Materials technology is being challenged to its limits in the deepwater and high-temperature applications the oil industry is heading into. Technip’s Alan Rutherford outlines work to make umbilicals fit for purpose in this area.
Thermoplastic hose. Photos from Technip.
The subsea umbilical market is evolving. The need for these products to go deeper and further continues and the requirement for higher temperature capable umbilicals is growing.
Umbilical technology is adapting to these new environments and continues to push the boundaries of the current technologies encompassed in the products, through new materials and innovation.
Market studies have shown a number of factors that drive elevated temperatures in umbilicals including; electrical power cores; hot fluid passing though tubes or hoses, solar radiation, umbilical burial adjacent to a hot pipeline, or a combination of these factors, especially in warmer water regions such as Asia Pacific, Africa and Middle East. The heat generated or experienced by an affected umbilical can lead to accepted temperature limits being exceeded for commonly used umbilical materials, resulting in de-rating of stress allowances or corrosion of metallic components, and mechanical degradation of polymers.
These factors leave umbilical manufacturers faced with many challenges; how do we design umbilicals to withstand these temperatures? Can these materials last for 25 years in service? Will new products be more cost efficient than current designs?
Facing these challenges led Technip Umbilicals to undertake several research and development initiatives on new technologies to ensure we can create products that are not only high quality but are also cost efficient for the current market.
Materials used within traditional thermoplastic umbilicals are able to cope in a wide range of applications and environments, including low-temperature and long-term continuous operations. However, using these materials in high-temperature environments, especially over long periods of time, can result in issues including; reduced collapse resistance due to material softening, reduced service life from degradation and difficultly in retaining the coupling on the end of the hose. In order to give the product long term viability, a fundamental change in all aspects of the hose design from liner material, reinforcement material, cover material and end fitting design was required.
To overcome the high temperature concerns in hose technology, Technip Umbilicals has developed a new thermoplastic hose design known as DUCOFlex HT (high temperature), creating a product which can operate continuously at temperatures from -40°C to +80°C and is capable of sustaining intermittent use at temperatures from -50°C to +100°C for longer periods of time, ensuring no degradation in performance.
Any new material must be more cost effective for future developments compared to the alternative; super duplex stainless steel tubes or coated tubes, which have a higher component and manufacturing cost compare to standard thermoplastic hose. Also, the additional benefits from easier termination and installation of thermoplastic umbilicals compared to steel tube umbilicals needs to be preserved to ensure the overall cost advantage is maintained.
Figure 1 – Traditional hose liner materials vs. DUCOflex HT.
Technip Umbilicals patented the use of the polyethylene based liner technology DUCOflex in 1994, which gave significant performance improvements in terms of fluid permeation and collapse resistance. At the outset of this development, a range of materials were evaluated in order to determine the optimal material for high temperature environments, whilst maintaining the proven advantageous properties of DUCOflex.
In order to determine the long-term suitability of candidate materials, samples of Technip Umbilicals’ traditional hose liner materials and the proposed new DUCOflex HT material were exposed to synthetic seawater at various temperatures for durations up to five years, and as can be seen in Figure 1. It is clear that the new DUCOflex HT hose material retains structural integrity during long term exposure to high temperatures, whereas the common alternatives, especially polyamide rapidly loose mechanical strength, resulting in significantly shorter life and reduced collapse resistance.
The increased temperature performance of the new DUCOflex HT hose also results in a more stable flexural modulus value as the temperature increases, this means that not only does the comparative collapse performance of the hose increase for a given temperature, it retains collapse performance over a significantly greater temperature band, meaning the useable water depth is significantly increased also.
The hose reinforcement design, both in terms of material and specific construction, has been modified to give enhanced long term performance at higher temperatures and has been tuned to give an optimal performance balance between working pressure rating and impulse fatigue. These modifications ensure the hose technical performance is improved, but critically remains fully ISO compliant, including a 4:1 factor of safety on burst pressure.
The metallic end fittings utilized with high-pressure umbilical hoses are typically the most challenging element in what is a very robust system. Therefore, in addition to the material changes to the hose construction, a new style of fitting needed to be developed to guarantee successful service at the higher temperatures. Elements of this fitting have been patented including a secondary lock nut mechanism to mitigate against loosening of the fitting during service. This fitting has been developed to allow connection with standard JIC or metric adaptors. However, as with traditional fitting designs, alternative joining arrangements can be provided to suit the clients’ requirements including stub pipe designs for welded terminations to remove any threaded seal interface.
Thermoplastic hose. Photos from Technip.
To reinforce the findings from research, a recent tender case study for high-temperature umbilicals focused on a 6km-long umbilical project located in 100-200 m water depth within Asia Pacific. The study highlighted that during planned maintenance the annulus lines were bled down via a vent line in the umbilical, allowing hot fluid to flow through and heat up the umbilical. During a typical operation, depending upon the water depth, constituency and temperature of the fluid, this could reduce the collapse resistance and limit the design life of a standard hose design. Traditionally this would have meant the hose was replaced with a stainless steel tube (lean duplex or super duplex), but this increases the cost and complexity of the umbilical. The steel tube is generally a longer lead time item, has higher bending stiffness, requires more time and cost to fit end terminations, is more challenging to install, has a finite fatigue life and is more costly especially if a corrosion protection coating (fusion bonded epoxy, for example) is required. By substituting steel tube with a new high temperature hose design this significantly increases the temperature and water depth performance without the costs and disadvantages of steel tube and enables the umbilical to remain fully thermoplastic with all of the associated benefits.
For this project study, a saving in the region of 10% was made by not replacing the affected hose with a super duplex stainless steel tube. Further savings could be realized through simpler termination hardware and less complex installation.
In the current market conditions, it is extremely important that companies adapt their products to fulfill market needs but with a strong focus on cost optimization. As subsea market demands change, Technip Umbilicals are meeting these challenges with cost effective, industry leading products.
Alan Rutherford has been working at Technip Umbilicals for over 15 years, during which time he has worked on various research and development projects including Super Duplex steel tube fatigue and long term ageing of materials. Rutherford is currently a principal engineer and lead of the team responsible for research and development of hose products and thermoplastic materials used within umbilicals.