(* data for B-spline surface, from example in docs *)
pts = {{{0.5, 0, -0.5}, {0, 0, -0.5}, {0, 1, -0.5}, {0.5, 1, -0.5}, {1, 1, -0.5},
        {1, 0, -0.5}, {0.5, 0, -0.5}},
       {{0.5, 0, 0.7}, {0, 0, 0.7}, {0, 1, 0.7}, {0.5, 1, 0.7}, {1, 1, 0.7},
        {1, 0, 0.7}, {0.5, 0, 0.7}},
   {{0.5, 0, 0.9}, {0, 0, 0.9}, {0, 1, 1.5}, {0.5, 1, 1.5}, {1, 1, 1.5},
    {1, 0, 0.9}, {0.5, 0, 0.9}},
   {{0.5, -0.1, 1}, {0, -0.1, 1}, {0, 0.5, 2}, {0.5, 0.5, 2}, {1, 0.5, 2},
    {1, -0.1, 1}, {0.5, -0.1, 1}},
   {{0.5, -0.3, 1}, {0, -0.3, 1}, {0, -0.3, 2}, {0.5, -0.3, 2},
    {1, -0.3, 2}, {1, -0.3, 1}, {0.5, -0.3, 1}},
   {{0.5, -1.5, 1}, {0, -1.5, 1}, {0, -1.5, 2}, {0.5, -1.5, 2},
    {1, -1.5, 2}, {1, -1.5, 1}, {0.5, -1.5, 1}}};
w = {{1, .5, .5, 1, .5, .5, 1}, {1, .5, .5, 1, .5, .5, 1},
     {1, .5, .5, 1, .5, .5, 1}, {1, .5, .5, 1, .5, .5, 1},
     {1, .5, .5, 1, .5, .5, 1}, {1, .5, .5, 1, .5, .5, 1}};
uk = {0, 0, 0, 1/4, 1/2, 3/4, 1, 1, 1};
vk = {0, 0, 0, 1/4, 1/2, 1/2, 3/4, 1, 1, 1};

Graphics3D[BSplineSurface[pts, SplineKnots -> {uk, vk}, SplineDegree -> 2,
                          SplineWeights -> w, SplineClosed -> {False, True}]] /.
           bs : BSplineSurface[pts_?ArrayQ, opts___] :>
           Module[{bsf = BSplineFunction[pts, opts]},
                  Cases[Normal[Cases[ParametricPlot3D[bsf[u, v], {u, 0, 1}, {v, 0, 1}],
                        _GraphicsComplex, ∞]], _Polygon, ∞]]

MakePolygons[vl_] /; ArrayQ[vl, 3] := Module[{dims = Most[Dimensions[vl]]},
  GraphicsComplex[Apply[Join, vl], Polygon[Flatten[Apply[Join[#1, Reverse[#2]] &,
                  Partition[Partition[Range[Times @@ dims], Last[dims]], {2, 2}, {1, 1}],
                        {2}], 1]]]]

Graphics3D[BSplineSurface[pts, SplineKnots -> {uk, vk}, SplineDegree -> 2,
                          SplineWeights -> w, SplineClosed -> {False, True}]] /.
           bs : BSplineSurface[pts_?ArrayQ, opts___] :>
           Module[{bsf = BSplineFunction[pts, opts], upts = 30, vpts = 18},
                  Cases[Normal[MakePolygons[
                        Table[bsf[u, v], {u, 0, 1, 1/(upts - 1)}, {v, 0, 1, 1/(vpts - 1)}]
                        ]], _Polygon, ∞]]