ADJUSTING FLOW STATION JOB TO REMOTE NIGERIAN LOCATION YIELDS SAVINGS
Ron Wooten
E.C. Williams OPI International Inc.
Houston
Installation-contract terms for an offshore flow station allowed Offshore Pipelines International Ltd., Houston, to cut costs by using construction methods suited to offshore Nigeria and taking advantage of superior marine equipment in Nigeria at the time of award.
In September 1991, Chevron Nigeria Ltd. and Nigerian National Petroleum Corp. contracted Offshore Pipelines to design, procure, construct, install, and commission the Opuekeba 30,000 b/d crude-oil flow station on an offshore platform near Olero Creek, Nigeria, approximately 22 miles from the nearest deepwater access (Opuekeba = [o-pwe-ka-ba]).
Chevron's original project plan included bringing the flow station to the site in small packages and then assembling it in a lengthy field hook-up process.
Offshore Pipelines developed a plan early in the project to maximize construction and hook-up in the fabrication yard, then transport the nearly complete structures to site by way of a newly dredged canal.
What proved to be most difficult was the site location in Nigeria. Job planning and communication were important in the successful completion of the project.
Keeping the components of the large and complex facility simple proved to be effective and efficient and played a key role in completing the project on time and within budget.
OVERCOMING OBSTACLES
The flow station consisted of a 30,000 b/d oil facility, including a power-control module complete with motor-control center and dual Solar turbines, quarters, treater, separators, flare pit, and all associated process equipment.
The platform and equipment designs and construction techniques used standard Gulf of Mexico technology.
In addition to Chevron, the Petroleum Council, and Offshore Pipelines, major subcontractors to Offshore Pipelines in the Opuekeba development were ABB Lummus Crest Inc. (ABB LCI) for design and procurement and Volker Stevin (Westminster) for dredging.
Offshore Pipelines used its Orange, Tex., and Onne, Nigeria, fabrication sites to build the decks, jackets, and piles and to install and connect most of the process equipment.
Offshore Pipelines' vessel DB-11 performed platform installations and support for hook up and precommissioning activities.
Other arrangements that proved essential in the project's success included the following:
- Offshore Pipelines' project and design team and Chevron and Lummus Crest personnel were all stationed in Houston.
- Gulf of Mexico fabricators were used for fabrication of the process equipment.
- A substantial portion of the work was performed in Nigeria.
With Offshore Pipelines' project team stationed in Houston, Chevron could draw from local divisional offices and assemble a team without unnecessary international relocation costs.
Offshore Pipelines' project team was also able closely to monitor fabrication and development of critical process equipment such as the production vessels (all to ASME code standards), and the generator/motorcontrol module, including two 900-hp turbines and pipeline pumps.
With all fabrication of process equipment centrally located on the U.S. gulf coast, Chevron's project team could easily and effectively monitor quality and progress.
As required from all foreign companies in Nigeria, Chevron's Nigerian business unit demanded use of domestic labor and contractors for fabrication. Offshore Pipelines was able to accommodate this requirement with use of its fabrication yard in Onne, Nigeria.
LIFT, DEPTH CONSTRAINTS
The facility consists of a 12-pile production platform (Fig. 1), 4-pile treater platform, 4-pile manifold platform, 6-pile utility platform, three connecting bridges, a flare pit and walkway, all associated process, utility, and fire fighting equipment; 18-man quarters; and two 1,000-kw, turbine-powered generators and building.
Initial constraints included a maximum lift of 200 tons and shallowwater access to the site.
Both constraints were joined in that Offshore Pipelines' designated lift barge DLB-I (maximum lift of 200 tons) required a draft of approximately 12 ft; access to the site was 22 miles up a creek with less that 12 ft of draft.
Offshore Pipelines hired Volker Stevin to dredge the entire creek as well as the site to 5 m (16.4 ft) and to install the flare pit and flare-pit walkway.
The 200-ton weight limitation concerned both Offshore Pipelines and Chevron because it would limit installation to bare decks without equipment. This in turn would severely lengthen derrick-barge exposure by dramatically increasing the offshore hookup time interval.
Offshore Pipelines' project team decided to take advantage of a recent mobilization of Offshore Pipelines' derrick barge DB-II (600-ton capacity) to West Africa from the Gulf of Mexico and assigned it to this project.
The advantage gained-use of the larger lifting capacity-allowed installation and hook up of virtually all of the process equipment in the Onne fabrication yard.
The new construction plan consisted of arranging access to the site for Offshore Pipelines' 350 ft x 100 ft derrick barge. Although the platform site is only 3 miles from the Nigerian coast, the shallow sandy coastline prohibits any channel dredging.
The decision was made to dredge the existing Olero Creek, a tributary of the Benin River, wider and deeper than originally planned for passage of the derrick barge and structures.
DREDGING
Dredging proved to be a challenging experience because 14 villages along the creek banks presented potential work stopping and ticklish situations. Offshore Pipelines therefore employed a full time community relations representative to interface directly with the villagers and obtain legal assistance from the Nigeria attorney's office to give official status to the contractor's presence. The strategy, though not perfect, did work, and dredging was completed with only minor delays.
The 8-month dredging project consisted of removing 1.5 million cu yd of material from the creek bottom and placing it in prepared spoil areas.
This type of dredging is standard for contractors in the area and presented no technical problems.
Keeping the dredges working despite interferences from local communities, however, required constant monitoring and compromise through Offshore Pipelines' representatives at the sites.
It was essential that down time be minimal because completion of the dredging was to coincide with arrival of the derrick barge.
Also, with the rainy season approaching, Offshore Pipelines anticipated that the newly dredged creek would stay passable for only 3 months.
FABRICATION
The process equipment was fabricated in skid units in gulf coast fabrication yards during the summer and fall 1992.
Each skid was complete including piping, instrumentation, and some electrical. The skids were also precommissioned before January 1993 shipment to Nigeria.
Offshore Pipelines maintains a fully equipped fabrication yard and Marine construction base in Onne, Nigeria.
All Nigerian elements of this project were based from this facility.
Offshore Pipelines planned portions of the work, such as the structural members cutting and coping, to be constructed at its U.S. facilities.
They were then shipped along with process equipment and electrical and instrumentation packages to Nigeria for assembly. The process equipment layout was simple but extremely effective in reducing offshore barge hookup.
The complex was oriented east-west with the process skids being north-south.
The pipe-walkway which runs east-west the entire length of the facility served as a clear access walkway and supplied ample room for the interconnect piping, electronics, and instrumentation.
All piping connections and instrumentation bulkheads were placed on the north end of the skids which allowed for minimum field connections.
Although most of the process skids were yard-installed, several skids required field installation due to deck lift interferences and center-of-gravity problems.
The field-installed skids were trial fitted in position in the fabrication yard. The piping was completed then disconnected at the skid edge.
The U-bolts on the pipeways were then loosened and the pipes unflanged and moved aside to allow for skid removal. These skids were later installed the field and required virtually no field welds for the piping tie-ins.
The pipes were wrestled into place rebolted and the U-bolts re-tightened.
Similarly, the instrumentation was for the most part installed in Onne.
The main production safety panel (CP-01) was placed in position in Onne. Tubing connections were branched out of the panel, routed to the main pipeway tubing trays, and branched out a second time at the final skid locations.
The tubing was completed and terminated at the skid bulkhead, then disconnected and moved aside similar to that of the piping. All bridge piping, electrical, and tubing trays as well as instrumentation tubing were installed onshore as well.
Upon the final installation of the field-installed skids by the derrick barge, the tubing was re-connected, again eliminating much field-installed tubing.
INSTALLATION
On May 14, 1993, Offshore Pipelines' DB-II traveled up Olero Creek just as the dredges were completing their maintenance dredging.
With the way cleared, material barges began transporting the platforms up river. jacket installation began immediately and progressed rapidly with the experienced offshore crew. The derrick barge's presence allowed large crews to accelerate a comfortable and efficient work platform.
Three large modules, 18-man quarters, a power-generation building, and a manifold-deck module were completed in the U.S., shipped directly to the work site, and unloaded onto the platform.
This was accomplished by shipping aboard a shallow draft roll-on/roll-off vessel that was able to make the ocean voyage and traverse the river without losing its cargo. Again, timing was critical, with the installation barge completing its work just as the modules arrived.
Scheduling would not be so difficult were it not for the potential delays clearing customs, which can take a month or more. A cargo such as this required quick clearing and was dealt with weeks in advance.
During the project design phase, Offshore Pipelines realized that earthen-pit wall construction in the soft, silty marsh would be difficult.
After several engineering efforts failed to prove the possibility of using a conventional flare pit wall from local soils, Offshore Pipelines chose sheet pile construction.
Material was locally available and offered quick installation. Although more expensive, this option solved a critical scheduling problem by completing the civil works prior to the derrick barge's arrival and maintaining an uninterrupted work schedule.
The flare pit and flare walkway were installed by the dredge contractor prior to derrick barge arrival.
BARGE-TIME PROBLEMS
Platforms were installed in the following sequence: 60-in. caisson, manifold platform, utility platform, production platform, treater platform, and communication tower.
The installations were straightforward; only two problems had to be solved with field ingenuity.
The cargo barges available were two 72 ft x 250 ft and two 50 ft x 180 ft. The minimum number of barges required was six; therefore, several round trips would be required.
The time required for each cargo barge trip was 4 days one way in addition to load out and tie-down time. This period created problems because "critical path" barge time was not being satisfied.
("Critical path" time is that time allocated to the barge when no other function which might advance the schedule can be performed.)
Barge standby time would result unless an alternative were considered. In order to shorten cargo-barge loading time, the decks and jackets were preloaded, while the derrick barge was docked at the yard.
Even with preloading, Bridge No. 2 created a problem because its cargo barge required a second trip. To meet critical path, the cargo barge would have to be unloaded in the field and released immediately which at first did not appear feasible.
During installation, Offshore Pipelines decided to store Bridge No. 2 on the shore in order to release the cargo barge. When the bridge was placed on the shore, it began sinking (Fig. 2).
It was evident that the soils would not support the six legs (10.75-in. pipe) that supported the bridge.
To solve this problem, wooden crating materials left over from the skid shipments and barge scrap plate were attached to the bottom of the support legs to increase the bearing area. Bridge No. 2 was then placed on the shore a second time without settlement.
Positioning survey services were limited in this area and although services were hired, survey personnel and equipment did not arrive until several days into the installation. The 60-in. caisson and the manifold jacket were positioned with only a transit and field ingenuity.
Positioning of the 60-in. caisson used only a tape and eyesight. The positioning of the manifold proved to be somewhat more complicated without survey.
The manifold jacket was positioned with only a transit and survey pole. Precalculated cable lengths were made up and configured as shown in Fig. 3. As the barge was lowering the manifold jacket, legs A1 and A2 were aligned directly behind each other along a line of the transit site.
This initial positioning survey, although crude, proved to be effective because all caissons and platforms were within Chevron's tolerances.
