Riser - Bouyancy Joint External Pressure

[ExRoughneck]

Hi,

Joined a new company a couple of months ago which had order a Low Pressure Riser. The given data for riser buoyancy joint (from manufacture) was that it would still have a negative uplift of 600kg. However, after my own calculation I got that it would have a uplift of 3.5MT. Which was confirmed by physical testing in water. Given the calculation mistake on the buoyancy joint I if the buoyancy joint can take the pressure. It is supposed to be submerged to 100m.

Running through Inventor simulation I am getting that the SF of 1.6, however I dont believe inventor is taking into account elastic or plastic collapse, only Von mises. Which I believe(after checking forum here,etc) would most like be the likely the scenario for thin wall pipe/cylinder with external pressure would fail.

I have done the following assumption for my;

1. neglected polyurethane foam in the buoyancy tank. Even tough the foam can take some pressure compression. I do not know the quality of the foam nor the application of the foam. therefore I have consider the buoyancy tank as filled with air.
2. I have not calculated the complete tank as full. I have assumed the tank as size of the biggest length of the ring stiffeners.
3. Tank consist of ABS material E=30*10^6
4. Parameters used. Diameter 1200mm = 47.2in, thickness 6mm = 0.23622 in, length 1243mm = 48.94in


I have then tried to calculated the critical external pressure based on the following.

A Roark formula for thin wall tubes (7th Edition - Table 15.2 - Case 19b)
q'= 0.807 * (Et^2/lr) * 4SQRT((1/(1-v^2))^3(t^2/r^2)
= 0.807 * (30*10^6 psi * 0.23622^2 in^2/ 48.9 in * 23.622 in) * 4SQRT((1/(1-0.3^2))^3(0.23622^2 in^2)/23.622^2 in^2)
= 125 psi

B Batdorf formula in A simplified method of elastic stability analysis for thin cylinder shell
pc = 0.926 * E * RT(gamma) / ((r/t)^2.5 (l/r))
= 0.926 * 30*10^6 psi * 0.75 / ((23.622in /0.23622in)^2.5 (48.9 in / 23.622 in)
= 100.6 psi

C AMSE VIII UG28 Thickness of shell and tubes under external pressure.
Using graph in section 2 subpart 3 part D I get Factor A = 4.2 *0.001 and Factor B = 6250

Pa = 4B/(3Do/t)
= 4 * 6250 /(3 * 47.2in / 0.23622 in )
= 41.7psi

Assuming that the ASME is having a SF of approximate 3 in their graphs and Batdorf is including a reduction factor of 0.75 based on experimental is collapsing on approximately 25% less than theoretical, these are almost the same.

D - DNV OS-F201 Here I get a much lower figure

Pel = (2 * E * (t/D)) / (1 - v^2)
= (2 * 30 *10 ^6psi * (0.23622in/47.2 in )) / (1 - 0.3^2)
= 8.2 psi (I get around similar using API 5C3)


E - API 5C3

Pe = 46.95 * 10^6 / [(D/t)(D/t - 1)^2}
= 46.95 * 10^6 / [(47.2in/0.23622in)(47.2in/0.23622in - 1)^2]
= 5.94 psi


The much lower figures for DNV and API is that due to the D/t range is very high (200) and out of their range?

Is it a far assumption that the riser will have a collaps pressure with external pressure around 125psi without SF? what should be the safety factor for this according to DNV-OS-F201? ALS and normal safety class?

Appreciate any assistance/guidance for the rated external pressure / water depth for this buoyancy joint.

 

[LittleInch]

ON the basis that no one has answered yet, I'm not sure that this outer skin is intended to be water proof or able to resist collapse.

If the material is ABS, I can't see it standing any significant external pressure, but this is where the foam will supply the resistance to collapse.

If this is a sealed system then it may need to be pressurized to resist collapse.

The drawing is good, but if this is a vendor supplied item, you really need to ask them.

It would be good to see where you think they were wrong on the buoyancy.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ExRoughneck]

LittleInch,

Thanks for the reply. The supplier wont answer my questions or regarding pressure rating. The wrong buoyancy I have attached, some simplifications, but are in line with actual floating midway and weight measured with certified crane. Which make me think they might have just put something together with no calculation at all.

The foam is suppose to take between 36psi-52 psi depending on grade. however not sure if the foam is applied correctly.

Yes it is ABS material and sealed.

 

[LittleInch]

To be perhaps fair to the vendor, given that this is seemingly being supplied as a complete unit in 12m lengths, you haven't included the weight of the pipe.

Taking a 20" 19mmwt steel pipe, you get very close to 600kg residual buoyancy (empty pipe). Of course if you remove the buoyancy from the steel pipe then you'll have a far higher buoyancy and the ring will float about half way, but it depends on how you ask the question,

I don't recognize those figure from the foam, but compressive strength of the PU foam they use for these items is really quite high - the density has a lot to do with it and the denser the foam the stronger the foam. Figures in excess of 5Mpa are not uncommon. Surely the vendor has a data sheet or website to get figures from??

The ABS is normally just there to prevent damage and act as a flexible sealing, not a pressure containment.

Ultimately it is up to you to be confident of what the vendor has supplied, but look at the data sheets, look at what he was asked to supply or conform to and what inspection or tests he's provided then decide or do some other tests.

I think you're being a little too worried and need to look iont things a bit more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ExRoughneck]

LittleInch,

Thanks for your reply. The weight of the pipe is included in the calculation and confirmed by weighing with certified crane. The whole joint together with the pipe is floating together midway, which confirm my calculations. Also confirmed by hanging weight below until the whole joint was submerged.

The foam used is Veracor CD 1028 Polyol. I couldn't find anything online so I have sent request to DOW for the compressive Strength, but no reply as of yet. A sister product Vercacor CD 981B have compressive strength of 150kPa.

I agree that you get what you request for, and the PO from my company issued is very poor and dont state anything really regarding standards required. My concerns is that the company build this is not like Aker Solution or Cameron. It is more like a small work shop. In addition bolt were supplied without stamps, drawings are referring to incorrect flanges (number holes dont match with class stated), seals containing asbestos and in addition the riser is not marked with pressure rating(water depth) as per API 16F.

 

[LittleInch]

OK,

I thought weight of buoyancy Joint was the weight of the buoyancy modules, not the metal work. In that instance it does look a little over specced. what's the size and wt of the pipe?

It sounds like you're right to be concerned. I would also like to see how these items are attached to the riser. If the work is in as unregulated a location as you suggest you could easily find 1 or 2 or more floating on the surface after a short time.

Density of the foam has a lot to do with the compressive strength so if they are mixing this stuff on site it could be anything...

Not sure where to go from here, If you have done a submerged test, can you do a submnerged test down to 100m? or 150m? sounds like physical testing is the only real way out of this.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.