Subsea Production System Decommissioning

An FPSO operating off the east coast of India was scheduled for shutdown following many years of service. Prior to sailing to a shipyard for refurbishment, all associated systems were required to be thoroughly cleaned of hydrocarbons, gas, and internal deposits, including sludge, scale, and naturally occurring radioactive material (NORM).

SANCCUS was appointed to support the decommissioning project, delivering tailored chemical solutions to ensure safe and effective cleaning.

Solution

Structures

There were 4 x 8" Production Risers (PR1, PR3, PR2 & PR4) to be cleaned/deoiled and 4 x 8" Gas Export Risers (GE1, GE2, GE3 & GE4) and 1 x 6" Gas Injection Riser (GIR) to be cleaned/degassed.

Step 1

Gel Train was pumped down to PR1 in the order of Aqualink Gel, Sandstorm 1, Aqualink Gel, J-Clean 1 and Aqualink Gel. Propelled by treated seawater, it was then pumped up to PR4.

Step 2

Repeat the same process with PR2 and PR3. The riser contents were routed to the slop oil tanks via the FPSO separators.

Step 3 & 4

Gel Train was pumped down to GE1 in the order of Aqualink Gel and J-Clean 1. Propelled by treated seawater, it was then pumped up to GE4. The same process was repeated with GE2 and GE3.

Step 5

Gel Train was pumped down to GIR in the order of Aqualink Gel and J-Clean 1. The riser contents were routed down to the Gas Injection Well.

Step 6

The infield flowlines were decommissioned by opening the Xmas Tree and Production Manifold valves in sequence to drive the flowline contents with seawater down to the Production Wells.

Step 7

The system is filled with seawater, isolated and depressurised to ambient pressure.

Result

Approximately 30 km of production, gas export, and gas injection risers were successfully treated using the SANCCUS Gel Train methodology to meet all specified decommissioning criteria:

1. <1% LEL of methane
2. <40ppm oil in water
3. Safe handling and disposal of NORM and mercury to industrial standards
4. FPSO systems fully prepared for preservation

Value

The SANCCUS Gel Train approach enabled the efficient treatment of a complex FPSO riser network through a single, coordinated methodology, addressing deoiling, degassing, and contaminant management without the need for multiple standalone operations.

Targeted use of proprietary Gels reduced reliance on extended treated-seawater circulation, resulting in shorter execution timelines, lower fluid volumes, and reduced offshore exposure. This streamlined approach helped control project costs while maintaining strict environmental and safety standards.

The successful delivery of this large-scale decommissioning project demonstrated the scalability and reliability of SANCCUS Gel technology, confirming its effectiveness in meeting stringent technical requirements across long, multi-riser systems.