junp-st-rep-23-0013 rev.c dropped object basis of assessment
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8/16/2019 JUNP-ST-REP-23-0013 Rev.C Dropped Object Basis of Assessment
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Project No Unit DocumentType
MaterialCode
Serial No Rev. Page
GF033672 000 RT 3632 0002 C 2/20
BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
ASSESSMENTBP Doc. No: JUNP-ST-REP-23-0013
Confidential: Do not disclose without authorization.
Copyright © Technip. All rights reserved. Technip USA, Inc. TBPE Firm Reg. No. F-3030
REVISION LOG
DATE ORREVISION NO.
SECTION ORPAGE NO.
CHANGE DESCRIPTION
Rev. C Section 5.2Included definition of variables for dissipation ofstrain energy figure
Rev. C Section 6.1 Included assumptions for technical approach
Rev. C Section 6.2.1Added explanations for strain limit and deflectionlimit
Rev. C Section 6.2.2 Included assumption for the dropped object
Rev. C Section 6.2.2
Updated information for topsides deck plate
assessmentRev. C Section 7.0 Updated information for subsea structure
Rev. C Section 7.2.2 Added definition of pipeline diameter
Rev. B No Change
HOLD STATUS
This revision has the following HOLDs:
SECTIONPARAGRAPH
NO.DESCRIPTION OF HOLD
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BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
ASSESSMENTBP Doc. No: JUNP-ST-REP-23-0013
Confidential: Do not disclose without authorization.
Copyright © Technip. All rights reserved. Technip USA, Inc. TBPE Firm Reg. No. F-3030
TABLE OF CONTENTS
1.0 EXECUTIVE SUMMARY ........................................................................................................ 5
2.0 INTRODUCTION ..................................................................................................................... 6
2.1 Project Description ............................................................................................................... 6
2.2 Purpose .................................................................................................................................. 7
2.3 Abbreviations / Designations ............................................................................................... 7
3.0 REFERENCE DOCUMENTS .................................................................................................. 8
4.0 ANALYSIS INPUT DATA ....................................................................................................... 9
4.1 Structural Drawings .............................................................................................................. 9
4.2
Structural Model .................................................................................................................... 9
4.3 Material Data .......................................................................................................................... 9
4.4 Lift Manifest ........................................................................................................................... 9
4.5 Crane Data ............................................................................................................................. 9
4.6 Lifting Path ............................................................................................................................ 9
4.7 No Lifting Zones .................................................................................................................... 9
4.8 Vulnerable Deck Areas ......................................................................................................... 9
4.9 Subsea Layout Architecture ................................................................................................ 9
4.10 Subsea Pipeline Data ............................................................................................................ 9
4.11 Metocean Data ....................................................................................................................... 9
5.0 MECHANICS OF DROPPED OBJECT ................................................................................ 10
5.1 Dropped Object Impact Energy ......................................................................................... 10
5.2 Dropped Object Energy Dissipation Mechanism ............................................................. 10
6.0 TOPSIDES DROPPED OBJECT ANALYSIS ...................................................................... 12
6.1 Technical Approach ............................................................................................................ 12
6.2 Hand Calculations ............................................................................................................... 12
6.2.1 Topsides Deck Beam Assessment ...................................................................................................... 13
6.2.2 Topsides Deck Plate Assessment ....................................................................................................... 15
6.2.3 Topsides Hatch Cover Assessment ..................................................................................................... 16
6.3
Non-Linear Finite Element Analysis .................................................................................. 16
7.0 SUBSEA DROPPED OBJECT ANALYSIS .......................................................................... 17
7.1 Probabilistic Assessment .................................................................................................. 17
7.2 Structural (Consequence) Assessment ............................................................................ 18
7.2.1 Impact Energy ...................................................................................................................................... 18
7.2.2 Impact Absorption Capacity ................................................................................................................. 18
8.0 REFERENCES ...................................................................................................................... 20
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BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
ASSESSMENTBP Doc. No: JUNP-ST-REP-23-0013
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LIST OF FIGURES
Figure 1-1: BPTT Juniper Project Layout ............................................................................................ 6
Figure 4-1: Dissipation of Strain Energy (1) ..................................................................................... 11
Figure 5-1: Fixed-Pinned Beam Plastic Analysis Schematic ........................................................... 13
Figure 5-2: Definition of Distance to Plate Boundary (1) .................................................................. 16
Figure 6-1: Angular Deviation Definition (2) ..................................................................................... 18
LIST OF TABLES
Table 5-1: DNV Recommended Values for εcr and H (Reproduced from Ref (1)) ............................ 14
Table 6-1: Angular Deviation of Dropped Objects (2) ....................................................................... 17
Table 6-2: Damage Classification of Pipelines and Risers (2) .......................................................... 19
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BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
ASSESSMENTBP Doc. No: JUNP-ST-REP-23-0013
Confidential: Do not disclose without authorization.
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1.0 EXECUTIVE SUMMARY
Technip was commissioned by BP to carry out the topsides and subsea dropped object
assessments at the Juniper platform. The purpose of this document is to explain the basis
of assessment for performing dropped object analysis on the Juniper topsides structure
and on the subsea architecture at the Juniper site. The purpose of the topsides dropped
object analysis is to characterize the ability of the Juniper topsides structure to resist
impact loading from potential dropped objects. The purpose of the subsea dropped object
analysis is to quantify the risk to subsea architecture and evaluate structural consequence
associated with potential dropped object impact.
This document discusses the required analysis input data, mechanics of dropped object,
analyses approaches for carrying out the topsides and subsea dropped object
assessments.
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BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
ASSESSMENTBP Doc. No: JUNP-ST-REP-23-0013
Confidential: Do not disclose without authorization.
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2.0 INTRODUCTION
2.1 Project Description
The Juniper Project will develop resources from the Corallita and Lantana fields located in
the Trinidad Columbus Basin. Both fields are located approximately 52 miles east of
Galeota Point, Trinidad, in a water depth of approximately 360 ft.
Figure 2-1 shows the field layout and the existing platforms. The development is an all
subsea scheme accessing both Corallita (3-wells) and Lantana (2-wells) and linking them
back to a newly constructed Juniper platform via individual flexible flowlines from each
well/tree which will be pulled through dedicated platform J-tubes.
The development will produce gas up to 590 MMscfd with first gas scheduled in Q1 2017
from the Corallita and Lantana reservoirs with the production gas to be exported through a
10 km - 26 in. export conventional riser and pipeline from Juniper. The subsea tie-in of the
export pipelines will be accomplished by a subsea wye that will be installed in the pipeline
system from Savonette to Mahogany B.
Figure 2-1: BPTT Juniper Project Layout
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The Juniper jacket will be a stand-alone, Normally Unmanned Installation riser platform(NUI) and will include temporary accommodations for 10 people. It will be a four-legged
fixed jacket type platform with two skirt piles per leg. The jacket legs at the top are spaced
90 feet apart on the long side and 60 feet apart on the short side. The Juniper platform
Jacket will be a welded tubular space frame, fixed to the sea bed by means of skirt piles.
There will be no boat landing on the structure; however, supply vessels will periodically
bring supplies to the platform.
This document includes the basis of assessment to perform boat impact analysis for the
BPTT Juniper platform.
2.2 Purpose
The purpose of this document is to explain the basis of assessment for performing
dropped object analysis on the Juniper topsides structure and on the subsea architecture
at the Juniper site. The purpose of the topsides dropped object analysis is to characterize
the ability of the Juniper topsides structure to resist impact loading from potential dropped
objects. The purpose of the subsea dropped object analysis is to quantify the risk to
subsea architecture and evaluate structural consequence associated with potential
dropped object impact.
2.3 Abbreviations / Designations
Term Definition
API American Petroleum Institute
ASTM American Society of Testing Materials
BPTT BP Trinidad and Tobago
DNV Det Norske Veritas
FEA Finite Element Analysis
GP Group Practice
RP Recommended Practice
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BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
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Confidential: Do not disclose without authorization.
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3.0 REFERENCE DOCUMENTS
The following codes, standards and design guidelines have been used in developing this
document. Unless noted otherwise, latest editions of these documents are recommended.
API RP 2A – “Recommended Practice for Planning, Designing, and Constructing Fixed
Offshore Platforms – Working Stress Design”, 21st Edition
DNV-RP-C204 – DNV Recommended Practice “Design Against Accidental Loads”
NORSOK Standard N-004 – “Design of steel structures”
BP GP 66-02 – BP Group Practice “Structural Design”
Full list of references used in preparing this document are provided in Section 8.0.
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4.0 ANALYSIS INPUT DATA
Input data considered in the dropped object analysis study are outlined in the subsequent
sections.
4.1 Structural Drawings
Structural drawings of primary and secondary members including connection details.
4.2 Structural Model
Latest in-place SACS structural model with operational load conditions.
4.3 Material Data
Structural material information will be obtained from the SACS structural model.
4.4 Lift Manifest
List of objects including tubular, containers and heavy objects that will be transferred to
Juniper platform.
4.5 Crane Data
Information on the physical capacity and dimensions of the operating crane.
4.6 Lifting Path
4.7 No Lifting Zones
4.8 Vulnerable Deck Areas
Information on topsides deck areas that are vulnerable to impact from dropped objects.
4.9 Subsea Layout Architecture
Information on subsea architecture with respect to platform location.
4.10 Subsea Pipeline Data
Dimensions and material properties for the pipelines vulnerable to impact from dropped
objects.
4.11 Metocean Data
Information on water depth, currents and waves.
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5.0 MECHANICS OF DROPPED OBJECT
5.1 Dropped Object Impact Energy
Impact energy associated with a falling object is estimated based on its kinetic energy.
The kinetic energy is governed by mass of object, including any hydrodynamic added
mass, and the velocity of the object at instant of impact as follows:
= × × (in air) Equation 1 (Ref (1))
= × × (in water) Equation 2 (Ref (1))
Where, m = mass of falling object
a = hydrodynamic added mass
v = impact velocity
For impacts in air, the velocity is dependent of fall height as given by:
= √2 × ∗ ℎ Equation 3 (Ref (1))
Where, g = gravitational acceleration
h = distance travelled from drop point
For impacts in water, the velocity depends on the reduction of speed during impact withwater and falling distance relative to the characteristic distance for the object. The
calculation of the velocity through water column and at the instant of impact will be
calculated per recommendations given in DNV-RP-C204 and DNV-RP-F107.
5.2 Dropped Object Energy Dissipation Mechanism
In most cases the kinetic energy of the dropped object is absorbed as strain energy. The
structural response of the dropped object and the impacted component by strain energy
dissipation can be represented as load-deformation relationships shown in Figure 5-1,
where Ro is the object resistance, Ri is the installation (e.g. platform) resistance, dwo is the
object deformation, dwi is the installation deformation, Es,o is the energy dissipation byobject, and the Es,i is the energy dissipation by installation. The areas under the load-
deformation curves equal the strain energy dissipation. Often the dropped object can be
assumed to be infinitely rigid so that all impact energy is absorbed by the impacted
component. In this, the energy dissipation involves large plastic strains and significant
deformations in the impacted component.
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Figure 5-1: Dissipation of Strain Energy (1)
Other forms of dropped object energy dissipation mechanism involve sound, heat, stress
waves and elastic deformation. Typically these dissipation mechanisms are very small
and can be ignored in the calculations.
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6.0 TOPSIDES DROPPED OBJECT ANALYSIS
This section describes the technical approaches that will be considered in topsides
response analyses due to impact loading from dropped objects.
6.1 Technical Approach
Topsides dropped object assessment will be performed using hand calculations and non-
linear finite element analysis. The hand calculations will employ plastic theory analysis for
beam response assessment, and empirical and yield line theory approaches for deck
plates and hatch covers, respectively. Using non-linear finite element analysis (FEA),
structural response of topsides deck as it undergoes large deflections can be explicitly
modelled. The FEA method can be used to verify the results obtained using hand
calculations. Subsequent sections describe the hand calculation and FEA approaches for
estimating the energy absorption capacities of the topsides deck.
Following are the assumptions considered in the analysis:
Simplified hand calculations assume that a member is compact.
Simplified hand calculations do not consider the local denting of a member.
Dropped object is assumed to be rigid and, thus, the energy dissipation by a
dropped object is negligible.
Impact energy is assumed to be dissipated through plastic deformation of the
topsides deck, secondary and primary members.
Only self-weights of the structural members are considered as the gravitation
loads in the analyses.
Additional assumptions considered in the analysis are mentioned in the subsequent
sections.
6.2 Hand Calculations
Topsides hand calculations are performed for the following structural components:
Deck beams
Deck plates
Conductor hatch covers
Following subsections describe hand calculation analysis approaches for the above-
mentioned structural components.
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6.2.1 Topsides Deck Beam AssessmentTo determine the response of the deck beam(s) to dropped object loading, the following
two approaches will be considered:
1. Beam tensile fracture
2. Beam gross deflection
The subsequent sections describe the two approaches in more detail.
Beam tensi le fracture
To determine the impact energy absorbing capacity of the beam due to impact from a
dropped object, simple beam plastic analysis can be performed. Plastic mechanism
schematic and the load-deflection curve for a point load applied at mid-span of the fixed-
simple beam is shown in Figure 6-1. The upper bound theorem of the plastic theory can
be utilized to compute the plastic limit load assuming the full plastic capacity of the beam
can be achieved.
Figure 6-1: Fixed-Pinned Beam Plastic Analysis Schematic
Deflection limit shown in Figure 6-1 can be estimated based on a critical rupture strain
limit using the recommendations provided in DNV-RP-C204. According to DNV
recommendations, for small axial restraint (conservative assumption), the deflection limit
can be computed using Equation 4:
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= × × Equation 4 (Ref (1))
= × [ × 1
3 × 4 × (1 ) × ] × ×
Equation 5 (Ref (1))
= −× × −× ×+
Equation 6 (Ref (1))
Where, w = deflection
dc = diameter of tubular beams, or height of cross-section for symmetric I-
profiles
cw = displacement factor calculated per Equation 5
εcr = critical rupture strain (see Table 6-1 for recommended values)
c1 = 2 for clamped ends, 1 for pinned ends
clp = plastic zone length factor calculated per Equation 6
W = elastic section modulus
Wp = plastic section modulus
εy = yield strain
Lbeam = beam span length
H = steel hardening parameter (see Table 6-1 for recommended values)
Table 6-1: DNV Recommended Values for εcr and H (Reproduced from Ref (1))
Steel Grades US Equivalent (2) εcr H
S 235 ASTM A36 20% 0.0022
S 355 ASTM A572 Gr. 50 15% 0.0034
Once the plastic limit load based on plastic theory and the deflection limit based on tensile
fracture are calculated, the impact absorbing capacity of the beam can be determined by
computing the area under the load-deflection curve shown in Figure 6-1.
Beam gross def lect ion
Where integrity of the piping, process equipment and their supports is of concern due to
deck gross deformation, beam deflection criteria approach can be used. In this approach
the deflection limit shown in Figure 6-1 can be limited by beam span/20 which is
approximately equal to a ductility ratio of 10 (assuming an elastic deflection limit of
span/200) and beam end rotation of 6 degrees.
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6.2.2 Topsides Deck Plate AssessmentThe approach for calculating the deck plate dropped object impact resistance capacity is
adopted from DNV-RP-C204 recommendations for stiffened plates subjected to rigid drill
collar impact. The energy dissipation in plating can be determined based on shear
deformation of the plate at the point of contact force. The plate energy dissipation is given
by:
= × × 1 0.48 × Equation 7 (Ref (1))
Where,
= × × × × +5×−×+.5× ×+ Equation 8 (Ref (1))
= −.5×− × Equation 9 (Ref (1))
= × × × Equation 10 (Ref (1))
= × × × Equation 11 (Ref (1))
= × 0.420.41× Equation 12 (Ref (1))
Where, R = contact force
k = stiffness of plate enclosed by hinge circle
mi = mass of plate enclosed by hinge circle
m = mass of dropped object
f y = steel yield strength
f u = steel ultimate strength
t = plate thickness
d = dropped object diameter
r = smaller distance from the point of impact to the plate boundary defined
by adjacent stiffeners/girders, see Figure 6-2
τcr = maximum shear stress for plugging of plates due to drill collar impact
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Figure 6-2: Definition of Distance to Plate Boundary (1)
The validity for the energy Equation 7 is limited to 7 < 2 r/d < 41, t/d < 0.22, and
mi/m < 0.75 (1).
6.2.3 Topsides Hatch Cover Assessment
The energy absorbing capacity of the hatch covers in the conductor bay area can be
estimated using a yield line theory approach. The hatch covers will require a specific
assessment due to their configuration and support details. In general the hatch covers
have insufficient edge restraint compared to the deck plates. Yield line theory does not
consider membrane action; therefore the energy absorption capacity of the hatches will be
derived from plastic bending.
6.3 Non-Linear Finite Element Analysis
Dynamic non-linear finite element analysis can be undertaken to analyze the response of
the platform deck to a dropped object. The explicit model of the dropped object or a
generalized object can be modelled with a mass and velocity on the onset of impact that
will have the required kinetic energy. The dropped object can be modelled as rigid object
such that all the impact energy is absorbed by deck, conservatively. This type of analysis
will take into account geometric and material non-linearity. Additionally, the structure
response in the FEA approach will account for tension membrane action. In the finite
element analysis, material failure (i.e., rupture) will be explicitly considered using plastic
strain limit. The plastic strain limit will be assumed as 15% in the elasto-plastic model.
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7.0 SUBSEA DROPPED OBJECT ANALYSIS
The subsea dropped object analysis will consist of two parts. The first part of the
assessment will assess the subsea structure risk from the accidental dropped objects.
The risk assessment approach will be performed probabilistically by considering the
frequency of exposure, mechanical drop frequency and probability of impact. The second
part of the assessment will consider the consequence of the dropped objects onto the
subsea structure through structural analysis perspective.
7.1 Probabilistic Assessment
The frequency for objects dropped from cranes impacting subsea structures will be
estimated using the guidelines provided in DNV-RP-F107. This method divides the
subsea into a set of dropped object excursion rings from a drop point and provides
guidelines for estimating the probability of object hitting the seabed. For objects dropped
into sea, the lateral excursion from a drop point is a function of object shape and mass.
The lateral excursion is also affected by currents, especially in deep waters (2). Dropped
object angular deviations as recommended in DNV and used for calculating the dropped
object lateral excursion are summarized in Table 7-1. The definition of the angular
deviation is shown in Table 7-1.
Table 7-1: Angular Deviation of Dropped Objects (2)
Object Description Weight (tonnes)Angular deviation (α)
(Deg.)
Flat/long shaped
< 2 15
2 – 8 9
> 8 5
Box/round shaped
< 2 10
2 – 8 5
> 8 3
Box/round shaped >> 8 2
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Figure 7-1: Angular Deviation Definition (2)
7.2 Structural (Consequence) Assessment
7.2.1 Impact Energy
The impact energy of the dropped object depends on the mass and the velocity of the
object. The velocity of the object through the water column is complex to calculate and
depends on the object shape and mass in water. When an object falling through water
reaches a balance between gravitational forces, displaced volume and flow resistance,
the object will fall with a constant velocity which is called its terminal velocity (2). Theterminal velocity of the object and impact energy that includes the contribution from the
hydrodynamic added mass will be calculated per recommendations presented in DNV-
RP-F107.
7.2.2 Impact Absorption Capacity
The impact absorption capacity of the steel pipelines will be calculated based on the dent
size formed due to impact from a dropped object. Empirical formula to estimate the dent’s
energy absorption capacity has been suggested by DNV-RP-F107 as shown below:
= 16× ×
9 .5
× ×
.5
× ×
.5
Equation 13 (Ref (2))
Where, mp = plastic moment capacity of the pipe wall
δ = pipe deformation, dent depth
t = pipe wall thickness (nominal)
D = steel pipe outer diameter
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BPTT JUNIPER PROJECTDROPPED OBJECT BASIS OF
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Confidential: Do not disclose without authorization.
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The above presented equation is based on a knife-edge load applied perpendicular topipeline and the indenting object covers the whole cross-section of the pipeline. The
effects of internal pressure in the pipes are not included, conservatively (2).
Detailed pipe impact resistance evaluation via finite element analysis can be individually
performed to better understand and capture the response of the pipe to a dropped object.
Recommended qualitative damage classification based on the dent size is presented in
Table 7-2 per DNV-RP-F107. This generalized damage classification will be used to
establish acceptance criteria when assessing the response of pipelines to dropped object
impact.
Table 7-2: Damage Classification of Pipelines and Risers (2)
Dent/Diameter (%) Damage Description
< 5 Minor damage
5 – 10Minor damageLeakage anticipated
10 – 15Major damageLeakage and rupture anticipated
15 – 20Major damageLeakage and rupture anticipated
> 20 Rupture
When estimating the impact resistance of the pipelines, additional impact resistance due
to concrete coating and blankets will be considered per DNV-RP-F107.
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8.0 REFERENCES
1. Det Norske Veritas. DNV-RP-C204 - "Design Against Accidental Loads". October
2010.
2. British Standard. Hot rolled products of structural steels. Part 2: Technical delivery
conditions for non-alloy structural steels. 2004. BS EN 10025-2:2004.
3. Det Norske Veritas. DVN-RP-F107 - "Risk Assessment of Pipeline Protection".
October 2010.
4. Bentley . SACS Software Suite Release 5.4 V8i . Version 5.4.0.12.
5. Technip. BPTT Juniper Project Structural Design Premise BP Doc. No: JUNP-ST-REP-23-001. 9 May 2014. Rev. A. GF033672-000-RT-3600-0001.
6. Veritec. Design guidance for offshore steel structures exposed to accidental loads.
Hovik, Norway : s.n., 1988.
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10. USFOS. USFOS User's Manual - Input Description USFOS Control Parameters.
2014.
11. Simulia Dassault Systèmes. Abaqus 6.13-1 FEA Software. 2014.
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Accidental Loads". 1 October 1981. Rev. 0. TNA 101.
13. BP GP 66-02. Structural Design.
14. Norsok Standard. Design of Steel Structures. October 2004. Rev. 2. N-004.