A robotic system for off-shore plants decommissioning

A ROBOTIC SYSTEM FOR OFF-SHORE PLANTS DECOMMISSIONING E. Cavallo, R. C. Michelini, R. M. Molfino PMAR Laboratory, Instrumental robot design Research group Dept. Machinery Mechanics and Design, University ofGenova via all'Opera Pia 15A - 16145 GENOVA Italy

The paper focuses on a robotic system conceived, designed, studied and built for cutting the legs of the off-shore platforms a few meters below the seabed. The work has been performed within the research project SBC Diamond Wire Cutting System Sub Bottom Cutter (GRDl 2000 25740) funded by the European Commission under the Fifth Research Programme and recently successfully closed. The proposed robotic system is an innovative prototypal machine able to anchorage on the seabed soil in front of the leg to be cut, to drill the soil at the given depth by limiting the volume of removed materials and to cut the sub-sea structure. Due to the complexity of the system and to the need of high operation reliability, the mechatronic modular approach has been adopted and the control modules have been distributed part on the sub-sea front-end and part on the support vessel remote-operated control stand. Simulation dynamic models of the robotic subsystems have been set-up and used for the knowledge base of the control system. Copyright © 2004 IFAC Keywords: Robotics, Interdisciplinary design, Marine systems, Ecology

1.

INTRODUCTION

In 1998, the OSP AR (Countries of the Convention for the Protection of the Marine Environment of the Northeast Atlantic) recognised that the 85% of the fixed oil and gas platforms in the North Sea should be removed completely. Now, decommissioning (see the Convention of Geneva, art. 5, and the "off-shore" regulations), requires: safe restoration of the marine habitat, non-interference of the removal activities with environment resources and resort to clean technologies with no risks for operators and third parties (LaBelle, 1999). The European Countries involved in offshore decommissioning (Zhiguo Gao, 1997) defined that proper re-establishment imposes removal depths of the sub-sea structures (jacket legs/piles, wellheads, etc.) down to 3-5 m below the seabed soil.

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The marine food industry profits of the on-duty conservativeness, and, later, of the re-establishment of natural equilibrium of the flora and fauna resources. The new robotic system is a prototypal machine, able to fulfil the recalled offshore decommissioning regulations, limiting (Twachtman, 1997) the underwater pollution. Moreover, the people working in contact or in close vicinity with the system and the marine resources are safeguarded to the highest level of health and safety standards, as dangerous duties are accomplished under remote control. The shearing task plays a key role in the dismantling of oil platforms and related structures, such as pipelines and loading terminals. The conventional cutting technologies, in use for sub-sea tasks, such as 'explosive' or 'high pressure water abrasive' systems,

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produce damages to the environment, also in consequence of the scattering at sea of dangerous materials and substances (Gerrard 1999). In this respect, the SBC robotic system is a significant improvement due to the new dig-and-saw concept, capable to avoid the effects of the suspension, by the combined actions of removing and convoying the sub-surface soil sediments, with negligible turbulence and without use of materials, resources and energy causing negative impacts on the environment. In fact a new dig approach that transfers on board the support ship all the seabed soil removed material and a saw approach based on an innovative diamond wire cutting technology have been adopted. The qualifying objectives and key advantages of the selected approaches in comparison with other usual technologies are: o the use of a clean technology, not interfering with the equilibrium of the marine habitat; o the absolute guarantee of the completion of the cutting task; o the conservativeness as for environment impact, with high overall efficiency and reliability of the technique for underwater use, with low energy consumption compared to the total power; o the automation process of the all, by intelligent remote control/supervision station on surface; o the integrated design of structural, power supply and underwater instrumental components (sensors, electro-hydraulic valves, data system etc ... ); o the unaltered overall efficiency of the removed structures and the characteristics of the materials involved in the cutting process, thus enhancing their life-span and allowing the re-use for the same or different work-scopes (absent in other cutting techniques like explosives and flame cutting, etc ... ).

2) new concept cutting machine different in design and characteristics from the ones available on the market, able to highly enhance all positive characteristics of the conventional diamond wires, mainly in resistance to axial loads, tolerance to the collapse of the cut materials, overall reliability, production and life-span; 3) new concept sub-sea work/deployment robotic platform, remotely operated, with its intelligent control/drive station on surface, stationed on board the vessellbarge supporting the sub-sea duty. The robotic system has been designed, based on the fundamental concept to simplify the component structures and interfaces to improve MTBF and intrinsic operational reliability. The interfaces have been standardised from mechanic and electronic point of view, to allow a fast installation of different and dedicated equipment fit for the specific sub-water cutting missions: the equipment have been thus designed with the ability to be interfaced to different deployment configurations by simple reconfiguration (Acaccia et alii, 1998). Figure 1 shows the robotic system architecture and its main modules.

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2.

THE ROBOTIC SYSTEM ARCHITECTURE

A top down approach is applied to share common design skeletal guideline and system architecture, to select effective sub-systems and components and to consider the interactions among them. To design a safe and reliable system, it is important to foresee every possible failures and to set contingencies for unplanned events that may happen during the mission, in order to set out rescue actions that avoid damages, having always under control the whole operation mission. The main operative modules of the proposed robotic system are: 1) new concept dredging system, based on the combined use of drilling heads and new generation turbines designed to avoid pollution effects in connection with the water turbulence generated by the dredging pumps, which produce the suspension of polluted sediments (Grant, Briggs, 2002) deposited on the upper sea-bed layer (hydrocarbons, heavy metals, micro-biological pollutants);

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Base

(r

Ct)

Cutting Assembly (twin pipes. trolleys. Pulley~ housing heads. wire)

Main Frame (base. tilt assembly. glides. support)

Excavation System (drilling & pumping mods)

Power & Control Station Power/Function Umbilical

Fig.l The robotic system CAD model. Competing modules and components alternatives for all the core innovative mechanisms have been considered, to reach full acknowledgement of technicalities supporting the final choice (Cavallo, et al.,.2004).

3.

MODELLING, SIMULATION AND VP ISSUES

The criticality of the operation surroundings and the deeply innovative requests of the prospected robotic system needed careful concern at the early project phases where modifications are cheaper and quicker.

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Indeed, cost oriented design methodologies, machine functional and operational characteristics, hostile marine environment, rescue missions in case of derangement and easy maintenance and disassembly for recycling operations have been duly taken into account, to develop highly life-cycle effective products. In this case, deep knowledge of physical environment phenomena was needed. Knowledgebased solutions have been devised and applied for innovative automation, with balanced integration of mechanisms, actuation, sensorisation, control remotemanipulation and supervision. Functional models for the innovative mechanisms have been implemented, taking into account the hazardous and severe operation environment, based on throughout deepened knowledge of the physical phenomena, to have a priori reliable idea of the realistic cinematic and dynamic behaviour. The modelling activities have been pushed to build complete mathematical descriptions and digital mockups of the core mechanisms and of the surroundings, in order to obtain suitable references, to be tested by simulation (Acaccia et alii, 1999).

MTBF, assemblability, disassemblability and other life cycle specifications. The project took advantage of different kinds of virtual prototyping and simulation techniques that allowed the validation of the analysed solutions at the subsequent design stages, as decisions support and performance verification. For this aim Simulink, Adams, ProlMechanica Motion and purposely developed C modules have been developed and interfaced. Figure 2 shows some snapshots of simulations performed on the robotic system digital mock-up during different tasks execution.

4.

CONTROL SYSTEM OVERVIEW

The careful engineering of the hardware and software systems, supervising the robotic system mission, has been performed, particularly considering the equipment installed on the vehicle and hosted in the surface vessel, considering the data exchange between them, and the performance to be achieved during the different kinds of mission. Several aspects have been carefully investigated: monitoring and control concept architecture, sensory system selection, monitoring and control modules design, communication system definition, operating and contingency procedures stating. The Figure 3 shows the standard operation layout: the machine is deployed on the sea bottom from a support vessel then, with the help of remotely operated vehicles (ROVs), is placed in front of the leg to cut and starts the dig and cut duty. An umbilical connects the submerged machine with the control unit located on the support, ship where the human operator continuously read the sensors values.

PLATFORM DECK

Start cutting

Cutting Complete

Fig.2 Snapshots from simulations on the robotic system mock-up A special simulation environment has been implemented and used for testing and verifying the control algorithms and logics. Dextrous workspace and singular points, kinematics, operational accuracy and stiffness have been considered as main performance criteria and balanced for the competing architecture alternatives vs . cost, maintainability,

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SUPPORT SHIP

Fig. 3 - SBC machine operation scheme

4.1 Control DeSign Basic Concepts To enhance the system reliability and maintainability, a modular approach has been adopted in the control design. Operative missions are performed as sequence of different tasks, accomplished by pertinent control modules. Every module is designed to achieve a particular goal and a suitable control system allows the human supervisor to check, in real time, the completion of the missions.

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Some modules are designed to be automatically controlled by the system, i.e. the anchoring and the cradle tilting, giving a graphic and numeric feedback to the human operator acting as remote supervisor. The overall cutting sequence, as well, is performed under local control, monitored by the operator located on the support ship. The control architecture has been defined and specific functions were distributed to each sub-system at the lower level. The intermediate control level has been structured in few blocks, with defined goals and functions. The human/machine interface functions have been established. The previously defined models have been translated into purposely written macros, interfaced with Matlab Simulink and ProlMechanica tools, to accomplish algorithms checks and to allow rapid prototyping actions. Using parametric values, the cutting machine and wire string behaviours are properly set. Full design of the control subsystems is devised, including the selection of hardware and software structure, with appropriate versatility and redundancy, to fit the software instructions according the mission targets. As far as possible and acceptable, standard equipment, software, and transmission protocols have been chosen from off-the-shelf components, in order to slightly impact the development price. The most innovative characteristics are: force control logic, position and force reflection based remote-handling, monitoring and data processing to create an expert archive modifiable and expansible, from which the user selects the mission parameters. An advanced man-machine interface is developed; it includes programming "by showing" functions (based on the system simulator issues) and task-oriented off-line programming modules. The project main criteria are to simplify as much as possible the underwater apparatus for reliability reasons, setting on-board the surface vessel all the sophisticated equipment. The driving and control variables are timely transmitted by very quick communication channels. Man Machine Interface is very important for the mission accomplishment. A simple and robust MMI has been realised, from which the tasks are defined and overseen, the machine actions remote-operated and the task monitoring are easily performed. The behaviour of the whole machine subjected to the defined control policy has been tested by virtualreality simulation, in order to evaluate the system dynamical behaviour and to prevent errors.

4.2 Control System Implementation The whole machine is controlled by a supervisory unit, which coordinates the included subsystems. The control system has been designed in order to be suitable for supervising, monitoring and controlling the many operations of modules, driving the main robotic subsystems: supporting base, mobile cradle

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arm, twin guide tubes and excavating heads forearm, cutting system end effector and the auxiliary systems: sea water pumping system and slurry management during five operational modes: o stand-by mode: this is the reference (idle) state, after deployment on the sea-bed; o emergency mode: if a failure arises, the alarm state is enabled, specifying the originating site; o posltloning (anchorage) mode: the robotic platform is located and its attitude set to start a cutting operation, the platform cradle is bent up to the selected engagement slope; o excavating mode: the twin pipe drill-and-dig heads perform the required digging beneath the sea-bed soil; o cutting mode: the diamond wire equipment accomplishes the planned task. as schematically represented in figure 4. CONTROL CONTROL MODES FUNCTIONS

SENSORS

EXCAVATION OEPTH

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RAME POSITION RAME FEEDING M

01. PRESSUlE FLOW IRE VELOCITY IRE TENSION RE ANGLE IRE TENSIONlNG M

01. PRESSURE. FLOW ENSIONING PULLEY OSITION

Fig.4 The robotic system control modes, functions and sensors The different states are, subsequently, reached by referring to the stand-by state. If any trouble or wrong value arises, the control system moves the machine in the stand-by (idle) status, waiting for the supervisor decision. The operator has access to the several monitored quantities, and disposes of a troubleshoot data-base and related hints to find out the most appropriate recovery actions. Only after the operator has checked the emergency and solved the problem, the machine can resume the automatic cycle or shall request further diagnostics, either, local actions. The sensors were selected, and suitable specifically designed signal processing algorithms have been studied, coded and checked by simulation to satisfy the stated performance requirements m nOIsy environment. The control system comprises the following units: surface control unit, including a touch screen

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computer, a surface-to-underwater interface and analogicaVdigital control unit; the sub-sea equipment, including actuators, sensors, alarms and various other equipment. The surface control unit reads all sensor signals from the sub-sea unit and presents the scaled values on the monitor. All outputs in the sub-sea unit can be activated from the surface control unit. This includes: camera control, light setting, and all hydraulic functions updating. A few control functions are reactively performed by the under sea control unit, while the most of them are remote-operated. Focussing, e.g., on the cutting task, the control system can be subdivided into three main dynamically coupled subsystems: - the motion of the frame supporting the diamond wire pulleys (cut feeding) , - the diamond wire stretching system (cut pressure), - the diamond wire speed (cut velocity), and a fourth one, the wire cleaning system, which is separately set.

CONSENSUS F'AOMMOTOR

PUllEY

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Fig.5 The reactive cutting system control In figure 5, a block diagram of the local MIMO control system (running on board the SBC machine) is given. The observed variables are further monitored by the central system console for remote-control. The operator is helped in his choices by model-based decision supports as shown in figure 6. 'R()~1

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I OR f~(i l 'n~SOI

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Fig.6 The cutting system remote-control scheme The reference cutting system model considers the steady state equilibrium of the wire, taking into account the friction on the idle and driving pulleys for the chosen cut feeding, pressure and velocity figures, the hydro-dynamic effects on the free wire segments, the wire-legs engagement cutting forces and friction, together with geometrical constraints, as can be seen in figure 7 that shows the cutting system layout.

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l:l _ _ _ l ",_ __

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DIAMOND WIRE

WIRE STRETCHING PULLEY

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Fig.7 The diamond wire cutting system layout during a pipe cutting The amount of information displayed and the control functions available to the operator are set individually for each control mode. In emergency mode, the value of all sensors are displayed, and all controls are available for the operator. Each mode will have direct access to the equipment necessary for its limited operation. If other equipment shall be operated, the manual override is necessary. The emergency mode has access to all functions and can automatically override current tasks by shut down procedures. The alarms are arranged for a number of parameters; the shut down is enabled only for a limited number of catastrophical accidents. This will also comprise the automatic control of the surface based high voltage starting equipment and power supply. The communication system is based upon the FIELD bus technology (PROFI bus or Can bus) and is connected through single or double twisted pairs. The video signals are directed through coaxial cables.

5.

SBC PROTOTYPE AND SEA TESTS

Figure 8 shows the SBC robotic system physical prototype. The system is quite big: it has been sized taking into account the transportation constraints. It is over than 10 meters long and 3,5 meters large; the twin pipes diameter is around 600 mm .. Some preliminary tests on the diamond cutting system stand-alone, performed in a suitably instrumented testing bench, allowed to complete the a priori knowledge of the new wires obtained by theoretical model simulation with the a posteriori knowledge gained through experimental checks. On this basis, new pearls and cutting system were designed and the control system algorithms have been set-up. Figure 9 shows the control stand, with the touch panel and the video console by which the operator, on board the ship, drives the SBC missions.

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As consequence of removing adverse environmental effects for the use of materials, resources and energy and of avoiding the waste of dangerous or polluting substances generated by the cutting process, the SBC guarantees that any human operators, working in contact with the system or profiting of the marine resources in close vicinity to it, are fully safeguarded, in confonnity to the highest level of health and safety standards.

7.

ACKNOWLEDGEMENT

The European Commission that funded the project SBC Diamond Wire Cutting System Sub Bottom Cutter (GRDI 2000 25740), the ITF consortium (Totalfina ELF, Shell, BP, HESS) and all the SBC partnership are gratefully acknowledged.

REFERENCES

Fig.8 The SBC physical prototype The first sea trials were perfonned in July 2003 on the sea bottom along the North - West reef of the Ulstein located in Ulsteinvik. and a steel pile of 910 mm diameter, 11 mm thick, has been successfully cut. Nowadays further open sea trials are programmed.

Fig.9 A view of the SBC control room

6.

CONCLUSIONS

The European environmental legislation, in particular, the Oil Operator and the National Authority act, often requires to convene on avoiding the resort to unsafe technologies, producing damages (explosives) or risks of contamination (water-abrasives) to the marine environment. Through the adoption of operation safe and environmentally acceptable standards, the project outcomes will benefit the oil and gas, as well as food industry by eliminating the risks of contamination of sea resources, providing an effective integration of the offshore regulations with the environment enacted rules.

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Acaccia G.M. , E. Cavallo, E. Garofalo, R.e. Michelini, R.M. Molfino, M. Callegari (1999), Remote manipulator for deep-sea operations: animation and virtual reality assessment, in Proc. 2nd Workshop on Harbour, Maritime & Logistics Modelling and Simulation (HMS99), 16-18 September, Genova, Italy, pp.57-62, ISBN 156555-175-3 . Acaccia G.M., M. Callegari, R.e. Michelini, R.M. Molfino, R.P. Razzoli (1998), Underwater robotics: example survey and suggestions for effective devices, 4th. ECPD Intl. Con! Advanced Robotics, Intelligent Automation & Active Systems, Moscow, Aug. 24-26, pp. 409416, ISBN 86 7236 0133 . Gerrard S., Grant A., Marsh R. , London C, (1999), Drill cuttings piles in the North Sea: management options during platform decommissioning Centre for Environmental Risk, Res. Rpt. No 31, Norwich, October, ISBN 1 873933 11 8. Grant, A., A.D. Briggs (2002), Toxicity of sediments from around a North Sea oil platfonn: Are metals or hydrocarbons responsible for ecological impacts? Marine Environmental Research, 53 , 95-116. Cavallo E., Michelini R.e., Molfino R.M. (2004), A remote-operated robotic platfonn for undewater decommissioning tasks, 35th Intl. Symposium on Robotics, ISR 2004, Paris, March 23-26, 2004, 16 LaBelle B. (1999), OCS Resources Management & Sustainable Development Report US Department ofInterior, September 24. Twachtman R. (1997), Offshore platfonn decommissioning perceptions change. Houston, Oil & Gas Journal Twachtman Snyder & Byrd Inc December 8. Zhiguo Gao (1997), Current issues of international law on offshore abandonment, with special reference to the U K Ocean Development and International Law 28, pp. 59-78.

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