10 replies to this topic

[dnrakesh2006]

Dear All,

 

We have offshore Gas + Oil production facility please refer to the attached excel sheets for a brief sketch.

 

there are typical 9 production wells and joining to the common production and test header as shown in schematic.

 

down stream of choke valve is designed for 70barg. each flow line is protected by PSH,PSL and PSV and a check valve.

 

PSV, PSH and PSL are not installed on Production and Test header.

 

but as per API 14C (Table B-1, SAC, A-3), we need to have PSH,PSL and a PSV for protection of Headers.

 

can we take credit of PSH, PSL and PSV which is already present on individual flow lines?  ( my worry is because of check valve on individual flow lines as shown in sketch will not allow to take credit)

 

Thanks in advance for valuable suggestions.

 

Regards

 

 Equipment_Drawings for Chere.xls   97KB   115 downloads

 

[ankur2061]

dnrakesh2006,

 

I am not going to dwell on what API 14C has written. I will only go by proper engineering logic. As per your sketch, the flow lines are designed for: 70 barg and 120°C. The production and test header are downstream of the flow lines and they are also have a design rating of 70 barg and 120°C.  

 

The above configuration means that your production system downstream of the choke valve and flowlines is "fully rated". If you are protecting the system (read flowlines) immediately downstream of the choke valve by instrumented protective system (PAHH) and a safety relief valve then it is obvious that the production header and the test header don't need any separate protection from high pressure due to the following reasons:

 

A. The production and test header is downstream of the flow lines (which are already protected) and the only source of flow to them is from the upstream flowlines as per the attached sketch.

 

B. The production and test header is "fully rated" i.e. the design conditions of the production and test header are the same as the upstream system as per the attached sketch.

 

This engineering logic is enough to justify that you don't need any instrumented protection or safety relief valve for the production and test header.

 

Hope this helps.

 

Regards,

Ankur.

[dnrakesh2006]

Thanks Ankur for the valuable input, I have few questions as mentioned below, please help me if my understanding is wrong,

 

1. during low low pressure case on production header and test header ( which can occur due to pipe rupture ) we need to close choke valve , master valve to stop the further spillage of hydrocarbon, since we have check valve between flow line and production header, because of this PALL on flow line will not sense the low low pressure on headers. may because of this we need a separate PALL on headers. 

 

2. during choke valve failure, pressure will increase in flow line, production and test header, PAHH can sense pressure on flow lines only because of check valve it will not sense, what is the pressure increase in the test and production headers. we may need a separate PAHH on test and production header during this case.

 

3. PSV on flow line will relive the fluid between check valve and choke valve. how to relive the fluid which is trapped in the production header? I think both flow line and headers can get pressurize during choke valve failure at same. may be first flow line will get pressurize and then header but time difference may be very little. we may need a PSV on headers to relive the trapped fluid between check valve and production headers.  overpressure in the system( flow line test and production header ) can also occur due to closer of  SDV on production line ( sorry this SDV I dint showed in the sketch)

 

please correct if my understanding is wrong.

 

thanks for the help and valuable input.

 

Regards

Rakesh

[curious_cat]

Rakesh:

 

Out of curiosity, can you say more about the valve you have shown immediately downstream of the check valve? In what situations can this be in a closed state?

[dnrakesh2006]

I just modified the sketch and attached to show SDV and check valve on production header.  the valve down stream of check valve ( on production header ) may close due to mechanical failure or instrument air failure.  please let me know if any additional information requires. 

Attached Files

•  Equipment_Drawings for Chere Rev01.xls   127.5KB   47 downloads

[ankur2061]

Rakesh,

 

Some questions from my side:

 

1. Why have you considered rupture as a selective scenario for the production header & test header? The flow is from the flow lines to the production and test header which has a higher pressure than the production/ test header. In my opinion it is difficult to justify a rupture only for the production / test header considering that the system is "fully rated". 

 

2.  Even if choke valve fails in an open position the highest pressure that will be seen would be on the flowline and not on the production and test header. To repeat, the system is "fully rated" downstream of the choke valve and you already have PAHH & PALL on the flow line, which makes it difficult to justify a PAHH / PALL on the production / test header.

 

3. Now that you have revised your sketch by showing another SDV on the production line you may require a "Thermal Expansion Relief Valve" (TRV / TERV) on the pipeline section isolated by two SDVs to cater for the thermal expansion of the trapped liquid between the isolated pipeline section. This TRV / TERV would be required only if your system is liquid-filled. If the flow is 2-phase or less than 95% liquid filled even a TRV / TERV may not be required. A detailed analysis would be required to ascertain the requirement of a TRV / TERV. Refer section 5.14.4 of API STD 521.  

 

Having presented my justification for not having a PAHH / PALL on the production / test header if you still have enough money to spend around you can go ahead and provide a PAHH / PALL on the production / test header.

 

Regards,

Ankur.      

[dnrakesh2006]

Thanks Ankur for your Valuable Input and time,

 

1. My thinking was PALL is required  to give alarm and ESD action when ever there is low low pressure in the system which may happen due to pipe rupture, flange leakage or it may be due to some other reason,  so that we can separate the system from source and stop further spillage of hydrocarbons. 

due of check valve presence between flow lines and headers PALL on flow line will not help to detect low pressure in production header and Test headers. 

 

2. I fully agree with you that flow lines will see the highest pressure during choke valve failure, mean time my thinking was even production header and test header can also see the almost same pressure may be lightly less than flow lines ( because flow lines are of only 15 m length), but because of check valve on flow lines we will not be able to see what is the pressure increase in the headers with the help of PAHH on flow lines.  we have PSV set at 70 barg on flow lines which helps to relive the fluid between choke valve and check valve on flow line. my concern was how to relive the fluid between check valve and SDV on production header if Headers are also reaches 70 barg along with flow line before PSV opens. 

 

3. PSV for thermal expansion is not required because of GOR is 25. PSV may be required during choke valve failure or blocked outlet case.

  

I raised this question to find out what is right method because In 2 of my previous projects ( offshore Plants) we considered check valve as restriction and provided the PAHH,PALL and PSV.  

 

this time I was doing another project for different client and I was referring to their FEED and other platform project that have the system which I showed in the sketch.

 

We are doing EPC job and  if we don't provide PALL, PAHH and PSV it is will be profit us ( and even client is not asking for it ) and I was just trying to find out which one is the right method to protect the system.

 

Thanks and Regards

Rakesh

[pankajranga]

Dear Rakesh,

 

As per the schematic provided by you,

 

If the system is fully rated, as stated by Ankur, there is no further need of protection on production/test header, as the PSHH and PSLL are provided on flow lines.

In addition you may also provide H and L alarm to alert the operator if the operation is taking place from DSC.

 

The requirement of PSHH and PSLL is not justified on the production header; however it can be provided if cost is not a constraint.

 

I am also attaching a modified schematic for the ESD valve that should be present on main pipeline with the LL pressure trip for pipeline rapture case.

 

 

The feedback provided is based on the various projects I have done; however, as you know, process engineering is very vast and our experienced seniors can always bestow the experienced they gained from their seniors.

 

 

New suggestions are always well come.

 

Regards

 

Pankaj 

Attached Files

•  Equipment_Drawings for Chere Rev01.xls   138.5KB   46 downloads

[adammusthafa]

Hi,

 

Assuming that all wells are similar:

 

1. I do not understand why the flowline is considered as fully rated? The WHSIP of the well is 450 barg, while the design pressure of the flow line is 70 barg. I think the flow line and downstream equipment is not fully rated and that is why PSV is installed.

 

2. Separate PALL is not required on the header as if the leak is downstream the check valve, the flowline pressure will also be reduced (unless, there are some flowline that has not protected by PALL sensors). It will be different story if you install PALL sensor only at the header. Any leak at flowline may not be detected as the check valve will try to maintain the header pressure correspond to the pipeline pressure as the PALL is installed downstream the check valve.

 

Thanks

[dnrakesh2006]

Hi,

 

Assuming that all wells are similar:

 

1. I do not understand why the flowline is considered as fully rated? The WHSIP of the well is 450 barg, while the design pressure of the flow line is 70 barg. I think the flow line and downstream equipment is not fully rated and that is why PSV is installed.

 

2. Separate PALL is not required on the header as if the leak is downstream the check valve, the flowline pressure will also be reduced (unless, there are some flowline that has not protected by PALL sensors). It will be different story if you install PALL sensor only at the header. Any leak at flowline may not be detected as the check valve will try to maintain the header pressure correspond to the pipeline pressure as the PALL is installed downstream the check valve.

 

Thanks

Thanks for your time and input. this is a very useful input and a good learning for me. I would like to  thank you all for their valuable inputs.

[jojeecares]

Dear Rakesh

 

My observation are:

 

1- How is the reservoir pressure lower than the wellhead shut in pressure. This seems really odd to me?

2- If you are going to consider choke valve to be stuck at fully open position, then your system is not fully rated and you are taking margin only by installing PSV.

3- My observation is that you dont need to install a low alarm switch at main header as any rupture/leakage would automatically lead to flow line shutdown or well closure.