Choke Skid Replacement

An ageing subsea choke skid in Tunisia was scheduled for replacement as part of an offshore infrastructure upgrade. The scope of work included the installation of a new choke skid together with associated pipeline spools, requiring careful management of the connected subsea lines.

Ahead of installation, the pipelines had to be safely deoiled and filled with an environmental barrier fluid to protect both divers and the marine environment. Once completed, the lines were required to be dewatered in preparation for recommissioning. SANCCUS Gel Pig train played an important role in the successful accomplishment of this project.

Solution

Structures

The choke skid was connected to the topside through a 10" flowline. Diver Support Vessel (DSV) arrived at the operation site.

Step 1

Glycostat Gel was injected via a hot-stab with 3/8" bore on the tree at lowered viscosity. It was then pushed towards the topside by treated seawater, displacing hydrocarbons in the lines.

Step 2

After depressurising the system, the temporary pig launcher was installd by removing the 10" tie-in blind flange. It had 4 valved nozzles as per blue arrows and was pre-loaded with 3 M-Gel Pigs and MEG. Each gel pig was shaped in a way that would create gaps so they can be launched individually by functioning the valves.

Step 3

1st and 2nd pigs were launched and propelled by slugs of Glycostat Gel from DSV.

Step 4

3rd pig was driven by treated seawater.

Step 5

Once the gel pig train had reached the pre-determined location and deoiling operation was completed, the topside was opened to atmosphere and temporary launcher valves were opened. This allowed the pig train to flow back and the pressure in the system was equalised naturally.

Step 6

With the gel pig train forming an environmental barrier, the new 6" choke skid and its tie-in spools were installed. They were then flooded with treated and coloured seawater for leak testing.

Step 7

Nitrogen gas was injected from the topside, propelling the pig train towards the new choke skid to dewater the lines. Gel pigs extruded through the 10" - 6" transition.

Step 8

The system ready to be back online.

Result

Glycostat Gels and M-Gel Pigs were successfully deployed to safely and efficiently displace hydrocarbons from the subsea pipeline, enabling the installation of the new 6" choke skid and associated tie-in spools.

The same Gel Pig train was subsequently reused to dewater the system, demonstrating excellent stability and performance throughout multiple operational phases. This ensured the pipeline was left clean and dry, allowing the asset to return to service without delay.

Value

The SANCCUS Gel Pig train delivered 3 critical objectives in a single, streamlined operation:

1. Effective deoiling of the pipeline
2. Creation of a reliable environmental barrier during the tie-in phase
3. Efficient dewatering of the system following installation

The flexibility of SANCCUS Gel technology enabled seamless treatment of multiple pipeline diameters within the same system, eliminating the need for additional tools or interventions. By tailoring gel viscosity and pig geometry to the specific project requirements, SANCCUS provided a solution precisely matched to the operational challenges.

This project clearly demonstrated the versatility, adaptability, and cost-effectiveness of SANCCUS Gel technology when applied to complex subsea replacement works.