Consider fluid selection, case construction, cooling and heat exchanger design, bearing development and more.
by Geoff Robinson & Rupert Knowles
May 24, 2016

As the topside space to undertake hydrocarbon processing continues to be at a premium, production from mature wells increasingly suffers from higher water cut and suboptimal reliability from in-well artificial lift systems. Subsea processing and, in particular, subsea pumping are never far from the agenda for most operators. The development of subsea multi-phase boosting solutions and centrifugal seawater injection systems at depths of up to 3,000 meters operating at pressures of 690 bar is creating the requirement for subsea motors that can match the robustness of the pumps they drive. Unlike conventional motors where airflow is used to cool the motor, these high-tech subsea motors require a water/glycol mix or oil fill to withstand the operational pressure and provide cooling. The challenge is to design high-powered motors up to and including 3.2 megawatts (MW) that achieve a five-year scheduled maintenance requirement with an ultimate goal of a 25-year service life. This article will explore the challenges of designing and manufacturing wet-wound water/glycol motors for these applications.

Motor Fluid Selection

An important consideration in subsea motor design is the selection of the barrier fluid for the motor fill. Traditionally, either oil fill or water/glycol mix is used. While both systems have been used for submersible and subsea applications, there are some fundamental differences that can have a big impact on system setup and operational performance.

The primary advantage of an oil-filled system is that the fluid provides insulation for the motor windings, negating the need for a separate insulation system on the winding cable and enabling a more compact design. From a topside submersible perspective, the infrastructure required to support the operation of the motor has a larger footprint than a comparable water/glycol system. In the subsea environment, with high viscosity leading to greater frictional losses in oil-filled motors, higher operating temperatures can cause an increase in corrosion levels, reducing operational lifetimes. With system integrity based on a double seal system, there is also the constant threat of seawater ingress, which would lead to immediate and catastrophic failure.

Adopting a water/glycol design overcomes these problems and delivers other advantages to operators of offshore and subsea installations. The water/glycol provides the lubrication for the hydrodynamic bearings, and it cools the motor and provides anti-freeze and anti-corrosion safeguarding. Combined with the use of fully qualified insulated cross-linked polyethylene (XLPE) cables and joints to meet the high-pressure demands of subsea applications, water/glycol is now a commonly used control fluid that allows subsea motors to run at operating speeds up to 6,000 revolutions per minute (rpm) with the lower frictional losses generated by the motor simplifying the cooling challenge. Lower-temperature operation also facilitates longer operational life and extended service intervals. With extensive qualification testing to validate the long-life characteristics of the motor at both a component and system level, the robustness of the water/glycol-XLPE insulated systems is now well-established, having been demonstrated theoretically through physical testing and in long-term operation.

With low environmental impact, the water-based coolant poses little threat to the subsea habitat in the unlikely event of any seal failure. Unlike an oil-filled unit in the case of seawater ingress, there is no immediate failure of the motor. Any failure would be gradual, providing several months of operational continuity to facilitate scheduled outage planning.

Motor Case Construction

All seawater-wetted applications require either a Norsok M650 approved super duplex or a 625 alloy clad carbon steel casing construction, which is dependent on the user preference and final application. At 1,035 bar test pressures (690 bar x 1.5), significant analysis of pressure requirements is required, using Pressure Equipment Directive (PED) and American Society of Mechanical Engineers (ASME) guidelines. Internal components also have to be suitable for a wet environment and will generally be carbon steel, aluminium bronze or stainless steel, with polyether ether ketone (PEEK) based bearing materials.

A 3.2-MW motor case presents a manufacturing challenge since these can be up to 3 meters long and 1 meter in diameter. This limits the manufacturing options available. Ideally, the preference is to manufacture a one-piece motor case, which will typically be forged.

Motor Cooling & Heat Exchanger Design

A wet-wound motor requires a high level of cable insulation (Class Y)—generally XLPE insulation. To achieve a minimum 35-year mean time between failure (MTBF) at these ratings of 3.2 MW, the internal temperature of the motor needs to be limited to about 140 F (60 C). One of the key areas that has to be considered is the end turns of the windings, because this is a typical motor hot spot and needs to be limited to a maximum copper temperature of 190 F (90 C). Failure to achieve adequate cooling in this area will have a detrimental impact on the 25-year service life.

Detailed thermal and electrical modeling of motor cooling characteristics generally employs motor design limited (MDL) electrical modeling and computational fluid dynamics (CFD) or similar tools. The model determines optimal internal motor temperature, and this is supported throughout the development phase by physical testing.

At 3,000-meter depths, it is difficult to predict the cooling currents available, so the normal assumption is that the only cooling method available will be convection cooling. Generally, the heat exchanger designs consist of parallel coils arranged external to the motor pressure case. The external surface of the coils is exposed to the seawater and cooled by natural convection. Evaluating the cooling coil at these elevated depths requires a detailed balance of analysis of structural strength, manufacturing processes and internal power losses to cooling coil surfaces.

Motor coolant heats as it is pumped through the stator/rotor gap (interestingly still called the air gap) and bearings, then cools as it passes through the cooling coils. Controlled leakage through wear rings cools the thrust disc and thrust bearing pads. The motor coolant is heated through a combination of approximately 86 percent frictional heating and internal pumping loss, with the remainder from electrical losses. Cooling coils account for 100 percent of the heat rejected because only a very small amount is lost through the motor case. The circulation pump flow rate is required to limit coolant temperature rise in the motor/stator gap to a maximum of 65 C for XLPE systems. Coolant emerging from the gap is in immediate contact with the electrical winding end turn insulation.

Cable Life & Splicing

For these applications, motor cable industry standard Subsea Electrical Power Standardisation (SEPS) is now applicable for evaluating insulation system life. The industry requirement stipulates more testing than would traditionally have been acceptable for these motor applications. Typical tests now include 80 C temperature cycling of penetrator and cable joints, 600 bar pressure cycling of penetrator and cable joints and advanced cable life aging tests

For a typical 2.5-MW machine, the winding cable consists of stranded copper wires of 1.2 millimeters (mm) each. The copper size accommodates the rated current with a current density below 12.5 ampere per square millimeter. A 0.25-mm-thick layer of semiconducting material surrounds the copper, which smooths the electrical field in the cable to reduce electrical stresses. The next layer is XLPE insulation that is 3 mm thick. This thickness insulates against 11 kilovolts (kV) and ensures that the electric field does not exceed 3.3 kV/mm at 11 kV (design limit).

The electric field is much lower at the normal operating condition of 6.6 kV, and constraining the electrical stress extends the lifetime of the cable.

Bearing Development

For wet-wound applications, significant analysis and testing are required to develop 6,000-rpm bearing technology.

At startup, the tilting pad thrust bearing operates with only boundary lubrication and does not achieve a fully hydrodynamic operation until it exceeds 200 rpm. During startup from 0 to 200 rpm, the friction in the bearing is relatively high and then drops once a hydrodynamic film has been established. A centrifugal pump impeller circulates the glycol/water mix around the motor to cool it. This impeller and thrust bearing absorbs about 7.5 percent of rated power at the max operating speed of 6,000 rpm.

For these applications, PEEK tilt pad bearings have been developed, qualified and further enhanced by significant development of mechanical keying to the steel substrate backing with the introduction of injection molding of the bearing materials. Enhanced endurance testing also includes the analysis of bearing temperatures.

Rotordynamic Modeling

Rotordynamic analysis is conducted on all motors to determine shaft critical speed, journal bearing stiffness, damping coefficients and shaft characteristics normally calculated using tools such as XL rotor. Analysis is a reliable tool to predict the base model but there is still no alternative to physical testing, and this requires a physical build of a prototype or scaled model to validate analysis.

Image 2. Subsea motors require a water/glycol mix or oil fill to provide cooling.Image 2. Subsea motors require a water/glycol mix or oil fill to provide cooling.

Motor Testing

In addition to wide-ranging qualification at a component level, motor system testing is a final prove-out of both the design and build quality of the finished system. Nominally governed by International Electrotechnical Commission (IEC) 60034 and American Petroleum Institute (API) 546, the requirement for full-load factor acceptance testing (FAT) in a subsea context tends to extend beyond the first article of a new design. Ensuring a rigorous shakedown of the motor system before deployment gives the best possible opportunity for identification of any potential causes of costly unplanned service interventions. With existing systems already topping 3-MW, 6,000-rpm specifications, the drive toward higher-power/higher-speed motors requires advanced specialized test infrastructure to provide testing capability comparable to in-service verticalized operation.

Moving Forward

As the subsea processing challenge increases in terms of depth, power and operational efficiency, the motor technology to support these applications is keeping pace. Current XLPE-based systems can efficiently meet most demand for applications up to 3 
MW, and alternative systems based 
on permanent magnet or higher-
class insulation systems allow the extension of the technology to higher-power applications.

The capability of these systems to deliver long-term operational reliability in an environment where the operational requirements can change significantly requires careful planning. The industry’s ability to model, qualify and test performance-critical motors ensures that high-quality subsea motor solutions will be available to meet the ever-increasing subsea challenge.