Development of wet-design power cables enables electrification and subsea renewable power projects to reach greater water depths, using dynamic umbilical and cable systems. These innovative wet-design power cores are qualified up to 132 kV, supporting future market growth into higher voltage levels. The power core design is simplified, and the electrification umbilical’s improved performance is demonstrated through a case study.
ROBERT WEEKS, JDR Cables
Power requirements are increasing, as demand for electrification grows. Higher voltage levels are required to transmit this increased power while avoiding increasing conductor sizes. Additionally, deeper depths are required to access resources, which lends itself to dynamic umbilical systems connecting floating platforms to subsea structures.
Challenge: Existing 132 kV cable technology utilizes a metallic radial water barrier within the power core construction, which is inherently heavy and poor in fatigue.
Solution: Wet-design cable solves the problem by eliminating this metallic water barrier completely.
DEEPWATER CHALLENGES
Consider a power umbilical connecting a floating production, storage and offloading vessel (FPSO) to a deepwater subsea structure.
In-service condition: Fig. 1 shows a typical general arrangement for a power cable/umbilical dynamic system—the umbilical hangs from the FPSO in a lazy wave configuration, using buoyancy modules or a mid-water-arch to maintain the heave of the lazy wave and touchdown protection to protect the umbilical from any movement at the touchdown point. For deepwater operations, typically there is a very high top tension, driven by the umbilical self-weight, which can require the umbilical tensile reinforcement to be increased or require additional buoyancy modules. Both measures have an associated cost increase for heavier umbilicals.
Installation condition: Fig. 2 shows a typical installation scenario with the umbilical being deployed off the back of an installation vessel and hanging in a catenary down to the seabed. The installation condition in deep water is considered more severe than the in-service condition, due to prohibitive costs associated with using buoyancy modules to reduce the top tension. As the umbilical is deployed off the vessel, it must be held by a tensioner applying a high lateral crush load to prevent it from slipping through and falling to the seabed.
The umbilical, therefore, needs to be designed to withstand very high lateral/radial crush loads. This can be a problem for steel tube components, which tend to ovalize when subjected to high crush loads. It is recommended that measures be taken to reduce the weight of the umbilical and thereby reduce the top-tension and the tensioner crush loads.
DYNAMIC DEEPWATER POWER UMBILICAL OPTIMIZATION
Figure 3 illustrates a typical dynamic deepwater power umbilical, containing power cores for transmitting power, fiber optic cables for transmitting signal and communications, and steel tube components for effective transmission of hydraulics or fluids over long distances. Shaped fillers may be used to distribute radial/lateral crush loading and improve the crush resistance. Additional layers of contra-helically applied galvanized steel wire may be applied to increase umbilical tensile load limit and further improve its crush resistance. However, this impacts both cost and umbilical weight significantly, which increases the in-service and installation top-tensions, as well as the installation crush loading.
As evidence shows, the weight of the umbilical must be reduced to reduce the tension and crush loading requirements. This can be done to some extent through material selection of polymeric filler elements, but primarily through careful consideration of the component design and removal of redundant weight.
Figure 4 illustrates a typical high-voltage (HV) AC power core. The typical design includes a metallic radial water barrier made of extruded lead. Due to its heavy weight, it contributes significantly to the umbilical self-weight and, therefore, cost. The lead layer is also very poor in fatigue and is not suitable for use in dynamic cables.
HV POWER CORES FOR DEEPWATER AND DYNAMIC APPLICATION
The purpose of the metallic radial water barrier is to protect the power core insulation from potential “water tree failure.” Figure 5 shows an example of typical tree growth in the XLPE (Cross-Linked Polyethylene) insulation, resulting from partial discharge from gas-filled voids under high electrical stress. This is a phenomenon that affected early XLPE cables and can only happen when the XLPE insulation is saturated with water vapor, and when high electrical stress is applied over a period of time.
The water trees initiate from voids in the insulation or from abnormalities in the contact surface between the insulation and conductor screen or the insulation screen semicon layers. Modern grades of insulation and modern manufacturing techniques provide opportunities to eliminate the problem of water trees.
DRY AND WET XLPE INSULATION GRADES
Modern grades of XLPE may utilize water tree retardant (WTR) additives to prevent water tree growth from occurring. These have a proven track record when installed subsea in medium-voltage (MV) power cores with max insulation electrical stress < 4 kV/mm. Dry-grade (super clean) XLPE insulation can be used at max insulation electrical stress of < 8 kV/mm, but this does not contain the WTR additive, and it requires a metallic radial water barrier to be in place to prevent permeation of water into the XLPE.
Figure 6 presents a comparison of dry-design power core options, using dry grade insulation and incorporating various designs of metallic radial water barrier. Lead sheath metallic barrier is very heavy and poor in fatigue and is, therefore, unsuitable for deep water and for dynamic umbilicals. Corrugated metallic water barrier uses corrugation to improve fatigue performance somewhat, but it causes a very large diameter, making it susceptible to collapse under hydrostatic pressure and therefore unsuitable for deepwater application. In comparison, the wet-design power core uses WTR grade insulation and does not require the metallic radial water barrier, therefore giving the best performance in deepwater and dynamic environments.
TYPE TESTING
Next-generation 132 kV WTR-XLPE polymers have been subjected to wet-age testing in line with CIGRE TB 722 regime A and regime B, Fig. 7. The test regime simulates long-term aging of the power cores, and step breakdown testing and Weibull statistical analysis is performed to assess the predicted lifetime of the power core in service. The results indicate a large factor of safety exists, and a lifetime prediction in excess of 40 years for the wet-design 132 kV power core.
JDR Cables has previously uprated WTR-XLPE from 33 kV to 66 kV as part of the OWA (Offshore Wind Accelerator) project, which has been reflected in the latest version of IEC 63026. Now that JDR has successfully uprated WTR-XLPE up to 132 kV, there is demand for this to be reflected when the international standards are next updated.
An 800 mm2, three-core 132 kV WTR-XLPE power cable has successfully passed mechanical testing, including 1.6M cycle fatigue testing and tensile bend preconditioning, and electrical and non-electrical type testing in accordance with IEC 63026, Fig. 8.
CASE STUDY: DEEPWATER 72.5-KV AC POWER UMBILICAL
Figure 9 presents a comparison of two dynamic 72.5 kV power umbilical options. The metal corrugated power core is the only viable dry-design power core for dynamic application vs. the wet-design power core, which uses a slightly thicker insulation system to accommodate the same voltage level. It can be seen that the wet-design power core umbilical has a lower weight in air and in water. Also, due to its smaller outer diameter (OD), more length fits on a 9.2 m drum, which is a typical installation medium for short lengths or if the dynamic section is a different design than the static section. Additionally, the smaller OD reduces the size and cost of associated dynamic bend stiffeners and other hardware in the dynamic system.
Figure 10 presents a comparison for alternative static umbilical options. The lead-sheathed dry-design power core is an option for the umbilical static section. However, the wet-design core produces an umbilical that is dramatically lighter in seawater, which is good for keeping the top-tension low. It is dramatically lighter in air and, therefore, more length can fit onto a 2,000 Te installation carousel, reducing the risk of requiring a costly offshore joint.
CONCLUSION
Reducing umbilical weight in water is key to cost optimization of a deepwater umbilical. Therefore, component specification should consider flexibility for potential lighter component options. The innovative HV wet-design power cores are optimal for static and dynamic umbilicals, due to their lighter weight and other factors that enable longer installation lengths. HV wet-design power cores are a key enabler for deepwater dynamic power umbilicals, as the alternative dry-design power cores are limited to either shallow water or static applications only. The 132 kV power cores are fully qualified and ready for the market. WO
ACKNOWLEDGMENT
This article is based on a paper presented at the Deepwater Development Conference, Madrid, Spain, March 27, 2025.
ROBERT WEEKS is a subsea umbilicals and cables expert with over 14 years of experience in umbilical systems. As chief engineer at JDR Cables, Robert is focused primarily on delivering new technologies to the market to expand JDR’s product range and technical offering. Mr. Weeks has previously held roles at JDR as principal engineer for Umbilicals and Technical Sales manager for Renewable Energy Power Cables, which provide him with both a broad range of experience in this sector and a deep understanding of umbilical and cable products. Mr. Weeks holds a master’s degree in engineering from the University of Cambridge, UK, and is a Chartered Engineer registered with the Institute of Mechanical Engineers.