SUBSEA TECHNOLOGY
conducted trials at Solent University tow tank over a three-day period.
VIV OPERATING CONCEPT Te operating concept of the subsea WITT is motion from VIV. Tis is achieved by connecting the WITT energy device to cylindrical cross section pipe sections, the periodic vortex shedding generates lift and drives the WITT generator. Te aim is to create a modular system that can be varied for sites with different conditions or power requirements with ease of installation and minimum maintenance at the forefront. High density polyethylene (HDPE) has been initially selected for the cylindrical pipe sections due to its low cost, fatigue performance and corrosion resistance. HDPE is readily available and can be easily joined using electro-fusion techniques offshore too. For different sites/sizes other materials could be used, such as composites or steel with the necessary level of scrutiny applied to fatigue life and corrosion protection. Te circular cross section pipe is also useful as a conduit for power/ communication cables, enabling an entirely self-contained system. A wide range of design options were studied to determine the parametric basis of the system as multiple parameters can be adjusted to affect the performance. Also, the natural frequencies and mode shapes of the system require consideration to ensure the WITT energy converters are placed at the antinodes where displacement is at a maximum.
POSSIBLE APPLICATIONS Te WITT device has many applications harvesting natural occurring motion energy (NOME) that is all around us. Tis subsea version driven by VIV has two primary applications as follows, although there may be many more: Subsea power: Where the primary aim is to power subsea electronics in remote
The WITT can be put to use for a number of subsea applications
locations where battery replacement is a costly exercise. Te low maintenance strategy and lack of external moving parts lend it to uninterrupted operation. Low ocean current speeds can be exploited by increasing the length of the system. Alternatively, areas with high current speeds can also be used by reducing the length of the system. Tuning is then achieved for a given power requirement by adjusting the pipe diameter and stiffness. Also, a top float can be used to adjust the line tension. Te application could be used to power ROVs or autonomous underwater vehicles (AUV) charging stations, or wherever a remote subsea power requirement exists, such as remote sensing in aquaculture and scientific data. Te WITT energy converter would charge local energy storage for demand when required. Some of this energy may also be used for impressed current corrosion protection systems. Riser/umbilical VIV reduction measures:
Here the WITT generator load can be varied to maximise the opposing force, effectively damping the VIV magnitude. Tis could be a pre-set load, or an active tuning performed remotely or onboard using an array of WITT devices along the length of the riser.
TANK TESTING
Patents for the WITT have been granted in the USA, China and Europe
Te purpose of the testing was to cover several aspects; observe VIV occurrence, determine the effect of the WITT energy device on the overall system dynamics, measure WITT displacements and frequencies over a range of operating conditions.
A full scale 5W WITT drivetrain was
available so a full-scale test programme, looking at the first section of the system comprising clamp, pipe and WITT in an inverted configuration, was carried out. A three-axis accelerometer and gyro was fitted to the WITT to determine the displacement amplitude and VIV frequencies.
Te extent of VIV present in the physical tests is very encouraging, the feedback of the WITT drivetrain and relatively short pipe section still results in an easily driven system. Te VIV prediction techniques would benefit from further investigation to improve their fidelity.
CONCLUSIONS
Te simulations, using computational fluid dynamics for a wide range of conditions and embodiments, were validated by tank testing and prove that the subsea WITT energy harvester using VIV is a feasible technology. Te solution has the following advantages: current direction independent; modular; scalable; tuneable; sealed generator; no external moving parts; and no shock loads Te scalability and modularity of the
subsea WITT allows for cost-effective tailoring for a site’s individual metocean characteristics and power requirements. A wide range of applications is possible, from powering subsea monitoring equipment through to reduction of riser fatigue by reducing VIV. l
Martin Wickett is CTO of WITT.
www.witt-energy.com
www.engineerlive.com 51
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