DP class of a vessel is determined by its Redundancy, i.e. to what extent the system is reliable to continue DP operation in case of a failure any of its component. However, the primary meaning of the term redundancy is the duplication of components or superfluity. 

It is referred to the ability of the system to maintain working condition or restore it in case of a Single Failure. A lot of things in the DP system are connected to the term Single Failure (a single breakdown or malfunction): when a single unit fails but the rest continue to function or there is a failure due to negligence or lack of attention. Worst-Case Failure means that due to a single failure or negligence the greatest number of the system components fails. The worst-case failure is determined for each individual vessel, e.g. if a breakdown of one thruster is a single failure on some vessels, then a closed valve from one service tank, feeding two of the four diesel generators and the main engine, is the worst-case failure for the other. 

It has already been mentioned that DP class of the vessel is determined by its redundancy, and required redundancy is set by the level of consequences followed after the loss of ability to keep position.  The worse consequences are, the more reliable DP system shall be. According to this principle, there are three DP classes:

DP class 1 system is expected nothing from. Vessel of such a class is able to maintain her position, but in case of a single failure may lose this ability. There is one console, one controller and such a set of thrusters, with which the system is unable to maintain position of the vessel if one of them fails.

In DP class 2 system there is a duplication of components and position loss shall not take place in case of a single failure. The power system shall be divided into two buses in such a way, that an overload or a short circuit on one bus is unable to affect the function of another. There are two consoles, two controllers, duplication of sensors and two DP operators (DPO). The worst-case failure (WCF) for DP class 2 system is defined as the one, when the vessel loses most of her generators and/or thrusters but continues to maintain her position. The case with the closed valve from the fuel oil service tank is a real WCF on an azimuth vessel with two bow tunnel thrusters, where the vessel loses one azimuth and one bow tunnel thruster, i.e. half of the propulsion system, and is still capable of maintaining her position. It is obvious, that in this case the consequences are the same for closing this valve or leakage on the fuel supply line from one of the tanks. It belongs to Static Components, and that is why there are at least two service tanks on board.

DP class 3 system is far more reliable – in addition to the reliability of DP class 2 system, it includes fire and flooding resistance of one compartment, as shown in Figure 4. It is achieved by installing components in a separate compartment. Thus, in case of a fire in one space, it will not affect operation of the component in other compartment (A.60 class division). When DP class 3 system components are located below waterline, it usually relates to the Power, the separation shall be watertight as well.

As it can be seen from the above, redundancy and the DP class are closely connected. DP class is determined by the levels of redundancy: the greater number of duplicated components the system has, the higher class is. The component duplication, i.e. redundancy, allows to prevent loss of position and/or heading in case of malfunction of any system component. In other words, the vessel will continue maintaining her position and heading during DP operation in case of a Single Point failure.

DP Classes (Classes of Dynamic Positioning)

As the DP system is installed on board a vessel, there is a term ‘Dynamically positioned vessel” (DP vessel).

It shall be underlined that it is done by means of thrusters (i.e. propulsion system); not sails, oars or anchors – propulsion system only!

Wind, waves and current constantly affect a vessel, making her drift, turn around or heel. Thus, the vessel may have six axes of freedom: surge, sway, yaw, pitch, roll and heave. It is advisable to learn them at once, always remember and never confuse.

Surge – motion of a vessel back and forth.

Sway – sideways motion of a vessel to port or starboard.

Yaw – rotation of a vessel in horizontal plane, change of heading.

Pitch – the up/down movement of the bow and stern of a vessel.

Roll – port-starboard tilting motion of a vessel.

Heave – vertical motion of a vessel up and down.

The DP system is able to control only tree axes: surge, sway and yaw – it will be referred as positioning. For this the system needs to obtain value of the forces influencing the vessel, her present position coordinates and heading. At this stage sensors and position reference systems are involved. Gyrocompasses feed data about the vessel’s heading to the DP system. One of the environmental forces which is possible to measure is the wind force. Influence of the waves and current is calculated by the mathematical modelling of the DP system and is called DP Current.

Position Reference System (PRS) provides the DP system with the data on vessel’s position coordinates. There are usually two or more PRSs, that is why their data is fed into one point, called Centre of Rotation. However, during the rough sea, antenna or scanning unit of PRS heels along with the vessel, continuously indicating data of its position on the coordinate system. In order to correct data input from the reference systems, the DP system requires measurements of vessel’s Roll and Pitch. Vertical Reference Sensor (VRS) measures Roll and Pitch angular values and feeds this data to the DP system.

To summarize, Dynamic Positioning – is an automatic position and heading keeping by means of vessel’s propulsion system (thrusters). The word heading shall be emphasized, as the word positioning is associated with the meaning “position” – i.e. latitude and longitude. Within the context of DP, Auto Position (AutoPos) – is maintaining both position and heading of the vessel.

DP vessel

DP states for Dynamic Positioning and the DP system is defined as:

From this definition, it is clear that the DP system includes some parts called components in the number of seven, as shown in, which shall be remembered:

Components of DP System: Overview and Functional Description.

Only proper functioning of all these components is able to ensure reliable functioning of the whole DP system. Upon arrival on board, it is necessary to study each component individually (its structure, layout, disadvantages, etc.) and monitor its functioning continuously during DP operation.

While speaking about DP class 2 and 3 vessels, duplication of each component is considered – a so-called Redundancy. It is, therefore, necessary to have clear understanding of the DP system in order to answer the question, if redundancy is provided, i.e. if the DP system complies with class 2 or class 3.

Before continuing to study some peculiarities of the DP system, it is necessary to understand the basics and principles of its work. The main international guideline for the vessels with the dynamic positioning system is IMO MSC Circular 645. The given above definition of the DP system comes from Circular 645 and is just an initial one. Later, to understand the concept of DP, it shall be considered from different angles and more terms shall be provided accordingly.

Circular 645 came into force in 1994. 20 years later, it was decided to make some amendments and, as a result, in June 2017 MSC.1/Circ.1580 was released. This new Circular is an updated and amended version of the previous one and contains basic requirements of the industry at present. The mentioned above definition can be found in Circular 1580 as well.

Considering this, the question may arise: which Circular shall the vessel and the system comply with? It is recommended that vessels built from the 1st of July 1994 until the 9th of June 2017 shall comply with Circular 645, while vessels built from the 9th of June 2017 and later shall comply with new Circular 1580. However, it is advised that Section 4 of the new Circular shall be applied to new and already built vessels.

Definitions containing in the amended Circular will be referred in this book. Similar definition of the DP system can be found in previous Circular. One of the difference is that meaning “to maintain a position” in Circular 645 is defined as “to maintain a position and/or heading” in Circular 1580 respectively. 

The previous Circular is mentioned as subject matter tests and competence assessments might still refer to Circular 645, however questions based on Circular 1580 may be encountered and it is necessary to be ready for any variants.

A TRA is a formal risk assessment for a specific task

It breaks the task down into steps, usually derived from the Work Instruction, and then:

• analyses the potential hazards for each step,
• records the existing controls already specified by procedures,
• and describes any additional controls required for each step to ensure the risks remain acceptable.

It is designed to be referred to before conducting the task as a reminder of the potential hazards and necessary controls.

Every task that is conducted regularly should have a TRA written for it.

If about to conduct a non-routine task that includes potential hazards, a TRA should be created if one does not already exist.

Definitions

As Low as Reasonably Practicable (ALARP): BGP Offshore has a legal duty to reduce risks to a level that is As Low as Reasonably Practicable (ALARP). Risk is shown to be ALARP when it can be demonstrated that the risk of activity has been reduced to a level acceptable to stakeholders. To reduce a risk to a level “as low as reasonably practicable” represents the point where the time, trouble, difficulty and cost of further reduction measures become unreasonably disproportionate to the additional risk reduction obtained.

In other words – no task is risk free; ALARP allows focusing of efforts on where they will have the most impact.

Consequence: Outcome of an event affecting objectives.

Hazard: A hazard is any object, situation, or behavior that has the potential to cause injury, ill health, or damage to property or the environment.

Hazard Effect: The consequences (harm) if a particular hazard is realized. Hazard effect in this context covers injury to people, ill-health or damage to property, equipment or the environment.

Initial Risk: The initial risk rating is based on the realistic outcome (severity) identified in the Hazard Effects and the likelihood of the Hazard Effects occurring, with all the BGPO existing controls in place. (E.g. A1 (Low), B4 (Medium), C5 (High)

Likelihood: Chance of something happening. This is targeted when trying to reduce the event from happening.

Residual Risk: Risk remaining after any additional control measures being applied which are excluding from BGPO existing controls. (E.g. A1 (Low), B4 (Medium), C5 (High)

Risk: The product of the chance that a specific adverse event will occur and the severity of the consequences of the event.

Severity: The numeric value assigned to a consequence.