How to Determine the Optimal Pigging Frequency

In a Nutshell:

WHAT IS FLOW ASSURANCE?

The term “flow assurance” mainly refers to ensuring the successful and economical flow of the hydrocarbon stream from the reservoir to the point of sale. Such studies often encompass an evaluation of the effects of a fluid and its transport (i.e. flow regime, hydrates, waxing, asphaltenes, slugging, naphthenates, scales, sand, corrosion, erosion and emulsions) and their potential to disrupt normal production due to excessive accumulation or catastrophic failure of the pipeline system.

Flow modelling is often performed by deploying standard industry software (such as OLGA, PIPESIM, Multiflash, etc.) in order to understand operating limits and evaluate mitigation strategies to minimize operational risks, maintain maximum production throughput and prolong pipeline life. The relationship between system integrity, flow assurance and external factors is depicted in Figure 1.

WHAT ARE THE POTENTIAL BENEFITS OF FLOW MODELLING FOR PIPELINE OPERATORS?

Flow modelling is an essential tool to optimize process conditions and cleaning pigging operations, which in turn benefits operators in the following ways [1]:

• Minimize liquid hold-up to reduce ∆P losses
• Clean sand/wax/scales/debris/hydrates
• Reduce internal corrosion (under deposit, MIC, etc.)
• Reduce the risk of leakage due to internal corrosion
• Effectively apply chemical treatments
• Complete a pre-inspection cleaning service

The impact of internal deposits and scaling – e.g. due to ineffective operational cleaning – on the loss of throughput and required increase of pressure to maintain pipeline constant flow is presented in Figure 2.

WHAT ARE THE BENEFITS OF INCLUDING FLOW MODELLING IN CORROSION ASSESSMENTS?

In crude oil pipelines, water starts to accumulate excessively when the operating flowrates drop below a critical (threshold) flowrate. This causes separation of the water phase due to gravity effects and phase slippage, especially in the upward-sloping pipeline sections. However, when a flowrate increases above this threshold, the water phase is effectively reduced, as it is carried by the oil phase due to interfacial friction.

Corrosion is typically not an issue at low water cuts because all of the water tends to be entrained by the flowing oil (water-in-oil dispersion). However, the higher the water cut, the more likely water break-out may occur, leading to a segregated flow of separate water and oil phases. Flow modelling is a useful tool to simulate the water accumulation effects, and thus corrosion risks, while also testing scenarios that aid in the development of strategies for operational cleaning and internal corrosion control.

HOW IS THE OPTIMIZATION OF THE OPERATIONAL CLEANING FREQUENCY AND CONFIGURATION ACHIEVED?

Cleaning frequency optimization requires a thorough evaluation of the historical debris returns, tool design, solids/water accumulation rate, debris/liquid handling capacity and operational constraints.

Cleaning for wax/liquid accumulation should be carried out at regular intervals to ensure the pig traps are not overwhelmed with the generated returns when receiving the tool. Flow modelling is highly recommended for the purpose of optimizing the cleaning strategy by performing sensitivities on cleaning frequency and optimized tool design (such as a bypass pig for effective removal of debris, as shown in Figure 3) .

CASE STUDY

A client in South America operates a 32-inch pipeline that transports crude oil over a distance of 585 km. Although the operator followed an active cleaning program with a frequency of twice a week, active corrosion features of up to 30-40% were detected at a few locations in the pipeline. Hence, ROSEN was asked to perform a flow modelling study to support the ongoing development and implementation of the Corrosion Management Plan (CMP) [2].

WHAT WAS THE CHALLENGE?

In line with the recommendations outlined in the CMP, the overall objectives of the flow modelling study were to:

• Establish the effect of the flowrate on the pipeline flow regime and identify a flowrate operating window that may be used to minimize and control pipeline internal corrosion.
• Understand the amount of water hold-up and identify its locations in the pipeline.
• Establish an optimized pig configuration and frequency in order to manage the water in the pipeline.

WHAT WAS THE OUTCOME?

Flow models representative of the pipeline system were developed using the industry-standard multiphase transient simulator OLGA software. The black oil compositional fluid model was used within OLGA to model the required PVT properties of the crude oil fluid.

Steady-state analysis showed substantial water drop-out when the flowrate drops below a threshold flowrate range of 1,250-1,500 m3/h (0.6-0.8 m/s liquid velocity). As the historical operating flowrates (2,050 m3/h) were well above the threshold flowrate range, there was a negligible effect of the flowrate increase on pipeline water content reduction (see Figure 4).

The pigging analysis (Figure 5) showed that increasing the scraping frequency to even 14 pig runs a week at historical operating conditions would not significantly improve the total water content in the pipeline. It was noted, however, that there were other benefits of an increased scraping frequency, including removal of deposits, debris and wax in the pipeline, such that it would significantly reduce the risk of under-deposit corrosion and microbiologically influenced corrosion (MIC) in the pipeline.

As part of this study, it also became evident that the historical pig configuration, which employed conical cups and was intended for light cleaning duties, had a tendency to run over stubborn debris. The cleaning tool design was thus not fit for the purposes of (i) maintaining an acceptable level of pipeline cleanliness, (ii) removing water and (iii) controlling corrosion.

CONCLUSIONS AND RECOMMENDATIONS

ROSEN recommended that the client continue with the implemented operational cleaning strategy – as long as a new cleaning pig design is adopted, which will be more effective in removing both water and debris. As shown in Figure 6, adding highly efficient sealing elements to the brush tools, combined with magnets, will enable a single pass to be performed for effective cleaning and water removal. A good pig design is key when it comes to the effective removal of the water film from the pipeline wall.

By implementing a systematic quantitative analysis of the cleaning debris recovered – i.e. volume, composition and bacterial analysis – the client was able to evaluate the results and, therefore, to optimize the cleaning frequency. Additionally, in order to evaluate the results of an initial schedule for the new operational cleaning strategy, ROSEN recommended that the operator conduct several runs with the newly developed cleaning pig. If the normal operating fluid velocities drop below 0.6-0.8 m/s, a significant amount of water may start to drop out in the pipeline, and an increased cleaning frequency should be considered.

References

ROSEN AP – Tech Talks, Flow Assurance and Pigging Feasibility Studies to Optimize Cleaning Operations, A. Pinto, P. Westwood.
[2] J. Soltis et al., 11182_CMP_YPF_32 Inch Line_Issue1.pdf, August 2019.