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Damage Assessment of Offshore Riser-guards under Accidental Vessel Impact
Zubair Imam Syed, Osama Ahmed Mohamed
Department of Civil Engineering, Abu Dhabi University,
Abu Dhabi, P.O. Box 59911, United Arab Emirates
Dinesh Palaniandy, Mohd Shahir Liew
Offshore Engineering Centre, Universiti Teknologi
PETRONAS, Tronoh, 31750 Malaysia
Abstract—Steel riser-guards are installed on offshore jacket
platforms to provide protection to risers against vessel
collision. As risers transport the crude oil which is highly
flammable, a collision between vessel and risers has the
potential of causing severe structural damage to the platform
and injuries to on-board service personnel. Due to lack of in-
depth investigation on the response and damage of
conventional riser-guards under an accidental vessel
collision, the current practice of design of steel riser-guards
is open to further improvement in terms of level of
protection and economy. This paper presents a new
approach in assessing the level of damage on conventional
steel riser-guards, in the event of a collision with offshore
supply vessels. Systematic numerical investigations were
performed using non-linear finite element analysis to explore
the structural response and damage under accidental vessel
impact. This investigation was aimed to assess the adequacy
of the protection provided by the conventional riser-guard
and to ascertain the damage level under its maximum
capacity in energy dissipation. The results from this
investigation show that the conventional riser-guard does not
only provide sufficient protection under accidental vessel
impacts, but also indicates toward an overly-designed
protection system which possesses much higher capacity
than the design requirements for offshore oil producing
regions in South China Sea.
Keywords- steel riser-guard, accidental vessel impact,
damage assessment.
I. INTRODUCTION
Risers in jacket platforms transport highly flammable
crude oil and natural gas extracted from the reservoir to
the processing facilities. Risers are often clamped to the
jacket legs, or to brace members for support and stability.
The location and the nature of risers make them a soft
target under accidental lateral impact from supply vessels.
Several past remarkable incidents and resulting
devastations had prompted the use of riser-guards to
prevent incidents similar to the Mumbai High North
platform explosion [1]. Supply vessels used for providing
services to the offshore platforms come in a variety of
sizes, typically measured in terms of displacement tonnes,
ranging from 1000 to 2500 displacement tonnes
depending on the region of operation. Aas et.al [2]
highlighted on the absence of a properly established ship
routing and scheduling to systematically manage the
operation of vessels in the vicinity of oil platforms, which
exposes the realistic probability of an accidental collision
between the vessels and the risers. The risk of a collision
involving vessels and platforms is closely related to the
density of traffic at the vicinity of platforms. In a special
research report published by the Health and Safety
Department of U.K [3], a total number of 353 incidents
involving supply vessels and platforms at the U.K
Continental Shelf (UKCS) were reported between the
years of 1975 to 2001. Over the years, numerous
protection systems have been employed for riser, and the
use of conventional steel frame attached to the jacket legs
are the most common and widely used. There is a gap in-
depth investigation on the response and damage of
conventional riser-guards under an accidental vessel
collision, the current practice of design of steel riser-
guards is also open to further improvement in terms of
level of protection and economy.
This paper presents a new approach in assessing the
level of damage on conventional steel riser-guards, in the
event of a collision with offshore supply vessels using
systematic numerical investigations which can improve
the understanding of structural response and damage
under accidental vessel impact.
II. COMMON DAMAGE ASSESSMENT
APPROACHES
In the oil and gas industry, commonly used qualitative
structural assessments are carried out periodically on all
assets, which are largely based on visual inspection. For
the case of steel tubular members under impact, the
maximum energy due to impact which a steel tubular
member could sustain is limited by the local buckling on
the compression side, or fracture along the tensile side of
the member. The ductility limit recommended by ISO
19902 states that a steel tubular member can be
conservatively assumed to have disconnected when the
extreme fibre experiences strain values higher than 5% [4].
However, in determining the actual capacity of a structure,
conservatism should be applied minimally to be able to
accurately assess the structural capacity against impact.
The various methods of performing damage assessment
and a comparison study on damage indexes for steel
moment frames was performed by Estekanchi and
Arjomandi [5].
In this present study however, plasticity based damage
assessment was performed on conventional steel riser-
guard by evaluating the percentage of damaged and
undamaged elements during accidental vessel impact. The
American Petroleum Institute (API) Recommended
Practice for Planning, Designing, and Construction of
Fixed Offshore Platforms [6] classifies the steel tubular
members used for offshore structures in to three groups
International Conference on Civil, Materials and Environmental Sciences (CMES 2015)
© 2015. The authors - Published by Atlantis Press
47
and among them Group 1 which consists of mild steel
with minimum yield strength of 240 MPa is commonly
used for offshore riser-guards. Primary members which
are made of high strength steel are designed to operate
within its elastic limit. Any form of permanent
deformation such as denting, would be considered to be
unacceptable as it may lead to a chain of events, causing
major structural failures.
III. THE LOWER SOLUTION BASED ON
INTERPOLATION
Conventional steel riser-guards were modelled using
general purpose finite element analysis software package,
ANSYS Mechanical. Transient non-linear analysis was
performed by applying triangular impulse loads,
equivalent to vessel collision on the conventional riser-
guard and the resulting structural response are used to
perform damage assessment. The method used to establish
the collision forces equivalent to vessel impact has been
previously highlighted in a separate publication by the
authors [7]. A similar approach was also used by Jin et al.
[8] in assessing damage due to barge impact on brace
members of jacket platform. The riser-guards modelled in
this study had a width of 14 meters and height of 10
meters. The riser-guard material was mild steel with yield
strength of 240 MPa, ultimate strength of 430 MPa
whereas yield and ultimate strains used were 0.0012 and
0.2 respectively.
Bilinear kinematic plasticity model was utilized to
take into account the structural behaviour beyond the
elastic limit. Two scenarios were simulated in this study,
which are the broadside impact and bow impact at the
mid-span of the riser-guard. Impulse forces for both
broadside and bow collision was idealised to be acting
uniformly across the highlighted members, which are
highly likely to come into contact during collision. The
probable locations of contact between vessel and riser-
guard were established based on the typical configuration
of supply vessel with bulbous bow, as shown in Fig. 1.
Fig. 2 shows the CAD model of the conventional steel
riser-guard, with the location of impact highlighted in red.
Figure.1. Typical supply vessel configuration [9]
Fig. 2. Location of vessel impact; (a) broadside impact and (b) bow
impact
Fig. 3 shows the maximum deformation of riser-guard
for both broadside impact and bow impact. The broadside
impact results in global structural deformation, indicating
that the impact energy dissipation was achieved by global
structural displacement. Impact energy dissipation for the
case of bow impact however, differs from the broadside
impact, where local denting was predominant in impact
energy dissipation.
(a)
(b)
Fig.3. Maximum deformation of riser-guard for: (a) broadside vessel
impact and (b) bow vessel impact
IV. DAMAGE ASSESSMENT
In the finite element simulation, two approaches are
used to identify damage. Total strain at critical joints
which has the potential to clearly indicate the damage
states of the riser-guard members were used along with
the distribution of damaged and undamaged elements to
map the damage level of riser-guards.
A. Total Strain at Critical Joints.
Prior to performing the analysis, critical joints were
first identified and the strain levels at these locations were
extracted. A failure strain of 0.2 m/m was selected to
(a) (b)
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describe the ultimate strain limit before fracture. Fig. 4
shows the total strain distribution along primary members
for broadside and vessel bow impact. The locations of
critical joints were highlighted with red circles in Fig.4 (a).
Failure of these critical joint would result in significant
reduction in the lateral stiffness of the structure, and
therefore it was assumed that when the failure strain limit
at critical joints were exceeded, the global structure of
riser-guard could be considered to have undergone overall
structural failure. Strain distribution Fig.4 (a) indicates
that during vessel collision, the damage primarily occurs
at the critical joints, where the strain values are much
closer to the failure limit. However, Fig.4 (b) shows that
the damage from vessel bow impact is concentrated only
at the location of contact, where maximum strain occurs at
mid-span of riser-guard.
.
(a)
(b)
Fig.4. Maximum strain distribution along primary members for (a)
broadside vessel impact, (b) vessel bow impact.
The critical joints show strain values much lower than
the failure limit, suggesting that in the event of a vessel
bow impact, the riser-guard is likely to experience local
damage while the global structure remains relatively
intact, unlike impact from broadside of a vessel. Fig. 5
and Fig. 6 show the time-history of the total strain
development at the critical joints.
Fig.5. Total strain at critical joint resulting from broadside vessel impact
Fig.6. Total strain at critical joint resulting from vessel bow impact
B. Total Strain at Critical Joints.
An element which exceeds the elastic limit of 0.12% is
considered to have experienced damage, whereby the
element remains permanently deformed. The distribution
of both undamaged and damaged elements for both
broadside impact and bow impact is presented in Fig. 7.
This figure shows that for the case of broadside vessel
impact, elements remaining within the elastic limit are
less than 40%. However, most of the damaged element
remains within the range of 0.0012 m/m to 0.005 m/m,
which barely exceeds the elastic limit and remains
relatively far from the failure limit. This suggests the
presence of additional capacity for higher impact energy
dissipation, provided that the critical joints remain intact.
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0
10
20
30
40
50
60
70
0‐0.0012 0.0012‐0.005 0.005‐.01 0.01‐0.05 0.05‐0.1
Percentage of damaged
elements, %
Strain range, m/m
Distribution of damaged elements
Bowimpact
Broadsideimpact
Fig.7. Distribution of damaged and undamaged elements along primary members for vessel broadside impact
From Fig. 7 it can also be observed that more than
60% of the elements remain undamaged as a result of
vessel bow impact. This also indicates to the damage
being concentrated at the location of contact between
vessel and riser-guard while elements away from the local
damage remain relatively undamaged.
V. CONCLUSIONS
The conventional riser-guards are designed to sustain
impact energy of up to 2.25 MJ, from accidental vessel
collision. Two possible scenarios were investigated in this
study, and the following conclusion can be drawn:
1. The protection provided by conventional riser-guard
for 2.25 MJ of impact energy is sufficient.
2. The conventional riser-guard possesses higher
capacity of impact energy dissipation than the
requirement for some regional operations like that of
South China Sea.
3. Protection against vessel broadside impact is much
more critical as it results in global structural damage.
Critical joints under broadside impact were much closer to
failure as compared to those from vessel bow impact.
4. The maximum deformation corresponding to 2.25
MJ is lesser than the available clearance between riser-
guard and risers on a typical jacket platform.
ACKNOWLEDGEMENTS
The first author would like to show his appreciation to
the Offshore Engineering Centre of Universiti Teknologi
PETRONAS (UTP) for providing resources and some
technical data for this research.
REFERENCES
[1] Mitra, N. K., Dileep P. K., Kumar A. Revival of Mumbai High
North - A Case Study. Proceedings of SPE Indian Oil and Gas
Technical Conference and Exhibition, Mumbai, 2008.
[2] Aas, B., Halskau, Wallace, S. W. On the role of supply vessels in
offshore logistics, Journal of Maritime Economics and
Logistics,11, pp. 302-325, 2009.
[3] Robson, J. K. Ship/platform Collision Incident Database. HSE
Books, Norwich, 2003.
[4] International Standard-ISO:19902. Petroleum and Natural Gas
Industries- Fixed Offshore Steel Structures. ISO, Switzerland,
2007.
[5] Estekanchi, H., Arjomandi, K., On the Comparison of Damage
Indexes in Nonlinear Time History Analysis of Steel Moment
Frames, Asian Journal of Civil Engineering (Building and Houses),
8(6), pp. 629-646, 2007.
[6] American Petroleum Institute. Recommended Practice for Planning,
Designing and Constructing Fixed Offshore Platforms - Working
Stress Design. API, Washington, 2007.
[7] Palaniandy, D. K., Liew, M. S., Karuppanan S., Syed, Z. I.
Estimation of Vessel Stopping Time During Collision with
Offshore Riser-Guard. Proceedings of FTSCEM, Bangkok, pp.
116-120, 2013.
[8] Jin, W., Song, J., Gong, S., Lu, Y. On the Evaluation of Damage 0n
Offshore Platform Structures due to Collision of Large Barge,
Journal of Engineering Structures, 27, pp. 1317-1326, 2005.
[9] Hartman Offshore, “Anchor Handling Tug Supply Vessel,” HO,
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offshore.com/fleet.php?l=en. Accessed 15 April 2014].
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