Hydrodynamic optimization of twin-skeg LNG ships by CFD and model testing

International Journal of Naval Architecture and Ocean Engineering.
2014.
Jun,
6(2):
392-405

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

- Published : June 30, 2014

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Twin-skeg
;
Single screw
;
LNG ship
;
Dimensional parameter study
;
Hull design parameter optimization
;
Computational fluid dynamic (CFD)
;
Model testing

INTRODUCTION

SSPA experiences a growing interest in twin skeg ship as one attractive green ship solution. The twin skeg concept is well proven with obvious advantages for the design of ships with full hull forms, restricted draft or highly loaded propellers. SSPA statistics, based on over 400 different twin skeg configurations in 200 projects, show that twin skeg hull forms in general require 6% lower propulsion power compared with a single screw ship with the same cargo capacity. For the best quartile of the same statistics the twin skeg hull forms still require 2-3% less propulsive power than the corresponding single screw designs. In addition, the maneuverability and lateral stability are improved, redundancy is added and the risk of excessive propeller induced pressure pulses can be reduced. Some of SSPA research works have been reported by
Williams (1975
;
1980)
and
Berlekom (1985)
.
In recent years large LNG ships with cargo capacity of 150,000
MAIN DIMENSION PARAMETER STUDY

SSPA has an extensive hull data base with over 7000 models tested, including over 400 twin skeg hull forms. The data bank contains information about every single hull form tested such as main dimensions, hull characteristics and – most importantly - resistance/propulsion performance, to which the results of an actual test can be compared, thereby establishing a sort of resistance and propulsion quality of the hull.
Every single model test from those of slender high-speed ships to highest blockage and slowest running tankers has been taken into the data bank from which samples of similar ships can be collected around combinations of hull characteristics that are of interest at the moment. A data output contains frequency distributions of resistance and propulsion quality figures of the sample ships. Accordingly it will be possible to statistically judge the model test results of a new project, the probability of further improvement and to help finding a better performing hull form. Alternatively the data bank can be used to perform statistical speed power predictions and analyze the probability to reach a certain performance.
Another application is to perform main dimension variations according to SSPA’s systematic model series, applying the quality figures and other propulsion characteristics from the data bank, as analyzed vs. the same basis. This will give statistical expectation of the power variation in relation to main dimensions and fullness. Conoco Phillips Co. was our first client to realize the value of such investigations; they commissioned SSPA in 2003 to perform parametric studies for single and twin skeg LNG vessels of various size and fullness. In 2005 investigations for BP Shipping Co. started within their hydrodynamic development program concerning parametric propulsion studies for two types of vessels. Today we are working on the 8
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HULL DESIGN PARAMETER OPTIMIZATION STUDY

From the selected sets of hull data, one twin skeg and one single screw LNG ships was designed. The ship/ propeller with twin skeg are denoted as Ship T/Propeller T and the single screw as Ship S/Propeller S. For proprietary reasons the exact lines can not be shown, but a perspective view of the hulls is given in
Fig. 2
.
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- • Distance between skegs

- Distance between skegs

The distance between the skegs is one of the most important design parameters governing propulsion performance. According to SSPA statistics, distances between 25% and 60% (for special reasons even more) of the vessel’s beam have been applied in a number of projects. In the present study, the distance of initial design (case 2) was increased and decreased by 12.5% (case 1 and case 3, respectively). It can be seen from
Fig. 6
that 2.4% improvement of propulsion performance can be achieved in case 3 (increased distance).
However, as for many regions in hull design the distance between skegs has to be determined by other practical considerations than purely hydrodynamic reasons. A larger distance between the skegs in relation to the hull will provide a possibility to apply larger, slowly rotating propellers with a higher efficiency and provides the possibility of slendering the outside part of the hull and decrease the wetted surface and wave resistance. The resistance of the skegs themselves and the risk for propeller air drawing when rolling in a seaway must be weighed against higher propulsion efficiency. It is often practical considerations that determine size and location of the propellers (e.g. the rpm of suitable engines or the aft ballast draught) and thereby the distance between the skegs. A small distance as in case 1 may also cause interference effects which affect the steering capability of the vessel. The case no 2 was selected for further optimization study.
- Longitudinal slope between skegs

The influence of the longitudinal angle of the center plane was investigated in case 4 and case 5 with a 2.5° lower angle and 2.5° higher angle, as compared to the initial design. The longitudinal slope of the hull bottom between the skegs must have such an inclination (to the horizontal plane) that flow separation in this area is avoided. 1.5% increase of drag for case 5 in
Fig. 4
may indicate some flow separation due to the higher longitudinal slope angle. Additionally a lower longitudinal angle moves the transition point at the flat of bottom further forward which might not be possible due to engine or tank arrangements.
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- Vertical skeg inclination

The vertical inclination of the skegs is another most important design parameter and should be chosen so that a pre-swirl is created in the flow to the propeller. The generated pre-swirl flow can produce favorable interaction with the propeller that improves the propulsion efficiency and results in a power reduction. Two variations were tested; case 6 (5° smaller angle) and case 7 (5° larger angle). It can be seen from
Figs. 5
and
6
that 2% improvement can be achieved in case 7 by increasing the vertical skeg angle (inclining the skeg further away outwards).
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- Longitudinal skeg inclinations

The influence of longitudinal skeg inclination is investigated by decreasing or increasing the angle by 1° in case 8 and case 9. The variations made in any case do not improve the propulsion performance and it can be concluded that the original angle seems to be a good choice.
- Skeg form

The skeg form affects the risk of separation and our experience indicates that an unsymmetrical form with a thinner body inside the skeg than outside is favorable. The influence of skeg form is investigated by slendering inside of skeg shape of case 2 and 3 in case 11 and 12. The results of self-propulsion simulation confirmed our previous experiences that 0.2-0.7% further improvement can be achievable from such a small modification applied to a limited part of skeg.
- Combined design parameter variations

The combined effect of the longitudinal and vertical inclinations is investigated in case 10; which is made by decreasing vertical skeg angle by 5° and increasing longitudinal skeg angle by 1° from the original design. No actual improvement was obtained from this combined variation, the design of twin skeg aftbodies is rather complex and individual as can be seen in
Fig. 3
as an example.
Experience has shown that more than one parameter for the change of skeg shape can be successful. Obviously using a formal optimization would have been beneficial for investigation of the combined effects by coupling the design parameters. However, the considerable CPU time required for combined parameter set renders such an approach not feasible within a reasonable time frame.
CONFIRMATION BY CFD AND MODEL TESTING

Based on the dimensional parametric and hull shape optimization study, the most suitable twin skeg hull design was selected for a comparison of the performances with a single screw ship by means of CFD simulations and model tests. Computations for performance evaluation were made with very dense grids with additional refinements around the stern of the ship including the propeller (s) and rudder (s). The used grids had in total 12.7
- Single screw lng ship-ship S

- Resistance

The predicted resistance components are compared with the test results in
Fig. 7
. The total resistance is slightly under predicted with a deviation of 1.7%.
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- Wake

The predicted model scale wake at V
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- Propeller open water tests

The open water characteristics of the propeller were calculated at a wide range of advance ratios from J = 0.2-0.7. The computed results are compared with the measurement data in
Fig. 11
. It is observed that the thrust and torque are well predicted for most frequent operating range between J = 0.4 and J = 0.6, but under predicted for a J lower than 0.2.
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- Self-propulsion

The predicted propulsion factors are well within 2.3-4.6% (see
Fig. 12
below).
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- Twin skeg lng ship-ship T

- Resistance

The predicted resistance components are compared with the test results in
Fig. 13
. The viscous resistance computed yields a form factor of 0.196, which is somewhat higher than the measured one (0.180). The total resistance is slightly over predicted with a deviation of 3.9%.
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- Wake

The predicted model scale wake at V
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- Propeller open water tests

The open water characteristics of the propeller were calculated at a wide range of advance ratios from J = 0.2-0.7. The computed results are compared with the measurement data in
Fig. 17
. It is observed that the thrust and torque are well predicted for most frequent operating range between J = 0.4 and J = 0.6, but under predicted for lower J than 0.2.
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- Self-propulsion

The predicted propulsive factors are well within 3-4% deviations for thrust, torque and rpm from the measured one. However, of more interest to the ship designer is the propulsion power, which is 8% under predicted (see
Fig. 18
below).
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- Propeller rotation direction

The propulsion performance can be different depending on propeller rotation direction. The propeller rotation direction is in general found to be preferably inwards turning at the top although it depends on the detailed design of the twin skeg hull form. 6 to 7% differences are predicted from the simulation and model test for the present case between inwards and outwards turning, see
Fig. 19
.
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- Rudder angle direction

SHIPFLOW simulations for varying rudder angles and propeller rotational directions were performed in order to find the optimum combination, with respect to required power. Similar to a normal self- propulsion test, the simulations were conducted for the design speed (V
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- Comparison between sinle screw and twin skeg designs

The comparison is made based on the results from the SHIPFLOW simulations and model tests performed for the single screw and the optimized twin skeg hull.
In
Fig. 21
the required delivered power is given and the model test result shows that around 11.4% power reduction can be achieved for the present investigated case with twin skeg design as compared to the corresponding single screw ship. SHIPFLOW prediction confirmed the conclusion from the model tests that twin skeg is the favorable design concept for a 170,000
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CONCLUSIONS

In the present paper, a design optimization study of a 170
Andreasson H.
,
Kaällman M.
,
Liljenberg H.
,
Olofsson H.
,
Trägådh P.
,
Wilske E.
2005
Design for safe and efficient LNG Carriers
2005 SNAME Maritime Technology Conference & Expo and Ship Production Symposium

Berlekom W.B.
1985
Twin skeg afterbodies can save fuel
Workshop on Developments in Hull Form Design

Fabian T.
2010
Parametric modeling and hydrodynamic analysis of twin-skeg vessels. Diploma Thesis
TU-Berlin
Germany

2009
SHIPFLOW 4.7 users manual
FLOWTECH International AB
Gothenburg

Kim H.C.
,
Lee D.Y.
2005
Core design issues of large LNG carrier
GASTECH 2005 conference

Lundstrom P.
2011
Model test report for 170k single screw LNG
SSPA
Gothenburg

Lundstræm P.
2011
Model test report for 170k twin skeg LNG
SSPA
Gothenburg

Williams Åke
1975
Single- and twin-screw propulsion of tankers and bulk carriers, SSPA Internal Report no 74
SSPA
Gothenburg
Proceedings First Ship Technology and Research Symposium

Williams Åke
1980
Some recent trends in hull forms for merchant ships
SSPA
Gothenburg

Citing 'Hydrodynamic optimization of twin-skeg LNG ships by CFD and model testing
'

@article{ E1JSE6_2014_v6n2_392}
,title={Hydrodynamic optimization of twin-skeg LNG ships by CFD and model testing}
,volume={2}
, url={http://dx.doi.org/10.2478/IJNAOE-2013-0187}, DOI={10.2478/IJNAOE-2013-0187}
, number= {2}
, journal={International Journal of Naval Architecture and Ocean Engineering}
, publisher={The Society of Naval Architects of Korea}
, author={Kim, Keunjae
and
Tillig, Fabian
and
Bathfield, Nicolas
and
Liljenberg, Hans}
, year={2014}
, month={Jun}