What is the maximum diameter of the Mid West Gas Pipeline (1999)?
CREATE TABLE "onshore_pipelines" (
  "name_year_commissioned" text,
  "owner_operator" text,
  "length" text,
  "maximum_diameter" text,
  "from_to" text,
  "licence_number" text
);
SELECT "maximum_diameter" FROM "onshore_pipelines" WHERE "name_year_commissioned"='mid west gas pipeline (1999)';