Embedment depth and uplift capacity of a cryogenic pipeline dynamically laid in a trench

Abstract

A cryogenic pipeline is a pipe-in-pipe with larger outer diameter, and is heavier and stiffer, compared to conventional deepwater steel flowlines or export pipelines. Cryogenic pipelines are being considered to be placed on shallow water sandy seabeds for transporting liquefied natural gas to shore. This paper reports a series of centrifuge tests to assess the as-laid embedment of a cryogenic pipeline during the dynamic laying process, and the uplift capacity of a trench-laid pipeline. Two seabed conditions were tested – a pipe laid onto the base of a trench and a flat silica sand deposit. A total of 12 dynamic pipe laying tests were conducted varying vertical force and pipe oscillation amplitude, frequency and number of cycles; encompassing typical laying processes. The embedment depth was found to be less than 0.5 diameters, owing to the resultant effect of lateral ploughing and sand densification. For the trench-laid cases, after the completion of simulating the pipe laying process, the trench was filled with a layer of sand and a layer of rock armour; varying the thickness of the layers. The pipe was then extracted vertically to evaluate the uplift resistance, which was shown to increase with increasing the thickness of the rock layer.