Richard W. Voight, Intec Engineering
Electric flowline heating (EFH) has become an important concept in today’s field development projects. Its significance can be summed up in two words - flow assurance. Flow assurance is an important topic of discussion given the increasing water depths and distances associated with today’s subsea production tiebacks.
A prominent problem for long-distance production flow assurance is preventing hydrates and wax deposits. Another problem is flow resistance resulting from high-viscosity oils. Historically, the most common solids formation prevention techniques have been thermal insulation to retain produced-fluid temperature coupled with chemicals (hydrate inhibitors and/or paraffin inhibitors) to lower the temperatures at which hydrate formation and wax deposition can occur.
For long flowlines, it is impractical to provide enough thermal insulation to hold flowing temperatures at acceptable levels without adding heat to the produced fluid. For high water-production rates, the cost of chemicals to prevent solids formation at seabed temperature can be very high. For these and other reasons, the industry is expending much effort to develop new, reliable, cost-effective methods and technologies for flow assurance for long-distance flowlines.
EFH options
EFH is the use of electric current to add heat to an insulated flowline system to retain fluid temperature at a level high enough to prevent solids from forming. Depending on the application, EFH can add heat continuously or intermittently.
Electrical current flow through an object heats that object. This is in accordance with a variant of Ohm’s law, which states that the amount of energy passed to an object by passing an electrical current through it is proportional to the square of the current flowing through it multiplied by the resistance of the object.
Mathematically, this is shown as:
P = I 2 * Rac, derived from Ohm’s Law,
Where; P: Real power in watts
I: Current in amps
Rac: AC resistance in ohms.
Several EFH variants applying this relationship have surfaced in recent years, and each deserves discussion. The variants can be subdivided into two major categories: wet- or dry-insulated. The thermal insulating material for wet-insulated flowlines and riser systems is in direct contact with open seawater. Dry-insulated flowlines (pipe-in-pipe or PIP) and riser systems use an outer rigid pipe to keep the thermal insulating material in the annulus dry.
Wet EFH systems can be classified further as closed loop, ungrounded, or grounded; open loop, cable, or seawater return; and heating tube, conductive, or inductive.
Closed loop, ungrounded, or grounded
In the closed loop concept, the flowline is one of the conductors in an electrical circuit. Electrically, the circuit can either be isolated completely from the environment or grounded at the remote end.
In the ungrounded configuration, where the entire system is completely isolated from the environment, including the power supply, no voltage potential difference exists between the flowline and surrounding media. In this situation, it is possible for the system to continue operating when there is a single failure in the electrical insulation. A second failure, however, cannot be tolerated as this leads to the flow of stray currents and possible damage to the system (i.e., one fault provides a reference to ground, a second fault creates a short circuit to ground).
The construction of a closed loop EFH system requires electrical isolating joints at the source and remote ends of the flowline. The joints provide electrical isolation between the flowline heating power supply and any facilities to which the flowline is mechanically connected.
A dense, non-hygroscopic thermal insulation is applied to the flowline to provide the required electrical insulation. On the outside of the insulation/outer jacket, a single-core electrical cable is “piggy-backed” onto the assembly during installation.
The grounded version closely resembles the ungrounded version. Removing the isolation joint at the remote end and grounding the flowline can achieve cost advantages. The system’s performance characteristics are similar to those in the ungrounded case.
Open loop, cable, or seawater return
In the open loop concept, seawater provides all or part of the current return path. In terms of hardware, the open loop is similar to the closed loop. The difference is that in the open loop, the flow of electrical current in the surrounding media forms part of the design. As a consequence, the potential difference between the surface of the flowline and the surrounding media is, in theory, equal to zero, and the insulation material has to provide only thermal insulation.
For the cable return option, part of the return current flows in the seawater, while the remainder flows in the return cable. The flowline is used to conduct an electrical current in parallel with the surrounding media, i.e. seawater and seafloor.
Because there is no potential difference between the flowline and the seawater, the entire flowline is effectively at ground potential. A potential gradient exists axially between the electrical connection points at the flowline ends. This apparent paradox results from stray currents in the seawater, which create a potential gradient in the surrounding media similar to that in the flowline.
This design is robust because imperfections or deterioration of the thermal insulation have no detrimental effect on the system’s electrical performance.
In the seawater return concept, all return current passes through the seawater surrounding the pipe instead of via a return path cable. To allow this concept to function correctly, the thermal insulation also must provide electrical insulation.
Heating tube, conductive, or inductive
The heating elements in the conductive tube concept are assembled from an insulated copper cable installed within a steel tube. The cable and tube are connected at the remote end to form an electrical circuit.
When an alternating electric current is applied to the cable and tube, the return current flows on the inside surface of the steel tube due to the coaxial geometry and the proximity effect. Depending on the magnetic permeability of the steel, this can cause the current to concentrate over a small surface area, giving rise to significant heating.
Thermal conduction transfers the heat from the steel tubes to the flowline. Effectiveness depends on the quality of thermal insulation, lay-up geometry, and relative temperature of the surroundings.
The heating element forming the induction tube heating concept is similar to that of the previous concept, with the exception of the end connection. In the induction tube heating system, all three power cables connect at the remote end to form a neutral point that is connected to ground.
An AC current passes through the cable running through the center of the steel tube to generate heat. This produces an alternating magnetic field that induces circulating (eddy) currents in the tube wall. The circulating currents are dissipated as heat due to the AC resistance and magnetic hysteresis of the tubing material.
Thermal conduction transfers the heat from the steel tubes to the flowline. Again, effectiveness depends on the thermal insulation, lay-up geometry, and the relative temperature of surroundings.
PIP end, center fed
Dry EFH systems can be categorized as PIP-end and center-fed, and cable heating.
PIP end-fed designs use a carrier pipe to transport the produced fluids, thermal insulation to help retain produced fluid temperature, and an outer pipe to protect the thermal insulation from the environment. EFH PIP designs differ from normal PIP designs primarily in that electrical connections must be provided, and the internal centralizers, water stops, etc., must be nonconductive.
The inner and outer pipes are the electrical conductors in this concept. The two pipes are connected electrically at the subsea end by a shorting bulkhead, allowing current to flow down one pipe and return up the other.
At the platform end, an isolating joint forms part of the flowline. This allows an electrical supply to connect across the flowline and carrier pipe without the near end bulkhead creating a short circuit and isolating the power supply from the topside structure.
When AC power connects to the system, current tends to flow on the outer surface of the inner pipe and inner surface of the outer pipe due to the distribution of the electromagnetic field set. This tendency is pronounced in carbon steel pipes because they have high relative permeability. The flowline pipe produces useful heat, while heat produced in the carrier pipe is lost to the environment.
For the center-fed case, the inner and outer pipes are used as electrical conductors similar to the end-fed concept. The difference is that in a center-fed system, steel shorting bulkheads are installed at both ends of the segment, and the power source is connected at the mid point via a mid line assembly (MLA). No topside isolating joints are required. In all other respects, the system is similar in configuration to the end-fed concept.
Cable heating
The cable-heating concept consists of a PIP system with heating cables installed between the inner pipe and the thermal insulation. The use of standard, single core, electrical power cables, installed in multiples of three along the outside of a flowline leads to a relatively simple heating method. The cables form a balanced three-phase load and produce heat by resistive heating.
When using copper cables, the AC resistance per unit length is relatively low, giving low heat output per cable. The cables already are limited by the properties of the electrical insulation. For this reason, multiple cables are used to deliver the required power. Currents induced in ferromagnetic flowline walls generate a small amount of additional heat.
There is one common theme tying all these EFH concepts together - Ohms Law. Some inherently take advantage of “skin effect” or “proximity effect” or both, but the lowest common denominator is that electrical energy is converted to heat.
The AC resistance of a conductor differs from the DC resistance of a conductor as a consequence of skin and proximity effects. Skin effect is the tendency of electrons to flow at the surface of a conductor subject to an AC signal. Proximity effect is the tendency of electrons to move in close proximity to those in a return circuit path in the presence of an AC signal. Both effects tend to increase current density thereby raising the apparent conductor resistance.
All of these concepts add heat to a flowline to retain fluid temperature at desired levels. However, they do not all function the same. The status of these solutions range from work in progress for some to fully qualified and installed systems for others.
Choosing the right EFH solution requires a system design process. There are a number of complexities and variables. Selection of the right EFH solution for any field development project requires the efforts of many, and all disciplines have to be considered. As field developments go forward, EFH solutions are poised to play integrally into their successful conclusions.
Richard W. Voight is a senior engineering specialist in the Subsea Systems Group of Intec Engineering. He has experience offshore with specialty subsea cable and umbilical applications, including remotely operated vehicles, multiplex deepwater BOP systems, installation, workover and control system umbilicals, and moored riser systems. Voight holds a Bachelor of Science degree in Electrical Engineering from the University of Houston.
