propulsion Tailoring thrusters to the task
Thruster manufacturers continue to refine their designs for an evolving offshore market, with some now offering condition monitoring systems to ensure sustained performance and reliability
H
mall changes in thruster location or orientation can, in some cases, produce a more effective manoeuvring force for the power used, with beneficial effects on fuel consumption and emissions, Rolls-Royce reports. This field of research – partly conducted through its University Technology Centres (UTCs) – is important for the group since it is relevant to offshore vessels and rigs as well as wind turbine transport and installation craft. The move to dynamically-positioned tonnage for offshore oil and gas project support favours a good understanding of factors influencing the positioning of thrusters. Such vessels may spend days at a time in dynamic positioning (DP) mode so the difference between optimum and sub- optimum thruster orientation can be a significant increase in fuel costs and higher emissions. For large vessels with multiple azimuthing thrusters fore and aft, Rolls-Royce advises, close study is necessary at the design stage of the location of the individual thrusters in plan view. One aim is to secure the greatest positioning thrust, including the case of designated failures. Another is to minimise the forbidden arcs, where slipstream from one thruster interferes negatively with another unit.
S L D α S Positioning thrusters to achieve the most effective force for the power used
rigs, mainly by model testing. Work in the Rolls-Royce UTCs combined with improved computational fluid dynamics methods allows proposed installations to be analysed at the design stage with considerable accuracy, and verified where necessary by model tests. Among the factors are the propeller-to- hull distance and the distance of the propeller from the hull edge, and the curvature in the bilge area, shown here diagrammatically. These become particularly important for tonnage with unconventional hull forms or barge-like forms, such as wind turbine installation vessels. The Coanda effect occurs in many areas of
It is also important to determine how far below the bottom of the hull the thruster propeller centre should be, and the tilt of the thruster or its nozzle.
Angling the thruster propeller slipstream downwards a few degrees is often beneficial; one way is to tilt the nozzle with respect to the propeller. In the case of anchor handlers with a swing-up azimuth thruster under the bow, Rolls- Royce practice is to swing the thruster down to a little less than 90 degrees.
The jet from the nozzle is thereby directed slightly downwards, avoiding impingement on the hull or interference with the main propellers and so increasing the straight-ahead bollard pull when the azimuth thruster is used to supplement the main propeller thrust. For most vessels having azimuth thrusters beneath the hull, the Coanda effect has to be taken into account; this was first studied many years ago with respect
to semi-submersible
www.osjonline.com
fluid flow, and is the tendency of fluid to follow a curved surface. The classic simple demonstration, Rolls-Royce explains, is to dangle the back of a spoon under a domestic tap when, rather counter-intuitively, the spoon is pulled into the water flow instead of being pushed away. In terms of vessel hulls under consideration here, this shows up as a tendency for flow from a thruster directed across the bottom of the vessel to curl up around the bilge, reducing the effective thrust (or station-keeping forces). Recent Rolls-Royce UTC work has included investigating various factors influencing the location of azimuth thrusters under a hull. Thrust losses are sensitive to the clearance between the nozzle and the hull and to the bilge radius. Increasing the vertical distance from the hull to the propeller axis (L) relative to propeller diameter (D) reduces the loss, as does reducing the bilge radius for a given L/D ratio. As usual with propellers, the depth of immersion is important.
Normally, propeller diameter is maximised to give the greatest amount of thrust but studies show that, in some cases, there is an advantage in reducing the propeller diameter if that means the distance can be increased. For example, for a particular vessel, cutting the azimuth thruster propeller diameter from 3.5m to 3.2m reduced the open water thrust by around five per cent; the corresponding increase in L/D, however, compensated for this loss, leading to an overall positive effect
since the propeller disc became more immersed and therefore less susceptible to aeration (ventilation). Understanding these and other
complex
interactions between propulsors and hull enables a better service to be offered by Rolls-Royce, which emphasises the importance of optimising the detailed location of azimuth thrusters at an early stage in vessel design, especially those with unconventional hull proportions. Voith Turbo claims to be the only producer of thrusters with ratings of 1,500kW based on permanent magnet
synchronous motor
technology. The principle of a thruster with a permanently-excited electric drive motor housed in the hollow shaft and with no maintenance requirements (thanks to seawater-lubricated bearings)
is increasingly appreciated, the
German designer reports. Since the start of series-production in 2008, the company has sold more than 55 units. Voith Inline Thrusters (VIT) and Voith Inline Propulsors (VIP) feature rim drives combining electrical, mechanical and hydrodynamic elements
Offshore Support Journal I June 2012 I 63 R
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200