Enhanced-Gel-Strength Concept Optimizes Chemical Use in Pipeline for Waxy Crude
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Banyu Urip crude contains 26% wax, which can lead to flow-assurance challenges in a crude pipeline exposed to lower temperatures. Injection of pour-point-depressant (PPD) chemicals has been considered an effective method to ensure flow of moderate waxy crude. For the Banyu Urip field, PPD injection was compared with other methods and found to be the best option. Nevertheless, it still contributes to approximately 20% of the operating costs. Optimization of this chemical use can bring benefits through lowering operating costs.
The Banyu Urip Field is in East Java, Indonesia. The facilities process a stabilized crude, which is then transported through a 20-in. export pipeline to a floating storage and offloading vessel. The export pipeline is divided into two sections, a 72-km onshore section and a 23-km offshore section.
High wax content is a key characteristic of Banyu Urip crude. The stabilized crude’s wax appearance temperature is 46°C, and its pour point is 33°C (±3°C). With a lowest ambient temperature of 27°C for the onshore section and 24°C for the offshore section, a risk exists of the crude temperature dropping below the pour-point temperature during a no-flow condition, which may cause gelling in the pipeline (Fig. 1 above).
Several options were considered for a flow-assurance strategy, including continuous PPD injection, pipeline displacement, and electrical heating along the pipeline. Continuous PPD injection was seen as the best option from cost and operability perspectives. Consequently, chemical-injection facilities were installed in all the well pads for continuous PPD injection. To maintain pour point at 24°C, initial calculations required approximately 500-ppm PPD injection. At current production rates, this equates to approximately 20% of the Banyu Urip operational expenditures.
Description of Processes
Continuous PPD injection with a 24°C pour-point target is an inherently conservative approach to keeping the crude pour point lower than the lowest expected ambient temperature. Furthermore, the PPD is not required in normal flowing conditions; rather, it is only required when the pipeline is shut in for a long period, which can cause the crude to cool to ambient temperatures.
In normal operation at design rates, crude enters the export pipeline at approximately 75°C and reaches the storage vessel at approximately 70°C. The export pipeline is insulated, which helps maintain heat in the crude for several days before reaching its pour point, even in no-flow conditions.
When the crude has reached pour point and starts forming a gel, the weak gel (within a certain gel strength threshold) is still breakable by application of export pump pressure within the pipeline maximum allowable working pressure (MAWP).
With a 20-in. pipeline, a radial temperature profile exists in which the center of the pipeline usually will have a higher temperature, making it easier to break the center with pressure.
The gel-strength concept is used to overcome pour-point-measurement limitations and better represent the actual conditions. However, in a conservative gel-strength concept, the required restart pressure is calculated using a force-balance equation. This assumes the gel will form uniformly in all sections of the pipeline and requires immediate plug flow movement of the crude body to restart the pipeline. For Banyu Urip export crude without PPD, the gel strength may reach 6100 Pa at 24°C. By this concept, to allow restart pressure within pipeline MAWP, the maximum allowable gel strength is 12 Pa.
Application of the Technology
Gel-strength measurement is conducted using polyether ether ketone (PEEK) plate-and-plate geometry in a rheometer. The PEEK is required because of its high adhesive strength to the crude as opposed to the standard stainless steel. At 24°C operating temperature, gel-strength measurement for naturally flowing Banyu Urip crude is less than 5 Pa. This is within the device accuracy threshold to represent no significant yield stress. A gel-restart model showed that, at the minimum operating temperature of 24°C, the maximum allowable gel strength is 140 Pa to allow pipeline restart within its MAWP (the export pipeline MAWP in the design phase is 1,300 psig). The increment of maximum allowable gel strength from 5 to 140 Pa correlates with a PPD injection dosage of approximately 300 ppm.
The Banyu Urip export pipeline is insulated using polyurethane foam and fusion-bond epoxy, which provides heat conservation. Moreover, the large diameter of the flow line and the wax’s natural insulation capability results in a radial temperature profile in the export pipeline. Results of a computational-fluid-dynamics model show that the pipeline center fluid temperature will reach approximately 24°C after 60 days.
A pilot experiment was conducted to validate the radial temperature profile. A segment of spare 20-in. export pipeline and stabilized crude from Banyu Urip field was used. This pipeline segment included actual radial insulation. Blind flanges with axial insulation were installed to contain the crude inside. The pipeline segment was then immersed into a circulating thermostatic bath (CTB) full of water to maintain an ambient temperature of 24°C, representing the lowest ambient temperature observed at the offshore section of the pipeline. This CTB is equipped with two evaporators, agitators, and an automatically controlled cooling system to maintain homogeneous water temperature. A crude-oil tank with a heater was used to condition crude oil to 75°C, representing the actual crude temperature entering the export pipeline. The heated crude was then pumped to the pipeline segment.
Sections of a thermocouple bracket were installed along the pipe segment to monitor heat transfer. Each section consisted of 13 measurement points to provide a radial temperature profile. These thermocouples were connected to data-acquisition software that captured data every hour. The experiment was monitored for 18 days until the crude reached a stable temperature.
The experiment data showed more-rapid temperature changes with no significant differences between the center and the outer layer. The following factors contributed to the results:
- The pipe segment length in the experiment is very short compared with the 95-km pipeline, causing significant axial heat loss in the experiment compared with the actual pipeline.
- Axial insulation in the experiment has higher thermal conductivity compared with a hot-crude interface in the actual pipeline, causing more-rapid axial heat loss.
- The pipe initial temperature during the experiment is ambient water temperature (24°C) compared with the flowing crude temperature in the actual pipeline (75°C).
- In the experiment, the pipeline is fully immersed in water. The actual pipeline is half buried (sitting on seabed), which can cause the experiment to be more conservative compared with actual& pipeline conditions.
Another CFD model was built to consider these factors and then compared with the initial CFD model. The comparison showed good data reconciliation.
Using a restart-pressure model based on an enhanced-gel-strength concept, the maximum allowable gel strength for the Banyu Urip export pipeline can be increased from 12 to 140 Pa, coinciding with approximately 300-ppm PPD injection dosage (200 ppm less than was originally used). This chemical saving can reduce potentially up to 40% of the PPD cost or almost 10% of the Banyu Urip operational expenditures in peak production.
On the basis of the radial temperature profile model and validated by heat-transfer experiment, the temperature of the inner portion of the Banyu Urip crude export pipeline remains above ambient temperatures after 30 days. This is sufficient for the core to remain easily breakable during a downtime event.
Enhanced-Gel-Strength Concept Optimizes Chemical Use in Pipeline for Waxy Crude
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