Tanking Control. C Gates BMT Design & Technology, Melbourne, Vic, Australia

Transcription

1 Tanking Control C Gates BMT Design & Technology, Melbourne, Vic, Australia cgates@bmtdt.com.au ABSTRACT The Royal Australian Navy tanker HMAS Success has recently undergone modifications to achieve International Maritime Organisation compliance. The complex modifications involved the installation of a second hull, internal to the ship, to reduce the risk of cargo spillage in the event of a serious incident. New control stations were introduced, and existing stations upgraded, to provide Human-Machine-Interfaces for the new system. The control stations interfaced with existing systems as well as the new systems, many in hazardous areas. The engineering design process for the new control system is discussed, including the key challenges encountered. Subsystems include tank level monitoring sensors, PLC systems, touch input displays, pump motor control, ventilation systems, communication systems and alarm and monitoring devices. Challenges faced were time constraints, interfacing an Ethernet communications protocol to a PLC controller, and dealing with obsolete equipment and hazardous areas. Conversion works to HMAS Success were undertaken, from December 2010 to April 2011, by Singapore Technology Marine (ST Marine). The designer was actively involved in set-towork activities for the control system during the conversion phase. In late May 2011, the ship returned to Australia where minor additions were incorporated into the control system to meet emergent requirements. INTRODUCTION Background The design task for the new ballast control system on HMAS SUCCESS was commenced well after the rest of the double hulling project, with an inevitable tight schedule. It was performed by an engineer who had just commenced working for BMT due to the unexpected loss of critical personnel, and the task had been inadequately scoped. Consequently, the design engineer was confronted with some of the key reasons for project failure. Purpose The purpose of this document is to summarise the approach used to deal with the perceived project risks and to provide details of the technologies and main components used in the control system. DESIGN TASK HMAS SUCCESS is a Durance Class replenishment oiler that (previously) did not comply with IMO double hull requirements. In addition to structural and mechanical changes (to provide a double hull) there was a need to provide a ballast control system (for new ballast pumps and pipe work) and a monitoring system for tank level indication.

2 The new control and monitoring system needed to: 1. Be compatible with ship operations and maintenance routines; 2. Provide local (pump room) and remote (control room) control of two new ballast pumps; 3. Provide operators with indication of the: a. tank levels, temperatures and pressures, b. position of ballast system butterfly valves, c. position of cargo system overflow valves, and d. pressure of ballast system transducers. Control functionality included limiting operation of the two ballast pumps to particular valve alignment arrangements, automatic shut-down of pumps in the event of certain conditions, and the provision of key-locked system overrides. Additional requirements included actuation of overflow valves dependent on control switch position, visual and audible alarms and communications between control and pump rooms. A picture of one the newly installed ballast pump motors is shown in Figure 1. Figure 1 - Installed Ballast Pump Motor Some of the equipment was located in hazardous areas, requiring special treatment. DESIGN APPROACH The basic design approach relied on improving the planning and organising phase of the task to facilitate goal achievement (see Ref. 1.). The task work-flow proceeded from the work breakdown structure by identifying and obtaining all identified prerequisites for the individual design activities. A buffer of work was created from the individual activities with met prerequisites, and these activities were completed in turn. When an impediment was encountered with one activity, another task was chosen from the buffer allowing work to continue while the impediment was resolved. There was a strong focus on key design issues and the design task was tailored to do only what needed to be done. The approach intended to limit the impacts (from the perceived risks) on project cost, schedule and quality. The design process commenced with an analysis of customer needs to establish what the system had to do (functional requirements) and how well it had to do it (performance requirements). These requirements were documented along with the identified method of verification, and traceability back to the customer's needs was recorded for each. Verification

3 of system requirements was by analysis, demonstration, test or inspection. The requirements were formally presented to the customer for review and acceptance. Next, the functional and performance requirements were assessed to define system functions and interfaces. A review of available technologies was conducted to create a list of feasible options that were available to meet these functions. The options were evaluated to determine the best solutions. Options were evaluated for: 1. Technical feasibility, in terms of people, processes and products; 2. Technical risk and the levels of maturity/uncertainty; 3. Prior acceptance within the Royal Australian Navy (standardisation); 4. Reliability (that the option would meet expectations, now and in the future); 5. Flexibility (that the option could still be used with changes in scope); 6. Quality; 7. Availability; and 8. Cost. The determined best solutions were combined to create the system design including subsystems, individual components and interconnections. The design process took into account system interfaces and applicable constraints. Documentation of the design followed and included a design description and installation details. Instruction details included functional overviews, arrangement, location and connection information as well as parts lists, label lists and data sheets. Additionally, a suite of test procedures was created for verification of component, subsystem and system performance. The design task was critical path (for the double hulling project) and, towards the end of the task, elements of the design were managed using methods normally employed for software development (Ref. 2.). These methods start by identifying outstanding activities (the backlog). Rather than using a traditional work breakdown structure, the backlog is used to decide on task allocation and project coordination. As work proceeds the backlog is continually assessed and tasking is assigned accordingly. Impediments are eliminated as a management function. Prior to commencing the task a search was conducted to identify (at a high level) the main reasons for project failure. Commonly, these were found to be lack of communications and information overload. Consequently, efforts were made to establish good communications with members of the customer's project office, suppliers and Navy regulators. Informal discussions were also had with ship system operators and maintainers. Additionally, (in an effort to reduce information overload) the design team concentrated on functional and performance requirements. Constraints were treated by exception unless they specifically related to the function or performance of the system. The design task provided an opportunity to improve document templates and mainline component lists for future tasks. It taught the importance of being flexible and the need to continually assess and reallocate work to meet project needs. COMPONENTS Choosing high quality, reliable components that are (technically) well supported and documented was a key part in successfully delivering the design task. The evaluation process considered mature and proven maritime solutions as priority criteria. Generally, these products are well supported by subject matter experts from within the supplier's organisation.

4 The value added by these subject matter experts assisted greatly in mitigating risks. Note: The use of Commercial-Off-The-Shelf (COTS) equipment had been approved (by the customer) as a method of reducing project costs. VEGA products were chosen for tank monitoring. VEGA Grieshaber KG is a world-leader in the supply of level, switching and pressure instrumentation and their measurement technology covers an extremely wide range of applications. VEGAPULS radar sensors were initially investigated for level measurement, but the manufacturer's recommendations for a smooth bore sounding tube would have been difficult to implement. Instead, VEGAWELL 52 pressure transmitters were chosen for the task. The VEGAWELL 52 transmitters met the operational requirements for tank level monitoring and have been proven to be robust and reliable in a maritime environment. They offer quick and precise monitoring characteristics and long-term stability. The VEGAWELL 52 transmitters were able to be retrofitted relatively easily into tanks and were connected to VEGASCAN 693 signal conditioning instruments in a multi-drop arrangement. The multi-drop arrangement utilises the HART communication protocol and was limited to five transmitters (instead of fifteen) per signal conditioning instrument, due to hazardous area limitations. Use of the HART communication protocol meant temperature information could also be captured from the transmitter's integral PT100 temperature sensor. VEGABOX 02 breather housings were used as the interface between VEGAWELL 52 pressure transmitters and VEGASCAN 693 signal conditioning units. VEGABAR 52 pressure transmitters were chosen for pressure monitoring. VEGABAR 52 pressure transmitters are well suited to monitoring gas pressures in product tanks. They provide precise monitoring and can withstand pressure shocks caused by rough seas. Like the VEGAWELL 52 transmitters, the VEGABAR 52 pressure transmitters were connected to VEGASCAN 693 signal conditioning instruments in a multi-drop arrangement, for pressure and temperature monitoring. The VEGASCAN 693 signal conditioning instruments also served as power supply units for VEGAWELL 52 and VEGABAR 52 transmitters and provided an Ethernet interface for connection to the logic controller. PACTware software is available for set up of VEGA products and provides a convenient method to adjust, test and record sensor parameters. VEGA products used are shown in Figure 2. Figure 2 - VEGA Products

5 A distributed Siemens SIMATIC S7-300 Programmable Logic Controller (PLC) was chosen for control of the system. Siemens automation systems provide a future-proof solution and are engineered to meet the needs of the maritime industry. The chosen controller arrangement provides an efficient, flexible and compact solution. Local and remote Control and Monitoring (C&M) is provided by an S7-315 Central Processing Unit (CPU) with digital and analogue ET 200M input/output (I/O) modules (for remote C&M) and ET200S I/O modules (for local C&M) in a decentralised arrangement. ET200 S I/O modules were used for local operation as they are certified for use in hazardous areas (Zone 2). The local (pump room) and remote (control room) modules are housed in steel control panels and interconnection is via a PROFIBUS field bus. The SIMATIC product range is shown in Figure 3. Figure 3 - SIMATIC Product Range The PLC is the core of the system and is connected to VEGASCAN 693 signal conditioning instruments, ballast system pressure transducers and butterfly valves, cargo system overflow valves, bar-graphs (for primary tank level indication) and ballast pump motor contactors. Generally, I/O signals are 24-volt DC and 4 to 20 milliamps with the exception of the VEGASCAN 693 instruments, which are via Industrial Ethernet. Seven VEGASCAN 693 instruments have been used and are connected to the PLC via a SCALANCE X-200 Industrial Ethernet Switch. The VEGASCAN 693 instruments and PLC use different communication protocols, and conversion from MODBUS (VEGASCAN) to PROFIBUS (PLC) is made via software in the PLC. Two SIMATIC MP 377 INOX 15" touch panels have been used for Human Machine Interfaces. One is installed in the pump room (for local C&M) and the other is installed in the control room (for remote C&M). The resolution of the MP 377 panel is 1024 x 768 pixels and it has a high-contrast thin film transistor (TFT) display and a stainless steel front. The MP 377 touch panel was chosen for its high ingress protection rating (IP66 installed) and suitability for hazardous areas (Zone 2). Primary level indication is provided by LED bar-graphs, which replaced obsolete units. HI- Q119 bar-graphs, from Precision Instrument Company, were chosen as replacements units. The bar-graphs incorporate programmable intelligent controllers and are rugged and reliable. The HI-Q119 bar-graphs were selected because they were available in a similar size to the obsolete units and for their compliance with MIL standards for shock, vibration and electromagnetic interference (EMI). The bar-graphs connect to 4 to 20 milliamp (analogue) outputs from the PLC. An installed HI-Q119 bar graph is shown in Figure 4.

6 Figure 4 - Installed HI-Q119 Bar Graph Schneider LC1D contactors are used for direct-on-line (DOL) starting of ballast pump motors. Start and stop signalling is enabled through touch panel inputs and PLC digital I/O. The motors are protected by GV3P motor circuit breakers and LRD thermal overloads that are designed for protection of motors used in explosive atmospheres. Figure 5 shows the motor switchgear during panel fabrication. Figure 5 - Schneider Motor Switchgear during Panel Fabrication The installed system worked well after some minor changes that were made during set-towork activities. BMT would like to thank the customer's project team for involving the designer in set-to-work activities. This improved the installer's understanding of the design and allowed for the required minor changes to be made quickly and simply. CONCLUSION The new ballast tank design task for HMAS SUCCESS, and associated project risks, has reinforced: 1. The need for a systematic design approach; and 2. The value of using high quality, reliable components that are supported by subject matter experts. The approach used for this design task has been summarised in this paper, along with an overview of the major components used.

7 REFERENCES 1. Ballard G B, 2000, The Last Planner System of Production Control, University of Birmingham 2. Schwaber K and Beedle M, 2002, Agile Software Development with Scrum, Prentice Hall