The objective is to investigate and analyze energy saving and process optimization opportunities in upstream surface facilities, from downhole all the way to the gas-oil separation plants (GOSPs), using value Methodology. Function analysis was used to identify those functions that can be reduced, eliminated, or synergized, to minimize GOSP operating and maintenance cost. 

In this article, various energy saving and process optimization opportunities in GOSPs were brainstormed, analyzed, shortlisted, simulated, and validated using actual plant data. Process simulation using Hysys was used to model and verify the feasibility of different process optimization opportunities in GOSPs. A 300 MBD production facility was used to benchmark the Hysys simulation model, and to verify the feasibility of these promising energy saving opportunities. All of the successful opportunities were selected, based on their minimum OPEX and CAPEX, using value engineering methodology.  

Introduction

In this paper, a gas-oil separation plant (GOSP) was optimized using value methodology concepts. Value engineering methodology is an important tool for every engineer, to use to optimize plant operation or project designs. Value methodology, also known as value analysis and value management, was originated by Lawrence D. Miles in 1940, while working at the purchasing department at General Electric (GE). Value methodology started when GE faced a shortage of strategic materials, and appointed Larry Miles to identify new materials to substitute for the existing ones, to reduce the overall cost. The alternative materials/options were selected if they provided the same, better, or best function at the least cost.

Value methodology has been implemented successfully in new projects to save costs, but rarely implemented in oil and gas operating facilities to minimize operating costs. The success of value methodology on reducing the capital project costs prompted the team to explore value methodology’s application to operating facilities, to minimize the operating and maintenance cost. Value methodology is used when the value for money is a concern and needs optimization. Value is defined as the ratio of function to cost. The value engineering methodology consists of a 6-step process as follows:

Figure 1: Value methodology major phases

1. Information Phase: During this phase, all information about the project or the plant, including the cost, is collected, reviewed, and analyzed. Also, the current conditions of the project are reviewed and defined, and the goals of the study are identified. These goals include: objectives, criteria, codes, mandates, program, budget, schedule, and cost estimate.
2. Function analysis phase: Function analysis is the core of value methodology, and defines the main function of the product or project in a simple format.
3. Creativity Phase: Generate the largest number of innovative ideas (brainstorming) without being judged or being controlled by standards and best practices.
4. Evaluation Phase: Evaluate the ideas generated from the creativity phase, and eliminate impractical ones, and select the most profitable and achievable improvement idea.
5. Development Phase: Develop further the selected best ideas from the evaluation phase and estimate the cost. The development phase includes cost benefit analyses, drawings, implementation steps, and responsibilities.
6. Presentation Phase: Present the selected ideas to the decision-makers and stakeholders for final approval.

Case Studies

Several energy saving opportunities were analyzed to highlight the need to continuously check the function of each equipment item and process in any operating facility. Many of the functions were found to be no longer needed, needlessly costing the plant. In some cases, these unnecessary functions were adversely affecting the plant operation. 

Case Study-1: Eliminating the Function of RO Reject Water

Plant-A is designed to divert the Reverse Osmosis (RO) 1st pass reject water to the water-oil separation plant (WOSEP). The objective of diverting the RO reject water to the WOSEP was to provide enough flow to operate the WOSEP and water disposal pumps during the early life of the project (low water cut). As the water cut increase went beyond 6%, there was no need to continue sending this clean water to the WOSEP. The main function of the RO reject flow was no longer needed. Function analysis was used to analyze the water injection and WOSEP systems, to come up with an alternative profitable use for the RO reject water. This reject water can be used for reservoir pressure maintenance, instead of being sent to the disposal reservoir. Disposing the RO reject water cost the plant 1.5 MW of electricity consumption, which could be saved if we reuse the RO reject water as power water injection for reservoir pressure maintenance. 

Figure 2: Eliminating the function of the RO reject water

Functional analysis in the value engineering methodology was used to assess the current need for disposing the RO reject flow to the WOSEP. The function analysis indicated that there is no need to continue disposing the RO reject water to the WOSEP, as the water cut is currently at 16-20%, which is enough to run the disposal water pumps and WOSEP. Functional analysis suggested utilizing this RO reject water for reservoir pressure maintenance. This resulted in an energy saving of 1.5 MW in addition to ground water conservation.

Total Power saving in Disposal Pumps
Total Power Saving, kw	1,515

 

Case Study-2: De-staging Crude Charge Pumps 

The capacity of the Crude Charge Pumps at Plant-A, as shown in Figure 3, was based on the total dry crude production capacity of 525 MBOD, with 12.5% water cut and 10% design margin. This resulted in an oversized pump (10,200 US gpm for each of the 3 x 50% pumps). Additionally, there is over-sizing in terms of differential head rating of the pumps. There is 54 psi extra differential pressure at the discharge of the pumps. This over-sizing was the result of higher pressure drop allowances for the three mixing valves (PDV-1, PDV-2, and PDV-3e) used to mix water with the crude to create a loose emulsion for the efficient crude dehydration and desalting. Initially, a pressure drop allowance of about 30 psi for each of the three mixing valves was used to size the crude charge pumps. Subsequently, a better-quality mixing valve with a maximum of 12 psi differential pressure was selected and installed after the pumps were sized and procured. This leaves the pumps with an  extra head of 3* (30-12)=3*18=54psi, which could be saved, and the unnecessary energy used, to pump the fluid to higher pressure than required, can be conserved.

Figure 3: GOSP-A process flow diagram with the crude charge pumps

Because of oversizing, the pressure in the system always exceeds the off-spec controller’s pressure setting. Therefore the off-spec valve in the wet crude handling unit opens continuously to recycle pumped crude back to the degassing tanks. This leads to the following:

1. Higher desalters wash water/demulsifier consumption.
2. Higher pumping energy consumption.
3. Higher electric current in dehydrators and desalters.
4. Higher steam consumption in preheating the crude and in the crude stabilizer.

	Flow, gpm	Head, ft	efficiency	BHP Power
Shutoff	0	1097	0	1800
Min Thermal Stable Flow	4400	1020	52.2	1990
Rated Flow	10232	754	80.4	2191.3
120% Rated	12278.4	605	78.3	2167
End of Curve	15500	330	75

Table 1: Crude Charge Pumps Design Conditions. The pump is designed to handle 120% of its rated capacity

Function Analysis of the multistage pumps. 

Each of the crude charge pumps consists of five stages with a total differential pressure of 295 psi. Function analysis of the 5-stages indicated the possibility of removing one of these stages, without affecting the overall plant operation and the minimum pressure requirement of 120 psig at the 2^nd stage Desalters. De-staging will have more impact on reducing the pressure and less/no effect on the flow. De-staging provided more flexibility of returning to the original design if needed in the future, but will provide less energy saving than the trimming, as shown below;

	No modification  5 stages in service	One Stage out (spare when needed) 4 stages in service/1 dummy
Fluid Specific Gravity (Sp.Gr)	0.903	0.903
Pump Rated Capacity,  gpm	10232	10232
Existing Pump Rated Head , feet	754	603
Existing Pump Rated Head , psi	295	236
Hydraulic horsepower, HHP	1759	1407
Pump efficiency, %	0.8	0.8
Brake horsepower, BHP	2199	1759
Motor efficiency, %	95	95
% saving	0 %	20 %
Total power consumption, KW	1,726	1,380
Energy Saving, kw	–	346

Table 2 De-staging of the crude charge pumps from five to four stages

Removing the extra DP of 54 psi will mean that the DP of about 241 psi would have been adequate. This is equivalent to removal of one of the five stages of each pump to conserve energy. De-staging the three crude charge pumps (2 running + 1 spare) will provide an energy saving of about 346 kW. In summary, the  function analysis lead to de-staging the pumps while maintaining the plant’s flexibility and saving electricity.

Case Study 3: Replacing the function of Disposal Water System to power water injection

In this case study, the Function Analysis will be used to sysnergize between the Water Injection Plant and the disposal water treatment and injection plant. GOSP-B is located north of Dhahran and have been designed to process Arabian Light and medium crude. GOSP-B producing facilities depend on ground water injection to maintain the oil reservoir pressure, but there is abundant produced/disposal water being disposed at high cost. Also, additional capital investment is required because the total disposal water will exceed the current installed disposal pumps capacity, and will double for GOSP-B facilities. Consequently, additional pumps, injection flanks, injection wells, and WOSEP vessels are required to meet the production forecast.  

Process Description of Existing Set-Up

The existing Water Oil Separation Plant (WOSEP) is designed to separate oil from the produced water coming from GOSP-B to a maximum oil-in-water specification of 50 ppm.The system consists of two vessels with total design capacity of 440 MBWD and four salt water disposal pumps (100,000 Barrel/day each) as shown in Figure 4. The current operating capacity is less than 200,000 Barrel/day and it is expected to reach 744, 000 Barrel/day in the future. The oil-in-water content to the WOSEPs is expected to be 1,000-5,000 vol ppm during normal operation.  The WOSEP has been designed for maximum oil in water content of 50,000 ppm. The separated oil is returned to Qatif Low Pressure De-gassing Tanks (LPDTs). Four salt water disposal pumps, are injecting formation (Produced) water into the wells with a discharge pressure exceeding 3,000 psig. The Salt Water Injection pumps (SWIP) have been designed to reinject formation water into a disposal reservoir. The WOSEP has been designed to operate at 84.7 psia to provide sufficient Net Positive Suction Head available (NPSHa) for the disposal water injection pumps. Operating temperatures ranges between 114 °F in the winter and 149 °F in the summer. The goal of the Water Injection Plant (WIP) is to inject the aquifer water into the oil reservoirs to maintain the reservoir pressure. Five pumps operate simultaneously in parallel, injecting aquifer water into reservoir to maintain pressure.

Figure 4: GOSB-B Process Flow Diagram after eliminating the function of disposal water

Function analysis of disposal water and power water injection:

Function analysis of disposal and power water injection indicated the feasibility of treating the disposal water and using it for power water injection. This will conserve both water and power. The produced or disposal water at GOSP-B is an untapped resource with great potential for use to maintain pressure in the oil reservoirs. This formation water, separated from LPPT, HPPT, LPDT, dehydrators, desalters, and other separation units in the GOSP-B plant, contains variable amounts of oil contaminant, and suspended solids. 

To conserve water, reduce operating cost, and minimize capital expenditure, installing a treatment package for the disposal water to meet the required oil reservoir injection specifications was found feasible. In this comprehensive study, disposal water samples collected from the WOSEP were characterized for TSS, oil content, and key ions composition. Coreflood tests were carried out on reservoir core plugs at reservoir conditions, to assess the effect of this disposal water on core permeability, and required filtration to improve injectivity. In addition, several water treatment proposals were identified to meet the reservoir injection specifications.

1. Maximum Oil in Water Content to be less than 20 ppm (wt.). 

2. Maximum Total Suspended Solids (TSS) to be less than 30 ppm. 

3. Mean particle size of suspended solid shall not exceed 5 micron. 

After finalizing the maximum suspended solids and oil content specifications that will not adversely affect the injectivity, different technologies were evaluated to achieve these specifications. An extensive survey of leading engineering/licensing companies specialized in water treatment technologies was completed, to determine the optimum proposal for treating GOSP-B disposal water. Five technology alternatives were generated to achieve the following:

1. Minimum capital cost.
2. Minimum operating cost.
3. Performance guarantee to meet the product specifications.
4. Minimum number of equipment and less spacing requirements.
5. More reliable and less complicated process.

Technology Alternatives:

Alternative-1: The system proposed consists of a Desanding Hydrocyclone (DSHC) followed by De-Oiler Hydrocyclone. Desanding and Deoiling Hydrocyclones is used to remove suspended solid particles and oil droplets. 

Option-2: The system proposed will consist of a DSHC to remove bulk suspended solids followed by dissolved or induced gas flotation to meet the oil specifications. Fine DSHC is used as finishing step to meet the tight TSS specifications. The Fine Desanders act as the second stage of cyclone solids removal. 

Option-3: The system proposed will consist of a DSHC to remove bulk suspended solids followed by dissolved or induced gas flotation to meet the oil specifications. Automatic Backwashable filter is used as finishing step to meet the tight TSS specifications of less than 20 ppm. Solids are diverted to the solid treatment package for concentration and shipment to outside disposing facilities. 

Option-4: The system proposed will consist of full upgraded WOSEP internals to help preserve and enhance the performance of the units as water rates increase. Oil in water can reach as low as 10 ppm by the WOSEP full upgrade, therefore there will not be a need to install the hydraulically induced gas flotation (HIGF) as separate unit. Simplified IGF will be installed inside the existing WOSEP. Also, additional baffles and coalescing packing will be installed. Upgraded WOSEP is followed by DSHC to remove bulk suspended solids. Fine Desanders Hydrocyclone or automatic Backwashable filter are used as finishing step to meet the tight TSS specifications of less than 30 ppm. 

Option-5: This alternative is the same as option-4, but with an enhanced DSHC to avoid installing a fine sand Hydrocyclone or Backwashable filter to meet the suspended solids specifications of less than 20 ppm. 

Option-6: This option strives to reduce the number of units in series used to treat the disposal water. Upon exiting the WOSEP, the produced water is routed to DSHC packages at a pressure of 84.7 psia. Each package consists of a single Desanders vessel containing multiple Desanders cyclones combined with a sand accumulator. The Desanders reduces the total suspended solids from a concentration of 250 mg/l to <30 mg/l. On leaving the Desander, the produced water flows into Induced Gas Floatation (IGF) where the 98% particles <5 microns are removed and the maximum free (dispersed, non-dissolved, non-emulsified) oil-in-water reduced to <20 ppm from a maximum oil-in-water content of 70 ppm at the package feed. Solids are diverted from the DSHC to the solid treatment package for concentration and shipment to outside disposing facilities. 

Cost Benefit Analysis

By reusing the GOSP-B disposal water as power water injection, there will be no need for installing new Water injection plant (WIP) or Disposal pumps to meet the required future forecast. Also, aquifer water will not be required as power water injection. A detailed cost benefit analysis was conducted, and by reusing GOSP-B disposal water for power water injection, the following benefits can be achieved:

• Power cost saving of up to 36 MW.
  • Ground water conservation
  • Improving the Plant’s availability and avoiding over pressurization of the disposal reservoir.
  • Capital cost avoidance of installing six additional pumps.
  • Cost avoidance of drilling ten additional disposal wells.

In conclusion, value methodology revealed that installing a new disposal water treatment plant to meet the maximum injection specifications was a viable option and cheaper than the current practice.

Case Study-4: Elimination the function of 1st Stage Desalter Wash Water Pumps:  

In GOSP-B, the original design setup recycles 250 to 460 gpm effluent water from the 1^st stage desalter to the dehydrator as shown in the figure below. This setup was beneficial during the early life of low water cut where the water cut is below 1%, and the minimum of 5% of water was required to efficiently operate the dehydrator.  Function analysis indicated there is no longer a need to continue recycling the desalter effluent water back to the dehydrator, as the water cut exceeds 28%. Therefore, the function of the 1st stage desalter recycle pump is no longer needed.

Figure 5: Function of the 1^st stage Desalter Recycle Pumps

Expected Benefits:

The expected benefits from applying function analysis include:

1. Defer reaching the dehydrator maximum water cut limit of 30%.
2. Avoid worsening the emulsion formation at the inlet mixing valve to the dehydrator, by fully opening the dehydrator inlet mixing valve.
3. Enhance the dehydrator operation and water separation efficiency, by increasing the density difference between the oil and formation water, as per Stoke’s Law.
4. Shutdown the first stage desalter recycle pumps and consequently conserve power and operating cost.

In summary, function analysis in value methodology has great potential to challenge the existing operation practices of the gas-oil separation plants (GOSPs) to minimize the operating/maintenance cost while complying with the product specifications.

References

1. SAVE International: http://www.value-eng.org/
2. Mohamed Soliman, Innovative Approach to Treat Produced Water for Re-Use in Saudi Aramco Reservoirs Pressure Maintenance, Abu Dhabi International Petroleum Exhibition & Conference, 7-10 November, Abu Dhabi, UAE, SPE-183340-MS
3. Al-Yousefi, Abdulaziz S., Value Management Concept and Techniques, 4th Ed, 2004.
4. SAVE International. (2015). Value Methodology Standard. Retrieved from https://c.ymcdn.com/sites/value-eng.site-ym.com/resource/resmgr/Standards_Documents/vmstd.pdf