Factors Influencing Causation Probability

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As seen from the Bayesian Network analysis of the ship-ship collision, Section 5.2.3 above, it is indeed possible to accurately model the causation probability. It is, however, very important that level of detail in the model is at a satisfactory level such that the results of the model becomes plausible. In this chapter we list some of the factors that influence the causation probability.

Reported causes for grounding and collision

Several researchers have published reports on causes for marine accidents. All studies define that the cause of a grounding or collision may be summarised crudely into the following four main groups:

  • 1. Due to failure in manoeuvring, including inaccurate positioning and poor lookout.
  • 2. Due to incapacitation of personnel such as doze, drunkenness engaged in other tasks and sudden illness. Doze has been identified as one of the main causes for grounding.
  • 3. Due to technical problems with engine, steering gear, or navigational instruments.
  • 4. Due to environmental causes, such as visibility, wind, or waves.

Group 1 and 2 in the list above represent the contribution from human errors. Unquestionable, human error is an important cause to navigational accidents – perhaps dominant, as it is quoted that human errors account for at least 80% of all accidents. (Some researchers even argue that 100% of all accidents are due to human error, since poor man-machine interface, failure of instrumentation (should have been checked more properly), under design, etc. all may be attributed as the result of some sort of human error. Any design is the consequence of human decisions.)

More precisely it could be stated that approximately 80% of navigational accidents involves at least some human errors or questionable judgements rounded in organisational factors. What complicates the assessment is that the blame (or cause) for an accident can be allocated in different ways according to the perspective of the investigator. Typically a serious accidents start from basic human errors but the seriousness of the accident is rather a compound of a set of technical failure, operators’ error, fundamental design errors, and management errors.

Therefore, any realistic modelling must provide a detailed representation of human error in order to be successful. Unfortunately, the human error mechanisms differ from technical or environmental cause (viz. the remaining 20%), and are – in fact – not yet well understood. A major problem in this respect is that there exists no such thing as a recipe for doing a specific task in the right way (e.g. performing a turn). In an examination of a series of manoeuvring simulation that have led to a grounding accident, Thau [29] found that the primary human error leading to the accident often occurred more than 10 minutes prior to the accident. Contrary, technical or environmental causes are generally simpler to model and understand.

Human and Organisational Errors

Human errors can be described as actions taken by individuals that can lead an activity (design, construction, and operation) to realise a quality lower than intended. Human errors also include actions not taken, as these also may lead an activity to realise a quality lower than intended. Many people typically think of human error as “operator error” or “cockpit error”, in which the operator makes a slip or mistake due to misperception, faulty reasoning, inattention, or debilitating attributes such as sickness, drugs, or fatigue.

However, there are many other important sources of human error. These includes factors such as management policies which pressure shipmasters to stay on schedule at all costs, poor equipment design which impedes the operator’s ability to perform a task, improper or lack of maintenance, improper or lack of training, and inadequate number of crew to perform a task.

The human error factors range from those of judgement to ignorance, folly, and mischief. Inadequate training is the primary contributor to many of the past failures in marine structures. Also boredom has played a major role in many accidents. Based on a study by Bea [1] of human error factors in marine engineering the following primary factors were identified:

20090405 Fig Causation Factors1.jpg

Organisation errors are a departure from acceptable or desirable practice on the part of a group of individuals that results in unacceptable or undesirable results. Primary organisational error factors includes, [1]:

20090405 Fig Causation Factors2.jpg

For example, the goals set by the organisation may lead rational individuals to conduct certain operations in manner that the corporate management would not approve if they were aware of their reliability implications. Similarly, corporate management, under pressures to reduce costs and maintain schedules, may not provide the necessary resources required allowing adequately safe operations.

Other types of organisation and management procedure that affect the system reliability include, for example, parallel processing such as developing design criteria at the same time as the structure is being designed – a procedure that may not be appropriate in economic terms according to the costs and uncertainties.

Human error evaluation

To date, four methodologies have been developed or adapted for maritime use. These are:

  • 1. The operator function model (OFM) type of task analysis
  • 2. Cognitive task analysis
  • 3. Skill assessment
  • 4. Error analysis

The OFM task analysis, developed in1986 by Mitchell and Miller, see Rasmussen [26], provides a breakdown of a function (such as avoiding collisions with neighbouring vessels) into the tasks that must be performed. This also includes the information needed to perform each task, and the decisions that direct the sequence of tasks. This type of task description is independent of the automation; that is, the same tasks, information, and decisions are required, regardless of whether they are performed by a human or by a machine. For example, in collision avoidance, other vessels must be detected, their relative motions analysed to determine whether there is a threat of collision, and a decision made regarding how to change own ship’s course or speed in order to avoid a potential collision. These tasks must be performed regardless of who (human or machine) executes them.

The cognitive task analysis method extends the OFM by considering the mental demands that would be placed on a human operator while performing tasks. For example, in order for a human to detect a new ship as soon as it appears, vigilance (sustained attention) and discrimination (the ability to spot a target against the background) are required. The mental demands of analysing the relative motion of the target vessel include plotting a series of target ranges (distance) and bearings (its angular position relative to own ship) and evaluating the ratio of change over time. Hollnagel [14] introduced a task transaction vocabulary that categorises mental demands, such as “search”, “detect”, “code”, “interpret”, and “decide/select”.

Assigning the appropriate OFM tasks to humans or machines can thereby represent different levels of automation. Then the cognitive impact of automation can be identified by comparing the number and types of cognitive demands placed on the human operator under the different levels of automation. For example, Froese et al. [4] found that when collision avoidance by manual methods was compared to the use of ARPA radar, then virtually all of the computational demands of the manual method had been eliminated through automation.

In order to evaluate the impact of automation on training requirements, a skill assessment technique was developed at US Coast Guard [30] by combining the OFM and cognitive task analyses with the Knowledge, Skills, and Abilities (KSA) analysis. The skill assessment is performed by taking each cognitive task (from the OFM/cognitive task analysis) and determining what types of knowledge or skill that is required for the proper performance of a task.

The hybrid analysis thereby focuses the knowledge and skill assessment on the task level. For example, when comparing the manual task in collision avoidance of plotting target range and bearing to the automated scenario that displays target information on the ARPA, then the basic knowledge requirements of collision avoidance do not change with automation. However, the procedural requirements change radically. That is, the mariner has to understand the theory behind collision avoidance regardless of the level of automation, but the specific set of procedural knowledge and skills the mariner needs is dependent on the level and type of automation.

Application of the described skill assessment technique has allowed both US Coast Guard [30] and Schraagen et al. [28] to distinguish changes in skill level as a result of automation.

The studies by Froese et al. [4] and by Scraagen et al. [28] concludes that the way an automated system is designed can also affect the mariner’s performance. Some automation “hides” information from the mariner, presenting only what the designer thought was needed. Unfortunately, many system designers do not fully understand the user’s task, and consequently we end up with less-than-perfect, error inducing designs.

By studying the types of errors commonly made by operators, and by understanding the ramifications of these errors (i.e., are they just nuisance errors or can they cause an accident?), important information is gained that further can be used in training and system redesign. Both error analyses adopted in [4] and [28] consisted of interviewing mariners and instructors, and observing the use of automation during routine shipboard operations.

Aspects that the risk analysis should include

When considering a risk analysis aiming at estimating the causation probability system knowledge is important.

First and most important – before system knowledge is applied – is a clear and unique definition of the purpose, extent and boundaries of the risk analysis.

Having clearly formulated the purpose, extent and boundaries of the risk analysis, the subsequent subsections discuss aspects of the system knowledge that becomes relevant when formulating the risk model for estimating the causation probability. In broad terms the system knowledge relates to:

  • Configuration of the considered navigational area
  • Composition of the ship traffic in the area
  • Environmental conditions, such as weather, visibility, current, etc.
  • Configuration of considered vessel, such as main particulars, manoeuvrability, bridge layout and procedures.

Today, many ships have periodically unmanned engine rooms connected by computerised alarm systems to the bridge. Further, microcomputers for accounting, general record-keeping, and e-mail to land-based operations, automated satellite positioning systems (e.g., the global positioning system or GPS), navigation and collision-avoidance systems like electronic charts (ECDIS) and automated radar plotting aids (ARPA). With this boom in technology comes the concern that not all mariners understand how to use the automation effectively and safely.

Indeed, there have been several “automation-assisted” accidents in recent years in which otherwise experienced mariners either did not know how to use the automated system or had trouble using it because of poor system design, Rothblum and Carvalhais [27]. The related human error modelling is best analysed using the cognitive task analysis. In a subsequent subsection the technical aspects of the different electronic systems is described.

Configuration of navigational area

System knowledge of the configuration of the navigational area concerns the arrangement of the route in the vicinity of the considered area and identification of all difficulties in following the route before the considered location. Routes in the considered region that crosses the route prior to the considered location may have influence on the navigational safety and may thus indirectly have influence on faults at the considered location.

The navigational markings, such as type of buoys that constitutes the routing system, must be identified. Further, presence and configuration of VTS system in the area as well as requirements for having pilot on board is part of the routing system. The purpose of the routing system is to improve the safety of navigation in converging areas and in areas where the density of traffic is great or where freedom of movement of shipping is inhibited by restricted sea-room, the existence of obstructions to navigation, limited depths and unfavourable meteorological conditions. This subsection describes some relevant aspects of the routing system.

Navigational route, markings, aids, and restrictions

Traffic lane - An area within defined limits in which one-way traffic is established. Natural obstacles, including those forming separation zones, may constitute a boundary.

Traffic Separation Scheme - A routing measure aimed at the separation of opposing streams of traffic by appropriate means and by establishment of traffic lanes.

Separation zone and lines - A zone or line separating the traffic lanes in which ships are proceeding in opposite or nearly opposite directions; or separating a traffic lane from the adjacent sea area; or separating traffic lanes designated for particular classes of ship proceeding in the same direction.

Inshore traffic zone - A routing measure comprising a designated area between the landward boundary of a traffic separation scheme and the adjacent coast, to be used in accordance with the provision of amendment to International Regulations for Preventing Collision at Sea, 1972 (Collision Regulations).

Deep-water route - A route within defined limits that have been accurately surveyed for clearance of sea bottom and submerged obstacles as indicated on the charts.

Precautionary areas - A routing measure comprising an area within defined limits where ships must navigate with particular caution and within which the direction of traffic flow may be recommended.

Navigational complications

Complications that may impinge on the operational safety, e.g. bridges, multiple routes, crossing traffic, etc.

Local and regional bathymetry

Have an influence on the vessel sizes that are able to operate in the area – or collide with specific obstacles. The distance from the route to the ground affects Pc for both collision and grounding.

VTS system

A VTS system is typically present in areas of high navigational complexity where an accurate monitoring or guidance of the vessels in the area is of importance. Typical such areas may be near location of large bridges, areas with high rate of icebergs, or highly trafficked areas. The main effect of a VTS system, for a ship in contact with the VTS system, will be on the selection of route and distribution of ships across the routes. Reportedly, Olsen et al. [22] found that the effect of the presence of a VTS system might reduce the causation probability for ship-bridge collisions by a factor of 2 to 3.

VTS systems may consist of the following equipment in different configurations, Olsen et al. [22]:

  • Radar installations
  • VHF radio and VHF direction finder
  • Closed Circuit Television
  • Infrared Television
  • Presence of a guard ship

A VTS system consisting of only radar, VHF radio and VHF direction finder constitutes the basic system. Closed Circuit Television and Infrared Television are additional equipment. In some areas a guard ship may be attached to the VTS system.

Ships participating in the VTS system must – if mandatory hen entering the VTS area – report to the VTS centre via the VHF radio. Local authorities define the requirement to the ship sizes that should participate in the VTS system. According to the IMO regulation it is mandatory for vessels above 300 GRT to have VHF radio onboard.

Some of the benefits of a VTS system is that the radar can detect navigational errors and thereby be corrected via the VHF communication. For ships violating the navigational regulations for the area, attempts can be made to establish contact with these over the VHF radio. Presence of Closed Circuit Television or Infrared Television allows for an improved surveillance of the navigation in the approach channels, for instance detecting a ship that omits to turn at a sea buoy or navigates of the channel. Presence of a guard ship may be able to help wandering vessels or like. This, of course, is highly dependent on the location of the guard ship and on the weather conditions.

The degree of vessel participating in the VTS system varies considerably for different locations and is highly dependent on the presence of identifiable hazards in the waterway (e.g. fishing boats, icebergs, bridges, etc.). Presence of identifiable hazards increases the degree of participation. It should be noted that ship owners might obtain lower insurance premiums if their vessels participate in local VTS systems. This aspect therefore presents incitement to participate in the VTS system. The following probabilities of a vessel not participating in the VTS system have been extracted from [22].

20090405 Fig Causation Factors3.jpg

Moreover, in the event of a vessel not reporting to the VTS system, then almost all (>99%, [22]) vessels respond to a direct call if the VTS system broadcast position, speed and course of the vessel. Some vessels, however, have proved impossible to contact by VHF or from a guard ship.

When receiving an advice by VHF from the VTS centre, Olsen et al. [22] also reports that an average of 90% to 95% comply with the VTS advice. It is noted that the compliance is dependent on the nature of the advice and on the credibility of the system with the mariner. Local conditions near the vessel and unknown to the VTS may prevent the ship operator from following the advice.

Requirements for pilot on board

In some navigational areas it is required that vessels above a specific size must take a pilot on board. Aspect that must be addressed relates to how well does the pilot inform the master of the vessel of navigational plans? What are standard procedures? Are there requirements to the pilot of specific knowledge of the manoeuvrability of the vessel? Etc.

Composition of ship traffic

The vessels that operate at general international routes range from traditional sailing ships, leisure crafts and fishing vessels (whose courses are unpredictable) to large tankers that are confined to deep-water routes only. The large diversity in the vessel traffic composition must be taken properly into account. This concerns bulk carriers and tankers in ballast having poor manoeuvrability; container ships with high cruising speed, hard pressed to arrive at their designated terminals just in time. Smaller petroleum, chemical and gas tankers feeding depots around the region, tow-boats and barges requiring plenty of sea-room to manoeuvre, and passenger ferries crossing the considered operational route.

Among the shipmasters of these vessels there is a wide variance in the interpretation of safety and the choice of accepting a particular standard, which varies from criteria used, the circumstances and in most cases opinion.

In gathering information on the ship traffic, focus will normally be on the commercial traffic, since these always will represent the primary threat to the navigational area.

Leisure traffic and local fishing activity, however, can disturb the commercial traffic and thus be a source of errors. The extent and pattern of this type of traffic should be quantified. Type, size, and frequency of vessels operating in the area should be registered. When combined with information of the configuration of the navigational area this information provides guidance of the possibility for performing evasive manoeuvres. In essence, more ships mean more risks!

For long-term design purposes forecasting of traffic intensity and composition is important. In this respect local bathymetry provides guidance for limiting vessel sizes, at least with respect to draft.

Environmental Conditions

The annual conditions for:

  • Weather condition,
  • wind variations, cross wind and in sailing direction
  • waves,
  • visibility (fog, precipitation)
  • current variations, cross current and in sailing direction
  • ice conditions

Major parts of these aspects were addressed in Friis Hansen and Pedersen [10].

Configuration of Considered Vessel

Aspects that should be described

  • Vessel type and particulars: speed, profile.
  • Manoeuvrability of considered vessel
  • Layout of Man-Machine interface
  • Number of officers on the bridge
  • Instrumentation: ARPA, ECDIS, GPS, collision avoidance, and track keeping, etc.

In the last few years, the Electronic Chart Display and Information System (ECDIS) has emerged as a powerful addition to the modern bridge. ECDIS offers the possibility for major changes in the navigation process and improves the safety and efficiency of maritime operations. By superimposing three items: a chart, the ship's real-time position, and radar on one display, ECDIS has the potential to improve the accuracy of navigation, increase awareness of dangerous conditions, and reduce the mariner's workload.

At US Coast Guard, [30], the potential effects of these systems on bridge operations were examined, using the controlled conditions possible on a full-mission simulator. Four issues were examined: the potential of ECDIS to contribute to navigational precision, its potential to reduce navigation workload, the chart features and navigation functions required by the mariner, and the potential contribution of the integration of radar features on ECDIS. The results provided support to the U.S. position on the International Maritime Organization's (IMO) Standards for ECDIS and recommendations for future system design and for the incorporation of the system in bridge operations, [30].

20090405 Fig Causation Factors4.jpg

Figure 16 Illustration of an ECDIS display, from [30].

Other relevant concerns relates to:

  • Navigational procedures and practice: voyage planning, pre-planning of actions and procedures in the event of evasive manoeuvres. Communication on the bridge
  • Human failures:
    • No action: absence, present but not attentive, attentive but problem not realised
    • Unintended wrong action: situation misunderstood, wrong action chosen, communication problems
    • Intended wrong action: navigational basis (charts) not updated, confusions of buoys and/or landmarks, manoeuvring capabilities overestimated, clearance requirements underestimated (relevant for ship-bridge collisions)
  • Technical failures:
    • Loss of propulsion
    • Steering system failures
    • Radar failure
    • GPS failure

Influence of the effects of automation (ARPA and ECDIS) on navigational functions:

  • voyage planning,
  • collision avoidance, and
  • track keeping.

What is the management attitude towards level of detail in voyage planning? Concerns should also be given on how training may affect the situation? Changes in training: less on computation and more on interpretation is needed given the wide usage of ARPA.

At USCG [30] the skills assessment and error analysis techniques identified several important types of skill and knowledge that were not fully covered in current internationally recommended training course objectives for ARPA. These same techniques also allowed the development of training course objectives for ECDIS, a relatively new piece of equipment for which no formal training courses exists.

In Froese et al. [4] and in Schraagen et al. [28] the cognitive task analysis and error analysis also proved valuable in identifying aspects of the user interface and equipment functionality which were inconsistent with the needs of the crew in the performance of the automated tasks. Taken together, these tools provide a powerful and comprehensive method of identifying the impact of automation on task and training requirements.