Submerged Density/Density of Composite Pipes

[sgdon]

Dear all

From a survey report of a subsea pipeline, i am managed to read off the weight of the pipe in per metre run (say X). This weight has taken account of the various components of the composite pipes (i.e. concrete, steel and anti corrosion coating etc) and the buoyancy.

However, i need the submerged density in kg/m^3 for input into a FE anaysis. How do i go about it. Using X and divide it by either the the steel area (main strucutral component) or the gross area of the composite section seems to yield relatively low density. Is this the right way to do it?

(Submerged weight is approximately 176 kg/m and the steel pipe is 665mm and 26mm thick, with a 50 mm concrete coating.)

thanks
sgdon

 

[DrillerNic]

I'd guess somthing like this:

Density = mass/ volume
Submerged Density = submerged mass/ volume

where the submerged mass is the mass in air multiplied by the seawater bouyancy factor (0.868 according to my Baker Tech Facts book). As a check, make sure it's greater than the density of seawater!

 

[billsumner]

To obtain the submerged density it is necessary to use the outside diameter of the concrete and allow for the density of seawater (usually 1.025 t/m³) and also for the weight of the contents.

I am a bit puzzled regarding the low weight you achieve with the input parameters. From what you provide, and ignoring any contents and coating, I get 208 kg/m not 176 kg/m.

You need to be careful with FE, because some packages then use the diameter for bending stress determination. They calculate this using the extreme fibre distance of the steel from the neutral axis.

So, I would then take the submerged weight and assume that it is contained within an equivalent pipe equal to the outside diameter of steel. You need to take account of the reduced volume of seawater to provide bouyancy.

So, the answer is a fudge depending on how the FE package is going to use it.

 

[sgdon]

Thanks all. I have use directly the load/m length to replace the submerged density. This is compared with the use of the submerged density derived from the steel area only. Both results are extremely close to each other. This shows that the FE is using the submerged density and multiply it by the steel area to get the gravity load of the pipe.

The problem is partly solved currently as far as implicit fe code is used. Same problem will surface again if an explicit code is used since then sqrt(K/M) will come into picture.