A study entitled "Long Subsea Tie-back Solutions for Pre-salt fields" was launched to compare different architectures concerning the hydrate and wax risks. In general, it aims the development of technical solutions and technologies applied to long subsea tie-back on pre-salt fields as a technically feasible and profitable solution. A fictive pre-salt field of two production wells located at 2,500m water depth tied back to a FPSO with a production flowline of around 30 km is considered.

This study started with a screening study to assess the technical feasibility of different "single line" concepts. A cost estimate study has been done in parallel to support the most cost-effective solution. Five architectures have been investigated:

  • Two architectures without subsea processing:

    • 1 trunkline;

    • 2 single lines.

  • Two architectures with Subsea Separation Unit (SSU):

    • SSU close to wells.

    • SSU at riser base.

  • One architecture with Multi Phase Pump (MPP)

    • MPP close to wells.

At the end of this phase, only three architectures Trunkline, Riser Base SSU and MPP architectures have been retained as the most attractive ones in terms of operability and costs (as indicated in the Fig 1). The concept Subsea Separation Unit (SSU) located close to wells even if inducing low costs was not kept as difficult to operate within the production field life. The two single lines concept was not competitive compared to the trunkline one. Moreover, in terms of costs, a strong incentive has been demonstrated for "lighter" architecture concepts (i.e. a flowline thermal insulation of a wet insulated flowline compared to a Pipe in Pipe (PIP) flowline and without flowline heating system such as electrical trace heating with pipe in pipe insulation (ETH-PIP) technology).

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