Subsea tie-in is typically made by diver-assisted flange connectors for shallow water pipelines and diverless remotely operated vehicle (ROV) connectors for deepwater pipelines.

There are three types of connectors available: flange, clamp (Graylok type), and collet connectors. Clamp or collet connector is more favorable over the flange connector due to ROV operability, offshore connection time, and available tie-in tools from contractors.

The methods of pipeline tie-in are explained below:

Divers measure and then assist the installation of a piece of pipe to fit in between the two ends of flowline to be tied together. Misalignment of the two pipes can be accommodated by installing bends into the spool, and inaccuracies in placing the pipelines down can be accommodated when the spool is made up (after diver measurements).

The connection method can either be by flanges or welding. The welding method requires a hyperbaric habitat. From a design viewpoint, should there be large flowline expansion, then this can also be accommodated by incorporating a dogleg in the spool. This will permit expansion of the pipe without transmitting high loads into the adjacent pipe.

Lateral deflection involves positioning the flowline end to one side of the target structure and then pulling it laterally into position.  The flowline may be pulled towards the target by a single wire, or a series of wires may be developed through dead-man anchors to give greater control of alignment. A bell mouth or stab-in guides usually assist final alignment.

For large diameters such as export lines or bundles, it is necessary to make a length of pipeline neutrally buoyant. This gives greater flexibility and reduces the pull forces, but can expose the pipe to large current forces.

This method requires the flowline to have the capacity to easily move axially over a relatively large distance. This limits the option to pulling the flowline directly from the pipelay vessel. The method of J-tube pull-in is to connect the flowline to a wire and, by pulling the wire, to pull the flowline through a riser (J-tube) up to the topsides of the platform. This method requires the J-tube to be of a reasonably large diameter compared to the flowline.

This method is very similar to the J-tube pull-in method with the exception that the tie-in is performed subsea. This method is usually applied in diverless operations, where a mechanical connecting system will be utilized to perform the connection. This system is mainly used at subsea manifolds/wellheads, where the water depth prohibits the use of divers. This is the only system developed for performing diverless connection of pipelines.

The stalk-on method is primarily used in shallow water applications (less than 40 m) and hence would only be applicable in the Southern North Sea. The method involves laying the flowline down adjacent to the jacket it shall be tied into. The vessel maneuvers over the flowline, lifts it up and welds on (or flanges on) the jacket riser. The pipeline and riser are then lowered onto the seabed/jacket. The jacket clamps are subsequently closed around the riser.

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Source:

Lee, Jaeyoung. Introduction to Offshore Pipelines and Risers. 2007.

Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.