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June 2004

TDI-Brooks Awarded a Metocean, Environmental, Bathymetry/Hazard Survey and Geotechnical Coring Project by Nigerian LNG Ltd.

TDI-Brooks was awarded by Nigerian LNG Ltd. (NLNG) a $1.4 million/8-month project on June 16, 2004.  The study is entitled “Metocean and Environmental Baseline Data Collection for BOD/BDP and ESHA Studies.”  The study involves metocean, environmental, bathymetric and site survey, and geotechnical services for the Nigerian LNG Ltd. Channel Deepening Project.

On behalf of Nigeria LNG Ltd, Shell Gas Nigeria B.V. is supervising the Channel Deepening Project near shore off Bonny Island in Nigeria.  The Liquefied Natural Gas (LNG) operations in the Niger Delta comprise (a) ‘upstream activities’, i.e. the production of natural gas by several oil & gas producers, (b) transport of the gas through a network of onshore and offshore pipelines to Bonny Island, and (c) ‘downstream activities’, i.e. the gas liquefaction at the LNG complex at Finima.  Nigeria LNG Limited (NLNG) was incorporated in 1989 and tasked with the harnessing of Nigeria’s vast natural gas reserves.  The company shareholding comprises Nigerian National Petroleum Company  (NNPC) 49%, Shell Gas B.V (Shell) 25.6%, Total 15.0% and ENI 10.4%.  The Nigeria LNG Plant is located on the western coast of Bonny Island, approximately 40 km south of Port Harcourt, Nigeria. The plant site is situated between Shell’s Bonny Crude Oil Terminal  (BCOT) and the Mobil NGL site.

Picture 1: Liguid Natural Gas Carrier being loaded at the Nigerian LNG
Terminal, Bonny Island, Nigeria

Vessels currently approach the jetties at NLNG from sea via an approximately 17 km long approach channel, which is 215 m wide between the toes (see Pictures 1 & 2). The approach channel depth is maintained by means of regular maintenance dredging operations. The channel connects into the Bonny River for the final approach to the plant.  To allow for sufficient capacity for the NLNG expansion and to allow for the new LNG carriers, the present approach channel has to be widened and deepened to remove the tidal window, which now exists for LNG carriers.   This study is part of a broader environmental assessment of these dredging activities. The proposed deepening of the channel is a continuation of the dredging that is already continually required, but aims also at a further deepening and widening to allow LNG carriers to berth during the whole tidal cycle.

Picture 2: LNG Carriers coming up the approach channel at the NLNG
Terminal as well as one carrier at the terminal loading facility. The NLNG
terminal is located near the mouth of Bonny River, south of Port Harcourt.

Figure 1 shows the study area of the project.  It is located in Rivers State southeastern Nigeria, some 40 km SE of Port Harcourt.  The figure shows the existing channel, NLNG terminal and dredge material disposal sites. 

Figure 1: Project Study Area

The area of interest includes the offshore and near-shore areas of the Bight of Bonny and the Bonny River (system) as far as potentially influenced by the dredging activities (including the disposal of dredged material) and the marine activities related to oil & gas production (and shipping) on Bonny Island.  The area of interest specifically refers to: 

·        Offshore approach channel to be dredged (widening and deepening). 

·        Potential offshore disposal sites for the disposal of dredged material, i.e. the disposal site currently in use for maintenance dredging and/or a new disposal site.

·        Coastal waters west and east of Bonny River potentially influenced by movements of sediments as a consequence of dredging and disposal of dredged material (siltation, erosion of sediments and geomorphology of the coast).

·        Approach channel and turning basin in front of NLNG jetties used by ships associated with the terminal activities.

·        Inland and near-shore waters as far as (potentially) influenced by routine (normal effluents) or acute (spills) discharges of ships associated with the oil & gas activities. Due to tidal movements this influence could well go far upstream of Bonny Town. 

·        Social environment (villagers) as far as dependent on the rivers and coastal waters for fisheries.   

The area of interest covers, therefore, the waters of the Bonny Local Government Area to the boundary of the Territorial Waters.  The waters in the Bonny LGA comprise a very dynamic system of – dependent on the moment in the tidal cycle – fresh, brackish or marine conditions. The Bight of Bonny has a high intensity of wave action. The prevailing SSW winds have a long fetch and generate large swell waves that strike the coast obliquely and generate strong alongshore currents running eastwards along the coast.  A distinct sea wave component is usually also present.  These factors, in combination with prevailing weather and tide conditions, promote erosion and sediment movement on stretches of coastal and tidal waters.  Tidal flats and mangrove areas (swamp) lie behind the beach ridge barriers and are inundated by tide water daily.

The field works includes several distinct elements including: 

1.      Physical and chemical characteristics of the seabed in the area to be dredged and the (potential) disposal sites;

2.      Physical, chemical and biological characteristics of the water column in the area to be dredged and the (potential) disposal sites;

3.      Benthos (animals and plants living in and on the seabed) in the area to be dredged and the (potential) disposal sites; and

4.      Hydrodynamic characteristics – particularly currents, tides, sediment transport, storm events, temperature, depth – in the area of interest.

Metocean Survey Component  - Metocean Services International (MSI) out of Cape Town, South Africa will undertake the metocean survey component as TDI-Brooks' subcontractor.  The main components of the metocean study include deployment of two current meter moorings near the mouths of the Bonny and Calibar Rivers, installation of a tide gauge for the 8-month bathymetry study and various ADCP transects at spring and neap tidal cycles.  Suspended sediment will also be measured by backscatter data from the ADCP, filtration and Seapoint turbidity sensors interfaced to a Valeport 606T tide gauge.  Tides will be measured at three sites. 

Picture 3: TDI-Brooks' R/V GEOEXPLORER, conducting bathymetry and
hazard survey, and environmental sampling in the Bonny River for
the NLNG Project

Bathymetric Survey - TDI-Brooks’ research vessel, the R/V GEOEXPLORER (GX), available in Onne Port, Nigeria will conduct the bathymetric surveys over broad sections of the survey area (Picture 3).  A local shallow draft vessel using the GX as its mother ship will be used to perform the bathymetric survey work in the shallow water (<5 meters) sections of area.  Both vessels will be outfitted with dual frequency fathometers and precision differential GPS navigation systems to determine water depth.  The unit will also determine if silt or sand is present on the bottom. 

Identification of Submarine Structures - A hazard survey of the approach channel will be conducted for subbottom objects such as wrecks, active and disused pipelines and other abandoned structures.  The hazard survey area in the approach channel is approximately 20-km long and 315 m wide. 

This survey will be conducted with TDI-Brooks’ vessel the R/V GEOEXPLORER outfitted with an EdgeTech subbottom profiler and a side-scan sonar/magnetometer. 

Geotechnical Survey Component – Twenty-one (21) cores will be taken down the axis of the channel to determine its geotechnical characteristics.  The geotechnical testing protocols call for a minimum of three (3) measurements per core for water content, bulk/dry density, Atterberg Limits, particle size analysis and shear strength. 

Environmental Survey Component - The environmental component involves sediment and water sampling at wet and dry seasons for chemical contaminants (hydrocarbons, trace metals, pesticides/PCBs, etc.), ancillary parameters, and sediment macro infauna (>1-mm in size).   The June/July sampling with the GEOEXPLORER will include the following components:

• Seabed sampling of 55 stations (71 samples) at channel, river and disposal sites;
• Water column sampling of 13 stations (mid-water) for physical, chemical and biological testing;and
• Sediment sampling at 24 stations for macroinfauna analysis.

The sediment analytical work will be conducted in TDI-Brooks’ laboratory.  Our laboratory conducts up to $2.0 million of analytical projects per year consisting of the analysis of several thousand water, sediment and tissue samples.  FugroNigeria will conduct some of the water column analysis.