Hot Tapping

Hot tap operation is a combination of many variables, welding is only one of them,though it's critical for any hot tapping operation. Note Hot Tap could lead to serious fatalities if not done properly. Since you mentioned about Shell's DEP, please note these DEP's are well respected in the industry. That does not mean what's written is absolutely unquestionable, but there're good rationale on what's written in the DEP's.  

DEP 31.38.60.10-Gen, HOT-TAPPING ON PIPELINES, PIPING AND EQUIPMENT is quite detailed. Some important extracts are very relevant to your queries and would clear the doubts why the practice of hot tapping is not permitted as a general rule. JUSTIFICATION FOR HOT-TAPPING:-Hot-tapping is potentially hazardous and therefore should only be undertaken when continuity of service is essential or shutdown of the system is impractical and when it is shown to be technically feasible and to offer a clear advantage over alternatives. The advantage may be economic and, in some situations, the safety and environmental risks may be less than those associated with more conventional methods.A decision on whether hot-tapping is to be applied shall be based on careful considerations including at least the following aspects: * safety;

* condition of the pipe/equipment under consideration;
* configuration of the connection;
* code/statutory requirements;
* operating conditions;
* technical capabilities of the drilling equipment under the operating conditions (pressure, temperature, nature of product); * related welding problems;
* economic aspects;
* environmental/pollution aspects.
If hot-tapping is to be applied under conditions approaching the technical or operational limits as defined in this DEP, specialist advice should be sought. MATERIALS:-This DEP is relevant only to hot-tapping on carbon, carbon-manganese and high-strength low-alloy steels (e.g. ISO 3183 L245 to L485 or API 5L grades B to X70). Materials for pipelines...