As subsea oil fields move further offshore and material use conditions become increasingly severe, it is necessary to have a fundamental understanding of how materials degrade over time in these harsh environments. It is important to make informed decisions regarding material selection in the initial stages of product design. Deploying critical subsea oil and gas systems to several thousand feet makes maintenance difficult or impossible, and assurance that systems will function as designed for their intended lifetimes is a key product differentiator.

Currently, there is no comprehensive material properties database that adequately captures the effects of simultaneous mechanical, thermal, and electrical loads applied in a corrosive environment for long periods mimicking conditions in the deep sea.

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Materials selection based on the Ashby plot with notional constraints defining the design space. (Images courtesy of Teledyne Oil & Gas)

Teledyne Oil & Gas is investing in a research and development program to address this need through rigorous evaluation of dielectric materials for subsea power applications and the development of a robust life-prediction methodology. The benefit of this program will be the ability to provide new products for more challenging environments faster and with quantifiably higher reliability.

The general approach is to begin with systematic characterization tests that measure the inherent properties of off-the-shelf materials. These tests augment and provide independent verification of manufacturer’s data. Next, researchers assess the sensitivity of material degradation to temperature, pressure, voltage, and chemical environment, separately and in combination. This information is used to develop accurate degradation mechanism models for aging in conditions relevant to subsea applications. With knowledge of the regimes over which various degradation modes dominate behavior, researchers perform appropriate aging (accelerated life) tests and use the data to develop accurate life-prediction models.

While well-established accelerated models exist for some key stressors such as temperature and voltage, there are other stressors such as high hydrostatic pressure in a fluid environment, where little data exists that elucidate the underlying degradation mechanisms. Furthermore, the combinations of relevant loads and their synergistic effects on property loss are also generally unknown. To address these knowledge gaps, Teledyne Oil & Gas has established a dedicated characterization facility co-located with multistress exposure testing equipment. The potential exposure conditions cover the requirements for most subsea systems. Identifying dominant degradation mechanisms is facilitated by the ability to directly monitor key physical property changes that result from stress exposure.

Materials selection

Selecting the best materials for mechanical, electrical, thermal, chemical, or other performance measures is an important part of the design process. Systematic selection of the best material for a given application begins with knowledge of the component requirements and the primary materials properties which affect those performance factors. When multiple criteria are considered, ranking candidates becomes a complex process.

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Figure 2. Snapshot of the Teledyne Oil &Gas materials database for subsea materials depicting the result of a tensile test and with post-mortem study of the sample.

To illustrate how one might begin the process, we’ll consider a single set of constraints associated with requisite mechanical performance. For example, high-power connector pin insulation needs to be stiff and strong to maintain its shape in a high-pressure subsea environment while also possessing a minimum prescribed failure strain for tolerance to handling.

The component design space can be conveniently represented using an Ashby plot, named after Professor Michael Ashby of Cambridge University.

The data of interest is depicted as a scatter plot displaying two or more properties of a variety of candidate materials. A simple example relevant to the subsea connector insulation is illustrated in Figure 1 in which Young’s modulus (stiffness) of various materials classes is plotted as a function of their respective strengths.

The granularity of such a plot can be adjusted to reflect the broad classes of materials shown or slight differences associated with changes in resin grade or processing method for a single material. As constraints on performance associated with the component requirements are defined, they are superimposed on the plot and used to eliminate from consideration materials that do not possess adequate combinations of properties.

Similar plots defining electrical, thermal, and cost figures of merit would all be considered during the materials selection process. However, an important caveat of materials selection for reliability is that the plots developed to define the design space comprise material properties data relevant for the end-of-life condition as opposed to the typical material datasheets provided by manufacturers. Therefore, accelerated aging testing is a critical part of developing a useful database for product designers.

Reliability testing

The time factor in reliability (aging over time) presents several interesting challenges, the first and foremost of which is the time required to complete the reliability tests. This becomes a greater challenge for high-reliability applications in which products are expected to last for a long period of time, such as subsea power components. Accelerated-life tests effectively solve this problem by simulating the life of the product under test, either by overstressing to accelerate the failure mechanisms or by simply compressing the time under life conditions (high usage).

In both cases, researchers need to evaluate failure modes and exclude any that would not occur during normal use. It is important to first identify variables (electrical, mechanical, thermal, etc.) that contribute to the key failure modes and to identify how the failure mechanism is accelerated by increasing the stresses (stress-life relationship). Existing relationships such as Arrhenius’ law and the inverse power law are frequently used for life predictions. When there are no models that explain the stress-life relationship, empirical relationships can be developed.

Since more than one failure mechanism exists in every product, proper care must be taken to investigate all predominant failure modes. Separate tests may be required to address each of these failure modes. Similar to having multiple failure modes, it is also possible for a single failure mode to be caused by different stresses, singularly or in combination. In other words, more than one stress may produce the failure mode. The complexity of this relationship presents significant challenges in developing acceleration models and estimating parameters. Understanding the underlying physics of failure by using characterization tests is critical in developing such accelerated-life prediction models.

Test facilities

Testing is conducted at two major labs: accelerated aging and analysis. They are co-located to ensure full control over all parameters and, equally important, the shortest time possible between aging and testing, to minimize recovery effects.

The core of the accelerated aging lab is a customized stainless-steel pressure vessel with high-voltage feed-throughs and a heating system. This unique

system currently enables aging of specimens at pressures up to 30?kpsi, temperatures up to 481°F (250°C) and electric loads up to 40?kV?DC/15?kV?AC, all of which can be applied simultaneously. This addresses the projected-use conditions envisioned for subsea power components for the next five years.

As requirements expand for other types of systems such as downhole instrumentation, the facility can be modified to accommodate more severe exposure conditions. The pressure vessel can be run with fresh or salt water, for limited exposure times, and with anaerobic ground-soil to simulate burial in a deep-sea environment. A computer controller and tracking system applies a custom load profile and logs all relevant process data.

The state-of-the-art materials analysis lab was established to support the testing requirements. The most important electrical parameters (resistivity, permittivity, breakdown strength), mechanical parameters (tensile modulus, tensile strength), and physical parameters (glass transition temperature, water absorption, specific heat, coefficient

of thermal expansion) are tracked for all materials in the program. The labs are staffed by materials scientists and chemists who not only perform analyses according to standards but are also able to identify and track anomalies that could indicate a new breakdown mechanism due to the combined loading. Without professional staff, significant anomalies might be discarded as operator error.

Testing and analysis of materials

The current focus of the materials reliability analysis program on subsea power systems includes a selection of dielectric materials with a prescribed minimum-use temperature of 302°F (150ºC). General material classes include thermoplastics, elastomers, and ceramics, with more than 77 different materials in the initial testing program, including new and established product grades from commercial vendors.

The most promising materials were selected for the testing program based on material properties reported by manufacturers. This list includes some of the most commonly used deepsea materials such as PEEK and polyetherimide, as well as novel blends with and without fiber reinforcements. Testing will clarify the role of reinforcement concentration, distribution, and orientation on material properties. Injection molded plaques were acquired for these initial evaluations, and redundant analysis samples were added to evaluate data scatter.

All materials are subjected to an accelerated aging and testing schedule. This is initialized with baseline measurements taken in the unstressed, as-received state. Aging tests first subject the materials to varying levels of load under single-stress conditions, encompassing the range of test conditions described previously and for various exposure times (pressure, electric field, temperature). Each sample is fully analyzed after every exposure to determine degradation mechanism regimes operating over the anticipated product use range. The result is a matrix of accelerated aging under appropriate combined loading scenarios, guided by a qualified reliability engineer after probing the single-stress-state responses of the materials.

All data, including analysis results and metallography, are loaded into a fully searchable, proprietary database accessible throughout the company (Figure 2). The database not only ensures complete traceability of every result from the individual raw material lot to the final analysis, including all handling and processing steps for every specimen, but also forms the basis of the materials selection and life prediction tools which will ultimately result from this effort.

Outlook

The proprietary material properties database under development by Teledyne Oil & Gas incorporates degradation mechanism identification for accelerated aging testing and analysis. The resulting materials selection and life-prediction tools will improve the life-cycle reliability of future subsea power products while simultaneously reducing their development cycle time and expense.

Ultimately, this unique capability will be expanded and used throughout the organization to develop a variety of next-generation subsea products and systems.