IHB Ship Design in numbers –  15 years of experience, 80 in-house engineers, 400+ completed projects

Case Study – Conversion from Bulker to Container carrier

Ships have been used to transport commodities for over three thousand years. Today ninety percent of goods traded around the world travel by sea, and the recent history of maritime logistics has seen spectacular increases in the size and capacity of vessels, with even bigger ships waiting to be built.

There are multiple solutions for increasing a ships’ capacity depending on the type of vessel and operation. Some of them are:

  • Increase of Reefer capacity of container vessel
  • Conversion from Bulker to Container vessel
  • Container vessel upgrade

The early decisions during the design phase are an important part that will later define the efficiency of the upgrade. The process involves many different steps and WE can ensure full support thru every process from the Feasibility study to the Engineering Part.

Shipowners can benefit from the advantages of the increased vessel capacity:

 

This Case Studies examine three different occasions where we have been asked to carry out projects for different types of capacity increase.

  1. Conversion from Bulker to Container carrier.

The main outline of the project consists of a full investigation and proof of concept design of stowage 12 TEU/6 FEU cargo container on deck. The vessel is a Bulk Carrier 58.000 DWT. Part of the project is a CO2 System Retrofit, where the existing CO2-bottles (120) are being re-arranged and additional 62 bottles are installed.

1.1. Position

Based on the requirements of the client firstly we determine the available container positions. In general, there are two options when deciding this:

  • On top of the hatch covers
  • In the Cargo hold.

After performing strengths calculations in this particular case the containers are positioned in the Cargo holds. Below is the General Arrangement showing the positions of the containers in the Cargo Holds.

1.2. Forced Ventilation System with Electric fans

One of the main requirements of the Client in this project was to take into consideration that the vessel is going to carry Dangerous Goods as per the Rules for Survey and Construction of Steel Ships Part R – Fire Protection, Detection, And Extinction, Chapter 19, p.66, Note №3 from 460_part_r_e_2019 Class NK.

Firstly we calculate the required airflow rate for each Cargo Hold which in this case is two air changes per hour. Required airflow will be provided by two air supply explosion-proof reversible fans per cargo hold.

Each air fan has approximately 16,000 m³/h air flow rate and it is installed in separate vent housing placed separately on PS and SB as shown in the picture below:

Тwo additional air supply pipe ducts of DN800 with rigid hinges closing hatch will be placed in each Cargo Holds as shown in the pictures below:

1.3. Container Socket Installation and Double Bottom Local Strength Check Calculations

In this phase, our Hull and Steel Outfit Department are integrating the Container Locking sockets in the Hold structures. There are a total of 372 Sockets

 

After the arrangement, the engineers are moving to perform the FEA study. The purpose of the analysis is:

  • to show the results of FEA for integration of the container socket in the Hold structures on the vessel.
  • to show the response of the structure to the increased load as a result of the added container weight and forces.
  • Strength calculation of the new double bottom structure reinforcement elements.

Calculations were performed in ANSYS – Workbench 19.0*

1.4. Analysis

The first part of the analysis is the Geometry data export:

                   Calculation Model – Double Bottom                                                  Container Sockets reinforcement assembled in Double Bottom

 

 

 

 

 

 

 

 

 Mesh existing structure

 

1.5. Force Distribution and Supports

The purpose of this strength calculation is to evaluate the behavior of the vessel’s double bottom in the cargo hold area, as a container load is applied to it. Load application points are raised breeches (container sockets). As raised breeches are not an object of this study, those are considered as bodies with rigid behavior, and those are removed from the model, ergo load application points are contacts between raised breeches and double bottom plating.

In order to simplify the load model, each container stack is presented by the forces applied on each socket as every stack load is distributed on four sockets.

Hydrostatic pressure, acting on the double bottom shell is applied calculated for conditional draft   5 m.

Each container stack contains 5 TEU with a weight of 75.0 t. Container load distribution and forces acting on the double bottom are provided by a raised breeches manufacturer.

Container load distribution is shown in the figure below:

 

Stack loads on deck are calculated according “Class NK RULES FOR THE SURVEY AND CONSTRUCTION OF STEEL SHIPS CONTENTS, Part C – HULL CONSTRUCTION AND EQUIPMENT” with conditions as follow:

Main particulars of the ship

  • Length (LPP)……………………… 185.00 m
  • Breadth (B) ……………………….. 32.26 m
  • Depth (D) ….………………………. 18.70 m
  • Draft scantling (d) ….………….. 13.00 m
  • Service speed (v) …….…………. 14.50 Kn
  • Displacement (V)……………….. 31772.25 t

Forces are calculated for CH1 – Bay 2 – most loaded with dynamic forces (most distanced from vessel LCG). Location of forces applied – existing structure of CH4 (whole CH area) where is the lightest structure.

1.6. FEA Calculations for Cargo Hold No.4

 

Calculation Model – Force distribution                                                             Calculation Model – Equivalent stress maximum

 

Calculation Model – Deformations 

                                                                                                       

Calculation Model – Equivalent stress

1.7. Conclusion

The main purpose of the stress calculations is to prove the sufficiency of the designed structure. Maximum calculated stresses for all loads are below the acceptable maximum values. Some peak extreme values are concentrated in very small areas on the surface of the elements or in areas with big swaps in the geometry parameters and are judged to be non-physical, but rather a result of numerical approximations. Therefore, such values are ignored and actual values are considered to be around the average values of the stresses outside the peak extreme values areas.

1.8. Fumigation store installation

Another stage of the project was to evaluate the behavior of the vessel’s main deck in the cargo hold area when the fumigation store device is installed on it on 4 seats.Fumigation store weight is 14.0 t.  It is placed on the main deck PS as shown below:

 

1.9. FEA results for Fumigation Store  

Calculation Model – Force distribution                                                                                                Calculation Model – Deformations

 

 

 

 

 

 

 

        

Calculation Model – Equivalent stress

The main purpose of this calculations is to prove the sufficiency of the designed structure. Maximum calculated stresses for all loads are below the acceptable maximum values.

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