the strip of material being helical and having joints at {HEAD ~ the edges} of {TAIL ~ the strip}, the pipe being of single thickness of material being the joints, 
of
the strip of material being helical and having joints at { HEAD ~ the edges } of { TAIL ~ the strip } , the pipe being of single thickness of material being the joints ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH