Various method exist for joining ends of subsea pipelines. These methods include the following:
- Flanged connection
- Atmospheric welding
- Hyperbaric Welding
- Mechanical Connectors
Flanged MethodFlanges are pre-installed on each pipe end during laying. The pipe ends are positioned approximately in line with the flanges 50-200 ft apart. An adjustable fixture (template) is lowered to the seabed and temporary attached to the flanges. The fixture is locked in position, released, and raised to the surface. A rigid pipe spool is prepared to match the exact dimensions of the fixture, lowered to the seabed, and bolted into place.
Figure 1. Flange Connection
A swivel-ring flange is used on one spool end to facilitate alignment of the bolt holes in the flanges. This method is generally limited to applications involving relatively small diameters and shallow water, although flanges have been used to at least 36-in diameter and in 500-ft water depths in the North Sea.
Flanges are low in cost, but they can take a long time to install and may leak during pressure testing. A leaking flange can be difficult to diagnose. In one installation, it took 2 weeks to locate the source of a small leak during a hydrotest, which was due to a leaking flange. However, flanges are considered trouble-free once they have been installed and tested. Flanges are sometimes used at the foot of risers to facilitate replacement of a riser.
The process of tightening large flanges has been made considerably easier and faster by the use of a hydraulic bolt-tensioning tool. The Hydra-Tight tool, sold in the U.S. by Flexatalic Gasket Co., has been used in the North Sea for several years. It consists of a series of hydraulically operated tensioners which are attached to protruding ends of the flange studs. Hydraulic power provided from the surface causes the tensioners to tension each stud uniformly. The nuts may then be tightened in as little as 3 hr using the Hydra-Tight tool. The primary advantage, however, is uniform tensioning of the studs. This reduces the likelihood of a leak, especially for large flanges.
A variation of the flanged spool that is gaining wider acceptance is the use of a ball flange to accommodate angular misalignment. Small diameter lines (10-12 in. or less) in 200-300 ft of water may often be lifted to the surface to make a connection using a ball connector. The pipe is dewatered, if necessary, and one end is raised by one or more lifting points. A ball-connector half is welded to this first pipe end. A joint or two of pipe are first welded on to bridge any gap between the two pipe ends.
The first pipe end is lowered to the seabed so that it overlaps the second pipe end. A measurement is taken on bottom, and the second pipe end is raised to the surface. The pipe is cut, the second ball half is pipe end is welded on the pipe, and the pipe is lowered to the seabed. The two pipe ends are then lifted slightly and the ball halves are mated. The connected pipe is lowered to the seabed and the bolts are tightened to lock and seal the ball joint.
If the pipes must be dewatered for lifting, temporary end caps are attached to the ball halves before the pipe ends are lowered. After the pipes are flooded, the temporary caps are removed.
The ball connectors may also be used in pairs at the ends of a rigid spool for new construction or for a long spool repair when pipe ends can be lifted to the surface. Measurement of the required spool length must be accurately made since the ball connectors will provide only limited length adjustment. Moreover, an axial movement of about one pipe diameter is needed to mate the halves of a ball joint.
Figure 2. Gas Pipeline Flange
Source:Mouselli, A. H. Offshore Pipeline Design, Analysis, and Methods. USA: PennWell Books. 1981.
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