Pipelines have been laid in the Nigerian swamp since the beginning of the exploration and production of oil in the Delta of the Niger River. Bouygues Offshore and its Nigerian subsidiary have been participating in the development of the fields onshore for more than 35 years, and has acquired experience in pipelaying in these areas.
The characteristics of this type of pipelaying project are driven mainly by the specific envirionment. All the construction activities take place in the swamp and are accessible only by boat on the rivers. This restriction requires the mobilization of a very important plant placed on board barges and pontoons. Several pipealying spreads are prepared for the job, each self sufficient on site. The work is organized with the spreads working in sequence. Accomodations are located on barges positioned close to the worksite and moved during the project execution.
One of the key points for this type of project is to maintain a detailed schedule, planning well in advance for delivery of the pipe joints, supplies, and catering. A very important parameter is the necessity of maintaining relationships with the local communities and villages along or on the route of the pipeline. A good relationship means utilizing their capacities for the pipeline work and employing a certain number of the villagers. Respect for the environment should be kept permanently in mind, along with the obligation to restore as much as possible the vegetation at the end of the project construction.
In general, the main tasks for pipeline installation are as follows:
- Surveying and bush clearing
- Trenching and dredging of river crossings
- Pipelaying in the swamp and river crossings
- Valve platform installation and pipe strings tie-in
- Trench backfilling and re-vegetation
Survey, right of way
An engineering survey is executed at the early stage of the project in order to check the routing, to verify the right of way corresponding to the permits obtained and the areas acquired by the company. This survey is also a verification of all the physical characteristics such as the rivers, water depth, type of creeks, pipe and road crossings, and other elements. Also, the survey checks the alignment of the pipeline over the distance, stakes the right of way width, and locates the positions of valves, markers, cathodic protection, test points, and platforms.
All this data are utilized for the engineering of the project. At the beginning of the construction, a second survey will be executed to verify that the previously recorded data is still valid and has not changed, and that the stakes are re-installed along the right of way.
The routing of the pipes determines an area (the right of way) which must be prepared since the vegetation is dense and doesn't allow easy access for the equipment. A right of way of 15 meters width is cleared of bush and timber. The timbers are cut and stacked alongside. This activity is usually subcontracted to local contractors. The work is usually done manually. Local communities are employed in this phase as much as possible.
The trenching work in the swamp area is performed by swamp excavators - backhoes mounted on marsh buggies. As soon as the trenches are excavated, they are flooded with swamp water which allows the floatation of pipes during the installation process. Usually, the swamp trench dimensions are the following, although they may differ slightly according to the specifications of the contract or the local rules:
- Bottom width: various (according to the number of pipelines in the same trench).
- Depth: 2 meters
- Pipe cover: 1.5 meters over the top of the pipe.
These dimensions are adapted to provide sufficient width for the pipelines and buoys as well as sufficient depth to allow floatation of the pipes based in particular on tidal movements. In order to prepare sites for the lay barge, several slots are dredged in riverbanks along the pipeline route. These slots are performed using a cutter suction dredge.
The pipelines are assembled on a shallow water lay barge. This barge, fitted with pile spuds, is positioned within the slot dredged in the riverbank at the river crossing. Pipe strings are then assembled on the swamp lay barge firing line. The firing line arrangement can be described as follows:
- Station No. 1: Line-up, clamp and weld first bead (manual).
- Station No. 2: Filling beads (semi-automatic).
- Station No. 3: Filling beads and cap weld (semi-automatic).
- Station No. 4: NDT and field joint coating preparation.
- Station No. 5: Field joint coating and infill.
- Station No. 6: Floats installation.
A crawler crane mounted on a working pontoon feeds the firing line conveyor. Pipe joints are welded by manual welding and semi-automatic techniques. NDT, field joint coating, infill, and float installation are performed on successive working stations.
As part of the pipe string production, pipe inside cleaning and beveling are performed on the lateral conveyor. Pipe joint length is cut to a minimum of 38 ft and a maximum of 42 ft, with the average as near to 40 ft as possible. During production, two air winches push the string fitted with floats into the flooded swamp trench. A swamp cargo carrier guides the string head within the floatation trench during the pushing operation.
Once completed, the string is hauled to the designed location in the trench with the help of the swamp cargo carrier and lowered on he trench bottom by floats removal. When several pipelines are installed in the same trench, the separation between two adjacent pipelines is 0.3 meters minimum.
Welding and NDT
We come back now to the welding itself. The pipe welding is performed on three working stations located on the lay barge firing line. The welding process is manual with cellulosic electrodes (SMAW) for root passes, and semi-automatic FCAW for filling and capping passes. When required, line pipe is beveled on the pipe lateral conveyor with two CRC-type facing machines. The joints bevels are usually as follows:
- API 60° for wall thickness joint 12.7 mm
- 2 by 30°/15° reduced bevel for wall thickness joint > 12.7 mm.
Each welding station is staffed with two welders and two grinders. The root passes are performed once the pipe on the lineup is aligned to the previous pipe and wedged with an internal clamp. The 4-in. pipeline does not require an external clamp.
All welds are usually inspected using radiography (up to 100%) unless the requirements are different. For large diameter pipelines, the source is X-ray, using a single wall, single image technique. An internal panoramic crawler is used. The weld control of the small diameter pipelines is performed with an external gamma-ray source.
Coating and tie-ins
After NDT operation, field joint surface preparation is performed at station No. 4. At the next station (No. 5) all joints are coated with a 3-layer polyethylene pipe coating solution. The field joint is coated with heat shrinkable sleeves. Alternatively, the field joints can be coated with laminated bitumen tapes. In both cases, the infill is performed with asphalt mastic coating.
The strings are tied together with the help of the welding spread, including swamp cranes. During tie-in operations, the strings remain afloat. After completion, control of the tie-in welds and field joint coating (heat shrinkable sleeves or laminated bitumen tapes), the pipe will be lowered to the bottom of the trench bottom by removing the floats. Field joint infill is not placed at the pipe section tie-ins.
After completion of string tie-ins, the trench is backfilled with the previously excavated material using swamp excavators. Then the re-vegetation is programmed in order to have the area as near as possible to the natural state.
River crossings are the major natural obstacles encountered in the swamp areas. The river widths may vary from a few meters to several kilometres. The means and equipment to be used are different for the different widths. Before trench dredging, a pre-survey of the river crossings is performed (bathymetry and a bottom mapping) using a survey vessel with an over-the-side mounted echo sounder and a side scan sonar, all of which being run simultaneously.
A corridor of minimum 50 meters width is covered on each crossing. Where possible, lines are run across the river, with additional lines along the banks for full coverage. On the narrower river crossings, line is run along the river. Line spacing and side scan sonar range are chosen to allow for full overlap and optimum resolution.
For river crossings under 30 meters width, an underwater trench is excavated with the help of a crane mounted on a working pontoon and fitted with a bucket. For river crossings over 30 meters width, the excavation includes several operations with the utilization of a large barge-mounted bucket dredge or a cutter suction dredge to excavate the trench. The dredged material is pumped through a floating pipeline to a spray pontoon and side-casted/dumped at a downstream location parallel to the trench.
The usual typical trench dimensions are:
- Bottom trench: 5 meters for minor, moderate, and significant crossings, and 10 meters for major crossings.
- Slope: 1:4. The chosen slope is subject to on-site verification.
- Depth: In accordance with the required cover (1.5 meters or 3.05 meters over the top of the pipe).
After completion of the dredged trench and before pipeline installation, a post-survey of the trench is performed. Access for the lay barge is provided by a slot dredged on one side of the river in order to install the line in a trench. On the other side, a slot will provide access for the tie-in pontoon.
Floating the string
The pipe string to be installed across the river is produced by the lay barge remaining on workstation within the slot. The entire length of the string is fitted with floats and pushed into the adjacent swamp trench.
Then, the string is flooded with filtered water taken from the river and hydrotested. The maximum test pressure shall be in accordance with the test pressure specified in the project specifications. After completion of the hydrotest and de-watering, the string is installed across the river. The two following techniques are considered:
- For moderate and significant crossings, the string is pulled afloat across the river. Then it is lowered on the trench bottom by float removal.
- For major river crossings, the string is usually installed using the bottom pull technique. The pipeline submerged weight is adjusted with floats before the pulling operation in order to comply with the pulling winch capacity and to provide lateral stability to the pipeline exposed to the river currents (subject to in situ verification). This point is studied during detailed engineering. A current meter is used in order to guarantee the integrity of the pipe during the pulling operation.
The pulling operation is performed as follows:
- Pulling cable connection the pulling head.
- Pulling cable paying out across river with help of the pulling pontoon.
- Mooring of the pulling pontoon.
- Bottom pull of the pipe string.
During pulling, the string is held back with a winch mounted on a pontoon. Once the string has been pulled across the river, the pipe is flooded with filtered water taken from the river in order to guarantee its stability. Before trench backfilling, the remaining floats are removed. If required, a corrosion inhibitor is used.
The profile of the pipe is designed with respect to the allowable bend radius. This is determined during the detailed design engineering on the basis of information collected during the survey at the beginning of the project.
The tie-in between the pipe section installed across the river and the swamp pipe section is performed within the slots on each side of the river. River and swamp pipe sections are lifted to the surface using a tie-in pontoon (flexi-float type) and welded.
After completion, control of the tie-in weld and field joint coating (heat shrinkable sleeve or laminated bitumen tape), the pipe is lowered to the bottom. No field joint infill is performed at pipe section tie-ins.
The trench is backfilled with material dredged with the cutter suction dredge from an upstream location within reach of the floating pipeline and then pumped to the spray pontoon for filling the trench. The required cover is obtained. After backfilling of the trench, a post-survey is performed.
Riverbank re-instatement is performed with previously excavated material. When required, banks of moderate, significant and major river crossings are reinforced with earth-filled bags.
The valve platforms are prefabricated in the yard, loaded, and transported with the piles to river crossing sites on cargo barges. The crane mounted onto a barge installs the platform at the specified location. The piles are driven through the platform legs to the specified penetration using a diesel hammer. Then, they are cut-off and welded to the platform legs.Pipe support piles are driven with a diesel hammer with the help of a piling guide. Then, they are cut-off at the required elevation.
Rivers markers are also prefabricated at the yard, loaded and transported with the support piles to river crossing sites on cargo barge.
The four support piles are driven with help of a piling guide and cut-off at the required elevation. Then the river marker structure is installed and welded onto the four support piles.
The pipe section is lifted to the surface using a tie-in pontoon (flexi-floats type). Then, bends and riser are welded to the pipe and fixed to clamp supports after lowering of the pipe into the trench bottom.
After installation, the pipeline is flooded, cleaned, gauged, and hydrotested. The pipelines are filled with filtered water taken from the river using a flooding pump and a polypig to ensure the removal of all air. Then the whole pipeline is cleaned using water driven brush pigs. The cleaning operation is accepted when the last brush-pig does not bring out solid debris resulting from construction activities.
After cleaning, a gauging pig is driven along the whole pipeline. The gauging operation is accepted if the gauging plate is in acceptable condition. If required, for lines longer than 5 km, a pig location device is used. After cleaning and gauging the pipeline is hydrotested. The maximum test pressure shall be in accordance with the specified test pressure.
For pipeline preservation until commissioning and once the hydrotest is completed, the pipelines are filled with treated fresh water (oxygen scavenger and biocide). The operator performs the de-watering of the line when the whole installation, and not only the pipelines, is ready. It may be that the pipelines could remain filled with water for several months.
Such a pipeline assembly and installation is not technically difficult. The only difficulties are related to the environment, the remote locations, and to some extent, problems with the local communities along the routing of the pipelines.
Success is conditioned by very detailed and meticulous preparation, a logistic based on the experience of the country, and a strong local organization. However, above everything, the expertise and experience of the personnel are the key to success.
Luc Poirson is the Commercial Manager in Nigeria for Bouyges Offshore