OMV Norge Iris-Hades Development Flow Assurance Study

Objective

OMV Norge engaged io consulting to assess the feasibility of operating the Iris-Hades (now known as Berling) subsea tieback to Equinor’s Åsgard B platform without the need for expensive electrically heated pipeline technology. The goal was to identify alternative flow assurance strategies that ensure safe and reliable production, with a focus on how to prevent hydrates during prolonged shutdowns and subsequent production restarts. (image courtesy of OMV Norge).

Project Scope

Key activities included:

1. Evaluating multiple flow assurance strategies, including pipeline heating, insulation, depressurisation, and chemical inhibition of portions of the system.
2. Conducting transient multiphase flow simulations using OLGA to model a carefully selected set of shutdown, depressurisation and restart scenarios.
3. Assessing the impact of Joule-Thomson (JT) cooling and identifying required mitigations.

Findings & Recommendations

Findings

• Restart-Induced Cold Temperatures:
  • High reservoir pressure and gas expansion across the choke during restart cause significant JT cooling.
  • Without mitigation, downstream choke temperatures could drop below minimum design temperatures.
  • Methanol injection can somewhat mitigate the cooling but is not, in itself, sufficient.
  • A warmup spool downstream of the choke can effectively increase restart temperatures, with the length determined to meet the required temperature.
• Depressurisation as a Hydrate Prevention Strategy:
  • Controlled depressurisation can be achieved, even in scenarios with relatively high liquid holdup, without liquid lock in the riser.
• Cold Restart Feasibility:
  • Restarting production by flowing the pipeline at low pressure and limited flowrate can be achieved without the contents entering the hydrate region.
  • Methanol injection before and during restart ensures cold fluids entering the pipeline during restart remain outside the hydrate formation zone.
  • Once methanol is swept through the system, pressure can be increased and production ramped up.
• Riser Base Slugging and Gas Lift:
  • Slugging during restart is minimal and no significant pressure build-up at the riser base is experienced.
  • Gas lift is not required, simplifying subsea infrastructure.

Recommendations

1. Electrical heating and riser base gas lift can be removed from the design, adopting the following hydrate management strategy:
   1. Depressurise the pipeline to prevent hydrate formation during prolonged shutdown periods.
   2. Inject methanol before and during production restart to prevent hydrate formation in the cold fluids entering the pipeline.
   3. Operate the pipeline at low pressure and limited flowrate during restart until methanol is swept through the pipeline to fully inhibit the contents against hydrate formation, at which point the operating conditions can be increased to normal operating pressure and production can be ramped up.
2. A warmup spool is required to manage low temperatures at the pipeline inlet during restart.