【作者】 徐继尚；

【导师】 李广雪； 曹立华；

【作者基本信息】 中国海洋大学 ， 海洋地质， 2009， 博士

【摘要】 使用双频侧扫声纳系统、浅地层剖面仪和相干声纳测深系统对东方1-1海底输气管道进行了综合调查，发现了百余个管下冲刷坑，这些冲坑已造成管道悬跨，严重威胁管道的安全。通过对东方1-1海底管道的实测资料分析，结合物理模型试验和数值计算方法，研究粘性海床管道周围冲刷机理，预测正常天气条件下平衡冲刷深度；研究管道自埋机理，讨论管道周围海底冲刷与管道自埋的关系；探讨管道周围底形演变过程对管道受力的影响；估算管道允许最大悬跨长度，评估管道的安全性；提出管道维护措施。依据实测管道周围海床的冲刷特征，将粘性海床管道周围冲刷分为管侧冲刷和管下冲刷两种，这两种冲刷现象分别是在常态水动力条件和台风条件下形成的。研究区潮流可导致管道两侧海底冲刷(管侧冲刷)，最大冲刷深度与前人提出的砂质底床管道周围最大冲刷深度吻合。海底冲刷过程中泥沙运动形式主要为悬移质，而较粗的颗粒滞留海底，从而使管道周围底质不断粗化。管道下方存在许多大规模冲刷坑(管下冲刷)，台风是管下冲坑的主要动力机制。虽然管道两侧冲刷与管下冲刷的机理不同，但二者之间是有联系的。管下冲刷坑的发展过程可分为四个阶段：泥沙起动阶段、水流隧道发育阶段、快速掏空阶段和掏空平衡阶段。管道两侧冲刷是管下冲刷的第一个阶段，为管下冲刷创造了条件，但常态水动力作用下管道周围海底冲刷只能停留在该阶段，只有在台风作用下，且管侧冲刷的累积作用达到管涌所需要的条件之后才会出现管下冲刷现象。DF1-1深水区(水深＞37m)管道自埋机理主要有重力下沉和冲刷自埋两种，管道自埋深度为重力下沉深度和冲刷自埋深度之和，其中冲刷自埋深度与管侧冲刷深度具有相同的规律。常态水动力造成的管侧冲刷可导致管道自埋，从而可减小台风引起的管下冲坑长度和管道悬跨长度，可能对管道具有保护作用。管侧冲刷既可导致管道自埋抑制管下冲刷，又可为管涌创造条件促进管下冲刷，具体哪种作用占主导取决于管道自埋速率、台风发生频率及台风强度。在DF1-1管道铺设完成之后的早期阶段管侧冲刷以导致管道自埋抑制管下冲刷为主，后期阶段管侧冲刷以促进管下冲刷为主。物理模型试验结果表明，在不同水动力条件、底质条件和管道初始埋藏深度条件下，管道周围海床演变过程表现出不同规律，主要分为四种类型：(Ⅰ)无冲刷；(Ⅱ)无沙纹的冲刷；(Ⅲ)不受沙纹影响的冲刷；(Ⅳ)受沙纹影响的冲刷。若沙波规模相对管道暴露部分较小，管道周围水流足以破坏管道前方沙纹的结构，被破坏后的沙粒从管下通过，因此小沙纹或沙波的活动性对冲刷过程几乎无影响。如果沙波规模相对管道暴露部分较大，管道周围水流不能够破坏管道前方沙纹的结构，管道被活动沙纹周期性埋藏。不同的海床演变过程对管道所受波浪力的影响不同。在相同水动力作用下，类型Ⅰ中管道在冲刷过程中受力比较稳定；类型Ⅱ与Ⅲ中管道在冲刷启动(管涌)阶段受力较大，然后逐渐减小，并很快达到稳定值；类型Ⅳ中管道受力状态受沙纹活动性的影响很大。为减小管道所受波浪力，不同类型海床上的管道需要不同的埋设深度。粉砂底床上半埋管道比裸置管道受力小很多，但砂质底床上二者相差不大，而对于第四种冲刷类型来说，活动沙波对管道受力的影响很大，半埋管道所受波浪力甚至比裸置管道受力大(浮动性也大)，故在砂质海床上铺设管道时应将管道埋至沙波活动层之下。分别从静态情况下管道弯曲变形和避免涡激共振的角度估算管道的极限悬跨长度。计算结果表明，静态条件下管道极限悬跨长度为56m，跨长超过该值时管道将会弯曲变形。为避免涡激振动，使用最高安全系数计算出常态水动力条件下的管道极限悬跨长度为30m；百年重现期台风条件下极限悬跨长度为20m，为保障管道安全建议消除长度大于20m的悬跨。小间距连续悬跨对管道的安全威胁很大，共发现8处连续悬跨点。安全隐患最大的管段主要集中在KP42-KP51之间。建议在CEP-KP42之间尝试“人工草”防护方法预防小间距连续悬跨的出现；KP42-KP51之间应该重新挖沟，将管道埋设，消除悬跨。本文研究成果的创新性主要体现在：1)依据实测管道周围海床的冲刷特征结合物理模拟试验，研究粘性海床管道周围冲刷机理和冲刷过程。研究表明，粘性海床管道周围冲刷分为管侧冲刷和管下冲刷两种类型，常态水动力造成的管侧冲刷为台风造成的管下冲刷所需要的管涌创造了条件，而台风对管侧冲刷又起到加强作用。将两种冲刷现象及其动力机制区别而又联系起来，更符合实际现场情况。2)研究常态水动力的双重作用，一方面可导致管道自埋阻碍管下冲刷，另一方面又可为管涌创造条件促进管下冲刷。管侧冲刷在DF1-1管道铺设完成之后的不同时期所起的作用不同：早期阶段管侧冲刷以导致管道自埋抑制管下冲刷为主，后期阶段管侧冲刷以促进管下冲刷为主。这对铺设于粘性海床上的管道维护具有重要参考价值。3)将管道周围海床演变类型分为四种类型，研究不同海床演变类型在冲刷过程中对管道受力的影响。管道周围海床演变类型有：(Ⅰ)无冲刷；(Ⅱ)无沙波的冲刷；(Ⅲ)不受沙波影响的冲刷；(Ⅳ)受沙波影响的冲刷。类型Ⅰ中管道受力在冲刷过程中比较稳定；类型Ⅱ与Ⅲ中管道受力在冲刷启动阶段较大，在冲刷初始阶段逐渐减小，并很快达到稳定值；类型Ⅳ管道受力受沙波活动性的影响很大。不同类型的冲刷过程及其对管道受力的影响不同，这对制定不同条件下的海底管道的具体维护方案具有参考价值。4)用不同方法计算管道在不同水动力条件下的极限悬跨长度，评估管道在不同重现期内的安全性，对海上油气资源丰富但经常遭受台风袭击的南海海域中的管道维护具有普遍意义。更多还原

【Abstract】 DF1-1 submarine pipeline was investigated using an integrated surveying system, including a dual-frequency side-scan sonar, a high resolution sub-bottom profiler, and a swath sounder system. More than a hundred of scour pits under the pipeline were found during the pipeline route survey, most of which have caused the span of the pipeline and threaten the safety of the pipeline.Using methods of field investigations, physical simulations and numerical calculations, the mechanism of local scour around submarine pipelines in cohesive soils was studied, and the maximum scour depth under ordinary weather conditions was estimated; the mechanism of the self-bury of the pipeline was studied, and the relationship between the self-bury and the local scour was discussed; the influences of the local bedform evolution on the hydrodynamic forces experienced by the pipeline were discussed; the maximum allowable free span length (MAFSL) of the pipeline was calculated, and the safety of the pipeline was estimated; Methods for protecting the pipeline were proposed.By analyzing the pipeline field investigation data, the concepts of ’side scour’ and ’down scour’ were proposed for the local scours around submarine pipelines in cohesive beds, which probably result from tidal currents and hurricane-induced currents respectively. The tidal current in the study area can cause the seabed scours on both sides of the pipeline (side scours), the maximum scour depths of which agree well with the calculated results according to the equations proposed by different researches based on experimental studies with sand or silt beds. The finer sediments were transported mainly in the form of suspended load in the scour process, while the coarser sediments were detained on the seabed, which caused the local sediments becoming coarser. There exists lots of scour pits under the pipeline (down scours), which may be mainly caused by hurricanes.The side scours may be related to the down scours, although they may be caused by different hydrodynamic forces. Experimental studies showed that the down scour process can be divided into four stages: (i) seabed scour on both sides of the pipeline; (ii) piping; (iii) seabed scour under the pipeline; (iv) scour equilibrium. The side scour is the first stage of the down scour process. The study results indicated that the tidal currents in the study area can not make the scour process reach the second stage (piping), which occurs only when the accumulative effects of the side scour reach the critical conditions by hurricanes.The DF1-1 submarine pipeline in the deep water regime (with water depth > 37m) maybe buried itself due to effects of the gravity and the side scours. The maximum burial depth of the pipe was the total effects of that caused by gravity and the side scours respectively, while the burial depth caused by scours corresponds well with the maximum side scour depth. The self-bury may protect the pipeline by reducing the down scours and span height caused by hurricanes.The side scours can either inhibit the down scours by inducing the self-bury of the pipeline or accelerate the down scours by making conditions for piping, the majority effects of which depend on the rate of self-bury, the frequency and intensity of hurricanes. The side scours inhibited the down scours at the early stage after the pipeline was laid, while accelerating the down scours now.Experimental simulation results showed that, depending on wave conditions, soil grain size and initial burial depth of the pipeline, different near-field bed evolution behavior were recorded. The observed behaviors were classified into four different basic regimes, namely: (I) no scour, (II) scour without sand ripples, (III) scour with small sand ripples, (IV) scour with large sand ripples. As long as the heights of ripples are smaller than that of the unburied section of the pipeline, the flow nearby the pipeline is able to destroy the ripples and transport the sand through the gap between the pipe and the seabed, so the moving small sand ripples have very limited effects on the scour process. However, if the heights of ripples are comparable to or larger than the pipeline’s unburied section, the flow nearby the pipeline is not strong enough to destroy the ripples. As a result, the scour process is significantly affected by the presence of the ripples with the pipeline being periodically buried either partly or completely by the moving ripples.The influence of scouring progress on wave forces was found to vary significantly in different aforementioned regimes. In regime I, the wave forces were quite stable; in regime II and III, the wave forces generally underwent a gradual reduction and reached their equilibrium force values at rather early stages of the scour process; in regime IV, the wave forces were significantly affected by the ripples activity and usually higher than that without the influence of ripples.Pipelines should be buried to different depths to reduce the wave forces depending on different soil grain size of the seabed. In silt soil, the wave forces on initially half buried pipelines were much smaller than that on initially fully exposed pipelines, but that were not necessarily true in sandy bed. In scour regime IV of sand bed, where the ripples activity can significantly influence the scour process, wave forces (with high fluctuation) on initially half buried pipelines are even higher than those on initially unburied pipelines. These results strongly suggest that the submarine pipeline needs to be buried below the active layer of sand ripples in order to reduce wave forces.Both static and dynamic analyses were used to study the maximum allowable free span length (MAFSL) of the pipeline. The study results show that, the MAFSL under static conditions is 56m. However, the MAFSL is 30m and 20m under ordinary weather conditions and hurricane-induced currents in 100-year return period respectively to avoid Vortex Induced Vibration (VIV) calculated using the highest safety class factor. It is suggested that the spans longer than 20m should be disposed. Additionally eight successive spans which may also threaten the pipeline were proposed in the study. The most hazardous scour pits are in the pipeline section from KP42 to KP51.Methods for protecting free spanning pipelines were analyzed and compared. A so called "artificial grass" technique is suggested for protecting the pipeline section from CEP to KP42, while the re-trenching method is suggested for the section from KP42 to KP51.更多还原