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A decommissioning project has become a reality in the Indonesian oil and gas market. As an oil and gas field reached its declining production, causing high operation costs, the Company had to perform a further evaluation and finally decided to decommission the Floating Storage Offloading (FSO) connected to the field. It is the first decommissioning project of the Company's offshore floating structure (FSO) and the first for the Indonesian oil and gas market.

The FSO location is approximately 2.2 km from the Floating Production Storage & Offloading (FPSO) facility at 90 meters of water depth, which is held by a turret mooring system that permits the vessel to weathervane freely. Two rigid export pipelines run from the FPSO to a Pipeline End Manifold (PLEM) located on the seabed near the FSO location, then continue with two (2) flexible risers to the turret installed on the FSO. The FPSO exports segregated propane and butane through the pipeline and flexible risers to the FSO.

The FSO has facilities to receive, refrigerate, and store the Liquified Petroleum Gas (LPG) in storage tanks slightly above atmospheric pressure. Boil-off gas will be recovered, reliquefied, and returned to the storage tanks. Refrigerated LPG is offloaded to export LPG carriers via a side-by-side offloading system.

The decommissioning scope includes offshore disconnection of two (2) 6-inch flexible risers, one (1) 5-inch subsea power cable, and six (6) mooring chains from the FSO, then secure and preserve them for offshore subsea wet storage. The FSO is towed to an appointed shipyard for re-conversion before returning to the owner.

This paper describes actual experiences in the engineering, procurement, and construction (offshore execution) phases. Offshore execution includes organization & manning, decommissioning vessel & equipment, decommissioning methodology, and hazard & risk management, which can be applied for future similar projects as a reference. Decommissioning methodology where FSO seakeeping and flexible riser, subsea power cable & mooring chains decommissioning are described in detail.
As the first decommissioning for the Indonesian oil and gas market, this project execution performance has achieved outstanding final project achievement in HSE, engineering, procurement and contract, quality, schedule, and cost.