Construction of Approach Channel & Turning Basins for the Black Point Location

 

C1          Construction of Approach Channel & Turning Basin

C1.1             General

In order to reduce dredging, it is necessary to construct the jetty for berthing of LNG carriers in water down to –14.3mPD and reasonably as close to the fairways as practical.  The jetty will be located in an area free from marine services and traffic.  Maintenance dredging will be required.

Dredging should be avoided in areas such as marine fairways, typhoon shelters and areas with submarine utilities.  The various boreholes indicate the dredging is likely to be wholly within the soft Marine Deposit layer, which will consequently require side slopes of about 1:4 for long term stability.  It has been assumed that the dredged sediment is not significantly contaminated and can be dumped at an uncontaminated mud disposal ground.  Rock excavation for the construction of approach channel and turning basin should be reduced to the extent practical in order to reduce impacts on the seabed and surrounding water environment.

The Black Point site is located where there is no marine service, and marine traffic is unrestricted.  The access for LNG carrier will be via a dredged channel to the main shipping channel to the west.  In order to locate the LNG jetty away from the main marine traffic, dredging for a turning circle and berth box is required as shown in Figure 2.1.

C1.2             Assessment Parameters

In order to assess the engineering implications of the marine dredging for each layout option the following assessment parameters have been used: -

·          Total length of approach channel and turning basin;

·          Volume of dredging;

·          Rock excavation in dredged zone;

·          Impact on existing utilities; and,

·          Siltation & maintenance.

C2          Total Length of Approach Channel and Turning Basin

C2.1             Option 1 – Base Case

The estimated length of approach channel and turning basin is approximately1.15km.

C2.2             Option 2 – Full Reclamation

The estimated length of approach channel and turning basin is approximately 1.15km.

C2.3             Option 3 – Full Excavation

The estimated length of approach channel and turning basin is approximately 0.65km.

 

C3          Volume of Dredging

C3.1             Option 1 – Base Case

Dredging is required for both the approach channel and the turning basin.  The total volume of dredging and subsequent rock excavation is estimated to be approximately 2.49 x 106 m³.

C3.2             Option 2 – Full Reclamation

Same as Option 1.

C3.3             Option 3 – Full Excavation

The total volume of dredging and subsequent rock excavation of the shorter approach channel and turning basin is estimated to be approximately 1.40 x 106 m³.

 

C4          Rock Excavation in Dredged Zone

From the available information, the chance of requiring excavating rock during dredging is low.

 

C5          Impact on Existing Utilities

Apart from the existing BPPS’s seawater intake located nearby, it is envisaged that no existing submarine utility is in the dredging approach channel and turning basin. 

 

C6          Siltation & Maintenance

Siltation study recently carried out suggests that siltation rate in the vicinity of Black Point is estimated at approximately 20cm per year, and hence the need for maintenance dredging is high.


C7          Summary for the Approach Channel & Turning Basin

A summary of the parameter values and relative scores derived from the engineering assessment for the construction of the approach channel and turning basin is given in Table C1 below.

Table C1 - Summary for Approach Channel and Turning Basin Construction

Parameter

Option 1
(Base Case)

Option 2
(Full Reclamation)

Option 3
(Full Excavation)

Total length of approach channel + turning basin (km)

RS = 3

(1.15)

RS = 3

(1.15)

RS = 5

(0.65)

Volume of dredging
(106 m³)

RS = 3

(2.49)

RS = 3

(2.49)

RS = 5

(1.40)

Rock excavation in dredged zone

RS = 3

(0)

RS = 3

(0)

RS = 3

(0)

Impact on existing utilities

RS = 3

RS = 3

RS = 3

Siltation and maintenance dredging (cm/yr)

RS = 3

(20)

RS = 3

(20)

RS = 3

(20)

RS = Relative Score

C8          Scoring for the Approach Channel & Turning Basin

Each of the parameters summarised above in Table C1 have been scored in accordance with the procedure described in Section 2.1.  The results are shown below in Table C2.  The table also shows the total score for each layout option derived using the weightings given in Table 2.5.

Table C2 – Scoring for Each Layout Option for the Approach Channel and Turning Basin

Parameter

Weight

Option 1
(Base Case)

Option 2
(Full Reclamation)

Option 3
(Full Excavation)

Score

WS

Score

WS

Score

WS

Total length of Approach Channel + turning basin

0.2

3

0.60

3

0.60

5

1.00

Volume of dredging

0.35

3

1.05

3

1.05

5

1.75

Rock excavation in dredged zone

0.2

3

0.60

3

0.60

3

0.60

Impact on existing utilities

0.15

3

0.45

3

0.45

3

0.45

Siltation & maintenance dredging

0.1

3

0.30

3

0.30

3

0.30

Total Weighted Score

 

3.00

 

3.00

 

4.10

Normalised Score

3.66

3.66

5.00

 

From the result of the assessment of all parameters for the approach channel and turning basin, Option 3 is the preferred layout.