The fluids flowing through many pipelines are two phase, i.e., a mixture of liquid and gas. Ideally the two phases separate out with the liquid at the bottom section of the pipe. The two phases flow together to a processing facility that separates the two phases. In practice, however, the two phases often travel at different rates which means that they can form slugs whereby the composition of the stream at different sections is almost all liquid or almost all vapor.
Many companies develop a Safety Manual as a means of organizing operations and maintenance work. Naturally, the details of what is in the Manual and how it is structured will vary depending on the type of process that is being operated and on local conditions. However, the outline provided here can be used as a starting point.