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