News

ISP Newsletter July 2020

General news

From the beginning of this year Mr. Martin van der Eijk has taken over the tasks of Mr. J.H. Vink as Editorial Office Manager. As announced earlier, the 4^th issue of Volume 66 was scheduled to be a special on the application of hydrogen in the maritime world. This had to be delayed until the first issue of Volume 67. Fortunately it was possible to make a good issue 4 of Volume 66 with articles on different subjects.

ISP Volume 66, Issue 4

ISP Volume 66, issue 4 of 2019, contains four articles. Two of these treat the application of RANS-VoF (Volume of Fluid) modelling. It has been applied to model air lubrication of ship hulls, and to investigate the effect of air pockets on the development of hydrodynamic impact pressures by green water. The two other articles are on sloshing loads on a LNG cargo containment system, and on a control strategy for combined DP station keeping and active roll reduction, respectively. Summaries, as well as the full articles (all open access) can be found on https://content.iospress.com/journals/international-shipbuilding-progress/66/4.

Exploring the limits of RANS-VoF modelling for air cavity flows

Air lubrication techniques have the potential to significantly reduce frictional drag, benefiting sustainable employability of ships. This article aims to explore the limitations and capabilities of RANS-VoF (Volume of Fluid) modelling to numerically model air cavity flows like typically would exist below a ship’s hull. Simulations were performed including numerical uncertainty verification and compared to experimental data for an external air cavity. The simulated velocity profiles at different stream-wise locations in the boundary layer around the cavity compare well to the experimental profiles. However, a mismatch was found in the determination of the required air flow rate for the cavity, which is hypothesized to be mainly caused by the incorrect turbulence modelling around the interface and the advection of a smeared air-water interface in the reattachment zone. This is a direct consequence of the used VoF method. The exact mechanism for air discharge at the cavity closure is still not clear.

A reduced order model for structural response of the Mark III LNG cargo containment system

Highly varying sloshing loads are a superposition of load components resulting from a sequence of different physical phenomena. However, not all features of spatial and temporal variations of sloshing loads and associated phenomena are equally important when failure of the structure is considered. Therefore, the prediction of sloshing loads should be focused on those load components which lead to failure. These components can be found by employing a structural model, which should be fast computationally considering the huge number of possible sloshing loads. This paper presents a reduced order model based on the beam-foundation model which is derived for the Mark-III cargo containment system. The model is validated against a detailed finite element model and it conservatively predicts the stresses at failure locations. The calculation time using the model is approximately two orders shorter in comparison to a finite element model computation, which allows the model to be applied for finding governing load components and associated physical phenomena.

A compressible two-phase flow model for pressure oscillations in air entrapments following green water impact events on ships

A significant part of all structural damage to conventional ships is caused by complex free-surface events like slamming, breaking waves, and green water. During these events air can be entrapped by water. The focus of this article is on the resulting air pockets affecting the evolution of the hydrodynamic impact pressure that loads the ship’s structure. In this study the COMFLOW method has been applied, which is an efficient method based on the Navier–Stokes equations with a Volume-of-Fluid approach for the free surface. It has been extended with a Continuum Surface Force (CSF) model for surface tension. The implementation was verified with benchmark cases and validated by means of a dam-break experiment, a characteristic model for green water impact events. Surface tension was found not to have an influence on entrapped air pocket dynamics of air pockets with a radius larger than 0.08 [m]. For wave impacts it was found that the effect of compression waves in the air pocket dominates the dynamics and leads to pressure oscillations that are of the same order of magnitude as the pressure caused by the initial impact on the base of the wall. The code is available at: https://github.com/martin-eijk/2phase.git.

A control strategy for combined DP station keeping and active roll reduction

Dynamic positioning (DP) systems are used for station keeping during offshore operations. The safety and operability of several offshore operations can be increased when the roll motion is actively controlled, especially in beam seas. A novel control strategy for combined roll motion control and station keeping is proposed, using only the installed DP thrusters. The control strategy is applied to an offshore construction vessel and the performance is demonstrated by time domain simulations. The DP footprint is compared to a conventional dynamic positioning control model. The proposed control model enables active roll reduction while the station keeping performance remains unaffected. The code has been made open source and is available on https://github.com/pwellens/3dp.git.

ISP issues in 2020

For 2020 (Volume 67) four issues of ISP are planned. The first issue (March) is a special issue on the application of hydrogen in the marine field. This issue is meanwhile available: https://content.iospress.com/journals/international-shipbuilding-progress/67/1. In the next ISP newsletter more detailed attention will be given to that issue.

Next meeting of the Editorial Board

Due to the COVID-19 pandemia no new meeting of the Editorial Board has been scheduled for the time being. Contacts will until further notice be made by E-mail, telephone etc.