From Mooring to Mooring: The Critical Issues of Out-Docking, Towing and Mooring of Immersed Tunnel Element in Hong Kong-Zhuhai-Macao Bridge Project

Lin, Ming (CCCC HZMB Island and Tunnel Project) | Lin, Wei (CCCC HZMB Island and Tunnel Project) | Huang, Weimin (CCCC HZMB Island and Tunnel Project) | Ning, Jinjin (CCCC HZMB Island and Tunnel Project)

OnePetro 

ABSTRACT

Thirty-three immersed tunnel elements of Hong Kong-Zhuhai-Macao bridge project had been installed in offshore condition without major accident: each typical element has a mass of around 76,000 t. In this paper, the author elaborated the special solutions and the critical details developed behind the work of plan, out-docking, towing, and mooring for immersion. Three work principles were learnt from this project, the redundancy design to the solutions, the rehearsal before real works, and the selection of appropriate timing and location in accordance with the detailed work plan.

INTRODUCTION

Background

In Hong Kong-Zhuhai-Macao Bridge project (HZMB), thirty-three (tunnel) elements with a large mass of 76,000t had been installed in offshore condition one after another from the year 2013 to 2017 (Lin, 2017) without a major accident. The HZMB tunnel is the longest roadway tunnel that has ever been built, even for shorter one accident during installation was not rarely seen; Walter (1997) presented some accidents learnt from US experience and Lars (2000) Øresund link. The thirty-five times installation (Lin (2017) reported that E15 had been installed for three times) with safety was thus not a coincidence. This paper elaborates the special efforts behind this good ending.

Among the works of installation of element in the HZMB project, the three steps of docking (warping) out, towing, and mooring for immersion were regarded as challenging works. To explain, the description starts with the completion of element production. Firstly, by inundation, element floated up with its position controlled by mooring lines that were connected to the winches on land. In times of installation, the element was then winched out through the dock gate. Then, to transport the element on the sea, the tugboats were used and tied to element by nylon ropes. Meanwhile, the mooring lines fixed on element were released to let element go. The towing distance is around 11 km. When element was towed to site, it needs to be moored again (i.e. connected with mooring lines which are anchored to the sea floor) for the subsequent operations of immersion and underwater connection. It can be seen that each element has to experience a transition state from the mooring in prefabrication yard to the mooring in tunnel site, the latter exposed to current and waves. Further, the natural water depth is shallow, so the element can only be towed in a confined channel (Fig. 1). If the element was out of control, severe consequence is expected: the project could suffer great loss in terms of cost penalty and time-delay, and the stranded element could block the already busy ship channel at Pearl River estuary.