The Liuhua29-1 gas field in the South China Sea is developed with a single 27km 12" flowline tieback to an existing subsea production manifold. The produced wet gas flow is assured by continuous MEG injection at the production manifold and trees. Due to the well arrangements and product compositions, the well jumpers, infield flowlines, and production flowline consist of flexible, carbon steel, and internal CRA lined flowline sections. This paper presents the flowline design, fabrication, and installation challenges encountered during the project execution, discusses the steps taken in order to address these challenges, and the outcomes of the project as a result of vigorous engineering efforts.
The Liuhua29-1 field development contains one drill center with 5 production wells and two satellite production wells which are 1.7km and 3.3km from the drill center manifold. The produced gas is transferred from the production manifold to the subsea tie-in location with a 27km 12" carbon steel flowline and carbon steel flowline jumpers. A 38km 6" carbon steel flowline delivers the MEG (mono-ethylene glycol) to the drill center for hydrate inhibition.
In order to overcome the significant design and installation challenges, which include internal corrosion from the high product temperature and flowrate, large quantities of critical spans that require rectification, and PLET dimension and valve delivery risk for inline installation, extensive detailed engineering design studies and modification to existing installation vessel were carried out. Through design iterations and continued improvement, the design and installation engineering is treated as an integral approach to minimize risks and costs while maximizing the reliability of the installed flowline system.
Through careful studies of product compositions, flow assurance analyses, choosing of corrosion inhibitors, and material selections; internal corrosion control of the flowline was achieved with inhibitor injection as well as lining the flowline interior with corrosion resistance alloy (CRA) material for the first 1km from the inlet to achieve the best overall results. In order to mitigate the valve delivery risk for PLET inline installation and reduce the critical span near the flowline end due to higher pipelay tension required, the S-lay vessel was modified with the addition of a PLET Handling System (PHS) to allow vessel-side PLET installation. FEED engineering concluded that there would be 71 spans which will require rectification prior to hydrotest as well as another 20 span rectifications prior to startup. To reduce the amount of deep-water flowline span rectifications, the project carried out numerous iterations of flowline bottom roughness analyses and span VIV analyses to optimize the flowline route and pipelay bottom tension. As a result of rigorous engineering and modification to the PLET installation method, the production flowline was installed successfully without a single span requiring rectification.
