2 CONSIDERATION OF ALTERNATIVES

2.1 INTRODUCTION

2.1.1 Background

Towngas proposes to install twin submarine pipelines to meet expected future demand and to supply natural gas as an alternative feedstock. The pipelines will convey gas from the Cheng Tou Jiao GRT to the existing Towngas GPP in Tai Po. At the outset of this study, three broad routing corridors were defined for locating the pipeline system, which involved a land route, a marine route and a combination of the two (ie land and marine route).

A desktop study was conducted to identify environmental and physical constraints, further define the route corridors and to undertake an initial environmental review and qualitative risk assessment of the three possible routes (Annex A). The purpose of the work was to identify the preferred routing corridor for analysis and further refinement as part of the EIA.

The conclusion of the desktop study was that when considering both the potential environmental impacts and the potential risk associated with the Project, a marine route, was preferred and should be the subject of further refinement as part of the EIA. The findings of the study were presented at a forum attended by various NGOs (1) and feed back was obtained on the preferred route for incorporation in the EIA.

2.1.2 Land or Marine Based Tee-off Point

During the desktop study one of the considerations was that the Project would operate with an intelligent pipe inspection gauge (PIG), which will be performed on a routine basis (once every ten years). As part of Project a tee-off point (2) provision with pigging facility and an emergency flare would be required at a location between the GRT and the Gas Receiver Station (GRS). Originally Towngas intended to have the flare located within the grounds of the GPP at the Tai Po Industrial Estate. However, upon review of the available space within the Industrial Estate it was apparent that land was constrained. Consequently, a location was sought along the pipeline route for siting a flare.

As discussed in Annex A, two options were identified, one on Tap Mun and one on Wong Chuk Kok Tsui. However, although the Tap Mun option was preferable to that at Wong Chuk Kok Tsui, construction would result in disturbances to the terrestrial environment and both the intertidal and subtidal habitats.

Consequently, Towngas held discussions with the Joint Executive Office of the LNG Terminal at Cheng Tou Jiao in Shenzhen and the plot layout was revised to incorporate a flare within the site in Shenzhen. A landing point at Tap Mun was, therefore, no longer required.

The Project Engineering Team consequently, proposed that a subsea tee-off point was utilised instead of a land based one.

Key issues in designing the subsea tee and fixing the location were:

* Construction of the subsea tee should avoid unacceptable impacts to the marine environment;

* Operation of the subsea tee should not impact or represent a hazard to marine users (eg shipping vessels fishermen and sports divers).

Taking the above into account a subsea tee was designed as follows:

* sited at a location in deep water (18 - 20 m);

* sited away from major marine traffic routes;

* designed to minimise the chances of fishing nets or anchors snagging on the structure by burying in the seabed; and,

* designed to protect from anchor drop, thereby reducing safety risks.

Further information on the subsea tee is presented in Section 3.

2.2 CONSIDERATION OF ALTERNATIVE ALIGNMENT OPTIONS

The EIAO Study Brief issued for this Project requires the examination of different alignment options (Clause 3.3.1) with a view to avoiding or minimising the potential environmental impacts of the Project, specifically:

* Potential impacts to water quality;

* Knock-on effects to the ecologically sensitive areas within Tolo Harbour, Tolo Channel and Mirs Bay; and,

* Avoiding impacts to coral communities and fisheries habitats.

The Study Team's approach to fulfilling the requirements of the Brief and selecting the preferred alignment is detailed below.

2.2.1 Minimise Impacts to Water Quality

An analysis of different construction methods and techniques to minimise impacts to water quality was conducted. The details of this analysis are presented in Section 2.3.

2.2.2 Identify the Presence of, and Avoid, Ecologically Important Habitats

The desktop study discussed in Section 2.1 (and presented in Annex A) identified the presence of ecologically important areas along the general marine routing corridor, including the following (presented on Figure 2.2a).

Marine Parks: There are three Marine Parks in the Study Area at Yan Chau Tong, Hoi Ha Wan and Tung Ping Chau. Marine Parks are gazetted for conservation, recreation and educational purposes and are under the control of the Country and Marine Parks Authority (CMPA). All three parks contain an abundance of coral and fish species and hence direct and indirect disturbances should be avoided as far as practically possible. Consequently, it was decided that the centreline of the pipelines alignment should not pass through the marine parks or be located within 500 m of the boundary (3).

Sites of Special Scientific Interest (SSSIs): Although there are a number of SSSIs located within the Study Area, only the mangrove stands at Ting Kok and Kei Ling Ha (and the Hoi Ha Wan and Ping Chau SSSIs which are located within Marine Parks) were designated for ecological reasons. The mangroves at Ting Kok and Kei Ling Ha are located within bays away from the marine routing corridor and hence will not be directly or indirectly affected by the proposed works.

Proposed and Gazetted Artificial Reefs: Artificial reefs (ARs) have been deployed in the Yan Chau Tong and Hoi Ha Wan Marine Parks and are proposed for deployment within Long Harbour. ARs are deployed to enhance fisheries and marine ecological resources and are under the jurisdiction of the Agriculture, Fisheries and Conservation Department (AFCD). ARs can be regarded as water quality sensitive receivers and impacts to them should be avoided as far as possible. The pipelines alignment, by avoiding the Marine Parks, also avoids the ARs located there. The Long Harbour ARs are located away from the marine routing corridor and hence will not be directly or indirectly affected by the proposed works.

Fish Culture Zones (FCZs): There are 13 FCZs within the Study Area, the majority of which are located within enclosed bays. Impacts to FCZs are controlled by the Water Pollution Control Ordinance and the Marine Fish Culture Ordinance. FCZs can be regarded as water quality sensitive receivers and impacts to them should be avoided as far as possible. Consequently, it was decided that the centreline of the pipelines alignment should not pass through an FCZ or be located within 500 m of the boundary(4).

Fisheries Protection Areas (Proposed): A Fisheries Protection Area (FPA) is proposed for designation under the Fisheries Protection Ordinance within Tolo Harbour, Tolo Channel and Long Harbour. Although designated to provide specific controls on fishing activities, the area can be regarded as a sensitive water body and any impacts should be minimised as far as possible during construction to achieve compliance with the Water Quality Objectives. As the FPA covers a large portion of the marine routing corridor, direct impacts to the FPA cannot be avoided. Consequently, the focus was on minimising the severity of indirect impacts arising from perturbations to water quality through an examination of construction methods and programmes.

Corals (Literature Review): Over 80 species of coral occur in Hong Kong, with the highest diversities recorded in eastern waters. Coral reefs support a range of species providing sheltering, feeding, spawning and nursery areas, resulting in a large and diverse community. The coral reef system has been shown to be sensitive to water quality. In order to fine-tune the alignment to avoid direct impacts to corals (and minimise indirect impacts) the locations of high ecological value coral communities were identified. These included the aforementioned Marine Parks and also Chek Chau (Port Island), Double Island, Gruff Head and Fung Wong Fat. Consequently, it was decided that the centreline of the pipelines alignment should not directly affect these habitats.

Hard Corals, Soft Corals and Black Corals (Dive Surveys): It was agreed with the AFCD and EPD that surveys should be conducted along the majority of the northern coast of Tolo Channel to update and fill the information gap in the literature on the corals (including hard corals, soft corals, gorgonians and antipatharians (black corals)). The results of the surveys are presented in Annex C with summary information presented in Section 6.

The Front End Engineering Design (FEED) component of the study included the conduct of detailed geophysical and geotechnical surveys of the marine routing corridor. To further verify that hard or soft corals would not be directly impacted by the Project, the results of the geophysical survey were analysed to determine whether hard substrate that may support the growth of these species is present away from the coastline.

Two small areas were identified, at Pak Sha Tau and in between Fung Wong Fat and South Wong Chuk Kok Tsui. At the former locations, the seabed consisted of fine mud a sand/gravel mixture. Two species of gorgonian were identified as colonising abandoned fishing nets. The gorgonians were sparsely distributed and small in size (< 30 cm). At the latter site no hard or soft corals were recorded and the only organism observed was one saucer anemone (Heteractis sp). Because of the sparse coverage of organisms the areas were not considered to be a constraint to the routing of the pipelines.

A Marine Archaeological Investigation (MAI) was conducted using the results of the geophysical survey. Although anomalies were identified from the side scan sonar surveys, none of them were close to the centreline of the pipelines alignment or were classified having high archaeological potential and hence they did not constrain the pipelines alignment.

2.2.3 Physical Constraints to the Alignment

Once the above considerations had been accounted for, a refined alignment corridor was proposed. This alignment was then examined against known physical constraints (Figure 2.1b of Annex A) and further refined as follows.

Mirs Bay Dangerous Goods Anchorage (DGA): There is a DGA located to the northeast of Chek Chau, which is used mainly by vessels entering Tolo Harbour. The alignment was shifted to the north to avoid the DGA and minimise the risk of anchor damage to the pipelines and the consequential risks.

Tolo Harbour Anchorages: There are five laid-up berths around A Chau Island, which are used by work boats in the harbour, as well as an anchorage area used by deep draft cement vessels to the east of A Chau Island. The alignment was shifted to the north to avoid the berths/anchorage and minimise the risk of anchor damage to the pipelines.

Water Supplies Department (WSD) Twin Main: There are twin water mains passing from Plover Cover Reservoir to the Sai Kung coastline at Nam Chung. The water mains cannot be avoided by the pipelines alignment. However, further to requests by the WSD and CED the angle of crossing has been set as close to 90o as possible and sited within deep water (approximately 19m) to minimise the risk of damage to the mains and facilitate the crossing.

Towngas Naphtha Pipe: This pipeline runs from a mooring dolphin off Ma Shi Chau to the Tai Po Industrial Estate seawall outside the existing Towngas GPP. The proposed pipelines would cross the naphtha pipeline instead of the large sewer and consequently the crossing angle was kept as close to 90o as possible (taking into account other constraints at the crossing point).

Drainage Services Department (DSD): The effluent export pipe from the Tai Po Sewage Treatment Works passes across Tolo Harbour and connects to the Shatin Sewage Treatment Works. Engineering information indicated that it would be technically challenging to cross this large pipe and consequently it was deemed preferable to avoid a crossing by landing the pipeline to the east of the DSD pipe at the Tai Po Waterfront Park seawall.

2.2.4 Onshore Routing Constraints

The proposed landing point is located towards the eastern end of the Tai Po Waterfront Park seawall. Two routes were identified to take the pipelines from the landing point to the GRS.

* Route Option 1 is located along the sea front walkway. The route is parallel to the seawall and cuts into the Towngas complex at the southeast point. The total length is approximately 1,040 m of which approximately 640 m is outside the Towngas complex (see Figure 2.2b).

* Route Option 2 cuts into the industrial estate travelling north beside the Motorola factory and turns west towards Dai Hei Street after 300 m. At the end of Dai Hei Street, it turns north again, into a car park area and eventually turns west and enters the Towngas complex at the northeast side of the tank farm. The total length is approximately 1,564 m of which approximately 960 m is outside the Towngas complex (see Figure 2.2b).

The engineering, environmental and risk considerations of the alternative route options were examined. From an engineering perspective, Option 1 presents less physical constraints and there were fewer existing utilities to cross. In terms of environmental implications both routes were similar; however, Option 2 is longer in length and has greater potential to impact landscape resources of the area as a result of possible disturbance to mature trees along the alignment. From a risk perspective Option 2 has increased potential for fatalities and pipeline damage due to the greater length and higher population.

2.3 CONSIDERATION OF ALTERNATIVE CONSTRUCTION METHODS AND WORKS SEQUENCES

The EIAO Study Brief issued for this Project requires the exploration of alternative construction methods and sequences of works for the Project (Clause 3.3.2) with a view to avoiding prolonged adverse environmental impacts to the maximum practicable extent.

2.3.1 Installation Techniques

The methods commonly used to install submarine pipelines include dredging, jet ploughing, jetting (trenching) and jetting (injection). For this Project the jet ploughing technique is not possible due to a combination of the size of the pipelines (18" diameter) and the required burial depth (minimum of 3 m) and hence is not discussed further.

Dredging

For submarine utility installations, dredging involves the removal of marine sediments from the seabed to form the trench, into which the pipelines are laid. Many dredging techniques, such as grab dredging, cutter suction and trailer suction dredging are available and chosen depending on the prevailing environmental conditions (eg shear strength of marine deposits). Dredging can be a comparatively fast way to construct a pipeline trench and is necessary in areas where extra pipeline protection is required (eg rock armour protection). However, the potential for impacts to water quality is higher than say jet ploughing or jetting, and the excavated sediments will, under Hong Kong's legislation (5) require disposal off-site at a designated disposal ground.

Jetting (Trenching)

Pipeline jetting is any method that uses water jets to break-up, remove or liquefy the soil from under a marine pipeline allowing the pipeline to settle at an elevation below the seabed. Jetting is a technique, which means the pipeline is pre-installed (laid) on the seabed before jetting is carried out. The jet machine ejects plumes of fluidised soil out of the eductors on the sides of the machine in the bottom portion of the water column (Figure 2.3a). Dependent on the site conditions these plumes of fluidised soil have the potential to become entrained in the water column.

Jetting (Injection)

The injection process is similar to the trench jetting in that it is a post lay technique. However, it differs in that the sediments are not excavated from the trench once they are fluidised. Instead the jetting machine uses two swords to cut into the sediments and then fluidise the marine sediments in front of the machine and between the two swords (Figure 2.3b). The pipelines are positioned between the two swords and hence settle down into the fluidised material. Depending on the strength of the sediments this technique may require up to three passes to lay the pipelines at the required depth of 3 m. However, as little material is entrained into the water column this technique in unlikely to cause adverse impacts to water quality.

Summary

In light of the above discussions the aim for this Project was to develop an installation methodology that minimised the amount of dredging in Hong Kong waters and maximised the amount of the pipelines alignment that could be installed using injection jetting. The intended installation method is discussed further in Section 3.

2.3.2 Installation Configuration

The Project involves the installation of twin submarine pipelines, which can be installed in separate trenches or bundled together in one common trench. The Study Team examined the engineering and environmental implications of these two options. From an engineering perspective a bundle lay would require modifications to the laybarge but would result in a shorter overall construction period. The bundle lay was considered, qualitatively, to result in less severe disturbances to the marine environment as it will result in less direct and indirect disturbance to seabed sediments and hence to water quality, marine ecology and fisheries resources.

Timing and Sequencing

The issue of timing and sequencing has been analysed as part of the water quality impact assessment in Section 4. Modelling scenarios have examined the impacts to water quality of undertaking the work in either the dry or the wet season. For both jetting works and dredging works both seasons have been examined to be acceptable in the sense that water quality, ecological and fisheries criteria are complied with.

2.4 SELECTION OF PREFERRED SCENARIO

The discussions presented in Sections 2.2 and 2.3 have examined the rationale behind the selection of the preferred alignment, the preferred working method and the issue of timing. Section 3 presents the details of the preferred construction methods and the installation sequence that has been assessed as part of this EIA.

Taking into account the examination of different alignment options a preferred alignment is presented in Figure 2.4a. This alignment for the pipelines has been studied in detail as part of this EIA Report. The selection of this position was taken after a holistic review of the environmental constraints (corals), physical constraints (navigation channel) and the results of the water quality modelling exercise. It was noted from the results of the water quality modelling (Section 4.6 and Annex B) that if the pipelines were shifted more to the north than the present position impacts may arise to corals along the northern coast of Tolo Channel. Similarly if the pipelines alignment was shifted further to the south than the present position, impacts may arise to corals along the southern coast of Tolo Channel and the Hoi Ha Wan Marine Park. Additionally, by moving the pipelines further south they will be located within the main navigation route into Tolo Channel which has the potential to increase the level of risk of damage to the pipelines from marine traffic impacts (eg external hazards such as anchor drop).

The environmental and physical constraints have been presented along with the preferred alignment for the pipelines in Figure 2.4a. As can be seen from the figure the pipeline alignment avoids direct impacts to the coral areas and Marine Parks.

The alignment presented on Figure 2.4a, therefore, represents the preferred alignment for the pipelines taking into account ecological, water quality and marine traffic constraints.

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(1) The forum was held in June 2001 and attendees included WWF Hong Kong, Friends of the Earth and Conservancy Association.

(2) To allow for possible future connections to an alternative source of gas.

(3) A conservative value of 500 m was used as suspended sediments were not expected, based on previous experience on other submarine utility projects, to travel further than 500 m assuming standard working rates and practices.

(4) Ibid.

(5) Dumping at Sea Ordinance dictates that excavated sediments should be disposed of at a designated disposal site. The selection of disposal site is determined by EPD and CED with reference to contaminant levels in the sediments (ETWTBCW 34/2002).