what is the sum of tonnage when the type of ship is twin screw ro-ro motorship and the date entered service is 11 february 1983?
CREATE TABLE "ships_operated_by_zeeland_steamship_comp" (
  "ship_s_name" text,
  "date_entered_service" text,
  "date_withdrawn" text,
  "tonnage" real,
  "type_of_ship" text
);
SELECT SUM("tonnage") FROM "ships_operated_by_zeeland_steamship_comp" WHERE "type_of_ship"='twin screw ro-ro motorship' AND "date_entered_service"='11 february 1983';