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TitleRisk Assesment Pipeline
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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
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© Det Norske Veritas

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Norske Veritas.

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Recommended Practice DNV-RP-F107, October 2010
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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.

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Recommended Practice DNV-RP-F107, October 2010
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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:

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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.

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Recommended Practice DNV-RP-F107, October 2010
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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 -

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