ASV operability at sea: Size matters as much as hull form design
The influence of vessel size on the performance of a second generation Autonomous Surface Vessel (ASV) in irregular head waves is investigated. The hull of the SWATH type ASV is optimized using a combination of high fidelity hydrodynamic numerical models to achieve minimal propulsion powering requirement as well as lower motions in waves compared to state of the art catamaran type (CAT) surface vessels currently referred to as first generation. Autonomous Underwater Vehicles (AUV) assistance at sea requires the ASV to possess exceptional seakeeping qualities which hull optimization procedures aim to achieve. However, the performance of the ASV may be sensitive to the most probable wave period of the ambient environment due to the asymptotic peaks typical of SWATH vessel responses in long waves. It is the objective of this study to assess the seakeeping behavior of five different sizes of the ASV at sea to determine the limiting sea state for each vessel size. Parameters such as the significant heave and pitch motions are used to examine the operability of the vessels in five sea states. The results highlight the importance of selecting appropriate vessel size based on the predominant weather conditions of the location.
Effects of Appendages and Small Currents on the Hydrodynamic Heave Damping of TLP Columns
A previous paper by the authors reported on the estimation of resonant heave (springing) damping of tension leg platforms (TLPs) and a method of scaling for full scale prototypes. The present paper is a sequel to this work, and examines the effect of adding an appendage in the form of a disk to TLP columns, and the influence of a small uniform current. Model tests conducted on a cylinder+disk in heave show that the heave damping induced by the disk is linear with the amplitude of oscillation. The disk is found to increase the form drag coefficient two-fold, in accordance with published results based on isolated edge theory. The effects of a small uniform current were also examined during the model tests. Results show an increase in heave damping ratio that is linear with the current velocity. In the presence of a disk, the damping induced by the current is doubled as well. Scaling laws are proposed in this paper that enable extrapolation of heave damping due to appendages and small currents to full scale. An example calculation shows that for a full scale TLP column the heavy damping is increased by about 300% due to addition of the disk,more » and by 87% due to a small current. The combination of the disk and the current increases the heave damping of the column by a factor of six.« less
Ocean thermal energy conversion: The promise of a clean future
Due to the world's heavy dependence on fossil fuels for electricity, pollution and global warming is on the rise. However, numerous countries are still relying on diesel generators as their main source of energy. There is a lack of practical alternative energy source that can meet the global energy demand without posing any threat to the natural environment. Ocean Thermal Energy Conversion (OTEC) is a concept that has the potential to address this growing issue. It is basically a mechanism that exploits the temperature difference between warm surface seawater and cold deep ocean water, to produce electricity. Although OTEC has a low energy density, the thermal energy in the ocean is vastly abundant. OTEC development has been dormant for a long time since it was first proposed in 1881. However, it has now regained recognition worldwide as a realistic solution to our world energy issue. Instead of having a great potential for power generation, it also carries the ability to produce high value products from the large volume of Deep Sea Water (DSW) that can be released as byproducts of OTEC plant operation. These products can hold a very profitable place in industries such as pharmaceutical, food, cosmetics and mineral water production. Further research on ocean current and DSW properties can someday lead to the commercial used of OTEC. This paper is a review of the basic concept, present status and future prospects of OTEC around the world.