A set of fluid mechanics equations in an arbitrary curvilinear coordinate system are obtained. The fully developed laminar flow in helical circular pipes subject to the influence of both curvature and torsion is analyzed. The solutions are obtained by the Galerkin method which is realized with the aid of computerized symbolic manipulation technique. The results indicate that the Galerkin technique can effectively overcome the limitation of small parameters for perturbation method. Based on the high-order semi-analytical solutions presented, the characteristics of secondary flow in pipes are discussed.