SPECIAL REPORT: Medgaz pipelay begins; S-lay, J-lay split duties

Saipem’s Castoro Sei arrived at Almeria, Spain, in early March and has since been prefabricating 12 m, 24-in. OD joints into longer lengths for installation as the shallower sections (maximum depth, 550 m) of the Medgaz pipeline. The vessel will begin pipelay from the Spanish coast in May 2008 and then move into position off the Algerian coast for similar operations. Medgaz expects work to be complete within 4 months (Fig. 1).

The dynamically positioned semisubmersible SSDV Saipem 7000 will complete deepwater pipelay (down to 2,160 m) during third-quarter 2008. DLB Crawler will perform the above-water tie-in off the Algerian coast.

This article describes the methods and technologies applied to each step of the Medgaz project, including Medgaz’s investigation into using S-lay in large-diameter ultradeepwater applications.

Background

Spanish gas consumption reached 33.4 billion cu m in 2006, up from 20.8 billion cu m in 2002.¹ By 2011, demand will exceed 44 billion cu m/year. Spain depends on imports for 99.6 % of its gas, of which 65% is LNG and 35% pipeline gas. LNG typically costs more than pipeline gas due to liquefaction, sea transportation, and regasification expenses.

Extensive studies by independent energy consultants show that the Medgaz pipeline will be the lowest long-run marginal cost supply option (excluding producing-country royalties) for future gas supplies to Spain (OGJ, Aug. 21, 2006, p. 57).

The Medgaz pipeline will carry 8 billion cu m/year between Beni Saf, Algeria, and Almeria. The €900 million project—with stakeholders Sonatrach (36%), Cepsa (20%), Iberdrola (20%), Endesa (12%), and Gaz de France (12%)—is to enter service in 2009.

Mitsui and Sumitomo began pipe manufacture in Japan in February 2007, with coating done by Bredero Shaw at its Kuantan, Malaysia, plant starting in June of the same year. The pipe is API Grade X70, with 0.9-1.2-in. WT. The thicker-walled pipe will cross at the greatest depths.

Groundbreaking and trenching at Beni Saf and Almeria started in July and September 2007, respectively. Pipe began arriving in Almeria in October 2007, with the final shipment arriving in February 2008 (Fig. 2).

The Castoro Sei measures 152 × 70.5 m and has a depth to main deck of 29.8 m. It uses 12 25-ton anchors and four azimuthal variable-pitch thrusters with 37 tons thrust. Three 110-ton capacity pipe tensioners work with a 300-ton abandonment-retrieval winch and two 60-ton capacity gantry deck-mounted fully revolving cranes.

Route selection

C&C Technologies completed some of the marine surveys defining Medgaz’ route, using its Odin Finder and Rig Supporter vessels. C&C also used its C-Surveyor I autonomous underwater vehicle, carrying out 50-hr missions at depths 40 m above the seabed.

Medgaz engineers supervised the surveys, with route selection guided by the following objectives:

• Minimization of environmental impact.
• Protection of marine flora and fauna.
• Avoidance of natural obstacles.
• Low geological, geotechnical risks.
• Minimal number of cable crossings.
• Ensuring the ability to use both J-lay and S-lay construction.
• Minimization of free-span lengths.

Medgaz’ offshore route measures 198.3 km, 49% at water depths greater than 1,000 m. It includes 19 curvature points and five places where the line will cross telecommunications cables, all below 1,000 m (Fig. 3). It also crosses the Yusuf fault and the steep slopes of the Habibas escarpment (71-77 km along its route), but 95% of the route has slopes less than 4° (OGJ, May 23, 2005, p. 59).

Pipe coating

Bredero Shaw applied both internal and external coatings to the Medgaz pipe joints. Flow efficiency coating consisted of a thin-film epoxy applied inside the pipe, reducing friction and turbulence and helping reduce both pipe diameter and power requirements.

Bredero applied a three-layer polypropylene anticorrosion coating to the outside of the pipes; a layer of fusion-bonded epoxy, followed by an extruded copolymer adhesive, and an outer layer of extruded polypropylene. The inner FBE layer provides resistance to cathodic disbondment, reducing cathodic protection costs.

The outer polypropylene layer normally protects the pipe during transportation and installation, as well as providing added installed resistance to shear forces, chemicals, and abrasive soil conditions. For Medgaz, however, Bredero applied an additional reinforced concrete weight coating, providing both negative buoyancy and mechanical protection. Besides the cage reinforcement applied to the outside of the coated pipe, the concrete itself mixed iron ore aggregate, cement, sand or granite aggregate, and water.

Pipelay

In 2004, Medgaz became concerned that the limited availability of J-lay vessels capable of deepwater pipelay might disrupt the project’s schedule, pushing start-up past 2009. The J-lay method, with the pipe leaving the vessel from a near-vertical orientation, places its only substantial stress during pipeline installation near the touchdown point in the sag bend, making it the method used in deepwater applications.

The S-lay method, by contrast, places initial installation stress on the pipe in the overbend region near the stinger, which is then compounded by the combined bending and external pressure experienced at the sag bend (Fig. 3).

Concerns that this combination of strains placed on high-grade, thick-walled, large diameter pipeline in a deepwater environment could lead to local buckling and reduced collapse strength have so far prevented it being used in such applications.

Medgaz commissioned C-FER Technologies to explore the possibility of an extension of the deepwater S-lay method from small-diameter pipelines to large, and to X70 grade pipe from previously used X65 material. Medgaz and C-FER were particularly interested in the effects on heavy section buckle arrestors of exposure to the S-lay overbend’s high bending loads.

Results suggested that S-lay could be successfully used for ultradeepwater pipelay of the sort encompassed by Medgaz, demonstrating that the line’s proposed pipe-buckle arrestor connection design would not cause premature buckling as the pipe crosses the stinger, nor would high bending strains at the overbend influence the pipe’s collapse strength.²

Medgaz ultimately booked both the Castoro Sei (S-lay) and the S7000 (J-lay) for installation work on the pipeline, but added flexibility to their selection process by investigating the limits of S-lay vessels in deepwater, large-diameter pipelay.

Integrity management

Medgaz has developed its own integrated CAD-GIS system for integrity management. The integrated system will combine all GIS spatial data, CAD design information, pipe procurement and tracking data, maintenance data, and pipeline survey data into a single source, which will then be able to capture and present different views of data acquired during any phase of the project.

Medgaz anticipates that the integrated system will avoid some of the problems associated with traditional systems; disjointed survey and CAD information, the need to manually correlate data at all stages; a resulting poor collaboration between survey, engineering, construction, and repair-maintenance functions; and a lack of project data flow between various applications (OGJ, Aug. 21, 2006, p. 57).

Power

Three Rolls-Royce RB211-6761 DLE gas turbine packages will power Medgaz. The 44,000-hp dry low emissions units will provide the front-end gas boosting from Beni Saf. Seven RB211 units at two compressor stations perform a similar role on the GME pipeline (between Algeria and Spain, via Morocco), which exits North Africa near Tangier before going under the Straits of Gibraltar and into Spain on the outskirts of Tarifa. GME entered service in 1996.

The Medgaz turbines will each power high-efficiency Dresser-Rand Datum barrel gas compressors. The compressor station’s inlet pressure will be 45 barg (about 650 psig), with a maximum 200 barg outlet pressure. Arrival pressure in Spain will measure 82 barg.

References

1. BP Statistical Review of World Energy 2007.
2. DeGeer, D., Timms, C., Wolodko, J., Yarmuch, M., Preston, R., and MacKinnon, D., “Local Buckling Assessments for the Medgaz Pipeline,” OMAE 2007, San Diego, June 10-15, 2007.