##### Document Text Contents

Page 1

RECOMMENDED PRACTICE

DET NORSKE VERITAS

DNV-RP-F107

RISK ASSESSMENT OF

PIPELINE PROTECTION

OCTOBER 2010

Page 2

FOREWORD

DET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,

property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and

consultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries

out research in relation to these functions.

DNV service documents consist of amongst other the following types of documents:

— Service Specifications. Procedual requirements.

— Standards. Technical requirements.

— Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:

A) Qualification, Quality and Safety Methodology

B) Materials Technology

C) Structures

D) Systems

E) Special Facilities

F) Pipelines and Risers

G) Asset Operation

H) Marine Operations

J) Cleaner Energy

O) Subsea Systems

The electronic pdf version of this document found through http://www.dnv.com is the officially binding version

© Det Norske Veritas

Any comments may be sent by e-mail to [email protected]

For subscription orders or information about subscription terms, please use [email protected]

Computer Typesetting (Adobe Frame Maker) by Det Norske Veritas

If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det Norske Veritas shall pay compensation to such person

for his proved direct loss or damage. However, the compensation shall not exceed an amount equal to ten times the fee charged for the service in question, provided that the maximum compen-

sation shall never exceed USD 2 million.

In this provision "Det Norske Veritas" shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents and any other acting on behalf of Det

Norske Veritas.

Page 22

Recommended Practice DNV-RP-F107, October 2010

Page 22

DET NORSKE VERITAS

The kinetic energy of the object, ET, at the terminal velocity

is:

2

2

1

TT vmE

(14)

Combining these to equations gives the following expression

for the terminal energy:

V

m

AC

gm

E

waterD

T

(15)

In addition to the terminal energy, the kinetic energy that is

effective in an impact, EE, includes the energy of added

hydrodynamic mass, EA. The added mass may become

significant for large volume objects as containers. The

effective impact energy becomes:

2)(

2

1

TaATE vmmEEE

(16)

where ma is the added mass (kg) found by ma = w· Ca ·V.

Tubulars shall be assumed to be waterfilled unless it is

documented that the closure is sufficiently effective during

the initial impact with the surface, and that it will continue to

stay closed in the sea.

It should be noted that tubular objects experiencing a

oscillating behaviour will have constantly changing velocity,

and it has been observed that for 50% of the fall-time the

object have a velocity close to zero (Katteland and

Øygarden, 1995).

5.3.2 Drag and added mass coefficients

The drag and added mass coefficients are dependent of the

geometry of the object. The drag coefficients will affect the

objects terminal velocity of the object, whereas the added

mass has influence only as the object hits something and is

brought to a stop. Typical values are given in Table 11.

Table 11 Drag coefficients

Cat. no. Description Cd Ca

1,2,3 Slender shape 0.7 – 1.5 0.1 – 1.0

4,5,6,7 Box shaped 1.2 – 1.3 0.6 – 1.5

All Misc. shapes

(spherical to complex)

0.6 – 2.0 1.0 – 2.0

It is recommended that a value of 1.0 initially be used for Cd,

after which the effect of a revised drag coefficient should be

evaluated.

5.3.3 Projected area

For long-shaped objects, the projected area in the flow

direction is assumed to equal the projected area of the objects

when tilted at a certain angle. This means that the projected

area of a pipe is:

Apipe = L D sin x (where x

o [0, 90] deg, measured from

the vertical)

As shown in Figure 7, a pipe will constantly change direction

when falling, and so the projected area will also change. A

uniform distribution of the angle should be used, or

alternatively the angle may be taken as 45 for object

categories 1, 2, and 3, respectively. Other objects are

assumed to sink in such a way that the projected area equals

the smallest area of the object.

5.3.4 Energy vs. conditional probabilities

In lieu of accurate information, Table 12 may be used for

energy estimates. Table 12 gives a suggested split of the

object’s energy into energy bands with a conservative

conditional probability of occurrence. The division for the

conditional probabilities is proposed for a pipeline with

normal protection requirement, and a normal distribution of

the impact energies. For pipelines that are required to resist

high impact energies and for which the share of objects that

give high impact energies is significant, a refinement of the

energy groups in the upper range should be considered.

Page 23

Recommended Practice DNV-RP-F107, October 2010

Page 23

DET NORSKE VERITAS

Table 12 Conditional probabilities of impact energies (see notes)

Energy band (kJ)8

Description

< 50 50 - 100 100-200 200-400 400 - 800 > 800

< 2 tonnes 1 30% 18% 14% 12% 11% 15%

2 – 8 tonnes 2 5% 8% 15% 19% 25% 28%

Flat/long

shaped 9

> 8 tonnes 3 - - 10% 15% 30% 45%

< 2 tonnes 4 50% 30% 20% - - -

2 – 8 tonnes 5 - 20% 30% 40% 10% -

Box/round

shaped

> 8 tonnes 6 - - - - 70% 30%

Box/round

shaped

>> 8 tonnes 7 - - - - 30% 70%

1 The distribution is made based on the following assumptions:

Only (open) pipes included.

The objects weigh 0.5, 1.0 and 1.5 tonnes, with 1/3 of all objects within each weight.

The angle at the surface is assumed equally distributed from 0 – 90 degrees.

The terminal velocity is assumed linear from minimum to maximum for 0 and 90 degrees respectively.

The length of the pipes is approximately 12 m.

2 The distribution is made based on the following assumptions:

Only pipes included.

The object weight is assumed equally distributed from 2 to 8 tonnes.

The angle at the surface is assumed equally distributed from 0 – 90 degrees.

The terminal velocity is assumed linear from minimum to maximum for 0 and 90 degrees respectively.

The length of the pipes is approximately 12 m.

3 The distribution is made based on the following assumptions:

The object weights are assumed to be within 9 to 10 tonnes.

Only pipes included.

The angle at the surface is assumed equally distributed from 0 – 90 degrees.

The terminal velocity is assumed linear from minimum to maximum for 0 and 90 degrees respectively.

50% of the pipes have length of approximately 6 m, 50% have length ~12 m.

4 The distribution is made based on the following assumptions:

Objects considered:

The object weigh 0.5, 1.0 and 1.5 tonnes, with 1/3 of all objects within each weight.

Container, baskets (large volume, low density) (30%), velocity ~ 5 m/s

Equipment, e.g. (small volume, massive, high density) (70%), velocity ~10 m/s

5 The distribution is made based on the following assumptions:

The object weight is assumed equally distributed from 2 to 8 tonnes.

Objects considered:

container, baskets (large volume, low density) (70%), velocity ~5 m/s

equipment, e.g. (small volume, massive, high density) (30%), velocity ~10 m/s

6 The distribution is made based on the following assumptions:

The object weigh 10 to 12 tonnes.

Objects considered:

container, baskets (large volume, high density) (70%), velocity ~5 m/s

equipment, e.g. (medium volume, massive, high density) (30%), velocity ~10 m/s

7 The distribution is made based on the following assumptions:

The object weigh above 8 tonnes

equipment, e.g. (massive, high density), velocity ~5 to 10 m/s

8 Added mass is included.

9 For objects dropped from the derrick more objects will have a surface entry angle closer to 90 degrees.

5.3.5 Hit frequency vs. energy

The frequency of hit can be estimated based on the number

of lifts, the drop frequency per lift and the probability of hit

to the exposed sections of the subsea lines. For a certain ring

around the drop point, the hit frequency is estimated by the

following:

rslhitliftliftrslhit PfNF ,,,, (17)

where:

Page 44

Recommended Practice DNV-RP-F107, October 2010

Page 44

DET NORSKE VERITAS

Appendix B. Impact capacity testing procedure

B.1 Introduction

For some components, the stated capacity formulations may not be applicable, or may result in estimates with large

uncertainty, etc. If it is necessary to establish the exact capacity, impact testing may be performed. A procedure for destructive

testing of components to establish impact capacity to be used in risk assessments is presented below. This procedure is focused

on determination of the impact capacity of steel pipes with diameter up to 10”-12”, flexibles and umbilicals.

The testing should reflect the accidental situations under consideration, and should aim to determine the capacity limits for the

different damage categories given in the methodology, e.g. D1 to D3.

B.2 Test energy

The test energy shall be based on the kinetic energy that is representative for the objects that are most likely to hit the

component, as calculated according to section 5.2, or if possible, the energy should be increased until a damage equal to

category D3 is obtained.

B.3 Test Equipment

B.3.1 General

The test rig should simulate a realistic situation. Such tests are not normally instrumented to record the material behaviour

during impact, only the final damage are measured. As the impact calculations for the risk assessment are not detailed, no

instrumentation is necessary.

In the simplest form, the test rig could be a crane with a remotely controlled release hook. It shall be ensured that the test

hammer will not rotate during the testing.

B.3.2 Hammer

The test hammer should normally have a mass of 1 tonnes, see Table B1. The front of the hammer should be made up with a

rectangular plate of 300 mm height/length and 50 mm width with a conical shape and an edge radius of 7 mm.

If the shape of the falling objects is known, e.g. an anchor chain, the actual shape can be used as the hammer front.

B.3.3 Support conditions

The support conditions should represent the most onerous case for the actual configuration, e.g. soil conditions similar to the

actual location, swan neck configuration, etc.

However, if the test is performed on stiff supports, then the test will reflect the true capacity of the component, i.e. all energy

will be absorbed by the component and none transferred to supports. In this way, the results will not be project specific and

may then be used for other projects.

B.4 Procedure

The testing should be repeated to ensure that the results are consistent. For design applications, the lowest reported value

should be used.

For risk assessment, the capacity will normally be the (mean) value found. However, for components where capacity is

sensitive to the shape of the hammer front, the capacity should be taken as 0.9 of the reported (mean) value. Examples of the

latter are multi-layer coatings for pipes, flexible pipes and umbilicals. In Table B1, the profile of the impacting object is given

along with directions to deciding the impact capacity.

Page 45

Recommended Practice DNV-RP-F107, October 2010

Page 45

DET NORSKE VERITAS

Table B1 Impact testing – applicable profile, mass and capacity

Description Test profile Test mass Applicable capacity

Simulating impact of any object

Steel pipes, protected or not R = 7mm 1 tonnes x

Steel pipes with coating (total capacity) R = 7mm 1 tonnes x or x = 0.9xR=7mm

1

Flexibles and/or umbilicals protected R = 7mm 1 tonnes x = 0.9xR=7mm

Any additional protection (not coating) R = 7mm 1 tonnes x or x = 0.9xR=7mm

1

Simulating impact of a 7” pipe (equal to tubing/liner) falling horizontally

Coating for steel pipes Simulate 7” pipe falling horizontally 0.6 tonnes x = 0.9x7” pipe

Flexibles and/or umbilicals Simulate 7” pipe falling horizontally 0.6 tonnes x = 0.9x7” pipe

1 If protection is sensitive to the test profile, R, the capacity should be reduced to 0.9 the observed capacity

Definitions:

x : observed impact capacity

xR = 7mm : observed impact capacity for test profile with R=7mm

x7” pipe : observed impact capacity for test profile that simulates a 7” pipe falling horizontally

R : profile as shown in Figure B1

Where nothing else is indicated, pipelines/umbilicals are considered not protected.

Use of Table B1

This table applies for activities in the vicinity of subsea

templates. The table is to be used as follows:

For the pipeline/umbilical/protection in question, the testing

requirements and applicable capacity can be read in the

relevant row. For example, for a flexible pipe to be tested for

any object hitting the pipe, the following data apply:

– Test profile: R = 7 mm

– Test mass: 1 tonne

– Applicable capacity: x = 0.9·xR=7mm (i.e. the applicable

capacity is 0.9 of the tested value)

R

v

90o

Figure B1 Profile for deciding impact capacity.

- o0o -

RECOMMENDED PRACTICE

DET NORSKE VERITAS

DNV-RP-F107

RISK ASSESSMENT OF

PIPELINE PROTECTION

OCTOBER 2010

Page 2

FOREWORD

DET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,

property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and

consultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries

out research in relation to these functions.

DNV service documents consist of amongst other the following types of documents:

— Service Specifications. Procedual requirements.

— Standards. Technical requirements.

— Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:

A) Qualification, Quality and Safety Methodology

B) Materials Technology

C) Structures

D) Systems

E) Special Facilities

F) Pipelines and Risers

G) Asset Operation

H) Marine Operations

J) Cleaner Energy

O) Subsea Systems

The electronic pdf version of this document found through http://www.dnv.com is the officially binding version

© Det Norske Veritas

Any comments may be sent by e-mail to [email protected]

For subscription orders or information about subscription terms, please use [email protected]

Computer Typesetting (Adobe Frame Maker) by Det Norske Veritas

If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det Norske Veritas shall pay compensation to such person

for his proved direct loss or damage. However, the compensation shall not exceed an amount equal to ten times the fee charged for the service in question, provided that the maximum compen-

sation shall never exceed USD 2 million.

In this provision "Det Norske Veritas" shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents and any other acting on behalf of Det

Norske Veritas.

Page 22

Recommended Practice DNV-RP-F107, October 2010

Page 22

DET NORSKE VERITAS

The kinetic energy of the object, ET, at the terminal velocity

is:

2

2

1

TT vmE

(14)

Combining these to equations gives the following expression

for the terminal energy:

V

m

AC

gm

E

waterD

T

(15)

In addition to the terminal energy, the kinetic energy that is

effective in an impact, EE, includes the energy of added

hydrodynamic mass, EA. The added mass may become

significant for large volume objects as containers. The

effective impact energy becomes:

2)(

2

1

TaATE vmmEEE

(16)

where ma is the added mass (kg) found by ma = w· Ca ·V.

Tubulars shall be assumed to be waterfilled unless it is

documented that the closure is sufficiently effective during

the initial impact with the surface, and that it will continue to

stay closed in the sea.

It should be noted that tubular objects experiencing a

oscillating behaviour will have constantly changing velocity,

and it has been observed that for 50% of the fall-time the

object have a velocity close to zero (Katteland and

Øygarden, 1995).

5.3.2 Drag and added mass coefficients

The drag and added mass coefficients are dependent of the

geometry of the object. The drag coefficients will affect the

objects terminal velocity of the object, whereas the added

mass has influence only as the object hits something and is

brought to a stop. Typical values are given in Table 11.

Table 11 Drag coefficients

Cat. no. Description Cd Ca

1,2,3 Slender shape 0.7 – 1.5 0.1 – 1.0

4,5,6,7 Box shaped 1.2 – 1.3 0.6 – 1.5

All Misc. shapes

(spherical to complex)

0.6 – 2.0 1.0 – 2.0

It is recommended that a value of 1.0 initially be used for Cd,

after which the effect of a revised drag coefficient should be

evaluated.

5.3.3 Projected area

For long-shaped objects, the projected area in the flow

direction is assumed to equal the projected area of the objects

when tilted at a certain angle. This means that the projected

area of a pipe is:

Apipe = L D sin x (where x

o [0, 90] deg, measured from

the vertical)

As shown in Figure 7, a pipe will constantly change direction

when falling, and so the projected area will also change. A

uniform distribution of the angle should be used, or

alternatively the angle may be taken as 45 for object

categories 1, 2, and 3, respectively. Other objects are

assumed to sink in such a way that the projected area equals

the smallest area of the object.

5.3.4 Energy vs. conditional probabilities

In lieu of accurate information, Table 12 may be used for

energy estimates. Table 12 gives a suggested split of the

object’s energy into energy bands with a conservative

conditional probability of occurrence. The division for the

conditional probabilities is proposed for a pipeline with

normal protection requirement, and a normal distribution of

the impact energies. For pipelines that are required to resist

high impact energies and for which the share of objects that

give high impact energies is significant, a refinement of the

energy groups in the upper range should be considered.

Page 23

Recommended Practice DNV-RP-F107, October 2010

Page 23

DET NORSKE VERITAS

Table 12 Conditional probabilities of impact energies (see notes)

Energy band (kJ)8

Description

< 50 50 - 100 100-200 200-400 400 - 800 > 800

< 2 tonnes 1 30% 18% 14% 12% 11% 15%

2 – 8 tonnes 2 5% 8% 15% 19% 25% 28%

Flat/long

shaped 9

> 8 tonnes 3 - - 10% 15% 30% 45%

< 2 tonnes 4 50% 30% 20% - - -

2 – 8 tonnes 5 - 20% 30% 40% 10% -

Box/round

shaped

> 8 tonnes 6 - - - - 70% 30%

Box/round

shaped

>> 8 tonnes 7 - - - - 30% 70%

1 The distribution is made based on the following assumptions:

Only (open) pipes included.

The objects weigh 0.5, 1.0 and 1.5 tonnes, with 1/3 of all objects within each weight.

The angle at the surface is assumed equally distributed from 0 – 90 degrees.

The terminal velocity is assumed linear from minimum to maximum for 0 and 90 degrees respectively.

The length of the pipes is approximately 12 m.

2 The distribution is made based on the following assumptions:

Only pipes included.

The object weight is assumed equally distributed from 2 to 8 tonnes.

The angle at the surface is assumed equally distributed from 0 – 90 degrees.

The terminal velocity is assumed linear from minimum to maximum for 0 and 90 degrees respectively.

The length of the pipes is approximately 12 m.

3 The distribution is made based on the following assumptions:

The object weights are assumed to be within 9 to 10 tonnes.

Only pipes included.

The angle at the surface is assumed equally distributed from 0 – 90 degrees.

The terminal velocity is assumed linear from minimum to maximum for 0 and 90 degrees respectively.

50% of the pipes have length of approximately 6 m, 50% have length ~12 m.

4 The distribution is made based on the following assumptions:

Objects considered:

The object weigh 0.5, 1.0 and 1.5 tonnes, with 1/3 of all objects within each weight.

Container, baskets (large volume, low density) (30%), velocity ~ 5 m/s

Equipment, e.g. (small volume, massive, high density) (70%), velocity ~10 m/s

5 The distribution is made based on the following assumptions:

The object weight is assumed equally distributed from 2 to 8 tonnes.

Objects considered:

container, baskets (large volume, low density) (70%), velocity ~5 m/s

equipment, e.g. (small volume, massive, high density) (30%), velocity ~10 m/s

6 The distribution is made based on the following assumptions:

The object weigh 10 to 12 tonnes.

Objects considered:

container, baskets (large volume, high density) (70%), velocity ~5 m/s

equipment, e.g. (medium volume, massive, high density) (30%), velocity ~10 m/s

7 The distribution is made based on the following assumptions:

The object weigh above 8 tonnes

equipment, e.g. (massive, high density), velocity ~5 to 10 m/s

8 Added mass is included.

9 For objects dropped from the derrick more objects will have a surface entry angle closer to 90 degrees.

5.3.5 Hit frequency vs. energy

The frequency of hit can be estimated based on the number

of lifts, the drop frequency per lift and the probability of hit

to the exposed sections of the subsea lines. For a certain ring

around the drop point, the hit frequency is estimated by the

following:

rslhitliftliftrslhit PfNF ,,,, (17)

where:

Page 44

Recommended Practice DNV-RP-F107, October 2010

Page 44

DET NORSKE VERITAS

Appendix B. Impact capacity testing procedure

B.1 Introduction

For some components, the stated capacity formulations may not be applicable, or may result in estimates with large

uncertainty, etc. If it is necessary to establish the exact capacity, impact testing may be performed. A procedure for destructive

testing of components to establish impact capacity to be used in risk assessments is presented below. This procedure is focused

on determination of the impact capacity of steel pipes with diameter up to 10”-12”, flexibles and umbilicals.

The testing should reflect the accidental situations under consideration, and should aim to determine the capacity limits for the

different damage categories given in the methodology, e.g. D1 to D3.

B.2 Test energy

The test energy shall be based on the kinetic energy that is representative for the objects that are most likely to hit the

component, as calculated according to section 5.2, or if possible, the energy should be increased until a damage equal to

category D3 is obtained.

B.3 Test Equipment

B.3.1 General

The test rig should simulate a realistic situation. Such tests are not normally instrumented to record the material behaviour

during impact, only the final damage are measured. As the impact calculations for the risk assessment are not detailed, no

instrumentation is necessary.

In the simplest form, the test rig could be a crane with a remotely controlled release hook. It shall be ensured that the test

hammer will not rotate during the testing.

B.3.2 Hammer

The test hammer should normally have a mass of 1 tonnes, see Table B1. The front of the hammer should be made up with a

rectangular plate of 300 mm height/length and 50 mm width with a conical shape and an edge radius of 7 mm.

If the shape of the falling objects is known, e.g. an anchor chain, the actual shape can be used as the hammer front.

B.3.3 Support conditions

The support conditions should represent the most onerous case for the actual configuration, e.g. soil conditions similar to the

actual location, swan neck configuration, etc.

However, if the test is performed on stiff supports, then the test will reflect the true capacity of the component, i.e. all energy

will be absorbed by the component and none transferred to supports. In this way, the results will not be project specific and

may then be used for other projects.

B.4 Procedure

The testing should be repeated to ensure that the results are consistent. For design applications, the lowest reported value

should be used.

For risk assessment, the capacity will normally be the (mean) value found. However, for components where capacity is

sensitive to the shape of the hammer front, the capacity should be taken as 0.9 of the reported (mean) value. Examples of the

latter are multi-layer coatings for pipes, flexible pipes and umbilicals. In Table B1, the profile of the impacting object is given

along with directions to deciding the impact capacity.

Page 45

Recommended Practice DNV-RP-F107, October 2010

Page 45

DET NORSKE VERITAS

Table B1 Impact testing – applicable profile, mass and capacity

Description Test profile Test mass Applicable capacity

Simulating impact of any object

Steel pipes, protected or not R = 7mm 1 tonnes x

Steel pipes with coating (total capacity) R = 7mm 1 tonnes x or x = 0.9xR=7mm

1

Flexibles and/or umbilicals protected R = 7mm 1 tonnes x = 0.9xR=7mm

Any additional protection (not coating) R = 7mm 1 tonnes x or x = 0.9xR=7mm

1

Simulating impact of a 7” pipe (equal to tubing/liner) falling horizontally

Coating for steel pipes Simulate 7” pipe falling horizontally 0.6 tonnes x = 0.9x7” pipe

Flexibles and/or umbilicals Simulate 7” pipe falling horizontally 0.6 tonnes x = 0.9x7” pipe

1 If protection is sensitive to the test profile, R, the capacity should be reduced to 0.9 the observed capacity

Definitions:

x : observed impact capacity

xR = 7mm : observed impact capacity for test profile with R=7mm

x7” pipe : observed impact capacity for test profile that simulates a 7” pipe falling horizontally

R : profile as shown in Figure B1

Where nothing else is indicated, pipelines/umbilicals are considered not protected.

Use of Table B1

This table applies for activities in the vicinity of subsea

templates. The table is to be used as follows:

For the pipeline/umbilical/protection in question, the testing

requirements and applicable capacity can be read in the

relevant row. For example, for a flexible pipe to be tested for

any object hitting the pipe, the following data apply:

– Test profile: R = 7 mm

– Test mass: 1 tonne

– Applicable capacity: x = 0.9·xR=7mm (i.e. the applicable

capacity is 0.9 of the tested value)

R

v

90o

Figure B1 Profile for deciding impact capacity.

- o0o -