Norwegian shipowner Torvald Klaveness says "extensive quarantine" of new arrivals is the only way to carry out crew changes.
In a post on its website, the company revealed details of how it has been coping with lockdown restrictions in the Philippines during the pandemic.
Over the last three months, the group has managed to swap seafarers over on most of the fleet of spin-off Klaveness Combination Carriers (KCC).
But the group has not been able to meet planned targets due to the "extraordinary circumstances".
It has deviated six of its Cabu-class caustic soda and dry bulk ships to carry out the changeovers in Manila Bay, where it has replaced 136 crew.
From August, it stepped up procedures with a "more extensive" quarantine practice for all new seafarers.
Before leaving their home countries, newcomers must spend 10 days in isolation, with tests being carried out in special hotel facilities arranged by Klaveness Ship Management.
This may also apply again in the country where they will board the vessels.
"This has so far been the most demanding requirement," Klaveness added.
This quarantine has been combined with online safety training for crew.
"Strict routines onboard the vessels and for on-signers before joining the vessels has been a considerable inconvenience to our crew," the company said.
"When we are unable to perform crew changes, the risk of exhaustion increases, which again poses a safety risk.
"As we never compromise on health and safety, we have put maximum efforts into making crew changes on all our vessels in extremely difficult circumstances, both with unreliable air travel, travel restrictions and quarantine measures."
Crews have effectively been blocked from taking over the group's newbuildings in China.
For the handover of the 83,000-dwt combination carrier Baleen at Jiangsu New Yangzijiang in August, Klaveness embarked a temporary Chinese crew to take the ship to South Korea.
A Klaveness crew is currently waiting in quarantine there to take over the vessel.
"We expect to make similar arrangements after the delivery of the fifth Cleanby, Bangus, in mid-October. "
Klaveness said: "Although we see that it is possible to perform crew changes, it is extremely demanding and difficult. In some areas like the Far East, crew changes have become increasingly difficult, and we expect this to be a challenge for months to come.
"Governments need to take action to allow crew change to happen far more swiftly than is the case today for our seafarers to continue securing supply lines in times of a global crisis."
TradeWinds has also been told of a specific medical problem involving an oiler who has been suffering tooth pain on a ship — not a Klaveness vessel — for three months.
Having first complained of the pain in May in Brazil, it took until August for a doctor to be allowed to board in Singapore.
The seafarer was prescribed medical painkillers and urgent dental treatment.
But this was not allowed in Singapore, so he was told to seek help at the next port of call in China.
Unfortunately, due to quarantine restrictions there, China shore leave appears impossible and treatment has been cancelled. The crewman needs painkillers to sleep.
