Surveying the SA Agulhas II - EE Publishers

0 downloads 0 Views 1MB Size Report
May 15, 2017 - The idea is that a hard hit with a sledge hammer would cause the propeller blade to vibrate for a while at frequencies, which are called its ...
Surveying the SA Agulhas II - EE Publishers

Switch To Mobile Version

Home

Magazines Events Production Services Subscriptions Advertising About Contact

EE Publishers

EngineerIT

EE Publishers Home

Energize

Vector

PositionIT

Search

Press Office

Surveying the SA Agulhas II

Surveying the SA Agulhas II May 15th, 2017, Published in Articles: EE Publishers, Articles: PositionIT

by Brendon Nickerson and Anriëtte Bekker, Stellenbosch University

  The Sound and Vibration Research Group from Stellenbosch University has been involved with research on the SA Agulhas II since 2012. The hull and propeller geometry of the ship was surveyed during routine ship maintenance in the dry dock at Simon’s Town Naval Base. This digitised geometry of the ship enables several engineering research studies. The SA Agulhas II is a state-of the-art polar supply and research vessel, owned by the Department of Environmental Affairs of South Africa (DEA). It is responsible for the re-supply of cargo and fuel to South African research bases in Antarctica, Gough Island and Marion Island. Each voyage is manned by about 50 crew members and 100 scientists who rely on this valuable platform to enable multi-disciplinary research in the Southern Ocean.

Most Popular 24 hours

This Week

This Month

Tags

Principles and testing methods of earth ground resistance The new electrical Certificate of Compliance Earth loop impedance and prospective short circuit (PSC) testing methods Mapping distribution of SA solar resources Calculate your own hourly charge-out rate Principles and testing methods of earth ground resistance Earth loop impedance and prospective short circuit (PSC) testing methods What you should know about the Unemployment Insurance Fund The effect of HIV/AIDS on your business The science, engineering and law of nuclear energy in South Africa Principles and testing methods of earth ground resistance Earth loop impedance and prospective short circuit

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]

Surveying the SA Agulhas II - EE Publishers

(PSC) testing methods What you should know about the Unemployment Insurance Fund Fig. 1: The SA Agulhas II in the dry dock at Simon’s Town Naval Base.

Biomass power projects in South Africa The effect of HIV/AIDS on your business

Aside from her logistic functions, the SA Agulhas II is an interesting research topic herself. Being a polar research vessel, she works in some of harshest environmental conditions on earth, including up to 12 m waves in the roaring forties and navigation through 2,5 m thick Antarctic sea ice. It is difficult to believe that a ship can sail in such tough and remote conditions based on engineering assumptions extrapolated from computer simulations and tests on scaled-down ships in ice and wave tanks. As the world of naval architecture is not thoroughly familiar with the conditions that polar vessels face, nor the true responses of ship designs to these conditions, an international research consortium has been performing full-scale measurements on the SA Agulhas II since her commissioning in 2012. The consortium comprises the DEA, Stellenbosch University, Aalto University, and the Finnish Meteorological Institute. The vessel is scientifically instrumented for measurements, which include operational parameters, shaft-line strain and vibration, and hull loading (ice-loads and flexure). Scientists and crew have also observed wave height and ice thickness to capture her operational environment. This network of engineering sensors could be likened to a nervous system that monitors critical “vital signs” to indicate the structural health of the ship. Monitoring data is valuable to improve the scientific basis for the design of ice-going ships.

ABB Application News Automation & Control news Automation News Comtest construction Data ECA

electricity Electronics news Endress+Hauser Energize energy

engineerIT Eskom Esri features Generation news GIS GIS & Data Processing news ICT news Industry news Installations & Contracting news Installations News Instrotech Lighting news Mapping & Data Visualisation News Measurement &

Instrumentation news measurement news Mechanical Technology news NewElec Osram Phoenix Contact PositionIT Power protection

Schneider Electric Siemens software South Africa Surgetek Surveying & Data Acquisition news Switchgear & Drives News vector

Views

Fig. 2: (a) A photograph and (b) a 3D scan of the stern of the vessel from the dry dock pit.

BROWSER-BASED E-ZINES The SA Agulhas II was docked at the Simon’s Town Naval Base in early March this year. This massive effort was managed by African Marine Solutions (AMSOL) under the coordination of Robert Hales. After five years of hard work, the ship needed a new coat of paint, buffing of her propellers and replacement of the anodes that keep her from rusting. This is also the opportunity to check the hull for damage and service rotating equipment, such as the propellers, bow and stern thrusters. Dry-docking of the vessel provided the opportunity to directly capture and digitise the hull and propeller geometry. The “reverse engineering” of the ship seemed a challenging task, until the Sound and Vibration Research Group (SVRG) at Stellenbosch University were placed in contact with Brent Godfrey from Optron and Ian Thomson from Cape Survey.

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]

Surveying the SA Agulhas II - EE Publishers

Surveying of ship geometry Thomson and Godfrey captured the geometry of the SA Agulhas II by using two Trimble TX8 Static Laser Scanners. They also used a Cannon SLR camera for the colourisation of the scanned point clouds. The scanners sample at a rate of 1 MHz, covering a 360° x 317° field, only a 43° cone around the supporting tripod is not captured. Fig. 2 presents a photographic image with the corresponding point cloud from a station at the stern.

BACK ISSUES Fig. 3: Ian Thomson is using a Trimble TX8 laser scanner at the bow of the ship from a station at the top of the dry dock basin.

The scanners were placed progressively along the port and starboard flanks of the vessel. The exact locations of the scanners were not important, as long as the scan data from a particular station overlapped with the one before it. Space around the ship was tight in the dry dock and the team had to ensure the incidence angle of the scanner on the geometry was not too large. As the scanner relies on line of sight, areas with more complex geometry were scanned multiple times from various angles. Once scanning along the side of the hull was complete, further scans were conducted from higher vantage points. These included scans taken from the edge of the dry dock (Fig. 3), to scans taken on-board the ship in various locations. The geometry of the propeller blades was also captured in high detail for the purposes of structural dynamic analysis on the blades. The gale-force South Easterly wind interrupted sandblasting operations on the hull, which allowed the team to complete the surveying in 57 scans of the expansive structure in only three hours. Registration and editing of surveyed data Post processing of the 3D point cloud and photographic information is performed in Trimble RealWorks v10.3. The individual scans are combined through a process known as plane-based registration, where common geometric features are used to overlap scans from different stations.

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]

Surveying the SA Agulhas II - EE Publishers

SERVICES

Fig. 4: The registered point cloud of the SA Agulhas II with a colourised photographic overlay.

The scans include both the geometry of the vessel as well as the surrounding environment. Fig. 4 shows a representation of the registered point cloud, with colourised overlay for the entire vessel. The colourisation clearly shows the sections of the ship where sandblasting and painting of the under-coat had commenced. The shadows of the dock-side cranes are also visible. Next, the ship geometry is isolated by cleaning away the surrounding environment using the RealWorks editing tools. The edited point cloud can then be meshed to create a surface geometry of the desired sections of the ship. Vibration testing of the propeller blades and shaft line Propeller-related damage is the cause of a high percentage of maritime accidents during ice navigation. Ice can cause damage to propellers through single or sequential impacts. When ice impacts a propeller, the blade will deflect which may cause permanent bending of the propeller or alternatively, the shaft rotation might slow down as a result of excessive ice resistance. This could cause damage to the blade and other propulsion elements. In order to investigate these complex ice loading scenarios, it is important to ensure that engineering models are as accurate as possible to begin with. To this end the SVRG investigated the flexural response of the blades in the dry dock. The idea is that a hard hit with a sledge hammer would cause the propeller blade to vibrate for a while at frequencies, which are called its natural frequencies. If a similarly supported computer model of a blade is exposed to an impact force, and the model responds with the same natural frequencies, this model is validated against the full-scale structure.  

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]

Surveying the SA Agulhas II - EE Publishers

Fig. 5: (a) Accelerometer sensors (indicated by dashed circles) are placed on the propeller blades to measure the vibration. (b) Clinton Saunders excites one of the propeller blades by impacts with a sledge hammer. (c) A photograph of the starboard ship propeller, and (d) the edited point cloud after scanning.

Fig. 5a shows that accelerometers were stuck to a propeller blade and the deformation of the structure was initiated by providing a force excitation through repeated hits with a sledge hammer as shown in Fig. 5b. The big advantage is that the structural aspects of the model could be validated through testing in the dry dock without having to model the hydrodynamic effects of water. With the propellers of the vessel exposed (Fig. 5c), their geometry could be captured through laser scanning. Fig. 5d shows a point cloud of the starboard-side propeller, which has been edited in Trimble Realworks. This geometry will assist to create computer simulation models for the investigation of icepropeller interactions in future research. Future research Structural fatigue models The SVRG at Stellenbosch University specifically studies how the motion of the ship influences the passengers on board and if the global flexure of the structure is likely to influence the vessel’s longevity. The ship bends and flexes as a result of extreme loadings from her operational environment. To use an analogy, one could imagine holding the ends of a ruler and repeatedly bending and twisting it. The ruler may survive many cycles of moderate bending or snap immediately if bent too far. In the same way, the SA Agulhas II is accruing bending and twisting loads during its operational lifecycle. The surveyed geometry will be used in conjunction with structural drawings of the vessel to create an updated structural model, thereby enabling detection of the local areas of the ship structure where damage is likely. Fig. 6a, c and e respectively show the first three bending frequencies of the SA Agulhas II as measured whilst she was moored in Cape Town Harbour. Figure 6b, d and f presents engineering estimates from a state-of-the-art finite element model by STX Finland during the ship design phase. Scale models A fully instrumented model-scale ship of the SA Agulhas II will be built to investigate ship responses in a controlled laboratory environment. Such a model requires that the submerged hull of the vessel should be geometrically accurate. The surveyed point cloud of the SA Agulhas II will be used to generate surfaces and machine paths for the manufacturing of such a scale model. The performance of the ice-going hull design of the SA Agulhas II can be evaluated in challenging wave conditions in the Stellenbosch University towing tank and the newly built ice tank facility at Aalto University. Engineering measurements from the scale model will be correlated to those measured on the full-scale ship to evaluate if model tests do indeed provide valid engineering guidance to naval architects.

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]

Surveying the SA Agulhas II - EE Publishers

It is particularly challenging to estimate the fuel required to operate ships in ice. Fuel consumption is directly related to global hydrodynamic resistance forces. Stellenbosch University will investigate a novel operational modal analysis algorithm through which global resistance can be directly measured from the vibration and flexure of the ship. This algorithm will be evaluated in a towing tank environment, where the drag force on the scale model can be evaluated through the towing force sensor.

Fig. 6: The global flexure of the SA Agulhas II as shown by a measurement model from a PhD study by Keith Soal (figures (a), (c) and (f) and a finite element model by STX Finland (figures (b), (d) and (e)).

Prediction of wave state from ship motions Butteur Ntamba Ntamba, from the Cape Peninsula University of Technology is working with Prof. Alex Härting from Jade University of Applied Sciences on a novel algorithm and motion measurement device to estimate wave conditions from vessel motion. The point cloud of the SA Agulhas II hull will be refined to a surface from which sectional contours will be extracted and used to cumulatively sum the contribution of wave forces that are likely to result in the measured ship motion. Next, an inverse problem is solved to estimate a 360° wave spectrum relative to the vessel orientation. The device is mounted inboard and is not exposed to hazardous wave impacts at the bow and will offer significant cost advantages over expensive wave radar. Conclusion The time-efficient surveying and registration solutions provided by the Trimble TX8 laser scanner and Realworks software has successfully resulted in the reverse engineering of the hull and propeller geometry of the SA Agulhas II. The acquisition of this geometry enables further research on the propeller and hull flexure, computerised simulation models and scale models, and environmental conditions which will benefit the scientific basis for the design of ice-going ships. Acknowledgements Brent Godfrey from Optron and Ian Thomson from Cape Survey are acknowledged for scanning the geometry of the SA Agulhas II. The on-site efforts of Hannes Swart and Clinton Saunders are appreciated. The National Research Foundation and Department of Science and Technology have funded this research through the National Antarctic Programme. The assistance of the DEA and AMSOL vessel management services crew have been instrumental to ship measurements and to facilitate work on the SA Agulhas II.

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]

Surveying the SA Agulhas II - EE Publishers

Contact Brent Godfrey, Optron, Tel 021 421-0555, [email protected]

Related Tags

Aalto University

Brent Godfrey

hull and propeller geometry

SA Agulhas II

digitised geometry

Ian Thomson

Simon's Town Naval Base

static laser scanners

Stellenbosch University

Surveying & Data Acquisition Technical

PositionIT

Finnish Meteorological Institute

routine ship maintenance

Sound and Vibration Research Group

survey

Trimble TX8

© Copyright 2017 - EE Publishers (Pty) Ltd. All rights reserved. Privacy policy Terms of Use Terms of sale Contact

Switch To Mobile Version

http://www.ee.co.za/article/surveying-sa-agulhas-ii.html[2017/05/19 08:43:13]