海底同组小区域沙波扭转迁移机理研究

A group of seabed sand waves’ reverse migration in small area, which is discovered as a new characteristics of seabed sand waves’ migration in recent years, can lead to dangerous subsea pipelines’ free spanning. Therefore, mechanism research on a group of seabed sand waves’ reverse migration in small area is very urgent. Currently, overseas and domestic related researches still in observation stage, and is unable to give a clear answer for its causes. This project focused on a group of sand waves’ reverse migration in the Beibu Gulf, established 3D high resolution wave-currents coupled ocean model for Beibu Gulf and the wave-current-sediment coupling model in the vertical section perpendicular to the crest lines of sand waves, simulated the reverse migration of sand waves in Beibu Gulf Area 1 and Area 2, constructed profile non-linear spectral model and field non-linear spectral model with transfer function concerning the characteristics of ocean current to present the migration process of sand waves, declared the characteristics of transfer function corresponding to the occurrence of sand waves’ reverse migration. The mechanism on a group of sand waves’ reverse migration was revealed and a method for predicting sand waves’ migration under complex ocean environment conditions in the project, which provides a more reliable means of evaluation on free spanning of submarine oil and gas pipeline induced by reverse migration of sand waves.

海底同组小区域沙波扭转迁移，是近年来发现的海底沙波迁移新特征，可能造成海底油气管线出现危险悬跨。因此，对海底同组小区域沙波扭转迁移机理进行研究十分迫切。目前国内外相关研究仍停留在观测阶段，尚无法就其成因给出明确答案。本项目拟以北部湾沙波区的同组沙波扭转迁移为具体研究对象，通过建立南海北部湾波流联合作用高分辨率三维海流模型和垂直于沙波波峰线的垂向断面内的波流-沉积物耦合作用模型，模拟南海北部湾1区和2区的方向相反的扭转迁移, 建立传递函数包含海流特征的断面非线性谱模型和全场非线性谱模型描述沙波迁移过程，给出沙波扭转迁移发生的传递函数特征，揭示小区域海底同组沙波扭转机理的同时发展了适用于复杂海洋环境条件的沙波迁移预测方法，为沙波扭转迁移致海底油气管线悬空风险提供更可靠的评价手段。

本项目以南海北部湾琼西南东方附近海域的沙波区内的同组小区域沙波扭转迁移为研究对象，以揭示其机理、提出预测方法为目的，综合利用了动力学数值模拟、调和分析、非线性谱方法等方法开展研究。通过项目的研究，建立了南海北部湾沙波区波流联合作用高分辨率三维流场模型，通过动力学数值模拟，成功复现了研究区内的沙波扭转迁移特征；提出了对沙波区近底摩擦速度进行调和分析的方法揭示扭转迁移机理，并提出了一种预测沙波运移方向的新方法；开展了台风作用下的沙波运移动力学模拟，揭示了台风对沙波运移的短时、作用强的影响特征。发展了考虑台风修正的非线性谱分析模型，并实现了沙波运移预测。项目所建立的高分辨率模型可迁移到其他相近沙波区，进行沙波迁移的动力学分析，具有一定的可拓展性。预测沙波运移方向的新方法适用于多分潮共同作用复杂往复流情况，只要掌握底部摩擦速度的时程曲线或者分潮调和常数等参数即可预测沙波运移方向，且确定周期性成分和定常流成分的贡献。通过该方法，可以很清楚地描述出流场特征的差异和对沙波运移的贡献。基于非线性谱分析方法识别出沙波运移常速度和台风致运移速度，可开展长时间的沙波运移预测。