Project Scope and Context
The Ærfugl MEG injection skid is designed to deliver monoethylene glycol to subsea flowlines to prevent hydrate formation. The system includes two high-pressure reciprocating triplex pumps and associated suction and discharge piping, interconnected with upstream and downstream facilities. While Phase 1 addressed static and sustained mechanical loads, the operational reality of reciprocating pump systems demands that dynamic effects—specifically, acoustically induced pressure pulsations—are fully characterized. The scope of this phase included the entire on-skid piping, relevant external tie-ins, and all connected acoustic paths, modeled in comprehensive detail to accurately simulate real operating conditions.
Analytical Methodology
The pulsation analysis was executed using BOSpulse software, with assessment criteria derived from API 674 DA2 (third edition). Both the suction and discharge sides of the system were modeled, incorporating detailed piping geometries, valve data, and boundary conditions reflecting the full operating envelope. The analysis addressed pulsation amplitudes up to the 12th harmonic of the pump’s base frequency (~80 Hz), as higher-order harmonics are known to be more difficult to excite and less likely to result in significant mechanical excitation.
Key assessment checks included:
- Verification that pressure pulsation amplitudes at each harmonic remained within the frequency-dependent allowables stipulated by API 674 DA2.
- Confirmation that minimum pressure at pump inlets stayed above the cavitation threshold under all conditions.
- Validation that maximum transient pressures at relief valves did not exceed setpoint margins.
- Assessment of unbalanced (shaking) forces generated by pulsations, with particular focus on resonance risks in connected piping.
Boundary conditions, fluid properties, and component settings were derived from as-built drawings and project specifications. The analysis included all relevant acoustic paths, extending beyond the skid where necessary to capture the influence of connected systems.
Results and Discussion
Suction System
The pulsation analysis revealed that, in the original configuration, pressure pulsations within the skid’s suction piping exceeded API 674 DA2 allowable limits at the 6th, 9th, and 12th harmonics. At the most critical locations, peak-to-peak pressure pulsations at the 12th harmonic reached up to 1990% of the allowable value, a level with clear potential to induce fatigue and operational instability. Notably, the minimum pressure at pump inlets was consistently above the required threshold, ensuring cavitation risk was avoided even in the presence of elevated pulsations.
To mitigate excessive pulsation amplitudes, a staged orifice solution was developed. The installation of a primary orifice at the skid inlet effectively reduced pulsation levels for harmonics up to the 12th, but isolated exceedances remained at the highest frequency. Additional orifice plates, installed in the suction lines directly upstream of each pump, successfully suppressed 12th-harmonic pulsations, bringing all measured values within API 674 DA2 allowables. This approach was validated through spectral analysis at critical nodes and demonstrated robustness across the full range of modeled scenarios.
Despite these mitigations, the analysis identified that pressure pulsations in the external suction line toward pump skid 46-XX-001 also exceeded allowable levels for the 12th harmonic.
The cut-off frequency for the analysis is not specified by the API 674. It is known that higher order modes (higher frequencies) are more difficult to excite. Therefore, the necessity for the inclusion of the 12th harmonic pulsations in the analysis could be debated.
The 12th harmonic pulsations are mitigated by installing orifice plates directly upstream of the valves in the suction lines of the two pumps.
Discharge System
On the discharge side, the original system exceeded API 674 DA2 pulsation allowables at the 12th harmonic, with peak-to-peak pressures up to 448% of the allowable at critical locations. Lower-order harmonics were within limits, indicating that the observed exceedances were a high-frequency phenomenon associated with the triplex pump dynamics and the acoustic characteristics of the discharge piping.
The introduction of an orifice plate in the discharge line from pump 46-PB-005B effectively reduced 12th-harmonic pulsation amplitudes to within allowable values. This solution was confirmed to be effective for both steady-state and worst-case operational scenarios, as validated by pressure spectrum analysis at the most sensitive locations. All maximum transient pressures at discharge relief valves remained below setpoint, confirming the absence of overpressure risk.
Unbalanced Forces and Mechanical Excitation
Pulsation-induced unbalanced forces, or shaking forces, were calculated for all relevant harmonics. Within the skid, these forces were moderate (up to 120 N in suction and 20 N in discharge at the 3rd and 6th harmonics, respectively) and not expected to induce significant mechanical excitation or fatigue risk in the primary structure. However, unbalanced forces in the external connected piping reached much higher values (up to 1930 N at the 6th harmonic), emphasizing the importance of system-wide acoustic design and the need for coordination with upstream and downstream system owners. The analysis concluded that mechanical excitation at harmonics above the 9th is unlikely due to the complexity of the vibration modes at such high frequencies.
Conclusions and Recommendations
The pulsation analysis confirmed that, with the recommended orifice mitigations implemented, the Ærfugl MEG injection skid meets all API 674 DA2 pressure pulsation criteria for both suction and discharge systems. Minimum and maximum pressures at all critical points remain within safe operating limits, and unbalanced forces are controlled within acceptable margins for structural integrity.
To ensure ongoing reliability and compliance, the following actions are recommended:
- Install all specified orifice plates in the suction and discharge lines as detailed in the analysis.
- Investigate and implement further pulsation mitigation measures in external piping where API 674 DA2 limits are exceeded, in coordination with adjacent system owners.
- Maintain strict adherence to the detailed support arrangement developed in Phase 1, as the dynamic loads identified in this phase reinforce the necessity of robust mechanical restraint, particularly for small-bore branches subject to vibration risk.