Design­ing Wind Assisted Com­mer­cial Cargo Vessels

by Siegfried Wagner

Within the frame­work of the project ​“Tran­si­tion­ing to Low Carbon Sea Trans­port” (TLCSeaT), focusing on the new-build of a sail assisted general cargo and island supply vessel for the Republic of Marshall Islands (RMI), CAESES has been used to create a para­met­ric design and cal­cu­la­tion envi­ron­ment that can be used for the eval­u­a­tion of dif­fer­ent design options during the vessel’s concept and early design phases.

The Uni­ver­sity of Applied Sciences Emden-Leer (HEL) acts as the tech­ni­cal coor­di­na­tor within the project group and is respon­si­ble for the design of this new vessel. The aim of the project is to support RMI in the tran­si­tion­ing process to reach a zero-emission goal in the maritime trans­port sector. RMI has defined nation­ally deter­mined climate goals (NDCs) to stepwise reduce emis­sions from sea trans­port with the final goal of net zero emis­sions in 2050.

CAESES was used for dif­fer­ent aspects within this initial design phase:

• Gen­er­at­ing a para­met­ric, and thus flexible, rep­re­sen­ta­tion of the vessel concept to inves­ti­gate and analyze dif­fer­ent design options. The flexible 3D model is used as geo­met­ric input for further ana­lyt­i­cal and numer­i­cal cal­cu­la­tions, as well as for demon­stra­tion purposes with project partners.
• Per­form­ing ana­lyt­i­cal cal­cu­la­tions using the feature pro­gram­ming envi­ron­ment con­cern­ing vessel capacity, sta­bil­ity, and per­for­mance pre­dic­tions for dif­fer­ent wind propul­sion systems and hydro­dy­namic layouts.
• Serving as a GUI for CFD cal­cu­la­tions with OpenFOAM to inves­ti­gate flow details of the hydro­dy­namic layout of the vessel, in order to validate and adapt ana­lyt­i­cal formulas that are used within the per­for­mance pre­dic­tion procedure.

Concept Design

The concept of this vessel has been devel­oped on the basis of an exten­sive field research within RMI, carried out in order to define vessel require­ments together with the Marshall Islands Shipping Cor­po­ra­tion (MISC) in 2020. Accord­ing to the findings of this field research, a design concept was devel­oped, which has been refined and worked out in detail in a joint effort with the company ​“Ship Design and Con­sult­ing” (SDC), a highly rep­utable ship design office based in Hamburg. A para­met­ric rep­re­sen­ta­tion of the cargo and island supply vessel for the Republic of Marshall Islands (RMI) is shown below.

The para­met­ric rep­re­sen­ta­tion of the model has been used in the early concept phase to inves­ti­gate and present dif­fer­ent design options, and to discuss possible solu­tions with the project partners in RMI. For this purpose, the layout of the vessel includ­ing super­struc­ture, cargo handling system, wind propul­sion system, as well as hydro­dy­namic layout and dri­ve­train has been inte­grated into the design.

The para­met­ric approach enabled easily trans­form­ing the model based on the dis­cus­sions with the project partners and thus, new ideas and dif­fer­ent solu­tions could easily be inte­grated into the design in a fast and effi­cient way by changing para­me­ters like vessel length, width, height, draft, keel-rake, dead-rise-angle, or the dimen­sions and posi­tions of super­struc­ture, rigging, and hull appendages.

The feature pro­gram­ming func­tion­al­ity within CAESES was used for fast cal­cu­la­tions of areas and volumes for accom­mo­da­tion, tank capac­i­ties, and cargo hold volumes, in order to meet the given require­ments. All relevant values could be mon­i­tored during the design process while the vessel was adapted.

OpenFOAM RANS CFD Simulations

Even though CFD sim­u­la­tions were carried out via a software con­nec­tion to OpenFOAM, a strong focus was put on the ana­lyt­i­cal cal­cu­la­tion pro­ce­dure that allows to predict average vessel per­for­mance (speed, power demand, and fuel con­sump­tion) for a wind assisted com­mer­cial vessel. Since the CFD sim­u­la­tions are based on the same geo­met­ric input that is used for the ana­lyt­i­cal com­pu­ta­tions, a direct com­par­i­son of ana­lyt­i­cal and CFD results for iden­ti­cal flow con­di­tions and hydro­dy­namic settings could be realized.

RANS CFD sim­u­la­tions are time con­sum­ing and thus not feasible if hundreds of con­di­tions must be cal­cu­lated to evaluate the benefit of changes made to the design. CFD com­pu­ta­tions were however used for com­par­ing single flow con­di­tions to evaluate the quality of the ana­lyt­i­cal formulas used for mon­i­tor­ing the design process.

Per­for­mance Prediction

The primary achieve­ment that could be realized is the per­for­mance pre­dic­tion tool, devel­oped in the feature pro­gram­ming envi­ron­ment within CAESES. The main benefit for using CAESES in this context is the direct geo­met­ric input from the vessel design that makes it possible to get imme­di­ate feedback to any changes made to the model. An ana­lyt­i­cal cal­cu­la­tion concept is espe­cially nec­es­sary for vessel per­for­mance cal­cu­la­tions, where hundreds of equi­lib­rium con­di­tions (equi­lib­rium of aero and hydro­dy­namic forces and moments) must be cal­cu­lated in an iter­a­tive pro­ce­dure for each design option that shall be evaluated.

The eval­u­a­tion pro­ce­dure starts by col­lect­ing all relevant data from the model and then using this data within the feature pro­gram­ming code for the per­for­mance cal­cu­la­tions. A 4 degree of freedom model has been pro­grammed for combined wind and engine thrust based on ana­lyt­i­cal formulae on the hydro­dy­namic side, as well as imported wind tunnel mea­sure­ments for the rep­re­sen­ta­tion of the wind propul­sion system. The code was able to solve the equation system for the whole wind-spectrum (around 400 equi­lib­rium con­di­tions) within a few minutes.

Results

The result of these cal­cu­la­tions is a vessel response matrix that yields speed and power demand as a function of wind force and direc­tion. If a specific wind profile (area or trade route depen­dent) is mul­ti­plied with this matrix, average values for vessel speed and power demand can be cal­cu­lated and used for the deter­mi­na­tion of average fuel con­sump­tion per nautical mile. This final value for fuel con­sump­tion per nautical mile has been used to compare and evaluate dif­fer­ent design options within the process, thus reducing complex cal­cu­la­tion results to the single value of increased or reduced fuel consumption.

Apart from the final value con­cern­ing fuel con­sump­tion and emis­sions, the approach allows detailed insights on vessel per­for­mance and dri­ve­train effi­cien­cies. It gives valuable infor­ma­tion on the layout and posi­tion­ing of appendages and helps in the process of opti­miz­ing size and ori­en­ta­tion of dif­fer­ent appendage solutions.

The example dis­played in the fol­low­ing diagrams shows the influ­ence of recu­per­a­tion (i.e., extrac­tion of power from a free milling pro­peller) on vessel speed and rudder effi­ciency. The ana­lyt­i­cal approach realized with CAESES feature pro­gram­ming makes it possible to evaluate the benefit of dif­fer­ent measures on the vessel’s per­for­mance. For this specific case, the design aim was to optimize the size of the rudder and posi­tion­ing of the side keels as to not overload the rudder in recu­per­a­tion mode. Reduced pro­peller stream veloc­i­ties lead to increased rudder angles that can become critical if the inflow angle is too close to stall.

Outlook

Further efforts will be made in the future to optimize the per­for­mance pre­dic­tion code in order to make it robust enough for auto­mated opti­miza­tion pro­ce­dures within CAESES. Part of this opti­miza­tion and val­i­da­tion process will be the inte­gra­tion of new maritime testing facil­i­ties (towing tank and wind-tunnel) that will com­ple­ment the existing numer­i­cal cal­cu­la­tion pos­si­bil­i­ties at the Maritime Tech­nol­ogy Centre at the Uni­ver­sity of Applied Sciences Emden-Leer.

About the Author

Special thanks to Siegfried Wagner for these insights into the design of wind assisted cargo vessels.

Siegfried Wagner studied naval archi­tec­ture and ocean engi­neer­ing and is a research assis­tant at the Uni­ver­sity of Applied Sciences Emden-Leer and the Fraun­hofer work­group for Sus­tain­able Maritime Mobility.