Reassessing Operational Strategies for Wax Management Reduces Expenses
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This paper presents a case study that is an example of how reassessing a flow-assurance risk-management strategy for operating assets can identify opportunities for optimization. Too often, flow-assurance risks are examined only after problems occur. In the absence of these events, many may assume the flow-assurance risks are managed optimally. Periodic reassessment of the risk-management strategies is a key concept for full life-cycle flow-assurance engineering.
Flow-assurance risks for offshore fields can be managed in a variety of ways that can be defined generally into risk-management or risk-avoidance strategies. As offshore developments progress into deeper waters with more-challenging technical issues, more-costly risk-avoidance strategies can jeopardize overall project economics. In contrast, risk-management strategies are considered through means of mechanical, thermal, operational, or chemical adjustments to manage the flow-assurance risk rather than attempting to remove it entirely.
Waxy crude oils are fairly common in offshore operations. As system temperatures drop below the wax appearance temperature (WAT), wax crystals will precipitate and cause deposition issues because of the thermal gradient between the bulk fluid and the pipe wall. A variety of mitigation techniques can reduce the effects of wax deposition on flowlines and pipelines offshore, including insulation, addition of heat, pigging, and the addition of paraffin inhibitors. In the development stages of a project, these risks are quantified by use of fluid properties and additional laboratory testing in order to estimate the required risk level and economic options for risk mitigation. The selected methods for managing the risk are used for first oil and adjusted and optimized while minimizing risk of compromising safe and reliable operations. Given the dependence of flow-assurance risks on various fluid and system properties, these risk-management strategies require periodic reassessment to ensure an optimized approach. This paper will present a case study where reassessment of a flow-assurance risk-management strategy resulted in an optimized approach that generated significant operational-expenditure savings.
An offshore, brownfield development in West Africa has two subsea export pipelines that transport a medium-gravity waxy crude oil to an intermediate platform and eventually onshore. The crude in the export lines is from multiple wells and commingled, with each crude featuring varying amounts of wax, resulting in a combined oil that has a WAT ranging from 70 to 85°F. Ambient water conditions are less than 50°F, resulting in a temperature gradient across the fluid and pipe wall to the seawater. Because of the length of the export pipelines and lack of insulation, the oil cools to temperatures below the WAT well before the export line terminates at the intermediate platform.
The initial risk-management plan involved a combination of pigging the export lines with a poly-flex soft pig every week and using a paraffin inhibitor.
Over time, the amount of chemical required to minimize pig returns slowly increased, which resulted in an effort to assess alternative chemical options. Although no increase in actual loss of production because of wax was seen, the field teams wanted to optimize the chemical to reduce pig return symptoms. The team sampled oil and performed screening tests for additional paraffin-inhibitor chemistries that could demonstrate improved inhibition over the incumbent chemical. The main goal was to reduce pig returns in an effort to run an intelligent pig through the export line for integrity measurements.
Although the original intention of the chemical assessment was to identify a new chemistry for improved inhibition to support intelligent pigging operations, the team capitalized on the opportunity to evaluate the wax-risk-management strategy as a whole.
Recently, the amount of paraffin inhibitor used in the two subsea export lines was increasing as a function of increasing severity of process indicators—namely pig return amounts and texture. Given the past successes and qualification of the paraffin inhibitor, the first step was to increase dosage levels in hopes of reducing the symptoms. Fig. 1 above shows inhibitor concentrations and pig returns for one of the lines, Export Line Alpha, over more than 12 months, with a general increasing trend in dosage level.
Fig. 1 reveals that any increase or decrease in the inhibitor-dosage rate did not correlate with a corresponding decrease or increase in pig returns. The sudden decrease in pig returns around January 2014 corresponded with the addition of a batch application of paraffin-dispersant chemical immediately before each pig run.
Caution should be taken when reviewing the effect of the dispersant on the pig returns. The chemistry of paraffin dispersants is such that they do not prevent wax from precipitating; they act to disperse a paraffin deposit once it has already formed, making it a remedial measure rather than a preventative one. Only paraffin inhibitors are designed to inhibit wax crystal growth and, therefore, reduce deposition rates. The use of the dispersant did not change the fact that increasing levels of the paraffin inhibitor were seemingly ineffective. This suggested a few possible issues: The wax characteristics of the oil had changed or the chemical was no longer effective for the specific waxes in the oil.
The laboratory study—aimed at identifying a new chemical—performed wax-characterization studies in addition to screening new chemicals. The team took the opportunity to attempt a comparison of historical information regarding characteristics such as WAT and wax composition of the oil to identify any changes over time. Because of the infrequency of these analyses, the data was inconclusive in determining whether the waxy oil had changed significantly. However, additional testing in the laboratory report provided valuable information with respect to the performance of the incumbent paraffin inhibitor.
Chemical screening was performed to compare several paraffin inhibitor chemistries alongside the incumbent paraffin inhibitor. Results across a variety of dosage levels indicated that another inhibitor had stronger performance than the incumbent chemical, especially at higher dose rates. However, a more-interesting conclusion from this data is the negligible inhibition of the incumbent chemical at dosage levels even 2–3 times higher than the actual concentration used in the field. If the chemical showed no efficacy in tests at dosage levels at or above field levels, and yet there was no significant increase in lost-production events because of wax, the team determined that a possible path forward would be to operate without a paraffin inhibitor.
To have confidence in a no-inhibitor solution for a waxy crude export line, nonchemical safeguards for managing the wax risk must be implemented reliably. In the case of these export lines, the aggressive pigging schedule (occurring approximately every week) was already providing sufficient protection against significant wax buildup in the lines, given that the chemical was ineffective.
The team created a plan for a field trial of the no-inhibitor option for managing the wax risk in the export pipelines. This involved a slow decrease of the paraffin inhibitor while monitoring the export line pressure drop (a lagging indicator of paraffin deposition), pig return amounts, and hardness. The chemical was decreased over a period of 1 week. Given the past successes with the paraffin dispersant, the team elected to maintain the use of the dispersant before pigging. This mainly served as a safeguard in the event of harder or larger amounts of wax deposits following the chemical decrease. Operations agreed to maintain a strict, once-per-week pigging schedule of the export lines to support the field trial and reduce risk of lost production from wax buildup in the lines. During the field trial, the team collected regular information on the export line pressure drop, pig return volumes, and hardness.
Data from the field trial for Export Line Alpha suggests a steady state was achieved, resulting in a successful field trial. The pressure drop does increase after injection of the inhibitor is stopped—approximately 20% over the course of the first 5 months of the field trial. However, it reaches a steady state and, even more recently, has exhibited a downward trend. After consulting with the team, an agreement was reached to turn off the chemical, and no lasting effect on the pressure drop was seen. The system since has returned to a steady state without the inhibitor with no signs of cumulative buildup in the lines.
As a result of the successful field trial, the offshore field has maintained the no-inhibitor approach to wax management with an aggressive pigging strategy. Overall, the field was able to reduce annual operating expenses (primarily related to the cost of chemical) by more than $5.9 million per year while maintaining reliable and safe operations of both export pipelines.
Reassessing Operational Strategies for Wax Management Reduces Expenses
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