Three oil rigs that were due to be towed to India and Bangladesh to be scrapped have been ordered not to leave the Cromarty Firth.
The Scottish Environment Protection Agency (Sepa) has issued an order banning the movement of the structures pending an investigation.
The agency said it had concerns about the rigs' "destination and disposal".
It said it was investigating whether the movement of the rigs would be in accordance with regulations.
The Cromarty Firth is used for parking rigs and platforms while the structures are not in use, require maintenance work or are awaiting a tow to another destination.
Sepa said it took "immediate action" to prevent the movement of the three rigs Ocean Princess, Ocean Nomad and Ocean Vanguard.
John Kenny, Sepa chief officer, said: "Last week Sepa was made aware of the imminent shipment of three oil rigs from the Cromarty Firth, and concerns about their destination and disposal.
"Sepa experts in the transfrontier shipment of waste immediately began investigations to establish whether movement of the vessels would be in accordance with European Commission Regulations for waste shipments and issued an immediate direction on 12 January preventing movement of the vessels."
He added: "Our investigations are still ongoing and until we are satisfied that there would be no breach of regulations we have directed that the vessels remain undisturbed.
"We have notified the harbour authority of this decision."
The Maritime and Coastguard Agency said the safe recycling of ships and offshore industry structures was a topic of "significant international concern", and had to be carried out in line with regulations.
A spokeswoman added: "We would expect operators to take this into account before planning the disposal of any vessel."
Highlands and Islands Scottish Green MSP, John Finnie, has praised the "prompt action" of Sepa.