Underwater pipe assembly and method for assembling underwater pipes

An underwater pipe assembly comprising:

• a first pipe made of a plastic material, extending along a longitudinal axis, and having an inner circumferential surface surrounding the longitudinal axis, an outer circumferential surface opposite to said inner circumferential surface, and an annular end face between the inner and outer circumferential surfaces;
• a second pipe made of a plastic material, abutting against said first pipe, and having an inner circumferential surface surrounding the longitudinal axis, an outer circumferential surface opposite to said inner circumferential surface of said second pipe, and an annular end face between said inner and outer circumferential surfaces of said second pipe, said annular end faces of said first and second pipes abutting against each other;
• and
• a coupling unit including an inner coil surrounding said inner circumferential surfaces of said first and second pipes at a junction therebetween, an annular inner cover layer covering said inner coil, an outer coil sleeved on said outer circumferential surfaces of said first and second pipes at a position corresponding to said inner coil, and an annular outer cover layer covering said outer coil, each of said inner and outer coils having a first end portion located on one side of the junction of said first and second pipes, and a second end portion located on the other side of the junction of said first and second pipes and opposite to said first end portion;
• wherein, when said inner and outer coils are energized, said annular end faces of said first and second pipes are melted to fuse together said first and second pipes along the longitudinal axis, and said annular inner and outer cover layers are melted to radially fuse with said first and second pipes, and
• wherein, said inner circumferential surface of said first pipe has a first annular recess extending inwardly and axially from said annular end face of said first pipe, said inner circumferential surface of said second pipe having a second annular recess extending inwardly and axially from said annular end face of said second pipe, said first and second annular recesses communicating with each other along the longitudinal axis and cooperatively forming an annular notch, said inner coil and said annular inner cover layer being disposed in said annular notch, said first end portion of said inner coil being located in said first annular recess, said second end portion of said inner coil being located in said second annular recess, said annular inner cover layer being flush with said inner circumferential surfaces of said first and second pipes.