All systems go!

In the late 1890s Guglielmo Marconi made the first successful wireless communication by radio surpassing the telegraph system. It quickly became obvious that this was what shipping had been waiting for. In a remarkably short time ships were able to communicate with land and each other. The equipment was perhaps bulky and little unreliable but these were early days. Since that time development has raced away and today not only can ships communicate with each other, but individuals can communicate to and within ships.

Furthermore the same basic principles have allowed the developments of radar, Global Positioning systems, satellite communications, devices to measure the depth of water and even the speed through the water. All of these work on the principle of sending out a signal and receiving it, the signal may differ but it still adheres to the basic transmission and reception criteria. All systems employ low power electronics, and are considered in many quarters to be almost ‘bolt on’ items to be installed toward the end of the ship building process. However, in recent years high power electronic devices have been developed that now permit the speed control of electric motors to a very high degree of accuracy. Along with the speed control comes thrust control and this has proven instrumental in furthering the cause of electric propulsion.

A selection of electronic devices can be made to communicate with each other and share information in a ‘network’ and it is the development and use of networks that has allowed for example the most versatile of facilities, Dynamic Positioning. When teamed up with the list of lower power devices listed above the power electronics device can control a vessel with an astonishingly high degree of accuracy.

The above developments are slowly changing the way that many designers and shipyards perceive electronic systems. Here we will look at dynamic positioning as a prime example of what can be done when electronic equipment is encouraged to talk to each other.

Dynamic Positioning

Dynamic Positioning (DP) has the ability to keep a ship in a given position or manoeuvre it within a very fine tolerance in the face of changing environmental conditions or navigational requirements. Many vessels in the offshore oil and gas industry spend much of their working time in very close proximity to each other. Imagine, for example, a rig supply vessel delivering equipment to an oilrig, which will be lifted from the vessel by the rig’s cranes. The vessel will have to remain in position with the rig towering over it while the lift is conducted in the face of often changing wind and tidal currents. Any contact with the rig’s structure will have catastrophic consequences for the supply boat and possibly even the rig itself. In another scenario a dive vessel is supporting divers from the surface via umbilicals as they conduct work to a rig’s underwater section. At the other end of the spectrum cruise vessels now routinely visit tropical ports and islands that are, due to their tourist industry, very environmentally conscious. Such vessels are increasingly called upon to sit on DP rather than deploy their anchors, which would damage the seabed, corals and marine life they are moored over.

DP was initially conceived the keep oil drilling n ships in position. The very first attempt at a DP vessel was Eureka built in 1961, which employed a system known as ‘taut wire’ to sense position and four steerable thruster to hold position. By the late 1970s DP was an established technique and by 1980 there were some 65 DP-capable vessels. The total now is well over 1,000 and expanding.

So what is DP and how does it work?

Essentially the vessel needs to know where it is, which way it is moving, and, what, if anything, to do about it? Two families of sensors, one positional and the other environmental, provide the ‘where it is, which way it is moving’ information. The positional information comes by means of a GPS, vertical reference unit and radar, etcetera. The environmental is provided by wind speed and direction (anemometer) and tidal drift indication. Positional requirement is provided by either operator input or often a remote reference input from an external source, for example the rig.

The processor in the DP system compares this information with an ‘electronic model’ of the vessel and its propulsion capability to calculate the required directional thrust to stay ‘on station’. This is then converted into demands to the propulsion systems. Let’s assume that our vessel is equipped with electric thrusters for propulsion. The DP system may have concluded that some of the thrusters need to increase speed, for example. Next question: is there enough power available? Enter the power management system (PMS), another relatively ‘new kid on the block’. The PMS recognises the loads on the ship’s power systems at any instant in time against the available capacity and decides whether to add to the capacity by starting another diesel generator and if so, which one to start. It will then start a diesel generator bring it up to speed, synchronise it with those already running before putting it, ‘on the board’. It will select the generator in such a fashion as to ensure that the total running hours are equally distributed over the number of generators the vessel has, thus contributing positively to the vessel’s planned maintenance routines.

At the same time all of these instructions are being copied to the ship’s voyage data recorder (VDR) in accordance with classification society and IMO regulations. With modern global and satellite communication it is possible to communicate these instructions along with a whole wealth of conditional data to the company’s shore-based office by means of wireless telemetry. If everything has worked to plan the indications from the positional sensors will confirm that the vessel has ‘held station’.

The electronics themselves

From the brief description above we can see a long list of electronic equipment and system including:

• GPS and its more sophisticated sibling Differential GPS,

• radar,

• anemometer,

• vertical reference unit,

• DP processor,

• power management system,

• thruster controllers,

• engine management system

(in the case of diesel main propulsion),

• pitch control system

(for use with controllable pitch propellers),

• voyage data recorder,

• satellite communications,

• sonar,

• laser alignment systems,

• taut wire.

They will have been produced by different companies, each of them a specialist in their own field. So it is now important to ensure that they can talk to each other with complete understanding and reliability. Like people of different nationalities these equipments often speak different languages, which is not as a result of any perverse sense of importance, but more because the various languages are industry standards and have been developed to fulfil specific requirements. These languages include Canbus, Modbus, Profibus and many others. As an example Canbus was originally developed and is therefore more suited to the control of machinery. How to get them to talk to each other? In everyday life we use an interpreter, in the electronic world the same thing is done by an ‘interface’, and it goes without saying that is another electronic device. The interface may reside in one of the items of equipment that need to talk to each other or it many be a ‘stand alone’ unit between the two.

Talk to any project manager and sooner or later he will probably tell you that integrating all the control systems is particularly difficult logistically because it is necessary to get all the various suppliers together at the same time to bottom out all the interfaces. Many times the writer has been assured by well respected manufacturers that their equipment will integrate seamlessly with another’s only to find that during commissioning one of the parties has made some latest modification and they do not communicate as seamlessly as one had been led to believe. On the other hand history and traditional thinking recognise the various disciplines that are required to build vessels, ‘hard skills’ like steel work, engineering and outfitting, but has not yet fully acknowledged the importance of electronic systems thought of perhaps ‘soft skills’ and they tend to be left until later in the project before being ‘brought on board’ design-wise. Integration discussions at the concept and design stages would provide better more user-friendly overall ship designs. However if the system was designed from the ‘top down’ and purchased as a single package, then it is possible for one company to market a complete system on behalf of all the various component manufacturers of the system. The Dutch company Alphatron is amongst the leaders in developing this approach they see the advantages to the customer and shipbuilder’s point of view in sorting out all the interfaces and relieving the ships designers of the potential headaches of integration.

Thrustmaster is a renowned specialist in providing thruster systems and complete DP capable propulsion systems. In producing the ‘complete package’ of thrusters, power generation and control system on a ‘one-stop-shop’ basis they appreciated early on the value of providing a fully integrated DP system and have worked with various manufacturers to develop fully integrated systems.

DP development

DP has over the years developed into different levels of performance. Largely the various levels address the issues of equipment failure. Clearly as can be seem in the examples above the need to maintain position when faced with failure is more critical for the rig supply vessel or the dive vessel than it would be for the cruise liner, and so the levels of DP and levels of redundancy designed into the various systems reflect this. Currently DP3 is the highest level of sophistication for such systems. In essence this would require a highly unlikely sequence of failures before the ships would have to be taken into manual control. Increasing use of failure mode effect analysis (FMEA) techniques are refining the requirements and thus driving the performance of such systems. For the future opinions differ slightly, but the principle is the same. This is that a ‘DP3+’ or perhaps DP4 standard is just around the corner. Certainly the DP manufacturers are ‘grasping the metal’ in moving towards fully integrated complete package systems.

Modern electronics gets its power from its ability to communicate. Like most people it wants to avoid conflicts, why should it be different in the world of ships? Many companies are working to develop this field still further and the future will doubtless provide exciting developments.

Andrew Rudgley