Swansea Magistrates' Court heard that Motherwell Bridge, which designs and builds storage tanks, heat exchangers and gasholders, had been contracted to build a gasholder at the Tata Steelworks in Port Talbot.
One of its employees, Philip Roberts, 63, from Burry Port, South Wales, was working on the project as a welder when the incident happened on 9 November 2013.
Mr Roberts was tasked with welding seams of the tank, which involved working at height from staging erected around the tank. However a colleague, who was also welding seams, had removed planks from the staging – as was accepted practice – to facilitate his own welding.
As Mr Roberts walked around his colleague to get to the next seam, he fell through the staging platform and landed on his back on a concrete plinth at the bottom of the tank more than two metres below. Two planks also fell with him, hitting Mr Roberts on the side of his head and his leg.
Mr Roberts fractured two ribs, suffered a bruised lung and a strained torso and spent four days in hospital. He was on crutches for around nine weeks and still suffers from backache and flashbacks. He has been unable to work as a welder.
A Health and Safety Executive (HSE) investigation found that Motherwell Bridge Ltd did not have suitable plans in place for working at height and did not supervise the work properly.
The company had been working to procedures that were nearly 20 years old and had not been updated despite significant changes to safety regulations and improved guidance relating to work at height.
Motherwell Bridge, of Drayton Hall, West Drayton, Middlesex, pleaded guilty to breaching Regulation 4(1) of the Work at Height Regulations 2005.
HSE principal inspector Phil Nicolle said: "This incident was entirely foreseeable and could have been prevented. If Motherwell Bridge had planned the task properly and identified the risks, they would have been able to implement safe procedures for working at height and Mr Roberts' painful injuries could have been avoided.
"In fact, it became apparent that the company's procedures were totally out of date and fell well below the standards expected in industry today. Whenever work at height is planned, companies should undertake suitable assessments of the risks specific to the site and work to be undertaken.
"Mr Roberts not only physically suffered due to Motherwell Bridge's failure to protect its employees, but he has also since suffered financially and emotionally."