This paper considers the design and development of a 30-inch piggable subsea pipeline Y-connection which a major operator intends to install in a central North Sea gas pipeline A Y-connection of this type enables two pipelines of the same diameter to be tied-in whilst maintaining full piggability. This will be the first known fully piggable Y-connection for a subsea gas pipeline and as such offers significant advantages over other tie-in methods, such as tee connections and riser platforms, both of which require some degree of intervention General aspects relating to the overall arrangement such as valving, piping layout requirements and protection and installation requirements are discussed although emphasis is placed on the Y-connection itself. The major stages in the design and development of the Y-connection are explained. These include design of the internal geometry to allow unhindered passage of pigs, structural analysis and material selection A method developed for predicting minimum flow velocities required in the pipeline to achieve successful pigging is also presented.


The concept of a fully piggable pipeline Y-connection can be considered to be relatively recent, with the first such example installed in an oil/condensate line in the Norwegian sector of the North Sea in 1985. The need for a tie-in method which permits pigging through both tied-in pipeline branches with no intervention or weather dependency has arisen due to the cost and complexity involved in utilising other solutions. These solutions are generally constituted by either riser platforms or subsea ‘tee’ connections Riser platforms by their nature are very costly and require intervention whenever any pigging takes place Considering piggable subsea ‘tee’ connections, two main types are currently in operation a ‘tee’ with removable subsea pig receiver which requires intervention and is weather dependent and a ‘tee’ with a return pigging loop tied back to the parent platform. Figure 1 presents illustrations of the three tie-in methods described.

The economic viability of a Y-connection depends to a large extent on its location within a system The capital cost of a wye is justifiable where an optimum pipeline tie-in position is located in an area remote from suitable existing platform facilities. Since all branches of the Y-connection must be the same diameter for pigging purposes, viability is also dependent on sufficient flow being available through the secondary branch of the Y-connection to ensure a suitable flow regime.

In the correct location the potential benefits of a Y-connection are substantial These benefits include the ease of pigging, capital cost savings over other tie-in methods, low maintenance costs and a simpler and more compact overall layout.

The Y-connection itself is either a cast or forged steel/component and is housed subsea in a supporting tubular steel framework assembly The Y-connection piping assembly generally comprises of two in-line ball valves on each upstream branch, although this may be varied depending on specific commissioning and isolation requirements. The piping system also includes bleed valves, pipeline expansion loops on each branch of the Y-connection and a protection structure with an integral support skid. Figure 2 illustrates the aforementioned layout. Where the Y-connection is located in an area of fishing activity, the associated protection structures are generally required to be steel framed, ‘deflection’ type assemblies which provide a low profile non-snagging shape for fishing equipment, anchor wires and so on. It is intended that the complete Y-connection assembly, including expansion loops, is installed in a singl

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