Offshore in depth

In previous columns, I have written about remotely operated vehicles and directional drilling, two relatively unsung technologies that have been critical to offshore field development. The subsea umbilical is another technology that hasn’t received enough credit for its crucial role in deepwater production.

Umbilicals contain control, power and communication lines, as well as conduits for injecting production chemicals. They connect surface production units to subsea wells and ancillary equipment via umbilical termination assemblies, jumpers and flying leads. Umbilicals seem very simple but, in fact, they require sophisticated engineering, advanced materials, and significant resources for manufacturing and quality assurance.

Umbilicals have been used since 1961, when Shell completed the first subsea well in the Gulf of Mexico, in 55 ft of water. Early umbilicals were essentially hydraulic control lines used to actuate valves on subsea wellheads. In the 1980s, with increasing water depths, the relatively slow response time of hydraulic controls led to electro-hydraulic systems and more complex umbilicals.  Stainless steel tubes were added to inject production chemicals. First-generation, braided-hose umbilical casing was replaced by thermoplastic hose and steel tubing, to contain large bundles of cables, tubes and fiber optic lines.

Design challenges. Umbilicals present a number of design challenges. To power and control electric circuits on the wellheads, low-voltage power cables are included. Fiber optic lines monitor wellhead conditions at high data rates. In deepwater fields, target reservoirs have high pressures, so the umbilical’s steel tubes have to match the subsurface safety valve rating of up to 15,000 psi, to provide hydraulic control and to inject chemicals into the production stream.

Dynamic loading of the umbilicals, due to vessel motion, can cause impacts on tubes, cables and reinforcement, leading to fatigue. A combination of buoyancy modules and internal ballast is used to stabilize umbilicals and reduce wear. Detachable turrets used on some FPSOs create highly dynamic conditions that must be addressed in the design of umbilicals and mateable connectors.

As water depths increase, greater demands are placed on umbilical performance. Electric power losses occur, due to resistance, and hydraulic pressure drops because of friction. Data communication signals can be attenuated. Ambient pressure in deep water can collapse umbilicals, and the weight of long umbilicals puts stress on connectors and seals.

Production from deepwater wells needs to be boosted to the surface with multi-phase or electrical submersible pumps, so medium-voltage cables (up to 33,000 volts) are included in the umbilicals to power them.

Because copper power conductors have a low strength-to-weight ratio, other components in the umbilical must provide enough axial strength to compensate. In addition, heat generated by current in the power circuits has to be considered in the design. Some umbilicals include non-traditional conductor materials, like aluminum and other alloys. Higher-strength tubing and alternate materials in the umbilical’s cross-section can make the umbilical more robust and reliable.

Designers also are reducing weight by using lighter materials like polymers and carbon fiber, or by implementing hybrid steel/thermoplastic outer casings. These lighter materials already have been used for intervention workover completion system umbilicals and flying leads.

“Deepwater umbilical systems require significant engineering time,” said Matt Smith, technical solutions director at Oceaneering International. “When I started work as an umbilical engineer in 1995, we might spend a few hundred hours designing an umbilical system. Today we can devote thousands of hours on the design and testing of a deepwater umbilical.”

For example, a dynamic umbilical, provided for a recent deepwater project, included three medium-voltage triads, supplying power to the subsea pumping units, 16 steel tubes, each rated at 15,000 psi, to supply hydraulic power and injection chemicals, as well as low-voltage power cables and optical fiber signal cables. “The project required more than 11,000 hours of engineering,” Smith said. Each of the two umbilicals provided for this project was 8,000 m long and weighed 750 metric tons.

Manufacturing of umbilicals for deepwater projects requires facilities that can produce long, continuous cables and tubes. These facilities also must conduct rigorous QA/QC to prove functionality and reliability, and place umbilicals on carousels that can be transported offshore. Steel tubes may have hundreds of welds over their lengths. Umbilicals that are thousands of feet long can cost up to $2,000/m.

Reliability. Operating in water depths up to 2,900 m, umbilicals have to operate reliably for 25 to 30 years. Newer umbilical systems have fault detection and remediation capabilities. For example, Technip is developing smart-sensing umbilicals that record hot spots detected along power cables, and strain caused by extreme storm conditions.

Reducing complexity. Operators strive to reduce the complexity, and cost, of umbilicals. For example, new valve technology may reduce the number of tubes needed, so a single tube can provide injection chemicals to multiple points. Future field designs may deliver chemicals from subsea storage tanks and hydraulic power from subsea hydraulic power units, reducing the number of tubes needed in the umbilical.

Total recently announced that it has decided to switch to all-electric control technology for its subsea wells in the North Sea and West Africa. By eliminating hydraulic controls, the company expects to reduce capital expenditures and protect the environment from potential hydraulic fluid leaks. 

About the Authors

Ron Bitto

Contributing Editor

Ron Bitto has more than 30 years of experience as a technology marketer and writer in the upstream oil and gas industry. RON.BITTO@GMAIL.COM