Elsevier

Ocean Engineering

Volume 60, 1 March 2013, Pages 149-162
Ocean Engineering

Review
Nonlinear structural consequence analysis of FPSO topside blastwalls

https://doi.org/10.1016/j.oceaneng.2012.12.005Get rights and content

Abstract

Hydrocarbons can be exploded by ignition with the mixture of an oxidizer when the temperature is raised to the point where the molecules of hydrocarbons react spontaneously with an oxidizer. The hydrocarbon explosion causes a blast or a rapid increase in pressure. This kind of accidents causes serious casualties, property losses and marine pollution. Topsides of offshore platforms are most likely to be exposed to such hazards as hydrocarbon explosion and a number of major accidents involving them have been reported. The aim of the present study is to develop practical procedure for nonlinear structural response analysis of FPSO topside blast wall under explosion loads. Two methods are adopted for nonlinear structural consequence analysis of FPSO topside blast walls. One has been computed based on the use of time-domain nonlinear finite element analysis and another is performed with single degree of freedom method based on resistance function. The relationships between blast pressure versus impulse of FPSO topside blast walls are developed. This paper introduces the procedure of two methods and describes benefit of each method from the result comparisons. Insights modeling techniques and procedures of this study will be very useful and practical for explosion risk assessment of offshore structures.

Highlights

► Topsides of offshore platforms are most likely to be exposed to hydrocarbon explosion. Structural damage can be categorized as a PI curve in the pressure/impulse space. ► The SDOF method was used primarily as a rapid tool to predict the structural response. ► The MDOF method produces reliable results under explosion actions.

Introduction

The offshore structures such as FPSO and Offshore Platform were developed due to moving focus on developing the natural resources from land to ocean. There are various loading conditions such as explosion, fire, extreme wave load and dropped object, etc. Especially, explosion in offshore structure is extremely hazardous. They involve extreme explosion actions which can have serious consequences for health, safety, and the surrounding environment (Paik et al., 2011). Therefore, considerable interests in explosion and blast wall have been increased since the Piper Alpha (6 July 1988). Fig. 1 shows photo of Piper Alpha accident, which resulted in tremendous damage (killing 167 men, $ 1.7billion loss) to the outlying area and huge fires (Cullen, 1990). Since the Piper Alpha accident took place, a substantial amount of effort has been directed towards the management of explosions and fire in offshore installations. Risk-based approaches have begun to replace traditional prescriptive approaches in offshore design, and 26 joint industry projects have been undertaken since 1990 (Paik and Chung, 1999, Paik and Czujko, 2009). In spite of these efforts, accidents occurred continuously, as evidenced by the recent Deepwater Horizon incident which occurred on 20 April 2010 in the Gulf of Mexico as shown in Fig. 2. Therefore many researches are required for the quantitative gas explosion risk assessment and management ofFPSOs.

Marine structures accidents like explosion and fire accidents have been serious matter of the industrial world and considerable interests in explosion and blast wall have been increased since Piper Alpha and Deepwater horizon accidents. However, there is dearth of reliable knowledge and understanding on the explosion accident. Explosion proof that can prevent or minimize accidents is divided into active explosion proof and passive explosion proof. Active explosion proof is the kind of protection method that prevents gas ignitions before gas is ignited and passive explosion proof is the method that minimizes consequence of explosion accidents after gas ignitions occur (Czujko, 2001).

While active explosion proof costs too much, passive explosion proof comes inexpensive. So that Passive explosion proof is considered efficient method in terms of cost-benefit. Our study in this paper focuses on nonlinear structural response analysis of blast wall structures that is typical passive explosionproof.

Corrugated blast walls are among some of the common passive protection systems that have been developed to protect personnel and/or important assets from hydrocarbon explosion. Most of the blast walls are designed using the single degree of freedom (SDOF) method as recommended in the design guidance and a time-domain finite element commercial software should use for predicting the response of blast wall panel by the Technical Note 5 (TN5) issued by the Fire and Blast Information Group (FABIG, 1996, FABIG, 1999, FABIG, 2002, FABIG, 2007).

This study presents the numerical study on the analysis of FPSO topside blast walls subjected to blast loading generated from typical hydrocarbon explosions for the above purposes (Czujko, 2005). One of the uncertainties in blast walls design remains in the accurate evaluation of the explosion loading. As such, they are normally designed against a nominal pressure of 0.5 bar (HSE, 2004) which often results in the elastic response of the section. However, recent large scale explosion tests on the blast walls have shown the possibility of as high as 4 bar overpressure with a shorter duration in a typical module (Fischer and Häring, 2009, HSE, 2003, HSE, 2004, HSE, 2006a). A variety of explosion loading cases consisted of diversity of peak pressure and duration time are considered for those reasons. Moreover, the PI curve comparisons between time-domain nonlinear finite element methods (NLFEM) and the single degree of freedom (SDOF) method based on charts that provide the maximum response are used. Particular attention is given to the plastic response of the blast walls (HSE, 2004, HSE, 2006a).

This paper is a summary of the results obtained techniques, using the nonlinear finite element method (NLFEM) and the single degree of freedom (SDOF) method that were performed in the present study. In addition, appropriate guidance for nonlinear structural consequence analysis of explosion will also be presented.

Section snippets

The procedure of nonlinear structural consequence analysis

Fig. 3 presents the procedure for the nonlinear structural consequence analysis of FPSO topsides in explosion scenarios (Paik, 2011). The structural system of the FPSO topsides is defined to include the decks, support members, blast wall and pipelines. The several explosion loading cases considered in this chapter include peak pressure and duration time determined on the basis of the exceedance curve and load curve. Three methods of determining the dynamic structural response of a structural

Pulse pressure test panel under explosion loading for validation

Drawing on HSE research reports 124 and 404, which are based on studies conducted by the University of Liverpool, the pulse pressure test results of 1/4-scale blast wall panels were considered suitable for validating between experimental and numerical study. Fig. 6 shows a typical blast wall construction made of corrugated stainless steel sheet with end connections at the top and bottom. They are designed to be efficient energy-absorbing systems (HSE, 2003, HSE, 2004, HSE, 2006a).

Fig. 7

Dynamic structural response of FPSO topside blast walls under explosion loads

Drawing on HSE research reports 124 and 404, which are based on studies conducted by the University of Liverpool, the pulse pressure test results of 1/4-scale blast wall panels were conducted for validating between experimental and numerical study. The FE simulation results for displacement–time histories and deformed shape were in good agreement compared with the experimental method (Fischer and Häring, 2009, HSE, 2004, HSE, 2006a). The experimental method is the best way of obtaining results,

Conclusions

The aim of the present study has been to develop a practical procedure of nonlinear structural consequence analysis for FPSO topside blast walls under explosion. In addition, appropriate guidance will also be presented on the use of the finite element numerical tool and be introduced of maximum response of single-degree elasto-plastic systems maximum for the above purpose. Based on the results obtained from the present study, the following conclusions can be drawn.

  • 1)

    The procedure of nonlinear

Acknowledgment

The present study is part of the EFEF JIP (Phase II and III). The authors are pleased to acknowledge the support of their partners in this project: American Bureau of Shipping, ComputIT AS, Daewoo Shipbuilding and Marine Engineering, Gexcon AS, the UK Health & Safety Executive, University of Liverpool, and Samsung Heavy Industries. This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education,

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