JUNP-ST-REP-23-0013 Rev.C Dropped Object Basis of Assessment

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Project No Unit DocumentType

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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

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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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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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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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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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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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))

= −× × −× ×+


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


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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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.

7. Jorgen Amdahl, Ernst Eberg. Ship collision with offshore structures. s.l. : Structural

Dynamics EURODYN, 1993. Vol. 93.

8. USFOS. USFOS Getting Started. 2001.

9. Tore Soreide, Jorgen Amdahl, Ernst Eberg, Tore Holmas, Oyvind Hellan. USFOS -

A Computer Program for Progressive Collapse Analysis of Steel OffshoreStructures. Theory Manual. Trondheim, Norway : s.n., 1993. STF71 F88038.

10. USFOS. USFOS User's Manual - Input Description USFOS Control Parameters.

2014.

11. Simulia Dassault Systèmes. Abaqus 6.13-1 FEA Software. 2014.

12. Det Norske Veritas. Technical Note Fixed Offshore Installations "Design Against

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.

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