Todd Kleperis explores the application of VIV-generated energy in the subsea sector
Constantly exposed to the elements, offshore piles and pipes are prone to problems related to fluid-to-structure interaction. Aside from corrosion, these structures are exposed to the effects of vortex-induced vibrations (VIV), which causes metal fatigue and stress.
But despite being an obstacle in the offshore industry, these problems (particularly corrosion and VIV) are the very conditions on which WITT (an acronym for whatever input to torsion transfer) can thrive, work and be applied.
What is a WITT?
WITT is a small energy harvesting device (only 350mm in diameter) that reaps chaotic motion and turns it into usable power. Sealed in a container with no exposed external parts, a WITT uses two pendulums connected to a flywheel going to a generator. When moved, the pendulums swing causing the generator shaft to turn. Thus, electricity is produced.
Specifically, WITT employs a unique transmission system. It converts bi-direction (maximum of six degrees of angular motion) into a rotational motion from the two axes. The rotational motion will then turn the flywheel in one direction, cranking up the generator in producing electrical power.
Each WITT is capable of producing 5W. For clients with specific power requirements, WITT can be customised (changing the pendulum mass) to meet certain power output and frequency.
Under The Sea
For subsea applications, WITT generates electricity through VIV. It is installed along with a high-density polyethylene (HDPE) pipe with a base clamp at the bottom and a submerged float at the top for tension. During vibration as a result of the vortex, the WITT device moves and the pendulum swings, cranking up the generator.
It only requires 0.5m/s of water flow for the entire WITT system to function. In the absence of water current (which is very rare in an open sea environment), the system’s electronics do not take all the power it generates, ensuring it will still work without stalling.
As for the design, the original configuration is a free-standing WITT unit. However, modular systems that can be customised for a variety of sites, conditions and power requirements are being developed. In other words, WITT is a scalable device that can be built initially from a small device to a large component.
It’s worth noting that not all areas in our ocean have enough VIV to power up the WITT. However, the company’s marine architects have prepared a global hydrology map to determine the best locations for installation.
Application For Cathodic Protection
There are two types of cathodic protection used as an anti-corrosion agent in metal structures. WITT can be applied on the impressed current cathodic protection (ICCP), which requires power to function as an anti-corrosion agent.
In the case of the ICCP method, a rectifier is used and installed on dry land. This instrument receives AC power and converts it into DC power, which will be supplied underwater through a wire connection going to the anodes. The power source of the anodes comes from a dry surface. These require electrical circuits and wiring for power to run. But what if we bring the power source right near the anodes? This is where WITT can be applied.
WITT can also be installed in existing structures with cathodic protection. Instead of wiring the anodes up to the surface for electrical connectivity, WITT can be the source of its power.
A single marine pile can be installed with one WITT system, which can then be increased in quantity to match higher power needs. Once installed, the unit will become a passive source of power without the need for further manpower and monitoring. On top of this, the construction mechanism is contained in a sealed vessel with no external exposed parts. It is made from standard industry components, offering a ‘fit & forget’ power solution.
Multiple Installations In Pipelines
Globally, our ocean floor is laid down with thousands of miles of pipelines. The Gulf of Mexico alone has over 24,000 miles of pipeline (mostly for gas distribution). However, much of it is abandoned or removed due to pipeline integrity failure caused by corrosion.
Although galvanic cathodic protection (anode bracelet that will degrade over time) is commonly used in pipelines, ICCP is still being used, especially in the decades-old and still functional pipelines. With new pipelines, multiple WITT devices (at regular intervals) can be installed simultaneously during deployment. This not only powers the ICCP method of cathodic protection, but will also accumulate enough energy to allocate for an ROV charging station. In this way, remote and robotic pipe monitoring and operation can be enhanced while lowering the risk of human-related accidents.
Finally, WITT can also contribute to the overall structural integrity of the pipes. With the use of a copper band, WITT can put a small charge in the system that will prevent marine life from adhering to the structures.
Other Potential Applications
As long as there is motion, the possibilities are endless for this easy-to-deploy power source device. For subsea applications, the R&D team is working on harvesting all forms of water motion: sea, river and tidal. Specifically, WITT is currently targeted to be used in powering up sensors and remote charging.
Todd Kleperis is with WITT Energy