structure of a container vessel steering gear C. Dellabiancia, M. - - PowerPoint PPT Presentation

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NDT tests and laboratory examinations on samples taken from the fractured starboard side foundation structure of a container vessel steering gear

• C. Dellabiancia, M. Dellabiancia, P. Del Chiaro, L. Rolla, V De Luise, S. Rinoldi,

C.T.S. Centro Tecnologico Sperimentale S.r.l. Ceparana (SP) ctssp@ctssp.com - ctssp.dir@ctssp.com – www.ctssp.com

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Marine steering gear system generalities Case of study Background Required inspection and laboratory analysis Main outcomes Results and observations Conclusion

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Marine steering gear system generalities

The main control of the steering operations is given from the helm of any ship, similar to an automobile where the entire control of the vehicle’s “steer-ability” rests on the steering wheel of the driver. The ‘control force’ for turning is triggered off from the wheel at the helm, which reaches the steering gear system. The steering gear system generates a torsional force at a certain scale which is then transmitted to the rudder stock that turns the rudder.

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Case of study - Background

The case of study is related to a VESSEL CONTAINER CARGO built in 2003 having overall length of approximately 212m by roughly 30 wide weighing circa 34.000 t.

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Case of study - Background

The vessel had been drydocked in La Spezia for normal refitting and repairing operations on board and for the replacing of rudder’s shaft bushings and couplings. Soon after the end of the operations on board the steering gear has been tested to verify proper functioning before flooding the drydock. During this test a loud noise was heard coming from the steering gear room. Inspecting the main maneuvering room technical personnel documented a severe damage on the STBD’s thrust side foundation of the maneuvering cylinders, which was partly ripped off from the aft peak sewage tank top.

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Case of study - Background

The vessel had been drydocked in La Spezia for normal refitting and repairing operations on board and for the replacing of rudder’s shaft bushings and couplings. Soon after the end of the operations on board the steering gear has been tested to verify proper functioning before flooding the drydock. During this test a loud noise was heard coming from the steering gear room. Inspecting the main maneuvering room technical personnel documented a severe damage on the STBD’s thrust side foundation of the maneuvering cylinders, which was partly ripped off from the aft peak sewage tank top.

The on board personnel declared that at the moment of the steering gear testing none of the safety devices for the pressurized maneuvering system, such as safety valves on hydraulic cylinders and pressure gauge on pump outlet, was activated

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Case of study - Background

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Case of study – STBD’s Thrust side foundation

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Case of study - STBD’s Thrust side foundation

During the first examination, performed directly on board, it was found evident the shiny appearance of the fracture surface; normally this morphological aspect is associated with poor toughness properties of the material

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Case of study – Required inspection – NDT – MPI

Nondestructive testing was required to verify the integrity of welded joints between PORT THRUST side foundation and AFT PEAK TANK TOP No evidence of macroscopic indication to be recorded was found with a superficial check performed by magnetoscopic examination, only marginally detected undercutting next to the beam’s toe.

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Case of study – Required inspection – NDT – UT

Volumetric nondestructive examination was required to verify the soundness of welded joints between PORT SIDE THRUST foundation and AFT PEAK TANK TOP The absence of fully penetration was observed by ultrasonic examination

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Case of study – Laboratory analysis

Soon after the NDT investigation on board it was decided for sampling part of the STBD foundation side to perform specific laboratory analysis with the main objective of verifying both chemical and mechanical characteristics of the material as well as the microstructure of the base material plate

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Case of study – Sampling position

M1 - M2 = macrographic examination of welded structure Tr1 - Tr2 = tensile specimens KV1 - KV2 = impact test specimens MICRO = micrographic examination

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Case of study – Tensile test results

POSITION Tensile strength Rm (MPa) Yield point Rp0,2% (MPa) Elongation A5 (%) Reduction Area Z (%) TR1 412 248 35,8 65,7 TR2 412 247 37,2 67,3 REFERENCE VALUES FOR STEEL GRADE [A] 400 ÷ 520 min 235 min 22

• Obtained values were within the prescribed limits for a normal

strength steel Grade A – According to IACS Rules

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Case of study – Charpy test results

POSITION Energy values (J) Energy average value (J) KV1 41 - 34 - 39 38 KV2 70 - 75 - 59 68 REFERENCE VALUES FOR STEEL GRADE [A] 27 min average value - longitudinal direction 20 min average value - transverse direction Obtained values were within the prescribed limits for a normal strength steel Grade A – According to IACS Rules

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Case of study – Macrographic examination

Macrographic sample M1 position and appearance Lack of penetration Macrographic sample M2 position and appearance Lack of fusion

Ammonium persulphate 10% Ammonium persulphate 10%)

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Case of study – Hardness test results

VERTICAL PLATE WELD METAL BASE PLATE

Hardness test Vickers HV 10

Point 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Line 1 182 180 178 237 285 291 218 213 219 183 188 181 157 156 157 Line 2 166 161 170 208 218 263 226 208 212 176 154 158 140 138 140 Line 3 161 160 171 246 236 273 210 203 199 155 154 148 145 143 142

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Case of study – Chemical analysis

Chemical composition verification has been carried

• ut on macrographic sample M2 in order to analyze

both base material and the weld metal

VERTICAL PLATE BASE PLATE WELD METAL

C (%) Si (%) Mn (%) P (%) S (%) Cr (%) Ni (%) Mo (%) Cu (%) V (%) Ceq (%)

BASE PLATE 0,141 0,13 0,60 0,0189 0,0089 0,30 0,12 0,005 0,03 0,001 0,31 VERTICAL PLATE 0,141 0,27 1,31 0,0139 0,0038 0,33 0,06 0,004 0,03 0.001 0,43 WELD METAL 0,049 0,48 1,13 0,0134 0,0074 0,32 0,08 0,003 0,02 0,001

• No evidence of composition anomalies was noticed.

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Case of study – Micrographic examination

The micrographic examination of sample without metallographic etching has allowed to detect the presence of discrete content of coalescing aluminium oxides (alumina) almost with globular form.

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20

Case of study – Micrographic examination

The micrographic tests have allowed to find a characteristic microstructure for the carbon steel examined, constituted by a regular distribution

• f predominantly ferrite and perlite according

with the tensile strength values obtained in the tensile tests Micrographic appearance of steel : Base Plate Microstructure: Ferrite – Pearlite distribution Etching solution : Nital 2%

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21

Case of study – Further toughness investigation

In order to verify the real behavior of welded structure in the presence of a dynamic load, as probably

• ccurred during the accidental event, three KV impact test specimens were taken, and the notch was

positioned on the same plane corresponding to the fracture progression.

VERTICAL PLATE BASE PLATE WELD METAL

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22

Case of study – Further toughness investigation - Results

POSITION Energy values (J) Energy average value (J)

KV3 14 - 10 - 14 13 REFERENCE VALUES FOR STEEL GRADE [A] 27 min average value - longitudinal direction 20 min average value - transverse direction The obtained values are decidedly lower than the minimum values required for the declared steel grade (A) The toughness of the material according to the test direction corresponding to the fracture progression is considerably poor The appearance of fracture surfaces of the individual impact specimens shows a morphology corresponding to a brittle material without evidence

• f

lateral expansion

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Conclusions and Observations

The base plate steel presented both mechanical properties and chemical compositions within the limits prescribed for a normal strength steel Grade A according to the IACS Rules for hull construction. Regarding the material’s toughness it had been documented tests values close to the lower prescribed limits and especially in the case of the KV3 test also below that specific limit; The welded joints showed no significant defects with the exception of a partial lack of fusion on the root face of the first pass on the base plate (confirmed evidence by UT examination conducted on the port side thrust foundation), it was considered that these minor flaws did not produced and/or influenced the progression of the fracture; regarding the hardness test the detected values are within prescribed limits for the type of joint and the steel in subject; The micrographic tests have allowed to find a characteristic microstructure for the carbon steel examined, constituted by a regular and predominant distribution of ferrite and pearlite, reasonable to be expected by the tensile strength values

• btained in the tensile tests; the micrographic analysis also allowed detecting a discrete rate of the inclusions consisting
• f aluminum oxides (alumina) which may have induced, due to a coalescence phenomenon, laminar defect in the body
• f the plate;

Following previous observations we think that the catastrophic failure of the welded structure has been caused by the effect of a considerable dynamic load applied to a welded structure with severely poor toughness by the beam’s toe on the welding between the thrust side foundation and the aft peak tank top, during the testing operation of the steering hydraulic system without any operating safety device

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23

Thank you for the attention

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Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588

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Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588

centro tecnologico sperimentale s.r.l.

Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588

centro tecnologico sperimentale s.r.l.

Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588

centro tecnologico sperimentale s.r.l.

Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588

centro tecnologico sperimentale s.r.l.

Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588

centro tecnologico sperimentale s.r.l.

Certified laboratory UNI EN ISO 9001 : 2008 BS OHSAS 18001: 2007 Accreditated laboratory UNI CEI EN ISO/IEC 17025 : 2005 Nr.1588