The short answer to this question is simple.
We do not have one port state MOU under IMO because it is tough to have such MOU.
There are number of reasons why it would be tough.
- All the port states need to have IT infrastructure that can connect the data of all the port states. IMO has no control of development of IT infrastructure of any country
- All the port states need to ensure that this data is not misused in any way. It is difficult for IMO to ensure that for each country.
- IMO would need to set up a centralised server to have all the data connected. IMO would need to invest a lot of money and resources in doing that.
Apart from that, the MOUs usually have similar expectations from the ships and its operations. For example Paris MOU will be concerned about ship's capability to use low sulpher fuel. But Indian Ocean MOU will not be concerned about that.
In this case, with a common MOU under IMO, Paris MOU would want to inspect the ships calling their ports.
Even if IMO like this idea of common MOU, it is never easy for them to implement this. This is because, for each new thing the IMO would want to implement, it need to convince all the flag states.
This is always a tough task unless something is absolutely necessary. More oftne than not this necessity is always initiated by a major accident or a major research work.
In past, Few students of world maritime university have taken up this topic as their thesis project. But unless these thesis show some urgency in the need of a common MOU, this would not be a realty. Not in near future at least.
Click here to Read one of thesis on this subject. It is interesting to read the view points of various MOUs on this subject in the thesis.