Feature Articles—October 2009 Issue
Challenges of Manufacturing and Installing Deepwater PipelinesMaximizing Pressure Tolerance of the Perdido Norte Pipeline, Installed at a Depth of More Than 2,500 Meters
By Martin Connelly
Technical Manager
Corus Tubes Energy
Hartlepool, England
Richard Freeman
Business Development Manager
Corus Tubes Energy
Corby, England
and
Mark Cizek
Senior Engineering Project Manager
Williams
Houston, Texas
In 2002, a major oil and gas discovery was made at the Great White Field in the Gulf of Mexico. The discovery, located about 350 kilometers south of Galveston, Texas, in the Alaminos Canyon Block 857, had water depths of more than 2,514 meters. The partners in the field held interests in several surrounding blocks in water depths up to 2,895 meters. They made the decision to utilize a common floating production facility, the Perdido Regional Development Host, and to export the oil and gas through a pipeline network.
Williams contracted with the oil operating company to build and operate the Perdido Norte oil and gas export pipelines. Both deepwater export pipelines were to be based on a 457-millimeter-diameter UOE design. Gas exports from the Perdido Regional Development Host would be delivered via a 168-kilometer line that would tie into Williams’ existing Seahawk gas gathering system, allowing transportation of approximately 265 million cubic feet of gas per day. Heavier hydrocarbon products would be exported via a 120-kilometer oil line to tie into Exxon Mobil Corp.’s (Irving, Texas) Hoover Offshore Oil Pipeline System.
As pipe diameter increases, it becomes more difficult to withstand hydrostatic pressures and resist unstable failure, or collapse. With a diameter of 457 millimeters, the Perdido Norte pipeline presented the substantial challenge of withstanding pressures at depths of more than 2,500 meters. In these extreme conditions, pipe specifications are critical, and it is paramount that each joint installed conforms exactly to the requirements that the conditions demand.
Williams contracted Corus Tubes for the manufacture of both lines, totaling more than 310 kilometers of 457-millimeter double submerged arc welded (DSAW) UOE pipe.
The UOE pipe manufacturing method uses steel plate feedstock that is progressively transformed into pipe through various forming operations (U-press, O-press and mechanical expansion), with an internal and external longitudinal weld to construct the finished pipe. Assessments on the Perdido Norte development resulted in the specification of a 457-millimeter trunk line for both the oil and gas products.
Deepwater Pipeline Design Although many factors impact a pipeline’s design, it is generally dominated by the need to withstand the internal pressure in the line. The higher the pressure that the pipe can withstand, the higher the maximum flow rate for the products can be, increasing revenue potential for the operator. However, for deepwater pipelines, the biggest challenge becomes the need to prevent hydrostatic collapse due to external water pressure, particularly during the installation phase.
There are two properties that govern the resistance of a pipeline to hydrostatic collapse: pipe shape ovality and pipe material compressive strength.
During the manufacture of UOE pipe, careful control is required to optimize these two attributes, thereby ensuring that the resulting pipe will offer performance suitable for deepwater conditions.
Perdido Norte Pipeline Challenges
While Williams has a considerable track record with pipeline from Corus Tubes in the deep and ultradeep waters of the Gulf of Mexico, the Perdido Norte project presented the greatest challenge to date. The pipeline route crossed numerous peaks and valleys and, despite careful routing, this resulted in a significant number of pipe spans ranging from 20 meters to 150 meters. This variability in length posed specific challenges to the project in terms of fatigue and bending stress performance.
The greatest challenge for the Perdido Norte pipeline was the sheer depth of water where the pipeline was to be laid. The American Petroleum Institute RP 1111 standard was applied to evaluate the pipe geometry and properties, as it contains guidance on the design criteria for hydrostatic collapse. Working with Houston, Texas-based OPE Inc. (Williams’ engineering consultants) and Corus Tubes, the deepwater design was thoroughly investigated, and opportunities for optimizing the required wall thickness of the pipeline were evaluated.
After much detailed consideration, a pipe grade of X65 was chosen, giving the best compromise of strength and weldability while allowing the installation of the 320-kilometer pipeline to be economical. The most critical design point for consideration in deep water occurs during installation. At this point, the pipeline is unpressurized and experiences the maximum net hydrostatic external pressure. In addition, the sag bend at the seabed induces axial tensile and bending stresses, forming a complex stress state that severely taxes pipe performance. To ensure the stress state was correctly determined, Williams collaborated with the installation contractor to confirm that the geometry and performance of its lay vessel was correctly modeled.
To ensure that the pipeline was of the utmost quality for deepwater installation, strict controls were imposed on the pipe manufacturer to ensure that the body ovality and strength of the material were suitable to withstand the installation process at these water depths.
The dimensions of the pipe are key to the successful installation and performance of deepwater pipelines. Williams needed to ensure that the lay contractor would have no problem with fit-up that would adversely affect the lay speed.
An extremely tight dimensional specification was agreed upon.
Once the pipe specification was decided upon and finalized, Corus Tubes developed a tailored plate specification suitable for successful pipe manufacture.
Manufacturing the Material
The performance of the plate feedstock used in modern line pipe is one of the key defining parameters in how a pipe is made and how it performs.
The forming and welding performance in the pipe mill is heavily influenced by chemical composition, mechanical properties, internal soundness, dimensional control and delivery condition.
The plate manufacturing delivered excellent mechanical property control within the stringent requirements imposed by the Corus Tubes specification on the plate suppliers.
The plates were 100 percent ultrasonically tested to Det Norske Veritas (Høvik, Norway) OS-F101 (2000) nonsour and cut to the correct size and shape.
After completion of the inspection checks and confirmation of acceptance, the plate feedstock was shipped to Corus Tubes’ pipe mill in Hartlepool for conversion into pipe. The pipe mill is located close to port facilities, enabling the import of plate and export of finished pipe. Corus Tubes’ project management team controlled all of the logistics.
Corus Tubes sought to optimize the shape of the pipe at all stages. The key factor in achieving excellent dimensional properties is the recognition that the most demanding areas of the pipe in terms of forming are the “shoulders.” The shoulder areas are the most difficult to form in the crimp press, O-press and expansion stages on small diameter/high-wall-thickness pipes. Failure to minimize these features can result in excessive strain and subsequent overloading of the mechanical expander due to the high level of pressure required to “round” the pipe, meaning it may not meet the requirements for a deepwater, high-pressure pipeline.
During the manufacture of the Perdido Norte pipe, it was paramount that pipe shape was controlled at all stages of the process to ensure that the pipe girth welding speed could be optimized during installation. The resulting pipe shape would also minimize any areas that could initiate hydrostatic collapse of the pipeline.
This successful control of diameter and roundness in both pipe ends and body is reflected in the narrow range of dimensions actually achieved during the manufacture of the Perdido Norte pipe.
Corus Tubes tested the pipe material for compressive strength as part of its ongoing development into deepwater collapse resistance. The results achieved show that Corus Tubes’ steel design and manufacturing route yielded a data population that was significantly improved beyond the minimum compressive strength that would be required to satisfy the collapse limit state concerns. This was achieved by a combination of steel design and subsequent control of forming operations.
Delivering the Pipeline
The post-production management and shipment of more than 26,000 line-pipe joints also presented challenges to the project team. The 76,200 metric tons of bare line pipe were shipped from the port of Hartlepool across the North Atlantic Ocean by 12 open-hatched, boxed vessels. To prevent increasing stress in the weld area of the pipe during shipment, all pipes were loaded into the ship’s hold in a nesting configuration with welds at the 12 o’clock position.
The pipes were coated by The Bayou Companies (New Iberia, Louisiana). To complete this 220-kilometer river journey, the bare pipes were loaded directly from the ocean vessels onto river barges, each with a capacity of between 1,200 and 1,400 tons. The complicated procedure of direct overside discharge from vessel to river barge required careful management of barge and tug movements as well as managing movements in and around the port area. Corus Tubes employed a dedicated management company to oversee and coordinate the process of transshipment.
Once loaded onto the river barges, the pipes were transported over six days up the Mississippi River to the coating facility. After application of an external fusion bonded epoxy coating, the pipes were again transported down the river before being loaded onto pipe feeder vessels for shipment out to the lay vessel in the Gulf of Mexico.
Laying the Pipeline
The Allseas (Châtel-Saint-Denis, Switzerland) Solitaire vessel performed the installation of the Perdido Norte oil and gas export pipelines using the S-lay method.
Both oil and gas pipelines, totaling 288 kilometers, were successfully laid in a total of 60 days for an overall average of nearly five kilometers per day. The 120-kilometer oil line was laid in 27 days, while the 168-kilometer gas line was laid in 33 days. The overall installation time frame of 60 days included installation of four inline structures (two on the oil line and two on the gas line), as well as installation of the gas pipeline end manifold and oil pipeline end termination.
Welding on board Solitaire was a highlight of the overall installation project. The welding defect acceptance criteria were extremely restrictive to ensure the resulting welds were as resistant to fatigue crack initiation as possible. Despite this criteria, the pipeline was completed with only one cutout due to ovality out of about 12,800 double joints.
With pipe out-of-roundness tolerances generally around one millimeter for the majority of the produced pipe, Allseas was able to perform welding consistently with optimum repeatability. In conjunction with a high-quality welding program, the pipe supplied by Corus Tubes played a significant role in the offshore success of Perdido Norte.
Conclusions
Pipeline technology has led the offshore industry into ever-increasing water depths. In 2009, pipelines have been designed for installation in 3,000 meters of seawater, and there are projects on the horizon that will require depths of 3,500 meters and beyond.
Although operating at the limit of deepwater experience, the Perdido Norte pipe was delivered and successfully installed more quickly, and with higher quality, than anticipated. Through planning, communication and the application of experience, the parties worked together to deliver an optimized design for the project that was manufactured and installed efficiently and effectively.
Corus Tubes’ UOE mill has greater pressing power than similar mills, and as such it was able to manufacture the Perdido Norte pipe with the thickest wall/smallest diameter produced by any UOE manufacturer.
Through the engineering advancements made during this contract, Corus Tubes has been able to increase its manufacturing capability to produce pipes in the same grade at thicknesses of more than 31.8 millimeters at 457 millimeters diameter. This capability opens up the possibility for export lines in even greater depths, providing lay capacity exists.
However, through the further optimization of the UOE manufacturing method, coupled with better design tools and increased manufacturing control, there is evidence to suggest that greater depths can be accessed even with the current fleet of vessels.
The quantification of hydrostatic collapse resistance also presents a unique challenge to the pipeline industry. The problem is one of system instability, and while empirical relationships exist that can be used to predict certain cases, a robust analytical solution for the problem has not yet been generated. It will not be possible to push the boundaries of deepwater designs with complete confidence until such an understanding has been achieved.
Corus Tubes continues to invest considerable time and resources into understanding pipeline performance in deep water. This research will enable the realization of a project that would currently be uneconomical and impossible to lay.
Martin Connelly joined Corus Group in 1993 after graduating with a first-class honors degree in metallurgy and engineering material from the University of Strathclyde. He worked in a number of technical, quality and operational roles before being promoted to Corus Tubes Energy technical manager.
Richard Freeman joined Corus Tubes Energy in 2004 as development manager and now works as business development manager with responsibility for sales into new markets and leading the product development function of the organization. Freeman graduated from the University of Leicester with a first-class honors degree in mechanical engineering.
Mark Cizek joined Williams in 2006 as a project manager and was later promoted to senior project manager for the Perdido Norte project. Cizek holds an Bachelor of Science in maritime systems engineering from Texas A&M University and a Master of Business Administration from Rice University.
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