Construction of Approach Channel & Turning Basins for the South Soko 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 as deep as –15mPD and as close as possible to marine fairways where the deep water exists.  The jetty will be located in an area free from marine services and traffic.  In all site layouts considered, the dredging of the turning circle and approach channels are undertaken to approximately –15mPD which will require significant maintenance dredging as well as affecting tidal flows in the vicinity.

The available investigation information indicates that the dredging is likely to be wholly within the soft Marine Deposit layer, which will 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 avoided in order to reduce impacts on the seabed and surrounding water environment.

South Soko Island is located where the marine service and marine traffic is unrestricted.  The potential access for LNG carrier will be from the deep waterway to the south of the island to the proposed jetty.  In order to facilitate the LNG berthing a significant amount of dredging is required for a turning circle and approach channel. 

C1.2         Assessment Parameters

In order to assess the engineering implications of the marine dredging at each of the sites the following assessment parameters have been used: -

·          Total length of approach channel & turning basin.

·          Volume of dredging.

·          Rock excavation in dredged zone.

·          Impact on existing utilities.

·          Siltation & maintenance.

C2          Total Length of Approach Channel and Turning Basin

C2.1         Option 1 – Base Case

The position of the jetty head results in a significant amount of dredging to give access to the jetty from the navigation channel.  The total length of the approach channel and turning basin is estimated to be 6.3km from the deep water with a seabed level of approximately -15mPD.

c2.2         Option 2 – Full Reclamation

The route of the approach channel and turning basin and the location of the jetty of Option 2 are the same as that of Option 1.

C2.3         Option 3 – SE Jetty

The location of the jetty at the southeast corner of South Soko Island helps reduce the length of the approach channel and turning basin to 2.2km from the deep water.

 

 

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 3.36 x 106m³.

C3.2         Option 2 – Full Reclamation

The approach channels and turning basins are the same as Option 1.

C3.3         Option 3 – SE Jetty

The total volume of dredging and subsequent rock excavation of the shorter approach channel and turning basin is estimated to be 1.07 x 106m³.

C4          Rock Excavation in Dredged Zone

The quantity of the rock required to be excavated is estimated at about 0.03 x 106 m3 for all options.

C5          Impact on Existing Utilities

Dredging of the approach channel and turning basin is not likely to encounter any existing submarine cables.

C6          Siltation & Maintenance

Siltation study recently carried out suggests that siltation rate in the vicinity of South Soko Island is estimated at 0.5cm/yr for all options, which is relatively minor, and hence the need for maintenance dredging is low.


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
(SE Jetty)

Total length of approach channel + turning basin (km)

RS = 3

(6.3)

RS = 3

(6.3)

RS = 5

(2.2)

Volume of dredging
(106 m³)

RS = 3

(3.36)

RS = 3

(3.36)

RS = 5

(1.07)

Rock excavation in dredged zone (106m3)

RS = 3

(0.03)

RS = 3

(0.03)

RS = 3

(0.03)

Impact on existing utilities

RS = 3

RS = 3

RS = 3

Siltation and maintenance dredging (cm/yr)

RS = 3

(0.5)

RS = 3

(0.5)

RS = 3

(0.5)

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 2.1.3.  The results are shown below in Table C2.  The table also shows the total score for each site derived using the relative 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
(SE Jetty)

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

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, it is found that layout option 3 is preferred.