Failure statistics show that mechanical pipe connections in general have been less reliable than welded. Therefore welding has been the preferred subsea pipeline connection method. In some cases, a welded connection is very unpractical. In particular for deep water pipelines beyond diving limits, welding has not yet been fully qualified. Consequently, mechanical couplings are an attractive alternative to replacement of the total length of the subsea pipeline in the deep area sections, provided that it is possible to make mechanical pipe connections sufficiently reliable. This paper describes a systematic presented in detail in DNV's Recommended Practice: RP-FI04, "Mechanical Pipeline Couplings". Its objective is to specify qualification methods, which show that Mechanical Pipeline Coupling reliability approaches that of welded joints. The systematic is based on the principle that each possible failure mode shall be identified, and that a reasonable safety margin to the failure is documented by analysis and tests.
Subsea mechanical pipeline connections for tie-in and repair are of particular interest. Tie-in to platforms or subsea installations using flanged and clamped connections are frequently used today but needs prepared pipe ends. A coupling can in principle be fitted on-to a pipe wherever the pipe has been cut or ends, and therefore also represents a practical component for pipeline repair. Couplings were frequently used in the early days before hyperbaric welding was fully developed. Hyperbaric welding requires divers and is therefore limited by both the availability of divers and the depth. The general health risks by diving are frequently debated even for shallow depths. Diving below 180 m requires particular precautions and below 300 m it is hardly feasible. Neither are the welding methods qualified for deeper depth nor are all the operations related to the welding fully automated. The latter is required for welding beyond diving depth.
