Blue Stream seismic, slope flows push limits for contractors

Spanning the Black Sea's marine gorges

Aug 1st, 1999

The Blue Stream Trans-Black Sea gas pipeline is moving from concept to reality as the project to carry Russian natural gas across the Black Sea to Turkey approaches the construction phase. The project includes twin 24-in. pipelines laid 390 km across the Black Sea from Dzubga, Russia to Samsun, Turkey, reaching depths of 2,150 meters.

Blue Stream is a partnership between the Russian gas transporter Gazprom and the Italian conglomerate SNAM.

Gazprom, is responsible for several large developments and has closely directed this milestone-setting project. Gazprom, through the engineering company PeterGaz has accomplished the necessary but extensive investigations of the marine crossing using specialists in the field of deepwater pipelines.

Intec was hired to do the preliminary and basic engineering work as a subcontractor to the PeterGaz Firm. Initially, Intec was hired to conduct a feasibility study, which constituted Phase I of the project. This study developed a viable concept for design and routing and laid the groundwork for the extensive test programs necessary for such an advanced project. The concept was well received and the project then moved into Phase II, in which basic engineering was performed. At this point, a detailed route survey was required to give a definitive map of the terrain and complete details of the environment the engineers were facing in the installation of this record-breaking pipeline. This phase also involved materials specifications and testing of coating, anodes, and line pipe. This work focused on the offshore portion of the line, leaving the landfall for later.

Contracts go out

Saipem was hired as the installation contractor after technical aspects of the company's ambitious plan to J-lay the pipe was evaluated and Gazprom and PeterGaz reviewed the commercial aspects.

With the stage set, the Blue Stream has now moved into a transition phase between the design work done up front and detailed design and construction. During this phase the engineering is being reviewed and the required equipment scheduled. This review and possible supplemental design phase is expected to last through the summer, at present procurement contracts are being awarded.

Sea bottom conditions

The Black Sea is practically land-locked. Its only outlet is the Bosporus Straits near Istanbul, and there is little communication with the Mediterranean Sea. At the same time, the Black Sea receives in flows from several rivers. These flows carry organic material that sinks to the bottom and breaks down in situ. Over time this has created a deepwater environment that is high in hydrogen sulfide-based bacterial colonies below the 100-meter level.

On or near the surface, the seawater is very clear and its beaches accommodate numerous resorts. That is because wind and currents are able to circulate the upper parts of the sea clearing it. But the deepwater environment contains concentrations of hydrogen sulfide in the mud as high as 2,200 ppm with a pH in the range of 6.0 to 6.2, making this a sour, slightly acidic environment.

For the purposes of the Blue Stream pipeline, the low pH and high hydrogen sulfide content pose a materials issue. This environment facilitates free hydrogen diffusing from the water into the steel of the pipe, causing hydrogen embrittlement. Hydrogen embrittlement occurs when the hydrogen in the water diffuses into the steel too quickly to be properly absorbed. This rapid absorption leaves the steel brittle and subject to stress corrosion cracks. Stress corrosion cracks and hydrogen embrittlement are a normal design concern in sour conditions but usually the corrosive conditions are seen on the inside, not the outside of the pipe.

To protect the pipeline from the corrosive environment, the engineers chose a three-layer polypropylene coating. In the coatings application the biggest challenge is efficiently coating the field joints. It is difficult to do an efficient job on triple coating the joints during J-lay installation because the work is on the critical path. A cast in place field joint coating was tested and found satisfactory during the basic engineering phase. Saipem, whose modified Saipem 7000 will perform the J-lay installation of Blue Stream, is considering the problem now and may opt for a different coating solution if that proves more comp ati ble with their equipment.

To protect the pipeline from hydro static collapse the wall thickness of this deepwater pipe is 31.8 millimeters. While the thickness of the pipe wall guards against structural failure, it also increases the residual strain left in the pipe wall after manufacture.

Sea bottom profile

Using the data gathered during the detailed survey of the proposed pipeline route, the engineers were able to calculate the physical limits of the pipe they plan to lay and project that onto a profile of the seafloor. The result is an optimal route that avoids major obstructions.

Such a routing plan would not be possible without the extremely detailed sea bottom profile gathered during the survey. The data collected were so dense that even small obstructions were detectable down to the square meter. While there is no idea straight line through the Russian and Turkish slope portions of the pipeline route, it has been possible, with the detailed data and analysis collected along the route to achieve a satisfactory combination of alignment and strength in place. While there will be numerous free spans, only a few will require remediation and some of these may ultimately be reduced by laying pipe a few meters offset to avoid specific obstructions. For areas where either the depth or length of a span is predicted to be beyond the limits of the pipe, plans were made to fill the gap with gravel or use mechanical supports.

Route surveying

Because there was a potential problem with soil instability on both Russian and Turkish slopes, an analysis of the sea bottom composition and past seismic events was done. The goal was two-fold:

  • Determine how the soft soil will react to currents or seismic events
  • Determine the likelihood of a seismic event during the life of the pipeline.

Using a remotely operated vehicle equipped with a side ban sonar, sub-bottom profiler, and upward looking Doppler sonar, a survey of the Russian slope was conducted. A bathymetry swath was used to map the surface of the seafloor along the proposed route.

Fugro Engineers BV, Holland, performed geotechnical tests for the project using technological innovations to conduct cone penetrometer (CPT) samples in more than 2,000 meters water depths. The tests were conducted from the vessel Bavenit, which is owned my Amige of Russia.

Because of the demanding seabed conditions of this project, Fugro had to develop a CPT, which had a very low bearing on the seabed. This allowed the unit to be supported by the weak soil. To accomplish this, Fugro used a combination of systems. The Fugro Seascout was modified to perform 5-meter pushes using a straight rod and a 33-cm cone. The unit also allowed Fugro to install an in-situ shear-vane module on the bottom of the Seascout.

New records

In the process of conducting these tests, two records were set. These records were set using the 40-meter CPT unit called the Wheeldrive Seacalf, which was deployed through the moon pool of the Bavenit. This CPT was developed to push a 40-meter straight rod made-up in sections. The CPT recorded data at the seafloor and back at the topside. The unit set a water depth record when it performed a 40-meter CPT in 1,840 meters of water, while measuring the stability on the Turkish slope. The second record was a 20-meter stroke shear vane in 1,600 meters water depth performed in the seabed mode, rather than on a drill string.

Data from these tests were essential to computer modeling of the potential strains this environment would put on the pipeline. Information was gathered on such things as the frequency of severe mudslides, how much debris was involved in slides, and the slide velocity. Using this information, it was possible to predict the effect on the pipeline. Intec was able to design a pipeline system that would withstand the 10,000-year event in this area. That means the pipeline might be moved slightly, but would survive the external loads the study could foresee over the next 10,000 years.

On the Turkish side of the survey, Carbon 14 dating was used to determine the age of ancient seismic events in an attempt to predict the severity and frequency of such events. The most recent events of any consequence were at least 2,000 years old.

Overall, the pipelay will be an expensive and massive undertaking, but the major environmental challenges constitute only a small portion (roughly 2-km) of the 390-km lay. The development, design, and survey work done up front on this project will provide operational cost saving benefits with a pipeline that is dependable and sturdy enough to provide Gazprom with an exceptionally long service life.

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