What are the resources available to you to build a project like this? Are you setup for welding aluminum or steel? Do you already have a background in boat assembly?
I have found it to be quite a challenge to build my boat in Fiji with materials and resources hard to come by but that was a sport fishing boat. Your project is utility so It should be easier to handle there with what is available locally, the challenge is building for strength. I will look up some resources and let you know if there is some help designwise
You can provide a bit more information that can help anyone who is interested to assist with your project:
How will you power the barge? Motor or towed?
A picture of your drilling rig with a person next to it for some scale and sense of weight distribution. This can help with stability issues in how top heavy the rig is vs your choice of beam for the barge
Are you transporting it currently? What are you using as the barge?
Are you biased towards putting something together yourself over importing a predesigned and cut kit from Aus or Nz
