Pipeline Ending Manifold (PLEM)/PLET

A typical subsea oil and gas field consists mainly of Xmas trees, manifolds, termination
units and pipelines. A termination unit can be called a “PLEM” or a “PLET.” These units are connection points between two or several pipelines. The Pipeline End Termination (PLET) comprises a single pipeline connection only, while the Pipeline End Manifold (PLEM) is supporting two or more pipeline connections.

Pipeline end manifold (PLEM)

Source: https://sal-heavylift.com/uploads/tx_salext/gallery/plem06_01.jpg

A “pipe-to-pipe connection” can take place when a riser is connected to a subsea installed pipeline, or when an export flowline is connected to a spool. These types of connections, compared to the “pipe-to-structure connections,” do not have a given substructure for support. For a pipe-to–structure connection, the Xmas tree, the manifold or the PLEM will provide the required support to the connection point, while a pipe-to-pipe connection requires a purpose-built substructure. The PLET is the required substructure for a pipe-to-pipe connection. It provides the support for the connection point.

Pipeline end termination (PLET)

Source: http://www.marinewellcontainment.com/wp-content/uploads/2015/01/PLET.jpg

For the subsea-to-shore solution, the PLET is positioned at the connection point between the export flowline and a spool as the flowline cannot be connected directly into the manifold. For the platform solution the PLET is positioned at the connection point between the riser and a spool as the riser cannot be connected directly into the manifold. A PLET can generally be categorized as following:

• Midline connection
• Horizontal connection
• Pull-in and connect method
• Subsea connection 

PLET and PLEM in subsea field layout

Source: http://brage.bibsys.no/xmlui/bitstream/handle/11250/183156/Thorsen,%20Steinar.pdf?sequence=1

 

The PLET is generally installed subsea pre-attached to the pipeline end. The assembly consisting of the substructure and the pipeline end, is called “PipeLine End Termination” or “PLET”. If the size of the pipeline or the substructure is large, it becomes not suitable to install them simultaneously as a completed PLET assembly. An “Open PLET” is a PLET assembly which requires to be assembled on the seabed after a separate installation of the pipeline and the substructure. This is due to the size and weight of the pipeline and or the substructure.

Open PLET in subsea field configurations

Source: http://brage.bibsys.no/xmlui/bitstream/handle/11250/183156/Thorsen,%20Steinar.pdf?sequence=1

The pipeline end used with the Open PLET system has to be specially designed due to the subsea assembly operation. In addition to the pipeline itself, and the hub, some alignment sleeves mounted on the pipeline are required to facilitate the integration with the substructure. This pipeline end is defined as “pipeline end termination,” and it must not be mistaken with the termination (movable part) of the connection system. The pipeline end termination is the pipeline end used with an Open PLET system. The main components of Open PLET are:

Open PLET components

Source: https://sal-heavylift.com/uploads/tx_salext/gallery/plem06_01.jpg

Skid: The structural framework for the Open PLET. The Porch, mud-mat, guide-post and alignment bumpers are mounted on the skid. The skid must withstand all the loads exposed to the Open PLET during design life.

Mud-mat: Prevents the Open PLET from sinking into the mud/seabed. The perforation ensures water current through the structure. This gives better maneuverability during installation.

Porch: The Porch on the Open PLET is referred to as “Porch open” due to the ability to engage and disengage with the pipeline end termination after subsea installation. The Porch on the Open PLET can be regarded to consist of the two alignment structures, Porch = MAS+RAS. The RAS is required, because the clamping of the end termination to the Porch open is not as good as for a regular Porch (for the Porch open, the MAS alone provides insufficient pipeline clamping capability to deal with applied moment loads).

Main Alignment Structure (MAS): The structure has interfaces towards the pipeline end termination for guiding purpose and for clamping. It provides locking pins for locking of the pipeline end termination in axial direction.

Rear Alignment Structure (RAS): The structure has interfaces towards the pipeline end termination for guiding purpose and for clamping. The RAS provides a guide-yoke which facilitates guiding, alignment and locking of the pipeline end termination.

Alignment bumpers: The bumpers are used for alignment of the Open PLET towards the pipeline end termination during subsea installation. One of the bumpers provides a marking system to facilitate positioning of the Open PLET in axial direction.

Pad-eyes: One set of pad-eyes is used for the installation of Open PLET from vessel to seabed. The other set of pad-eyes is used for a pull operation at seabed when repositioning the Open PLET in axial direction.



Source:
http://brage.bibsys.no/xmlui/bitstream/handle/11250/183156/Thorsen,%20Steinar.pdf?sequence=1