Patterson said, ‘We are in discussion with several operators on a range of three new AHTS designs. They range from 120 to 150 tons bollard pull (tbp) at the smaller end, to around 160 to 180 tbp for the mid size and around 300 tbp for the biggest vessels. All of them need to get the biggest possible winch onto the smallest feasible ship. When you are looking at a winch that is 16m wide and has three main drums, each carrying 5,000m of 74 mm wire, and two secondary winch drums each carrying 4,000m of 74 mm wire, you can no longer have the current standard size vessel.’

New environmental regulations are another factor pushing up hull sizes. From August new contracts with more than 600m3 of ship’s fuel will need to be built with a double skin with all fuel tanks arranged inboard. That puts pressure on fuel capacity. AHTS vessels must have the capacity to carry out a long distance tow with a range of up to 60 days at sea. Patterson continued, ‘Previously we could arrange to use the ship’s fuel and then use cargo tanks located in the side and double bottom for long haul fuel reserves. As the new rules stand, connection to the cargo tank system unless contained within the double skin will no longer be allowed, so we are having to find ways around that.’

The new generation AHTS vessels will be beamier than the current generation. Patterson added, ‘That gives us better initial stability, but until we see the outcome of the Bourbon Dolphin report we don’t want to fix the design parameters. Owners are asking what angles of heel they will get with a certain winch size and size of vessel, but we all have to wait to see what the changes to rules might be. Our intention in the meantime is to think through new safety requirements we can design in to stop the sudden side load occurring. We are looking at improving safety of working by putting in better intermediate stopping devices, so that if the shark’s jaw fails, the load cannot come suddenly abeam.’