In 1980 there were more than 630 of these specialist ships with a tonnage of 7.4 million grt (gross registered tonnage) in service on the world's oceans.
Ngonyaka ka1990 kwakukho ngaphezulu kwamakhulu amathandathu anamashumi amathathu ezi nqanawa zikhethekileyo ezinomthamo wenqanawa -oyilenganiswe ngeetoni omalunga nescaidi ezisixhenxe (umthanu wenqanawa -oyilenganiswe ngeetoni obhalisiweyo) ezisebenza kumanxweme elizwe.