flexible riser stiffness?

[sun5454]

hi everybody

i am looking into possible high pressure (about 4000psi) flexible riser. 200m long . possibly 18 inches ID . the riser connected to Submerged Turret Loading (STL).

When high pressure gas passes through the hose , it will try to straighten , i.e. like the fire hose . How can we determine whether the riser when such high pressure is applied will or will not straighten and pop up the STL .

Thank you

 

[BigInch]

pressure creates a closed end axial tension stress = Pressure * inside cross-sectional area of pipe / cross-sectional area of pipe material.

Bending stress = M * c/I
Moment M
Distance from neutral axis to outside wall of pipe
Moment of intertia of pipe I

Total axial stress = tension stress +/- bending stress
Strain = stress * E, Young's modulus.
You will have a stress on one side of the riser equaling axial stress + bending stress
On the other side of the riser, stress = axial stress - bending stress

Calculate the stresses at every point along the riser
Calculate the resulting strains
Sum the strains on one side of the riser and that is the change in length of that side of the riser
Sum the strains on the other side of the riser and that is the change in length of that side of the riser
Assume pipe ends remain square 90º
From the difference in lengths, find the change in shape of the riser, assuming curvature must be such that all sections through the pipeline remain square with the pipe outside diameter at each differential element.

"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics

http://virtualpipeline.spaces.msn.com

 

[sun5454]

Thanks for the info biginch .

I understand the axial tension stress part .

1. In my calculation with 16 inches ID and 4800 psi i get 437t load acting on a closed end (i.e. Valve). Dose this mean If I dont have a buoy much heavier than that the buoy will pop out of water.

2. If the risers is conneted to a FPSO (displ 13000t), and the valve is closed with 4800 psi (i.e.437 t of load) will the fpso move . (i.e. like a tug pushing)

2. bending stress-how can we get bending moment and bending stress in a flexible hose (i.e.The flexible hose from seabed to STL)and also how can we estimate the the youngs modulus of syntheic hose with steel reinforcement.

 

[BigInch]

Buoy size depends only on its submerged volume and the riser's weight in water being supported by that buoy. If either one of them plus any weights they support weigh less than the water they displace, they could surface.

The closed end force on one end of the riser is counteracted by and equal and opposite closed end force at the other end of the riser; these only create a tension stress in the riser itself.

I suppose that theoretically the FPSO could move, if everything was just right. I haven't thought too much about that though. The FPSO might deflect the riser too. Any wind or current loads the FPSO has at the time will must tend to pull the end of the riser connecting to it.

I would think you want to have enough unsupported length of riser such that it will still have bending deflection and not straighten out and try to move the FPSO. In other words, any change in arc length of the riser will not change the total horizontal displacement of the beginning and ending point of the riser (in the global coordinate system), it will just increase the curvature of the riser.

I would approach the problem from a composite materials point of view, such as reinforcement steel in concrete. We need something like an effective modulus of a composite material. If the strains are similar, which they should be in order to prevent one material from disbonding from the other, the cross-sectional area of each should be proportioned to the total cross-sectional area of both and according to their individual moduluii of elasticitities to give a stress in each such that strains are equal. Can you estimate an "effective modulus" on that basis?

If that doesn't work, and/or you can't get an allowable strain from the manufacturer, you might have to test some of the composite material. Young's modulus might be estimated by measuring the force to stretch it a certain amount and divide that by the amount it stretches. Stress * Length/E = strain, so solve for an average or effective E.

can you post a configuration of this riser buoy and FPSO arangement? I'd like to see what I'm trying to talk about here.



"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics

http://virtualpipeline.spaces.msn.com