On-Bottom Stability Research Of Subsea Non-metallic Pipeline

On-bottom stability of submarine pipeline has been technical problems which limit the applications of submarine pipeline. The stability research for steel pipeline is very broad and theory is mature, but very few people or institute worked on non-metallic pipe stability. This paper will take the non-metallic submarine pipeline as the research object, to do on-bottom stability study through code design, numerical simulation and experimental study.Additional weight calculation in this paper is mainly reference to on-bottom stability design codes of steel submarine pipeline, however, the following differences has to be considered:due to limited strength of non-metallic pipeline material, pipe diameter will not be too large, the hydrodynamic force calculation is difference with steel pipe; non-metallic pipe is applied in shallow water, and wave is dominant force, Airy wave theory will be used.In order to further understand the deformation behavior of the non-metallic pipeline on seabed, numerical simulation method is used in this paper. Use the calculated additional weight results and the selected weight method to do modeling and analysis, and get the deformation and displacement results under wave loads, From the results, the maximum lateral displacement is1.121m, which is exceeding the10D range of the dynamic stability method in DnV codes, however, the maximum stress is only0.864MPa and the maximum strain is about0.96%which is safety for non-metallic pipe. These results indicate that the DnV codes exist some shortcomings in dealing with small-diameter pipe and non-metallic pipe on-bottom stability problems. The new codes specially focused on non-metallic pipeline on-bottom stability design are needed.Finally, the cyclic water tank test focused on non-metallic pipe on-bottom stability is carried out, two kinds of different depths, four groups of different weights and a total of eight experiments are carried out. The following conclusions are outlined:The non-metallic pipe without additional weight will loss stability in0.4m water depth with50mm wave height, and loss stability in0.6m water depth with70mm wave height. It is shown that the movement of water particle is attenuation with increasing water depth, which is consistent with the theory of water particle movement trajectory. When compared the similar weight with different weight types (the second and third set of experiments), it is shown that the larger spacing of additional weight is significantly better than the dense spacing for pipeline stabilization. Before pipeline instability, the surrounding soil will be eroded, the greater the weight, the more serious the erosion, until reaches a certain level when the post-stable phenomenon appear, pipe penetration is large at this condition.