GLOBAL STRENGTH ANALYSIS USING FEM

The finite element method (FEM) is a numerical method for solving complex engineering problems. A structural 3D model built up from 2D shell and beam elements can be loaded and the resulting deformations and stresses can be analyzed.

Why a global strength analysis?
Because ships become more complex due to discontinuities in the structure, like large openings in the shell or deck structure and complex loading conditions, ship owners and class societies demand proof of strength of the vessel and local stability (buckling) structural items. A global finite element calculations can provide this proof to a high degree of accuracy and therefore higher allowable stresses can be used. Furthermore, these calculations provide a lot of information to optimize the structure and/or perform more detailed calculations.

How to perform a global strength analysis?
Structural finite element calculations are ideal to simulate these complex structures and/or loading conditions. To perform such an analysis a good geometrical model is crucial. At DEKC Maritime we use Rhinoceros to create our models. In-house developed scripts help us to create a more accurate model within less time. Examples of these scripts are automatic splitting tools for crossing areas and an automatic edge splitting tool to provide the necessary common edges. After completion, the model can be exported to Ansys. Our export script creates an identical model in Ansys, including all thicknesses, used materials and identification. This model is then meshed, for a global model a coarse mesh is normally used. Interesting areas can already be meshed using a finer mesh.

Global loading usually includes a light weight and buoyancy distribution together with a wave load distribution. For a sailing yacht the rigging load and for a cargo vessel the cargo distribution is added. This cargo distribution can be exported from our ship stability program NAPA and applied to the global model.

What results can be obtained from the analysis?
From the calculated results valuable information can be extracted. Of course, the global stress results can be evaluated in an envelope of the stress results. These envelopes (including corresponding load case) give a direct overview of the occurring stresses. Together with the global deflection and the reactions, they also provide a good way to check the results. Based on the results, areas in the model can be identified where reinforcement may be necessary, or where structural material may actually be removed. Further deflection results can be extracted to calculate cargo hold deflections or determine the relative deformations of window frames.

Are there more options?
The global results can be the base for further sub model analysis. In this case the deflections from the global model are used as prescribed translations/rotations at the sub model boundaries to include the global deflection effects.
These sub models are normally provided with a more detailed geometry and a finer mesh to calculate the detailed stress results. In case of stresses above the yield limit an analysis, using nonlinear material properties, could prove the structural integrity of the structure and allow local yielding. To calculate the stability of an unstiffened plate panel with unusual shapes and/or loading patterns an “in house” developed method can be used. This method isolates the unstiffened panel and load pattern obtained from the larger FE model. After applying an imperfection, a nonlinear analysis determines the load bearing capacity of the panel.

Very usefully
Recapitulating: Finite element analysis is a powerful tool that can provide very useful detailed information to analyze and optimize new built structures or existing structures. At DEKC we can perform these finite element analyses and more.
At DEKC Maritime we use Ansys APDL to perform our finite element calculations. Our licenses provide us with all the necessary simulation options: structural non linearities (stability analysis), nonlinear material behavior (calculate local yielding), applying strain (simulate weld shrink) and contact analysis (friction analysis). We use APDL because this offers the possibility to use scripting. These scripts can be used for multiple projects and automate time consuming operations: applying loads, making envelopes etc.

Ballast water treatment refits

When time is essential, there is no room for errors in the installation process. Especially in the refit business where sailing schedules are leading. Due to emission and ballast water legislation changes, the refit business is booming. These refit projects come with the major challenge that there might be a gap between the real life situation and the documentation.
Where DEKC as a knowledge center excels in shipbuilding engineering, we do not limit ourselves to the theoretical side of the project. In previous projects we were able to give guidance before and during the installation of the engineered systems. One of our engineers / project managers who was involved in the design of the new systems was assisting the preparation of the refit and placed on board to assist with the installation.
There are several advantages in combining project coordination and engineering of a system refit:

• Single point of contact; from engineering to commissioning

• Assistance with documentation and class approval process

• Assistance in finding suitable subcontractors for the job at hand

• Making sure the complete scope is covered

• Making sure the right materials and equipment is ordered in time

• A clear planning is delivered before the project and checked during the project

REFERENCE PROJECT

FEASIBILITY STUDY

In previous projects, DEKC Maritime assisted customers with the selection of suitable systems, which are conform the client specific needs with a feasibility study. The main goal of this study was to select a suitable system for the ship and their operations. Based on existing documentation of the ship, a technical review was performed for several options of new ballast water treatment systems.
Within our study we used mainly these comparing parameters:

• Power consumption

• Footprint large components

• Pressure drop

• Technical reservations/implications in existing system

REFIT COORDINATION

During the preparation phase and the installation phase of the ballast water refit one of our project managers assisted in the project coordination of the refit. With the aim of effectively reducing the workload from the management of the client, DEKC took care of the communication with several subcontractors needed during this refit. These subcontractors were primarily piping manufacturers, electrical installation companies, construction companies and system integrator of alarm monitoring and control system.

LASER-SCAN AND ONBOARD SURVEY

In preparation for the basic engineering for the refit of the ballast water treatment system a 3d scan and survey were performed. When 3D information is not available and only 2D drawings are available, a 3D laser scan is performed to get an accurate 3D image of the engine room and its surroundings. From the scan a point cloud file is generated which can be used as a starting point for the basic engineering of the ballast water treatment system.

PREPARATION PHASE

Within the preparation phase, customers are unburdened by DEKC by means of a specialist who makes sure the right material and services are ordered, all documentation is updated and submitted to class.
In the preparation phase, the DEKC project manager prepared a detailed schedule. Including start and end date with all major milestones of the refit project. In addition to the schedule, a plan of action is made of the refit period. In this plan of action, the installation steps and required personnel were described.

BASIC AND DETAIL ENGINEERING

Within this project system design and engineering is done using 3D Cadmatic software; from basic routing for space reservation and early material take off to detailed production information and every step in between. This way engineering and models from different packages can be combines in to one model and customers can review during the engineering project by means of the eBrowser, giving full insight in every step.

KICK OFF MEETING

A kick-off is essential for the execution of a refit project. It should be made clear in the kick-off what is expected of the different parties. Within the kick-off all scheduled actions and milestones are being discussed. When all activities are discussed with the different parties, clashes can be detected and resolved before they appear.

INSTALLATION PHASE

During the installation, DEKC has performed the onsite coordination of the ballast water treatment refit. With the experience and knowledge, DEKC project managers will give practical guidance and consultation during the installation of the new ballast water treatment system.
 

CONCLUSION

To finally close the gap between theory and practice DEKC unburdens the customer with a single point of contact who oversees the whole system refit scope from early design to commissioning. It has proven that the knowledge and focus on the system refits, gives projects a higher chance of success.

COMMUNICATING SHIP DESIGN IN VIRTUAL REALITY

Designing and building ships is one of the oldest professions known to humankind. Throughout most of its history, the basic approach has remained unchanged: the ship would be designed on a number of 2D plans, after which it was up to the craftsmanship of the builder to make the design a reality. In the last several decades, ship design has made the transition from paper drawings and hand calculations to CAD files and digital analysis tools. However, at first, the computer was simply used as an easier way to draw in 2D, and the end result of this hard work was still nothing more than a large stack of 2D drawings.

Considering how computer technology has progressed in the past decade, 3D computer models became the order of the day, and we could start using 3D information in every step of the design process. In this philosophy, the 3D model of the ship has become the basis of all information, from which the 2D drawings are derived and delivered for approval. This is valid from the very first general arrangement to all construction plans and workshop drawings for the shipyard. Nonetheless, communicating the design is still done mostly in 2D.

With all this 3D information at hand, the next logical step is to change the way we communicate our designs. Instead of 2D drawings, now the 3D model itself can be easily shared across platforms. Key to this development is coupling the engineering model to a gaming platform, Unity. This allows essentially unlimited freedom in how to present and interact with the information from the engineering model. By combining this with virtual reality (VR) hardware, the design can be communicated in a much more immersive and intuitive way. In turn, this allows the designer to better accommodate the customer’s wishes, or to communicate more effectively with other technical experts.

Currently, computer calculation results, production information, or maintenance data can be coupled to components of the 3D model in Unity. Sharing this data in an intuitive way could significantly streamline the class approval process of a design. In addition, this technique could be used during repairs or retrofits, by overlaying the digital model on top of the actual vessel in augmented reality (AR) for the engineer on-board. Ultimately, by incorporating a real-time data stream from sensors on the vessel, a complete virtual representation of a vessel could be made on-shore, which would provide a platform for operators of unmanned ships.

The coupling of 3D engineering and the Unity platform has provided us with a very powerful toolkit for changing how a ship design is communicated. Using these tools creates a much greater understanding of the design in an intuitive way for clients, classification societies, and shipyards, and allows us to design better ships. Furthermore, the future applications of these tools are virtually limitless, and can help pave the way for radical changes in the shipping industry.

Author: Harry Linskens

NOTE: this article is a summary of a paper written for the COMPIT conference. Please contact us if you would like to receive the full paper: info@dekc.nl

THE USE OF POINT CLOUDS IN REFIT ENGINEERING

If for any reason engineering for a refit needs to be done, a good comparison between the digital engineering model and the real world is essential for an accurate engineering package. Pictures and measurements of the existing situation can be taken but these will never be as accurate as desired and most of the time more questions will rise afterwards. In addition, in most instances there is only one possibility to take pictures and measurements on board so there is only one chance to get it all.
 

To make sure we do not miss anything during a visit on board it is preferred to make a laser scan that results in a point cloud file. A scan of the complete area where engineering is required will be made so the risk of missing information afterwards will be as minimal as possible. The data from the laser scan will be processed to be compatible with Cadmatic software, and exported for use in the eBrowser. This way the point cloud data will be available for everyone working on the project, both at the engineering company and client office.
 

With a point cloud file you create a virtual environment of the real situation available at all times without having to visit the job site, which in the case of a vessel can be all over the world. Benefits of this are of course that you do not have to make expensive trips to take measurements and the point cloud will be more accurate and not subject to human errors which will result in a more accurate engineering package.
 

The goal is to deliver an accurate and detailed engineering package from which the shipyard or construction company can make the necessary piping and outfitting with as few as possible errors within their planning and budget. The time and money spent making the point cloud will pay itself back numerous times during the project and production.

WINTERIZATION

In recent years, interest for maritime operations in the Arctic region has risen substantially, highlighted by projects such as Yamal LNG. While the changing climate does help with opening up such opportunities, designing vessels and systems for such extreme conditions is still far from trivial. At DEKC Maritime, we have broad experience working with the different regulatory codes governing extreme-cold maritime engineering, such as the IMO Polar Code, the IACS Polar Class rules, and the Russian RS regulations.

On the practical side, we have been involved in a number of projects for operations in extreme-cold conditions:

• Design and engineering of a winterized equipment bay for a walk-to-work system

• Feasibility study for the winterization of heavy-lift vessels and operations in the Arctic Ocean

• Structural design of a shallow-draft icebreaker

THE MANY FACES OF PARAMETRIC HULL DESIGN

Green shipping is the single most important topic in the maritime industry today. This umbrella-term is used to describe solutions to all aspects of the environmental impact of shipping, ranging from ballast water treatment and marine noise to reducing harmful emissions. Particularly the latter seems to capture the public imagination like no other: any Google search on the topic of “green shipping” will produce many pages of big ideas to tackle the problem, often visualized in calming shades of blue and (of course) green.

With so many solutions being proposed, it is easy to get lost in the myriad of alternative fuels, alternative means of propulsion, or technological gadgetry designed for cutting emissions. Also, many of these developments still require significant investment. With current state-of-the-art tools, however, ships can be much more effectively designed (or redesigned) to do their job at a low cost and still respect all environmental regulations. Most important among these tools at DEKC is the use of parametric hull models.

A parametric hull model, very briefly, is a 3D hull shape that can be altered using a single numerical value, called a parameter. Parameters can include very basic values such as length or beam, but they can also be used to afford precise control over the hull shape, for instance the waterline entry angle or bulb tip height. Most importantly, by using this method, a large spread of many different design options can quickly be generated and evaluated. This ensures that the best possible combination of design parameters is chosen.

Coming back to cutting emissions, the most direct method of achieving this is simply by reducing the amount of power, and thus fuel, required to travel at a certain speed. This has the added benefit of reducing fuel costs as well. Parametric hull design is very well suited for optimizing a hull shape to minimize fuel consumption. To do this, the parametric hull model is coupled to a CFD[1] solver to obtain accurate power results for each design. Either local modifications, such as a bulb redesign, or a complete hull can be optimized in this way.

Less interesting to the public at large, but no less critical to owners and operators, is designing an efficient ship to perform its job as efficiently as possible right from the very start. This requires designers to consider how best to achieve the desired stability, capacity, and speed, while minimizing the size, weight, and installed power of the vessel. Many of these properties of the vessel are 100% dependent on the hull geometry: therefore, parametric hull models are incredibly helpful for the designer to find an optimal combination of design parameters during the earliest concept design.

In addition, having these calculations coupled to the parametric hull model opens up possibilities for automated optimization of the hull shape. Literally thousands of different design combinations can be automatically generated overnight by advanced optimization algorithms. These designs can then be compared with respect to various (often-conflicting) targets, such as low fuel consumption, low freeboard and tonnage, and high cargo capacity. Thus, the designer ensures that the selection of main particulars is already contributing to an optimal design.

With any type of parametric study, the key is that many different design options can be quickly evaluated by the designer. Thus, promising combinations of parameters can easily be identified, for instance to lower the required power. Furthermore, an optimal hull form can very quickly be generated to suit the needs of the design during the first phases of the design process. Therefore, parametric hull modeling has become an integral part of our design process at DEKC, and helps ensure we design the best ship possible.

VR MODELS FOR TU DELFT AND THE ROTTERDAM MAINPORT INSTITUTE

Over the years DEKC has specialized in communicating new designs in Virtual Reality (VR) with her customers. The VR model allows you to walk through the vessel before it is built and therefore giving the new owners or crew a comprehensive view of the vessel, including the underlying structures and systems. These models are built with the aid of gaming technology on the UNITY gaming platform.  

In the attempt to implement and promote VR and gaming technology in our industry DEKC is sharing some VR models with the TU Delft and Rotterdam Mainport Institute. This will help the institutes to improve their curriculum and provides a model for the students where they can walk through endlessly without traveling to a ship. In addition sharing the models enables the development of VR tools, simulations and games for our industry. We hope to use the “to be developed” tools and games in the future to make better ship designs and improve maritime projects.

At DEKC we believe the current use of VR is only the beginning. When converting existing vessel designs in to a VR environment the possibilities become endless. For example, simulating critical offshore operations to minimizing risk with the use of gaming software, assessment of modification possibilities in a virtual environment or making instruction and training games to allow personnel to get familiar with the vessel before stepping aboard. We are excited to see what the future brings in this aspect and encourage others in the maritime and offshore industry to collaborate with us on these developments.