J Fail. Anal. and Preven. (2015) 15:474–479
DOI 10.1007/s11668-015-9977-9
CASE HISTORY—PEER-REVIEWED
Investigation on Failure of a Drilling Rig Mast Structure
D. Ghosh . H. Roy . S. Ray . A. K. Shukla
Submitted: 27 November 2014 / in revised form: 16 April 2015 / Published online: 24 June 2015
Ó ASM International 2015
Abstract A drilling rig that was used for exploration of
gaseous fuels was inspected. During its operation the rig
mast suddenly collapsed and the structure got twisted from
the middle. The failure investigation was carried out to
determine the probable causes of failure in the rig. Estimation of mechanical properties, optical microscopic
analysis along with scanning electron microscopy examinations were necessary supplements to this investigation.
Detailed stress analysis of the structure for different conditions of loads and anchors was examined. It was
ultimately concluded that non-functioning of record-ometer was responsible for rapid increase in operating load
and one or more cables/anchoring points did not have
sufficient strength to withstand the operating load. The
anchoring cable broke with a jerk and the mast structure
collapsed under its weight.
Keywords Defect analysis Failure analysis Fractography Structural Integrity
Introduction
In spite of the best efforts of design engineers and material
scientists, engineering components fail in service. Occurrence of failure of engineering components may lead to
serious consequences such as significant financial loss,
environmental contamination, and even loss of life. In the
event of a failure, it is sometimes essential to investigate
the root cause of failure in terms of design, quality of
D. Ghosh (&) H. Roy S. Ray A. K. Shukla
NDT & Metallurgy Group, CSIR-Central Mechanical
Engineering Research Institute, Durgapur 713209, India
e-mail: dghosh@cmeri.res.in; dbs1012000@gmail.com
123
material, fabrication procedure, or operational error in
handling. This investigation primarily deals with the
assessment of probable cause of damage of an in-service
drilling rig mast structure for exploration of methane gas
from a coal bed. Photographs of mast structure before and
after failure are shown in Fig. 1. There are few available
reports on failure analysis of mast structures [1–5], however, failure of these types of drilling rig mast structures
are very rare as they are too expensive (millions of Euros)
and are handled with utmost care. It is evident from Fig. 1a
that during operation, the mast structure was anchored with
cables and were fixed at different ends such that resultant
of the stresses were concentrated on the main structure.
The complete mast structure was inclined toward the
borehole at an angle of 4°. Four cables, identified (RTG1,
RTG2, FTG1, and FTG2) from the rear and front top ends
of the mast were anchored at four points on the ground.
These points were located at equal angles with symmetry
along the line joining the borehole point and center of the
winch drum. All the cables were positioned at angle of 45°
in the vertical plane. There were two cables (RTC1 and
RTC2) connecting the top end of the mast to the anchors on
the carrier. Two more cables (RMC1 and RMC2) are
connected between the middle portion of the mast and two
anchor points on the carrier. Two cables (RMG1 and
RMG2) are anchored to the ground and connected to the
middle part of the mast. The anchor points on the ground
have also been located in similar manner as of the cables,
used to anchor the top of the mast. Other cables, which do
not share the load on the mast structure, have not been
considered in the analysis.
While in operation, the drilling pipe jammed at a level
of 1175 m, which was slowly brought to a level of 1153 m.
Thereafter, a jarring action was followed with no success.
Hence, an additional pull on the cable was applied to
J Fail. Anal. and Preven. (2015) 15:474–479
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release it. During this operation, the rig mast suddenly
collapsed and the structure twisted from the middle
(Fig. 1b). Circumstantial evidences revealed that the
record-o-graph was non-functional at that time, which
therefore could not record the level of loading applied at
the moment of failure. The root cause of the failure of the
different engineering components has been conducted to
prevent the repetitive failure and at the same time remedial
measures are also highlighted to avoid similar failure in
near future [6–20].
The basic objective of this work was to identify the root
cause of the failure and to ascertain whether it was due to
problem related to materials or the failure was deficiencies
in the design, fabrication, installation, error in functioning,
etc. Apart from preliminary visual examination, chemical
analysis of the chain link material, magnetic particle
examination of welded joints, mechanical testing, fractographic analysis, and detailed finite element analysis using
ANSYS software package form the different steps of this
investigation in order to identify the probable cause of
failure of the mast structure.
(a)
Cables
Cables
Cables
Cables
Carrier
Carrier
Investigations
Visual Examination and Preliminary Findings
Visual examination of the failed mast structure was carried
out and it revealed the following:
(i) The maximum damage and bending/twisting were
localized near to middle portion of the mast
structure (Fig. 2).
(ii) Plastic deformation in the form of bending and
twisting of the failed structure was evident as shown
in the Fig. 3.
(iii) A number of cracks open to the surface of various
magnitudes and direction were present at many of
the welded joints away from the damaged locations
of the mast structure (Fig. 4). It is possible that
appropriate non-destructive testing was not carried
out during its scheduled inspection.
(iv) There was inadequate reporting of previous NDT
inspections.
(v) The stress record-o-graph was not functioning
properly during the event of failure.
Material Characterization
The failed material is subjected to analyze chemically by
spectrometer (Model: Q4 TASMAN, Bruker, Germany).
The results of the analysis of the materials are provided in
Table 1. The chemical analysis confirmed to the specification ASTM-595 grade steel [21]. Microstructural
analysis was carried out from the samples extracted from
region shown in Fig. 3. Tensile tests were carried out as per
ASTM standard E-8m [22]. Flat specimens of 25 mm gage
length and 5 mm thickness were fabricated from the as
(b)
Fig. 1 Drilling rig mast structure during (a) in-service condition and
(b) after collapse
Fig. 2 The middle portion of the mast structure is the region with
maximum damage
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Materials twisted due to plastic
deformation
Location for sample
removal for material
characterization
Fig. 3 Deformed and twisted angle near the middle portion of the
structure
the specimens was taken at 10 kgf load and is reported in
Table 2. The obtained values (mechanical properties) of
the mast structure confirms to the specification of ASTM
595/572A, though the UTS values (528 Mpa) are on the
higher side in comparison to the specified value (450 Mpa).
However, ASTM 595/572A grade materials are recommended for mast structure [6]. The fracture surface was
saw cut and carefully extracted from the failed mast
structure (from location marked in Fig. 3). The fracture
surface was subsequently cleaned by ultrasonic cleaner
(acetone is used as solvent) followed by surface replication.
It is to be noted that the fracture surface being exposed to
open atmosphere for a long time resulted in the formation
of severe corroded layer on its outer surface. An attempt
was made to remove the corroded layer using HCL as
cleaning media. The fracture surface was then examined
using scanning electron microscope (SEM), S-3000N,
Hitachi Ltd, Japan (Figs. 5, 6).
Results and Discussion
Metallographic analysis of a damaged and undamaged
region revealed a microstructure consisting of pearlite and
widmanstatten ferrite (Fig. 5). There was no abnormal
material degradation in the damaged location of the failed
mast structure. Thorough examination of the fracture surfaces (Fig. 6) revealed a texture indicative of cleavage-type
cracking with river line patterns at places along with ductile voids around the vicinity. The crack initiation zone was
observed in Fig. 6b. It can be concluded from the SEM
image that the failure was due to application of the stress
that exceeded the strength of the part. The sudden jerk
during the collapse of the mast structure showed that the
material had undergone high rate of loading and then
plastically collapsed. Some type of steels are stain rate
dependent and may exhibit cleavage-type cracking under
high strain rate, while exhibiting dimple rupture under
slower strain rate. The evidence of ample plastic deformation in the failed component is shown in Fig. 3.
Structural Analysis Using FEM
Fig. 4 Magnetic particle examination revealed presence of fine
cracks in the weld joints at many location of undamaged part of the
mast structure
received failed sample. The tensile properties, yield
strength (YS), ultimate tensile strength (UTS), and percentage elongation are shown in Table 2. The hardness of
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In order to identify probable cause of failure of the mast
structure, simulated analysis of the structure for different
conditions of loads and anchors was examined. The analysis was carried out by finite element method (FEM). The
FEM is a numerical method, being used extensively in the
areas of Solid Mechanics, Heat Transfer, Fluid Mechanics,
Aero-elasticity, coupled field analysis etc. [23]. The
method is based on discretization of a structure in finite
number of inter-connected elements. The physical behavior
of the field variables of the elements, viz., displacement,
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Table 1 Chemical analysis (weight percent) of the samples extracted from the failed mast structure
Element
C
Mn
Si
S
P
Weight percent
0.21
0.85
0.027
0.022
0.012
Specified ranges
0.15–0.25
0.30–0.90
0.035 max
0.035 max
0.060 max
Table 2 Observed tensile properties along with average hardness value
Sample No.
Y.S, MPa
U.T.S, MPa
% Elongation
1
338
528
31
2
340
528
28
3
324
527
27.5
Avg. hardness, 10 Kgf, HV
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Fig. 5 Microstructures of the (a) undamaged part and (b) damaged part of the mast structure
slope (degree of freedom) in case of structural analysis, are
defined in terms of variables at the nodes by suitable
interpolation function. The elastic characteristics of an
element is defined from the geometrical and material
properties of the element and expressed in the form of a
matrix, called element stiffness matrix. The stiffness
matrices of the elements are assembled suitably to form the
global stiffness matrix of the complete assembly. Subsequently, the simulated loads and the support conditions are
applied and the resulting matrix is solved to obtain the
unknown nodal variables, loads, and reactions at supports.
Different conditions have been used to simulate possible
failure causes. Examining different possibilities of failures,
different structural conditions, which may lead to final
failure, have also been examined. Five different configurations have been analyzed to examine the possibility of
failure of any member of the structure or anchoring cable.
All the configurations have been analyzed for 160 T load
on the mast structure.
The probable total cases have been considered are as
follows:
(i) Case-1: The complete structure before failure with
all the anchored cables in positions and 160 T load
on the structure.
(ii) Case-2: The complete structure without one
anchored cable connecting top of the mast and
anchoring point (RTG1).
(iii) Case-3: The complete structure without one
anchored cable connecting top of the mast and
anchoring point (RTC1).
(iv) Case-4: The complete structure without one
anchored cable connecting the middle of the mast
and anchoring point (RMC1).
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Fig. 8 Simulated FEM of the
deformed mast structure without
cables
(v) Case-5: The complete structure without one
anchored cable connecting the middle of the mast
and one anchoring cable (RMG1).
Fig. 6 Fracture surface showing (a) river line and high magnification
image showing (b) river lines and ductile voids along crack initiation
zone
Fig. 7 Finite element models
developed for stress analysis
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Depending on the results, subsequent load cases have been
defined for examination.
The physical properties or real properties of the elements, such as plate thickness, cross-sectional area, and
moment of inertia of different members have been calculated from the measured dimensions of the corresponding
structural members and cables.
The following material properties have been used for
different structural members:
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Modulus of elasticity, E = 210,000 MPa;
Poisson’s ratio = 0.3;
Yield stress = 334 MPa. The finite element model
developed for stress analysis is shown in Fig. 7.
The detailed FEM analyses reveal the following:
The maximum deformation of the structure, maximum
cable tensions and maximum equivalent von Mises stress,
is given by the relation,
p p
req ¼ 1= 2 ðrx ry Þ2 þ ðry rz Þ2 þ ðrz rx Þ2 þ ð6s2xy
þ 6s2zx þ 6s2yz Þ:
ðEq 1Þ
Here rx, ry, and rz represent normal stresses and sxy, szx,
and syz represent shear stresses.
From simulated studies under different conditions of the
mast, it was observed that with all ropes in proper anchored
condition, the mast would not have failed even with
enhanced load or jerks. Therefore, initiation of deformation
must have started owing to failure of some cables. Therefore, the sequence of operations, conditions of the mast,
anchored cables and loads, which have led to failure of the
mast, is concluded as follows:
(a) One of the anchoring cables of the top of the mast
has failed due to either improper anchoring or poor
service condition of the cable.
(b) Excessive load on the mast due to jerks has led to
failure of the anchoring cables of the top of the mast.
(c) Without anchors at the top, stress near the interconnection between the lower and upper parts of the
mast structure goes beyond yield strength and the
mast deforms plastically.
(d) With higher inclination of the mast, effective
bending load on the structure increases substantially
and bending continues further.
(e) Finally, even in absence of load tension, because of
self weight, the mast fails as shown in Fig. 8, similar
to Fig. 1b (actual failure).
Conclusions
Based on the findings of this investigation, it is concluded
that non-functioning record-o-meter did not provide adequate warning that the load applied to the mast was applied
at the dangerous levels. This ultimately led to rapid increase
in operating load. One or more of the cables or the anchoring
points did not have sufficient strength to withstand the rapid
increase in operating load exerted due to the sudden additional pull. One or two anchoring cables at the top failed first,
thereafter stress between the lower and upper parts of the
mast structure was beyond the yield strength and mast
deformed plastically failing under its own weight.
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