Subsea shut down valve

[maew]

I have some problem about breaking design pressure of the subsea gas pipeline. Our pipeline length is 600 km long, inlet and outlet pressure is 2500 and 700 psig. As you know, we don't want to design all 600 km pipeline with the design pressure of 2500 psig. So we need some equipment to break the design pressure of the pipeline. One method is to use subsea shutdown valve with umbilical to the near existing platform. However, this method have some disadvantage if the SSDV is malfunction so we need diver to open the valve and this mean the long period of interruption. So, can anybody suggest me for this issue?
Thank you very much.

 

[SooCS]

Since the pipeline is near the platform, why not use surface SDV. You will need two level of protection one SDV and another PSV.

 

[roscoe]

Maybe I don't understand the question correctly, but I don't understand the need for a valve on your pipeline. What is the purpose of the valve?
Why not just break up the pipeline wall thickness design based on the actual pressure it will see along say 100 km intervals. At the interface joint between the thicker wall and thinner wall joints, I believe the welding specs such as API 1104 allow you to weld pipes of dissimilar thicknesses. If not you could fabricate a transition piece. This seem like less cost and less maintenance than a subsea valve.

 

[DrillerNic]

subsea ESDs are a legal requirement in the North Sea after Piper Alpha, so why not find out the reliabilty of susbsea ESDs first from UKOOA?

Without a vlave in the line, you'll have to work out how you're going to hydrotest eh line, and also, having different design pressures along the line may cause problems in the upset, line pack condition.

 

[StephenA]

Just to reply to roscoe.

I presume that the reduction in pressure is due to friction losses along the 600kms

If there is no valve the design pressure of 2500psi occurs throughout the pipeline if the flow stops.

Hence the need to protect the remainder of the line.

StephenA

 

[sailoday28]

Dependent upon the flow velocity, even a slow closing valve in a long line can lead to sever "gas" hammer effects. Is this accounted for?

 

[DrillerNic]

I don't think you can get dynamic surge (water hammer) effects in a gas line as gas is compressible....

 

[sailoday28]

Hammer, pressure surge, will occur when flow is changed. For simplification purposes, The severity of the hammer is strongly dependent upon the product of density*sound speed*change in velocity. AND how sudden the change in velociy occurs. In very LONG pipes, and for the moment, neglicting friction, even what seems to be a slow closing valve, LARGE SURGES can occur.