Sub Project 4 - Performance in a Seaway

Introduction

Ships are traditionally optimized for operation in calm water, at design draught. Wind and waves are important for the operation of the ships, and might mean that the ships should be designed differently to be optimal in their actual operation. In addition comes the need to design for safety and operability in really harsh weather conditions. Both purposes require methods for reliable prediction of the performance in a seaway. To predict the powering performance of ships in a seaway, one need to predict the added resistance due to wind and waves, and the change of propulsive efficiency. Furthermore, the prediction methods must be computationally efficient, something that currently rules out complete RANS CFD simulations. With respect to computation of added resistance, the main shortcomings of existing practical computational methods are:

• following seas,
• effect of above-water geometry,
• accuracy in general
• added resistance due to steering and manoeuvring

With respect to prediction of speed loss and power increase, in addition to predicting the added resistance, main challenges are related to:

• Voluntary speed reduction – criteria and typical practice
• Effect of use of rudder, related to course-keeping, which makes it difficult to predict.
• Change of propulsive efficiency – the effect of change of propulsion point depends on added resistance and is well-known, but the effect on hull interaction factors like thrust deduction and wake is less well known.
• Propeller-engine interaction, impacting both propeller efficiency and engine specific fuel consumption

There is a general need for end-to-end validation of speed loss and power increase due to wind and waves, since there are so many inter-related effects, each of them being quite complicated.
Currently, speed loss due to wind and waves is accounted for in the design by use of a sea margin on power, which is routinely set to 15%. However, the trend to make ships with less power in order to gain a good EEDI ranking probably means that the sea margin should be set higher. Effectively the same problem is related to predicting expected sea margin in slow steaming operation (Eco and SuperEco). Thus, there is a need for establishing methods and guidelines for rational determination of sea margin.

Objective:

• To obtain full scale data for validation of computational methods
• Learn more about factors that are important for speed loss and added power due to operation in waves
• Validation of computational methods for speed loss and power increase due to wind and waves
• Review the current practice with respect to sea margin, and propose improved approach
• Investigate the effect of waves and off-design operation on Energy Saving Devices


Tasks

3.1 Full scale monitoring

The objectives are to obtain data for validation of computational methods, and to learn more about factors that are important for speed loss and added power due to operation in waves.
The activity will mainly be based on collection of data from continuous monitoring of ships that are either owned by SMART Maritime participants or where the monitoring system is operated by a SMART Maritime participant. The activity is suited for in-kind work by these industry participants.
What vessels to use and therefore what data variables we can have will have to be determined early, but is currently not known. It is of course beneficial to have as many proper measurements as possible, and it is of interest to also be able to measure fuel consumption accurately. Adding instrumentation to some vessels might turn out to be very beneficial, but this must be done in cooperation with vessel owner and/or operator of the data logging system.
Uncertainty of measurements and not least uncertainty about the operating conditions (wind, waves, current, and ship loading condition) are main obstacles, and have to be dealt with. This is a very challenging topic, but fortunately, there is a lot of research going on in this field, both in other projects at NTNU and MARINTEK and elsewhere.

Wanted contributions from industry partners:

• Data and measurements from ships in operation
• Analysis methods

Expected outcomes:

• Advisory guidelines on practical procedures for instrumentation, logging and checks for the full scale performance monitoring of ships in service
• Quantification of the various sources of uncertainties and expected accuracy with respect to performance monitoring
• Validated methods for obtaining ship performance data from onboard monitoring systems
• Cleaned, validated and anonymized datasets for further research
• Data on performance change due to waves for different ships
• Data cleaning, filtering and analysis routines for processing ship monitoring data

3.2 Validation of computational, model test and hybrid methods for speed loss and power increase

Various existing computational methods for speed loss and power increase due to wind and waves shall be applied to the ships where we collect monitoring data, and comparisons between computations and full scale measurements will be performed. Preferably relevant model test results should be available for the vessels to be investigated so that the computations can be compared to existing model test results. It is also an option to supplement by additional tests with existing models.

It is a point to test different computational methods, and possibly introduce correction factors in the methods, in order to identify the best methods for the different ship types. Hybrid approaches, such as tuning numerical models with more or less simplified model tests will be investigated. The work within this project will be supported by ongoing activities in WP2 – Hull and propeller, related to best practice methodologies for model tests, effect of steering losses and evaluation of new numerical prediction methods.

Wanted contributions from industry partners:

• Input to survey on existing calculation methods
• Performing some calculations
• Supply data and model test results for vessels where ship monitoring is performed
• Supply datasets where model scale investigations of speed loss and power increase due to waves have been performed

Expected outcomes:

• Report on state-of-the-art of prediction of speed loss and power increase due to waves, and how it can be detected from ship monitoring.
• Journal publication on the outcomes of the correlation study

3.3 Prediction of sea margin

Sea margin is a simple way of expressing the need for additional power to maintain speed in typical sea conditions. An accurate prediction of sea margin requires extensive computations of added resistance due to wind and waves, combined with knowledge of the ship route and weather conditions. Therefore, the sea margin is often set to 15%, without any detailed calculations. The objectives of this activity are:

• To make a review of current practice with respect to sea margin
• To propose a rational, but simple calculation scheme for sea margin
• To make example calculations of sea margin for ships operated by project participants, including sea margin at reduced operating speeds.

This activity is planned to be performed by a master student, intern or similar resource, in close cooperation with researchers at NTNU and MARINTEK, as well as the industry partners in the project. It shall be built on the outcomes of the two previously described tasks; 3.1 Full scale monitoring and 3.2 Validation of computational methods.

Wanted contributions from industry partners:

cInformation on sea margin for ships studied in this project
• Information on slow-steaming, ship routes etc.

Expected outcomes:

• Report or thesis containing review, description of calculation method for sea margin, as well as calculation examples from the fleet of SMART Maritime participants


3.4 Effect of waves on Energy Saving Devices

Energy-Saving Devices (ESD) are increasingly used on standard merchant vessels. They are optimized for calm water, and there is an almost complete lack of knowledge of how operation in waves and other off-design conditions impact their effectiveness. Thus, a study of effectiveness of Energy-Saving Devices in waves and off-design conditions (mainly significant deviations in propeller operating point) is of interest. Within the current scope, it is assumed to investigate devices which have a high market share and reputation, i.e. the benefits in calm water are already reliable. The study will have to focus on a limited number of principally different ESDs. The choice of types shall be done in cooperation with the industry partners, currently Becker Mewis Duct and PROMAS are suggested.

Experience from operation of ships with ESD shall be collected as far as possible, but since data from full scale which makes direct comparison of performance with and without relevant ESDs is expected to be rare, if at all accessible, the study can’t rely on this alone.

Therefore, studies of relevant ESDs in waves shall be performed by model testing. The activity relies on the use of existing models, or to be added to new commercial model tests.

The activity aim at providing insight into how waves and off-design conditions influence the working of the studied ESDs, and on that basis recommendations for how to select ESD for different ship types and operations will be given.

 

Wanted contributions from industry partners:

• Experience and/or data from operation of ships with ESD. Before-after experiences are particularly welcome
• Model tests of ESD at MARINTEK. The project can only finance the extra tests in waves. It relies on an influx of new project(s). Alternatively, allowing use of still existing models from previous test campaigns, can be acceptable.

Expected outcomes:

• Model test results comparing ESD performance in calm water at over- and underload conditions and in selected head wave conditions.
• Report with guidelines on selection of the investigated ESDs taking effect of waves and off-design operation into account.
• Journal publication on the outcomes of the ESD in waves study

Deliverables:

• Advisory guidelines on practical procedures for instrumentation for the full scale performance monitoring of ships in service

Validated methods for obtaining ship performance data from onboard monitoring systems

Data cleaning, filtering and analysis routines for processing ship monitoring data

Report or thesis containing review, description of calculation method for sea margin, as well as calculation examples from the fleet of SMART Maritime participants

Model test results comparing ESD performance in calm water at over- and underload conditions and in selected head wave conditions

 

WP – involvement

WP 2 Hull and Propeller: Hydrodynamic simulation modelling
WP 3 Power Systems and Fuel: Power system simulation modelling
WP 4 Ship system integration, validation and monitoring: Integration of simulation models and validation of virtual prototypes against full-scale data

Schedule:

Duration: 2 years. Kick-off: Jan 2016


Participants and resources

Research:

MARINTEK, NTNU


Relevant industry partners :

 
Partners: Whilhelmsen, KGJebsen, Grieg, Vard, Havyard, DNVGL, Jotun

Personnel:

SP leader: Sverre Anders Alterskjær
Personnel: Sverre Steen, Prasad Perera