Elsevier

Ocean Engineering

Volume 114, 1 March 2016, Pages 236-249
Ocean Engineering

Methods for determining the optimal arrangement of water deluge systems on offshore installations

https://doi.org/10.1016/j.oceaneng.2016.01.010Get rights and content

Highlights

  • Proposed a new procedure of optimal arrangement method for water deluge systems.

  • Proposed the Water deluge Location Index (WLI).

  • Provided a demonstration and validation of the new method using CFD simulations.

Abstract

Offshore installations are prone to fire and/or explosion accidents. Fires have particularly serious consequences due to their high temperatures and heat flux, which affect humans, structures and environments alike. Due to the hydrocarbon explosions caused by delayed ignition following gas dispersion, fires can be the result of immediate ignition after gas release. Accordingly, it can be difficult to decrease their frequency, which is an element of risk (risk=frequency×consequence), using an active protection system (APS) such as gas detectors capable of shutting down the operation. Thus, it is more efficient to reduce the consequence using a passive protection system (PSS) such as water spray. It is important to decide the number and location of water deluge systems, thus the aim of this study is to introduce a new procedure for optimising the locations of water deluge systems using the water deluge location index (WLI) proposed herein. The locations of water deluge systems are thus optimised based on the results of credible fire scenarios using a three-dimensional computational fluid dynamics (CFD) tool. The effects of water spray and the effectiveness of the WLI are investigated in comparison with uniformly distributed sprays.

Introduction

The operation of offshore facilities such as FPSOs, TLPs, SPARs and semi-submersibles in shallow or deep water is prone to hazardous risks. Fires and explosions account for more than 70% of accidents on offshore installations (Christou and Konstantinidou, 2012). Fires with high temperatures and heat flux result in catastrophic consequences that lead to casualties, property damage and pollution. The Piper Alpha (6 July 1998) and Deepwater Horizon (20 April 2010) accidents are typical examples of fire events (Fig. 1), and numerous fire accidents have been reported on offshore installations (Christou and Konstantinidou, 2012).

To prevent fire accidents and/or reduce their consequences, the importance of fire risk assessment and management has been magnified (Czujko and Paik, 2012a, Czujko and Paik, 2012b). The risk assessment and management of fire are noted in the rules, recommended practices and design guidelines (Spouge, 1999, NORSOK, 2010, ABS, 2014, LR, 2014) and relevant guidelines have been established accordingly (Nolan, 1996, Walker et al., 2003, Vinnem, 2007, Paik and Czujko, 2009, Paik and Czujko, 2010, Paik and Czujko, 2011, Paik and Czujko, 2012, Paik et al., 2011). The risk can be defined asRisk=Frequency×Consequence.

The two most commonly implemented risk control options are active protection systems (APSs) and passive protection systems (PPSs). APSs such as gas detectors and showdown systems are used to prevent accidents and PPSs such as water sprays, heat shields, fire and blast walls and passive fire protection (PFP) are used to address the consequences after the accidents. PPSs are usually preferred over APSs due to the latter׳s higher cost (Lei et al., 2015, Sohn et al., 2015), and the optimal placement of protection systems also influences their cost effectiveness.

Seo et al. (2013) introduced a methodology for optimising gas detector locations among APSs on offshore installations using a quantitative approach and a two-dimensional analytical method. Paik (2011) investigated the effects of fire walls and PFP, optimising them using quantitative fire risk assessment and management.

Although there have been numerous studies on protection systems, they have not successfully optimised the effects. Thus, the 3D CFD simulation is needed to improve the accuracy and effectiveness.

The objectives of this study are to (i) suggest a procedure to optimise water spray systems using the water deluge location index (WLI) – a new approach to selecting the optimised locations of water spray systems using the 3D CFD simulation; (ii) investigate the effects of water spray systems; and (iii) compare the proposed system with water spray systems distributed by traditional methods. Among the three types of water spray systems shown in Fig. 2, the water deluge system (Fig. 2(a)) is examined in the present study. After selecting probabilistic fire scenarios, fire CFD simulations are performed by Kameleon FireEx (KFX) CFD simulation. Then, optimised locations are suggested for water deluge systems and their performance is compared with that of uniformly distributed water sprays.

Section snippets

A procedure for the optimisation of water deluge system locations

Fig. 3 shows a procedure for the optimisation of water deluge system locations using the proposed WLI.

The procedure is composed of the following steps:

  • 1)

    selection of credible fire scenarios;

  • 2)

    fire CFD simulations and/or experimental tests;

  • 3)

    obtaining the consequences of fire loads (e.g., temperature-time history, temperature distribution and temperature escalation);

  • 4)

    definition of the operation temperatures for water deluge systems (e.g., reference temperatures);

  • 5)

    calculation of WLI; and

  • 6)

    selection of

Introducing the water deluge location index (WLI)

In this study, a ranking index – the WLI – is proposed for use in selecting optimised locations for water deluge systems. The WLI rates each space, and can be calculated asWLIi=[n=1N(TR,nTr)/tR,nN]×FN,(i=1,2,3,,totalnumberofspaces),where (TR,nTr)/tR,n is the slope of the temperature until the reference temperature is reached, TR is the reference temperature, Tr is the room temperature, tR is the time at which the reference temperature is reached, n is the scenario number, N is the total

Target structure

A hypothetical floating liquefied natural gas (FLNG) topside structure constructed by the Korea Ship and Offshore Research Institute (KOSORI) at Pusan National University was selected as the target structure to perform an applied example. It consists of three decks: the upper (solid), mezzanine (0.7 porosity) and process (solid), as shown in Fig. 6.

Selection of fire scenarios

It is important to select credible fire scenarios when making decisions about optimised water spray positions. There are numerous methods for

Efficiency of the proposed method

To verify the efficiency of the WLI in the present study, the uniformly distributed and optimised (by WLI) positions are compared. In this case, 9 water sprays are used. Fig. 22 shows the selected water deluge system locations obtained by the WLI, which is calculated with a 121 °C reference temperature, as suggested by the NFPA (1996). Fig. 23 indicates the uniformly distributed water sprays.

Fig. 24 shows the comparison of the results, which are temperature distributions 40 s after ignition

Concluding remarks

The objectives of this study were to (i) suggest a new procedure for selecting efficient water deluge system locations to prevent and reduce the consequences of fire accidents on offshore installations; (ii) investigate the effects of water sprays; and (iii) compare the aforementioned results with those of the traditional method (uniformly distributed water sprays). Although the water sprays in the traditional method are uniformly located, the WLI is proposed to optimise water deluge system

Acknowledgement

This research was financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institutes for Advancement of Technology (KIAT) through the Promoting Regional Specialized (Grant no.: A010400243) and also was conducted under the project to establish the foundations of industrial technology which is funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (Grant no.: N0000003).

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