Modelling And Simulation Of An Offshore Hydraulic Crane

This paper presents a modeling approach based on Bond Graph (BG) method for offshore hydraulic crane focusing on its hydraulic system characteristics. A hydraulic library is built in the modeling software tool 20-sim using BG elements. The hydraulic submodels are designed according to one specific type of offshore crane, however, they can be easily modified and reused for other similar systems. BG method is a modelling technique for modeling of complex system by describing the energy flow inside the physical system. One of the main benefits of modeling using BG for the hydraulic system is the model provide interfaces to systems of other domains, for example, cooling system, mechanical model, control unit, etc. In this paper it is shown how an integrated BG model of the hydraulic system for a knuckle boom crane is derived and used for simulation. The simulation results proved the validation and effectiveness of the presented modeling approach for simulation of multi-domain systems. INTRODUCTION Cranes are found onboard almost all kinds of vessels and platforms for handling personnel and cargo. Cranes onboard vessels and platforms handling goods between the quayside and vessel or between vessels are normally referred to as offshore cranes. Cranes that are used for handling submerged loads as well e.g. launch and recovery of submersibles or installation of subsea hardware, are normally referred to as subsea cranes. Compare to land based cranes with a solid fixed base, offshore and subsea cranes are subject to significant dynamic forces from the resulting payload sway directly or indirectly caused by the vessel motion. As field testing in offshore industry is expensive and time consuming to carry out and constrained by many factors such as weather condition and vessel availability, modeling and simulation become a crucial part for product design, testing and analysis. On one hand, offshore cranes are mostly hydraulic actuated due to the consideration for stable performance and safety redundancy. On the other hand, it is rather delicate to model and control hydraulic systems because of the complex dynamic behavior and nonlinear aspect of fluid energy transfer. Many studies on hydraulic system modeling dedicated to one or several specific components. There are many software tools available for modelling and simulation of hydraulic systems. Modelling tools used in former researches include SimHydraulic from MathWorks (Vĕchet and Krejsa 2009), Easy5 from MSC (Li et al. 2011), SimulationX from ITI (En et al. 2013), 20-sim from Controllab (Aridhi et al. 2013), etc. These programs provide standard libraries for hydraulic components which can be parameterized and modified to certain levels. The generalized models are not designed for a specific system which means they might be over-complicated thus compromise the simulation efficiency. It is possible, to a certain level, to create new specific models for components that are not included in these libraries from these software tools, but that’s not always the best way. Take 20-sim as example, a hydraulic library is developed according to the Modelica hydraulic library. The library doesn’t include all the valves in a crane system. Instead of using BG elements, the models are written in a way which is difficult for the users to understand and edit. In this paper we present a modeling approach for offshore hydraulic crane system based on BG method. The submodels are created from scratch using basic BG elements and are completely open for editing as detailed as necessary depending on the simulation purpose. Another reason of choosing 20-sim as the modelling tool is using BG method complex systems, e.g. an offshore hydraulic crane, involving multiple energy domains can be modelled and integrated. The rest of the paper starts with introducing the basics of the BG method and the hydraulic system of the Proceedings 28th European Conference on Modelling and Simulation ©ECMS Flaminio Squazzoni, Fabio Baronio, Claudia Archetti, Marco Castellani (Editors) ISBN: 978-0-9564944-8-1 / ISBN: 978-0-9564944-9-8 (CD) kunckle boom crane. Then, the modelling of the main components using BG is described and the results from the simulation of the model are presented. Finally, the conclusion and future work is discussed. BOND GRAPH METHOD BG method as a general approach for modeling interacting systems is based on identifying the energetic structure in a system. A system can be decomposed into a few basic physical properties depending on what is going to be studied, and then the system can be described by interrelated idealized elements. The energy or power interaction between two elements is called a “power bond” represented by a half arrow. Another type of bond called “signal bond” represented by a full arrow indicates a signal flow at negligible power. A power bond is defined by two variables with generalized names of “effort” and “flow”, of which the product is power. Table 1 lists a number of energy domains and their corresponding power variables. Table 1 Common used BG energy domains Energy Domain Effort (e) Flow (f) Name Unit Name Unit Mechanical translation Force N Linear velocity m/s Mechanical rotation Torque Nm Angular velocity rad/s Electrical Voltage V Current A Hydraulic Pressure Pa Volume flow m/s Thermal Temperature K Entropy flow W/°C Magnetic Magnetomotive force A Flux rate Wb/s Chemical Chemical potential J/mol Reaction rate Mol/s Roughly speaking, the basic elements account for energy supply based on supply of effort and flow (Se-element and Sf-element), potential and kinetic energy storage (Celement, I-element), energy dissipation (R-element) and energy transform (TF-element) or conversion (GYelement). In addition to the basic elements describing the boundary components, the interconnection in between two elements is described using an ideal 1junction or 0-junction element, which neither store nor dissipate the energy. In brief, a 1-junction has equal flow on all bonds adjoining and the sum of efforts equals to zero, while a 0-junction is just the opposite: the effort is the same and the sum of flow is zero. The essence of defining an element is to establish the relation of the energy variables. Below Figure 1 so-called tetrahedron of state, illustrates the basic 1-port elements relating the energy variables (Pedersen and Engja 2008). Figure 1: Tetrahedron of state for basic 1-port elements OFFSHORE CRANE HYDRAULIC SYSTEM The hydraulic system of a common offshore knuckle boom crane is studied in this paper. The crane consists of three joints actuated by a hydraulic motor and two hydraulic cylinders (Figure 2). Figure 2: Offshore hydraulic knuckle boom crane When considering the complexity of the model, it is vital that the simulation can be done in real time. Thus the hydraulic system schematic is simplified to include only the main components at a level corresponding to the characteristics that shall be studied (Figure 2). The main components of the crane hydraulic system include a Hydraulic Power Unit (HPU), pipelines, valves (compensator, 4/3proportional direction valve, load control valve), cylinders, and motors. Figure 3: Hydraulic system schematic BOND GRAPH MODELING OF CRANE HYDRAULIC SYSTEM After identified the main components of the hydraulic system, in this chapter modeling of these components using BG elements is described. The hydraulic submodels are created based on the basic principles of fluid dynamics (ASSOFLUID 2007). To reduce the complexity of the overall model, the model of each component is also simplified. Fluid inertia and flexibility are dominant in the pipeline and cylinder chambers, thus neglected in the other components. As mentioned, BG method is modelling approach by describing the energy flow of the system. In the hydraulic domain, the key principle is to establish the connection of pressure and flow through the system. HPU (pump) The HPU of the crane mainly consist of a pressure compensated pump, which maintains a preset pressure at its outlet by adjusting its delivery flow in accordance with the pressure at any given time. If the system pressure is less than the pressure set point, the pump outputs its flow proportional to the pressure deviation. In the BG method a pump is modelled as a flow source element (Sf-element). The Sf-element has one output power port associated with the pump outlet. The effort and flow relationship is given by the following equations: