API RP 1133:2005(R2010) pdf download.Guidelines for Onshore Hydrocarbon Pipelines Affecting High Consequence Floodplains
4.1.2 Buried Crossings
In locating the route vertically, the potential for future deg- radation or scour of the channel should be considered by reviewing the hydrology of the channel, conducting a geo- technical investigation, as well as reviewing site records and existing topographic maps. The pipeline should be placed at a depth below the expected level of scour. Minimum burial depth of trenched crossings shall comply with U.S. Army Corps of Engineers, Department of Transportation (DOT), Office of Pipeline Safety, and other applicable regulatory requirements. The lateral location of the pipeline should be determined after a careful review of channel hydrology. The lateral stabil- ity of the channel should be determined. Characteristics of channel instability include the following: • Bank erosion, • Migration of the channel within the floodplain, and • Migration of meanders downstream. The rate of bank erosion and migration can be projected by reviewing the hydrology and forecasting the changes in the channel for the design life of the pipeline. The lateral location of the pipeline and effective length of the crossing are then determined based on these projections. Adequate depth should be maintained to eliminate impacts from the future migration of the channel. For example, if the migration rate is 2 ft per year and the design life is 50 years, the depth below the level of scour should be maintained for at least 100 ft from the bank in the direction of channel migration. Special consideration should be given to previously existing channels that may scour as a result of secondary overbank flows during flood conditions.Significant savings can be realized if several pipelines are bundled together in one crossing. Directionally-drilled cross- ings may be bundled if the diameter of the bundled pipelines does not exceed the largest diameter bore that can be installed. One or more spare pipelines within the bundle may be consid- ered to allow for future expansion. For pipelines that are bun- dled together, provisions should be made for adequate corrosion control to prevent interference of cathodic systems or to prevent one or more lines from acting as a sacrificial anode. See the following publication for further guidelines: Instal- lation of Pipelines by Horizontal Directional Drilling, an Engineering Design Guide (PRCI no PR-227-9424).