沉管隧道管节拖航水阻力原型试验

Water resistance proto-type test of immersed tunnel element towing

港珠澳大桥沉管隧道的管节在海中拖运时的体积达到76 000 m^3。在评估阶段计划采用4艘拖轮进行拖航。施工准备期间进行了拖航试验,结论是需要6艘拖轮。第一次实施拖航作业时,实际的拖航编队用了8艘拖轮,尽管有了这个"冗余"的考虑,管节与船队在拖运途中不进反退了700～800 m。这说明了管节与船队受到的实际阻力与物模试验存在差异。工程连续安装近30个沉管时,团队操作协同进入佳境,作者利用这一时机,进行了5次实体管节拖航的实验,观测管节的速度以及相应的拖航阻力。发现管节在受限航道拖运,拖力增加,速度增大。但是到达了一定的程度,即便拖力继续增大,速度的增加趋势变缓。并将该观测数据与前期的1:40物理模型试验进行了对比,发现物理模型试验结果对拖运速度的估计偏大,也即估计的水阻力偏小,可能原因是边界条件的差异,也可能是尺寸效应的影响。

In the Hong Kong-Zhuhai-Macao Bridge( HZMB) project construction,the immersed tunnel element reached up to 76 000 m^3 in volume by the time of transportation at sea. In the early design stage,4 tugboats were planned to be applied to the towing works. In construction preparation stage,physical model test was conducted and the result showed that 6 tugboats were to be required. In the first time of towing,8 tugboats were used but the elements under towing fell backward around 700 ~ 800 m instead of moving forward, which demonstrated the deviation between the actual resistance encountered by the towing fleet and the result of physical model testing. By the time when nearly 30 elements were installed,the operation team had formed a good cooperation and everything was going on safely. Taking advantage of this opportunity,the test of towing the real tunnel element was orgnized and the advancing speed of the element and the corresponding resistance were observed in five towing operations. It was found that the element,being towed in limited navigation channel,had its speed increased with the increasing of towing power. However,when the advancing speed of the element reached to a certain limit,its speed increasement tended to become slow even if with increase of towing power. In addition,this observed data was compared with the result of previous physical model test with scale of 1: 40 and it was found that the results of the physical model testing overestimated the towing speed,i. e. underestimated the water resistance.The reasons may be the differences of boundary conditions and the impact of scaling effect.