Floating Production Storage and Offloading (FPSO) units play a crucial role in offshore oil and gas production. A critical component of these systems is the compressor skid, which compresses natural gas for processing or export. This case study focuses on the pulsation and vibration analysis of a three-stage reciprocating compressor system installed on an FPSO in the North Sea.
The objective of this project was to evaluate the compressor system’s compliance with API 618 (6th Edition, 2024) Design Approach 2 requirements, identify areas of non-compliance, and propose mitigation measures to address excessive pulsations and vibrations. This analysis is crucial for ensuring the reliability, efficiency, and safety of the FPSO’s gas compression operations.
Problem Definition
The primary issue investigated in this study was the occurrence of high-pressure pulsations and excessive vibrations within the compressor system. In the past damage had already been observed on some of the vessel nozzle connections. Specific challenges included:
- Pressure pulsations in the piping and compressor flanges exceeding API 618 allowable limits.
- Excessive non-resonant acoustic shaking forces on piping and pulsation suppression devices (PSDs).
- Design constraints related to the pressure drop across pulsation dampeners, limiting the effectiveness of certain mitigation measures.
The analysis was constrained by limited flow rate data, which was derived from a flow meter downstream of the second-stage separator and upstream of the recycling loop. As agreed with the client, the study utilized a combination of measured and computed flow rates to model the system’s behavior.
Methodology
The study employed the API 618 Design Approach 2, which consists of five key checks:
- Pressure amplitude check for piping and compressor flanges.
- Evaluation of pressure drops across pulsation suppression devices.
- Assessment of non-resonant acoustic shaking forces.
- Analysis of the impact of scrubber filling levels on acoustic performance.
- Piping restraint analysis
The analysis utilized BOSpulse software to model the compressor system, which operates at a nominal speed of 845 RPM. A frequency sweep of ±10% was applied, resulting in an operating range of 760.5 RPM to 929.5 RPM. The first 12 harmonics of the compressor system were analyzed to identify resonances and potential standing waves.
The methodology included simulating the system’s acoustic behavior under varying operating conditions, comparing computed pulsation levels and shaking forces with API 618 allowable limits, and evaluating the effectiveness of potential mitigation measures.
Analysis
Pulsation Amplitude Check
The analysis revealed multiple locations where pressure pulsations exceeded the API 618 allowable limits:
- Piping: Excessive pulsations ranged from 102% to 1815% of the allowable limit, with the highest levels observed in critical sections of the piping systems.
- Compressor Flanges: All stages of the compressor experienced pulsations above the allowable limits, potentially impacting valve lifetime and compressor efficiency.
These high pulsation levels were attributed to acoustic resonance, where the natural frequencies of certain pipe sections aligned with the compressor’s harmonic frequencies.
Non-Resonant Acoustic Shaking Forces
Excessive shaking forces were detected in multiple pipe sections, with the highest force exceeding the allowable limit by over 1500%. These forces were caused by pressure differences across pipe sections, amplified by resonance at specific harmonics.
Scrubber Filling Levels
The filling levels of the scrubbers were found to influence the acoustic properties of the system. As liquid accumulated in the scrubbers, their effective volume decreased, altering their natural frequencies. This effect underscored the importance of regular draining to minimize pulsations.
Pressure Drop Across Pulsation Suppression Devices
While pressure drops across the pulsation suppression devices were within allowable limits, the addition of orifice plates as a mitigation measure significantly increased pressure drops, rendering this solution infeasible in some locations.
Proposed Solutions
Based on the analysis, the following solutions were proposed:
Redesigning Pulsation Dampeners:
- Replace existing pulsation dampeners with units of different volumes and lengths.
- Conduct a detailed pulsation bottle sizing analysis to ensure compliance with API 618 requirements.
Installing Orifice Plates:
- Introduce orifice plates at critical locations to disrupt standing waves and attenuate pulsations.
- Five proposed locations were identified for potential implementation.
Optimizing Scrubber Drainage:
- Regularly drain the scrubbers to maintain their acoustic properties and minimize pulsations.
Evaluation and Conclusion
The proposed solutions were evaluated for their effectiveness and feasibility:
- Redesigning the pulsation dampaners was deemed the most effective solution, as it addressed the root cause of excessive pulsations. However, this approach required significant design changes and cost considerations.
- The orifice plates provided localized relief but introduced unacceptable pressure drops, particularly near pulsation bottles.
- The scrubber drainage offered marginal improvements and was recommended as a supplementary measure.
The pulsation and vibration analysis of the compressor system on the FPSO highlighted critical areas of non-compliance with API 618 DA2 requirements. The study concluded that the most effective mitigation strategy was to replace the existing pulsation dampeners with redesigned units tailored to the system’s acoustic characteristics.
This approach, combined with regular scrubber drainage and selective use of orifice plates, would ensure compliance with API 618 requirements and improve the reliability and efficiency of the compressor system.
