Anchor Loads on Pipelines

89 Pages·2015·2.99 MB·English

by  Ying Wei

Anchor Loads on Pipelines
Ying Wei
Maritime Engineering
Submission date: June 2015
Supervisor: Svein Sævik, IMT
Norwegian University of Science and Technology
Department of Marine Technology

NTNU
MASTER THESIS
Norwegian University of Science and Technology
Department of Marine Technology

Title: Delivered:
10.06.2015
Anchor loads on pipelines
Availability:

Student: Number of pages:
Wei Ying 86
Abstract:
Anchor hooking on a subsea pipeline has been investigated in this thesis. Anchor loads on
pipelines is in general a rarely occurring event, however, the severity when it occurs could
easily jeopardize the integrity of any pipeline. It is considered as an accidental load in the
design of pipelines.
Pipeline Loads, limit state criteria and anchor categories are defined by the DNV standards.
For pipeline, DNV-OS-F101 (08.2012), Submarine Pipeline Systems is adopted. Offshore
standard DNV-RP-E301 Design and Installation of Fluke Anchors and DNV-OS-E301 Position
Mooring are adopted for the anchor system analysis.
SIMLA models are established to check whether the pipelines can be hooked by anchors. A
short rigid model is established first, which allows efficient parameter studies with respect
to pipe diameter, span height, anchor size and chain length to find the circumstances
where hooking can take place. Then “long” pipe models are investigated for selected cases
to show the pipeline response curves.

Keyword: Advisor:
Acidental load Prof. Svein Sævik, NTNU
Dr. Erik Levold, Statoil
Anchor hooking

Address: Location Tel. [phone]
NTNU Marinteknisk Senter Fax [phone]
Department of Marine Technology O. Nielsens vei 10
N-7491 Trondheim
NTNU Trondheim
Norwegian University of science and technology

THESIS WORK DESCRIPTION
MASTER THESIS SPRING 2015
Stud. tech. Ying Wei

Anchor loads on pipelines
Ankerlaster på rørledninger

Anchor loads on pipelines is in general a rarely occurring event, however, the
severity when it occurs could easily jeopardize the integrity of any pipeline. It is
considered as an accidental load in the design of pipelines. In the Norwegian Sea
there are several locations where the subsea pipeline density is high, also in
combination with high vessel density. The vessels usually know where pipelines are
located and avoid anchoring, but anchors might be dropped in emergencies, lost in
bad weather or due to technical failures. In these cases, the drop might not be
noticed before the anchor hooks, e.g. in a pipeline.
The master course is to be based on the project work carried out in the fall semester
2014 which included the following activities:
1. Literature study on pipeline technology, relevant standards for pipeline design,
with particular focus on impact loads. Aspects related to vessel size, frequencies
and corresponding anchor equipment is to be included.
2. Study the theoretical background for and get familiarized with the computer
program SIMLA
3. Define the basis for a case study considering anchor geometry, pipeline
mechanical properties, soil interaction parameters, wire chain capacity, water
depth and hydrodynamic coefficients
4. Establish a SIMLA model for the hooking event and perform simulations to
demonstrate the performance of the model
From that basis, the study is extended to include:
1. Establish a short generic model that allow efficient parameter studies with
respect to pipe diameter, span height, anchor size, water depth, chain length to
find the circumstances where hooking can take place (the pipe might be short
and rigid)
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NTNU Trondheim
Norwegian University of science and technology

2. Then make “long” models for selected cases and establish the pipeline response
curve until the anchor chain fails.
3. Conclusions and recommendations for further work
All necessary input data are assumed to be delivered by Statoil.
The work scope may prove to be larger than initially anticipated. Subject to approval
from the supervisors, topics may be deleted from the list above or reduced in extent.
In the thesis the candidate shall present his personal contribution to the resolution of
problems within the scope of the thesis work
Theories and conclusions should be based on mathematical derivations and/or logic
reasoning identifying the various steps in the deduction.
The candidate should utilise the existing possibilities for obtaining relevant
literature.
Thesis format
The thesis should be organised in a rational manner to give a clear exposition of
results, assessments, and conclusions. The text should be brief and to the point,
with a clear language. Telegraphic language should be avoided.
The thesis shall contain the following elements: A text defining the scope, preface,
list of contents, summary, main body of thesis, conclusions with recommendations
for further work, list of symbols and acronyms, references and (optional) appendices.
All figures, tables and equations shall be numerated.
The supervisors may require that the candidate, in an early stage of the work,
presents a written plan for the completion of the work.
The original contribution of the candidate and material taken from other sources
shall be clearly defined. Work from other sources shall be properly referenced
using an acknowledged referencing system.

The report shall be submitted in two copies:
- Signed by the candidate
- The text defining the scope included
- In bound volume(s)
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Norwegian University of science and technology

- Drawings and/or computer prints which cannot be bound should be
organised in a separate folder.
Ownership
NTNU has according to the present rules the ownership of the thesis. Any use of the
thesis has to be approved by NTNU (or external partner when this applies). The
department has the right to use the thesis as if the work was carried out by a NTNU
employee, if nothing else has been agreed in advance.
Thesis supervisors:
Prof. Svein Sæ vik, NTNU
Dr. Erik Levold, Statoil

Deadline: Juni 10, 2015

Trondheim, Januar, 2015

Svein Sæ vik

Candidate – date and signature:
15, 01, 2015

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NTNU Trondheim
Norwegian University of science and technology

PREFACE
This master thesis is performed as a part of my Nordic Master degree in Marine
Technology, with specialization in Ocean Structures. The thesis accounts for 30
credits in the spring semester.

My supervisor of NTNU is Professor Svein Savik, I would like to thank him for the
excellent guidance during my thesis work. Especially his guidance for the software
Simla helps me a lot for modeling and code debugging. My supervisor of Aalto is
Professor Romanoff Jani, I would like to thank him for the answers of my questions. I
would also like to thank Naiquan Ye from MARINTEK for the installation of software
and Eril Levold from Statoil for the model data.

The main objective of this thesis is to establish hooking models by Simla to perform
the simulation. The main content includes the DNV standard to define loads, limit
criteria and theoretical understanding of hooking scenarios. In addition, modelling
process and results are presented.

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NTNU Trondheim
Norwegian University of science and technology

CONTENT
1 Introduction .......................................................................................................... 14
1.1 Thesis outline.............................................................................................. 15
2 DNV standards ...................................................................................................... 16
2.1 Pipeline ....................................................................................................... 16
2.1.1 Loads ................................................................................................ 16
2.1.2 Failure modes of pipelines ............................................................... 17
2.1.3 Limit State Criteria ........................................................................... 19
2.2 Anchor system ............................................................................................ 22
2.2.1 Mooring Equipment ......................................................................... 22
2.2.2 Limit State Criteria ........................................................................... 24
3 Literature study ..................................................................................................... 25
3.1 Pipeline Stresses (4) ................................................................................... 25
3.1.1 Hoop Stress ...................................................................................... 25
3.1.2 Longitudinal Stress ........................................................................... 26
3.2 Environmental loads ................................................................................... 27
3.2.1 Hydrodynamic forces ....................................................................... 27
3.3 Accidental loads - Impact loads .................................................................. 29
3.3.1 Anchor direct impact loading ........................................................... 29
3.3.2 Anchor Hook Loading ....................................................................... 29
4 SIMLA .................................................................................................................... 31
4.1 Finite Element method ............................................................................... 32
4.1.1 Basic principles (8) ........................................................................... 32
4.2 Static analysis ............................................................................................. 36
4.3 Dynamic analysis ........................................................................................ 37
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Norwegian University of science and technology

4.3.1 Explicit Methods .............................................................................. 37
4.3.2 Implicit Methods .............................................................................. 37
5 Modeling ............................................................................................................... 39
5.1 Input data ................................................................................................... 39
5.1.1 Anchor geometry ............................................................................. 39
5.1.2 Hooking frequency ........................................................................... 41
5.1.3 Pipelines parameters ....................................................................... 43
5.1.4 Material Mechanical Data of Pipelines ............................................ 44
5.2 Cases definition .......................................................................................... 45
5.3 Model process ............................................................................................ 46
5.3.1 Pipeline............................................................................................. 47
5.3.2 Anchor .............................................................................................. 48
5.3.3 Cable ................................................................................................ 48
5.3.4 Seabed .............................................................................................. 48
5.3.5 Roller ................................................................................................ 49
6 Results ................................................................................................................... 50
6.1 “Short” pipe model SIMLA result ............................................................... 50
6.1.1 Hooking behavior ............................................................................. 50
6.1.2 Unhooking behavior ......................................................................... 56
6.1.3 Simulation results ............................................................................ 59
6.2 “Short” pipe model results analysis ........................................................... 60
6.2.1 Scenario a ......................................................................................... 60
6.2.2 Scenario b ......................................................................................... 67
6.2.3 Comparison of scenario a and b ...................................................... 69
6.3 “Long” pipe model SIMLA results ............................................................... 70
6.3.1 Case l1114 ........................................................................................ 70
6.3.2 Case l3114 ........................................................................................ 71
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Norwegian University of science and technology

6.3.3 Case l4114 ........................................................................................ 72
6.3.4 Case l4214 ........................................................................................ 73
6.4 “Long” pipe model SIMLA results analysis ................................................. 75
7 Conclusion ............................................................................................................. 76
8 Recommendations for further work ..................................................................... 77
9 Bibliography .......................................................................................................... 78

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