Innovating subsea umbilicals

The US DOE's Los Alamos National Laboratory in New Mexico. Photo: Los Alamos National Laboratory.

Recent developments in subsea umbilicals technology were highlighted at this year’s Offshore Technology Conference (OTC). Greg App reports.

Umbilicals are literally the lifeline from the surface to the majority of subsea structures that make offshore exploration and production possible. However, multiple factors associated with ultra-deepwater environments have posed various challenges in regards to the construction, installation, and preservation of traditional subsea umbilical systems.

A technical session at the 2014 OTC in Houston included presentations discuss- ing some of the more pertinent technological innovations regarding this essential component of the offshore industry.

Carbon nanotube composite cables

Terry G. Holesinger of the US Department of Energy’s Los Alamos National Laboratory presented key findings of his team’s progress regarding the continuous research and development of carbon nanotube (CNT) composite cables for ultra-deepwater oil and gas field.

Long-term applications of this technology includes the possibility of subsea floor power distribution, ultra-deep- water dynamic power umbilicals, and improved efficiency in regards to hangar penetrations.

Some more immediate applications regarding deepwater include the improvement of downhole electrical submersible pumps (ESPs), operation at lower voltages to improve reliability, and increasing electrical transmissibility through the wellhead space.

“There is an ever-increasing need in the field of deepwater exploration for the development of conductors that are superior to copper and able to trans- mit more power to the subsea floor,” Holesinger said. “Experimental proce- dures regarding the development of CNT composite conductors are extremely promising.”

CNTs aim to fill the demand for a superior conductor that can operate at the extreme temperature and pressures that characterize deepwater exploration.

In fact, the authors of OTC paper “Carbon nanotube composite cables for ultra-deepwater oil and gas fields” note that the conductivity of various experimental cables were shown to have a higher specific conductivity that exceeded all metals except sodium (DePaula, Holesinger, Pappas, Rowley and Sperling).

“To date, we have successfully produced wires that display high conductivity values for CNT coatings. Additionally, these conductors are much lighter compared to pure copper wires and offer additional advantages for appli- cations needing lightweight materials,” Holesinger continued.

Since their discovery in 1991, CNT conductors have been known to have remarkable properties: electrical conductivity, strength, and ther- mal conductivity. “The challenge now is to produce these CNT products on a larger scale so they can be utilized by the deepwater offshore industry,” Holesinger said.

Instability of umbilicals

McDermott Subsea’s Miguel Pereira presented research regarding the company’s approach to the key challenges presented by the structural mechanics of umbilical systems.

One solution is to simplify the umbilicals’ installation analysis method by using 3DUST, an in-house umbilical simulation tool capable of suitable finite element meshing of geometrics, analysis of material assignments, and defining boundary conditions while presenting relevant results. Pereira and his co-authors P. Ramar and M.A. Dixon discuss in their OTC paper “Instability of umbilicals” that this method has already been applied to many deepwater umbilicals in West Africa, the Gulf of Mexico, and Brazil.

Pereira, Ramar, and Dixon propose a new method for assessing the effects of the installation procedure in regards to the structural integrity of the umbilical system by using the 3DUST umbilical simulation program. This method models the installation of the umbilical system by simulating loads that occur dur- ing the installation process These loads include bend radius, con- tact force, tension, and squeeze loads from caterpillar tracks and tube internal pressure.The paper states the installation of off- shore umbilicals without compromising their functional and structural integrity is a key challenge for the umbilical installation contractors and manufacturers. (Dixon, Pereira, Ramar). “A balance has to be struck between greater cross-section requirements...and stricter installation constraints,” Pereira told the OTC audi- ence during his presentation. “The key factors that define this balance involve the quantity and size of chemical, power, and signaling lines within a single umbilical, as well as the amount and size of equipment through which the umbilical travels during the installation process.”

Assessing fatigue life of flexibles and umbilicals DNV GL’s Mayuresh Dhaigude presented progress highlighted in the OTC paper “developments in computational methods for assessing fatigue life of flexibles and umbilicals,” written by Dhaigude and P.P. Sharma. “Flexible risers and dynamic umbilicals are being used increasingly in harsh environments which require reliable prediction of fatigue life,” Dhaiguda said. While it is extremely important to be able to predict fatigue life to ensure continuous and efficient operation, these predictions are extremely difficult due to layered construction and helical components such as tensile armors and steel tubes. In the presentation, Dhalgude and Sharma outlined an efficient fatigue analysis scheme involving cross-sectional analysis techniques required for fatigue stress calculations in helix elements subjected to bending, tensile and pressure loading, with special attention given to the effects of friction stresses caused by the stick/slip behavior of helix elements in bending.

“This analysis scheme will enable operators to establish fatigue damage and fatigue life by considering corresponding probability of occurrence for short term conditions,” Dhaigude said. The program being used to accomplish this analysis scheme is called Helica, an umbilical analysis tool whose calculation efficiency is due to analytical calculation of helix bending stresses assuming loxodromic helix geometry. Helica is capable of being applied to umbilicals and flexible risers while featuring a load sharing analysis for combined loading.

Additionally, Helica can calculate cross sectional stiffness properties, consistent fatigue stresses on the umbilicals, capacity curves for an umbilical cross section in compliance with applicable design codes, and both long- and short-term fatigue life expectancy. As factors such as installation, power efficiency, and simulation techniques are continually being developed to address and predict the various challenges facing the successful operation of subsea umbilical systems, it will be extremely interesting to observe how the offshore oil and gas industry adopts and applies these new technologies to reach new production and exploration depths.