Introduction to Dynamic Positioning
7 - Information for Key DP Personnel
7.1 - Failure Mode and Effects Analysis
For all DP vessels, all failure modes and their effects should be considered in a formal FMEA (failure modes and effects analysis) study16. The presence of an FMEA document is often a requirement of the pre-charter auditing and inspection process, as well as being a requirement of the classification society for DP class notation. The modes that should be considered are the sudden loss of major items of equipment, the sudden or sequential loss of several items of equipment with a common link, and various control instability failures. Faults that can be hidden until another fault occurs should also be considered. Also to be considered are the methods of detection and isolation of the fault mentioned. Operator responses to the types of failure considered should be reflected in the vessel's operations manual. The FMEA should consider likely operational scenarios of the vessel, such as shallow water, high tidal stream rates and limited provision of position reference. See Ref. 16 for further information on FMEAs.
Redundancy levels are defined by the IMO document MSC/Circ.645 - "Guidelines for Vessels with Dynamic Positionin Systems"17 and the IMCA document "Guidelines for the Design & Operation of Dynamically Positioned Vessels"18. Three ‘equipment classes’ are defined, summarised in the IMCA guidelines as follows:
Equipment Class 1
Loss of position may occur in the event of a single fault
Equipment Class 2
Loss of position should not occur from a single fault of an active component or system such as generators, thruster, switchboards remote controlled valves etc. But may occur after failure of a static component such as cables, pipes, manual valves etc.
Equipment Class 3
Loss of position should not occur from any single failure including a completely burnt fire sub division or flooded watertight compartment.
A single fault includes a single inadvertent act by any person on board the DP Vessel.
In basic terms, equipment Class 1 refers to non-redundant vessels, Class 2 relates to vessels with full redundancy of systems and equipment, while vessels built or fitted to equipment Class 3 are able to withstand the loss of all systems in any one compartment from the effects of fire or flooding.
7.2 - Classification Societies
A number of classification societies issue class notations for DP-capable vessels. The notations from each of the societies vary, but refer to the compliance with the equipment classes. The following table lists the class notations and corresponding equipment classes for Lloyd’s Register, DnV and ABS:
|
Description |
IMO Equipment Class |
Corresponding Class Notations |
||
| LR | DnV |
ABS |
||
|
Manual position control and automatic heading control under specified maximum environmental conditions |
|
DP(CM) |
DNV-T |
DPS-0 |
|
Automatic and manual position and heading control under specified maximum environmental conditions |
Class 1 |
DP(AM) |
DNV-AUT DNV-AUTS |
DPS-1 |
|
Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault excluding loss of a compartment. (Two independent computer systems). |
Class 2 |
DP(AA) |
DNV-AUTR |
DPS-2 |
|
Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault including loss of a compartment due to fire or flood. (At least two independent computer systems with a separate backup system separated by A60 class division). |
Class 3 |
DP(AAA) |
DNV-AUTRO |
DPS-3 |
7.3 - Consequence Analysis
One of the requirements of the IMO Class 2 and 3 guidelines, is a system of Online Consequence Analysis to be incorporated in the DP system. This function continually performs an analysis of the vessel's ability to maintain its position and heading after a predefined, worst case failure during operation. Possible consequences are based on the actual weather conditions, enabled thrusters and power plant status. Typical worst-case single failures are:
- • failure in the most critical thruster
- • failure in one thruster group
- • failure in one power bus section
If the consequence of the predefined failure is a loss of position, it is reported to the operator via the DP alarm system. The consequence analysis can operate for different configurations and give Class 2 or Class 3 alarms and warnings. A typical alarm message is "Consequence Analysis Drift-Off Alarm". The associated description reads: "Single worst case failure will cause drift-off". The analysis function typically runs every minute and averages over the last minute.
7.4 - Watchkeeping
There are many different DP vessels and DP operations. Some tasks require the vessel to maintain a static or relatively static position for days or even months on end (drillships, flotels). Other vessels will be continually manoeuvring in order to execute their work. Irrespective of the work the Watchkeeping principles are similar and some general watchkeeping procedures are included here18.
Some Class 1 vessels operate with one DPO on watch, but the majority of DP operations are carried out with two operators manning the bridge. On some vessels, one DPO mans the DP desk exclusively, while the other watchkeeper carries out all other bridge functions. These two individuals then swap roles every hour. The watch relief arrangement should allow staggered watch change-over such that there are never two fresh DPOs taking over at the same time. When taking over the watch, DPOs must familiarise themselves with certain aspects of the management of the vessel at that time. The list of information that the bridge team must acquire at this time includes (but is not limited to) the following:
- • Position and heading of the vessel
- • Status and recent performance of the DP system and its peripherals
- • Details of Position Reference Systems in use and their performance
- • Availability of further PRS on failure of the above
- • Level of redundancy
- • Status of the operation in hand. Planned changes/progress for the coming watch.
- • Details and status of any operational elements (e.g. if the vessel is a DSV and diving operations are underway, then the status, position, depth of the diving bell or basket, the number of divers in the water, their umbilical lengths and expected return times, also details of their operational task)
- • Weather conditions and forecasts
- • Communications, on-board and external
- • Traffic in the area. Any planned traffic movements that may affect the vessel and her operation or positioning
- • Any planned helicopter operations
7.5 - Checklists
Checklists are an essential and accepted feature of most DP operations. It is essential that checklists are treated as an aid to memory and not as a complete substitute for ‘thinking’. It is very easy for one person in a hurry to fill out a checklist without checking many of the items contained therein. Checklists need updating from time to time, as new important points are found and equipment is modified or updated. Checklists are usually controlled documents within the shipowner’s quality assurance system, where alterations may be seen as a ‘non-conformance’ and change takes too long.
Typical checklists to be maintained by the watchkeeping DPO include:
- • Pre-DP checklist
- • Pre-operational checklist
- • Watch hand-over checklist
- • Periodic DP checklist
- • MCR checklist