submersible platforms is necessary. Until now, several safety measures have been developed and its performance of gas explosion mitigation on various offshore platforms has been analyzed as well (Li et al., 2017). Water spray is one economical and environment-friendly alternative among the safety measures. Researches on gas explosion suppression of water spray started in the 1990s. Acton etc., analyzed the performance of the water spray with two different nozzles on gas combustion rate and explosion overpressure reduction for fixed offshore platform module (Acton et al., 1991). The results demonstrated that apart from the mitigation effect, the water spray could enhance the combustion rate and overpressure as well under some conditions. Van Wingerden and coworkers investigated the water spray-inducing turbulence effect on the combustion flame speed in a 1.5 m3 cubic box and a 50 m3 fixed offshore platform module, respectively (Van Wingerden and Wilkins, 1995; van Wingerden et al., 1995). The results showed that the break-up of droplet is the critical step to ensure the mitigation performance of water spray. Moreover, water spray with the droplet size less than 10 μm and larger than 200 μm is more useful due to the fact that the smaller droplets could directly evaporate in the flame, which then reduces the flame speed, and the larger droplets are easier to break up into the smaller droplets. In addition, Van Wingerden and coworkers validated the accuracy of the CFD tool namely FLACS to mimic the gas explosion process with water spray configurations which have been widely used on offshore platforms (van Wingerden et al., 1995). During the recent decade, most researches focused on investigating the performance of ultrafine water mist with droplet size smaller than 200 μm on explosion mitigation of various gas in various confined spaces such as pipeline and cubic chamber by experimental test and CFD tool, respectively (Cao et al., 2017a, 2017b, 2019; Qin et al., 2012; Song and Zhang, 2019; Jing et al., 2020). Significant progress has been achieved on the physical effect of such water mist on the gas explosion process. However, since being difficultly produced in the real world, the superfine water mist may not be currently applied for offshore platforms. Water spray with droplet size larger than 200 μm is still common on the offshore platforms (Van Wingerden and Wilkins, 1995).

However, to the best of the authors’ knowledge, very limited studies have been carried out on mitigation analysis of the commonly used water spray on the ultra-deep-water semi-submersible platform, which has different characteristics with congestion level and open space from the aforementioned research objectives, e.g., fixed offshore platform module. This study aims to fill this gap by using an explosion risk analysis (ERA) approach. ERA is one of the critical steps for the safety design and analysis of semi-submersible platforms, which can provide the design accidental loads (DALs) in a probabilistic manner (Shi et al., 2019a). Compared to the worse-scenario-based approach, ERA approach not only ensures the capability of the ultra-deep-water semi-submersible platform to resist a certain number of explosion scenarios, but also significantly saves the platform construction costs. Until now, different ERA approaches have been developed and applied for the gas explosion mitigation analysis of various safety measures for offshore platforms (Jin and Jang, 2018a, 2018b; Hansen and Middha, 2008; Talberg et al., 2001; Qiao and Zhang, 2010; Davis et al., 2011; Azzi et al., 2016; Gupta and Chan, 2016). Moreover, our previous studies proposed Bayesian regularization artificial neural network (BRANN)-based ERA approach and also verified its efficiency and accuracy by different types of offshore platforms (Shi et al., 2019a, 2019b).

This study conducts the water spray mitigation analysis of ultra-deep-water semi-submersible platform based on our previously proposed ERA approach. Gas explosion numerical model with and without water spray configurations of an ultra-deep-water semi-submersible platform is constructed by using the validated FLACS codes. Sensitivity analysis of water spray configuration on exceedance frequency of maximum overpressure is performed by conducting 5000 gas explosion simulations. An accurate and efficient optimization approach is proposed to quantitatively determine the optimal water spray configuration. The outcome of this study could provide the qualitative and quantitative guidance for water spray mitigation design for ultra-deep-water semi-submersible platforms.

Section snippets

Numerical modeling of gas explosion with water spray

Water spray system could reduce the consequence of gas explosion in several situations, which has been verified by some experiments. The evaporation of the droplet plays a significant role in the gas explosion consequence mitigation since the generated water vapor could dilute the flammable mixture resulting in a reduction of the burning rate or even the flame quenching. The droplet diameter is one of the most important factors influencing the evaporation process of the droplet. Previous

Gas explosion numerical modeling of ultra-deep-water semi-submersible platform with water spray

Normally, we require to first conduct the ventilation and gas dispersion simulations prior to the gas explosion numerical modeling for explosion risk analysis. However, since the effect of the water spray on the gas dispersion consequence is very limited, we thereby directly apply the gas dispersion numerical model and the associated simulation results from our previous study, which did not consider the water spray configuration (Shi et al., 2019b). On the contrary, we demonstrate the gas

Sensitivity analysis of water spray configuration on exceedance frequency of maximum overpressure

This part analyzes the sensitivity of the water spray configuration on the exceedance frequency of maximum overpressure of the ultra-deep-water semi-submersible platform. However, in order to determine the exceedance frequency of maximum overpressure under various water spray configurations, BRANN-based ERA procedure proposed in our previous study needs to be repeated. Since conducting this procedure in detail is not the focus of this study, we briefly describe such procedure by four steps.

Water spray configuration optimization

After the sensitivity analysis, the optimal water spray configuration should be quantitatively determined given the desirable exceedance frequency according to the ALARP principle and the expected design accidental load that the platform structure could resist. In this regard, the most accurate strategy in which ERA process is iteratively repeated until the optimal droplet size and WAR are obtained should be first recommended. However, this strategy requires large number of computational

Conclusions

This study conducted the water spray mitigation analysis of ultra-deep-water semi-submersible platform based on our previously proposed ERA approach. The brief conclusions are as follows.

  • (1)

    Water spray with droplet size 700 μm and WAR 26 L/m2 min has the capability to mitigate more than 90% explosion scenarios. For one certain number of scenarios, by increasing the droplet size from 200 μm to 700 μm, the mitigation degree is first significant and then keeps stable. However, there is no monotonous

CRediT authorship contribution statement

Jihao Shi: Conceptualization, Methodology, Writing – original draft, Supervision. Junjie Li: Formal analysis, Data curation, Visualization, Weikang Xie: Data Curation, Visualization. Faisal Khan: Conceptualization, Writing – review & editing. Yuanjiang Chang: Methodology, Writing – review & editing. Yuan Zhu: Validation, Investigation. Guoming Chen: Project administration, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Faisal Khan thankfully acknowledges the financial support provided Natural Sciences Engineering Council of Canada (NSERC) and Canada Research Chair (Tier I) Program to help participate in this work.

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