Seawater return flowline analysis

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Year:
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Our client was experiencing integrity issues with their seawater cooling return pipework. Our client observed excessive movement in the pipework and found a crack at the caisson inlet weld. It was suspected that this was due to the adequacy of piping support and specific transient behaviour of the fluid within the pipe causing the movement.

Our client had previously carried out remediation work to try and minimise the movement of the pipework, however these measures have had minimal impact.

Our client requested Apollo’s support to investigate the problem and help them gain a better understanding of the fluid behaviour within the pipework. Apollo carried out a process review of the problem alongside a computational fluid dynamics (CFD) analysis to understand the fluid behaviour. This work informed our client why they were finding excessive movement in the pipework. Apollo then re-analysed the problem using CFD with the inclusion of a restriction orifice before the caisson inlet and proved to the client that this eliminated the transient behaviour within the pipework and thus would significantly reduce the pipework movement.

 

The Problem

· Seawater return pipework that connects to a caisson is moving excessively

· A crack had formed at the weld connecting the flowline to the caisson

· Our client was concerned that the persistence of this issue would further threaten the integrity of the pipework

 

The Solution

· Process review and CFD analysis was carried out.

· Process review identified that seawater inside the flowline was expected to cavitate

· CFD analysis verified the finding of the process review by identifying the areas of the pipework experiencing cavitation. CFD analysis also found that the forces on the pipework varied cyclically due to the behaviour of the fluid

· Remedial measures including the implementation of a pressure control valve or restriction orifice were identified in the process review and assessed based on their likely effectiveness, relative cost, operability and installation effort required

· A restriction orifice was identified as the best option

· CFD analysis repeated with the inclusion of a restriction orifice and proved that it eliminated the transient behaviour and cavitation

 

And for all you engineers who want more details:

 

The background

Our client had discovered a crack at the weld where a 14” seawater return line enters a 24” caisson. The seawater return line falls from an elevation of 54.7 m LAT to 24.5 m LAT

where it enters the caisson. The caisson is vented to atmosphere and incudes a small purge. Clamps were previously installed to prevent excessive movement of the flowline but proved unsuccessful. The client was unsure of the cause of the problem and requested Apollo to investigate the behaviour of the fluid within the flowline and to develop a solution to eradicate the problem.

The solution

Apollo looked at the fluid mechanics within the pipework from both a process perspective and using CFD. The friction head loss for the flowline was calculated and compared with the pressure developed from the change in elevation. As the gravity head was at least 10 times larger than the friction head loss, cavitation was predicted all the way up 14” flowline from 9.8 m above the inlet to the caisson. The CFD analysis confirmed that the static pressure of the fluid in the flowline would fall to the vapour pressure at 9.8 m above the caisson inlet, confirming that cavitation would occur in the flowline. The CFD analysis also highlighted that the forces exerted on the pipework varied cyclically.

In summary the flowrate was outside that for self venting flow and thus created a vacuum and transient behaviour. Two options existed. One to move from a self venting to a vented arrangement or two, to move to a flooded pipe arrangement where a downstream restriction is added to ensure the system remains fully flooded and limit fluid acceleration. Adding a vacuum breaker was not deemed desirable due to the complex pipework arrangement and hence a resistance to flow was added.

A 125 mm diameter restriction orifice was then added to the CFD model just upstream of the final bend before the flowline enters the caisson. The CFD analysis showed that the inclusion of the restriction orifice had the desired effect by increasing the back pressure within the flowline above the fluid vapour pressure, preventing the fluid from cavitating. The forces exerted on the pipework were also found to be constant when the restriction orifice was included.

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