Ship Capability Assessment

Ship capability in this sense is the ability of the vessel to perform its intended tasks in a defined wave environment without exceeding predefined motion criteria limits.  It is often defined in terms of percentage operability or operational effectiveness.

To calculate the operability it is essential to know the vessel’s operating route and understand its type of operation so that the appropriate wave statistics and motion criteria limits are selected.

The vessel’s capability can only be rationally assessed against predetermined criteria limits calculated at relevant ship board locations.  They include all motion types described in the section Motion Analysis, such as MSI and MII.

However, these performance criteria are now assigned a value that defines an acceptable level for human habitation and the vessel’s operation in waves.  These limiting criteria values are independent of the wave environment.  Table 1 below shows some of the criteria that might be applicable to various ship subsystems, whilst table 2 shows typical comfort criteria levels appropriate to various vessel operations and personnel.

Table 1:  Common criteria applied to different ship subsystems
(Background Source: Marintek)
 

 

Table 2: Suggested comfort criteria limiting levels for passengers and crew
(Background Source: Marintek)
 
 

The wave scatter diagram usually reflects the wave statistics for the route on which the vessel is to operate.  Detailed wave scatter diagrams are not always readily available for every part of the world’s oceans but this data is improving through extensive wave monitoring programs and mathematical models.  These statistics will vary with each day, month, season or even year.  Therefore, where possible it is preferable to take an averaged three element data record for a year of wave height, period and direction.  This level of detail allows the wave statistics to be applied specifically to a vessel’s operating route.  Should this information not be available or economically feasible to obtain, other options remain such as using a more generic scatter diagram or conducting a different type of analysis such as limiting sea state analysis.  This can also be performed whilst taking into account the wave spectrum, wave heading and ship speed.

Conducting a vessel performance assessment is well suited to design tasks related to tender submissions, new vessel selection, route analysis, benchmarking and model testing programs.  This may be required to satisfy a client, a design process or for regulatory requirements.

This type of task might involve reporting of one or more of the following types of analysis

  1. percentage operability or operational effectiveness;
  2. limiting wave height boundaries;
  3. limiting sea state, or
  4. motion criteria response assessment based on specific motion characteristics eg. MSI, MII. 

 Limiting wave height analysis is about defining the maximum wave heights in which a vessel can operate without exceeding any given criteria. The information generated from this type of analysis is often too much for general reporting but can be useful for highlighting particular problem areas of a vessel's seakeeping characteristics. 

Operational Effectiveness is about defining the ability of the ship to go to sea and accomplish its mission tasks in whatever weather conditions may prevail at the time. Through computational procedures the operational effectiveness of a vessel is expressed as the proportion of time the ship can successfully accomplish its mission tasks in a given combination of sea areas and seasons. Whether a task is considered to have been accomplished depends on the criteria limits that define it. Therefore, the proportion of time the vessel can operate without exceeding these criteria limits defines its Operational Effectiveness or Operability where a score of unity or 100% means a vessel is capable at all times of fulfilling its tasks. The probability for this event is defined by the variables that contribute being the proportion of time the ship spends in a given season, sea area, wave direction, wave period and wave height at a given speed and course or combinations thereof.

Operability

The overall process to obtain an operability comprises all aspects previously described and are presented in the figure below.

 (Background Source: Marintek)