Buoyancy for Pipeline Equipment 1

[HotStab]

I am working on the design of subsea pipeline equipment: PLETs and In-line ESDV Sleds. PLETs shall be connected to rigid pipes, but In-Lines shall be between flexible flowlines. They shall be installed at depth up to 2000 meters.
Due to lay-out configuration, for all assemblies the center of gravity will be above the center lines of pipeline connection flanges. During installation or recovery of In-Lines, or PLETS in first extremity, this high COG shall impose an undesirable torsion in the pipeline.
To eliminate this problem counter weight should be added to the bottom part of the assemblies. Or buoyancy shall be attached to the top of the structure.
As far as I could see, the first alternative is not a good one, because buoyancy should be required anyway to compensate the dead load of equipment and reduce the bending moments it impose in pipeline. The use of counter weight will increase the dead load, and will imply in more buoyancy required. Other collateral effect: extra weight would demand more foundation area.

Please, I need help to understand:
1- Could In-Line equipment be installed without buoyancy?
2- Is it possible to install a PLET at first extremity without buoyancy?
3- How to evaluate the buoyancy quantity required?
4- Is lowering the COG using counter weigh a valuable approach?
5- Which are the main considerations that should be take in account for rigid and flexible pipeline installation?
6- Can rigid pipelines be installed at 2000 meters by S-lay method?
7- Which are the special requirements that should be considered in equipment design regarding different installation methods (S-lay and J-lay)?

Unfortunately, we have no access to the installation contractor. So, I would appreciate your help very much.

 

[BigInch]

Lots of complex questions that have many complex answers.
I think you might need to find an engineering contractor first.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[HotStab]

BigInch,
I have looked for help in the web and conference papers. It seemed to me that what the customer is asking for (a self balance equipment in which the vertical position can be guarantied by o low COG only) is not a good solution.
Unfortunately, as I said, I have no contact with the future installation contractor yet and might not even have access to them. So, I am trying to find arguments to justify the usage of buoyancies, that in fact, as I guess, is a common practice.
I know I post questions that would require long and detailed answers. But, I would appreciate any help, any answer, even a partial one, is welcome.

 

[BigInch]

Too early for installation contractor. Where's the feasibility study?

I surely can imagine that the instability of a CG above the CP is not going to be easy to maintain. Go with the extra weight to lower the CG below CP with a safety factor.

Too deep for S lay. J is the only possibilty. Check the hanging weight against equipment capacity.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[HotStab]

Thank you, BigInch, for your advice.
We are only supplying the some pieces of pipeline equipment. We have no access to field development, neither to interface technical information like flowline and pipeline data or installation procedures.
Please, let me know your opinion about the usage of buoyancy for compensation of submerged equipment weight. Is it a fact? Is it usual? Is it always required?

 

[BigInch]

2000 m is way out of my depth, but using the same principles that work for lesser depths,

I'd think you'd want to guarantee they won't move, either by somehow fixing to the bottom, or by supplying sufficient weight to provide stability in all axes.

I'd also think that you should provide fixed positions (an anchor point or guides for attached pipe) and any pipe attached to your equipment should be designed as flexible as possible (including "rigid" steel pipe) and attached to your mounting and support locations via slotted connections or other support elements using configurations that allow movement if necessary so as nothing becomes overstressed. You should probably also consider that the attached pipe will be loading your equipment and foundation/support mechanism, rather than your equipment loading the pipe.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[LSThill]

Pipieline publication around the late 1970 eray 1980 check HoustonMain Libary or University of Houston Libary.

This Reference was use on the replacement of Sea Pipeline to the SBM in Libya 1981.

Leonard Stephen Thill

 

[BigInch]

LSThill, 2000 m depth?

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization