Cement rings below wellhead

Good Morning All, 
On one our deep water well, we found one cement ring in both our 13â…œ" and 10¾" casings at the exact same depth ±0.5m below where the cement wiper plugs are. 
We suspect that during cement job, cement managed somehow to pass the top plug and get into the void between the top plug and the casing hanger, then is "sucked" down when the top plug is released. 
We are not filling this void as the cement plugs are prepared offshore (offline before the job), then the casing hanger assy + cement swivel is laid down on deck horizontally. 
We are using an Halliburton SSR swivel and plugs. No issue with the swivel seals (tested after job), no sign of cement there. 
Has anyone faced this issue before (cement rings at / below wellhead area) ? What were the root causes and actions taken to avoid re-occurrence ? 

Thanks in advance, 

1 Answer(s)

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