12                 hazard to life

Introduction

12.1            For both Aberdeen and Ap Lei Chau project sites, potential damages to the Potential Hazardous Installations (PHIs), namely the Hongkong & China Gas Company’s Gas Holder (PHI No. H4) (hereinafter referred to as “HKCG Depot”) and the Shell LPG Transit Depot/Bulk Domestic Supply (PHI No. H5) (hereinafter referred to as “Shell Depot”). can be avoided with implementation of safety measures and close monitoring procedures. Operation of these PHIs would not be disturbed and gas supply would not be disrupted provided that vibration and ground settlement caused by construction works can be controlled within allowable limits.

Mitigation Measures

Ground Settlement

12.2            Ground settlement is to be monitored by a series of geotechnical instrumentations. A series of ground settlement markers and piezometers are to be installed in the vicinity of the proposed tunnel and near the HKCG Depot. Monitoring of groundwater and settlement will form an important part of the strategy to confirm the effectiveness of the mitigation measures and to adjust the inflow limits if necessary. In order to cater for unforeseen problems encountered during the construction stage, additional monitoring and instrumentation system would be installed in the vicinity of the HKCG Depot at a later stage by the Contractor before the commencement of construction works. Typical allowable limits for well maintained buildings and historic buildings are 25mm and 5mm respectively. Allowable limit of 13mm is proposed and the actual allowable limit is subject to further requirements of Civil Engineering and Development Department (CEDD) and relevant authorities. Gas facilities should be able to withstand ground settlement (including differential settlement) within the allowable limit.

 

Ground Vibration

12.3            Monitoring plan should include setting up monitoring points at fixed locations adjacent to more sensitive structures which have lower vibration tolerance. The monitoring would be carried out at ground level with sensors mounted on small concrete bases embedding into undisturbed ground. Each sensor would be able to record ground vibration. For some particularly sensitive structure within the Shell Depot such as LPG compound, sensor would be located at the structure to ensure the vibration limit at the structure is not exceeded. Allowable limit of 5mm/s PPV was proposed to HKCG. The same criteria would be also applied to the Shell Depot.

 

Gas Leakage Detection for Ap Lei Chau Site

12.4            It was advised, during the HAZOP meeting, that no gas leakage detection alarm is installed in the Shell Depot. It is recommended to install gas detector/ alarming system to provide warning to Shell operator and Construction Site staff in case of gas leakage. This provides an early warning for operation staff and construction staff in case of gas release. Specific emergency procedures should be developed, together with Shell Depot Operation Team, and documented in an emergency plan to cater for gas leakage scenarios before the construction stage.

 

Emergency Plan and Procedures

12.5            The following measures are recommended as “Best Practice” for DSD to implement during construction stage.

 

(i)         Emergency evacuation procedures should be formulated and DSD should ensure all workers on site should be familiar with these procedures as well as the route to escape in case of gas release incident occur. Diagrams showing the escape routes to a safe place should be posted in the site notice boards and at the entrance/exit of site;

 

(i)         The emergency procedures should specify means of providing a rapid and direct warning (e.g. Siren and Flashing Light) to construction workers in the event of gas release in the gas facilities; and

 

(ii)       The construction site officer of DSD should establish a communication channel with the gas operation personnel during construction stage. In case of any incidents in the gas facilities that need site evacuation, operation staff of gas facilities should advise the DSD’s site officer to evacuate the construction workers; and

 

(iii)      Induction Training should be provided to any staff before working at the both work site; and

 

(iv)      Periodic drills should be coordinated and conducted to ensure all construction staffs are familiar with the evacuation procedures. Upon completion of the drills, a review on every step taken should be conducted to identify area of improvement.