Feature 3 | RO-RO FERRY REPORT Figure 12: Turning in 0.17m level ice.


relative to the ship’s centreline, and the propeller blade pitch angles for both units. A system of rosette (three element 45degs), uni-axial strain and shear gauges were installed by LR at critical locations on the hull structure at the nominal locations of the ship. Te measurements embraced both the ship’s general structure and also a limited distribution on the pod unit’s foundation structure. Propulsor observations were achieved


through the use of LR’s borescope observation capability. A small number of tapped holes were inserted in the hull at predetermined locations to enable observations of the propeller action to be made during normal open water and ice operation. These observations were synchronised with the other measurements described above.


Hull structural measurements The hull structural measurement system was one of the largest projects undertaken on an ice class ship and involved a high level of complexity and utilised some of the latest measurement technologies available. The measurement system needed to have a clearly defined purpose to provide useful data and the following measurements were identified:


• Ice loads acting on the hull structure


• Local ice loads due to icebreaking in the forward region


• Corresponding midship loads • Corresponding aſt loads


longitudinals and primary girders located in the fore/aſt directions around the azimuth thruster were fitted with strain gauges. For all of the frames that were to be


instrumented, shear gauges were applied to each end of the frames and on both sides of the frame directionally opposing each other; that is, four gauges per frame. Te gauges in the ballast tank followed a similar application. However, the gauges and cables had additional water protection measures against ballast water ingress. In addition, a number of frames were


• Local ice loads in way of shoulder while turning and proceeding ahead


• Ice loads acting on the propulsion and transmitted to the hull structure.


To identify the critical locations a finite


element model of the aſt end of the ship was created with ice loads applied through level ice interaction along the waterline (Figure 1), ice impact on the forward bow region and ice impact on the azimuth thruster. As a result of the analysis and considering


the purpose of the measurement system, the critical locations were identified. Tis resulted in a measurement system which sub-divided the aſt port side into a system with gauges applied to the frames and adjacent plate panels. In addition, the port side primary


52


fitted with an additional two shear gauges located at either end of the frame on the same side. This was to investigate the influence of additional ice loads acting on the frames. Traditional ice load measurements have adopted the installation of two shear gauges; however, this method may include other loads such as torsional and bending components. By installing four gauges per frame these loads are removed. Te difference between these two readings will give an insight into any additional ice load moments acting on the frames. Due to the machinery and structural


arrangement in the midships region, the same frame could not be used for shell plating and bottom plating locations in the void space and were subsequently offset. In the forward region, the gauges were installed on a section of two frames in the forward azimuth space on the port side. Te frames and sections corresponded to similar ones instrumented in the aſt azimuth space. Rosette strain gauges were installed at


the mid span/spacing of plating adjacent to the frames on the forward side (except midships, as detailed above), while six uni-axial strain gauges were installed on primary longitudinal members on the port side of the aſt azimuth space. All the strain gauges were temperature compensated.


Figure 13: Ice block just before impact with the propeller. A strong cavitating tip vortex was also observed, indicating a bollard- push loading condition.


Machinery measurements Both the stern and forward azimuth thrusters had a series of measurement equipment fitted. Te thruster angle and propeller blade


pitch angle was obtained through the Rolls-Royce central processing unit in the azimuth space. The shaft rpm and torque measurement system were fitted to the intermediate shaſt in the azimuth machinery space. Te rpm recording system used an infrared pickup, while the torque was recorded using a twin chevron torque strain gauge arrangement via telemetry and demodulator. The pitch control and steering gear pressures were connected, via the Rolls-Royce azimuth system, to the test point connectors on the thruster unit terminal blocks.


General measurements and visual observations Te ship speed, position and wind speed were recorded from the Sam Electronics navigation system installed on the bridge. Acquisition of external bridge wing camera data for ice interaction observations was obtained from the central ships computer system once the trials were concluded. A centralised control station was set up


on the bridge. Te main computer and fibre optic systems were located below the bridge floor, with the monitor, keyboard, mouse, internet cable connections and a CD drive connected for periods to back up data on the workstation. In addition, a series of external hard drives was installed along with an internet connection to remotely provide a summary of the data acquisition status while the ship was in service post ice trials.


The Naval Architect April 2011


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