Carrying out a pre-FEED study for a wellhead platform and tie-ins

Scope

Jee conducted a pre-FEED study to assess the technical and commercial viability of installing a new wellhead platform and connecting 12” export and 2” piggyback methanol pipelines to an existing offshore pipeline system.

A topsides riser tie-in had previously been ruled out due to complexity and cost. The study therefore focused on subsea tie-in options at the base of the existing platform.

The objectives were to:

• Assess pipeline isolation and pigging/sphering requirements
• Develop a high-level plan and cost model for subsea tie-in execution
• Identify risks and propose mitigations
• Define testing and commissioning requirements

Solution

The team reviewed subsea tie-in options for both pipelines, with emphasis on constructability, operability, and long-term integrity. Key considerations included:

• Isolation and Pigging: Evaluation of isolation plug deployment, pig launcher/receiver suitability, and compatibility of existing pipeline components.
• Corrosion Management: Recognition that subsea tie-in would preclude conventional sphering, requiring a revised corrosion strategy such as CRA spools, tethered inspection tools, or gel pigs.
• Risk and Safety: Consideration of the increased inventory in the existing export system and the need to update the pipeline safety case accordingly.
• Costing and Execution: Development of a high-level cost model alongside identification of risks and mitigations for construction, commissioning, and long-term operation.

Conclusion and Recommendation

The study confirmed that a subsea tie-in is both technically sound and commercially viable. One preferred option was identified for inclusion in the field development plan.

Progression is subject to:

• Verification of pipeline integrity at the isolation point
• Confirmation of component compatibility for plugging and pigging
• Development of a corrosion management strategy suitable for operation without conventional sphering

Once these conditions are met, the subsea tie-in offers a more practical and cost-effective solution than the previously considered topside riser tie-in.

For more information, visit www.jee.co.uk/design

To contact our Head of Design, John French, email john.french@jee.co.uk, or call +44 (0)1732 371 371.