A solution with titlesec and tabularx:
\documentclass[a4paper,12pt]{report}
%------------------------------------------------------------
\usepackage{amssymb,amsmath,amsthm,latexsym,mathrsfs,amsfonts,dsfont}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{titlesec}
\usepackage{tabularx}
\usepackage{lipsum}
\usepackage{titlesec}
\titleformat{\chapter}[display]{\filcenter\Huge\scshape\bfseries}{\begin{tabularx}{\textwidth}{@{}XcX@{}}\titlerule[5pt]& \large\mdseries\raisebox{-1.05ex}{\chaptername\enspace\arabic{chapter}} & \titlerule[5pt]\end{tabularx}}{-1ex}{\hrule\vspace{1ex} }[\vspace{0.75ex}\endgraf\hrule]
\titlespacing*{\chapter}{0pt}{-8ex}{20ex}
\begin{document}
\chapter{Preliminary}
\lipsum[1]
\end{document}

For unnumbered chapters, you can add this code to your preamble:
\titleformat{name=\chapter, numberless}[display]{\filcenter\Huge\bfseries}%
{\titlerule[5pt]}{-2.2ex}{\hrule\vspace{1.5ex}}[\vspace{0.75ex}\endgraf\hrule]
