Stability, safety and operability of small fishing vessels

Ocean Engineering 79 (2014) 8 1 - 9 1

ELSEVIER

Contents lists available at ScienceDirect

Ocean Engineering

j o u r n a l h o m e p a g e : w w w , e l s e v l e r . c o m / l o c a t e / o c e a n e n g

Stability, safety and operability of small fishing vessels

Francisco Mata-Alvarez-Santullano ^•*, Antonio Souto-Iglesias ^•'^

' Maritime Accident and Incident Investigation Standing Commission (ClAiM), Ministry for Development and Transport, Government of Spain, Paseo de la Castellana 67, 28071 Madrid, Spain

^ Model Basin Researcli Croup (CEHINAV), Naval Architecture Department (EJSIN) Technical University of Madrid (UPM), 28040 Madrid, Spain

A R T I C L E I N F O A B S T R A C T Article history:

Received 4 June 2013 Accepted 18 January 2014 Available online 7 February 2014 Keywords: Safety Operability Fishing vessel Seakeeping Intact stability Ship motions I n t h i s paper, t h e r e l a t i o n s h i p b e t w e e n s t a b i l i t y , s a f e t y a n d o p e r a b i l i t y f o r s m a l l f i s h i n g vessels is i n v e s t i g a t e d . T o t h i s a i m , a r e l e v a n t s e t o f s m a l l f i s h i n g vessels is s e l e c t e d . T h e s e h a v e s i m i l a r m a i n d i m e n s i o n s a n d c a p s i z e d i n s t a b i l i t y r e l a t e d a c c i d e n t s b e t w e e n 2 0 0 4 a n d 2 0 0 7 . T h e s t a b i l i t y a n d o p e r a b i l i t y c h a r a c t e r i s t i c s o f s u c h vessels are c o n f r o n t e d w i t h t h o s e o f t h e vessels t h a t h a d b e e n d e c o m m i s s i o n e d i n o r d e r t o b u i l d t h e m , o p e r a t e d b y t h e s a m e c r e w s , i n t h e s a m e areas a n d u s i n g t h e s a m e f i s h i n g g e a r t y p e s . S u c h v e s s e l s a r e c o n s i d e r e d as r e f e r e n c e s a f e v e s s e l s s i n c e t h e i r o p e r a t i o n a l l i f e c a m e t o a n e n d w i t h o u t a n y h a z a r d s . W i t h r e g a r d t o s t a b i l i t y , f u l f i l m e n t o f t h e i n t a c t s t a b i l i t y c r i t e r i a i n f o r c e w h e n t h e v e s s e l s w e r e d e s i g n e d a n d b u i l t is v e r i f i e d . O p e r a b i l i t y c r i t e r i a are s e l e c t e d a n d t h e i r f u l f i l m e n t is a n a l y z e d f o r a r a n g e o f sea s t a t e s , h e a d i n g s a n d v e l o c i t i e s u s i n g l i n e a r s e a k e e p i n g a n a l y s i s . I n l i g h t o f t h i s a n a l y s i s , o p e r a b i l i t y is d i s c u s s e d as a v a l i d i n d i c a t o r o f s h i p s a f e t y . T h i s s t e p is c o n s i d e r e d r e l e v a n t p r i o r t o a n a l y z i n g t h e s e s e t s o f v e s s e l s t h r o u g h s e c o n d g e n e r a t i o n s t a b i l i t y c r i t e r i a u n d e r d e v e l o p m e n t b y t h e I n t e r n a t i o n a l M a r i t i m e O r g a n i z a t i o n , a s u b j e c t o f i m m e d i a t e f u t u r e r e s e a r c h . © 2 0 1 4 E l s e v i e r L t d . A l l r i g h t s r e s e r v e d . 1. Introduction

Commercial fishing is one of the most hazardous occupations today, w i t h fatality rates being widely documented in the specia-lized literature. In the US, they are around 130 fatalities per year for every 100,000 fishing sector workers, compared to 4 for the rest of the sectors (Lincoln and Lucas, 2010); similar alarming figures are reported in the UK (UK MAIB, 2010).

In Galicia, the region of Spain where the fishing sector is most prevalent, w i t h more than one half of the fishing fleet and workers of the whole country, fishing accounts for a very high number of fatal accidents during the working day, second only to the construction sector which employs a much larger percentage of the Spanish working force. It can be seen that most of the casualties happen in vessel-related accidents (or maritime acci-dents) and among these, losses due to stability problems (capsiz-ing or large heel(capsiz-ing) account for half, mainly in vessels of length below 24 m (Miguez Gonzalez et al., 2012). This analysis is consistent w i t h Jin et al. (2001) who demonstrated that capsizing is the type of accident where crewmembers have the highest probability of dying.

* Corresponding author. Tel.: + 3 4 915977159; fax: + 3 4 915978596. E-mail addresses: fmataOfomento.es (F. Mata-Alvarez-Santullano), antonio.souto@upm.es (A. Souto-Iglesias).

' Tel.: + 3 4 913367156; fax + 3 4 915442149.

0029-8018/$-see f r o n t matter © 2014 Elsevier Ltd. All rights reserved. http;//dx.doi.org/10.1016/j.oceaneng.2014.01.011

Between November 2004 and September 2007, five Spanish-flagged fishing vessels capsized due to loss of stability resulting in a large part of their crew dead. Examining the five accidents side by side, it is noticeable that the vessels had similar characteristics, in particular that their lengths ranged between 15 and 24 m and that they had all been built between 1999 and 2001 in accordance w i t h a recent tonnage distribution regulation (Mata-Alvarez-Santullano and Souto-Iglesias, 2013).

The shipowners, masters and crews of the capsized vessels were the same that had been operating the vessels decommis-sioned i n order to build the lost ones. The decommisdecommis-sioned vessels are referred to hereinafter as "predecessors". Moreover, it can be reasonably argued that the uses of the capsized vessels were analogous to the predecessors' since they operated in the same area, using the same fishing gear and in the same social frame-work. The predecessors had been in service for many years, while the five fishing vessels which succeeded them sank, after a short operational life, in stability related accidents. It may therefore be interesdng to try and study the differences, at various levels, of these two sets of vessels.

All these lost vessels complied w i t h the IMO (International Maritime Organization) stability regulations, implemented in 1970 in the Spanish legislation. Despite this fact, the lack of stability caused all accidents. Given the growing complexity and specializa-tion of vessels and also given that the current stability criteria could not properly cover part of the dynamic phenomena present in several stability related accidents, the IMO has recognized that

8 2 F. Mata-Alvarez-Santullano, A. Souto-Iglesias / Ocean Engineering 79 (2014) 8 J - 9 I

Nomenclature KG center of gravity height over base line [m]

LBP length between perpendiculars [m]

B breadth [m] LOA length overall [m]

D depth to main deck [m] T mean draft [m]

DISF displacement [t] RT register tonnage

GM transversal metacentric height [m] VA vertical acceleration [m/s^]

the current stability framework can be improved. It has therefore established working groups to develop the so called "Second Generation Intact Stability Criteria" (SGISC) (IMO, 2008). This new regularion is still under development and has not yet been approved by thé IMO. Surely those works w i l l constitute the basis for future stability regulations, which sooner or later w i l l apply to fishing vessels. The proposed new stability regulation under discussion contemplates the use of CFD codes to assess the ship vulnerability to some stability failure modes. Nevertheless, the IMO working group undertaking this task, has not yet agreed on the stability direct assessment methodologies, that is, the seakeep-ing calculations w i t h CFD codes are put in question by some delegations, mainly due to the lack of validation of the codes (IMO, 2013, 2012).

Mantari et al. (2011) looked at the links between stability and safety by considering the actions of fishing gear, w i n d and beam waves from the perspective of intact stability. They compared the inclining and righting arms for different operational conditions and evaluated the balance of energy of these external forces. They came to the conclusion that the inclining moments deriving from the fishing gear are, in many occasions, at least as important as the heeling moments produced by rough weather scenarios.

The links between safety and seakeeping has been analyzed by some authors under several approaches. Tello et al. (2011) pro-posed a methodology based on seakeeping calculadons for the analysis of fishing vessels operability. They studied several vessels of the Portuguese fishing fleet, proposing operability criteria w i t h their corresponding limiting values. They concluded that roll and pitch criteria are the most often exceeded ones, and identified some trends in hull shape that optimize the fulfillment of those criteria.

The idea that arises is that the relationship between safety and operability needs to be studied.

The masters operate their ships when motions experienced onboard are below certain levels and interrupt fishing operations only when those are surpassed and operation is not possible. They are hence the first to assume that a ship w i t h a larger operability range is a safer ship. This relation needs a rigorous assessment, which we aim at conducting in this paper by analyzing the aforementioned reference case studies, namely, by comparing some stability and operability characteristics between the five vessels lost and their predecessors. It is worth noting that this work does not intend to model specifically the accidents suffered by the lost vessels, neither to assess their stability in rough weather. The aim of the paper is to investigate the relationship between the regulatory stability and operability of these t w o relevant sets of vessels as one necessary step in understanding the limitations and prospects of the former.

The paper is organized as follows: methodology of the analysis is first presented by briefly reviewing IMO regulations on stability and by selecting operability criteria for a seakeeping analysis. Second, the case studies are presented looking at their main dimensions, weights, etc. Third, results for intact stability and operability based on seakeeping analysis are presented and compared between the families of new vessels and decommissioned ones. Finally a discus-sion is provided concerning the limitations of the IMO transversal

stability criteria w i t h respect to prevention of stability failure and the suitability of operability based criteria to help in fishing vessels safety assessment.

2. Methodology

2.J. General

Five fishing vessels that sank due to stability failure are studied. The studied fishing vessels are similar in size, relatively small (LOA between 16 and 20 m), built between 1999 and 2001, and were lost from comparable stability causes between 2004 and 2007. Building any of these vessels meant that one or more existing fishing vessels had to be decommissioned refraining the tonnage of the whole fishing fleet f r o m increasing, according to the European fishing effort reguiations in force. In the five cases studied more than one vessel had to be decommissioned. The largest among the decommissioned vessels w i l l be the selected "predecessor" of the capsized fishing vessel and w i l l be considered a reference safe vessel for comparisons. Normally, and what was the case for the considered pairs of vessels, a fishing vessel and hèr "predecessor" share many characteristics such as master, crew, fishing zones, gear type, base port, etc., since the shipowner is usually the same person.

For each lost fishing vessel and her respecrive predecessor, a characteristic loading condition is established. Each vessel has been studied in one loading condition only, chosen from the available information, normally being the full load condition. In the case of vessels for which no stability booklets were available (most predecessors) using the best available information a loading condirion close to the full load is estimated. Then, stability and operability calculations for both vessels are performed.

2.2. Regulatory stability 2.2.1. General

The vessels stability is checked against the IMO stability criteria for fishing vessels proposed in the "Code on intact stability for all types of ships covered by IMO instruments", approved by the IMO Assembly Resolution A.749(18).

Spain adopted these stability criteria in 1970 which were therefore mandatory when the five vessels studied were designed and built. Since i t was not mandatory when the five predecessors were built, the IMO Severe W i n d and Rolling Criterion (weather criterion) has not been considered in the present study. Regarding the five lost vessels, according to the Spanish stability regulations, compliance w i t h the weather criterion has to be checked only i f the area under the stability curve up to 30° is below 0.065 m rad in the most unfavorable loading condition. All five vessels concerned had larger area stability curves and therefore the previously mentioned weather criterion did not apply in any case.

Intact stability calculations have been performed w i t h state of the art naval architecture software, considering free t r i m . No free surfaces in tanks have been considered. The center of gravity has been considered to be at midship. For the cases where the stability

F. Mata-Alvarez-Santullano. A. Souto-Iglesias / Ocean Engineering 79 (2014) 81-91 83

booklet is available small differences have been found between the calculated cross-curves and the ones in the booklet. These differ-ences are mainly attributed to the hull modeling process when introducing hull form data into the naval architecture software.

2.2.2. Stability index

The stability curves (GZ) give a quick and good idea of the stability characteristics of a vessel and, in our case, noticeable differences have been found between the stability curves of some of the vessels. These differences may be quantified by comparing each single stability criterion. For comparison purposes, a single magnitude to better quantify the differences in regulatory stability between each fishing vessel and her predecessor is desirable and an index related to the KG margin has been devised.

For each vessel, the largest KG for which all the stability criteria are fulfilled is computed; this is the Limiting KG. Then, the stability index (Sl) used to compare each vessel is the ratio between the Limiting KG and the actual KG of the loading condition studied. This SI is presented in percentage terms, and gives an idea of the reserve of stability of the vessel.

2.3. Seakeeping and operability

(1989), on the basis of the relative motions between the vessel and the sea surface given by the seakeeping code.

For the green water on deck criterion, the probability has been calculated at two points for each vessel (deck level fore and aft) and the largest value has been chosen. Vertical and lateral accelerations have been computed at three points for each vessel (working deck fore and aft, and the bridge). The largest value amongst the three is chosen to check the criterion fulfillment. Calculations have been performed for headings from 0° to 180° (head seas) i n steps of 30° and vessel speeds f r o m 0 to 10 knots in steps of 2 knots.

Although, strictly speaking, the proposed criteria limit oper-ability and not safety it is assumed that, for comparative purposes, these operability values may be valid indicators of the vessels safety. It w i l l later be seen, by comparing capsized vessels w i t h their predecessors, that this could in general not be the case. However, it is necessary to be aware of the several limitations of the method used, since a linear seakeeping approach cannot capture some dynamic phenomena in waves - e.g., broaching-to or parametric rolling, and the amplitude of the ship motions in large amplitude waves can be questioned. These limitations must be taken into account when considering the relationship between the calculated operability and ship safety.

2.3.7. Genera/

As mentioned in the introduction, the current stability criteria might not properly cover part of the dynamic phenomena present in several stability related accidents. We propose to explore the link between operability and safety by analyzing the degree of fulfillment of several operability criteria through seakeeping computations. With this aim, the seakeeping performance of the ten vessels has been analyzed. A short-term seakeeping analysis has also been carried out checking ship motions against a series of criterion limiting the ship operability. Tello et al. (2011) proposed seakeeping criteria to assess fishing vessels operability. For the operability analysis presented herein, and their criteria have been used. These are enumerated in Table 1:

Morions response operators have been calculated using the PRECAL linear seakeeping code. PRECAL computes ship motions in the frequency domain using a 3D panel boundary element method formularion (Chow and McTaggart, 1996; PRECAL version 6.6 User Manual, 2010). A non-linear roll damping coefficient of 0.12 has been considered for the roll motion, akin to the value chosen by Tello et al. (2011), considering that the same types of vessels are under analysis in our research. The x, y, z inerda radius rados vs. B, Lbp, Lbp have been esdmated by the PRECAL code, and values between 0.32 and 0.38 are taken for roll, 0.25 for pitch, and between 0.27 and 0.29 for yaw. These latter values are similar to the ones used by Tello et al. (2011) (0.4, 0.25 and 0.25 respectively).

Green water on deck and propeller emergence probabilides have been computed according to the formulation given by Lloyd

Table 1 Operability criteria. Criterion Prescribed m a x i m u m value Cl Roll 6 ' (rms) C2 Pitch 3= (rms)

C3 Lateral acceleration (at the previous three points) 0.1 g (rms) C4 Vertical acceleration (at bridge, w o r k i n g deck fore 0.2 g (rms)

and w o r k i n g deck a f t )

C5 Propeller emergence 15% (probability) C6 Green water on deck (at w o r k i n g deck fore and 5% (probability)

w o r k i n g deck a f t )

2.3.2. Sea state

The operability study has been performed in t w o sea states defined by the significant wave height and modal wave period according to the standardized scale adopted by NATO (Military Agency for Standarizadon, NATO, 1983). For all vessels, SSN4 and SSN5 have been studied, corresponding to significant wave heights of 1.88 m and 3.25 m w i t h modal periods of 8.8 s and 9.7 s respectively.

A Bretschneider sea spectrum has been used. Three o f the five studied vessels were lost in the Adandc Ocean, close to the Spanish north coast; one sank in the Gulf of Cadiz, close to the Gibraltar strait, while the fifth was lost in the Mediterranean Sea, Those sea states chosen have been found to represent the condi-dons of those areas, where the fleet of small coastal fishing vessels operates most of the time. With the RAOs obtained w i t h PRECAL and the previously mendoned spectra, the motion spectra have been computed and used to obtain the rms values necessary to assess the fulfillment of the operability criteria presented in Table 1.

2.3.3. Operability index

For comparative purposes, an operability index (01) has been defined and is calculated as the percentage of combinations speed-heading at which the operating vessel complies w i t h all operability criteria. The 01 can be rigorously established using an auxiliary funcdon Z which depends on speed and heading and is defined as a Boolean function, taking value 1 when at least one criterion is not met and 0 for the safe zone:

01 = 1

io Jo

z(e, v)dv de/(\QK) ₍₁₎

An 01 equal to zero means that at least one operability criterion is surpassed for every combinadon of ship speed and heading. An 01 equal to 1 means the vessel would operate safely at any speed and heading, meedng all operability criteria.

Operability indexes have been obtained for the five pairs of studied ships and their predecessors for speeds f r o m 0 to 10 knots in 2 knot steps and headings from 0° (following seas) to 180° (head seas) in 30° steps. The values of the operability indexes have been interpolated for those points lying inside the intervals defined. This approach is necessary to obtain accurate operability graphs

F. Mata-Alvarez-Santullano, A. Souto-Iglesias / Ocean Engineering 79 (2014) 81-91 84

and hence meaningful OIs. The loading conditions studied are the same as in the stability study.

3. Fishing vessels studied

As mentioned i n the introduction, between November 2004 and September 2007 five Spanish-flagged ships capsized due to transversal stability related causes. Main dimensions and other characteristics of those vessels are presented in Table 2. In this paper, they are labeled as F1-F5. According to the European common fisheries policy, in order to build one fishing vessel, one or more vessels had to be decommissioned accounting for the same gross tonnage and power as the ship to be built. The vessels that were retired f r o m service to build F1-F5 are referred to as "predecessors". Table 3 summarizes the main characteristics of the predecessors, labeled as P1-P5.

The ships in these tables are referred to using the European Fishing Fleet Register (EFFR) code (http://ec.europa.eu/fisheries/ fleet/index.cfm?lg=en).

The case studies LOAs range between 16 and 20 m. They are classified in three categories according to their fishing gear: seines, hook and lines, and gillnets and entangling nets. Predecessors belong to the same categories and are slightly smaller in their main dimensions (e.g. LOA= 11.3-16 m compared to 16-20 m).

The body plans of the ten vessels are shown (not to scale) in Table 4.

Main characteristics of both sets of vessels and loading condi-tions studied are included in Table 5.

4. Results

4.1. Regulatory stability

The stability curves of the vessels studied are presented in Fig. 1. Comparison of stability curves of vessels (F1-F5) and their predecessors (P1-P5) have been plotted jointly.

Regarding the stability curves calculated for each vessel, all predecessors had, i n general, a larger GZ and; thus, better dynamic stability than the lost vessels. The capsized fishing vessels had also a lower GM than their respective predecessors.

Table 2

Fishing vessels lost.

Label Boat code i n European Gear type fleet

Fl ESP25057 Seines 17,00 34.18 F2 ESP24593 H o o k and lines' 16.02 29,97 F3 ESP24391 Seines 18.00 44,83 F4 ESP2435S Gillnets and 20.00 87.03

entangling nets

F5 ESP24199 Seines 19.40 59.01 ^ W h e n capsized the ship was operating as a pot vessel.

The stability index computed for all vessels is presented i n Table 6. As mentioned, the stability index (Sl) for comparison is the limiting KG margin (in percentage terms) over loading condition KG. The values of the stability index for the lost vessels (Sliv) and for the predecessors (Sip), as well as the ratio between the two magnitudes are included. It is remarkable that most predecessors show larger stability indexes than the vessels substituting them. For the pairs F l - P l , F3-P3 and F5-P5 the differences are quite noticeable. It is also remarkable that vessels Fl and F5, although complying vyith the criteria, had very little stability margin. The reason, for the F5 case, is the short range of positive stability, which causes the heel angle corresponding to maximum GZ to be quite close to 25°. This is due to a large part of F5 superstructure not being considered waterright by the designer and not con-triburing to the intact stability. Therefore, the F5 stability curve stops growing when the deck edge gets submerged. It must be highlighted that the marginal compliance w i t h the angle at which the maximum GZ occurs may lead to a rapid capsize in case a strong wind gust is applied to the vessel.

In any case, all the case studies and predecessors fulfilled all applicable IMO stability criteria. Considering that similar opera-rional factors applied for capsized vessels and predecessors, a question mark on the suitability of intact stability criteria for these sets of vessels can be placed. The idea of looking at operability criteria in order to investigate the vessels safety thus arises naturally.

4.2. Operability

In this section, the results of the operability calculations are presented. Figs. 2-6 present a graphical representation of oper-ability w i t h areas where each criterion is exceeded for each vessel, predecessor and sea state. Safe zones where no threshold is exceeded are also shown. Global operability graphics in SSN4 and SSN5 considering simultaneously roll, pitch, lateral accelera-tion and vertical acceleraaccelera-tion (excluding green water on deck and propeller emergence) are presented in Figs. 7 and 8. The oper-ability indexes (01) (Eq. (1)) calculated for all studied vessels are presented in Table 7.

Table 7 summarizes the calculated operability index (01) for all vessels, considering ship motions only, excluding green water and propeller emergence.

2001 2004 Lack of stability; probably s u r f - r i d i n g and broaching 2 0 0 0 2004 Lack o f stability, probably overloading

1999 2004 Lack o f stability, probably s u r f - r i d i n g and broaching 1999 2006 Lack o f stability, probably dead ship c o n d i t i o n and fishing

spaces flooded

1999 2007 Lack of stability, probably inadequate w e i g h t distribution Length overall Tonnage Year of Year of Possible accident causes ( f r o m the official investigation ( m ) (GT) build loss reports)

Table 3

Fishing vessels predecessors to the lost ones.

U b e l Boat code in European fleet Gear type Length overall ( m ) Tonnage (GT) Year o f build Year o f retirement Notes

Pl ESP16060 Seines 15 17.11 1989 2001 Predecessor to ESP25057

P2 ESP11830 Hook and lines 11.3 5.86 1963 2000 Predecessor to ESP24593

P3 ESP05969 Seines 14.1 28.7 1978 1999 Predecessor to ESP24391

P4 ESP00251 Gillnets and entangling nets 16 47 1983 1999 Predecessor to ESP24358

F. Mata-Alvarez-Santullano, A. Souto-Iglesias / Ocean Engineering 79 (2014) 81-91 85

4.2.1. Results analysis

Regarding tiie operability graphs and OI's calculated in the previous section, some conclusions can be drawn:

(1) Most of the lost vessels had greater operability than their predecessors. This difference is most noticeable in SSN4, where significant differences are found for Fl, F2 and F4 w i t h respect to Pl, P2 and P4.

Table 4

Case studies and predecessors' body plans.

Fishing vessels (F1-F5) Predecessors (P1-P5) Fl-Pl

F2-P2

F3-P3

F4-P4

F5-P5

(2) For all vessels, operability deteriorates w i t h increasing sea states. In SSN4 all lost vessels (F1-F5) maintain high levels of operability, w i t h the 01 ranging between 0.81 and 0.96. The lost vessels operability deteriorates significantly in SSN5, except for F4 and F5 that still have 01 of 0.62 and 0.59 respectively. These two vessels being the largest ones amongst the ten studied, them having a larger operability is expected. (3) Regarding the predecessors, more heterogeneity is found.

Apart from P2, the 01 of the other four vessels varies f r o m 0.47 to 0.90. Regardless of the wave height being considered, excessive pitch responses above the limiring criterion have been found for the P2 case, which significantly reduces the global operability of the vessel. This might be attributable to this vessel being the smallest one, w i t h 11 m LOA, well below the rest of the studied vessels. Therefore, operability results for P2 must be cautiously taken.

When examining the operability results in Figs. 2-5, which present criteria separately, some points arise,

(1) In general, predecessors present lower operability than the lost vessels regarding the pitch morion criteria.

(2) Green water on deck and propeller emergence are quite sensitive to the sea state. Most vessels fulfill these criteria i n any combination of heading and speed in SSN4 (Hs=1.88 m) but fail to comply w i t h them in SSN5 (Hs=3.25 m) for most heading/speed combinations. Therefore, the operability of all vessels regarding this criterion drastically deteriorates w i t h relatively small increases in wave height. For this reason, green water and propeller emergence are not useful criteria for comparative purposes between the two sets of vessels studied, since in all cases, the drastic operability deterioration occurs i n a relatively small wave height interval.

(3) The most often exceeded criteria are pitch, roll, and lateral acceleration.

(4) Pitch becomes a problem for ship headings close to 180-' (head seas) and high speeds. When increasing wave height, the pitch in the following waves (0°) also limits operability.

(5) In general, vessels P1-P5 do not present larger roll motions than vessels F1-F5. Roll is not a very limiting criterion in SSN4, except for vessel P2. Regardless of the speed of the vessel being considered and for headings around 9 0 ' (beam waves), roll criterion is a major limiting factor of operability of most of the vessels in SSN5. Notice this feature might pose a problem for vessels that operate at zero speed without manoeuvring capability, especially, purse seiners while pulling the net and pulling catches onboard. A common scenario for those vessels, while pulling the net in bad weather, is to be pushed by wind and waves, ending transverse to the waves and suffering rolls that can put the vessel in danger (Mantari et al., 2011).

Table 5

Main dimensions o f the lost vessels (F1-F5) and their predecessors (PI-PS).

Lbp ( m ) B ( m ) D ( m ) T ( m ) - DISF ( t ) Freeboard ( m ) KG ( m ) RT Nat. roll period (s) GM ( m )

F l 13.5 5 2.35 2.127 71.21 0.223 2.191 30.15 5.2 0.579 Pl 12.6 4.08 1.54 1.05 25 0.49 1.185 13.37 3.3 0.842 F2 13.8 4.57 2.15 1.8 53.23 0.658 1.8 26.03 4,4 0.68 P2 9.5 4.38 1.57 1.17 21.65 0.4 1.334 10.1 3.5 0.93 F3 13.5 5.2 2.35 1.775 80.67 0.336 2.21 23.69 4.8 0.76 P3 12.5 5.12 1.7 1.264 43.6 0.436 1.56 26.4 3.3 1.38 F4 16.2 5.3 2.3 2 97.82 0 3 2.238 34.46 4.3 0.62 P4 14.46 4.7 1,902 1.302 49.2 0.6 1.98 20 3.3 1.1 F5 15.5 5.75 2.5 1.73 90.8 0.759 2.17 32.45 4.8 0.96 P5 14 5 2.068 1.616 68.14 0.455 1.104 32.45 3.4 1.2

86 F. Mata-Alvarez-Santullano, A. Souto-Iglesias / Ocean Engineering 79 (2014) 81-91 Vessels F l - P l D.3 <]Xi ;,':;}..: - t - f i - zsos; • • . P l - 1 C « 0 . — j -^ ! 1 _ T „ il "•".-f-"rr . ! .. • / - • i : --- \ : r--; • • : i fl Mi 20 30 i!> 50 7t) hsel angle (Ï) I • ; | - . : ; .

1

•A

i f ]

"4

1 , 'j

Stability index (SI) o f the lost vessels (F1-F5) and their predecessors (P1-P5).

Fig. 1. Comparison o f stability curves o f vessels (F1-F5) and their predecessors (P1-P5).

(6) Regarding vessel accelerations, some conclusions can also be drawn. Degradation of operability due to the lateral accelera-tion criterion being exceeded w i t h increasing wave heights is significant for most vessels, while vertical acceleration criter-ion being exceeded is less dependent on sea state. Therefore, in general, lateral accelerations limit operability more than vertical ones. Lateral acceleration responses are significant for vessel headings around 90", which is a foreseable result, being strongly correlated w i t h the roll motion.

Lost vessel S I F ( % ) Predecessor Sip (%) Ratio SIp/Slp

Fl 2.7 Pl 11.0 4.6

F2 9.9 P2 19.7 2.0

F3 6.7 P3 51.0 7.6

F4 10.1 P4 8.0 0.8

F5 2.0 P5 37.0 18.5

When looking at operability and taking into account all criteria simultaneously (Figs. 6 and 7), the following conclusions can be drawn:

(1) In general, operability of all vessels is more significandy impaired when facing bow waves (90-180°) for both sea states SSN4 and SSN5.

(2) In SSN5 vessels operability region gets confined to quartering seas (headings from 30° to 90°).

Comparing the operability w i t h the stability characteristics of the ten vessels studied in Secdon 4, some conclusions are reached. (1) The lost vessels have, in general, larger operability but less

stability than their predecessors.

(2) When comparing stability and operability between a lost vessel and her predecessor, large differences in operability do not always imply large differences i n stability. For instance, F4 and P4 have stability indexes quite similar (see Table 6) but the operability differences between F4 and P4 are quite significant (see Figs. 5, 7 and 8). This suggests that there is no direct and obvious correlation between stability and operability.

Summarizing the previous results, it can be stated that oper-ability studies based on linear seakeeping calculations may not be enough to assess ship safety. A consistent relation between ship stability and ship operability, calculated from linear seakeeping methods, has not been found. Due to the particular nature of the case studies (all vessels capsized in stability related accidents) the conclusion can be drawn that a direct link between operability and safety in the five case studies and by extension for these types of vessels cannot be established.

4 . 3 . Additional remarl<s

Some additional issues, which do not directly affect the main conclusions, are worth presenting:

(1) The stability curves for vessels Fl and F3, presented i n Fig. 1, have a peculiar behavior. These curves grow regularly between 0 and 10-20°, then, the curves remain almost horizontal up to 30-40°, and finally the growing ratio increases again. The reason is that the main deck is submerged because of a reduced freeboard, and then the stability increases again when the watertight superstructures are submerged as heel increases.

(2) According to the operability results, in SSN5, for most speeds studied, ships tend to be more operative in headings of 3 0 - 6 0 ° (following waves). It is interesting to realize that navigating w i t h these headings may impose additional risks since the dynamics of vessels in stern seas is not contemplated in the operability studies. It is well documented (IMO, 2007) that for those headings and speeds, fishing vessels may be at risk of surf-riding and broaching. In addition, w i t h respect to Table 3,

F. Mata-Alvarez-Santullano, A. Souto-Iglesias / Ocean Engineering 79 (2014) 81-91

SSN4 ' SSN5

E Mata-Alvarez-Santullano, A. Souto-Iglesias / Ocean Engineering 79 (2014) 81-91 89

it is stated that some of the vessels studied which sank could have experimented surf-riding and broaching.

(3) According to the authors' professional experience, it is not uncommon that fishing vessel masters identify stability w i t h a

low level of motions on board. A vessel w i t h high operability may generate false safety perceptions on the crew, and the master in particular. Small amplitude ship motions and reduced pitch/roll/accelerations may be perceived by the

90 F. Mata-Alvarez-Santullano, A. Souto-Igtesias/ Ocean Engineering 79 (2014) Sl-91

Fig. 8. Operability comparison o f vessels F1-F5 and P1-P5, in SSN5, considering roll, pitch, lateral acceleration and vertical acceleration.

Table 7

Operability indexes of the lost vessels (F1-F5) and their predecessors (P1-P5). F l - P l F2-P2 F3-P3 F4-P4 F5-P5 01 SSN4 Lost vessels 0.94 0.81 0.86 0.92 0.91 Predecessors 0.47 0.00 0.90 0 5 2 0.83 Ratio O I P / O I F 0.50 0.00 1.05 0.57 0,91 01 SSN5 Lost vessels 0.17 0.15 0.30 0.62 0.59 Predecessors 0.08 0.00 0.31 0.08 0.32 Ratio Olp/Olp 0.47 0.00 1.03 0.13 0.54

masters as a symptom of good stability, while according to the findings of this paper i t is not necessarily true; that is to say, there is no strong relation between stability and operability. This suggests that fishing vessel safety cannot be assessed neglecting the human element, only by the study of ship motions or ship stability. This reinforces the idea that more investigation about ship motions on a seaway is needed. The relationship between ship stability, safety and motions must be investigated. These conclusions also suggest that an ade-quate training in stability for fishing vessels masters is needed.

5. Conclusions

In the present paper, the intact stability and short term operability of ten small fishing vessels have been studied. Five vessels built in the same time frame, which sank due to stability related causes in a short period of time, and the five respective vessels which were decommissioned to build those (referred to as "predecessors") have been compared. These predecessor vessels, which ended their service life in a regular way, are considered as a safety reference. It is relevant to stress that the shipowners, masters and crews of the capsized vessels were the same ones that had been operating the predecessors, in the same fishing

areas, using the same fishing gear type, and in the same social framework.

The intact stability of each vessel has been characterized by her stability curve i n a characteristic loading condition, and by a stability index, defined from the limiting KG that allows the ship to fulfill the IMO intact stability criteria. The Weather Criterion has not been considered since it was not applicable to the selected vessels when these were designed and built.

The stability of each lost fishing vessel has been compared w i t h the stability of her predecessor. It has been found that the new vessels had i n general larger stability than the predecessors. Notwithstanding that, the ten vessels fulfilled the IMO stability criteria and hence, f r o m the stability point of view, they can be considered equally safe.

Considering that the capsized vessels and their predecessors were operated in similar contexts, this stability analysis opens a quesrion mark on the suitability of intact stability criteria for these cases and therefore the possibility of analyzing the vessels oper-ability in order to characterize the vessels safety has been explored. The masters operate the ships responding to the f u l f i l l -ment of operability criteria and interrupt fishing operations only when those are surpassed and operation is no longer possible. They are hence the first to assume that a ship w i t h a larger operability range is a safer ship.

Operability of each vessel is established by calculating her short-term motions in two typical sea states w i t h linear seakeep-ing analysis, and checkseakeep-ing these motions against a set of oper-ability criteria. A global operoper-ability index has been defined for comparison purpose. The operability of each pair of vessels (sunken vessel and predecessor) has been compared, resulting in the capsized vessels having more operability than the predecessors.

As a main conclusion, the comparison between the stability characteristics of two sets of vessels and the comparison between the operability characteristics of the same two sets o f vessels throw opposite results. While the predecessors had i n general more stability, the lost ones had in general larger operability. Thus

F. Mata-Alvaiez-Santullano, A. Souto-lglesias / Ocean Engineenng 79 (2014) 81-91 91

the masters of the new fishing vessels could have considered them to be safer as they experienced, in general, lower motions and accelerations, while in fact the new vessels were less stable than their predecessors, and might have required a more careful operation.

Overall, these results indicate that usual operability criteria may not contribute much to assess ship safety during design phases. It also suggests that masters should be strongly trained in stability, maldng them able to adequately manage their vessel's stability regardless of the operability behavior.

As a final remark, taking again into account that the sunken vessels fulfilled IMO stability criteria and had larger operability than the predecessors, we believe that more effort is needed towards developing and validadng new and more complex stabi-lity criteria, able to capture the reastabi-lity of the dynamics of fishing vessels at sea.

Acknowledgments

The authors are grateful to Hugo Gee, José L. Cercos-Pita and Clara Mata for assisting in the preparation of the manuscript. The authors are also grateful to one anonymous reviewer for her/his detailed analysis of the manuscript and subsequent recommendations.

References

IMO, 2008. Report to the M a r i t i m e Safety Committee (SLF 51 Meetings Period No. SLF 51/17), stability, Load Lines and Fishing Vessels Sub-committee. Interna-tional M a r i t i m e Organization, London,

IMO, 2012. Development o f second generation intact stability criteria. International M a r i t i m e Organization, London (Report of the Correspondence Croup on Intact Stability (Part 1) (No, SLF 55/3/1), Stability, Load Lines and Fishing Vessels Sub-Committee),

IMO, 2013. Comments on Present Status of Development of Second Generation Intact Stability Criteria (No. SLF 55/3/7), Stability, Load Lines and Fishing Vessels Sub-Committeelnternational M a r i t i m e Organization, London.

Jin, D., Kite-Powell, H., Talley, W., 2 0 0 1 . The safety o f commercial fishing: determinants of vessel total losses and injuries. J. Saf Res. 32, 2 0 9 - 2 2 8 . Lincoln, J.. Lucas, D., 2010. Commercial fishing d e a t h s - U n i t e d States, 2 0 0 0 - 2 0 0 9 , j .

Am, M e d . Assoc. 304, 1437-1439.

Lloyd, A.RJ.M., 1989. Seakeeping: Ship Behavior in Rough Weather. Ellis H o r w o o d Limited, Market Cross House, Cooper Street, Chichester, West Sussex P019 lEB, England.

Mantari, J,L, Ribeiro e Silva, S„ Cuedes Soares, C„ 2011, Intact stability o f fishing vessels under combined action of fishing gear, beam waves and w i n d . Ocean Eng. 38, 1989-1999.

Mata-Alvarez-Santullano, F, Souto-lglesias, A., 2013. Fishing e f f o r t control policies

and ship stability: analysis o f a string o f accidents in Spain i n the period 2004¬

2007. Mar. Policy 40, 10-17.

Miguez Gonzalez, M . , Caamaiio Sobrino, P, Tedin Alvarez, R., Diaz Casas, V., Martinez Lopez, A„ Lopez Pena, E, 2012. Fishing vessel stability assessment system. Ocean Eng. 4 1 , 6 7 - 7 8 .

M i l i t a r y Agency for Standarization, NATO, , 1983. Standardized Wave and W i n d Environments and Shipboard Reporting of Sea Conditions (STANAG No. 4194). PRECAL version 6.6 User Manual, 2010.

Tello, M . , Ribeiro e Silva, S., Guedes Soares, C, 2011. Seakeeping performance of fishing vessels in irregular waves. Ocean Eng. 38, 763-773,

UK MAIB, 2010, MAIB Annual Report 2010. Maritime Accident Investigation Branch, Government o f UK.

Chow, D.L, McTaggart, K.A., 1996. Validation of SH1PM07 and PRECAL w i t h a Warship Model (Technical M e m o ) . Defense Research Establishment Atlantic Dartmouth, Nova Scotia, Canada,

IMO, 2007. Circular MSC.1/Circ.l228. Revised Guidance to the Master for Avoiding Dangerous Situations i n Adverse Weather and Sea Conditions.