Innovation in the oilfield has long been driven by composites. Tailor-made to withstand the high pressures and harsh conditions of sub-sea and offshore environments, composites offer creative solutions while keeping weight at a minimum. Improvements are seen in critical areas such as installation expenses, environmental resistances, and overall part longevity. In addition, composites offer integration to metal components, magnetic transparency, and non-conductive materials.
As exploration expands in scope and depth, and pressures continue to rise, composites will play a critical role as an enabling technology. Below, we break down some key composite factors from material properties to the manufacturing process.
Most projects begin with selecting a fiber that will meet electric and stiffness requirements. Continuous glass fiber thermoplastics are generally RF transparent, non-conductive, and about a third the stiffness of steel in the fiber direction. In contrast, carbon fiber is stiffer, stronger, and tends to be used where electric phenomena are not a primary concern. Resin choice is usually a result of service temperature, chemical compatibility, and cost.
Service Life & Fatigue
Service life and fatigue often go hand in hand. In general, thermoplastics are better in fatigue than both metals and thermosets. For a more details, check out our handy comparison chart for thermosets and thermoplastics.
Some customers use our reinforced PEEK structures upwards of 525°F, but service temperature ratings depend on sustained loading. PEEK creeps at sustained load above 300°F, but the deflections may be inconsequential to your design. Keep in mind, these materials are 60% by volume continuous fiber, which has temperature limits far exceeding oilfield demands. Furthermore, molten PEEK at 650°F has the consistency of chewing gum, not water.
Failure Modes & Safety Factors
Continuous fiber plastics tend to fail catastrophically without prior indication, so safety factors above 1.5 are usually overkill. Some designs require margins as low as 1.05 since yield and rupture strengths are usually the same, practically speaking. And while API and other governing specs must be addressed, they do not always account for use of composites and pose challenges when trying to implement under guidelines designed for metals.
Connecting Composites to Metals
Despite the common challenge to join dissimilar materials, usually tubular in the oilfield, each design is unique and requires a highly tailored solution. The extreme non-linearity of stresses emerging in a laminate does not allow application of only a general solution. Designs must encompass common guidelines such as transition stress gradually across materials, thermal expansion, and localized stress while providing a solution for the given geometry, material choice, and loading.
Through Automated Fiber Placement, a laminate’s CTE can be tailored in two directions (often hoop and axial are a concern). There are many reasons to design with CTE in mind.
Engineering-grade polymers such as PEEK survive most oilfield conditions better than metals. For example, composites have higher H2S resistance than common metals. PEEK is not resistant to highly concentrated fluorine and a few other common chemicals. For such applications, sometimes fluoropolymer or PEI provides a better solution. Long-term seawater exposure is a common concern in this industry, and while some companies require their own tests, much data has been published promoting the longevity of PEEK materials used sub-sea.
Residual stress is present in all composites, as the fiber and resin vary by orders of magnitude in their stiffness. Implementing heat and pressure to process them imparts residual stress than cannot be removed in the part, only altered and managed.
The highest performing composites entail continuous fiber as opposed to chopped or short fibers. Through automated fiber placement, strips of prepreg tape build the structure layer by layer. This offers significant design customization and repeatability by tracking individual plies and laying fibers along predetermined load paths, minimizing wall thickness, valuable real estate in the oilfield. Considering manufacturing upfront can bring tremendous benefits, such as making dozens of parts at once and reducing overall project costs.
For more information about our work in the oilfield, contact:
Business Development Manager