One of the greatest areas of growth in the global oil and gas industry is within the subsea sector, yet the continuing drive to access reserves in ever deeper and more inaccessible locations creates significant challenges.

Even the term “deep” has taken on a different meaning as the industry has developed. Forty years ago we may have considered “deep” to be 153 m (500 ft) or so of water. Today it is not uncommon for activities to be undertaken at depths of up to 3,000 m (10,000 ft) in high temperatures and pressures of up to 20,000 psi. In the not-too-distant future we will see depths of 4,000 m (13,123 ft) and 5,000 m (16,404 ft) reached.

What’s next in umbilicals?

The pressure challenge in deepwater wells relates to both external and internal pressure. At greater depths, external pressure increases one atmosphere for every 10 m (33 ft) of water depth. For example, at a depth of 5,000 m the pressure will be 500 times greater than the pressure at sea level, meaning that the mechanical strength of equipment deployed at such depths has to display a greater robustness than those used in more benign environments.

In an effort to access ever more remote locations, it is natural that the industry will consider greater step-outs from shore and longer tiebacks, in some cases stretching up to 274 km (170 miles), creating significant obstacles for the industry to overcome.

As the “nerve system” of any subsea oil and gas assembly, umbilical tubing is a mission-critical part of production, connecting hydraulics and electrical components from offshore platforms or floating production systems to the equipment below. Future trends to develop deeper wells in more remote locations while at the same time providing a safe environment present an increasing array of challenges in this sector and to the industry as a whole.

Since umbilicals are either floating (dynamic) or positioned on the ocean bed at great depths, the industry is continually exploring ways to optimize their design. The focus is constantly toward safer, smaller, more compact products that are lighter and corrosion-resistant with reduced risk of wall collapse, no migration of fluids, good protection of cables and no changes in hydraulic response times and flow resistance over many years.

Safety first

The first consideration when working in any aspect of the oil and gas industry is always safety. This goes from “working safe” to other safety issues as the oil majors explore and drill in deeper and more remote environments with extreme temperatures, including Arctic cold, hot wells, etc. Failures or accidents are unthinkable, so every effort is made to make sure products are safe and fit for purpose.

In considering the development of an umbilical, three key factors must be taken into account. Today’s umbilicals must be stronger and lighter to handle the significantly higher pressures and forces found at greater depths. The advance toward widespread subsea processing means umbilicals have to be compatible with and effectively connect to subsea processing units that include electrical and fiber-optic elements. Finally, they must be suitable for linking longer subsea tiebacks.

Such longer tiebacks demand greater umbilical lengths, larger weights and larger production volumes. For example, the company was commissioned to provide 855 km (531 miles) of umbilical tube to secure the safe and reliable control of 21 wellheads at depths of 250 m to 345 m (820 ft to 1,132 ft) on the Snøhvit oil and gas project in the Barents Sea.

Snøhvit is the first major development on the Norwegian Continental Shelf without a fixed or floating unit. Instead, a subsea production system on the seabed feeds a land-based plant on the northwest coast of Melkøya at the entrance to the shipping channel into Hammerfest. Output is transported to land through a 143-km (89-mile) long subsea gas pipeline.

It was important that the stainless steel material used in the umbilicals maintained high mechanical strength even in subzero temperatures, which the customer also wanted to protect in trawlable waters. The umbilical design had a total delivery weight of 4,000 tons, a max unit weight of 135 kg/m (41 kg/ft) and an outer diameter of 8.4 in.

Fatigue strength also is extremely important. The spirally wound tubes in the PVC matrix slide freely and must handle wear, crush and axial compression to safeguard the hydraulic and service fluids, especially for dynamic umbilicals connecting to FPSO vessels.

Sandvik SAF 2507 super-duplex coiled umbilical tubing has become established as an industry standard in umbilical tube, with more than 100,000 km (62,137 miles) installed globally. Its high mechanical strength makes it suitable for high-pressure applications, and minimal welding is required. Its thin tube walls enable lighter construction while it is corrosion- and hydrogen-induced-stress-cracking-resistant.

There is a growing need in the industry for safer, stronger, lighter, corrosion-resistant materials to optimize the design of next-generation deepwater umbilicals with easy fabrication and efficient installation in longer tiebacks.

Stronger solution

As part of the advance in the development of umbilicals technology, the company has developed the new high-alloy Sandvik SAF 3207 hyper duplex stainless steel, able to provide a 20% to 25% increase in yield strength compared to super duplex. The enhanced strength enables the coiling of tubes into smaller reels with longer umbilical lengths, making more efficient use of installation vessels.

Thinner walls and lighter installations also help in accessing and operating ultradeep wells, which were previously too costly or complex to exploit.

The benefits of the hyper duplex stainless steel are numerous but include excellent resistance to pitting, crevice corrosion and stress corrosion cracking in hydrogen sulphide- and chloride-containing environments; a high resistance to general corrosion in acidic and caustic environments; excellent resistance to erosion corrosion and corrosion fatigue; exceptionally high mechanical and fatigue strength; and good weldability.

With a minimum pitting resistance equivalent value of 50, excellent strength and pitting and corrosion resistance, the Sandvik SAF 3207 has huge potential to enable the industry to tackle increasingly challenging deepwater projects.

Maintaining momentum

The continued evolution in technological advancements within the oil and gas industry as a whole and the subsea industry in particular will mean that the capabilities of umbilical tubing will forever be challenged and the umbilicals sector will be constantly called on to question the performance of its products at increasing pressures and temperatures.

The focus for the sector, however, should not just be to match the current ambitions and expectations of operators in the creation of systems to assist in the extraction of more oil from increasingly difficult locations but to stay ahead of them. Working with companies to identify future trends and their potential impact on umbilical development will help the industry as a whole maintain its current momentum.