(595d) Challenges and Potential Solutions in Gtl FPSO Process Design for Monetization of Offshore Stranded Gas

Approximately half of global natural gas reserves are considered to be stranded gas that is uneconomical for market delivery due to the remote location of potential markets, lack of economic transportation and infrastructure, or lack of conversion technology. GTL (Gas-to-Liquid) FPSO (Floating Production Storage and Offloading) offers an attractive solution to monetize these remote, offshore and stranded gas fields.  A FPSO vessel is a large vessel which can be permanently moored over a remote offshore gas field. The gas is brought on board the vessel via a flexible riser coming from the wells on the sea bed. Shuttle tankers are used to offload the product from the FPSO vessel and take it to a shore-based refinery.

DSME has recently completed Conceptual Design Package of GTL process for FPSO application producing 20,000 BPD (barrel per day) of a Fischer-Tropsch liquid syncrude product. Compared to land-based application, several technical challenges need to be addressed for offshore FPSO application. An offshore site must be completely self-sufficient in utilities, and all the required repairs/maintenance must be performed while the vessel is on-station. The entire process and all product storage must fit onto the topside deck of the ship or within its hull. The ship will be in constant motion as waves pass under it. The design of structures and process equipment must take this into account. The safety, maintenance and material handling requirements unique to FPSO must be considered in the design.

These design challenges have been addressed in this conceptual design of GTL FPSO. For example, the novel concept of a single pumpable synthetic crude product has been implemented in the process design. Compared to multiple products such as light gases, naphtha, distillate and wax typically found in land-based GTL processes, the concept of single product offers several advantages for FPSO application such as much simpler product upgrading unit which saves topside deck space and capital, and only one type of onboard storage tank and shuttle tanker required. The single product concept has been realized by dilution and minimum wax treatment by low severity hydrocracking or hydroisomerization. Another design challenge addressed in this conceptual design is to eliminate the need for ASU (Air Separation Unit) for syngas production and hydrogen production for product treatment typically required for land-based GTL processes. The combination of compact reformer based on Steam Methane Reforming and hydrogen-specific membrane separation is employed to address this design challenge.  The elimination of both ASU and additional hydrogen production unit saves topside deck space and capital as well as relieves safety concerns.

In this presentation, these design issues and their solutions in GTL process development for FPSO will be described and some of them will be illustrated with 3D modeling and animation. Furthermore, several potential areas will be discussed that can further enhance process efficiency and economics in the next engineering phase of the GTL process development for FPSO.