Human Centered Transportation Systems

On Guidance and Decision-Support Displays in Ship-to-Ship Lightering Operations

Dagfinn Husjord and Egil Pedersen
Norwegian University of Science and Technology, O. Nielsens vei 10, Trondheim, Norway
E-mails: ,


Ship officers in charge of lightering operations with tankers have answered questionnaires focusing on the decision-making process and aspects on the ongoing development of a navigation support and guidance system for ship-to-ship operations. The potential for improving the process of decision-making and proposals for GUI displays that are aimed to enhance the operational safety are presented.

Index Terms - STS Lightering, Decision-support Display.

I. Introduction

A SHIP-TO-SHIP lightering operation is typically involving two tankers that are maneuvering in close proximity in order to come in position for operation alongside to commence cargo transfer. This is a challenging task for the officer in charge of the decision-making process during the approach as it is based on visual observations and radar measurements. Experienced officers are necessary to minimize the risk of miscommunication between the ships, [1].

A ship-to-ship (STS) lightering operation can either be carried out while two ships are under power making way through water, or when one ship is already positioned at anchor. In the case of two ships under power, the ship-to-be-lightered (STBL) is supposed to maintain speed and course, and is also referred to as the Constant Heading Ship. The Maneuvering Ship, also called the service ship (SS), will approach until it is parallel with the Constant Heading Ship before commencing the final approach phase which is to maneuver until the ships are side by side and mooring can take place, Figs. 1 and 2.

In an STS lightering operation the Captains are, as always, in charge of their respective ships. However, it is normal to have a Mooring Master onboard the Maneuvering Ship and in some cases an Assistant Mooring Master onboard the Constant Heading Ship. The Mooring Master will act as a Pilot and advise the Captain and his crew on how to navigate and maneuver, but is normally also authorized to terminate the operation if the safety is at risk.

This paper systematizes and proposes a model of the decision-making process in STS lightering operations. Challenges and weaknesses in the process are highlighted. Proposals for GUI displays that are aimed to enhance the operational safety in STS approaches have been made.

Fig. 1. The approach phase of a lightering operation; the Constant Heading Ship is underway at minimum controllable speed. (Courtesy of SPT Ltd.)

Fig. 2. The principle of an approach maneuver in STS lightering, [2].

II. Decision-Making in STS Lightering

The process of taking a decision is carried out by the Mooring Master and/or the Captains of the ships. They mainly use the radar as a guidance navigation system, in addition to visual observation when the two ships are getting close. The speed and course can be controlled and monitored, but the relative movement and angle between the ships are more difficult to deduct from the information supplied by the standard navigation systems onboard. The decision-making process has input from human factors, technical guidance systems and user interfaces.

Decision-making in navigation can be regarded as an outcome of mental cognitive processes, leading to the selection of a course of action among several alternatives. Every decision-making process produces a final choice. The output can be an action or an opinion of choice. From a cognitive perspective, the decision-making process must be regarded as a continuous process integrated in the interaction with the environment. It may also be regarded as a problem-solving activity which is terminated when a satisfactory solution is found. Therefore, decision-making is a reasoning process which can be rational or irrational, [3].

In STS lightering the input from navigational systems on the bridge to the decision maker, (e.g. the Mooring Master), comes in digits of characters as position, course, speed, bearing and range. Both the characters itself and the degree of changes in the characters give information to the decision maker. In some cases, the relative change in the character is more important than reading of the character, e.g. a high degree of change in the course, and therefore a high rate-of-turn (ROT), communicate the fact that the ship is turning around its vertical axis (yawing) which is a momentum that takes time to reduce and stop on a ship of this size. An STS lightering operation is an excellent example where ROT can be more important than the actual course itself.

The six steps of this natural and intuitive decision-making process which is used in navigational training and practice are:

After Step 5, Implementation, the evaluation brings the cycle back to Step 1 again. Thus, the decision-making process can be regarded as a continuous loop.

In Fig. 3 the perception in STS lightering operation is visualized. This is, in general, not different from the perception in normal navigation at sea. The whole task is influenced by the person’s experience which is identified in the bottom layer of the figure. Above the experience is the situation awareness which is defined as “…the perception of the elements of the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future." [4].

In STS lightering operations the main elements are results from visual and instrumental observations which are collected through a selective process in which it is possible to imagine situational awareness and the experience as filters influencing what to select. Information obtained through communication is also filtered in the same way, along with the necessary information from regulations, procedures and operation manuals.

[5] describes the naturalistic decision-making process as recognition primed, and states; “In this process, the experience and knowledge of the decision makers sub-consciously primes an appropriate response to a recognized problem. This occurs without any conscious optimization of the solution.” The practical experience build-up during several years of conducting lightering operations is partly conscious and partly not. This can be observed when Mooring Masters, pilots and other navigators are asked how a procedure, in a navigation task, is put into practice. It is much easier for them to describe what they do, than describe what they are thinking when they are doing it. In some literature this human characteristic is named tacit or implied knowledge. The original tacit knowledge held by individuals is unique to them, a product of their whole experience, and not a direct source of generalizable knowledge, [6].

Fig. 3. Perception as first stage in the decision-making process.

This knowledge is an important element of controlling a lightering operation. Essentially, decision performance reflects the decision-maker’s level of situational awareness. In comparing different situations, the decision-maker has to gather necessary key-information to compare, and take into consideration, the influence of the different environmental forces on the vessels. The information is processed unconsciously and is selected and shaped by the experience and the situational awareness. Included in the experience are the person’s knowledge, training foundation and attitude. The output on this stage of the process is the selected information needed to solve the actual task, [7].

The next stage in the decision-making process is to do a comparison of the perceptual situation with the pictures of memorized experience. Similar situations, or close to similar situations, can be compared with a ‘database’ of earlier experienced situations and a memory of what control measures used at that time. (Input from regulations, procedures and operation manuals can also occur on this stage.)

In order to be able to make a decision, or even to set up the necessary goals in navigation, a spatial (3D) awareness needs to be present to navigate and maneuver. In STS lightering operations, as in most navigation operations, the decision-making process can be seen as an embodied reasoning system. The most important instrument for a navigator is the eye measuring height, width and depth. In combination with the cognitive unconscious, experienced memory, which is only possible through the embodiment of the mind, [8], the navigator experiences the spatial awareness. The experienced way of moving in time and space from a known position A to a destination B demands a spatial prediction or a spatial feeling present in the navigator’s control. In one way or another during a lightering operation the Mooring Master has to imagine “before his inner eye” where the two ships will be in the future, [9].

Fig. 4 illustrates how the spatial prediction is a part of the spatial awareness. Alternative solutions are outcome of the comparison, but spatial prediction is required to shape the final decision. In the example describing Steps 1 ~ 6 the ship was yawing to port with an alternative to use rudder to stop the movement.

Gathering all the necessary information in the present (and history) will together with the experience give the navigator an “a priori” solution where both ships will be in the future, with some degree of accuracy. A spatial prediction is possible if there is enough information and experience available. A good navigator will calibrate his spatial prediction process like a “human Kalman filter” by improvising and adjust the control forces to meet the changing environmental forces. This is especially necessary in the final approach where the two ships are so close that asymmetrical hydrodynamic forces (interaction) are experienced by both ships.

Fig. 4. In the final stage of the process the decision is made under influence of the spatial presence.

The complete decision-making process is illustrated in Fig. 5. Shaped like a flow diagram the process moves in steps from the top where several input sources are available. The decision-maker chooses what to observe and selects the information needed. This information is compared with other similar experienced situations to try to discover differences and similarities. The results from this comparison are different solutions, which through the navigator’s spatial understanding and prediction give us a final decision. This decision is implemented and then the navigator is observing how the vessel is moving. The process is the same for a small correction in speed or course as for a larger maneuver with significant changes in propulsion and rudder.

One important factor and dimension that needs to be under control is the time. The time-factor is acting from an observation starts to a decision is made and is important for all navigation. Time is needed to observe, select, compare, decide and implement. It can be seen as several parallel circular processes acting on, or at, different elements, but also moving beyond or outside the decision-making process itself. The time-factor is present in every step of the process as shown to the right in Fig. 5. In the last part of the decision-making process the decision is more directly connected to the time factor and the navigator’s capability to predict the position as a function of time. This is the navigator’s spatial awareness and spatial prediction.

Lack of control of some of these different time factors will by navigators often be referred to as “bad stomach feeling” and can reduce the decision-makers performance, e.g. stress. To be in control, the Mooring Master needs to be a step ahead of the tasks expected of him and he must be capable of doing several decision-making processes simultaneously.

Fig. 5. The complete decision-making cycle: The situational awareness is in the first part while spatial awareness is in the last part of the process. The experience is acting through the whole process.

In a lightering operation where one ship is navigating and maneuvering up and alongside another ship, the most important parameters are the relative range, distance and angle and the respective time derivatives. The Mooring Master observes the radar for information about range and bearing from own ship, Fig. 6. The ARPA and the Automatic Identification System (AIS) provides information about the other ship’s speed, position and course. The AIS information may be presented on an Electronic Chart and Information Display System (ECDIS) where the Mooring Master also collects information about the surrounding waters. In the final phase of the approach the relative movement and the angle between the ships are deducted through visually observations only in the present practice.

The Mooring Master selects necessary information by using his situational awareness and compare this with similar experienced situations, education, training and standing procedures. The ships involved have a great mass displacement and momentum and the Mooring Master’s spatial awareness and spatial prediction need to be shaped through operating experience. The spatial experience enable to “see the ships movement before his inner eye”, i.e. to see where the ships will be in the future as a result of the actions being made at present. Then, the Mooring Master will make the decision.

Fig. 6. Radar image from the final approach phase, [10].

III. Challenges and Weaknesses in the Decision-Making Process

To navigate and maneuver with limited maneuverable tankers and moor together in open waters demand a high level of mental and problem solving activities. The task needs to be analyzed, prioritized, chosen and decided. Activities on this level have a large consequence in case of error and “the process can be especially prone if a new element shows up and the task becomes non-routine,” [11]. This new element can be changes in the environmental conditions such as wind, swell or current. Such changes need improvisation and can therefore be a demanding mental activity which is capable of putting the safety of operation in danger resulting in cancellations and delays.

The human as a decision-maker, under the influence of several different factors as stress, fatigue, noise etc., has a variation in the performance which, at one point in time, can be on the lower level of the performance scale. Low performance is often called “human error”, but some cognitive systems’ engineers states that this must be seen as a natural variation in the human performance, [12]. For an operator like a Mooring Master it may be difficult to relate to the level of its own performance. Frequently training is necessary to gain and maintain a high level of performance. Motivation and attitude are key factors.

Knowledge, system insight and long experience are a fundament to achieve a robust decision process that can result in prediction of the ships position several minutes into the future with only minor adjustments in the control system. This can be compared with an automatic control system based on inertial navigation which can be vulnerable for even a small error in the measurement of the velocity. Even an input error in the velocity in some cm/s can result in a significant error in the position estimation for the ships in the future, [13].

IV. Guidance and Decision-Support Displays

A. Requirements

Real-time information of the relative movement of the ships can improve the decision-making process. The input from a dedicated decision-support system [10] goes fast through the steps in the process and does not need to be selected or filtered. Key features that must be satisfied are:

Radar will remain the main support system for the Mooring Master in the initial phase of an STS approach. However, for distances below about half a nautical mile separation the radar system will decrease in performance. From an operational safety viewpoint it is obvious that a dedicated decision-support system must give input to the decision-maker in terms of range and bearing so the same way of thinking throughout the operation can be maintained. It is mentally difficult for a navigator to switch to different mediums and parameters during one operation, [9].

A feasible technology to provide relative positioning data with sufficient accuracies (~1 cm/s for speeds) could be the Velocity Information GPS (VI-GPS), [14]. Two GPS receivers on each ship are needed at the bow and stern, respectively, in order to enable a good relative measurement of the movement. VI-GPS can, with an additional (third) receiver, provide information of the roll motion as well which, in swell seas, can be critical in order to avoid steel-to-steel contact.

A system based on VI-GPS technology can provide information about the relative speed as well as bearing and range to reference points at the bow and stern. This enables that course and rate-of-turn (ROT) of the Constant Heading Ship can be monitored in real time and not just through VHF communication or AIS which have time delays. The graphical presentation can then provide a direct input to the Mooring Master that strengthen the situation awareness and enables correct spatial feeling from observing the ships movement on a screen.

The Mooring Master needs more precise information about how the ships are moving when the distance is less than ½ nautical mile, i.e. range, bearing, relative speeds in the longitudinal and transverse directions, opening angle between the ships.

In the final approach phase, when the ships are side-by-side and separated a distance about the ship’s breadth, the Mooring Master is located on the bridge wing and the modus in the decision-making process changes to ‘near field’ in which receiving information by a hand-held device would be more suitable. Information about ranges, relative longitudinal movement and the speed vectors are also to be presented on this display mode.

B. Usability Study

Mock-ups of different proposals of support displays have been made for the purpose of user surveys. These took place at the Ship Maneuvering Simulator Center (SMS) in Norway with experienced Mooring Masters throughout 2009. Different types of information and graphical presentations were discussed. Separate meetings were held with experienced simulator instructors at the SMS training facility with a focus on the decision-making process.

The user surveys included open and closed type of questions of which the former varied from the description of the daily work on board and during the journey, to the next operation and how the users considered the observations they received and how they made a decision on this basis, [15].

Fig. 7. View from the Service Ship during the approach phase. (Courtesy of the Ship Maneuvering Simulator Center AS.)

C. Proposed GUIs

Figs. 8 – 12 show the various ‘bird eye view’ displays that have been proposed based on the outcome of the usability study. The ships are to be in correct scale with respect to the length-to-breadth ratio. It should be noted that some of the displayed information assumes that the VI-GPS system [14] is implemented with GPS receivers on both ships as well as a direct communication line for data transfer. Fig. 12 shows a second alternative of a graphical presentation.

The rudder angles are displayed with colors (red to port and green to starboard) and digits to make it easier to perceive the information. The relative movements are all labeled with an arrow and digits. The amount is also indicated by the thickness of the arrow and the color around it. Two decimals have to be presented on all speeds, at least when the ships are closing in. Opposite relative movements will have different colors.

The numerical information is linked to the particular ship through having the same color as well as placement on the corresponding side of the display. The pitch, roll and heave information have appropriate units.

Fig. 8. GUI Alternative 1: Display in the final approach phase.

As the ships are closing in during the final approach, the information about the relative movement can be placed on a top layer with transparent background so the graphical movement of the ships can still be observed, Fig. 9.

The handheld device must have a switch or a sensor that enables day/night modus. Colors like light green and yellow are easy to spot in the darkness, Fig. 10. Light blue colors must, however, be avoided because their frequency will destroy the night sight. The light purple color between the stern of the ships can be changed.

The graphics of the ships can be small in the initial phase of the approach. It is then only needed to have two relative measurements ahead and abeam of the service ship. These are marked with arrows on green and orange backgrounds, Fig. 11.

The 2nd alternative (Fig. 12) of graphical display presentation has a grid in the bottom that gives the distance between the ships and a longer arrow, representing the course, to provide a better indication of the relative heading. The speed and course of each ship is presented at the origin and tip of the course-arrow, respectively. The relative distances are marked with dots and digits. This has to be presented on top of the ships when they are closing in. The relative movements (abeam, forward and stern) are indicated with arrows which increase in size when the relative speeds are increasing.

Fig. 9. GUI Alternative 1: Display when the ships are closing in.

Fig. 10. GUI Alternative 1: Night vision display.

Fig. 11. GUI Alternative 1: Display when the ships are in the initial phase.

Fig. 12. GUI Alternative 2: Display when vessels are closing in.

V. Concluding Remarks

A step model of the decision-making process in STS lightering operations has been proposed. Situation and spatial awareness are platforms for the process in which the sequential steps are; observe, select, compare alternatives, spatial prediction, decision and the implementation. The time-factor associated with the decision-making process is critical to navigators and Mooring Masters as each step needs a minimum time of operation or reasoning.

The Mooring Master as a decision-maker in STS lightering needs to be one step ahead of the situation to maintain control. Difficulties in measuring the relative range, movement, angle as well as the variation of the human performance, are possible weaknesses of the decision-making process.

The key information needed by the Mooring Master has been revealed through usability studies. Two alternative graphical presentations are proposed and will be implemented in a tailor-made decision-support system for STS operations on a full-mission ship manoeuvring simulator. Test programmes with experienced Mooring Masters and non-experienced navigators will be carried out to establish the overall improvement ratio by introducing a decision-support system in STS lightering.


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About the Authors

E. Pedersen is Professor of Marine Technology at the Norwegian University of Science and Technology and part-time Professor of Nautical Science at the University of Tromsø.

He is a fully trained merchant ship navigational officer and holds a doctoral degree in Nautical Science from the Norwegian University of Science and Technology. His practical marine experience includes various positions on board ocean going fishing vessels and marine seismic surveying vessels.

Prof. Pedersen has international engineering experience from the upstream oil service industry and has spent several years in Japan as post-doc researcher to National Maritime Research Institute, Kobe University and Marine Technical College. He has also been a visiting Professor to Tokyo University of Marine Science and Technology.

Prof. Pedersen’s research interests are at present on ship-to-ship lightering operations, maritime decision-support systems and recovery of spills to the environment in Arctic waters.

D. Husjord is PhD student in Nautical Science at the Norwegian University of Science and Technology and Assistant Professor (on leave) in Nautical Science at the University of Tromsø.

He is educated as a ship navigation officer and holds a degree in Nautical Science at Masters level from the Norwegian University of Science and Technology. He has served several years onboard merchant ships in senior officer positions.

Husjord’s research is related to the development of a guidance and decision-support system for ship-to-ship operations. He also investigates the safety potential in different navigational systems with respect to improve operational safety at sea.