ABSTRACT INTRODUCTION

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1 An integrated approach to optimising the performance of a blast wall system G. Schleyer," D. Galbraith^ & I. Gillarf ^Impact Research Center, Department of Mechanical Engineering, University of Liverpool, UK (formerly of British Gas pic) ^Mobil North Sea Ltd., Aberdeen, UK ^Research and Technology Division, British Gas pic., Newcastle upon Tyne, UK ABSTRACT A detailed non-linear dynamic analysis has been combined with a series of large scale experiments using controlled, partially confined, vapour cloud explosions to provide an assessment of the performance of a blast wall system on an offshore platform. The work was undertaken to accurately predict the capacity of the blast wall and provide suggestions of how to improve its blast resistance. INTRODUCTION The Piper Alpha Inquiry by Lord Cullen [1] and following Safety Case legislation for offshore installations has increased awareness of hazards and their consequences. As a result, structures are being assessed more rigorously than before to determine their capacity to withstand hazardous loading in the event of an incident. For some structures, extreme loading such as blast was not a design consideration. Consequently, operators now face the problem of either justifying that the probability of a hazardous load which could cause structural failure is acceptably low or modifying the structure to resist the hazardous loading. As part of the Offshore Safety Case requirements, Mobil North Sea Limited (MNSL) has identified a need to determine the blast resistance of the protective wall system on the Beryl B platform. Beryl B was installed in 1983 and is a fixed steel oil and gas platform on a "traditional North Sea" K-braced jacket. The topsides is "semi-integrated" and includes fire walls between process, well heads and ultilities areas. The platform is operated by MNSL on behalf of co-ventures Amerada Hess Limited, BG North Sea Holdings, Enterprise Oil pic and OMV (UK) Limited. Using conventional analysis, the wall system is predicted not to have the capacity to resist the required blast loading.

2 i Transactions on the Built Environment vol 8, 1994 WIT Press, ISSN Structures under Shock and Impact British Gas has theoretically and experimentally assessed the performance of the protective wall system on Beryl B for Mobil. DESCRIPTION OF THE PROTECTIVE WALL SYSTEM Figure 1 shows the construction of a typical Beryl B wall which consists of a steel framework of vertical posts and horizontal stiffeners overlaid with 6 mm plate. The vertical posts, which are 4 and 4.5 metres apart, are of 356 x 171 mm Universal Beam (51 kg/m) section and the horizontal stiff eners are 1.1 metres apart, of 152 x 76 mm Rolled Steel Channel section. All welds are of continuous fillet type. The wall spans vertically 8 m between the main 2 m deep plate girders and extends horizontally 20 m between the main tubular columns. d PL.ATEGIR i DER rh h 1 8m 1.1m I PCATEGIRDER! i 4.5m 4.0m 4.5m -f i 20.0m Figure 1: Construction of Beryl B Blast Wall The weakest part of the wall is considered to be the attachment to the upper plate girder. Attention was therefore focused on the fixation of the wall to the plate girder and the restraint offered by the girder. To ensure the wall had the required capacity, sufficient fixity restraint at the plate girder was essential. Another important consideration in this assessment was the dynamic reaction imparted by the wall onto the supporting plate girder, in particular the reaction due to the in-plane membrane forces induced by the blast in the wall. These factors were considered the key to optimising the performance of the wall. From these considerations, a test programme was devised first of all to study the influence of membrane restraint on the overall response of the wall and secondly to provide evidence to validate theoretical predictions of blast response of the wall. With high levels of blast overpressures being

Structures under Shock and Impact 23 predicted in the space bounded by the wall, the overall objective of the assessment was to re-examine the capacity of the wall, previously assessed to be

3 Structures under Shock and Impact 23 predicted in the space bounded by the wall, the overall objective of the assessment was to re-examine the capacity of the wall, previously assessed to be approximately 0.2 bar. An acceptable capacity was considered to be a blast load of 0.6 bar. The validated theoretical model was used to investigate a range of proposed scenarios. TEST PROGRAMME The experimental programme was undertaken at the British Gas test site at Spadeadam in the North of England. The main objective of the test series was to study the influence of membrane restraint on the overall response of the wall, this being considered as the key to improving the resistance of the blast wall. Four test walls, similar in construction to the blast walls on Beryl B, were individually mounted in an explosion chamber and subjected to blast overpressures ranging from 0.1 bar to 1 bar with load durations of between 100 and 200 msec. Both elastic and elastic-plastic deformations were recorded over the series of tests. Pressure and displacement transducers attached to the test walls were used to capture the loading and response data for analysis. A description of the test walls, experimental procedure and a selection of results is presented below. Description of Test Walls The four steel test walls were of similar construction and differed only in the method of attachment to the framework of the explosion chamber, as shown in Figure 2. The general construction of the test walls was based on that of the blast walls on the Beryl B platform, the overall size was however limited by the 4.5 m x 4.5 m opening of the explosion chamber. To overcome this limitation, a method of representing the elastic behaviour of an 8 m span section of the blast wall arrangement of Beryl B was devised for the first of the walls to be tested. The first test wall was designed to represent the lower half of the 8 m span of the Beryl B wall and enabled comparisons of the centre deflection in the as-built wall to be made with predictions made using analytical methods. Results from this test were thus used to validate computer models which could then be used, with some confidence, to predict the response of the full size blast wall to a range of likely overpressures. Subsequent tests were carried out on blast walls which were designed solely to investigate the effects of membrane action, no direct attempt was made to represent the 8 m span of the as-built platform wall. The base of the test walls were hinged to prevent high bending moments being generated by the high stiffness of the floor of the explosion chamber. The top of the test walls were constrained by the frame of the explosion chamber with either hinges, or a lateral restraining bar depending on whether membrane forces were induced or not.

4 24 Structures under Shock and Impact 4m Test Wall 1 Test Wall 2 Test Wall 3 Test Wall 4 Figure 2: Test Wall End Conditions Experimental Procedure The experiments took place in a 9 m long by 4.5 m square cross-section, open ended explosion chamber, constructed from stiffened steel walls as shown in Figure 3. Each test wall was secured at the rear of the chamber, the front of the chamber was left fully open. Obstacles, in the form of an array of plastic pipes, were used to create congestion in order to generate the desired explosion loading characteristics. Natural gas was mixed with air inside the chamber until the required concentration was achieved. The mixture was then ignited to produce an explosion and impart a load on the test wall at the rear of the chamber. The point of ignition was just in front of the centre of the test wall. The test procedure in general was to apply three levels of explosion loading to each wall, the first two levels to cause mainly elastic response and little or no permanent deformation and the final level to cause elastic-plastic response with permanent large deformation. Pressure and displacement transducers were positioned on the walls to give comprehensive information about the loading and response of the wall. The data was recorded on a signal analyser at a sample rate of 10 khz. The principal results are described below.

Structures under Shock and Impact 25 Figure 3: Explosion Test Chamber Experimental Results The first wall, representing an 8 m span section of the existing Beryl B blast wall, was the least

5 Structures under Shock and Impact 25 Figure 3: Explosion Test Chamber Experimental Results The first wall, representing an 8 m span section of the existing Beryl B blast wall, was the least restrained of all the walls, consequently, the dynamic pressure loading on this wall was limited to an average overpressure of 90 mbar. This load produced a maximum elastic deflection of 87 mm at the top middle of the test wall at a point corresponding to the centre of the 8 m span as-built Beryl B platform blast wall. The second wall, with lateral restraint across the top but no direct membrane restraint and hinged along the bottom, was subjected to an average overpressure of 850 mbar. This loading produced plastic deformation in the wall with a maximum centre deflection of 258 mm. Local plastic buckling of the vertical beams occurred at the mid-span position which contributed to the large permanent deformation. The third wall with hinges top and bottom and membrane restraint offered by the box frame was subjected to a load of 1090 mbar. A local failure occurred in the upper support channel on the wall to which the hinges were attached due to the offset between the plating and the centre line of the hinges. The maximum deflection at the centre of the wall was 353 mm. The local failure relieved the wall from developing high membrane forces resulting in higher deflections than were initially predicted. However, the test demonstrated that this type of failure mechanism could be used to advantage in limiting the membrane reaction in the supporting structure. This test was repeated on a modified wall in which the offset was removed.

26 Structures under Shock and Impact An average overpressure of 1070 mbar was applied to the fourth wall which was also hinged top and bottom with modifications to the alignment of the hinges.

6 26 Structures under Shock and Impact An average overpressure of 1070 mbar was applied to the fourth wall which was also hinged top and bottom with modifications to the alignment of the hinges. In this case the membrane action of the plate was allowed to develop and significantly improved the resistance of the wall to the blast loading compared to the previous result with a maximum recorded centre deflection of 232 mm. The results from the test programme were compared with the analytical predictions to give confidence in using the analysis to optimise the performance of the blast wall system. THEORETICAL ANALYSIS Research into blast response of structures at British Gas has produced a number of computer-based models for the assessment of plant and structures subjected to blast loading. The emphasis has been on developing simplified models which are easy to use and capable of predicting the overall response to dynamic loading in the elastic-plastic regime. Special attention has been given to modelling the behaviour of corrugated walls and stiffened plates subjected to large dynamic pressure loads. The mechanism of load transfer to the supporting structure has been studied in some detail. Large scale experiments, similar to those described in this paper, and finite element analyses have been used to validate the models. The British Gas structural model for stiffened plates was considered appropriate to this problem of the blast walls on the Beryl B platform. Modelling Technique The model was derived from first principles using classical energy methods and assuming specific global modes of deformation. Lagrange's equations were used to formulate the non-linear differential equations of motion, which were solved by the Runge-Kutta time-stepping technique. Membrane restraint, if appropriate, and the formation of plasticity were modelled in the analysis to represent as closely as possible the plastic behaviour of the test walls under large dynamic loading. The plastic behaviour of the wall was assumed to be largely dominated by the behaviour of the vertical stiffeners. The stiffeners work primarily in bending and the plating works largely in the membrane mode to resist the loading. In addition, the plating acts with the stiff eners to increase their plastic moment capacity and this was taken into account in the modelling. The moment at the centre of the stiff eners was monitored and when it exceeded the plastic moment capacity, a plastic hinge mechanism was introduced and the analysis continued in this mode. This approach enabled the global response of the wall to be predicted. Comparison with Test Results A comparison of the test results and the computer modelling results are given in Table 1 below. The results reveal that the analysis enables the global response at the centre of the wall to be predicted. The large

7 Structures under Shock and Impact 27 difference in one of the results for the deflection of the I-beam reveals the dependency of the beam deformation on the global modes of deformation. While the overall performance of the analysis remains satisfactory, the results indicate that specific areas of the analysis could be improved. Table 1: Comparison of Results Test Conditions Max Deflection of vertical I-beam (mm) by: Expt Analysis Max Deflection at centre of wall (mm) by: Expt Analysis Equivalent to 8 m span without membrane restraint at 90 mbar m span without membrane restraint at 850 mbar m span with membrane restraint at 1070 mbar NEW DESIGN OF WALL ATTACHMENT Having established confidence in the analytical approach, it was subsequently used to investigate the design of the wall attachment to the plate girder. The work had shown that the calculated blast resistance of the wall system could be improved by utilising the membrane action of the wall plating. However, this imposed high reaction loads on the supporting plate girder structure. A new attachment concept was proposed which aimed to utilise the membrane action of the wall but limit the membrane forces transmitted to the plate girder. A computer model of the entire wall, 8 m high and extending 20 m between columns was used to generate performance and design data for different stiffnesses representing the new attachment. Table 2, below, gives some of these values for an average blast load of 0.6 bar magnitude and load duration of 150 msec. The analysis allowed the membrane force to increase with time using a constant stiffness up to a limiting value at which point a plastic failure mechanism is assumed to occur in the connection at the top of the wall. Thereafter, the wall continues to deflect with constant membrane resistance.

8 28 Structures under Shock and Impact Table 2: Blast Wall Performance/Design Data for 0.6 bar load and 100 kn/mm support stiffness Max Deflection (mm) Max Shear Reaction (kn/m) Max Membrane Reaction (kn/m) The results of this analysis were used to assist the design of the connection at the top of the wall. These end connections must be capable of resisting shear loading and allow displacement in the plane of the wall to avoid generating longitudinal membrane forces. The new connections are considered to provide adequate restraint for the wall to withstand an average blast load of 0.6 bar magnitude and limit the membrane reaction in the supporting steelwork. CONCLUDING COMMENTS Large scale experiments on blast walls have shown that, if a blast wall is connected to the plate girder structure to enable advantage to be taken of the membrane restraint, the capacity of the wall to resist blast loading is increased. However, the membrane action induces high reaction forces at the supports which has to be taken into account with regard to the load transferred to the primary structure. Increasing the capacity of the protective wall on Beryl B to resist the predicted blast loading at the expense of generating high membrane forces in the primary structure was considered unacceptable. Instead, complementary large scale tests and computer modelling were employed to develop a new design of connection between the wall and supporting steelwork which enabled advantage to be taken of the membrane restraint and involved minimal structural alterations. ACKNOWLEDGEMENTS The work presented in this paper was carried out by British Gas pic, Research and Technology Division under contract for Mobil North Sea Limited. British Gas wish to thank Mobil North Sea Limited for their agreement to publish the results of this work. REFERENCES 1. Cullen, The Hon Lord. The Public Inquiry into the Piper Alpha Disaster, HMSO, November 1990.

Scale testing of profiled stainless steel blast walls

Scale testing of profiled stainless steel blast walls G. S. Langdon & G. K. Schleyer Department of Engineering, University of Liverpool, UK Abstract A series of laboratory tests were carried out at the