For details about dynamic positioning systems, see Rules for Dynamic Positioning Systems.
1.2 The arrangement and design of the dynamic positioning system shall take into consideration, i.e.:
– the type and purpose of the unit
– the environmental conditions and water depth at the operations site
– the motion restrictions imposed by elements connecting the unit with the sea bottom (i.e. in first line, risers)
– the distance and kind of other
structures/installations in the vicinity
– the consequences of system failure or malfunction.
1.3 Definitions: A dynamic positioning system is usually composed of the following main subsystems:
– measuring and sensor system (environment, position and motion reference)
– control system (evaluation of sensor information, thruster activation)
– propulsion system (position adjustment)
– power generation
1.4 As a dynamic positioning system is designed to suit a particular unit at a defined location, this restriction will be clearly indicated in the certification or classification documents.
1.5 Control station: All essential data/information about position/heading of the unit, environment conditions, functioning/status of measuring equipment and alarms, status of thrusters, etc., shall be displayed in a separate control station, arranged at a suitable location overlooking the operations and surroundings of the unit.
1.6 Documents for approval
The following documents shall be presented to BKI for approval:
– general description of the system configuration and characteristics, including general arrangement drawings, e.g., control panels, control consoles, location of control station.
– thruster characteristics.
– response of the unit to environment influences, as described under C.6., and to thruster action
(response analysis).
– proof of the unit's ability of maintaining position after specific single failures. See Section 2.A. of the Rules mentioned under 1.1.
– functional block diagrams of the control system(s).
– functional block diagrams of the position reference system(s) and the environmental sensor(s).
– wiring diagrams.
– drawings showing the electrical power supply of all units and the internal power distribution.
– drawings and descriptions of monitoring functions of control, sensor and reference systems.
– list of equipment identifying manufacturer, type, type approval number etc.
1.7 Detailed information on the positioning system, its operation, maintenance, safety checks, alarms and contingencies in case of component failure or malfunction shall be contained in a special DP Operation Manual.
2. Class Notation
2.1 Dynamic positioning installations complying with the Rules may be assigned one of the following Notations (additions) to the Character of Class of the vessel or unit:
Class Notation DP1 Class Notation DP2 Class Notation DP3
2.2 The Class Notations corresponding to DP1 to DP3 are associated with different failure modes and degrees of redundancy, DP 1 meaning the lowest, and DP3 the highest grade of sophistication.
For details regarding equipment components and failure suppositions see the Rules mentioned under 1.1 above. In the following, some general requirements are listed and indications given, in addition to the Rules mentioned.
3. Measuring and sensor system
3.1 Definition: The installations/devices referred to under this heading comprise the following items:
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– position reference/measuring system, indicating at regular intervals the position of the unit in relation to fixed reference points on the sea- floor (e.g., beacons) and/or land based reference stations (radar), or satellites;
mechanical systems (e.g., taut wire, strain measurement) may be used additionally – measuring devices continuously indicating the
heading of the unit (usually, gyrocompass) – continuous recording of water depth
– inclination and accelerations measuring devices (roll, pitch, heave), depending on type of unit
– environmental forces: Wind speed sensors and in particular cases, water velocity (currents) sensors.
3.2 Generally, a combination of different measuring methods is used for position reference, to ensure an acceptable level of redundancy and reliability. For operations far from land (radar) stations, DGPS will usually be incorporated in a position reference system.
3.3 Certification, rule applicability
3.3.1 Measuring devices and sensors for which offshore operation experience is proven will generally not be required to be certified by BKI. However, all equipment forming part of the DP system shall be included in the function testing procedure of the whole system (see 7.).
3.3.2 Mechanical, electrical and hydraulic components of measuring and sensor installations shall comply with Rules for Materials, for Machinery and for Electrical Installations, as far as applicable.
Suitability for the particular ocean environment shall be proven.
3.4 The data transmission frequency of position reference measurements shall be sufficient to ensure the reliable functioning of the control system.
Acoustic measuring systems shall be designed/
adjusted so that interference with noise/signals from other sources on the unit or neighboring installations will not impair the reliability/operability of the DP system.
3.5 Alarms shall be provided to indicate
– malfunctioning or breakdown of any single measuring device or sensor,
– surpassing of allowed range limits.
See also Rules for Electrical Installations, Volume 5, Section 9, and the Rules mentioned under 1.1.above.
3.6 Where the system design requires redundancy of sensors and/or measuring devices, these must be
fully independent (power supply, data transmission lines, etc.) See Rules mentioned under 1.1 above.
4. Control system
4.1 Definition: The control system is meant to be a computer or array of computers and interfaces, together with programs, capable of storing and processing the information received from the measur-ing/sensor system, and deriving there from suitable input for the activation of the thrusters. Furthermore, the system must be capable of controlling the availability/functioning of all components and automatically switching/passing on functions and duties to other (stand by) components in case of failure.
4.2 The implied functions of determining the unit's actual position and calculating the forces and moments necessary to restore the desired position shall be performed by the computer programs in a stable, convergent manner. This capability, essential for the reliability and efficient of a DP system, shall be confirmed by trials (see 7).
4.3 Systematic investigations such as failure mode and effects analysis should be carried out to determine the viability and reliability of different possible/contemplated system concepts. The corresponding investigation for the concept finally chosen shall be part of the approval documentation.
4.4 For control/safety systems in general, see also Section 8 and Rules for Electrical Installations, Volume 5, Section 9.
4.5 Power supply of the control system: See 6.
Where the degree of redundancy chosen provides for a second (back-up) control system, the power supply of the latter shall be completely independent.
5. Propulsion system
5.1 Definition: Depending on the type of unit, the propulsion may in principle be effected by
– fixed propellers (free or ducted; fixed/variable pitch; for ship type units, also transversely arranged tunnel),
– thrusters (azimuthing unit with gear, usually ducted, sometimes retractable).
5.2 Propulsion unit components (propellers, gears, couplings, piping) shall generally conform to the relevant BKI Rules (Rules for Materials, Vol. V, Section 11; Rules for Machinery Installations, Volume 4, Sections 5 and 6, or Rules for Machinery Installations, Volume III, Sections 4, 5, 6).
See also Section 13, D.
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5.3 Where a dynamic positioning installation is provided as a sole means for station-keeping, it shall be possible to maintain the position and heading of the unit within the predetermined limits also after failure of any one of the thrusters.
5.4 Thruster/propulsion control shall generally be possible manually as well as incorporated in the computer-based control system (see above). The control mode actually in operation shall be indicated on a separate thruster control panel in the control station(s).
5.5 The possibility of correcting the heading and position of the unit by the combined action of propellers and rudders will generally not be considered as an option within the dynamic positioning concept.
5.6 The arrangement, design and hydrodynamic configuration of the propulsion equipment shall be such that disturbance of the position monitoring (measuring) devices - particularly acoustic systems - is minimized.
5.7 Special provisions shall be made for propellers forming part of the positioning systems, which are also intended for propulsion during transit operations.
6. Power generation/supply
6.1 The electrical power needed for the positioning system may be provided by the general power generation system of the unit, or by a separate system.
The following requirements are valid for both cases.
See also Section 15.
6.2 Regarding general design and safety requirements, reference is made to the relevant BKI Rules (Seagoing Ships: Volume IV, Section 3; Offshore Installations: Volume 5, particularly Sections 3 and 4).
6.3 A "Power Management" scheme should be set up to determine, i . e.,
– the total amount of power needed in extreme conditions;
– the necessary number of generators running, depending on environment conditions;
– power distribution between positioning system and other users/power needs on board, under normal and under emergency conditions (local selection and limitation);
– power-related input to the control system (see 4.
above), including reaction to component failure/breakdown;
6.4 In case of a power supply breakdown, the control system shall automatically be supplied by an
emergency supply system (battery, emergency generator).
6.5 Redundancy: Regarding availability and redundancy requirements for power supply, depending on the Class Notation, see Rules mentioned under 1.1 above.
7. Testing, trials
7.1 Hardware components of the DP system shall be tested before being installed, either according to the BKI Rules where they are applicable, or according to an approved test plan. Standard components may be exempted, if testing in a satisfactory manner is incorporated in the production process.
7.2 Testing of sub-systems shall be carried out before installation on board, as far as practicable, by simulation of real operating conditions. This applies also to computers and software. Component and system failures considered in the design procedure are part of the simulation program.
7.3 Any apparatus/device forming part of the DP system installed on board in such a way that it can be affected by vibrations may have to be subjected to a relevant investigation and test.
7.4 Trials upon completion and installation on board shall be carried out in presence of a BKI Surveyor, according to an approved trials plan/program.
Acceptance criteria shall be established and agreed upon.
The trials program, which may be divided into dockside and on-location testing, shall include verification/testing of the following:
– functioning of the measuring/sensor systems – reaction of the control system(s) to specific
signals received from the measuring system – functioning of alarms and status signals
– functioning of automatic switch-over and substitution procedures/actions (failures of equipment to be simulated)
– functioning of all relevant information flow between the control system(s) and the thrusters/propellers
– position keeping ability of the system after any single failure associated with the assigned Class Notation.
7.5 The propulsion and power supply systems are additionally subject to the test procedures prescribed by
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the BKI Rules for Seagoing Ships, as far asthey are not covered by the trials program mentioned above.
For tests and surveys, see also Rules for Dynamic Positioning Systems mentioned under 1.1.
2. Structural design
Local forces exerted by the drives shall be taken account of in the structural design of the turret andthe adjacent structures of the unit, considering also stiffness requirements/tolerable deformations. See Section 4, C.2.5.
3. Mechanical and electrical installation F. Turret - Active Heading Control
1. General design considerations
Where positioning is effected using a turret installation, it must be decided during the design procedurewhether an active turning installation (drives) mustbe provided to ensure the required heading ("weathervaning") capability of the FPSO unit.
Note:
This decision may have to be made at a relatively late stage of design, as it depends on a number of factors, such as turret diameter, bearing configuration, environment forces acting on the unit, tolerances of riser system.
The machinery components and electrical installation (drive units, gears, power supply, cabling, etc.) shall generally correspond to the requirements of the applicable BKI Rules (Seagoing Ships: Volume III and IV;Offshore Installations: Volume 4 and 5), and/or to acknowledged standards and codes.
4. The activation of the drives and control of the relative heading shall be effected from the bridge and/or central control station. System details shall be submitted for approval.
5. Testing of the turning installation and control instrumentation is to be carried out under BKI supervision, according to an approved test plan.
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Section 8 - Lifting Appliances 8 -1