Archives

RANS-Based Simulated Ship Maneuvering Accounting for Hull-Propulsor-Engine Interaction

By Ould el Moctar, Udo Lantermann, Philipp Mucha, Jens Höpken & Thomas E. Schellin

Model-scale measurements and full-scale sea trials were compared to numerical simulations of different ship maneuvers. Simulations were performed using codes that solve the Reynolds-Averaged Navier-Stokes (RANS) equations coupled with the nonlinear rigid body equations of motion. Different time scales existing between ship motions and fluid flow as well as diverging spatial discretizations of hull and propulsor in conjunction with the propulsor’s rotation increased the required computational efforts, rendering the model unsuitable for comprehensive parametric investigations. To overcome these difficulties, a mixture between mesh morphing for small-scale motions and global displacement of the mesh for large-scale motions was developed. Previous investigations mostly replicated captive model tests or resolved the viscous flow more precisely to explore various dedicated effects. RANS simulations offer valuable insight into scale effects involved in maneuvering prediction as they allow the study of both ship and model scale flows. This paper explores the reliability of both direct maneuvering simulation (using a combination of a freely moving ship including the rotating propulsor) and simulations based upon the virtual replication of captive model tests (using Taylor-series expansions to model the external hydrodynamic forces). The influence of engine-propulsor interaction in maneuvering motion behavior and scale effects is discussed. Predictions are compared to model tests and sea trials.

Technical Note: PreSquat – Workshop on Numerical Prediction of Ship Squat in Restricted Waters

By Philipp Mucha, Ould el Moctar & Carl-Uwe Böttner

In September 2013, the PreSquat workshop on numerical prediction of dynamic squat of ships in shallow and restricted waters was held by the University of Duisburg-Essen, the Federal Waterways Engineering and Research Institute (BAW) and Germanischer Lloyd (GL) in Mülheim, Germany. The purpose of the workshop was to benchmark the capabilities of available numerical methods for squat prediction through comparison with experimental data for a large container carrier of Post-Panamax type. With the exception of one test case the comparison with model test results was blind, i.e. the test results were not provided prior to the workshop. This technical note describes the model tests and the numerical results submitted to the workshop.

Investigations of the Safety of Flight of WIG craft

By Nikolai Kornev & Andreas Groß

The paper presents motion simulations in time domain of a WIG craft with a Lippisch configuration under wind and wave perturbations. The aerodynamic characteristics were obtained using the vortex lattice method. The motion is considered in the longitudinal plane. A novel approach is proposed to generate artificial wind turbulence with prescribed kinetic energy and spectra. The wave surface is modeled by a plane performing linear and angular oscillations. Motion simulations were performed to show the limitations of flight safety analysis based on the classic stability theory. Simulation of the take-off motion was carried out to determine the conditions for safe transition from the skimming to flare mode. It was shown that the application of the flight control is necessary for safe flight in the proximity of the ground.

A Two-Stage Optimisation Method for Full-Scale Marine Propellers Working Behind a Ship

By Stephan Berger, Markus Druckenbrod, Markus Pergande & Moustafa Abdel-Maksoud

A procedure for the optimisation of full-scale marine propellers working behind a ship is presented.
The procedure consists of two stages, the actual optimisation and a detailed investigation of selected propeller variants. In the first stage, an evolutionary algorithm in combination with the in-house panel code panMARE is used to solve the multi-objective propeller optimisation problem.
Efficient methods for the evaluation of propeller variants and parameterisation of propeller shape are presented. Proceeding from the results of the optimisation, a set of selected propeller variants is investigated in the second stage taking into account all relevant propeller-hull interactions. The
applied algorithm is based on coupling of panMARE with a viscous flow solver. An enhancement of this method allows investigating several propeller variants sequentially without stopping the solvers. In this work, the optimisation method is applied to improve the propeller of a container vessel.

Modelling Operation of Service Vessels in Offshore Wind Farms Using Stochastic Activity Networks

The complexity and uncertainty involved in current offshore wind energy projects creates a need for decision support models. By applying stochastic activity networks (SANs), this article introduces a new way to model the operation of service vessels in an offshore wind farm. The SANs enable not only building a physical composition of the offshore wind farm, but also integration of the maintenance demands from various components, maintenance planning and maintenance processes at the same time. With sufficient input data, the complete model can be used to evaluate the availability of the offshore wind farm and work load on the service vessels, subsequently to determine or improve the maintenance plan. Based on their graphical rules, the SAN models are concise and easy to understand. The established SAN models can be applied individually for the evaluation of different maintenance plans, or can also be an add-on in more comprehensive decision support models.

Classification of Design Variables Based on Constraint Conditions and Structural Optimal Design – Application to Structural Optimal Design of Midship Section

A structure optimization of midship section is under consideration in this paper. A double hull oil tanker is selected for optimization. Since there are many design variables describing the midship section, usually, design variables are divided into some groups concerning design and manufacturing. When the classification of design variables is not proper, there is a problem that the optimal solution cannot be obtained. Classification method of design variables proposed here is intended to overcome this drawback. Influence of design variables on constraint conditions is estimated based on principal component analysis and used for classifying design variables. The classification of design variables obtained by the proposed method is different from a conventional
group. Selected problems show that the proposed method can produce a better solution than the
conventional method.

Second-Order Seakeeping Analyses using Perturbators

Perturbation methods up to first order are common in seakeeping predictions. For higher than first order, lengthy theoretical analyses are retired; they result in complicated formulae requiring high programming effort, and often the well-established numerical methods for first-order quantities fail when applied to second-order flow quantities. Both the derivation of the required expressions and their programming can be simplified substantially by using mathematical entities called perturbators. Here the concept of perturbators is described, and their application is demonstrated for a two-dimensional test problem: the force and moment up to second order acting on a fixed, partly immersed cylindrical body in a regular wave.

Marine Construction and Operation Challenges in the Barents Sea

The Western and Southern Western Barents Sea is the offshore area south of Bear Island and east towards the newly agreed delineation boundary between Norway and Russia, limited by the Norwegian mainland to the south. In this area, the Snøhvit gas field is in production and the Goliat oil field is being developed. Recently, encouraging oil finds (Skrugard and Havsul) have increased the interests in the geology and the oil and gas potential of the area. This paper summarises challenges faced by the marine construction contractors working in these areas and also discusses various phenomena that affect marine operations at extreme cold climate conditions. It is considered important to present a review of all aspects of working in these cold climate areas as construction and operation activities in the areas for a large part will be carried out by personnel unfamiliar with the challenges of the areas. Working under unfamiliar conditions could lead to undesirable events that could escalate into severe accidents.

CFD Prediction of Wind and Current Loads on a Complex Semi-Submersible Geometry

The paper presents results of steady simulations based on Reynolds-averaged Navier-Stockes (RANS) equations of the loads due to wind and current on a semi-submersible oil platform. Flow angles range from 0 to 360 . The loads due to current and wind are treated separately. The simulations of current are performed in a uniform flow, whereas in the simulations of wind, the atmospheric boundary layer is taken into account. The computational mesh is locally refined to include several levels of detail. Viscous boundary layers are not modeled. The computed results are compared to wind tunnel measurements.

Computation of Free Surface Flows around Box-Shaped Ships by an Unstructured Navier-Stokes Solver

A novel ship concept which is called ultra large block coefficient ship (ULBS) to enable efficient environment-conscious sea transportation is under investigation at Yokohama National University. Since ULBS is supposed to have a very blunt hull, flow field analysis around a ship is crucial for a design of hull forms with better hydrodynamic performance. Computational Fluid Dynamics (CFD) is expected to be an efficient design tool for unconventional hull forms such as ULBS. However, it is desirable to examine applicability of the CFD method before the actual design application. Thus, free-surface flow computations of two box-shaped ships which can be considered as the extreme cases of ULBS are carried out. Grid convergence study is performed with respect to resistance for the verification of the results. Total resistance coefficients are compared with each other and also with available experimental data. The pressure and velocity distributions of the two ships are compared with each other. The flow structures with large separations are observed and the influences of the box geometry to the flow fields are discussed.