Second International Symposium on Fishing Vessel Energy EfficiencyE-Fishing, Vigo, Spain, May 2012

Improvement of trawl efficiency using measurements at sea and numerical simulations

Priour Daniel1,

1 RDT/HO, Ifremer, Brest, France.

Abstract—This work has been carried out during the French national project EFFICHALUT. The objective of the project was to improve the energy efficiency of a bottom trawl of a fisherman.

This was achieved through a better understanding of the behaviour of the trawl. This better knowledge comes from i) sea trials during which the shape of the trawl was apprehended via measurement of warp tensions and bridles, and ii) numerical simulations. The measurement showed an imbalance between the upper and lower wings: the tension on the upper bridle was too large compare to the lower, contrary to what was expected. The numerical simulation showed the same imbalance and further exaggerated deformation of the upper wings, again not expected. From this information the fisherman has proposed improvements to the design based mainly on the replacement of part of the netting of the top wing by ropes. The measurement by the fisherman of consumption of fuel during few months shows a decrease of 17% of the fuel consumption of the improved trawl relatively to the reference one. The plan of the trawl from this project has already been distributed to 15 boats. The saving for the entire 15 boats is estimated at 800K€ per year.

Keywords-: Trawl, Energy efficiency, sea trials, numerical simulations

I. INTRODUCTION

The project EFFICHALUT [1] aims to improve energy efficiency of trawls. It is based mainly on i) measurements at sea and simulations in order to improve knowledge of trawl behaviour, ii) improvement of trawl by fisherman from this knowledge.

The measurements at sea are carried out on the reference trawl. These measurements are about shape, tension and consumption. The measurements on the improved trawl are only about fuel consumption. Simulations are dedicated to reference and improved trawls.

II. DEFINITION OF THE REFERENCE TRAWL

The plans for the reference bottom trawl are provided by the fisherman and are in the figures 1 & 2. The doors, from Morgere company, are of 2.9m by 1.85m wide and weigh 1.4 t each.

Figure 1. Design of the back of the reference trawl. The mesh size and twine diameter are on the left. The panels size is noted.

valid. The average speed is about 1.3m/s for valid values

B. Mean values

The table 1 summarizes the mean values and standard deviations on the reference trawl for the second week.

TABLE II. DIFFERENCE BETWEEN THE SIMULATIONS AND THE AVERAGE OF MEASUREMENTS AT SEA DURING THE FIRST WEEK.

Starboard difference

Port difference

Warp tension -2.00% -34.00%

Top bridle tension -36.00% -25.00%

Bottom bridle tension 6.00% -2.00%

Vertical opening -28.00%

Given the inaccuracies in the measurements related to the SCANMAR sensors (poor acoustic connections with the boat, sensors masking) and NKE sensors drift, the simulations can be considered representative of reality. These simulations, or more precisely the numerical model can then be used for the behaviour phase.The simulation shows that the upper wing is largely deformed and that the tension of the upper bridle is larger than the tension of the lower bridle, contrary to what was expected (figure 12).

Figure 12. Simulation of the reference trawl. The upper wing has a too large deformation and the tension in the top wing is too large

In this simulation the tension of the upper bridle is 980Kg and in the lower 830Kg. The tension in the warp in front of the door is 2300kg. These values are consistent with measurements at sea, it is notable that the simulation of the upper wing shows a too large deformation like a bag

V. TRAWL IMPROVED BY THE FISHERMAN

The data collected at sea and the simulation work show that:1. tensions of the upper bridle are too large relatively to

lower bridles,2. simulations show that the upper wings have a large

deformation,The professional has proposed to amend the trawl according to the following figure. The modification is to replace the top wings by ropes. It is expected that this modification :

1. reduces the overall drag, 2. reduces the tensions of the upper bridle 3. reduces the deformation of the upper wing .

Figure 13. Design of the back of the improved trawl. The net of the top wing is replaced by ropes of 1m long. The design is to be compared with figure 1.

The simulation of this improved trawl shows an almost total disappearance of the large deformation of the upper wings (figure 14)

Figure 14. Simulation of the improved trawl. The ropes in the top wing are visible. The large deformation of the wing is gone and the tensions of the upper bridle approach the tension of the lower bridle. This figure is to be

compared with figure 12.

This improved trawl was carried out and used by the fisherman. These improvements are positive: a gain of approximately 17% was measured between the improved trawl (used since beginning 2011) and the reference trawl used up to 2010 (table 3). The trawl is lighter and therefore easier to hang without loss of fishing. Since the improved trawl was used as the first 5 months of 2011, the comparison is carried out only over the same period in 2010 (table 3).

TABLE III. CHANGE IN FUEL CONSUMPTION MEASURED ON THE TRAWLER FOR THE FIRST 5 MONTHS OF 2010 AND 2011. REFERENCE TRAWL WAS USED UP TO 2010 AND IMPROVED TRAWL SINCE 2011. THAT MEANS A

DECREASE OF 17% AND AN ANNUAL ECONOMY OF 55K€ FOR THE BOAT

Year Trawl Consumption on 5 months

Days at sea

Daily consumption

2010 reference 232 612 litre 100 2 326 litre

2011 improved 183 404 litre 95 1 930 litre

VI. DISCUSSION-CONCLUSION

The point that seems the most positive of this project EFFICHALUT is the effectiveness of collaboration between experience of professionals, tests at sea and simulation. In fact, professionals know their gear and they are therefore best placed to improve their nets. Tests at sea and simulation provide new information on trawl allowing fishermen to imagine improvements. Regarding the project EFFICHALUT, simulation has shown both that the top wings were too largely deformed, probably related to excess net. On the other hand it showed that, contrary to what was expected, the tensions in the top wings were larger to the tensions in the bottom wings. This finding was confirmed by measurements at sea. With this information, the professional has proposed the amendment

which was to replace the netting in the top wings by ropes. Use of this trawl showed a decrease in consumption of 17%.It should be noted that already this improved trawl is distributed in the profession: 15 boats in Boulogne sur mer (North France) use it. It is reasonable to assume that fuel economy after this project is around 800 000 € per year based on 15 boats using this improved trawl, consuming 500m³ per year with the reference trawl, a savings of 17% and a fuel price of 60 cents per litre.

ACKNOLEDGMENT

The author would like to thank the partners of the project : Wacogne family, CME, Morgère, LeDrezen, Aemi; the

European Fisheries Fund and the French Ministry of Agriculture and Fisheries for funding this field of research.

REFERENCES

[1] Priour, D., 2012. Rapport final du projet EFFICHALUT , 01 - 2012 – Rapport DCB/RDT/HO/R12-001

[2] Priour, D., 2005. FEM modelling of flexible structures made of cables, bars and nets. In: Proceedings of the 12th international congress of the international maritime association of the Mediterranean IMAM 2005, Lisbon, September 26–30, V.2, pp. 1285–1292, ISBN 0 415 39036 2.