When gas pipelines go through oceans or rivers, according to KOGAS (Korea Gas Corporation) design criteria, buried pipes are encased in concrete due to heavy water pressure and sudden impacts. Especially, the submarine pipes for HVL (High Volatility Liquid) in the deep-seabed are recommended to be encased in assistant structures. This study includes the behavior of pipelines encased in rectangular concrete which suffer from external pressure according to the depth, as well as the inner pressure generated by fluid inside. Based on classical theory of elasticity, the interface stress for the steel-concrete composite pipeline was defined using results from a FEM analysis. Investigations were done on local deformities of the diameter at each part of the pipelines. After calculation the local deformed diameter by using an existing equation for computing the stress in double walled cylinders, which considers the internal and soil pressure, the rate of diameter change is determined. The results were then compared with the behavior of the pipeline encased in rectangular concrete. Through this study, it was verified that pipelines encased in rectangular concrete are about 50% more effective in stress reduction than pipelines without any complementary structure and almost all the external loads are supported by concrete encasement. This study is an elementary part of the research for a new efficient integrity estimation method of concrete-covered submarine pipelines. With this result, the basic design element needed in designing submarine pipelines with protective structure was found.