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Chapter 10 363 Marine Installations shut-downs, and also during sudden pump stops caused by emergencies, such as loss of power. Guidance for selecting a PE pipe with sufficient surge pressure strength is also presented in Chapter 6 of this Handbook. A sudden pump stop can sometimes also result in flow separation, giving rise to a momentary reduction in pressure along some portion of the pipeline. Since underwater pipelines can be subject to relatively large external hydrostatic pressure, flow separation can sometimes lead to a significant net negative internal pressure. A check needs to be made to ensure that the pipe DR that has been selected based on maximum internal pressure considerations is also adequate to safely resist buckling, or pipe collapse, under the largest net negative internal pressure that could ever develop from whatever cause. Guidance for this design check is also provided in Chapter 6 of this Handbook. The ballast weights that are attached to PE pipe for purposes of its submersion also fulfill an important role as ring stiffeners that tend to enhance a pipe’s inherent resistance to buckling. Common design practice is to accept this benefit as an added “safety factor,” but not to directly consider it in the design procedure for selection of a pipe of appropriate ring stiffness. Step 3 Determination of the Required Weighting, and of the Design and the Spacing of Ballast Weights The determination of these parameters is made in accordance with the following sub-steps. Step 3a Maximum Weighting that Allows Weighted Pipe to be Floated into Place The buoyant or vertical lift force exerted by a submerged PE pipe is equal to the sum of the weight of the pipe and its contents minus the weight of the water that the pipe displaces. This relationship can be expressed mathematically as follows: (1) FB = [WP + WC] – WDW WHERE FB = buoyant force, lbs/foot of pipe WP = weight of pipe, lbs/foot of pipe WC = weight of pipe contents, lbs/foot of pipe WDW = weight of water displaced by pipe, lbs/foot of pipe Since the density of PE (~59.6 lbs/cubic foot) is only slightly lower than that of fresh water (~62.3 lbs/cubic foot) the pipe contributes somewhat towards net buoyancy. However, the major lift force comes from the air-filled inner volume of the pipe. Since, for a pipe of given outside diameter, the size of the inner volume is determined by the pipe’s wall thickness – the greater the thickness, the smaller thePDF Image | Marine Installations PE
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