These can be compliant with DNV 2. Figure 3 shows an example of such design. Figure 3: A fully welded steel container. Some examples of sizes and weights: Dimensions L Standard sizes of general purpose containers are 20'x8'x8'6" or 40'x8'x8'6 Imperial units. Figure 6 shows a few examples of containerised building. Figure 6 and Table 2 give some common shapes, sizes and weights. Modular buildings are designed for ease of transportation.
An alternative to this is flat-pack design as shown in Figure 7. Custom built containers which are designed to DNV 2. The floors are of sandwich type with 4mm top layer and 6 mm bottom layer with 50mm Rockwool in between to achieve A60 fire rating. Walls and ceiling are also insulated using 75 mm Rockwool held together by 9mm thick fire retardant boards. Intumescent fire coating is becoming more common for PBs used for offshore installations.
The Nature of Blast Loading The PBs can be idealised as a cube, where it is assumed that one of its sides faces toward the explosion. It is also assumed that the PB is a rigid structure and is rigidly fixed to its foundations. The side facing towards the explosion is normal to the direction of propagation of the blast wave.
When the blast wave strikes the front of the PB, reflection occurs producing pressure, which may be two or more times greater than the incident wave This is discussed in more detail in Section 5.
The blast wave then bends or diffracts around the PB exerting pressure on the sides and top, and finally on its back face. The pressure on the sides and top of the PB builds up when the blast front arrives at the point in question.
This is followed with a short period of low pressure caused by a vortex formed at the front edge during the diffraction process and which travels along or near the surface behind the wave front. When the blast wave reaches the rear of the PB, it diffracts around the edges, and travels down the back face.
Loading on the PB during this process is a function of position as well as time. The blast wave attenuates as it propagates outward from the explosion epicentre. Consequently, the value of peak overpressure and impulse decreases with distance, while the duration tends to increase. As the pressure wave moves radically away from the center of the explosion, it contacts the PB and upon contact, the initial wave pressures are reinforced and reflected.
The variation of the pressure on the PB surface is a function of the "angle of incidence": The angle of incidence is formed by a line which defines the normal distance between the point of detonation and the door, and a line which defines the path of shock propagation between the center of the explosion and any other point of the structure in which the door is located see Figure 8. As the blast wave continues to propagate outward, a front known as the "mach front" is formed by the interaction of the initial wave and a reflected wave which resulted from reinforcement of the incident wave by the ground.
The height of the mach front increases as the wave propagates away from the center of the detonation. This increase in height is referred to as the path of the triple point and is formed by the intersection of the initial reflected and mach waves. A structure is subjected to a plane wave uniform pressure when the height of the triple point exceeds the height see Figure 9.
Figure 9: Air burts 3. In a surface burst, the initial wave of the explosion is reflected and reinforced by the ground surface to produce a reflected wave. Unlike the air burst, the reflected wave merges with the incident wave at the point of detonation. This forms a single wave similar in nature to the reflected wave of the air burst, but essentially hemispherical in shape See Figure The Basic Parameters of Blast It is generally assumed that the distance to the explosion and the length of the PB are such that the overpressure and duration do not change significantly over the length of the PB.
This is a conservative assumption for portable buildings. It is further assumed that there is no glass breakage, so the possibility of developing overpressure inside the structure can be ignored. The main dimensions of the example PB are noted in Figure The relative position of the example PB is such that the long face is facing the oncoming blast wave. This is shown in Figure The negative phase peak side-on pressure is assumed to be zero. This blast load is assumed to act normal to the long side of the PB.
Calculations are shown only when the blast load is normal to the longer face, and should be repeated by assuming that the blast wave is normal to the shorter side. External Blast on PB Figure 13 shows three extreme cases of the relative size of the incident blast wave and the obstruction in its path.
In case I, the blast wave strikes a large surface without impediment and the load on this surface is then equal to the overpressure of the incident wave. In case II, the blast wave collides perpendicularly with a surface of very large dimension, so that the low pressure wave around the edges the rarefaction wave does not play any role rarefaction is the reduction of a medium's density or the opposite of compression. In this case the load on the surface is equal to the overpressure in the reflected blast wave.
In Case III, we are dealing with an object with small dimensions. The rarefaction progresses so quickly that it does not have to be considered. Furthermore the difference between the pressure on the front and on the back part is so small that the load only consists of the dynamic pressure. PBs, which are generally rectangular in shape, are a combination of the first two cases. Furthermore, existence of other obstacles complicates the matter.
Thus, when the blast wave reaches the front surface, it engulfs the PB. Strictly speaking loads on the PB are both position and time dependent. But the speed of the blast wave is quite high in comparison with the PB dimension, so that the time lag of loads between faces can be neglected. For the same reason the variation of load on any surface can also be ignored. As shown in Figure 14, the original blast wave is represented by its equivalent shock wave with the same peak overpressure and impulse.
In this example, it is further assumed that the PB is completely enclosed. However, any opening would cause the inside pressure to rise and thus partially compensate the external pressure.
As there is no opening in the front wall, the blast cannot traverse the structure and thus the back wall will not experience the reflected pressure loading. The initial reflected overpressure 2. The general overpressure 3. The drag loading. The drag loading includes the effects of the drag pressure that is related to the dynamic pressure and the drag coefficient. The drag coefficient can be either positive or negative and is dependent on the size, shape, and orientation of the structure [4, 5].
When the blast wave meets a surface or an obstacle, then such blast wave is locally disturbed. Due to this disturbance, the loading on the obstacle is not equal to the time-pressure path of the undisturbed blast.
Behaviour of a shock wave upon striking a closed rectangular structure is shown in Figure 15a to c. This figure shows the position of the shock front and the behaviour of the reflected and diffracted wave over the centre portion of the structure. As the shock wave strikes the front face of the building, a reflected shock wave is formed, and the overpressure on this face is raised to a value in excess of the peak overpressure in the incident shock wave.
This increased overpressure is called the incident shock front and is a function of the peak overpressure in the incident shock wave. The angle of incidence of the shock front with the front wall is zero degrees in this case.
At the instant the reflected shock front is formed, the lower overpressure existing in the incident blast wave and adjacent to the top edge of the front face initiates a wave of lower overpressure than that which exists in the reflected shock wave this is known as rarefaction wave or suction wave, Figure 15b, which travels in the opposite direction to that of a shock wave directly following an explosion.
This rarefaction wave travels with speed of sound in the reflected shock wave towards the bottom of the front face. Within a short time, called the clearing time, the rarefaction wave causes the reflected shock wave to disintegrate and reduces the overpressure existing on the front face to a value which is in equilibrium with the high velocity air stream associated with the incident wave.
After the shock wave strikes the front wall of the structure, at a time equal to the length of the structure divided by the shock front velocity [4, 5], the shock front reaches the rear edge of the structure and starts spilling down toward the bottom of the back wall Figure 15c.
The back wall begins to experience increased pressures as soon as the shock front has passed beyond it. The maximum back-wall overpressure develops slowly as a result of vortex shedding and the time required for the back wall to be enveloped by the blast wave.
As the shock front passes beyond the front wall the overpressure exerted on the roof of the structure is initially raised to a value nearly equal to the overpressure existing in the incident shock wave. The pressure is largest at the front and tails off at a point over a distance Lw , the blast wavelength.
The velocity of the air particles, and hence the wind pressure, depends on the peak overpressure of the blast wave. In the lower overpressure range with normal atmospheric condition, the peak dynamic pressure can be calculated using the following empirical formula [2].
The drag coefficient depends on the shape and orientation of the obstructing surface. For a rectangular building, the drag coefficient may be taken as 1. The dynamic pressure exerts the dominant blast effect on open frame structures, framed structures with frangible cladding, and on small structures or components, such as masts, poles, etc.
This statement is not relevant to the PB, which is a closed box, but any critical attachment should be designed for the dynamic pressure. The effect of this blast wave reflection is that the surface will experience a pressure much more than the incident side-on wave value. The reflection coefficient depends on the peak overpressure, the incident angle of the wave front to the reflecting surface, and on the type of blast wave.
See References 4, 5 and 6 for the reflection coefficient for shock waves and pressure waves for various incidence angles. This upper limit corresponds to the total reflection of the entire blast wave without any diffraction around the edges of the reflecting surface.
See References 4 to 7 for further details. The reflected overpressure depends on the incidence angle and the rise-time of the side-on overpressure. For design purposes, the normal shock reflected condition should be assumed unless the specified design explosion scenario dictates otherwise. The reflected overpressure decays to the stagnation pressure, Ps , in the clearing time, t c as defined below and illustrated in Figure This equivalent load is computed by equating the impulse for each load shape and using the same peak pressure Pr.
These walls will experience less blast loading than the front wall, due to lack of overpressure reflection and attenuation of the blast wave with distance from the explosion source. As a blast wave travels along the length of a structural element, the peak side-on overpressure will not be applied uniformly. It varies with both time and distance. A reduction factor C e is used to account for this effect in design.
Values of C e Figure 19 are dependent on the length of the structural element, L1 , in the direction of travelling blast wave. The sidewall is spanning from the foundations to the roof. The front wall sees the highest load, the side wall load calculation is only necessary to check the side walls themselves to account for interaction of in-plane and out of plane forces.
However if PB is made of stiffened plate, then L1 should be assumed equal to distance between stiffeners. The overall duration is equal to this rise time plus the duration of the free field side-on overpressure. Consequently, the roof will experience the side-on overpressure combined with the dynamic wind pressure, the same as the sidewalls.
This RP was developed for use at refineries, petrochemical and chemical operations, natural gas liquids extraction plants, natural gas liquefaction plants, and other onshore facilities covered by the OSHA Process Safety Management of Highly Hazardous Chemicals, 29 CFR Buildings covered by this RP are rigid structures intended for permanent use in fixed locations.
Tents, fabric enclosures and other soft-sided structures are outside the scope of this document. Significant research and development of technology pertinent to building siting evaluations has been performed since the publication of the previous editions of RP Examples of updated technology include prediction of blast damage to buildings, determination of occupant vulnerabilities, and estimates of event frequencies.
Prior versions of RP and the technical data included in them should not be used for building siting evaluations. This edition of RP does not cover portable buildings. Portable buildings are now covered by RP It is recognized, however, that portable buildings specifically designed for significant blast load represent a potential area of overlap between RPs and In accordance with 1.
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