"Such incidents are rare, however, and in the case of Princess Cruises it was a rogue engineer who was found to be at fault rather than a systematic failure of procedures."
What I don't quite understand is why did that engineer take it upon himself to set up an illegal discharge pipe?
What would he have to gain by doing so?
Do they get paid by the gallon of waste material or was it just to make himself look better than the engineers on other ships?
It would likely be fear of retribution or duress from a senior engineer, or other authority.
The OWS is a cost factor, and the cruise industry is exquisitely fine tuned in cost control.
One scenario would have the engineer being questioned as to why so much oil was showing up in Bilge water (ie, you mustn't be caring for the equipment to allow leaks, and we'll have to implement some new checks and balances, aka scrutiny of the engineers performance)
Another scenario would be questioning the engineer as to why the OWS is being used so much (ie costing th e company more money), and that 'we' (the company) have never had these difficulties with any other of our engineers.
The magic pipe is not unique to Princess vessels, nor is knowledge of its use, or why it would be used.
At any rate, sufficient layers of protection for executives exist in the tiers of employees on the ladder.