The Petrojarl I Floating Production, Storage, and Offloading (FPSO) facility, in operation since 1986 in the North Sea, underwent critical modifications to prepare for deployment at the Atlanta oil field offshore Brazil. The relocation entailed adapting the crude oil offloading system to manage Brazilian crude with higher viscosity and to operate with a longer floating hose. These changes prompted a comprehensive surge analysis to ensure system integrity during both normal and transient flow conditions, focusing particularly on pressure transients (surges) that could exceed the mechanical design limits of the offloading system components.
System Configuration and Modeling
The modified offloading system comprises six submerged cargo pumps transferring crude oil from the FPSO to a receiving vessel via a 400-meter flexible floating hose. The system is designed with a manifold combining the outputs of all pumps, which then feeds the single offloading line. A butterfly valve is installed at the receiving vessel’s connection point; for North Sea operation, a marine breakaway coupling was also included but was deemed not applicable for the Atlanta project.
Physical and operational modeling parameters were rigorously defined. The crude oil properties for simulation were set as follows: density of 977 kg/m³, viscosity of 519 cP, bulk modulus of 2.0 GPa, vapor pressure of 0.1 bara, and temperature of 65°C. The offloading line includes steel piping (wave speed 1200–1300 m/s) and a flexible hose (wave speed 385 m/s). The pumps, running at 1770 rpm for rated flow, deliver between 450–480 m³/hr each, with a total system flow rate approximating 2750 m³/hr during peak operation.
Surge Analysis Methodology
The analysis employed BOSfluids, with model geometry and parameters validated through direct data and site visits. The simulations addressed five critical transient scenarios:
- System filling with oil (flushing inert gas) using a single pump.
- Sequential start-up of the remaining pumps to reach maximum flow.
- Rapid closure of the marine breakaway coupling (not applicable for Atlanta but analyzed for completeness).
- Rapid closure of the butterfly valve at the receiving vessel.
- Simultaneous trip of all six pumps.
Boundary conditions were carefully set to reflect operational realities at both the start and end of offloading, incorporating realistic tank levels, hose routing, and elevation changes. Allowable pressure for the system was determined by combining the design pressure (16 barg) with a surge factor (1.33 per ASME B31.3), resulting in a maximum permissible surge pressure of 21.3 barg.
Results and Key Findings
Steady-State Performance
Under steady-state conditions, system pressures and flows remained within design expectations, with total flows just below 2800 m³/hr at full operation. No steady-state conditions risked exceeding component limits.
Transient Scenario Outcomes
- Scenario 1: System Filling No significant surges were observed during the initial oil filling, as the absence of significant restrictions meant pressure increases were negligible.
- Scenario 2: Pump Start-Up Incremental opening of pump discharge valves caused minor surges, with a maximum pressure of 15.4 barg—well within allowable limits. Each pump engagement caused a transient increase in flow and pressure, quickly stabilizing.
- Scenario 3: Breakaway Coupling Closure (Not for Atlanta) Simulating a rapid five-second closure of the breakaway coupling during maximum flow produced a surge pressure of up to 30 barg, exceeding the allowable by 41%. This result highlights the critical importance of actuation speed and flow conditions in surge management (not applicable to the Atlanta field, but instructive for similar systems).
- Scenario 4: Butterfly Valve Closure Rapid closure (16 seconds) of the butterfly valve at the receiving vessel resulted in a maximum surge pressure of 23.8 barg, exceeding the allowable by 12%. The surge pressure peaked as the valve approached full closure, with pressure waves reflecting and gradually damping throughout the system.
- Scenario 5: Simultaneous Pump Trip The maximum surge pressure following simultaneous pump trip occurred at the end of offloading, reaching 16.6 barg—within the allowable. This event also saw significant vapor pocket (cavitation) formation at high points, with the risk of implosion-induced pressure spikes. Careful restart procedures, including venting, are necessary to avoid subsequent damage.
Additional Observations
- Unbalanced Forces: While not the primary focus, the analysis provided time histories of unbalanced loads, which must be evaluated separately for mechanical integrity.
- Entrapped Gas Effects: The presence of inert gas pockets, while difficult to quantify, would likely dampen surge pressures, acting as a natural pulsation damper.
- Thermal Assessment: The crude’s temperature drops slightly (about 1–4°C) during transit through the insulated hose, with the lowest temperatures occurring at the lowest flow rates. These minor losses do not significantly affect surge behavior but could influence viscosity and pump performance under slow flow conditions.
Recommendations
To mitigate surge risks identified in the analysis, particularly for rapid valve closure events, the following measures were proposed and validated through simulation:
- Increase Closure Time: Extending the butterfly valve closure to 33 seconds reduces maximum surge to 21.1 barg, within design limits.
- Reduce Flow Rate: Limiting the maximum flow through the hose (to 2450 m³/hr, or a pump rpm of 1600) achieves a similar reduction, with peak surge at 20.2 barg.
- For the breakaway coupling (not applicable to Atlanta), increasing closure time and/or reducing flow rate can also bring surge pressures to within allowable values.
Additionally, mechanical evaluation of piping and supports under transient unbalanced loads is essential, as these were outside the current scope.
Conclusion
The surge analysis for the modified Teekay Petrojarl I FPSO crude offloading system demonstrated that, under most operational and upset scenarios, transient pressures remain within safe design limits. However, specific transients—namely rapid closures of the butterfly valve—can generate surges exceeding allowable pressures if not properly managed. The study provides clear recommendations for operational modifications (closure time, flow limitation) that ensure system integrity. These findings underscore the necessity of detailed transient analysis whenever system modifications or new operational environments are introduced, particularly when handling viscous fluids and long transfer lines.