Pressure surge during crude unloading

How to avoid pressure surge problems when dissimilar ship to shore offloading operations are interconnected.


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Article Summary

Most oil operators have transfer facilities transporting oil from crude carrying tankers to onshore storage tanks through unloading pipelines. In the current scenario of the oil and gas industry, operators are struggling to reduce capex and are placing more effort on efficient utilisation of existing infrastructure. To increase the availability of an oil unloading facility, operators tend to interconnect their neighbouring oil unloading pipelines. Owing to the huge quantity of hydrocarbons and the great lengths of pipelines, the risks involved in such operations are very high. Hence, these pipelines must be protected against all abnormal events that can damage them. ‘Pressure surge’ during unloading operation is one of the critical scenarios where pipeline pressure can increase beyond the mechanical strength of the pipeline. Pressure surge is a steep increase in pipeline pressure due to a sudden change in fluid velocity.

Generally, the design pressure of pipelines is based on steady state calculations. Transient analysis of the pipeline needs to be conducted for various scenarios to evaluate and mitigate pressure surge. Whenever a pipeline’s operations differ from the scenarios for which it was designed, transient behaviour of the pipeline needs to be analysed again to evaluate pressure surge, and to validate the suitability of existing surge mitigation measures. It should be noted that each surge study needs careful attention to the circumstances, to the main assumptions made, and to the pipeline network configuration.

Ship unloading operations are intermittent, typically twice a week, hence the unloading pipeline infrastructure is under-utilised. In the case of breakdown in the unloading facility, the liquid transport operation is affected for multiple days. Hence there is a need to have a back-up option to increase the availability of the unloading facility. Interconnecting nearby unloading pipelines can provide increased availability of an unloading facility. Once interconnection is implemented, even if one unloading facility breaks down, the tank farm can use another unloading facility through the interconnection.

This article investigates a case study of neighbouring oil unloading pipelines, which were originally designed for individual oil unloading operations and later the two pipelines were interconnected as part of an integration of infrastructure for increased availability. Pressure surge behaviour for the altered operating scenario of the pipelines is discussed here.

Causes and possible mitigation of pressure surge
Pressure surge occurs due to the sudden closure of a terminal valve, the shutting down of pumps, or any sudden blockage in the path of the moving fluid. For instance, the quick closure of a valve causes the fluid stream to go into reverse, which in turn creates a transient pressure wave upstream and downstream of the valve. The wave upstream of the valve is much higher due to pressure build-up and is the main cause of the surge.

However, it is important to note that downstream of the valve can be equally alarming as it witnesses sudden low pressure, which can reduce the pressure below the vapour pressure of the liquid and thus cause cavitation.

This pressure surge depends on multiple factors like the nature of the fluid, the rate of velocity change, and pipeline system characteristics such as length and elevation.

Some of the mitigation measures for pressure surge are: increasing the design pressure of the pipeline by increasing the thickness of the pipeline; installing a surge vessel; changing the closing characteristics of the valve; or installing a surge relief valve with a surge relief tank. The kind of mitigation measure followed is decided based on the nature of the system and its cost factors.

Case study: unloading pipeline system
This study describes two ship tanker unloading facilities. Each of the unloading facilities is equipped with single point mooring (SPM) for unloading tankers, and a dedicated oil transport pipeline to the tank farm. Figure 1 depicts the two unloading pipelines along with their possible interconnection. During normal steady state operation, a crude carrier is connected to the SPM end of the pipeline and, using pumps on the tanker, oil is transported to the onshore tank farms through the unloading pipeline. The two pipelines operate independently during a normal unloading scenario.

Both of these unloading pipelines are rigid and are designed for unloading similar crude carriers. However, pipeline sizes, pipeline routings, and the types of valves on these two systems are different. Hence, the surge behaviour of the two pipeline systems is different, resulting in their having surge relief valves (SRV) set at different pressures. The 42in line is provided with SRV 12 barg set pressure, while the 48in line is provided with 15 barg set pressure. Also, these two systems have surge relief tanks of different capacities at their respective tank farms.

The pressure surge scenario of the terminal valve being closed during ship tanker unloading was studied for the 42in pipeline. A transient analysis of the pipeline was performed for light crude transport using commercial pipeline transient analysis software. The pipeline is allowed to operate in the steady state condition for an initial period of 10 seconds and then the isolation valve at the tank farm is closed slowly. As the pipeline valve starts closing, pipeline pressure starts rising, resulting in pressure surge. The pipeline valve’s closing time can be as high as 120 seconds or more, which is decided on surge mitigation measures. The faster the valve closes, the greater the resulting surge pressure. Figures 2 and 3 indicate the tank farm side pressure of the pipeline, upstream of the isolation valve.   

The initial pressure of the pipeline (the steady state pressure) is about 2 barg at the tank farm. However, during a pressure surge scenario a steep rise in pressure up to 52 barg was estimated. To mitigate this pressure surge, a SRV was installed close to the tank farm and minimum valve closure time is fixed at 120 seconds. The SRV set at 12 barg relieves excess fluid to a surge relief tank, resulting in reduction of maximum surge pressure to 23 barg. ASME B31.4 design code allows 10% higher pressure above design pressure during pressure surge for a liquid pipeline. Similarly, the 48in pipeline is also provided with a SRV to mitigate pressure surge.
Interconnected pipelines
Major points need to be taken into account while interconnecting two pipelines:

Hydraulics study: normally a hydraulics study is the first basis for interconnecting two unloading pipelines. The unloading capacity of an interconnected pipeline may increase or decrease while transporting through the interconnection. Any decreased unloading rate can still be accommodated by changing the ship’s unloading time to a certain extent.

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