Artikel 27 Pipa Bawah Laut

Februari 14, 2016

Artikel 27 Pipa Bawah Laut

PIPELINE END TERMINATION (PLET)

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. Figure 1 is an example of a PLET

Figure 1 - PLET, example Skarv field

Figure 2 illustrates a difference between a platform solution and a subsea to- shore solution with respect to the use of a PLET. 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.

Figure 2 - Open PLET in subsea field configurations

Generally, a PLET is positioned where a flowline is connected to a spool. Most of the flowlines in a subsea field are rigid pipelines, and spools are then required between the flowlines and the connection points on the subsea facilities to compensate the thermal expansions in the flowlines. Figure 3 shows how PLETs are used in a subsea field configuration. The red clouds indicate the positions.

Figure 3 – PLETs in a subsea field, example Gorgon field

The PipeLine End Termination (PLET) is a substructure required in a midline connection. While the connection point on a subsea facility, the Porch, is supported to seabed by the facility itself, the midline connections are stand-alone units on the seabed, and therefore require their own substructure.

PLET can generally be categorized as following:

- Midline connection

- Horizontal connection

- Pull-in and connect method

- Subsea connection

Figure 4 shows the PLET (fixed end), the termination (movable end) and the configuration before and after the connection operation. The termination is landed on the PLET, close to the Porch. A ROV operated connection tool executes the pull-in operation by pulling the termination towards the Porch. A torque tool operated by the ROV closes the clamp connector.

Figure 4 - PLET example 1

Figure 5 shows the PLET (fixed end), the termination (movable end) and the configuration before and after the connection operation. The termination is landed on the seabed in proper distance from the Porch. A ROV operated connection tool lands on the Porch, attaches a rope to the termination, and executes the pull-in operation. The termination is aligned towards the Porch. Closing of the clamp connector is also executed by the same tool (integrated pull-in and connection tool.

Figure 5 - PLET example 2

Open PLET

The PLET systems provided by GE Oil & Gas can roughly be differentiated as following:
1. Integrated structure PLET
2. Open structure PLET (Open PLET)

If the substructure is attached to the pipeline end prior to subsea installation, the PLET is called an “integrated structure PLET". This type of PLETs can be used for pipelines and substructures of limited sizes. When the pipeline dimensions become large, the size and the weight of the connection system components increase. A larger substructure is then required for support. The installation loads will then be too large to install the pipeline and the substructure simultaneously. The ability for the seabed to carry the weight of the PLET is also vital with respect to the size of the substructure. If the soil has low carrying capacity, the bearing surface on the substructure has to increase.

For pipelines of larger dimensions (approximately above 25 inches), and or in situations where the seabed has relatively low carrying capacity, the required size of the substructure will result in the open structure PLET solution. The pipeline end and the substructure then have to be installed separately, and assembled on the seabed. This system is called “Open PLET.” 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.
Substructure + pipeline end termination = Open PLET (= fixed end of connection system).

Figure 6 is an illustration showing examples of the two types of PLET (Open PLET to the left). For the Open PLET the pipeline end and the substructure (yellow structure) are separable. The integrated structure PLET has a substructure (yellow and blue structure) which is pre-attached to the pipeline end.