Manoeuvring and positioning a ship
As the command and control centre of the ship, the bridge must have a comprehensive control system that connects to the engine and steering gear as well as other systems on the ship such as the navigation lights. On a modern ship this is achieved by a combination of hydraulic, mechanical, electronic and electrical means and a system of indicators that returns information to those controlling the ship.
The controls of the various systems are often supplied by the bridge manufacturer for an integrated system or they can come from an independent third party. Operation of the controls will vary and in many ships, the once familiar system of wheel, knobs and levers has been replaced by a combination of joysticks and touch screens.
In 2010, Kvant introduced a haptonomic module which can be used with its controls to give a positive feedback to the operator. This gives back some of the ‘feel’ that was inherent in older methods of control and allows the navigator to better understand what is happening at the rudder or propulsor. Although this may seem to be an unnecessary gimmick since the navigator has the readout display in clear sight, it has been shown to prevent excessive use of the control and thus eliminate the hazards and undesirable effects of systems being overloaded. Kvant’s innovation has since been followed by others.
Designed with ergonomics in mind
The layout of the controls is considered to be part of the ergonomics of the bridge and placement will take into account the position from which they are normally operated. On some ferries and offshore vessels, the controls may even be partially incorporated in to the captain’s chair.
The physical controls are normally sufficient for general use but emergencies do arise and all vessels are required to have a means of emergency steering which is invariably located as close to the rudder as possible and thus very far from the bridge. Those operating the emergency steering need to receive orders and so as well as voice communication to the engine room, there must also be a means to communicate directly with the emergency steering room from the bridge. There must also be a tannoy system for allowing commands to be broadcast throughout the vessel.
Many of the information display systems required under SOLAS vary depending upon the ship’s size. All vessels above 300gt and all passenger ships are required to have a Speed and Distance measuring device for measuring speed and distance through the water (SOLAS regulation V/184.108.40.206). In addition, ships of 50,000gt and over require a similar device for measuring speed over the ground in the forward and athwartships direction (SOLAS regulation V/220.127.116.11). Both devices if fitted should be connected to the ship’s VDR. The devices are usually referred to as Doppler Speed Logs for the very simple reason that they operate using Doppler radar and a transceiver fitted to the ship’s hull.
Until very recently, it was considered sufficient for the requirements for the larger ships to be covered by installing a single device capable of both measurements. However, in July 2013 following MSC90, the IMO issued a clarification that on ships requiring both devices (i.e. ships of 50,000gt and over) the requirement should be fulfilled by two separate devices: one speed and distance measuring and indicating device capable of measuring speed through water; and one separate speed and distance measuring and indicating device capable of measuring speed over the ground in the forward and athwartships direction. These amendments are published in IMO resolution MSC.334(90) and the IMO circular MSC.1/Circ.1429 and apply to devices installed on ships constructed on or after July 1, 2014.
Above 500gt, ships are also obliged to be fitted with a rudder angle indicator which, as the name indicates, provides information displaying the angle of the rudder. The same ships must also have a display indicating the thrust and pitch of the propeller(s) if they are fitted with controllable pitch propellers. On all ships above 50,000gt there is an additional requirement for a Rate of turn indicator to give information on how fast the ship is turning at a steady rate. The display is normally shown as number of degrees turned.
As a means to prevent the helmsman suffering from fatigue, ships of 10,000gt and above are required to be fitted with an autopilot. Autopilots, or heading control systems as they are commonly referred to, are also common on smaller vessels where their installation is not mandatory simply because of the benefits they can confer. The autopilot should not be used in high traffic areas and it is essential to keep a lookout whenever it is in use for obvious reasons.
The performance standards for autopilot systems are quite extensive and have adapted over time to cover evolving technology such as ECDIS. As a consequence autopilot systems are now capable of more than just maintaining the vessel on a pre-set heading with minimum operation of the ship’s steering gear. Being connected to the gyro compass and GPS as well as the ECDIS, an autopilot can now be part of a track control system making turns and following a pre-determined passage plan. If the system is to make turns it should be connected to a suitable source of speed information and be able to perform turns, within the turning capability of the ship, based either on a pre-set turning radius or a pre-set rate of turn. Since some systems will be fitted with remote stations to which control can be delegated, it is also a requirement that the master station should have a means to regain control at any time. Finally the system must also incorporate alarms that operate if the vessel deviates from the pre-set course or if there is a failure or a reduction in the power supply to the heading control system or heading monitor, which would affect the safe operation of the equipment.
For most vessels complicated manoeuvres are only made when berthing or unberthing but for vessels working in certain sectors – particularly the offshore sector – the ability to accurately hold station for long periods is essential. To accommodate this need, vessels are fitted with some form of Dynamic Positioning (DP).
DP systems evolved over time and did so outside of the IMO regulations. So as to bring some uniformity to the development of DP systems, the IMO issued guidelines in 1994 in MSC/Circ.645 and this has evolved into an established international standard for DP systems. The guidelines have successfully provided the framework on which national regulations and classification society rules are based.
Since MSC/circ.645 was first published, DP has evolved from being a tool primarily for mobile offshore drilling units (MODUs) maintaining position over offshore wells, to being employed for a wide range of position keeping operations, with systems being fitted on much larger numbers of new vessels and on an increasingly diverse set of vessels, from offshore units to shuttle tankers and passenger vessels. In March 2016 at SSE 3, it was agreed that further revisions were needed to the guidelines and work is continuing on this with a view that changes will be finalised in time for SSE 4 in 2017.
A vessel fitted with DP has a computerised control system that automatically maintains a vessel’s position and heading by using the ships propellers and thrusters to offset wind and current forces. Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the computer pertaining to the vessel’s position and the magnitude and direction of environmental forces affecting its position.
Although DP is related to navigation, it is normal for the DP operation centre to be located away from the main bridge. DP operators spend most of the working time in front of computer screens and there would be no advantage for them to be housed on the bridge.
The computer program needs to be populated with a mathematical model of the vessel that includes information pertaining to the wind and current drag of the vessel and the location of the thrusters. This knowledge, combined with the sensor information, allows the computer to calculate the required steering angle and thrust output for each thruster. Dynamic positioning may either be absolute in that the position is locked to a fixed point over the bottom, or relative to a moving object like another ship or an underwater vehicle.
As dynamic positioning systems are so specialised the manufacturing sector is quite small with Kongsberg Maritime, Rolls-Royce Marine, Navis Engineering and GE Power Conversion, DCNS and EMI being some of the best-known names. There are presently three different classes of dynamic positioning systems recognised within the IMO guidelines with increasing degrees of sophistication made possible by technological developments that have evolved since the first DP system was devised.
- Equipment Class 1 has no redundancy. Loss of position may occur in the event of a single fault.
- Equipment Class 2 has redundancy so that no single fault in an active system will cause the system to fail. 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 which also has to withstand fire or flood in any one compartment without the system failing. Loss of position should not occur from any single failure including a completely burnt fire sub division or flooded watertight compartment.
For ships to remain in one position requires highly accurate position referencing. This can be supplied either by satellite positioning or from a fixed point on the seabed if the vessel involved is a drilling unit or similar. If satellite position fixing is used it should be noted that the position obtained by GPS is not accurate enough for use by DP and Differential GPS (DGPS) is required. DGPS makes use of a fixed ground-based reference station (differential station) that compares the GPS position to the known position of the station. The correction is sent to the DGPS receiver by long wave radio frequency. For use in DP an even higher accuracy and reliability is needed.
Companies such as Veripos, Fugro or C&C Technologies supply differential signals via satellite, enabling the combination of several differential stations. There are also systems installed on vessels that use various Augmentation systems, as well as combining GPS position with GLONASS.
The choice of DP class for particular tasks is a matter for the shipowner and the client although in territorial waters the state involved may impose rules. In Norwegian waters for example, the Norwegian Maritime Authority(NMA) has specified what Class should be used in regard to the risk of an operation. Unlike the IMO guidelines, in the NMA guidelines four classes are defined:
- Class 0 Operations where loss of position keeping capability is not considered to endanger human lives, or cause damage.
- Class 1 Operations where loss of position keeping capability may cause damage or pollution of small consequence.
- Class 2 Operations where loss of position keeping capability may cause personnel injury, pollution, or damage with large economic consequences.
- Class 3 Operations where loss of position keeping capability may cause fatal accidents, or severe pollution or damage with major economic consequences.
Although DP systems employ computers to make the rapid adjustments to thrusters needed to maintain position, the overall operation must be monitored and controlled by trained DP Operator (DPOs). The main role of the DPO is to determine whether there is enough redundancy available at any given moment of the operation and to take appropriate action in the case of any equipment failure. In 1996 the IMO issued MSC/Circ.738 (Guidelines for dynamic positioning system (DP) operator training).
Although the IMO has issued the guidelines for DO Operator training it is a fact that in many of the main offshore oil and gas fields, rules other than SOLAS and MARPOL take precedence within a set distance from offshore facilities. These rules are drawn up by national health and safety authorities and there are differences depending on where ships are operating. In addition the charterers of offshore ships often have their own requirements for DP operator experience and qualifications.
The IMO guidelines were in fact the work of the International Marine Contractors' Association (IMCA) and in June 28, the IMO approved Rev.2 of MSC.1/Circ.738, which provides a reference to guidelines issued by IMCA. The new guidelines are available (link: http://www.imca-int.com text: here).