The parameters required to assess the desanders were very clear. A set of questionnaires were sent to various vendors and service providers having desander service for separation of sand with following details:
- Details on the hydrocyclone technology and accumulator depending upon company’s application requirement. The below parameters were considered as one of the essential parameters for practical functionality. References with d50c calculations can be referred for more details on hydrocyclone designing and the main parameters effecting the design.
- Pressure drop for separation of a particular particle size.
- Minimum inlet pressure required for functioning: To achieve a particular operational efficiency, desanders work at a minimum inlet pressure. In simple terms, the pressure / energy provides the rotary motion of hydrocarbon and slurry mixture.
- Gas Separation capability: Due to gas lift nature of the wells and fluctuation in gas rates.
- Automated flushing: Mainly for remote platform and jacket installations.
- Footprint : Due to offshore nature, and desanders being more a retrofit option, so, not much space reserved for any installation therefore footprint compactness was highly desirable.
- Designing and CFD: This was more to assess the designing capability of the service provider.
- Details on additional components of the system i.e. scope of cleaning the sand and haulage as well.
- Details on Company’s experience and capability of company to provide a complete package rather than just the desander system.
A total of 18 Companies participated (responded) in the survey.
After the results of market survey were in place, team was able to get clearer picture on strategizing. In terms of easing out the process of findings and strategizing, the desander system were classified under these categories (A, B and C type) depending on their capability of handling the variation in flow rates for similar efficiency:
- Category A: Static fixed desander: Desanders with fixed liners or inserts and have a fixed envelope of separation around a particular size separation efficiency.
- Category B: Static desander with changeable inserts: Desanders with changeable inserts. Depending on the flow rates, the inserts can be changed manually to handle variation in flow rates.
- Category C: Dynamic Desanders (Motor / regulated flow): cyclone connected to a motor and high speed rotation of the cyclone claims to handle variation in flow rates over large ranges of fluid.
Table-1 show a quick comparison of the manufacturers / principal companies supplying Category of each type and also their respective parameters in general to the case provided.
|Liquid handling ranges||Limited since driven by the well pressure||Limited since driven by the well pressure however the insert / liner can be replaceable||Able to handle bigger range due to motor driven and regulated inlet flow|
|Pressure drop||Ranging from 5-50 psi(pounds per square inch)||Ranging from 15-200 psi||Ranging from 5-40 psi|
|Min inlet pressure||Ranging from 5-100 psi||Ranging from 15-200 psi||Make C1: very minimal Make C2:17psi|
|Gas separation||None of the vendors have the technology except Vendor C but claim would be able to outsource the equipment.||None of the vendors have the technology but claim would be able to outsource the requirement.||Yes Make C1: Not Proven Make C2: Patented|
|Automated flushing system||Prefer to supply only manual flushing system but claim would be able to supply if that is the requirement. * Prove of reliability / proven / patented technology||Yes||C2: Compact footprint. Standard 2*2m|
|Footprint||Standard 2*2*5m||Depends on the flow rates. Typically higher flow rates require bigger vessel||Principal|
|Design & CFD analysis||3rd party / principal||3rd party||Vendor A – 18 to 20 Vendor B - Minimal for flow control only|
|Multi-liners||Not available||Work more efficient for single phase||Not available|
|No. of principals||6||7||2|
It was also observed that within the Categories especially A and B categories, there are also variations of technologies as follows:
- Variation in design of the cones: One of the supplier provided hydrocyclone with sharper upper cones followed by longer angled lower cones. This combination maximizes tangential velocity in the upper part of the cyclone. It then provides a long residence time in the critical separation zones in the lower part of the cyclone. This results in a substantially finer separation with fewer fines in the underflow.
- Multi-liner systems: Concept of multi-liners being used to handle large variation of fluid rates for a single phase separation system (especially for separation of finer particles) was also observed. Such system, though have high pressure drop but can separate very fine size particles and are well apt for cases like downstream of separator wherein gas has already been separated.
- One of the desanders in C category used a motorized unit to rotate the cyclone. The other system in Category C used special technology of varying the inlet flow area to handle separation across variety of hydrocarbon rate range with same efficiency.
Hydrocyclone related topics
- Cyclonic Technology
- Design of Hydrocyclone for Drilling Solid Separation
- Desanders, Desilters And Hydrocyclones Arrangements
- Hydrocyclone Desander
- Hydrocyclone （Mud Cleaner, desander, desilter）Troubleshooting
- Why Desander Desilter (Mud Cleaners) Important In Solids Control
Mitigation of Challenges / Gaps
After a complete understanding of the challenges in past and also the status of technology available in market, the team was able to make effective decisions and planning on how to mitigate the challenges in future. Table-2 summarizes how each and every gap or challenges with reference to desander selection were proposed to be tackled.
|Challenge / Gap||Mitigation Plan and Revised strategy for new scope|
|Gas Fluctuations||GLCC / Flow stabilizer|
|No defined process||ISM is updated with a process flow for desander selection|
|Pressure drop||verify the allowable pressure drop with Integrated Network Modelling|
|Mainly Category A being used||To shift the methodology of hydrocyclone more on having a large envelope of flow rates|
|Ad-hoc solution||Having contract for desander where only technically assessed and qualified vendors selected|
|No focal discipline||Involvement of CST and RST|
|Minimal Performance Monitoring||Enhancement on contracting strategy|
|No long term contract||Establishment of a contract with various vendor under one umbrella considering all the lessons learnt from earlier gaps incorporating a comprehensive work scope|
Though the table above briefly and clearly explains the challenges / gaps identified and their mitigations suggested, but it is worthwhile to detail few of the in more detail one of the main technical gap during the assessment of past installations, was a proper systematic process for the selection of desander in a particular environment or conditions.
As stated earlier, this was one of the main reasons for several desanders not functioning as desired. The desander core team within CST came up with an improved flowchart for selection of the desanders. The flowchart constructed was more on the basis of practical challenges being faced within the Company on improvement of process. Users for this flowchart can view it as a reference, but should modify depending on the needs for a particular company or objective. The flowchart not only highlighted and decide on the type of desander being used but also an overall location of the desander.
In addition, the team came up with all the important parameters that would affect the desander design and selection and tabulated into “Integrated Sand Management” guidelines of the Company. With a proper process of selection in form of flowchart and an idea to the user on the important parameters to be considered, the technical process for the engineer, whether production technologist, process engineer or even operations engineer.
Such a guideline, in general, narrows the gap between various disciplines, clearly stated their responsibility and help them to align in a more structured way. The challenge of having fixed static desanders resulted in many desanders being out of envelope and therefore the mitigation was to preferably use the desanders wherein hydrocyclone comes with changing inserts.
Another main challenge was no focused or responsibility for any particular discipline or team for the entire installation. This mitigation was clearly mitigated with CST taking efforts and onus in not only analysing the earlier gaps but also coming up with the final deliverables of a proper contract.
In terms of variation in technology and with more than 10 principals available with some uniqueness in their system, it was apparent that not one system can suit all the environment as best fit. Depending on the location, gas conditions, pressure drop requirement and even robustness of the liner itself every condition or field might have different installation preference. This was one of the key drivers to establish a contract with multiple bidders rather than competitive bid contract (detailed in next section).
Not only this, team also came up with a table and set of parameters for the first pass of the desander technology for any cases. Table – 3 below shows an example of one of the technical evaluation of various technologies for a particular field. Please note that table-2 is an example and the main highlight here are the set of parameters / criteria to be considered during technical evaluation.
|1||Type of desander||single liner||single liner||single liner||single liner||single liner||single liner||single liner|
|2||Desander need to work within the liquid rates||12 – 20 kblpd||Yes||Yes||Yes||Yes||Yes||Yes||Yes|
|4||Desander need to work within the gas rates||3 – 5.2 MMscfd||Yes||Yes||Yes||Yes||Yes||Yes||No|
|5||Smallest particle size separation||20 micron||Yes||Yes||Yes||Yes||Yes||Yes||Yes|
|6||Desander need to work at min inlet pressure||100 – 120 psi||Yes||Yes||Yes||Yes||Yes||Yes||Yes|
|7||Desander need to have min pressure drop||< 30 psi||33||14.5||20||20||50||26||15|
|8||Pressure drop monitoring device||Yes||Yes||Yes||Yes||Yes||Yes||Yes|
|9||Experience in production||Yes / No||Yes||Yes||Yes||Yes||Yes||Yes||N/A|
|10||Size within the available deck space||4.5m × 5m||2.1 × 1.2 × 5.4||2.5 × 2.5 × 7.2||2.5 × 2 × 4||2 × 2 × 7||2 × 2.5 × 3.5||3 × 3 × 7.3||2 × 2 × 2.5|
|11||Accumulator capacity||500kg/ day @ 0.2083 m3||1300kg||500kg||500kg||500kg||1000kg||500kg||500kg|
|13||Ability of supplying the flow stabilizer or degasser||Mandatory||NR||Yes||Yes||Yes||Yes||NR||Yes|
|15||Weightage||crane capacity 5MT||6||12||8||10||8.5||17.5||N/R|