Offset Surface / Offset Mesh - Creates Self-Intersection

Hello All, 

I am trying to offset a wavy surface by 3mm. (The surface is originally sweep2 but also works when I loft between the two rails - have tried both and none work). 

When I offset by less distance (say 0.15mm) it works - I can do an offset surface without self-intersecting faces. However when I offset the amount I need (3mm), it intersects with itself, as you can see in the image attached. I have tried OffsetLoose but as I am fairly new to Grasshopper I am not quite sure which variables I am playing around with... 

I suspect this has a fairly simple solution and would appreciate your help. Bear in mind I need to do 35 offsets like this (the final aim is one airtight mesh). 

Thanks in advance

ptr