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An essential indicator of a merchant ship design’s sustainability and cost-efficiency is the relationship between its payload capacity and fuel consumption. The more this relationship can be enhanced, the fewer emissions will be generated and the greater will be the earning capability achieved by ship owners. Deltamarin’s objective in the development of the B.Delta designs has been to improve on the hull form and structural design of existing vessels in order to minimise resistance and lightweight, consequently decreasing consumption and increasing payload.

New challenges in hydrodynamics

Through Deltamarin’s history the field of hydrodynamics has been the one of its strongest disciplines. Hydrodynamics plays an important role at the earliest stages of every project. Low fuel consumption, combined with the ability to keep tight schedules and still have maximum payload are key factors to cost-effective ship types. The best practices to achieve the optimum hull form and propulsion were developed in cruise ship and ferry projects. Deltamarin’s projects have been infused with the attitude of aiming higher and higher to get the best results after every successful design.

When the development of bulk carriers – B.Delta designs - started, Deltamarin faced new challenges in hydrodynamics. Bulk carriers have an extremely full-bodied hull compared to the designs that Deltamarin’s designers were used to seeing. The demands and goals remained the same. The ship should make reasonable speed with a low fuel consumption and high deadweight capacity. As a first step we made a comparison to similar vessels and their main dimensions to set a goal for the design. Based on that comparison there seemed to be some room for improvement and the goal was set to achieve 10% better fuel consumption, while at the same time the vessel should have 10% more payload capacity than any other design. To meet these two goals at the same time requires optimised hull form and propulsion combined with low steel weight.

A traditional claim is that with a low Froude number wave resistance is almost negligible. Another claim is that lowering the wave resistance is the only effective way to optimise the hull for resistance. If these presumptions were true, there would not be too much room to optimise the hull form of a bulk carrier for resistance. We realised that neither of these claims held true in the case of an extremely full bodied hull.

Wave formation already starts to be significant with clearly low Froude numbers on very full bodied hull forms. Typically the wave around the fore shoulder is very deep and it causes a strong wave system along the hull. This kind of wave system with many transversal waves creates significant wave resistance, which has a great impact on the total resistance of the hull. Another aspect of full-bodied vessels is that due to the bluntness of hull form the flow around the hull is more three dimensional and viscosity related effects, such as vortices and flow separation get stronger. These also have a significant impact on resistance. During the development of standard designs we have seen reductions in resistance even while wave formation has remained unchanged.

Innovations in hydrodynamics

Every challenge can be seen as an opportunity for innovation. The major and first challenge with the B.Delta designs was to optimise the hull form for wave resistance. From the very beginning it was clear that the main goal was to get the wave around the fore shoulder as smoothly as possible. That is the only way to get the wave formation minimised and that guided the optimisation of the hull form towards low resistance.

The initial design was optimised with potential flow solver Nu-Shallo that Deltamarin uses in hull form development on a daily basis. During the first model tests Deltamarin realised that there was still room for further optimisation of the hull form, although the performance was already on a very good level at the beginning. Being slightly better on fuel consumption than any other similar design was a good starting point. Nevertheless the goal had not yet been reached.

Based on the experiences from the first model test round, Deltamarin started the re-optimisation of the hull form. It was quite clear at this stage that a break-through to a new level would require something totally new. During the re-optimisation the main goal remained to reduce the wave formation.

Various shapes were calculated with Nu-Shallo and several ideas were studied to get the wave more smoothly around the fore shoulder. After dozens of modifications we had an idea of what kind of impact all these shapes had on the wave formation. The best ideas were combined to get the guidelines for the new design. The design was optimised based on the guidelines before the next round of model testing.

The new model test round showed that our design was developing in the right direction and the results were encouraging. Deltamarin decided to continue the effort, as it was showing good results to reach the given goals. At this stage only small fine tuning on the hull form was made, purely based on observations during model tests as we realised that the limits of the potential theory solver had already been reached. Together with these minor modifications several propulsion aids were tested as well to reach the last percentages on power demand.

After all these efforts Deltamarin finally managed to get to the target of model tests being 10% better on fuel consumption than any other design. The process accumulated knowledge and experiences which were later on spread to all B.Delta designs and other high block coefficient vessels. Recently Deltamarin has started to use more powerful tools to reach an even deeper understanding of flow phenomena around a full bodied hull. We have had very good results with the RANS-solver Finflo, as this tool gives the advantage to study every little detail of the flow. Through that we are looking forward to develop our B.Delta designs even further regarding hydrodynamics.

Optimal structural design

In order to meet the goal of increased payload capacity in addition to lowering fuel oil consumption, Deltamarin looked into what could be done to optimise the structural design of the B.Delta bulk carriers. In recent years the classification societies’ rules for merchant ships have seen a shift from prescriptive scantling requirements to direct analysis procedures. The IACS Common Structural Rules for bulk carriers and oil tankers are examples of this development. Deltamarin has recognised that since scantlings will now mainly be determined by analysing structural response to global hull girder loads, the key to structural optimisation is the minimisation of these loads.

The most important global load effect in these single deck ships is the vertical hull girder bending moment. While the wave component of hull girder bending is typically based on rule values for unrestricted ocean service, the still water component depends on the distribution of weight (cargo, water ballast, etc.) along the ship’s length. Deltamarin customizes software tools developed by Napa to optimise the general arrangement of a hull for minimum still water bending.

The optimisation procedure is managed with a multi-objective genetic algorithm, which is suitable for engineering problems where there are multiple, and sometimes conflicting, objectives. The algorithm creates design variations and determines which variations are favourable with regard to the objectives. Design parameters of promising variations are mixed to create new variations and ultimately a pareto group of variations is formed. In the design variations that belong to this group, one objective cannot be improved further without a trade off on another objective. A solution that is optimal with regard to the engineering problem at hand can be selected from this group of variations.

In the case of the bulk carrier the optimisation task was formulated in such a way that the variables were the locations of transverse watertight cargo hold bulkheads in the ship’s stability and hydrostatics model. The objectives were the positive (hogging) and negative (sagging) hull girder bending moments under relevant loading conditions, including light and heavy ballast conditions as well as homogeneous and alternate cargo conditions. As a result of the procedure, a design variation (i.e. a compartmentation layout) that yields the least severe design bending moment can be identified and implemented as a basis for general arrangement. The solution that is brought forward from a large number of systematic variations may be one that a designer would not arrive at intuitively.

Verification of structural design

Once hull girder loads have been minimised, initial hull scantlings are designed for adequate strength. The following step is to analyse front the design using the finite element method and thus verify its merit. It is important to begin structural 3D modelling and class-required analyses at an early phase in the ship design project, particularly where a solid standard ship concept is being developed. For this purpose Deltamarin makes use of state-of-the-art structural modelling tools by Napa. The integration of direct strength analysis to the structural design process is facilitated as a logical continuation to the initial structural optimisation. This means that any individual design variation, which is created by the optimisation algorithm, can be meshed into a finite element model. The element meshing of structures is carried out in accordance with rule requirements concerning the size, quality and shape of the mesh. The model can then be transferred into classification societies’ finite element pre/ post-processing tools, which take care of analyzing the model against rule loads and criteria.

A 3D strength analysis will commonly reveal that the initial hull scantlings require strengthening in way of highly stressed areas. This may be due to stress concentrations in way of openings or discontinuities, bi-axial buckling stresses of plating and the deflections of fatigue-sensitive structural details. In the case of a standard ship, which must offer great versatility in service, this structural response needs to be studied under a multitude of loading conditions. Once the necessary strengthening is implemented in the design, the lightweight calculation of the ship can be refined. This will in turn be a factor in the evaluation of payload and earning capability.

Another application for the results of concept-phase strength analysis is found in the ship’s product coordination 3D model, which exists in the Catia software environment. The coordination model is used to manage and transfer design information between the hull, outfitting and machinery disciplines. The coordination model, as a compilation of design input from the various disciplines, is effectively an up-to-date general arrangement and develops as the ship project progresses. It is in the coordination model that those hull members, which are found to be critically stressed, can be indicated as prohibited areas for large access or pipe openings. This information is then utilised in the arrangement of access ways and pipe systems within double hull spaces. The early availability of such information in a project will help eliminate unnecessary design modifications at later stages.

The hydrodynamic and structural developments are merely examples of the holistic work that took place developing the new B.Delta standard bulk carrier designs. The goals for all the disciplines were ambitious and the resulting designs have proved their competitiveness in the market. The designs offer lower fuel consumption together with better payload capacity, thus making the investment more lucrative to ship owners. On the other hand the designs are also simplified to enable easy building anywhere in the world. The designs truly combine cost efficiency with a sustainable solution.