DNV-OS-H206: Loadout, transport and installation of subsea objects ...

DET NORSKE VERITAS AS

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

OFFSHORE STANDARD

DNV-OS-H206

Loadout, transport and installation of subsea objects

(VMO Standard - Part 2-6)

SEPTEMBER 2014

© Det Norske Veritas AS September 2014

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

This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document, and is believed to reflect the best ofcontemporary technology. The use of this document by others than DNV is at the user's sole risk. DNV does not accept any liability or responsibility for loss or damages resulting fromany use of this document.

FOREWORD

DNV is a global provider of knowledge for managing risk. Today, safe and responsible business conduct is both a licenseto operate and a competitive advantage. Our core competence is to identify, assess, and advise on risk management. Fromour leading position in certification, classification, verification, and training, we develop and apply standards and bestpractices. This helps our customers safely and responsibly improve their business performance. DNV is an independentorganisation with dedicated risk professionals in more than 100 countries, with the purpose of safeguarding life, propertyand the environment.

DNV service documents consist of among others the following types of documents:

— Service Specifications. Procedural 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

U) Unconventional Oil & Gas


Offshore Standard DNV-OS-H206, September 2014

CHANGES – CURRENT –

CHANGES – CURRENT

General

Det Norske Veritas AS, company registration number 945 748 931, has on 27th November 2013 changed itsname to DNV GL AS. For further information, see www.dnvgl.com. Any reference in this document to“Det Norske Veritas AS” or “DNV” shall therefore also be a reference to “DNV GL AS”.

This is a new document.

General

This is a new document in a series of documents replacing the DNV Rules for Planning and Execution ofMarine Operations (1996/2000); this standard replaces Pt.2 Ch.6. Extensive revisions and/or amendments havebeen made, with the following main changes:

— Sec.2 General Requirements is new, combining new content with some original text from Section 4 of theprevious Rules.

— The simplified method for estimation of dynamic lift loads and relevant soil force/capacities is covered inDNV-RP-H103 and the items covering these parts in section 2 and 3 in the Rules are hence omitted.

— Section 2 and 3 are now section 3 and 7 respectively and they have been considerably re-written.— Three (3) new sections have been added:

1) Sec.4 Loadout and transport

2) Sec.5 Subsea lifting

3) Sec.6 Installation of pipelines, risers, cables and umbilicals.



Contents –

CONTENTS

CHANGES – CURRENT ................................................................................................................... 3

1 Introduction ............................................................................................................................... 8

1.1 Application .................................................................................................................................................... 8

1.1.1 General ............................................................................................................................................... 81.1.2 Complementary standards ................................................................................................................. 81.1.3 Conditions for use .............................................................................................................................. 8

1.2 References....................................................................................................................................................... 8

1.2.1 Numbering and cross references ........................................................................................................ 8

1.3 Definitions..................................................................................................................................................... 10

1.3.1 Verbal forms..................................................................................................................................... 101.3.2 Terminology..................................................................................................................................... 101.3.3 Abbreviations ................................................................................................................................... 101.3.4 Symbols............................................................................................................................................ 11

2 General requirements ............................................................................................................. 12

2.1 Planning ........................................................................................................................................................ 12

2.1.1 General ............................................................................................................................................. 122.1.2 Operation period............................................................................................................................... 122.1.3 Environmental conditions ................................................................................................................ 122.1.4 Critical design parameters................................................................................................................ 122.1.5 Installation site survey ..................................................................................................................... 132.1.6 Route survey..................................................................................................................................... 132.1.7 Risk management ............................................................................................................................. 13

2.2 Documentation ............................................................................................................................................. 13

2.2.1 General ............................................................................................................................................. 132.2.2 Design documentation...................................................................................................................... 132.2.3 Operation manual ............................................................................................................................. 14

2.3 Lifting appliances......................................................................................................................................... 14

2.3.1 Crane ................................................................................................................................................ 142.3.2 Other lifting appliances .................................................................................................................... 14

2.4 Load and motion limiting systems.............................................................................................................. 15

2.4.1 General ............................................................................................................................................. 152.4.2 Active heave compensation systems................................................................................................ 152.4.3 Passive heave compensation systems .............................................................................................. 16

2.5 Lifting equipment ........................................................................................................................................ 17

2.5.1 General ............................................................................................................................................ 172.5.2 Design considerations ...................................................................................................................... 172.5.3 Wet-storage of lifting equipment ..................................................................................................... 182.5.4 Custom-made lifting equipment....................................................................................................... 182.5.5 Lifting tools ...................................................................................................................................... 182.5.6 Test lift ............................................................................................................................................. 18

2.6 Guiding and positioning systems................................................................................................................ 19

2.6.1 General ............................................................................................................................................. 192.6.2 Control of lift.................................................................................................................................... 192.6.3 Guide lines/guide wires.................................................................................................................... 192.6.4 Bumpers and guides ......................................................................................................................... 19

2.7 Installation aids ............................................................................................................................................ 20

2.7.1 General ............................................................................................................................................. 202.7.2 Design considerations ...................................................................................................................... 202.7.3 Design factor .................................................................................................................................... 20

2.8 Miscellaneous systems ................................................................................................................................. 20

2.8.1 Dynamic positioning systems .......................................................................................................... 202.8.2 Ballasting systems............................................................................................................................ 212.8.3 Atmospheric diving systems ............................................................................................................ 21

2.9 ROV operations............................................................................................................................................ 22

2.9.1 Planning............................................................................................................................................ 222.9.2 Schedule and contingency................................................................................................................ 222.9.3 Maintenance and tests ...................................................................................................................... 222.9.4 ROV Tools ....................................................................................................................................... 232.9.5 Operation.......................................................................................................................................... 232.9.6 Navigation ........................................................................................................................................ 23



Contents –

2.9.7 Launching restrictions...................................................................................................................... 232.9.8 Monitoring........................................................................................................................................ 242.9.9 Deep water ROV operations ............................................................................................................ 24

2.10 Operational requirements........................................................................................................................... 24

2.10.1 Application....................................................................................................................................... 242.10.2 Operation criteria ............................................................................................................................. 242.10.3 Pre-installation surveys ................................................................................................................... 252.10.4 Testing.............................................................................................................................................. 252.10.5 Organization .................................................................................................................................... 252.10.6 Safety and contingency .................................................................................................................... 25

3 Loads and structural design ................................................................................................... 26

3.1 Loads ............................................................................................................................................................. 26

3.1.1 General ............................................................................................................................................. 263.1.2 Loadcases and analysis .................................................................................................................... 26

3.2 Vessel motions and accelerations................................................................................................................ 26

3.2.1 General ............................................................................................................................................. 263.2.2 Characteristic vessel motions generated by wind seas..................................................................... 273.2.3 Characteristic vessel motions generated by swell ............................................................................ 27

3.3 Loads ............................................................................................................................................................. 27

3.3.1 Weight and buoyancy....................................................................................................................... 273.3.2 Hydrostatic loads.............................................................................................................................. 273.3.3 Environmental loads......................................................................................................................... 283.3.4 Accidental loads ............................................................................................................................... 283.3.5 Pull-down and pull-in loads ............................................................................................................. 283.3.6 Off-lead, side-lead forces and horizontal offset ............................................................................... 283.3.7 Loads during positioning.................................................................................................................. 283.3.8 Other loads ....................................................................................................................................... 29

3.4 Structural design.......................................................................................................................................... 29

3.4.1 General ............................................................................................................................................. 293.4.2 Object ............................................................................................................................................... 293.4.3 Bumpers and Guides ........................................................................................................................ 293.4.4 Rigging lay down and securing........................................................................................................ 293.4.5 Seafastening and supporting structures ............................................................................................ 293.4.6 Inspection ......................................................................................................................................... 29

4 Loadout and transport ............................................................................................................ 31

4.1 General.......................................................................................................................................................... 31

4.1.1 Application....................................................................................................................................... 31

4.2 Submerged towing ....................................................................................................................................... 31

4.2.1 General ............................................................................................................................................. 314.2.2 Submerged tow of objects attached to installation vessel ................................................................ 314.2.3 Submerged tow of objects attached to towed buoy.......................................................................... 314.2.4 Surface or sub-surface tow of long slender elements....................................................................... 324.2.5 Loads and analyses........................................................................................................................... 32

4.3 Bundles.......................................................................................................................................................... 32

4.3.1 General ............................................................................................................................................. 324.3.2 Load-out of bundles ......................................................................................................................... 334.3.3 Towing of bundles............................................................................................................................ 33

4.4 Pipelines, risers, cables and umbilicals ..................................................................................................... 34

4.4.1 General ............................................................................................................................................. 344.4.2 Load out by lifting............................................................................................................................ 344.4.3 Load-out by spooling ....................................................................................................................... 344.4.4 Sea transport..................................................................................................................................... 34

4.5 Pipe joints ..................................................................................................................................................... 35

4.5.1 General ............................................................................................................................................. 354.5.2 Load out by lifting............................................................................................................................ 354.5.3 Sea Transport ................................................................................................................................... 354.5.4 Offshore pipe loading....................................................................................................................... 35

5 Subsea lifting............................................................................................................................ 36

5.1 General.......................................................................................................................................................... 36

5.1.1 Application....................................................................................................................................... 36

5.2 Loads and analysis ....................................................................................................................................... 36

5.2.1 Loads ................................................................................................................................................ 365.2.2 Combination of loads ...................................................................................................................... 36



Contents –

5.2.3 Lift analysis - General ...................................................................................................................... 365.2.4 Simplified method for estimation of hydrodynamic forces acting on submerged objects............... 37

5.3 Acceptance criteria ..................................................................................................................................... 38

5.3.1 Acceptance criteria – Simplified method......................................................................................... 385.3.2 Acceptance criteria - Alternative .................................................................................................... 38

5.4 More accurate estimation of hydrodynamic forces .................................................................................. 39

5.4.1 General ............................................................................................................................................. 395.4.2 Documentation ................................................................................................................................. 39

5.5 Lifted object.................................................................................................................................................. 39

5.5.1 General ............................................................................................................................................. 395.5.2 Pipelines, risers, cables and umbilicals ............................................................................................ 405.5.3 Spools ............................................................................................................................................... 405.5.4 Retrieval of damaged objects ........................................................................................................... 40

5.6 Operational aspects ..................................................................................................................................... 40

5.6.1 General ............................................................................................................................................. 405.6.2 Installation tolerances....................................................................................................................... 415.6.3 Wet parking...................................................................................................................................... 415.6.4 Safety and contingency .................................................................................................................... 42

5.7 Deep water ................................................................................................................................................... 42

5.7.1 Deep water lowering operations....................................................................................................... 42

6 Installation of pipelines, risers, cables and umbilicals ......................................................... 43

6.1 General.......................................................................................................................................................... 43

6.1.1 Application....................................................................................................................................... 436.1.2 Risk management ............................................................................................................................. 43

6.2 Operational planning................................................................................................................................... 44

6.2.1 General ............................................................................................................................................. 446.2.2 Operation period............................................................................................................................... 446.2.3 Continuous operations...................................................................................................................... 446.2.4 Safety and contingency .................................................................................................................... 446.2.5 Operation manual ............................................................................................................................. 44

6.3 Installation spread, aids and ancillary equipment.................................................................................... 44

6.3.1 Installation spread ............................................................................................................................ 446.3.2 Calibration and testing .................................................................................................................... 456.3.3 Installation aids and ancillary equipment......................................................................................... 456.3.4 Abandonment and recovery system ................................................................................................. 456.3.5 In-line and termination structures .................................................................................................... 466.3.6 (Platform) Pull-in winch systems..................................................................................................... 46

6.4 Loads and design.......................................................................................................................................... 46

6.4.1 Loads ................................................................................................................................................ 466.4.2 Load effects ...................................................................................................................................... 476.4.3 Limit states ....................................................................................................................................... 476.4.4 Failure modes ................................................................................................................................... 47

6.5 Installation - General................................................................................................................................... 48

6.5.1 General ............................................................................................................................................. 486.5.2 Initiation .......................................................................................................................................... 486.5.3 Laying ............................................................................................................................................. 486.5.4 Lay monitoring................................................................................................................................. 486.5.5 Lay-down ......................................................................................................................................... 496.5.6 Shore pull ......................................................................................................................................... 49

6.6 Product specific installation requirements ................................................................................................ 50

6.6.1 Pipeline system installation.............................................................................................................. 506.6.2 Riser, umbilical and cable installation ............................................................................................. 506.6.3 J-tube pull-in of flexible risers, flexibles pipelines, umbilicals and cables ..................................... 506.6.4 Bundle and pipe string installation................................................................................................... 516.6.5 Tie-in of pipe strings and bundles ................................................................................................... 51

6.7 Tie-in operations .......................................................................................................................................... 51

6.7.1 Application ...................................................................................................................................... 516.7.2 General ............................................................................................................................................. 51

7 Soil and foundations ................................................................................................................ 52

7.1 Soil capacity and on bottom stability ......................................................................................................... 52

7.1.1 General ............................................................................................................................................. 527.1.2 Stability calculations ........................................................................................................................ 527.1.3 Material factors ................................................................................................................................ 52



Contents –

7.2 Loads and installation aspects .................................................................................................................... 52

7.2.1 Positioning loads .............................................................................................................................. 527.2.2 Installation effects on the soil .......................................................................................................... 527.2.3 Penetration and levelling of skirted foundations.............................................................................. 53

7.3 Miscellaneous ............................................................................................................................................... 53

7.3.1 Effects of conductor installation and shallow well drilling ............................................................. 537.3.2 Retrieval of object ............................................................................................................................ 54



Sec.1 Introduction –

SECTION 1 INTRODUCTION

1.1 Application

1.1.1 General

1.1.1.1 This standard, DNV-OS-H206 - Loadout, transport and installation of subsea objects, providesrequirements, recommendations and guidance for loadout, transport and installation of subsea objects.

1.1.1.2 The standard applies to subsea objects being lowered to their final position on the seabed by cranes orother means, or pulled down or ballasted from the sea surface. Typical objects covered are subsea structures,pipelines, umbilicals, bundles, cables and risers.

1.1.2 Complementary standards

1.1.2.1 DNV offshore standards covering marine operations, i.e. DNV-OS-H101, DNV-OS-H102 and DNV-OS-H201 through DNV-OS-H206, are collectively referred to as the VMO Standard.

Guidance note:

The “VMO Standard” supersedes and replaces “DNV - Rules for Planning and Execution of Marine Operations”. See

also Table 1-2.

---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

1.1.2.2 General recommendations for planning, loads associated with and the design of marine operations aregiven in DNV-OS-H101 and DNV-OS-H102.

1.1.2.3 Complementary guidance and recommendations for lifting operations in air are given in DNV-OS-H205.

1.1.2.4 For positioning and station keeping of installation vessels, relevant requirements in DNV-OS-H203should be considered.

1.1.3 Conditions for use

1.1.3.1 The objectives of this Standard are stated in DNV-OS-H101 Sec.1 A.

1.1.3.2 The general conditions for use of this Standard are stated in DNV-OS-H101 Sec.1 B200.

1.2 References

1.2.1 Numbering and cross references

1.2.1.1 Table 1-1 defines the numbering system used throughout this standard, in comparison with thatadopted in some of the DNV-H series of offshore standards, published to date. See Table 1-2.

1.2.1.2 The text in this standard includes references to the documents listed in Table 1-2. If indicated wherethe references are given, the referenced text shall be considered as part of this standard.

1.2.1.3 Requirements herein are based on the document revisions listed in Table 1-2; however the latestrevision shall normally be applicable, unless otherwise agreed.

Guidance note:

The agreement should be made (normally through contracts) between the parties involved, typically Company,

Contractors and MWS.


Table 1-1 Numbering

Level Numbering Numbering in some published DNV-H standards

Sections 1 Sec. 1, 2, 3…

Sub-Sections 1.1 A., B., C.….

Paragraphs 1.1.1 A 100, A 200, A 300…

Items 1.1.1.1 101, 102.., 201, 202.., 301, 302…




1.2.1.4 The documents listed in Table 1-3 include information that through references in this text, clarify andindicate acceptable methods of fulfilling the requirements given in this standard.

1.2.1.5 The latest revision of the informative references should normally be considered.

Table 1-2 Normative references

Reference Revision Title

DNV-OS-E407 Oct 2012 Underwater Deployment and Recovery Systems

DNV-OS-F101 Oct 2013 Submarine Pipeline Systems

DNV-OS-F201 Oct 2010 Dynamic Risers

DNV-OS-H101 Oct 2011 Marine Operations, General (VMO Standard Part 1-1)

DNV-OS-H102 Jan 2012 Marine Operations, Design & Fabrication (VMO Standard Part 1-2)

DNV-OS-H201 Apr 2012 Load Transfer Operations (VMO Standard Part 2-1)

DNV-OS-H202 See note Sea Transport (VMO Standard Part 2-2)

DNV-OS-H203 Feb 2012 Transit and Positioning of Offshore Units (VMO Standard Part 2-3)

DNV-OS-H204 Nov 2013 Offshore Installation Operations (VMO Standard Part 2-4)

DNV-OS-H205 Apr 2014 Lifting Operations (VMO Standard Part 2-5)

Note: Publication of the complete DNV-OS H-series is planned during the period October 2011 - January 2015. Each OS will enter into force on the date of publication. Until the OS is published the relevant requirements in “DNV - Rules for Planning and Execution of Marine Operations” shall be considered governing.

Table 1-3 Informative references

Reference Title

DNV-RP-H101 Risk Management in Marine- and Subsea Operations

DNV-RP-H102 Marine Operations during Removal of Offshore Installations

DNV-RP-H103 Modelling and Analysis of Marine Operations

DNV-RP-C205 Environmental Conditions and Environmental Loads

DNV-RP-A203 Qualification Procedure for New Technology

DNV-RP-J301 Subsea Power Cables in Shallow Water Renewable Energy Applications

DNV-RP-H201 Subsea Lifting (Planned issued October 2014)

DNV 2.7-3 DNV Standard for Certification No 2.7-3 – Portable Offshore Units

DNV Ship Rules DNV Rules for Classification of Ships

DNV-OS-E303 Offshore Fibre Ropes

DNV CN30.4 DNV Classification Note 30.4 Foundations

ND/0029 GL Noble Denton – Guidelines for Submarine Pipeline Installation

ND/0035 GL Noble Denton – Guidelines for Offshore Wind Farm Infrastructure Installation

IMO MSC/ Circ. 645 Guidelines for vessels with dynamic positioning systems

NORSOK U-102 Remotely Operated Vehicle (ROV) Services

IMCA AODC 032 Remotely Operated Vehicle Intervention During Diving Operations

IMCA D 014 International Code of Practice for Offshore Diving

ISO-13628-2 Design and operation of subsea production systems - Part 2: Unbonded flexible pipe systems for subsea and marine applications

ISO-13628-5 Design and operation of subsea production systems - Part 5: Subsea umbilicals

ISO-13628-11 Design and operation of subsea production systems - Part 11: Flexible pipe systems for subsea and marine applications

API 17E Specification for Subsea Umbilicals, Fourth Edition (ISO 13628-5:2009, Identical Adoption)

API 17J Specification for Unbonded Flexible Pipe

API 17B Recommended Practice for Flexible Pipe




1.3 Definitions

1.3.1 Verbal forms

1.3.1.1 Verbal forms of special importance are defined as indicated below in this standard.

1.3.2 Terminology

1.3.2.1 Terms of special importance are defined as indicated below in this standard. See also DNV-OS-H101for general terms and DNV-OS-H205 for lifting related terms.

1.3.3 Abbreviations

1.3.3.1 The list below defines abbreviations used within this standard:

Table 1-4 Verbal forms

Term Definition

Shall Verbal form used to indicate requirements strictly to be followed in order to conform to the document.

Should Verbal form used to indicate that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others, or that a certain course of action is preferred but not necessarily required.

May Verbal form used to indicate a course of action permissible within the limits of the document.

Table 1-5 Terms

Term Description

Characteristic condition: A condition which has a defined probability of being exceeded within a defined time period, see also DNV-OS-H101 Sec.3 A300.

Characteristic load: The reference value of a load to be used in the determination of load effects. See also DNV-OS-H102, Table 3-1.

Design load: The design value of a load found by combining the relevant characteristic load(s) multiplied by the appropriate load factor(s).

Design sea state: The short term wave condition which forms a basis for the design and design verification.

Object: The structure handled during the marine operation, typically a structure, pipeline, cable, riser, umbilical etc. that will be permanently installed subsea.

Product: This is used as a collective term for the various objects covered in Sec.6.

Short term wave condition:

A wave condition where significant wave height and zero crossing wave period are assumed constant in the duration time, typically 3 hrs.

Significant wave height: Four times the standard deviation of the surface elevation in a short term wave condition (approximately equal to the average wave height associated with the highest third of all waves).

Snap force: Short-duration dynamic force associated with sudden changes in velocity of a lifted object, or sudden tensioning of a slack cable system, e.g. in the case of uncontrolled ‘lift-off’ from a supply vessel or sea-bed, and/or during uncontrolled deployment/recovery through the splash-zone.

Zero crossing wave period:

Average wave period, i.e. average time interval between upward or downward crossings of the still water level by the water surface.

ADS Atmospheric diving systems

AHC Active heave compensating

ALS Accidental limit state, see DNV-OS-H102

CoB Centre of buoyancy

CoG Centre of gravity

DAF Dynamic amplification factor

DAFconv Converted DAF, i.e. DAF calculated as a function of weight in air (m g) of the object

DP Dynamic positioning

FMEA Failure mode effect analysis

HAZOP Hazard and operability study

MBL Minimum breaking load

MPI Magnetic particle inspection

PHC Passive heave compensating

ROV Remotely operated vehicle




1.3.4 Symbols

1.3.4.1 The list below defines symbols used within this standard:

TLP Tension leg platform

ULS Ultimate limit state, see DNV-OS-H102

UT Ultrasonic testing

VIV Vortex induced vibration

AROV Projected cross sectional area of ROV.

dcab Diameter of (submerged) cable.

Fcur Horizontal current force on ROV

Fhyd Characteristic hydrodynamic load.

Fpd Forces on object when pulled down in lock-in position.

Fsnap Characteristic snap load

Fstatic Static submerged weight of object.

Fstatic-min Minimum static submerged weight

Fstatic-max Maximum static submerged weight

Ftotal Total (static + hydrodynamic) characteristic load on the object

g Acceleration due to gravity.

Hs Significant wave height of design sea state.

K Stiffness of hoisting system.

lcab Projected length of submerged cable.

m Mass of object in air.

TR Operation reference period, see DNV-RP-H101.

vcur Maximum current velocity.

ηct Characteristic single amplitude vertical motion of crane tip.



Sec.2 General requirements –

SECTION 2 GENERAL REQUIREMENTS

2.1 Planning

2.1.1 General

2.1.1.1 Subsea operations shall be planned and documented according to the requirements and philosophiesgiven in DNV-OS-H101 Sec.2.

2.1.1.2 Operational requirements/restrictions, see [2.10], shall be duly considered in the planning phase.

2.1.2 Operation period

2.1.2.1 The required operation reference period TR (defined in DNV-OS-H101 Sec.4 B200) should bethoroughly evaluated at an early stage.

2.1.2.2 The start and end points for subsea installation operations shall be Safe Conditions. The SafeConditions and point(s) of no return, if any, should be clearly defined. Safe Condition is defined in DNV-OS-H101 Sec.2 A102 Guidance Note. See [6.2.3] for continuous operations.

Guidance note:

The time expected for the removal of seafastening should normally be included in the operation reference period. Thestart of seafastening removal will normally be defined as a point of no return unless equipment and procedures forreinstatement of seafastening has been planned and accounted for in the operation reference period.


2.1.3 Environmental conditions

2.1.3.1 Subsea installation operations will normally be weather restricted; planning should include a thoroughevaluation of the expected environmental conditions to ensure that there will be adequate weather windows forthe planned operations.

Guidance note:

A subsea installation operation could comprise several sub-operations, each with different limiting environmentalcriteria.


2.1.3.2 All possible environmental conditions (see DNV-OS-H101 Sec.3) shall be evaluated and consideredduring planning.

2.1.4 Critical design parameters

2.1.4.1 When evaluating a subsea operation, the parameters listed below should as found relevant, be takeninto account prior to establishing the operation and design criteria (see also DNV-OS-H101 Sec.4 B).

a) water depth

b) tide

c) on bottom visibility

d) accuracy of survey equipment

e) available current data

f) wave/wind statistics for area in question

g) the expected operation reference period

h) expected time to reverse the operation

i) type of operation

j) contingency procedures, e.g. retrieval or abandonment of object

k) type of installation vessel/equipment

l) weather forecast and monitoring uncertainties

m) vessel response characteristics

n) deck handling/over-boarding restrictions

o) type of lifting gear

p) crane capacity and specifications

q) weight of crane wire (in deep water)




r) crane tip motion

s) crane hoisting/lowering speed

t) hydrostatic and hydrodynamic effects (air filled structure or not)

u) entrapped air

v) submerged weight

w) tugger line forces

x) operational restrictions on tugger line disconnection

y) guide wire forces and winch speed limitations

z) soil conditions and soil properties

aa) seabed topography

ab) load reducing systems (heave compensation capacities)

ac) vessel DP capability /position keeping systems

ad) ROV station keeping capability

ae) ROV working range

af) complexity of ROV tasks (e.g. ROV Interfaces on structure).

2.1.5 Installation site survey

2.1.5.1 The planning process shall incorporate information gathered from site surveys to account for prevailingsoil conditions, see [2.1.4.1] item z) and aa).

2.1.5.2 In general, installation site surveys should be carried out as described in DNV-OS-H204 Sec.2 [4.3].

Guidance note:

The below listed aspects could be of relevance for subsea installations and the survey should hence give adequateinput to evaluate properly these aspects:

a) Limiting set-down velocity. I.e. to estimate soil-structure interaction effects due to installation impact loads.

b) Suction forces (reverse end bearing) during rapid pull-out.

c) Off-target position of object, both due to alternative permanent positions and due to contingency set-down.

d) Scour/build-up caused by current.


2.1.6 Route survey

2.1.6.1 For pipelines, umbilicals, flexibles, cables and submerged tows, route surveys shall be carried outalong the total length of the planned route to provide sufficient data for design and installation related activities.

Guidance note:

More information regarding route surveys and their purpose can be found in the following documents/sections:

— For pipelines: DNV-OS-F101 Sec.3.

— For subsea cables: DNV-RP-J301 [3.4]

— Submerged pipeline towing: DNV-OS-F101 Sec.10 F500

— Bundles: [4.3.3.3]


2.1.7 Risk management

2.1.7.1 Operational risk should be evaluated and handled in a systematic way see DNV-OS-H101 Sec.2 C.

2.2 Documentation

2.2.1 General

2.2.1.1 General requirements for documentation are given in DNV-OS-H101 Sec.2 B.

2.2.2 Design documentation

2.2.2.1 Depending on type of structure the following design documentation is normally required as a minimum:

— Design load evaluations/calculations/analysis including motion response characteristics for installationvessel(s).




— Structural strength analysis and stability calculations for the object.— Strength and capacity calculations for all equipment and (temporary) structures.— Technical specifications, certificates and test reports for equipment.— Documentation of soil characteristics.— Vessel data, stability and strength verifications.

Guidance note:

For lifting appliances the design documentation should normally be given in the form of certificates - however, seealso [2.3.2].


2.2.2.2 If monitoring is used as a means of operational control, expected target monitoring results should bedocumented by calculation. Target monitoring values and acceptable tolerances on them should be clearlydefined.

2.2.3 Operation manual

2.2.3.1 An operation manual shall be prepared, see DNV-OS-H101 4 G200.

2.2.3.2 The items listed below should be adequately covered in the manual. A clear reference to the documentswhere this information could be found may normally be considered as adequately covered.

a) Operational organisation chart(s) and responsibilities of key personnel.

b) Description of limiting operational environmental criteria and requirements for weather forecasting andwind/wave/current monitoring.

c) Detailed operation schedule and weather window requirements, ref. DNV-OS-H101 Sec.4 B.

d) Pre-launch/deployment checklists ensuring that all required preparations have been carried out.

e) Clearly defined and measurable installation tolerances.

f) Target position of the object and vessels during all phases of the operation.

g) Procedures for handling of possible contingency situations (see [2.10.6]).

h) Object limiting structural criteria (e.g. max allowable tension, min. allow. tension, MBR, etc.)

i) Description of equipment limitations.

j) Detailed description of operational steps, supported by relevant drawings and sketches.

2.3 Lifting appliances

2.3.1 Crane

2.3.1.1 Crane and crane vessel shall comply with the requirements in DNV-OS-H205 [2.2].

2.3.2 Other lifting appliances

2.3.2.1 The capacity and quality of underwater deployment and recovery systems should generally bedocumented as adequate according to the principles described in DNV-OS-E407.

2.3.2.2 If a winch is chosen for deployment of structures to the seabed, the winch shall be regarded as a liftingappliance.

Guidance note:

Documented capacity and load-testing as for a crane, see e.g. DNV 2.22, of deployment winches are mandatory. Otherrequirements to cranes e.g. to monitoring and alarm systems can be evaluated on a case by case basis.


2.3.2.3 Structures, such as A-frames, forming part of a lifting appliance intended for subsea lifts shouldnormally fulfil the design, fabrication and test requirements applicable to cranes; see DNV-OS-H205 [2.2].

Guidance note:

Alternatively, it can be acceptable to define these parts as structures, see DNV-OS-H205 Sec.5. Normally thisapproach would apply to temporary lifting appliances, used for specific operation(s). Note also Table 5-1 in DNV-OS-H205.


2.3.2.4 If traction winches are used for deep water installations, due considerations shall be made to thepossible failure modes of the rope.




Guidance note:

System performance will depend on the type of rope and winch design. Discard criteria for the rope should beestablished based on relevant failure modes identified. See DNV-OS-E303 Offshore fibre ropes (ConditionManagement Program).


2.4 Load and motion limiting systems

2.4.1 General

2.4.1.1 Load and motion limiting systems are devices used to minimise relative motions and dynamic loadsexperienced by a lifted object, during subsea lifting operations.

Guidance note:

Load and motion limiting systems can consist of active or passive heave compensation systems, shock absorbers, softsprings and fenders. Heave compensating systems are generally described DNV-RP-H201 App.C.


2.4.1.2 Load and motion limiting systems shall be designed, fabricated, installed and tested in accordance withrelevant recognised codes and standards, see DNV-OS-H101 Sec.1 B305.

2.4.1.3 Adequate capacity and functionality for the intended use shall be documented. A thorough descriptionof the system and its use during the planned lifting operation shall be provided.

Guidance note:

Typical elements to be evaluated are structural capacity, hydraulic capacity, sufficient stroke length, power supply,adequate cooling etc.


2.4.1.4 Unless adequate reliability can be documented, contingency cases considering malfunction of the load/ motion limiting system shall be investigated. Calculations may be carried out assuming accidental limit state(ALS).

Guidance note:

Adequate reliability implies a documented risk of catastrophic failure less than 1/10,000 per operation. Hence, if therisk of malfunction is greater than 1/10.000 the possible consequence of malfunction, which could be “catastrophicfailure”, should be analysed. All possible failure modes of load limiting systems should be identified using applicablerisk identification techniques and methods as described in DNV-OS-H101 Sec.2 C200. It should be documented thatthe installation can be safely completed or abandoned at all times, without jeopardizing the integrity of the object.


2.4.2 Active heave compensation systems

2.4.2.1 A subsea lifting operation that relies on an active heave compensation (AHC) system shall be carefullydesigned, ensuring that the operation is carried out in accordance with the system’s operational limitations andoperating procedures.

Guidance note:

Consideration of an accidental case (as described in [2.4.1.4]) will normally be required. As a base case therefore, itis recommended to calculate hydrodynamic loads without considering AHC systems.

For sub-operations mentioned in [2.4.2.3] it is normally acceptable to take into account the motion (and if applicableload) reducing effect of an AHC system. The AHC function should be checked before the sub-operation andcontingency procedures in case of an unsuccessful check should be established in order to fulfil the requirement in[2.4.1.4].


2.4.2.2 Efficiency of the heave compensator (in terms of stroke length and /or max pay out/in speed) shallgenerally not be taken higher than 80% of the theoretical operational values.

Guidance note 1:

A safety factor of 0.9 on the stated and documented efficiency reduction factor is recommended, i.e. if the heave compsystem has 90% stated / documented efficiency, the maximum efficiency factor should be 0.9 ⋅ 0.9 ≈ 0.8, or 80%.





Guidance note 2:

The test results and operational records used to derive the efficiency factor should be based on data from operationswhere the environmental conditions, depth, weight of object and other effects influencing the performance of theheave compensation system are comparable with those of the planned operation.


2.4.2.3 The motion (and if applicable, load) reducing effect of an AHC system should normally be appliedduring sub-operations of limited duration only, e.g. final landing, final positioning or initial phase of retrievalof a subsea object.

Guidance note:

Acceptable duration of AHC operations should be evaluated based on the criticality of the operation and the reliabilityrecord of the AHC.


2.4.2.4 Characteristics and performance of AHC systems shall be documented (see [2.4.1.3]). Performancemay be documented by testing and relevant operational records.

Guidance note:

Guidelines for performance of AHC systems are presented in DNV-RP-H201 App.C.


2.4.2.5 If there is a high risk of significant suction forces and/or soil friction, the appropriate crane mode andits limitations should be duly considered as indicated below:

— AHC mode shall only be used in combination with measures to avoid excessive loads.

Guidance note:

Lifting off in AHC mode can give the crane operator control of the lifting speed. There is however a risk of excessiveloading.


— Tension Control mode shall be used only in combination with measures to avoid excessive speed.

Guidance note:

Whilst lifting off in Tension Control mode can provide the crane operator with tension control, there is a risk ofexcessive lifting speeds.


2.4.3 Passive heave compensation systems

2.4.3.1 The effect of passive heave compensation (PHC) systems (e.g. spring/damper devices) may beaccounted for in hydrodynamic load calculations.

Guidance note:

The dynamics of PHC systems can be calculated following the guidelines in DNV-RP-H103 Sec.5. The effect of suchsystems may also be implemented as soft springs in snap load calculations (see DNV-RP-H103 [4.7]).


2.4.3.2 The characteristics and performance of the PHC system shall be documented (see [2.4.1.3]). Theperformance of the system can be documented by testing and/or operational records.

Guidance note 1:

Guidelines for performance of PHC systems are presented in DNV-RP-H201 App.C


Guidance note 2:

PHC systems can be set up in different ways e.g. with full stroke available to resist loads above the threshold settingor with the stroke divided so that both load increases and reductions can be absorbed (within the available part-strokelengths).


Guidance note 3:

If the rigging includes multiple PHC systems the stability of the system should be demonstrated.


2.4.3.3 If relevant, efficiency of the heave compensator in terms of stroke length and /or max pay out/in speedshould generally not be taken higher than 80% of the theoretical operating range.




Guidance note:

To use an efficiency factor of 80%, system performance of 90% of theoretical values should be documented.


2.5 Lifting equipment

2.5.1 General

2.5.1.1 See DNV-OS-H205 Sec.4 for general definitions and requirements.

Guidance note:

This sub-section includes clarifications and additional recommendations regarding use of lifting equipment for subseaoperations.


2.5.1.2 Subsea lifting equipment materials must be carefully selected to take account of the potential failuremechanisms that may be caused or accelerated by the environment such as galvanic corrosion, stress corrosioncracking or the effects of cathodic protection systems.

2.5.2 Design considerations

2.5.2.1 Lifting equipment shall be verified for the worst combination of dynamic hook load and all reasonablyforeseeable load effects (including unintentional ones).

2.5.2.2 Lift points should be configured such that the risk of damage and/or accidental release of slings (dueto possible impact loads) are negligible.

2.5.2.3 Lift point layout and rigging design shall ensure adequate stability and acceptable tilt of the objectduring all phases.

Guidance note 1:

If adequate lift stability is not obvious by inspection, the risk of overturning should be evaluated, documented andmitigated. Adequate stability of the object should be ensured considering:

— all possible unfavourable combinations of sling loads, buoyancy, CoB and CoG (CB and CG), see also DNV-RP-H103 [3.6] and [5.6.1.6]

— vertical wave loads

— horizontal (differential) wave loads

— current loads

— lift dynamics.


Guidance note 2:

Due to buoyancy, the tilt of the lifted object can change when being submerged. This should be considered whendefining the optimal tilt in air and water.


2.5.2.4 Lifting equipment should be designed with attention given to the planned subsea release/connection ofthe rigging.

2.5.2.5 Lift points and exposed areas of the lifted object should be designed to allow slackening of lifting wiresand release and controlled recovery of rigging items without snagging.

2.5.2.6 All lifting equipment shall as a minimum incorporate one safety barrier / retention mechanism (safetylatch, split-pin/cotter-pin etc.), itself being adequately secured and protected against accidental release.

Guidance note:

These safety barriers / mechanisms should not be affected by lifting or external loads.


2.5.2.7 For lifting operations with dynamic forces that are large relative to the static weight of the object, it isconsidered normal practise to incorporate a minimum of two safety barriers, again suitably protected againstaccidental release. The primary safety barrier should have adequate strength to accommodate any possible loaddirection.




Guidance note:

ROV spring safety latch hooks should be avoided if there is any possibility of slack slings/snap forces. This becauseeven if the latch has a secondary release barrier the hook may come out of position and the latch take the load whichit is not dimensioned for.


2.5.2.8 Lifting equipment containing hydraulic, pneumatic or other remotely operated release mechanisms,shall be designed to fail safe.

2.5.2.9 Structural lifting elements, like spreader bars, lifting frames, etc. should preferably be free flooding. Ifnot, free flooding maximum depth rating to be calculated and marked on the equipment, see [3.3.2.1].

2.5.2.10 If trunnion-type lift points are used, slings should be mechanically secured against significantdisplacement and unintended release during phases of variable sling load.

2.5.2.11 The lifting arrangement should have sufficient length to allow crane hook to be connected at decklevel in order to avoid working at height on board the installation vessel/barge.

Guidance note:

If not possible e.g. due to lifting height a proper plan for the hooking on including if required physical means asrigging platforms, etc. needs to be in place.


2.5.3 Wet-storage of lifting equipment

2.5.3.1 Lifting and other temporary equipment stored on the seabed shall have adequate resistance against allpossible mechanisms of degradation - material properties shall be justified and documented accordingly.

2.5.3.2 If storage periods beyond normal inspection intervals are anticipated, the means for satisfying anyformal and regular inspection requirements should be agreed.

2.5.3.3 The destructive effect of cyclic loading shall be considered for equipment subject to such loadingduring wet-storage (e.g. pick-up lines connected to buoys).

2.5.4 Custom-made lifting equipment

2.5.4.1 Lifting equipment designed for case-specific usage shall comply with requirements in DNV-OS-H205[5.1.5].

2.5.5 Lifting tools

2.5.5.1 Lifting tools shall comply with requirements in DNV-OS-H205 [4.3.3].

Guidance note:

A lifting tool in this sub-section is defined as a hydraulic tool, internally or externally connected to a tubularreceptacle.


2.5.5.2 It shall be documented that the tool cannot accidentally release due to varying loads (typically lowtension) in the lift system.

2.5.5.3 Lifting tools designed for remote subsea release shall have a back-up release mechanism.

2.5.6 Test lift

2.5.6.1 The need to perform test lifts shall be considered.

Guidance note:

An onshore test-lift is recommended. The actual rigging configuration and lifted load should be used to confirm thatsling lengths and tilt are within specified tolerances and that slings / loose gear can be hooked-up and laid down, safelyand without damage.


2.5.6.2 If the required tolerance on tilt of a submerged object is small (e.g. due to the need to engage with guideposts), a subsea test-lift should also be considered.




Guidance note:

The main purpose of this test is to find the tilt of the object in submerged condition. See also [2.5.2.3] GN 2. Theevaluation of need for testing should consider possible corrective action based on test results and the level ofconfidence in CoG/CoB positions.


2.6 Guiding and positioning systems

2.6.1 General

2.6.1.1 General requirements for guiding and positioning systems are given in DNV-OS-H101 Sec.6 C.Requirements for the operational control of lifts in-air can be found in DNV-OS-H205 [2.3.2]. This sectionclarifies some of the requirements in these documents and where appropriate should be considered tosupplement their requirements.

2.6.2 Control of lift

2.6.2.1 Adequate control of any lift shall be ensured during all phases. See DNV-OS-H205 [2.3.2] forrequirements relating to the lifting in-air phase.

2.6.2.2 In cases where the retrieval of a lifted object to deck is necessary, the following should be considered:

— anticipated weight increase and instability due to the effects of entrapped water, debris and drainage duringlifting

— available deck space— guides and bumpers— tugger wire system— reinstatement of seafastening.

Guidance note:

The above is also applicable for retrieval/backloading of heavy rigging, spreaderbars, installation tools etc.


2.6.2.3 Subsea disconnection of tugger lines should be suitably planned. If ROV’s are to be used fordisconnecting tugger lines, consideration should be given to the limitations and recommendations in [2.9.1].

2.6.3 Guide lines/guide wires

2.6.3.1 Guide wires should be used to prevent rotation of the lifted object during installation. It will normallybe sufficient to prevent rotation during only the final stages of lowering. Other means of preventing rotationcan be acceptable.

2.6.3.2 If guide/pull-down lines fixed to a pre-installed subsea template or similar require a fixed vesselheading, the weather criteria specified for the operation should reflect this.

2.6.3.3 Guide wire tension shall be adjusted to suit the weight of the installed object and possible current forceson the object/lifting gear.

2.6.3.4 If guide wire winches are used to provide wire tension, the weight of the guide wire shall be accountedfor when defining the required winch tension/capacity.

2.6.3.5 Guide wire winch speed shall be considered when defining the operational limiting criteria for theoperation.

2.6.3.6 Capacity of guide-wire attachment points on subsea structures shall satisfy the structural strengthrequirements given in DNV-OS-H102.

2.6.3.7 The guide-wire system shall include a weak link. The capacity of the weak link shall not exceed thedesign load of the guide-wire attachment, or 80% of the system MBL, whichever is less.

2.6.4 Bumpers and guides

2.6.4.1 If a guide system on a subsea structure incorporates more than one guidepost, the use of guide posts ofdiffering length should be considered to facilitate landing of the object.

2.6.4.2 If a guide system consists of guide funnels or similar, the connection of the guide receptacle to thestructure should be designed with consideration given to installation loads (e.g. overload / impact) that coulddamage the integrity of primary structural elements of the object.




2.6.4.3 The design of guiding systems should consider contingency cases and should not limit retrieval of theobject.

2.6.4.4 Due consideration shall be paid to the possibility of the object/structure becoming jammed in the guidesystem.

Guidance note:

Primary and a secondary guiding systems can be required to install structures with smaller tolerances than can besafely achieved (e.g. without risk of jamming) by one system alone. Two independent guiding systems can also berequired in cases where large motions are expected. The primary system should be designed according [2.6.4.1] and[2.6.4.2]; the secondary guiding system being designed to resist residual forces and achieve final alignment/installation.


2.6.4.5 Design requirements for bumpers and guides are given in [3.4.3].

2.7 Installation aids

2.7.1 General

2.7.1.1 General requirements for system and equipment design are given in DNV-OS-H101 Sec.6 A.

Guidance note:

Installation aids are defined herein as purpose-built equipment, used to assist and control a specific phase of a liftingoperation, in turn making it safer and more efficient.


2.7.2 Design considerations

2.7.2.1 Installation aids should be located such that they are not damaged during preceding operations, e.g.lifting of structures, handling of piles, opening/closing of hatches, etc.

2.7.2.2 Temporary attachments having the potential to damage the structure or other equipment should beremoved after final use without undue delay.

2.7.2.3 The use of surface-supplied gas/hydraulic power to connect/lock the object to a pre-installed seabedunit should be avoided. If used, the risk of sustaining of mechanical damage during lowering/positioningshould be assessed and minimised; a sufficient back up system can be necessary to mitigate undue risk.

2.7.2.4 Subsea sheaves, blocks and other equipment that require lubrication during operation should haveclosed or pressure-compensated lubrication systems.

2.7.3 Design factor

2.7.3.1 Rigging equipment used for purposes other than lifting should be used with safety factors adequate forthe intended use.

Guidance note:

If the consequence of failure is considered tolerable by all involved parties, a reduced consequence (safety) factor canbe acceptable. E.g. if the rigging is used for pulling/hold-back only and the consequences of rigging failure areregarded as negligible, a lower consequence factor may be applied, see DNV-OS-H205 [4.1.5].


2.8 Miscellaneous systems

2.8.1 Dynamic positioning systems

2.8.1.1 General requirements for the operation of DP (Dynamic Positioning) vessels are given in DNV-OS-H203 Sec.5.

2.8.1.2 DP operations requiring DP equipment class 2 and 3 shall be restricted (planned, see also [2.8.1.3] and[2.8.1.4] below) based on the power/thrust available after worst single failure.

Guidance note:

The worst single failure concept is further described in DNV Ship Rules Pt.6 Ch.7- Dynamic Positioning Systems, andin IMO MSC/Circ. 645.


2.8.1.3 DP capability should be continuously monitored throughout the operation, and the operation should be




safely terminated if the DP-vessel will no longer be able to keep position if the single failure criterionapplicable to the equipment class should occur. In this context deterioration of environmental conditions andthe necessary time to safely terminate the operation should also be taken into consideration. Possible increasein current forces and uncertainty in the weather forecasting (see DNV-OS-H101) shall be accounted for.

Guidance note:

DP capability plots should be used to verify power/thrust availability based on expected environmental conditions.


2.8.1.4 Should a stand-off mode be impossible, preparations for abandoning or retrieval of an object should bemade in due time, prior to reaching the consequence analysis alarm.

Guidance note:

Stand-off mode is defined as a situation where operation is discontinued and vessel/product/object is temporarily heldin a safe condition. The Stand-off mode shall be designed as a safe condition as defined in DNV-OS-H101 Sec.2A102. Adequate planning should be made to accommodate any vessel heading limitations and object handlingrestrictions.


2.8.1.5 Minimum clearances between a DP vessel and any fixed or floating structures shall be definedconsidering the minimum clearances indicated in DNV-OS-H203 Sec.5 C200, see also DNV-OS-H205[2.3.3.5].

2.8.1.6 For complex and/or close proximity DP operations involving one or more DP vessels, a DP operationprocedure shall be presented.

Guidance note:

The DP procedure shall as a minimum include:

— a description of the work that is planned performed

— weather criteria (force and direction)

— minimum distances between vessels

— pre-operation DP testing requirements

— foot print testing should be included if found relevant based on the required station keeping accuracy

— reference systems setup, including evaluation of possible shadow effects on aerials and thrust interference onhydro-acoustic transducers

— engine room and switchboards configuration

— communication procedures, internally and between vessels

— copy of HAZOP/risk analysis findings and risk reducing measures

— training/competence level of key DP personnel.


2.8.2 Ballasting systems

2.8.2.1 For operations requiring ballasting of an object, a suitable ballast control and monitoring system shouldbe provided.

2.8.2.2 Ballast systems utilising external umbilical power supply are subject to the same recommendations asin [2.7.2.3]. The ballast system should be designed to fail safe in case of umbilical damage.

2.8.2.3 All ROV operated valves should be clearly marked according to function and with open/shut (O/S)positions. Valve indicators on critical valves can be considered necessary for visual verification purposes.

2.8.2.4 Analogue pressure gauges to be monitored by ROV shall be of adequate size and have easy-to-readindication and figures.

2.8.2.5 Special back-up or monitoring equipment can be required to avoid uncontrolled ballasting and over-pressurisation.

2.8.3 Atmospheric diving systems

2.8.3.1 Atmospheric diving systems (ADS) shall be certified in accordance with recognised standards.

2.8.3.2 The system and operational procedures should be adequate for the intended work scope. IMCA D 014can be consulted for detailed advice and recommendations.

2.8.3.3 ADS should in general incorporate adequate back-up, enabling 24 hours (around the clock) operabilityif required.




2.8.3.4 Operational reliability should be documented through presentation of dive logs, maintenance recordsetc.

2.8.3.5 It should be documented that the ADS system is capable of operating under the given design andoperational criteria.

2.9 ROV operations

2.9.1 Planning

2.9.1.1 ROV systems and tooling should be selected based on the environmental conditions expected at theworksite during the planned and contingency intervention/observation tasks.

2.9.1.2 When planning for a subsea operation, the following ROV limitations and recommendations should benoted:

a) Minimum practical operational depth in the expected wave conditions, also considering possible wake fromvessel thrusters.

b) ROV working range, i.e. maximum horizontal offset vs. available tether length, considering the worstexpected current conditions.

c) Planning and design of the ROV operation shall as far as possible minimise the operational influence of theROV operator's skill and experience.

d) Poor visibility due to e.g. disturbed soil conditions, stirred up by contact or thruster use close to seabed.

e) Access to working site.

2.9.1.3 The station keeping capability and manoeuvrability of the ROV during operation shall be considered.If the ROV is carrying equipment or is equipped with tooling packages/skids, this needs to be accounted for.

Guidance note:

ROV operations involving moving targets should not normally be undertaken and any ROV manipulator or toolingoperation that requires the pilot to actively control the position of the ROV during performance of the task should beavoided.


2.9.1.4 All essential ROV interfaces should have appropriate grab bars or other means for stabilizing ROV.

2.9.2 Schedule and contingency

2.9.2.1 ROV downtime, both planned and possible/unforeseen (see DNV-OS-H101 Sec.4 B300 and B400),should be taken into consideration when establishing the required weather window.

Guidance note:

ROV contingency procedure(s) developed to ensure that one ROV down failure will not (significantly) compromisethe time to safe position may be considered. See also [2.9.2.3].


2.9.2.2 Realistic ROV recovery time, both for planned maintenance and repairs shall be taken into accountduring planning of the operation.

2.9.2.3 Subsea operations, where operation reference period is based on there being at least one operationalROV at all times, should be equipped with at least two independent ROV spreads. The need for backup ofessential ROV tools should be assessed and if applicable the time needed to switch ROV tools/skids betweenROVs should be accounted for in the planning.

2.9.2.4 The ROV crew should be sufficient to provide 24 hours (around the clock) operability.

Guidance note:

Time/schedule critical ROV operations always implies 24hrs coverage, but such coverage may be deemed notnecessary on some ROVs and OBSROVs.


2.9.3 Maintenance and tests

2.9.3.1 Prior to acceptance of ROV operations, maintenance records and dive logs for each ROV should bepresented. Sufficient spares should be available.

2.9.3.2 For complex and critical stages of the installation that are dependent on ROV operations, Client/Contractor shall demonstrate ROV capability of executing the planned intervention. This may be demonstratedby used of 3D models, mock-up tests, previous experience, etc.




2.9.3.3 Function testing of ROV, ROV equipment and survey spread should be part of the test programdescribed in [2.10.4.1].

2.9.4 ROV Tools

2.9.4.1 All tools shall be adequate for the intended work task.

2.9.4.2 Tools shall be adequately tested and calibrated. Cutting tools shall be tested on deck (using similar wiretype) prior to operation.

2.9.4.3 Tools can influence the ROV operability and power consumption. This should be duly considered inthe ROV/tool selection process.

2.9.5 Operation

2.9.5.1 If complex operations reliant on the skill of the ROV operator alone cannot be avoided, ROV operatorexperience shall be evaluated - training sessions specially adapted for the proposed operation can beappropriate.

Guidance note:

See also [2.9.1.2]c, [2.9.3.2] and [2.10.5.2].


2.9.5.2 ROV thruster capacity for time critical operations should be at least 30% higher than the maximumexpected current force acting on the ROV and its umbilical.

Guidance note:

The horizontal current force on the ROV and the submerged cable may be taken as:

Fcur = 0.615(dcab ⋅ lcab + AROV) vcur 2[kN]

where

dcab: diameter of submerged cable [m]lcab: projected length of submerged cable [m]AROV: projected cross sectional area of ROV [m2]vcur: maximum current velocity [m/s]


2.9.5.3 For operations using both ROV(s) and diver(s), any restrictions on simultaneous working should beclarified and considered in advance.

Guidance note:

For guidance on safety considerations that should be taken into account when divers are working with or in the vicinityof ROVs, see AODC 032 ‘Remotely Operated Vehicle Intervention during Diving Operations’. These considerationsinclude entanglement of umbilicals, physical contact and electrical hazards.


2.9.5.4 Wire cutting by use of ROVs should only be performed on slack wire ropes or on ropes with very lowtension, e.g. carrying own weight only.

2.9.6 Navigation

2.9.6.1 Means for locating and tracking of the ROV from the surface are required for navigational purposesand emergency recovery.

Guidance note:

There is a potential risk of acoustic interference, such as shadowing or noise under several conditions, for example ifseveral vessels are operating in the same area. Frequencies for acoustic beacons should be selected to avoidinterference.


2.9.7 Launching restrictions

2.9.7.1 ROV launching and recovery restrictions shall be defined based on the capacity of the launch andrecovery system, including capacity of the umbilical. In addition any restrictions related to operational aspectsneed to be considered.

2.9.7.2 The over-boarding system shall be safely operated within its intended design limit and dueconsideration of ROV recovery needs to accounted for in the definition of the weather criteria.




Guidance note:

— The launch and recovery system should incorporate a guide/cursor system to ensure controlled clearance withvessel side during lowering through the splash zone.

— Overboard launching and retrieval of large ROV's should not take place in sea states exceeding 2.5-3.0 m (Hs) ifnot the ability to operate in a safe manner under more severe conditions has been documented.

— Moon-pool ROV operations may be extended to Hs < 5-6 m, depending on the motion characteristics of the vessel.


2.9.7.3 Launch and recovery shall as much as practically possible take place at safe distance from sensitivesubsea infrastructure.

2.9.8 Monitoring

2.9.8.1 Video monitoring of all Subsea operations should in general be provided, e.g. ROV, diver-operated,etc. Any critical part of the operation should be performed with such monitoring.

2.9.8.2 All diving and complex Work-ROV operations should be monitored by independent ROV withmonitoring as its only task.

2.9.8.3 The ROV used for monitoring subsea operations should, as far as practically possible, be operated fromthe installation vessel.

2.9.8.4 If the ROV operation has to be performed by a vessel other than the installation vessel, the stability andreliability of the video-link system between the vessels shall be proven under the given conditions.

Guidance note:

Some operations can require a large horizontal distance between the installation vessel and the observation ROV, thusnecessitating a separate ROV vessel. The video-link should be tested prior to start of operation.


2.9.9 Deep water ROV operations

2.9.9.1 ROV equipment capacities shall be chosen to suit the relevant depth.

Guidance note:

Both the ROV and any ROV tooling should be “depth rated”, and their stated depth limitation should not be exceeded.General wear on the complete ROV spread during deep water operations is more extensive than during moderatedepth operations, it is important therefore that all required maintenance is done prior to operation. During deep wateroperations special attention shall be given to lubrication systems which can be affected by the external water pressure.


2.9.9.2 Current forces acting on the umbilical and ROV shall be defined, see [2.9.5.2].

2.9.9.3 Potential effects due to resonance in wires, cables, umbilicals, etc. shall be investigated and accountedfor in the design.

2.10 Operational requirements

2.10.1 Application

2.10.1.1 Requirements in DNV-OS-H101 Sec.4 will generally apply. This section should be consideredsupplementary to the requirements for subsea operations.

2.10.1.2 For lifting operations in air the requirements to operational aspects given in DNV-OS-H205 [2.3] areapplicable.

2.10.2 Operation criteria

2.10.2.1 Operational limiting criteria and required weather windows shall be clearly defined for all parts ofsubsea operations.

Guidance note:

Subsea operations are often comprised of several sub-operations each with different operational limitations, or caninvolve a continuous interruptible operation that can be paused due to deteriorating weather conditions. Due attentionshould be paid to the definition of limiting criteria forecast for such operations.


2.10.2.2 Effects of unexpected environmental conditions such as swell and current could be critical - theforecast and/or monitoring shall include all relevant parameters.




2.10.3 Pre-installation surveys

2.10.3.1 A pre-installation survey of the work-site or pipeline route can be required in addition to theinstallation site survey or route survey required for design purposes (covered by [2.1.5] and [2.1.6]) if:

— time elapsed since the design base-survey is significant

— a change in seabed conditions is considered likely

— the installation site/route is located in areas with heavy marine activity

— new installations or facilities are present in the area

— seabed preparation work is performed on the installation site/within the route corridor after previoussurvey.

2.10.3.2 The pre-installation survey, if required, shall determine/confirm:

— potential new hazards

— location of wrecks, submarine installations and other obstructions such as mines, debris, rocks and bouldersthat might interfere with, or impose restrictions on, the installation operations

— the size as well as the location of the seabed objects and infrastructure

— that the present seabed conditions confirm those of the survey required in [2.1.5] and [2.1.6]

— previously unidentified hazards related to the nature of the installation operations.

2.10.3.3 The extent of, and the requirements for, the pre-installation installation site survey/route survey shallbe specified.

2.10.4 Testing

2.10.4.1 Applicable integration testing should be carried out. Equipment subject to such testing shall be clearlyidentified in the test program, see DNV-OS-H101 Sec.4 F200.

2.10.4.2 If required system integration testing should be carried out onshore, proving that integration of allcomponents and tooling can be achieved. This can involve the manufacture / mock-up of simulation models.If mock-ups are used, great care shall be taken to ensure that they replicate the actual item.

2.10.5 Organization

2.10.5.1 General requirements for the organisation, personnel qualifications and communication duringoffshore installation operations are given in DNV-OS-H101 Sec.4 E.

2.10.5.2 The feasibility of subsea operations often relies on the correct completion of tasks by ROV - it shouldtherefore be ensured that ROV operators have the necessary experience and skills.

Guidance note:

Recommendations for manning level and ROV crew qualifications are given in NORSOK U-102, Section 6.


2.10.6 Safety and contingency

2.10.6.1 Operations shall be carried out with due attention to safety, to minimize the risk of emergency andcontingency situations.

2.10.6.2 Detailed procedures for critical operational steps should be established, to define environmental limitsfor possible contingency operations necessary to bring the object to a safe condition e.g. by abandoning orreversing the operation, see also DNV-OS-H101 Sec.2 A400.

Guidance note:

Special considerations for contingency planning under various types of operation are provided in relevant sections.


2.10.6.3 For lifting/handling objects over the side of a vessel, risks relating to dropped objects should beconsidered, see DNV-OS-H102 Sec.3 F300.

Guidance note:

For lifting operations taking place over a vessel side it is normally recommended to establish a safe over boardingdistance from any subsea assets following the principles in DNV-OS-H102 Sec.3 F303.




Sec.3 Loads and structural design –

SECTION 3 LOADS AND STRUCTURAL DESIGN

3.1 Loads

3.1.1 General

3.1.1.1 A thorough evaluation shall be made to identify the nature of and derive the magnitude of all possible/relevant load effects, their appropriate characteristic value/s and combinations.

Guidance note:

Loads applicable for the type of subsea operation proposed (as indicated in [3.3]) should at least be considered. Loadsare also described for various types of operations in Sec.4, Sec.5 and Sec.6.


3.1.1.2 Loads associated with lifting in air (described in DNV-OS-H205 Sec.3) will normally be applicablealso for subsea lifting operations.

3.1.1.3 All loads should be assessed and categorized as indicated in DNV-OS-H102 Sec.3.

3.1.1.4 Vessel motions will normally have a significant effect on loads experienced during offshore operationsand should be assessed accordingly. See [3.2].

3.1.2 Loadcases and analysis

3.1.2.1 Load analyses should be carried out as described in DNV-OS-H102 Sec.4 A.

3.1.2.2 DNV-OS-H102 Sec.4 C describes how to combine individual loads into appropriate load cases/combinations.

3.1.2.3 Requirements for loadcases and analysis of forces for lifting in air (described in DNV-OS-H205 [3.4])will normally be applicable to subsea lifting operations, in addition to the recommendations herein.

3.1.2.4 Further requirements for different types of subsea operations are given in Sec.4, Sec.5 and Sec.6.

Guidance note:

Recommendations for modelling and analysis of various types of marine operations are also given in DNV-RP-H103.


3.2 Vessel motions and accelerations

3.2.1 General

3.2.1.1 General requirements for motion analysis are given in DNV-OS-H102 Sec.4 B.

3.2.1.2 The characteristic motions, velocities and accelerations of relevant reference points (e.g. crane tip) onthe installation vessel should be calculated for the defined environmental design conditions.

Guidance note:

Calculations may be done either by a refined analysis, or by acceptable documented simplified calculations, see DNV-RP-H103 for guidance.


3.2.1.3 Moderate and low sea states in open sea areas are often composed of both wind wave systems andswell. All relevant combinations of wind seas and swell should be considered, ref. DNV-OS-H101 Sec.3 C100.

Guidance note 1:

The wave conditions in a sea state can be divided into two classes, i.e. wind seas and swell. Wind seas are generatedby local wind, while swell is not. Swell seas are waves that have travelled out of the areas where they were generated.Note that several swell components can be present at any given location.


Guidance note 2:

For subsea lifting operations it is normally sufficient to consider the most unfavourable relevant combination(s) ofsimultaneous wind seas and swell. As a minimum the combination of wind seas and swell acting with 90° (or 270°)difference in propagation direction should be considered. The combination of head (or stern) wind seas and swell withpropagation direction towards the crane side of the vessel/object deployment side of vessel will normally be moresevere than swell propagating in the opposite direction.





3.2.2 Characteristic vessel motions generated by wind seas

3.2.2.1 For subsea (lift) operations dependent on a fixed vessel heading, vessel responses for all wavedirections should be analysed. Spacing between the wave headings analysed should not exceed 22.5°.

Guidance note:

Analysis for 45 degrees spacing may be acceptable supported by interpolation calculations for intermediate headings.


3.2.2.2 For subsea (lift) operations that are to be performed independent of vessel heading, vessel responseshould be analysed for wave directions at least ± 15° off the vessel heading stated in the procedure.

3.2.2.3 Short crested sea with spreading n = 2 used in the directional function, ref. DNV-OS-H101 Sec.3 C902,should be applied for operations that are independent of vessel heading.

Guidance note:

DNV-RP-H103 indicates that, for some cases, it can be acceptable to perform calculations for long-crested seas only.If long-crested sea is used for simplicity, the following applies:

Vessel response should be analysed for wave directions at least ±20° outside the vessel heading range. The headingrange should normally be taken as a minimum of ±15°, but in some cases ±10° can be acceptable. Hence, in the lattercase the analysis should be carried out with vessel target heading ±30°.

Note that if neither other headings nor swell (see [3.2.2.1] and [3.2.3.1]) is considered, the operational limiting swell(from other directions than the wind generated sea) should be set to “zero” (i.e. negligible).


3.2.3 Characteristic vessel motions generated by swell

3.2.3.1 Critical swell periods should be identified and considered in the design verification, see DNV-OS-H101 Sec.3 C1000.

Guidance note:

For vessel lifting operations, the following swell periods can be critical:

— Swell periods coinciding with vessel roll and pitch natural period.

— Swell period coinciding with natural period (eigenperiod) for horizontal motion (pendulum motion) of the liftedobject in air, given by DNV-RP-H103 [9.2.1.6].


3.3 Loads

3.3.1 Weight and buoyancy

3.3.1.1 The object weight, CoG, buoyancy and CoB should be defined according to DNV-OS-H102 Sec.3 C.

3.3.1.2 Maximum tolerances on CoG and CoB shall be defined with due consideration given to object stability,criticality and sensitivity of the same during lowering through the wave zone, see [5.2.3.6].

3.3.1.3 The buoyancy and CoB will vary when passing through the splash zone. This shall be taken intoaccount whenever relevant.

3.3.2 Hydrostatic loads

3.3.2.1 Hydrostatic and differential pressures on submerged objects shall be taken into account.

Guidance note:

Maximum expected external water pressure for objects and compartments should be conservatively assessed. Forlifting equipment (e.g. closed spreader bars) the consequence factor of 1.3 (See DNV-OS-H205 Table 5-1) should beapplied to the maximum pressure.


3.3.2.2 Objects flooded during submergence shall have sufficient openings to allow the effective escape of air,considering planned lowering speed.

Guidance note:

It may be necessary to suspend the object for a period of time below water (at a depth unaffected by waves) to allowcomplete flooding, before lowering to depth.





3.3.2.3 The possibility of entrapped air should be evaluated and included in calculations when relevant. Lossof stability, risk of implosion and slack slings should at least be investigated.

3.3.3 Environmental loads

3.3.3.1 Environmental loads should be determined, based on the defined environmental conditions, see [2.1.3].

3.3.3.2 Relevant wave, wind and current loads acting on installation vessel(s) and object(s) should beconsidered as described in DNV-OS-H102 Sec. 3 D and E.

3.3.3.3 Hydrodynamic loads should be calculated according to DNV-RP-H103.

3.3.4 Accidental loads

3.3.4.1 Possible accidental loads shall be considered (see DNV-OS-H102 Sec.3 F for guidance).

3.3.5 Pull-down and pull-in loads

3.3.5.1 Forces on a buoyant object pulled down by a line from the surface, by means of a sheave or similardevice on the seabed, may be computed in accordance with the principles for a subsea lift. For the final lock-in stage, see [3.3.5.2].

3.3.5.2 When an object is pulled in/down, into lock-in position on a sea bed structure, the pull force on theobject may be taken as:

Fpd = 1.2 ηct K [N]

where

ηct: characteristic single amplitude vertical crane tip motion, [m]

K: the stiffness of the hoisting system, see DNV-RP-H103 [4.7.6] [N/m]

3.3.5.3 In general, the hoist line should constitute the weak link in the system. The ultimate capacity (MBL)of attachment brackets, e.g. attachment of hoist line to the object, attachment of sheave to the bottom structure,etc., should as a minimum be 1.3 times the MBL of the attached line.

Guidance note:

Alternatively, for welded steel structures the above requirement could be documented by calculations showing thatthe (plastic) design capacity in ULS of the connection is equal or greater than the hoist line MBL. The main inputparameters are in this case:

— Design load in ULS = 1.0 × MBL of hoist line.

— Steel characteristic resistance, see DNV-OS-H102 Sec.5 A500.

— Material factor, see DNV-OS-H102 Sec. 5 B400.


3.3.6 Off-lead, side-lead forces and horizontal offset

3.3.6.1 Off-lead and side-lead forces on cranes/lifting appliances should be calculated on the basis of vesselmotions, environmental and operational loads on the object and the resulting inclination of the hoisting linefrom the vertical.

3.3.6.2 The horizontal forces or the horizontal offset and corresponding off-lead and side-lead angles relativeto the vertical (as documented) shall be within the specified design values of the crane/lifting appliance.

Guidance note 1:

Off-lead and side-lead forces are forces on the lifting system occurring when the lifted object is pulled away from thevertical through the crane tip. Off-lead means in the direction away from the crane, side-lead being perpendicular tothe direction of the crane boom.


Guidance note 2:

For an axially stiff cable with negligible bending stiffness the offset of a vertical cable with a heavy weight at the endof the cable in an arbitrary current with unidirectional velocity profile may be calculated according to DNV-RP-H103[5.2.2.1].


3.3.7 Loads during positioning

3.3.7.1 Loads related to translation and rotation of the object during lowering, positioning and setting shouldbe considered, see [7.2].




3.3.7.2 Reaction forces from the soil should be determined and accounted for. See Sec.7.

Guidance note:

Such loads may be foundation reactions at seabed impact and suction forces, if an object needs to be recovered orrepositioned.


3.3.7.3 Both ULS and ALS loads from the guiding and positioning systems shall be defined as foundapplicable, see also [2.6.3] and [2.6.4].

3.3.8 Other loads

3.3.8.1 When relevant, due consideration should be given to special loads such as;

— tugger line loads— loads due to redistribution of ballast,— loads due to CoG changes— entrapped air— entrapped water— other relevant loads.

3.4 Structural design

3.4.1 General

3.4.1.1 General recommendations regarding structural design and fabrication are given in DNV-OS-H102.

3.4.2 Object

3.4.2.1 Adequate structural strength should be documented for the object involved in the subsea operation.

3.4.2.2 Lifted objects should be designed to withstand hydrostatic, hydrodynamic and any other loadsexperienced during transportation and installation, as described in Section [3.3.1] to [3.3.8].

3.4.2.3 Appropriate consequence factors, see DNV-OS-H205 [5.1], should be applied to lift points, primaryand secondary structural elements.

3.4.2.4 Due consideration should be given to skew load cases, the effects of which are not normally coveredby in service design conditions.

3.4.2.5 Attention should be given to possible horizontal load components acting on the lift points.

3.4.3 Bumpers and Guides

3.4.3.1 Bumpers and guides should be designed according to requirements in DNV-OS-H101 Sec.6 C.

3.4.3.2 The maximum entry speed of the object onto the guiding system shall be defined taking into accountthe characteristics loads as described in DNV-OS-H101 Sec.6 C200.

3.4.4 Rigging lay down and securing

3.4.4.1 Requirements for design of rigging lay-down and securing arrangements are given in DNV-OS-H205[5.1.8].

3.4.5 Seafastening and supporting structures

3.4.5.1 Requirements for design of seafastening and vertical support structures (grillage) for transportation ofobjects are in general covered in DNV-OS-H202.

3.4.5.2 The seafastening and grillage should allow for easy release and provide adequate support andhorizontal restraint until the object can be lifted clear of the transportation vessel/barge.

3.4.5.3 Elements providing horizontal and/or vertical support after cutting/removal of seafastening shall beverified for the environmental conditions applicable for the operation.

3.4.6 Inspection

3.4.6.1 Inspection of lift points and lifting equipment should comply with the requirements relating to “specialstructural steel” in DNV-OS-H102 Sec.6 B.

3.4.6.2 Lift points shall be inspected for each subsequent lift.




Guidance note:

The extent of inspection should be defined based on the available information about the lift point load history and theoriginal NDT. If no relevant information is available the inspection should be as for a new lift point, see [3.4.6.1]. Liftpoints can be accepted for subsequent lifting based on a visual inspection only if;

a) the load history (since last MPI/UT inspection) of the lift points is known,

b) no excessive or uncontrolled loading of the lift points has occurred, or is suspected to have occurred duringprevious lifts, and

c) no damages are detected during the visual inspection.

Lift points satisfying items b) and c) only, should be subject to 100% MPI before any subsequent lifting.


3.4.6.3 Requirements for inspection of objects stored subsea should be agreed before lifting. Demonstrationof appropriate load history since the most recent NDT is normally required.

Guidance note:

See DNV-RP-H102 for specific guidance regarding removal of objects.




Sec.4 Loadout and transport –

SECTION 4 LOADOUT AND TRANSPORT

4.1 General

4.1.1 Application

4.1.1.1 The requirements in DNV-OS-H201 and DNV-OS-H202 generally apply for loadout and transportoperations.

4.1.1.2 For load-out by lifting, requirements in DNV-OS-H205 generally apply.

4.1.1.3 This section should be considered as additional and/or clarifying requirements for objects andoperations that are not covered by the conventional load-out and vessel transports methods in the referencedVMO-standards.

Guidance note:

The following operations and products/objects are covered in this Section:

— Towing of submerged small volume structures and long slender elements.

— Loadout and towing of bundles.

— Loadout and transport of pipelines, risers, cables and umbilicals transported on reels or carousels.

— Load-out and transport of pipe joints.


4.2 Submerged towing

4.2.1 General

4.2.1.1 Recommendations in this sub-section are applicable for towing of submerged small-volume structuresand long slender elements.

Guidance note:

Three different tow configurations are covered in this sub-section:

— Submerged tow of objects attached to Vessel

— Submerged tow of objects attached to Towed Buoy

— Surface or sub-surface tow of long slender elements (see also [4.3] which covers pipe bundles).


4.2.1.2 The operation reference period (TR) for the towing operation shall be assessed taking into account thefollowing:

4.2.1.3 Safe conditions before and after the tow shall be clearly defined.

4.2.1.4 The low towing speed that is normally expected for submerged towing.

4.2.1.5 Considering TR, the operation shall be classified as weather restricted or weather unrestricted, seeDNV-OS-H101 Sec.4.

4.2.1.6 All parameters that could be critical shall be considered during the modelling and analysis of asubmerged tow. See DNV-RP-H103 [7.3.2] for examples of critical parameters.

4.2.2 Submerged tow of objects attached to installation vessel

4.2.2.1 Design considerations for submerged tow of object attached to installation vessel are given in DNV-RP-H103 [7.3.3].

4.2.2.2 Hang-off rigging shall be designed in accordance with DNV-OS-H205 Sec.4.

4.2.2.3 Possible fatigue of hang-off point(s) on vessel and lift points / elements supporting lift points on towedobject shall be considered.

4.2.2.4 Possible fatigue of components on towed object, e.g. internal piping, due to VIV shall be considered.

4.2.2.5 Measures shall be taken to prevent abrasion of hang-off rigging and any anti-rotation line(s).

4.2.3 Submerged tow of objects attached to towed buoy

4.2.3.1 Design considerations for the submerged tow of an object attached to towed buoy are given in DNV-RP-H103 [7.3.4].




4.2.3.2 Design of the towing arrangement shall comply with the requirements in DNV-OS-H202 Sec.4.

4.2.3.3 Hang-off rigging between the buoy and the object shall be designed in accordance with DNV-OS-H205 Sec.4. The possibility of fatigue should be considered.

4.2.4 Surface or sub-surface tow of long slender elements

4.2.4.1 Examples of slender objects that can be towed to field are;

— pipelines, bundles, spools— TLP tethers— riser towers / hybrid risers.

4.2.4.2 Design considerations for surface or sub-surface tow of long slender elements are given in DNV-RP-H103 [7.3.5].

Guidance note:

Several slender objects can be towed as a bundle arrangement (e.g. strapped together or within a protective casing).[4.3] covers load-out and transport of pipe bundles, however these requirements can also be applicable to other typesof slender objects as outlined in [4.2.4.1].


4.2.5 Loads and analyses

4.2.5.1 The modelling and analysis of submerged tows should be performed according to therecommendations in DNV-RP-H103 [7.3.9].

4.2.5.2 All relevant vessel headings shall be analysed, see [3.2.2] for requirements and guidance.

4.2.5.3 All possible (relevant) loads and their appropriate characteristic value shall be identified according tothe principles in Sec.3.

4.2.5.4 Calculated characteristic values of loads should have maximum 10% probability of being exceeded. Inorder to verify that the corresponding design load is adequate it is recommended to also check the tail of thedistribution.

Guidance note:

E.g. the 1% probability of being exceeded load could be calculated and verified against the design load and the overallstructural safety requirement, see DNV-OS-H101 Sec.1 A200.


4.2.5.5 Acceptable clearances to vessel side or moonpool sides should normally be documented bycalculations.

4.3 Bundles

4.3.1 General

4.3.1.1 This sub-section covers controlled depth tow and off-bottom tow of pipe and riser bundles.

4.3.1.2 The bundle design should be based on direct calculation of required bollard pull.

4.3.1.3 Bundle towing connections and wires should be designed based on dynamic analysis of the launch,towing and holdback forces.

4.3.1.4 Appropriate design factors should be defined with reference to the following alternatives:

— As for towing (DNV-OS-H202) if the object is floating in a controllable manner after a structural and/ortowline failure.

— As for lifting (See Sec.5 and DNV-OS-H205) if the object is not controllable after a structural and/ortowline failure.

4.3.1.5 Bundle break-out forces shall be conservatively estimated. The effects of launch track slope/settlement, mechanical resistance, launch bogie / roller condition and other relevant parameters that influencethe break out force shall be considered.

4.3.1.6 The stability of the total bundle and tow heads/structures shall be calculated for all stages of the launch,tow installation and flooding. Side current forces, hydrodynamic effects during tow and free surface effectsduring flooding operations should be considered.




4.3.1.7 Bundle behaviour during tow should as far as possible be estimated during design. Inline structuresshould as far as possible be designed in a way that will minimise the generation of hydrodynamic drag and liftforces that could cause an instable/fluctuating bundle configuration during tow.

4.3.1.8 Sensitivity studies shall be carried out for essential parameters such as weight, ballast, buoyancy,salinity, cross current, towing speed, back tension, internal pressure loss etc. for relevant phases.

4.3.2 Load-out of bundles

4.3.2.1 Local environmental conditions at the launch site, such as wave directions/patterns, tide and currentforces should be considered.

4.3.2.2 The launch area, including an adequate corridor to allow for the necessary deflection of bundle, shallbe surveyed prior to the operation.

4.3.2.3 Bundle deflection due to side current shall be analysed for different stages of the launch. The towingvessel offset positions required to counteract the predicted bundle deflection shall be established.

Guidance note:

When a bundle is towed in its axial direction in an “off-bottom tow mode”, friction between the ballast chain andseabed cannot be used to counteract lateral deflection of the bundle.


4.3.2.4 Bundle support and bending restrictions shall be defined, based on structural pipe analysis,consideration of local soil conditions, the launch track characteristics and bundle weight and stiffness. Variableconditions such as scour and erosion in wave effected zone and consolidation of the soil shall be considered.Acceptable departure angles from launch way, in both horizontal and vertical direction, shall be defined.

4.3.2.5 Adequate means of monitoring environmental conditions and limiting load-out parameters shall beestablished and tested before commencement of load-out.

4.3.3 Towing of bundles

4.3.3.1 Adequate means of monitoring environmental conditions, tow parameters and bundle configurationshall be established and tested before commencement of tow.

4.3.3.2 Adequate back-up systems shall be available.

4.3.3.3 The tow route, including an adequate corridor to allow for the necessary deflection of bundle andtemporary lay-down areas, shall be surveyed prior to start of tow.

4.3.3.4 Prior to commencing the tow the bundle shall be ballasted to an acceptable configuration for the tow

4.3.3.5 Bundle parameters, configuration and feedback shall be systematically checked after commencementof the tow. Deviations from expected values shall be recorded and any possible effects on the towing procedureand bundle evaluated.

4.3.3.6 Bundle deflection and anchorage forces (required to stabilise the bundle in any predefined holdinglocations) shall be analysed for the characteristic current conditions and loads.

4.3.3.7 Current speed (and direction) should be monitored at regular intervals during tow and holding periods,unless extreme current values are used in the analysis of bundle behaviour. Contingency procedures should beavailable and mitigating actions employed in case the current speed exceeds the design values.

4.3.3.8 Bundle behaviour following towline failure should be assessed and used as a basis for evaluating andgenerating and appropriate contingency procedures.

Guidance note:

Qualification testing should be based on a product and configuration representative of the actual bundle and towingconditions anticipated.


4.3.3.9 The strength of the bundle should be documented as adequate for all potential situations, including thatwhere it is hanging freely supported only at each end.

4.3.3.10 If external ballast is used (normally chain) the bundle must be sufficiently robust to accept some lossof ballast during tow, without undue effect on the bundle configuration.

4.3.3.11 Adequate abandonment equipment shall be carried on-board the lead tug(s) and trailing tug, to enablecontrolled laydown and abandonment if necessary.




4.4 Pipelines, risers, cables and umbilicals

4.4.1 General

4.4.1.1 This subsection applies to pipelines, risers, cables and umbilicals transported on (and installed from)reels or carousels. Relevant requirements are also applicable for products transported on (and installed from)baskets.

4.4.1.2 Requirements for load-out/shore pull including reeling of pipe strings (‘stalks’) are covered in DNV-OS-F101 Sec.10 F700 and Sec.10 F400 (if applicable).

Guidance note:

Guidance regarding loadout and transport of cables can also be found in DNV-RP-J301 [5.3].


4.4.2 Load out by lifting

4.4.2.1 Lifting equipment shall comply with the requirements in DNV-OS-H205.

4.4.2.2 Heavy items, structures and end terminations stored with the product can influence the CoG of the reel/basket/carousel significantly and shall be taken into account when undertaking the design for lifting andtransportation.

4.4.3 Load-out by spooling

4.4.3.1 An operation-specific procedure for the load-out operation shall be prepared, including a detaileddescription of the planning and step-by-step procedure for the execution of spooling itself.

4.4.3.2 Spooling operations shall be designed such that the associated handling procedures do not result intension, twisting or bending of the product (including terminations and ancillary equipment) in excess of itsoperational limitations.

4.4.3.3 Continuous monitoring of the tension, pulling force and other relevant parameters by means ofmeasuring devices shall be performed during spooling; the accuracy required is given in DNV-OS-H101 Sec.4D.

4.4.3.4 Critical steps in the spooling operation should be visually monitored continuously, including transitionpoints (rollers, deflectors, sheaves, chutes, spans etc.) to monitor for the presence of excessive twist - themaximum allowable twist shall be defined.

4.4.3.5 Product lift/transfer arrangements and procedures shall be designed so that product limiting criteria arenot exceeded. Tension shall be maintained in free spans such that the resulting catenary does not infringe theminimum bending radius defined for spooling operations.

Guidance note:

The product may be supported at each end by bend shoes, sheaves, chutes or bellmouths of a suitable radius.


4.4.3.6 Any lifting gear/equipment and attachment points shall be configured such that the product remainswithin all its design limiting criteria.

4.4.3.7 Lifting equipment used to handle in-line and end structures / end terminations shall be designed inaccordance with DNV-OS-H205 Sec.3. The design of lifting equipment and lift points on such structuresshould consider all possible load directions and distributions between different parts of the rigging.

4.4.3.8 Methods involving for example soft slings, Chinese fingers or similar choked around the product, usedfor pulling and/or hold-back of the product, shall be qualified prior to operation. Qualification should includetesting carried out on a configuration representative of the actual product and load conditions/directions.

4.4.3.9 The design of load-out vessel moorings should consider reaction loads during the spooling operation.Vessel mooring design should consider tidal range, necessary offsets and heading variations, with reference topipe spans and allowable product loads.

4.4.3.10 Operators of the spooling units (winches, carousel, turntable, etc.) shall be in continuous radiocontact. Spare radio sets shall be available.

4.4.4 Sea transport

4.4.4.1 All products, including in-line assemblies, end structures/terminations, buoyancy modules, clamps andother accessories shall be seafastened according to requirements in DNV-OS-H202.




4.4.4.2 The structural integrity of equipment such as reels, carousels and baskets shall be documented asadequate. The actual transport condition and product parameters including, but not limited to, for examplecontent, packing arrangements, ‘locked-in’ spooling tension, CoG, load distribution and weight/lengthcontingency shall be considered.

4.4.4.3 When transport is on board the installation vessel and the installation equipment also acts aspermanent/temporary seafastening, e.g. reel drive systems, the mechanical/hydraulic capacity of theinstallation equipment shall be documented as sufficient for the relevant transportation loads. Special attentionshould be given to systems depending on hydraulic pressure, gears and roller/bearing systems exposed to highloads in a static condition.

4.4.4.4 The seafastening arrangements for storage reels not supported in a reel-drive system should bedocumented for all phases.

Guidance note:

A seafastening release procedure should document that the reel is adequately secured until the reel drive system ismounted.


4.5 Pipe joints

4.5.1 General

4.5.1.1 This subsection is applicable to the load-out and transport of individual and multiple pipe joints.

Guidance note:

A pipe joint is a length of steel pipe, approx. 12 m long, used in the manufacturing of offshore pipelines. A doublejoint consist of two pipe joints welded together, reducing the number of offshore welds and thereby the offshoreinstallation time.


4.5.2 Load out by lifting

4.5.2.1 Handling and lifting of pipe joints shall be performed according to recognised procedures and methods.Equipment used for handling and lifting shall be designed to prevent damage to coatings and/or prepared pipeends.

4.5.2.2 The capacity of lifting equipment shall comply with requirements in DNV-OS-H205.

4.5.3 Sea Transport

4.5.3.1 All products, including pipe joints, in-line assemblies, end structures/terminations and otheraccessories shall be seafastened following the principles in DNV-OS-H202.

4.5.3.2 Acceptable stacking heights shall be established and documented for transportation loads. Each pipejoint and pipe stack shall withstand relevant weight and environmental loading. Drainage of water shall beensured. Potential icing shall be considered and appropriate measures should be taken where necessary.

4.5.4 Offshore pipe loading

4.5.4.1 Pipe loading offshore shall not be done in areas where a pipe dropped overboard could damagepipelines or other subsea assets, see DNV-OS-H102 Sec.3 F303 for guidance on the safe distance required.



Sec.5 Subsea lifting –

SECTION 5 SUBSEA LIFTING

5.1 General

5.1.1 Application

5.1.1.1 Requirements for lifting in air (see DNV-OS-H205) are generally also applicable to subsea liftingoperations.

5.1.1.2 Subsea lifting is defined as installing, moving or recovering an object subsea by means of a crane orother lifting appliance.

5.1.1.3 General requirements for subsea operations are given in Sec.2. The following sub-sections should beconsidered as additional/supplementary requirements for subsea lifting operations.

5.2 Loads and analysis

5.2.1 Loads

5.2.1.1 General requirements are given in Sec.3; all loads relevant to a subsea lift shall be taken into account.

5.2.1.2 Both minimum and maximum values should be applied in calculation of static weight, ref. DNV-RP-H103[4.2.2].

Guidance note:

Variation in weight/mass due to flooding/draining of subsea structures as described in DNV-RP-H103 [4.2.2] shouldnot be considered as accidental conditions and the associated loads are hence applicable in ULS loadcases.


5.2.1.3 The estimation of hydrodynamic coefficients should follow the recommendations given in DNV-RP-H103 [4.6]. See also DNV-RP-C205 for guidance.

5.2.1.4 The stiffness of the hoisting system can be calculated according DNV-RP-H103 [4.7.6].

5.2.1.5 If unavoidable, snap loads shall be analysed thoroughly.

Guidance note 1:

If the hoist line or one or more slings become slack, significant snap forces may be experienced. A slack conditionnormally occurs if the hydrodynamic forces exceed the force caused by the static (submerged) weight of an object.Appropriate limiting weather conditions should be defined where possible such that slack sling criteria are fulfilledand snap loads are avoided, see [5.3.1.1].


Guidance note 2:

Characteristic snap loads may be calculated according to DNV-RP-H103 [4.7.2] or by more advanced methods, seee.g. [5.3.2.4].


5.2.2 Combination of loads

5.2.2.1 An appropriate and structured method of combining all relevant loads shall be used. See DNV-OS-H102 and DNV-RP-H103 for requirements and guidance.

Guidance note:

If time domain analyses are applied, various combinations of irregular waves (wind seas) and swell may be includedfor direct calculation of combined load effects and the resulting forces on the lifted object.


5.2.3 Lift analysis - General

5.2.3.1 Lifts shall be analysed for all relevant loadcases. See DNV-OS-H102 Sec. 4 for general requirements.

5.2.3.2 Adequate control of the lift in air should if necessary be documented by analysis. See DNV-OS-H205[2.3.2].

Guidance note:

The formula presented in DNV-RP-H103 [9.2.1.6] can be used to check if horizontal resonance is likely to occur. Ifsuch amplification can occur it should be documented by calculations/analysis that horizontal motions are prohibited




by physical means (DNV-OS-H205 [2.3.2.1]). Note also that spreader bars and/or crane hooks can be subject tocritical (double) pendulum motions.


5.2.3.3 All critical vertical positions of the object relative to the sea surface shall be included in the liftanalysis. See DNV-RP-H103 [4.5] for further guidance.

5.2.3.4 If clearances during deck handling operations are considered critical, see [5.6.1.4], lift off from thedeck of the installation vessel should be analysed to estimate the minimum clearances between the lifted objectand other structures/vessel side.

5.2.3.5 If tugger wires are used to control a lifted object, the required capacity of the tugger wire arrangementshould be calculated; both the winch wire tension and winch pay-out/pay-in speed should be considered.

5.2.3.6 The lifted object shall be documented as having adequate stability when lowering through the wavezone. See [2.5.2.3].

5.2.3.7 Hydrodynamic forces on objects lowered through the water surface and below the wave influencedzone, can be estimated using the simplified method described in [5.2.4]. Other methods may be applicable, see[5.4].

5.2.3.8 The local strength of lifted equipment (including hatch covers, ancillary equipment, etc.) should bechecked for the effects of wave slamming.

5.2.3.9 Guidance for estimating loads associated with deepwater operations is given in Section [5.7.1].

5.2.3.10 Positioning loads are covered in [3.3.7], whilst geotechnical aspects related to landing on and retrievalfrom the seabed are covered in Sec.7.

5.2.3.11 It shall be documented that for landing operations relying on active heave compensation (AHC), thecrane tip heave motion and heave velocity remains within the compensating capacity of the AHC system. Seealso [2.4.2].

5.2.4 Simplified method for estimation of hydrodynamic forces acting on submerged objects

5.2.4.1 The Simplified Method described in DNV-RP-H103 [4.3], may be used to estimate the forces onobjects lowered through the water surface and down to the seabed. The method is equally applicable forretrieval.

5.2.4.2 The intention of the Simplified Method is to provide a basic, albeit conservative, estimate of the forcesacting on a lifted object. Alternative calculation methods can be considered acceptable provided they followthe general guidelines given in [5.4].

5.2.4.3 The Simplified Method may be adopted assuming the following criteria are fulfilled:

— The size of the lifted object (overall length) is relatively small compared to the wave length.— The vertical motion of the object follows the crane tip motion.— The load case is dominated by relative vertical motion between object and water – other modes of motions

can be disregarded.

Guidance note 1:

The horizontal extent of the lifted object (in the wave propagation direction) should be less than 1/4 of the typicalwave lengths of the waves exciting the structure. The force in the hoist line will normally be conservatively estimatedif the horizontal extent of the lifted object is larger than the above limitation. Forces experienced in individual slingshowever can be underestimated as unsymmetrical loading can be more pronounced for larger objects.


Guidance note 2:

The method is not applicable if the crane tip oscillation period or the wave period is close to the resonance period ofthe hoisting system. For further guidance see DNV-RP-H103.


Guidance note 3:

Note that the lifted object may be divided into main items and surfaces contributing to the hydrodynamic force, seeDNV-RP-H103 [4.3.9.6].


5.2.4.4 More accurate estimations are required if the criteria in [5.2.4.3] are not fulfilled, see [5.4].




5.3 Acceptance criteria

5.3.1 Acceptance criteria – Simplified method

5.3.1.1 The following criterion should be fulfilled in order to ensure that snap loads are avoided in the slingsand hoist line:

Fhyd ≤ 0.9 × Fstatic-min [N]

where

Fstatic-min = Force due to minimum static weight of object [N], see [5.3.1.2].Fhyd = Characteristic hydrodynamic force [N]

Guidance note 1:

A 10% margin to the start of slack slings is assumed to be an adequate safety level given the load factors andcombinations stated in the ULS criteria.


Guidance note 2:

When deploying objects that are close to neutrally buoyant (e.g. ROVs, etc) it will normally not be possible to fulfilthe slack sling criteria in moderate sea states. In such cases the hoisting system should incorporate a guide system toensure controlled clearance from the vessel side. Further, unless means to avoid snap loads are available, thecharacteristic snap load should be accounted for in the design. The characteristic snap load may be calculatedaccording to DNV-RP-H103 [4.7.2].


5.3.1.2 The minimum static weight of an object as described in [5.2.1.2] should be applied when checkingslack sling criterion.

5.3.1.3 In addition to the slack sling criterion, the capacity of lifted object and lifting equipment should bechecked according to DNV-OS-H205.

5.3.1.4 The Characteristic total force on an object should be established by applying the maximum staticweight, see [5.2.1.2].

5.3.1.5 The capacity checks described in DNV-OS-H205, Lifting Operations, relate to the weight of the objectin air. Hence, a converted dynamic amplification factor (DAF) should be applied equivalent to a factor valid inair. The following relation should be applied in the equations given in DNV-OS-H205:

DAFconv = Ftotal / mg

where

DAFconv = is the converted dynamic amplification factorm = mass of object in air [kg]g = acceleration of gravity = 9.81 [m/s2]Ftotal = is the largest of: Ftotal = Fstatic-max + Fhyd or Ftotal = Fstatic-max + Fsnap [N] Fstatic-max = Force due to maximum static weight of object [N], see [5.3.1.4] .

5.3.1.6 The base case method in DNV-OS-H205 [5.1.2] should be used to define the load factors (and thedesign load) only if the Simplified Method is applied.

5.3.2 Acceptance criteria - Alternative

5.3.2.1 Acceptance criteria in this paragraph are applicable if the methods described in [5.4] are used tocalculate hydrodynamic forces.

5.3.2.2 Acceptance criteria in [5.3.1] (for avoidance of snap loads) are applicable for the main hoist line. Forindividual slings Fhyd ≤ 1.0 ⋅ Fstatic-min is acceptable.

5.3.2.3 The capacity of lifted object and lifting equipment shall be checked as indicated in [5.3.1.3], [5.3.1.4]and [5.3.1.5].

5.3.2.4 Snap loads may be established using time domain analyses; this requires appropriate and accuratemodelling of hoisting system and lift rigging characteristics.

5.3.2.5 The probability of exceeding the calculated extreme characteristic load (hydrodynamic + static load orsnap load) in the operation period shall not exceed 10%. In order to verify that the corresponding design loadis adequate it is recommended to also check the tail of the distribution. See [4.2.5.4] GN.

5.3.2.6 The design load could be calculated based on the two ULS load conditions “a” and “b”. See DNV-OS-H102 Sec.5 B (and DNV-OS-H205 [5.1.3]). Note that the load factors shall be applied to the characteristichydrodynamic and static loads acting on the object.




Guidance note:

The acceptance criterion in [5.3.1.1] is assumed to provide an adequate safety level given the load factors and loadcombinations stated in the ULS criteria. If slack slings are encountered, the design loads should preferably becalculated as indicated. However, the alternative given below in [5.3.2.7] can be found acceptable.


5.3.2.7 If the approach in [5.3.2.6] is not followed, a load factor of 1.3 shall be applied on the totalcharacteristic lift force including snap load (i.e. characteristic static load + characteristic dynamic load). Seealso GN above.

5.4 More accurate estimation of hydrodynamic forces

5.4.1 General

5.4.1.1 If the criteria for using the simplified method, given in [5.2.4], are not fulfilled or more accuratecalculation methods are desired, alternative calculation methods can be acceptable, providing they follow therecommendations given in DNV-RP-H103 [3.4].

5.4.1.2 See [5.3.2] for accept criteria.

5.4.2 Documentation

5.4.2.1 General requirements for documentation are given in [2.2]. If alternative calculation methods are used,additional documentation as described in this sub-section shall be made available.

5.4.2.2 Documentation based upon regular design wave approach should be comprehensively and thoroughlydescribed, such that the calculations are reproducible.

5.4.2.3 As a minimum, for time domain analyses the following documentation is required:

— Description of how the hydrodynamic model of the vessel has been derived, load condition, etc.— Description of how the object’s hydrodynamic coefficients have been derived. Values should be included

for both individual elements and the total added mass/damping.— Description of how slamming has been modelled, comparison of slamming loads versus analytical results,

results from model tests or CFD analyses.— Description of how extreme forces have been estimated.

5.4.2.4 As a minimum for CFD analyses, the following documentation is required:

— Numerical method applied.— Applied boundary conditions and size of computational domain.— Applied turbulence model.— Results from spatial and temporal convergence tests.— Description of how transient effects are accounted for.— Computed forces, pressures and velocities should be checked and compared with approximate hand

calculations.

5.4.2.5 If model test results are available, numerical simulation results should be compared and validated withthe model test.

5.5 Lifted object

5.5.1 General

5.5.1.1 Requirements for the structural design of lifted objects are given in [3.4].

5.5.1.2 DNV Standard for Certification No 2.7-3 may be used as an alternative standard for structuralverification of objects lifted subsea.

Guidance note:

DNV Standard for Certification No 2.7-3 covers all objects defined as portable offshore units and it is recommended used:

— where units are intended to be transported, lifted and installed repeatedly

— and/or for units where generic handling procedures are intended to be used

— and/or for units that require certification.





5.5.2 Pipelines, risers, cables and umbilicals

5.5.2.1 For objects that are partly supported by a lifting appliance, the load distribution on the object shall bethoroughly evaluated.

5.5.2.2 Loads for risers, umbilicals and cables are generally covered in [6.4.1]. Relevant loads and analyticalmethods should consider possible (dynamic) effects due to the geometry of the termination.

5.5.2.3 The lifting structure shall fulfil the requirements for structural design given in Sec.3.

5.5.2.4 Additional loads due to handling and positioning of end termination assemblies shall be considered.

Guidance note:

Riser-like products will normally be fitted with termination assemblies designed for subsea installation by lifting,seabed support and for tie-in. Such termination assemblies can be difficult to position due to the lateral, vertical androtational stiffness of the product attached.


5.5.3 Spools

5.5.3.1 Spools are typically slender structures, with concentrated masses at the ends. Relevant loads andanalyses methods should be adopted to consider possible dynamic effects associated with such geometry.

5.5.3.2 For objects with large horizontal extent e.g. long slender structures like spool pieces, spreader beams,etc. more refined analyses (than the Simplified Method described in [5.2.4]) are needed to adequately establishloads in individual slings.

5.5.3.3 Local reaction forces at support points should be considered.

5.5.3.4 Free flooding of objects such as spools, spreader beams, etc. can cause large shifts in CoG, subsequentinstability and possible overturning. These effects should be duly considered.

5.5.3.5 Trial lifting of spools and/or jumpers shall be carried out to verify the rigging geometry prior to load-out. If the trial lift reveals that a sling is slack or the tilt angle is unacceptable, sling lengths shall be adjustedand the test lift repeated.

5.5.4 Retrieval of damaged objects

5.5.4.1 The status and integrity of a damaged object shall be established prior to retrieval, to avoid collapseand further damage during recovery.

5.5.4.2 In many cases objects are not designed for retrieval loadcases in a damaged state, hence applicableretrieval load cases, taking into account water filling, soil forces, suction, grouting, marine growth, etc. shallbe considered.

5.5.4.3 The loads experienced by a damaged object during retrieval and re-installation can affect its futurefatigue resistance and design life. This shall be reduced accordingly.

5.6 Operational aspects

5.6.1 General

5.6.1.1 See [2.10] for general requirements relating to the planning and execution of subsea operations.

5.6.1.2 Operational aspects for all phases of the subsea lift operation shall be considered.

Guidance note:

A typical subsea lift consists of the following main phases/steps:

— removal / release of seafastening

— lift-off from deck and manoeuvring object clear of transportation vessel/barge

— lowering through the wave zone

— lowering to seabed

— positioning and set-down

— disconnect and retrieve lift rigging.


5.6.1.3 In addition to the onshore test lift described in [2.5.6], a trial over boarding lift during mobilisation isrecommended to: verify available hook height, confirm that the intended lift path is free from obstructions,confirm clearances to other objects/vessel side, check routing of tugger lines/tag lines, etc.




5.6.1.4 The operational aspects for lift operations in air, see DNV-OS-H205 [2.3] are normally applicable alsofor subsea objects.

Guidance note:

[2.3] in DNV-OS-H205 includes recommendations for:

— control of lift

— clearances

— lifting (operational criteria and procedure details)

— monitoring of lifting operations

— cutting of seafastening.


5.6.1.5 When lifting an object from a barge or other vessel on board a crane vessel positioned side by side, therelative motion between the crane hook and the barge at the position of the lifted object is critical. The risk ofinterference between barge/vessel and the lifted object after lift-off should be considered.

5.6.1.6 Assumptions, see [5.2.3.6], regarding stability of an object should be considered during loweringthrough the wave zone. This can be particularly relevant in relation to:

— tilting of partly air-filled objects during lowering— the effect of free water surface inside an object— the possibility of entrapped air.

5.6.1.7 During lowering of an object to the seabed the following should be considered:

— Horizontal offset due to current (where the current velocity can be time-dependent and its magnitude anddirection variable with water depth).

— Dynamics and possible resonance effects due to wave induced motion of crane tip on vessel.

5.6.1.8 The object’s maximum allowable landing velocity shall not be exceeded, regardless of whether thestructure is landed onto the seabed or another structure.

Guidance note:

Landing velocity is the sum of the objects vertical heave velocity and the crane/hoisting system pay-out speed.Adjustment of the weather criteria might be necessary to limit the landing velocity.


5.6.2 Installation tolerances

5.6.2.1 Installation tolerances shall be clearly defined. The means for controlling and ultimately confirmingthese shall be available.

5.6.2.2 The feasibility of achieving the installation tolerances should be thoroughly reviewed based on factorssuch as:

— experience from similar operations— manufacturing tolerances— dimensional control surveys — trial fits— environmental conditions— ability to assist positioning of the object by use of guiding or orientation systems— soil conditions — expected visibility— accuracy of survey/measuring equipment.

5.6.2.3 Once the correct installation position has been confirmed, the lift rigging should be disconnectedwithout undue delay. ROV (or diver) access and snag free release of lift rigging are critical aspects that shouldbe considered early in the design process.

5.6.3 Wet parking

5.6.3.1 The seabed condition shall be assessed if wet parking is planned or included in contingency procedures.The need for seabed preparation shall be evaluated.

5.6.3.2 Assessment of seabed condition should normally include calculations of soil capacity, on-bottomstability and if relevant necessary break-out forces due to suction.




Guidance note:

The maximum length of time that the structure could be wet stored should be considered. Assessments should evaluatelikely seabed penetrations. Further guidance for calculating these effects are given in Sec.7.



5.6.4.1 General requirements for contingency planning are given in [2.10.6].

Guidance note:

Typical contingency situations can relate to damage of lifted object due to unexpected loads, failure of vesselpositioning system, failure of object guiding and/or positioning system, out of tolerance installation position, failureof crane, failure of lift rigging, ROV breakdown, deteriorating weather conditions or any other contingency situationidentified from risk identifying activities.


5.6.4.2 Contingency procedures shall, where necessary, consider and address whether the object could betemporarily abandoned on seabed, temporarily suspended or recovered to deck of the installation vessel.

5.7 Deep water

5.7.1 Deep water lowering operations

5.7.1.1 For lifting operations in deep water the following effects shall be considered:

— Cable stretch due to self-weight and weight of lifted object.— Horizontal offset due to current where the current velocity can be time-dependent and its magnitude and

direction can vary with water depth.— Dynamics and possible resonance effects due to wave induced motion of vessel crane tip.— Methods for controlling vertical motion of lifted object.

Guidance note:

See DNV-RP-H103 Sec. 5 for advice on how to calculate these effects. Note also that deep water is not specified asa specific limit in this Standard as a reasonable fixed limit will depend on prevailing environmental conditions andapplication, e.g. drilling, diving, subsea construction, renewable energy, etc.


5.7.1.2 The permissible installation tolerances shall be determined taking into account the increased difficultyin accurate seabed positioning caused by large water depth and environmental conditions (current).

5.7.1.3 Deep water lowering and landing/docking operations must be planned with due consideration of thetime needed for deployment / recovery of units, tools and ROV’s, for handling, manoeuvring, guiding andpositioning.



Sec.6 Installation of pipelines, risers, cables and umbilicals –

SECTION 6 INSTALLATION OF PIPELINES, RISERS, CABLES AND UMBILICALS

6.1 General

6.1.1 Application

6.1.1.1 This Section gives requirements and guidance for installation of both rigid and flexible submarinepipeline and cable systems including pipelines, risers, cables and umbilicals. Both static and dynamicapplications are covered.

Guidance note 1:

DNV-OS-F101 Submarine Pipeline Systems Sec.10 gives requirements for installation/offshore construction ofsubmarine pipeline systems. Parts of DNV-OS-F101 Sec.10 are also generally applicable for flexible pipes and risers.Hence, requirements in DNV-OS-F101 are widely referred to and/or repeated in this Section where applicable.


Guidance note 2:

Detailed guidance regarding installation of cables may be found in DNV-RP-J301 Sec. 6. Additional guidance forwind farm cable installations may be found in ND/0035, Section 10.


Guidance note 3:

In this section, the installation of submarine pipeline and cable systems including pipelines, risers, cables andumbilicals is referred to as “laying operations”; “product” is used as a collective term for the various objects coveredin this section.


Guidance note 4:

Further guidance on subjects not explicitly covered in this Section may be found in ND/0029. Examples are:

— Installation of deep water Steel Catenary Risers (SCR).

— Burial of pipeline by trenching and backfill, jetting, rock placement or dumping.


6.1.2 Risk management

6.1.2.1 Operational risk should be evaluated and handled in a systematic way. See DNV-OS-H101 Sec.2 C.

6.1.2.2 Risk identification techniques and methods shall be used as applicable for the intended operation, seeDNV-OS-H101 Sec.2 C.

6.1.2.3 The extent of risk evaluations shall depend on the criticality of operations and experience fromprevious similar operations. For laying operations risk analyses should normally include a failure mode effectanalysis (FMEA) for equipment and hazard and operability studies (HAZOP) for critical operations.

Guidance note:

Typical items to be covered in HAZOP are:

— simultaneous operations

— lifting operations including pipe joints transportation and storage

— dry and wet buckles including flooding of pipe

— initiation and lay down including shore pull

— operations inside safety zones

— critical operations (laying in short radii curves, areas with steep slopes etc.)

— crossings

— failure of equipment and measuring and monitoring devices

— tie-in operation

— pre-commissioning activities

— environmental conditions and weather criteria

— emergency abandonment

— loss of station keeping capabilities

— survey.





6.2 Operational planning

6.2.1 General

6.2.1.1 General requirements for the planning of subsea operations are given in Sec.2. The followingparagraphs should be considered supplementary for laying operations.

6.2.2 Operation period

6.2.2.1 The required operation reference period, TR, see [2.1.2] should be thoroughly evaluated at an earlystage.

Guidance note:

— Marine operations may either be classified as weather restricted or as unrestricted, DNV-OS-H101 Sec.2 A200.See also [6.2.3].

— In case operation limits are stricter for ceasing the operation than for normal laying this should be evaluated indetail and accounted for in the operation planning.


6.2.3 Continuous operations

6.2.3.1 Laying operations with a planned duration exceeding the limitation for weather restricted operations,see DNV-OS-H101 Sec.4 B500, may still be defined as such subject to the following conditions:

— Continuous surveillance of actual and forecast weather conditions is implemented.— The operation can be halted and the handled object brought into Safe Condition within the maximum

allowable period for a weather restricted operation.

Guidance note:

Safe Condition is defined in DNV-OS-H101 Sec.2 A102, GN.



6.2.4.1 General requirements for contingency planning are given in [2.10.6].

6.2.4.2 Detailed contingency procedures for each critical operational step should be established.

Guidance note:

For laying operations typical contingency procedures can include failure of dynamic positioning system, failure ofanchors or anchor lines, coating repair, anode repair, failure of tensioning system, ROV breakdown, breakdown/failure of position reference systems/ navigation reference system, weather conditions in excess of operating limitconditions, third party marine activity and critical or emergency situations identified in FMEA or HAZOP studies.


6.2.4.3 The schedule shall include contingency time for possible repair/s (e.g. of damaged sheathing/coating).

6.2.5 Operation manual

6.2.5.1 General requirements for the content of the operation/installation manual are given in [2.2.3].

6.2.5.2 For laying operations the installation manual content described in DNV-OS-F101 Sec.10 L should beconsidered.

Guidance note:

The definition of installation manual in DNV-OS-F101 Sec.10 L is different from the definition applied in the VMO-Standard. The definition in DNV-OS-F101 is more comparable with what the VMO-Standard requires fordocumentation at site. See DNV-OS-H101 Sec.4 G.


6.3 Installation spread, aids and ancillary equipment

6.3.1 Installation spread

6.3.1.1 The installation spread for vessels performing installation of products as described in [6.1.1.1] shallcomply with the requirements in DNV-OS-F101 Sec.10 D.




Guidance note 1:

DNV-OS-F101 Sec.10 D gives requirements for:

— vessels

— position reference systems/ navigation reference systems

— anchor systems, anchor patterns, anchor handling

— dynamic positioning

— cranes and lifting equipment

— lay vessel arrangement, laying equipment, instrumentation

— mobilization activities

— qualification of vessel and equipment

— calibration and testing.


Guidance note 2:

For further guidance regarding installation spread for cables see DNV-RP-J301 [6.2].


6.3.1.2 The tensioner system shall be qualified for the actual product dimension to be installed. Variations inouter diameter due to both production tolerances and dimensional transitions shall be within the working rangeof the tensioner system.

6.3.2 Calibration and testing

6.3.2.1 Testing and calibration of laying equipment shall be done according to DNV-OS-F101 Sec.10 D.

6.3.2.2 Applicable integration testing should be carried out, see [2.10.4].

6.3.3 Installation aids and ancillary equipment

6.3.3.1 The installation vessel shall carry a sufficient amount of spares for the lay operation, especially forcritical equipment/tools used over and close to vessel side.

6.3.3.2 Temporary product hang-off shall be well planned, using dedicated equipment only. The hang-offsystem should normally consist of well supported hang off collar/clamp.

Guidance note:

Product hang off using soft slings, Chinese fingers or similar choked around the product can be accepted if the methodis properly qualified, see [6.3.3.3], prior to operation.


6.3.3.3 Clamps and other attachment devices shall be qualified. Qualification should include testing carriedout on a configuration representative of the actual product and load conditions/directions.

Guidance note:

When clamping on coating materials with undocumented creep properties/shear properties, an endurance test shall beincluded; the holding time should reflect the operational requirements. See [2.5.5] for requirements to lifting tools.


6.3.3.4 All work in the proximity of the clamping area that could influence material properties or the load, suchas heat, chemicals, grease, vibration, aligning pipe/product with side force etc., shall be specially considered.

6.3.3.5 Clump weights used during initiation or pull down, shall be of suitable weight and design. Sharp edgesthat could cause excessive point loads on or abrasion of the product and/or installation rigging shall be avoided.

6.3.3.6 Clamps/swivel joints shall be qualified for the product, configuration and installation method to ensurecorrect functioning during all phases. Special attention should be given to self-aligning swivel joints which canbe required to accommodate very different angles under installation and in-place conditions.

6.3.3.7 Emergency hang off facilities shall be readily available at all times. Moving, mounting, operation,holding capacity and power supply shall be tested and/or documented prior to commencement of the operation.

6.3.3.8 All clamps, protection frames, anchor flanges etc., shall be installed in accordance with thespecification and drawings, using appropriate bolt torque and to the specified tolerances.

6.3.4 Abandonment and recovery system

6.3.4.1 The abandonment and recovery system (A&R) should be able to abandon the product safely if water-filled. If the recovery system is incapable of recovering the product, alternative methods should be available.




Guidance note:

For Pipe in Pipe systems it is normally acceptable to assume only that the inner pipe or the annulus is flooded, not both.


6.3.4.2 Rigging components used to attach the A&R winch wire to the product lay-down tool shall be designedin accordance with DNV-OS-H205 Sec.4. See also [2.7.3.1].

6.3.5 In-line and termination structures

6.3.5.1 This sub-section applies to product in-line structures such as tees, wyes, junction boxes etc. and toproduct end terminations/end modules.

6.3.5.2 In-line structures shall incorporate means of preventing overturning caused by pipeline rotation;structures with their COG above the pipeline centre shall be specially considered.

6.3.5.3 Lifting equipment used to handle in-line and end structures/end terminations shall be designed inaccordance with DNV-OS-H205 Sec.4. The operation shall be planned to ensure that handling of the structures/terminations does not introduce unacceptable levels of tension, twist or bending into the pipeline/product at thetermination.

6.3.5.4 Design of lifting equipment and lift points on in-line and end structures should consider all possibleload directions and load distribution between the different parts of the rigging.

6.3.6 (Platform) Pull-in winch systems

6.3.6.1 The required capacity of the pull-in winch, winch wire and other parts of the pull-in system should bebased on maximum expected dynamic load during operation and relevant safety factors.

6.3.6.2 The safety factor philosophy shall be based on a consequence analysis. If pull-in wire failure is likelyto result in loss of the pull-in object, a safety factor comparable with the lifting equipment should normally beused.

6.3.6.3 Winches (including sheaves, blocks, foundations, etc.) used for product pull-in towards platform/otherinstallations should be commissioned according to approved procedures. See DNV-OS-H101 Sec.4 F.

Guidance note:

Winch commissioning should normally include dynamic load testing with 110% of the maximum expected loadduring operation. Testing should be carried out using the number of wire rope layer(s) that will be present when themaximum expected load occurs.


6.3.6.4 Rigging components used to attach the winch wire to the product/pull-in head shall be designed inaccordance with DNV-OS-H205 Sec.4. See also [2.7.3.1].

6.4 Loads and design

6.4.1 Loads

6.4.1.1 General requirements for loads are given in Sec.3. All loads relevant for the objects covered in thissection shall be taken into account.

6.4.1.2 A general definition of load categories and description of loads are given in DNV-OS-H102 Sec.3.

Guidance note:

It should be noted that DNV-OS-F101 and DNV-OS-F201 categorize loads somewhat differently to DNV-OS-H102as indicated below:

— Both G and Q loads are called functional loads in DNV-OS-F101 Sec.4 B.

— E loads have the same definition, i.e. environmental loads - see DNV-OS-F101 Sec.4 C.

DNV-OS-F101 Sec.4 E defines interference loads. These could be considered as accidental loads with probabilitygreater than 1/10 000 per operation, see DNV-OS-H102 Sec.5 D204.


6.4.1.3 This section considers loads related to the installation (construction) phases of the product. Loadsrelated to the operating phases of the product shall be treated in accordance with recognized design codes.




Guidance note:

Construction loads are described in DNV-OS-F101 Sec.4 D and should be categorised as P, Q or E loads.


6.4.1.4 The most unfavourable load scenario for all relevant installation phases and conditions shall beconsidered.

6.4.1.5 All loads and forced displacements which can influence the product shall be taken into account. Foreach cross section or part of the system to be considered and for each possible mode of failure to be analysed,all relevant combinations of loads which can act simultaneously shall be considered.

6.4.1.6 When considering the environmental design load the most unfavourable relevant combination, positionand direction of simultaneously acting environmental loads shall be used when documenting the integrity ofthe system, see also DNV-OS-H102 Sec.4 C.

6.4.1.7 Any possible accidental loads shall be considered.

Guidance note:

Accidental loads are described in DNV-OS-F101 Sec.4 F and interference loads in Sec.4 E.


6.4.2 Load effects

6.4.2.1 A load effect is the resulting cross sectional load due to the applied loads (e.g. weight, pressure, drag).

6.4.2.2 All loads and load effects occurring during the marine operation that could influence the operationalprocedure, the design or the dimensioning of structures shall be analysed and considered in planning andpreparation for marine operations according to the principles in DNV-OS-H102 Sec.4 A. See also DNV-OS-F101 Sec.4 G for further guidance.

6.4.2.3 The characteristic load effects from the different load categories are combined with the inclusion ofload effect factors to constitute the design load effect. See also [6.4.3].

6.4.2.4 Requirements for vessel motion analysis are given in DNV-OS-H102 Sec.4 B.

6.4.3 Limit states

6.4.3.1 All relevant limit states shall be considered during design for all relevant phases and conditions. SeeDNV-OS-H102 Sec.5 A200 for limit state definition and for limit states to be considered in design.

6.4.3.2 The ULS condition b) in Table 4-4 in DNV-OS-F101 and the ULS condition in Table 5-2 in DNV-OS-F201 should be considered whenever relevant in addition to the load combinations defined in DNV-OS-H102.

Guidance note:

The above requirement may be fulfilled by applying the load combinations in DNV-OS-H102, with a load factor of1.1 on G and Q loads in ULS combination b).


6.4.4 Failure modes

6.4.4.1 All relevant failure modes (for the installation/construction phase) shall be investigated. A failuremode is relevant if it is considered possible and the anticipated consequence(s) of the failure cannot bedisregarded.

6.4.4.2 The relevant failure modes may be grouped according to their nature, either as global (total system) orlocal (individual member) modes of failure, see DNV-OS-H102 Sec.2 C200.

Guidance note 1:

Reference to relevant standards describing potential failure modes for various types of products are indicated below.Use of alternative recognized standards may be accepted;

— Submarine pipelines: DNV-OS-F101

— Dynamic risers: DNV-OS-F201

— Flexible pipe systems: ISO 13628-2 (API 17J) or ISO 13628-11 (API 17B)

— Umbilicals: ISO 13628-5 (API 17E)

— Subsea power cables: DNV-RP-J301





Guidance note 2:

Flexible pipe systems can include flowlines, risers and/or jumpers. I.e. flexible pipes may be used for both static anddynamic applications. A flexible dynamic pipe will normally be categorized as a dynamic riser.


6.4.4.3 The allowable MBR (minimum bending radius) shall be defined for all relevant loads.

Guidance note:

The definition should include at least the following information:

— Tables/curves showing allowable bending (curvature) as a function of tension and pressure if applicable.

— All possible failure modes.

— Safety factor(s) included in the indicated MBR.


6.5 Installation - General

6.5.1 General

6.5.1.1 General operational requirements are given in [2.10]. The requirements of this subsection are generallyapplicable to installation of all products described in [6.1.1.1], regardless of installation method. Additionalrequirements pertaining to specific installation methods/products are given in the Subsection [6.6].

6.5.2 Initiation

6.5.2.1 The initiation point shall be of adequate design and have sufficient structural capacity to resist theinitiation load in any possible direction.

Guidance note:

The characteristic initiation load should be calculated as the maximum characteristic functional (P & Q) loadmultiplied with a factor not less than 1.5 to include dynamic (environmental) effects.


6.5.2.2 Purpose made initiation points, such as piles or dead man anchors, shall be load tested to 1.5 timesmaximum characteristic bottom tension (functional load) during start-up prior to commencement of layoperation. The direction of pull during test shall reflect the actual conditions during initiation. Holding timeshould be at least 15 minutes.

6.5.2.3 If a diverless latch/sheave type of initiation system is used, ROV monitoring of the sheave is imperativethroughout the whole initiation phase. Further, crossings between running wires and other wires, pipelines,mooring equipment etc. shall be continuously monitored.

6.5.2.4 The initiation head structure should have means of preventing overturning caused by product rotation.Structures with COG above pipeline centre shall be specially considered.

6.5.3 Laying

6.5.3.1 Crossings shall be prepared according to the findings from a risk assessment.

6.5.3.2 Requirements for anchor positioned lay vessel operations are given in DNV-OS-F101 Sec.10 D400.

6.5.3.3 If anchors are used for positioning of laying vessel, the anchor handling tugs shall keep the anchorsecured on deck when manoeuvring above pipelines and other subsea equipment.

6.5.3.4 Weather limitations for anchor handling tugs should be defined and taken into consideration whenplanning lay operations.

6.5.3.5 Vessel lay operations using dynamically positioned vessels shall comply with the requirements inDNV-OS-H203.

6.5.4 Lay monitoring

6.5.4.1 The lay configuration and loads shall be controlled in order to ensure that these are within establisheddesign parameters during installation. The configuration and loads may be controlled by various means andshall be clearly described including allowable ranges for the specific installation. Redundancy is required.




Guidance note:

The lay configuration may be controlled by tension, stinger tip clearance and lay back distance/touch downmonitoring. Depending on the installation vessel and type of product, the preferred method can alter.


6.5.4.2 The product touch down point shall be monitored as well as other interface points that are critical tothe integrity of the product or represent a risk for fixed installations or other subsea installations.

6.5.4.3 In order to enable continuous lay operations, adequate contingency measures for touch downmonitoring shall be established in case of primary monitoring system failure.

6.5.4.4 The ROV monitoring laying operations shall have sufficient working radii to observe critical areassuch as touchdown point, turning points etc. during laying. See also [2.9].

6.5.5 Lay-down

6.5.5.1 The laydown head structure should have means of preventing overturning caused by product rotation,structures with their COG above the pipeline centre shall be specially considered.

Guidance note:

In cases where orientation of lay down head structure is not relevant for the preceding operations, this requirementcould be disregarded. The requirement in [6.5.5.3] will apply in this case.


6.5.5.2 Laydown tools shall be designed to resist the maximum laydown forces from all radial directions incase of pipe rotation or include an effective swivel system.

6.5.5.3 Laydown tools should have a release mechanism that can be operated even if landed upside down oron their side.

6.5.6 Shore pull

6.5.6.1 The requirements of this subsection are applicable to the execution, inspection and testing of shore pullwhen products as described in [6.1.1.1] are pulled either from a vessel onto the shore, or vice versa.

6.5.6.2 Detailed requirements for the execution, inspection and testing of shore pull shall be specified,considering the nature of the particular installation site.

6.5.6.3 Measuring devices shall be used to control the integrity of the product during execution of the shorepull. The product tension and pulling force shall be continuously monitored, and shall be kept within allowablelimits. Monitoring with ROVs can be needed.

6.5.6.4 The winches shall be equipped with wire tension and length indicators and recorders. All measuringequipment shall be calibrated, and an adequate amount of spares shall be provided to ensure uninterruptedoperation.

6.5.6.5 It shall be documented that ROVs are able to operate under the seastate expected for the operation inquestion. Factors such as water depth, visibility and effect of breaking waves should also be considered.

6.5.6.6 Satisfactory abrasion resistance of the product coating shall be demonstrated for the installationconditions.

6.5.6.7 Buoyancy aids or pre-installed seabed rollers may be used to maintain pulling tension within allowablelimits.

6.5.6.8 Buoyancy aids shall be designed providing sufficient redundancy. The number and capacity ofbuoyancy elements shall be sufficient to ensure that the product will stay afloat in case of damage to or loss ofa realistic number of elements.

6.5.6.9 If buoyancy elements are used, handling of buoyancy elements and control of product catenary arenormally controlled by light workboats. The limiting environmental conditions for these operations shall bespecified so that work carried out from small workboats is feasible and safe.

6.5.6.10 Visibility and adequate lightening shall enable full overview of the floating product.

6.5.6.11 Workboat crews should be especially instructed/trained in order to control and handle the productwithout the risk of imposing damages

6.5.6.12 A shore pull / shore initiation operation will in most cases be a shallow water operation, which canlimit the ability of the installation vessel to position freely, and can also limit the use of thrusters due to draft




limitations. The effect on the DP capacity and the system redundancy shall be carefully considered, to maintainthe required positioning capabilities.

6.5.6.13 Requirements and considerations related to design and preparation of landfall/onshore part of theproduct system may be found in DNV-OS-F101, Submarine Pipeline Systems and in DNV-RP-J301 for CableSystems.

6.6 Product specific installation requirements

6.6.1 Pipeline system installation

6.6.1.1 The installation of submarine pipeline systems shall comply with the requirements in DNV-OS-F101Sec.10 F.

6.6.2 Riser, umbilical and cable installation

6.6.2.1 Termination head transfer from storage position to tensioner system shall be done in a controlledmanner, special attention should be given to minimum bending radius (MBR) of the product and the axialtension maintained during transfer. Dynamic behaviour (swinging) of heavy crane hooks etc. during handlingof product shall be specially considered.

6.6.2.2 Product handling should be considered step by step. Governing combinations of load directions andmagnitudes shall be identified and included in calculations.

6.6.2.3 Product handling on a vessel will imply that several tension systems are in operation at the same time(winches, cranes, tensioner, friction forces etc.); this fact places special demands on the rigging design bothwith regard to minimum breaking strength and load direction and distribution between the different parts of therigging.

6.6.2.4 The clearance to installation vessel (moonpool or vessel side) shall be monitored and maintainedduring all phases of the installation. In case of small clearances between product and vessel, contact points shallbe provided with suitably radiused protection.

6.6.2.5 Laydown of product shall be engineered ensuring that the product can be abandoned withoutcompromising any parameters specified by the manufacturer. During project preparations, the productabandonment and recovery arrangement shall be designed, including, for example, grappling lines forrecovery, marker buoys, etc.

6.6.2.6 Prior to planned abandonment of product, the ends shall be sufficiently protected from water ingressso that the cable can be safely abandoned on and retrieved from the seabed.

6.6.2.7 Further recommendations to installation of subsea power cables may be found in DNV-RP-J301. Therecommendations in DNV-RP-J301 can also be applicable to other types of subsea cables such as fibre opticcables.

6.6.3 J-tube pull-in of flexible risers, flexibles pipelines, umbilicals and cables

6.6.3.1 Prior to pull-in of products into J-tubes, the position of the J-tube, clamps, supports and product shallbe confirmed and evaluated with respect to assumptions made in the design. Potential damage shall beidentified and relevant measures taken if necessary.

6.6.3.2 To prevent the pulling head and product from jamming and to ensure that the J-tube is clear of debrisand obstructions, the diameter, roundness and cleanliness of the J-tubes shall be inspected by gauging pigs,pulling a test pipe or similar.

6.6.3.3 The entry of the product into the bellmouth shall be continuously monitored, and the tension in the pull-in cable shall be within specified limits.

6.6.3.4 Upon completion of the J-tube pull-in, a survey shall be performed to confirm the position of theproduct including supports etc. Potential damage shall be identified and appropriate measures taken if required.

6.6.3.5 Any possible skew loads on pull-in heads associated with entry into the bellmouth, through bends etc.shall be considered in the design.

6.6.3.6 Friction effects shall be calculated according to the principles in DNV-OS-H102 Sec.4 A600.

6.6.3.7 If lubricant is planned to be used during pull-in to reduce friction, the compatibility of the lubricantwith the surfaces in question both with respect to chemical reactions and friction reduction must bedocumented. Testing may be carried out in order to establish applicable friction coefficients following theprinciples in DNV-OS-H102 Sec.2 E400.




6.6.4 Bundle and pipe string installation

6.6.4.1 In general, the forces and configuration for off bottom tow (slow speed tow of bundle close to seabed)shall be accurately established in order to keep the full length of the bundle including structures clear of theseabed. It is of great importance that load variations can be readily detected as this could indicate uncontrolledcontact with seabed/obstacles.

6.6.4.2 Bundle deflection due to side current shall be analysed for the characteristic current conditions inapproach and installation areas as well as the necessary pull force/catenary length to keep the bundle within thecorridor. When a bundle is moved in “off bottom tow mode” lateral friction between the ballast chain and theseabed cannot be utilised for counteracting the lateral deflection.

6.6.4.3 Fatigue analysis shall be carried out to verify that fatigue ‘damage’ as a result of transport loads isacceptable in relation to the total operating fatigue life of the bundle.

6.6.4.4 If a pull-in system is being using for the final bundle positioning, the necessary pull in force should becalculated. Friction between seabed and bundle/ballast chain, general seabed topography, back tension, etc.should be considered.

6.6.4.5 Ballast calculations shall be presented for the complete water filling/ballast operation. If the bundle ispressurised prior to installation it shall be demonstrated that the bundle is stable during the bleed off to ambientpressure and that weight loss due to escaping gas does not cause the bundle to ascend.

6.6.4.6 Crossings should be kept to a minimum; post flooding contact forces shall be determined for each case.Potential chafing effects for pull in wires shall be evaluated.

6.6.5 Tie-in of pipe strings and bundles

6.6.5.1 It is assumed that integration tests, if applicable, have verified the operability of the various tools andequipment. Test reports should highlight and reflect critical operational sequences and their limiting factors.

6.6.5.2 In general, procedures for the operational sequences listed below should be established, includingcontingency plans, and limiting weather criteria:

— pull in tool installation/retrieval

— connection tool installation/retrieval

— pull head disconnection/retrieval

— connection of pull-in wire to pull-in head

— guide wire installation

— flooding of bundle

— chain and buoyancy tank removal.

6.6.5.3 Suitable arrangements shall be provided for release of towing wire from pull head. Residual tensionand/or torsion should be considered.

6.7 Tie-in operations

6.7.1 Application

6.7.1.1 The requirements of this sub-section are applicable to subsea tie-in operations using mechanicalconnectors. Tie-in operations above water and tie-in operations using welding are covered in DNV-OS-F101.

6.7.1.2 Tie-in operations by means of hot or cold taps are subject to special consideration and agreement.

6.7.2 General

6.7.2.1 The alignment and position of the tie-in ends shall be within the specified tolerances before completingthe tie-in.

6.7.2.2 During all handling, lifting and lowering into final position, open flange faces shall be protected againstmechanical damage.

6.7.2.3 Tie-in tool capabilities shall be sufficient to overcome forces from friction, soil build up, holdback andmisalignment.

6.7.2.4 Tie in tool capabilities shall be based on documented values achievable on site.



Sec.7 Soil and foundations –

SECTION 7 SOIL AND FOUNDATIONS

7.1 Soil capacity and on bottom stability

7.1.1 General

7.1.1.1 Installation shall be planned to ensure that the object can be properly seated at the intended site withoutexcessive disturbance of the supporting soil.

7.1.1.2 It should be documented that during all phases of the installation operation the object remains stableon the sea bed, without experiencing unacceptable displacements due to soil failure.

7.1.1.3 A general reference is made to DNV CN30.4 for selection of soil properties and determination offoundation capacities.

7.1.2 Stability calculations

7.1.2.1 Whether the permanent foundation solution is based on mat foundation or piled foundation, there willoften be a temporary phase during installation where the object will be supported on mats, possibly equippedwith skirts.

7.1.2.2 Stability should be checked for load combinations including gravity loads, environmental loads wheresignificant, and any other loads applied to the structure during installation, e.g. during stabbing of piles.

7.1.2.3 For reasonably homogeneous soil conditions, stability may be checked using conventional bearingcapacity formulae, accounting for inclined loading combined with checks for pure sliding. Recommendationsfor idealised soil conditions are given in DNV CN30.4 [4.4].

7.1.3 Material factors

7.1.3.1 For foundation failures which can have unacceptable consequences, such as structural damage orirrecoverable, unacceptable displacements, material factors should be applied according to relevant designstandards, e.g. DNV CN30.4.

7.1.3.2 The adequacy of the normal load and material factors should be evaluated in relation to the proposedinstallation procedure and governing boundary conditions.

Guidance note:

Provided that the installation is performed under controlled conditions by use of heave compensator, which effectivelyprevents excessive impact velocity, standard safety factors can be used. Otherwise, adjustments should be agreed ona case by case basis.


7.2 Loads and installation aspects

7.2.1 Positioning loads

7.2.1.1 Forces due to horizontal and vertical impact velocity between the object and sea bed or bottom supportstructure should always be evaluated from case to case and a maximum effective impact velocity should bespecified, which accounts for the boundary conditions in [7.2.2].

Guidance note:

In cases where the consequences of high impact velocity are documented to be minimal and insignificant the verticalimpact velocity can be determined based on other governing boundary conditions. The maximum vertical impactvelocity need not be taken greater than the free fall velocity of the object in calm water.


7.2.1.2 Characteristic ULS positioning loads in vertical and horizontal direction should normally not be takenless than 3% of the installed object's submerged weight including added mass.

7.2.2 Installation effects on the soil

7.2.2.1 To avoid risk of foundation failure during installation an impact analysis should be carried out.

Guidance note:

Subsea structures are often placed on the sea floor from an installation vessel. Vertical heave motions in combinationwith the lowering velocity result in an impact between the sea floor and the structure. This impact is normallycontrolled by limiting weather criteria and carefully planned, well controlled installation operations. Nonetheless,




subsea structures can and still do on occasion suffer foundation failure during installation, mainly in soft soilconditions.


7.2.2.2 Impact analysis should consider and establish relationships between the following factors:

— submerged weight of object— tension in installation wire— flexibility of installation wire— heave motion— effective lowering velocity considering sensitivity of winch mechanism (slow running or star-up) — hydrodynamic (added) mass forces— water evacuation areas (for foundations with skirts)— governing soil properties.

Guidance note:

Impact analyses are described in DNV-RP-H103 Sec.6.


7.2.2.3 Based on the impact analysis, the allowable maximum set-down velocity can be determined. In orderto control that the actual set-down velocity is less or equal, it will in most cases be necessary to use a heavecompensator, see [2.4], which controls and limits the heave motion prior to touch-down. Without heavecompensation, failure due to global displacement and/or local soil disturbance can occur. The consequences ofsuch incidents should be investigated.

7.2.2.4 When deciding upon allowable heave, the possible magnification of heave motions due to resonanceshould be taken into account.

7.2.2.5 The possibility of snap loading in the lifting wire when the object is set on the seabed should beconsidered. This should in particular be focused upon moderate water depths with correspondingly stiff liftingwire and when the maximum heave velocities are higher than the crane lowering velocity. If the wire is not paidout sufficiently once the lifted object is set on the seabed, the force in the wire due to snap loading will in mostcircumstances be increased due to seabed suction.

7.2.2.6 Deceleration associated with impact induces additional dynamic forces on equipment and structuralelements which should be evaluated; this effect is most relevant when installing an object on hard ground.

7.2.3 Penetration and levelling of skirted foundations

7.2.3.1 The self-penetration of a skirted foundation should be estimated prior to installation operations.

7.2.3.2 When suction is required for obtaining full penetration of a skirted foundation the expected range ofsuction should be based on expected range of penetration resistance. A maximum allowable suction should bedefined based on the concerns to avoid buckling of skirts and to avoid piping and soil heave inside the skirtsdue to soil failure (reversed bearing capacity failure).

7.2.3.3 A structure supported by three or more skirted foundations may be levelled by application of suctionand/or overpressure. Procedures for levelling should be prepared in advance including limitations on theallowable suction and overpressure.

7.2.3.4 The entire installation including penetration and levelling shall be monitored and reported. In case theactual self-penetration or suction is outside the predicted range, a re-evaluation of the calculations made shouldbe performed. It can be necessary to re-evaluate the foundation design.

7.2.3.5 Recommendations for estimating and controlling penetration and levelling are given in DNV-RP-H103 Sec.6.

7.3 Miscellaneous

7.3.1 Effects of conductor installation and shallow well drilling

7.3.1.1 Conductor installation and shallow well drilling require attention from a subsea structure integritypoint of view.

7.3.1.2 Planning for conductor installation should take into account the potential for disturbance of existingfoundation soils and the risk of reducing stability of the structure (or of adjacent conductors) further.




Guidance note:

— Soil disturbance during drilling operations can result from hydraulic fracture, washout (uncontrolled enlargementof the drilled hole), or shallow gas pockets.

— Hydraulic fracture occurs where drilling fluid pressure is too high and fluid is lost into the formation.

— Washout generally occurs in granular soils and can, in part, be induced by high drilling fluid circulation rates ordrilling without mud. Washout can produce large voids in the soil structure and lead to stress relief in thesurrounding soils.

— When pumps are applied for disposal of drill mud the bore hole can collapse due to excessive suction.

— These incidents can be accompanied by loss of circulation of drilling fluids or in creation of sea floor craters.Thereby the stability of foundations can be reduced and displacements increased. These detrimental effects canoccur whether the drilling takes place after installation of the structure or before, e.g. through a pre-installedtemplate or for an exploration well.


7.3.1.3 In case of pre-drilled wells, records of conductor installation and shallow well drilling shall beavailable to the designer of the structure. The cuttings from the well drilling operation, if allowed to accumulateon the sea floor, should be taken into account both in the object installation and retrieval procedures.

7.3.2 Retrieval of object

7.3.2.1 If retrieval without overpressure (inside skirted foundations) or jetting is anticipated, the forcesanticipated during retrieval should be determined, to ensure that retrieval can be accomplished with availablemeans.

7.3.2.2 For re-positioning or retrieval of an object placed on the sea bed, forces due to suction should becalculated. This may be done by using bearing capacity formulae as given in Classification Note 30.4 section4.4. It should be noted that the pull-out resistance in cohesive (clay) soils will increase with increasing loadingrate (or pull-out velocity), which should be accounted for.

7.3.2.3 Retrieval forces are dependent on soil parameters, foundation geometry, lifting velocity, consolidationtime, contact pressure, etc.

7.3.2.4 Retrieval force calculations/analyses shall be carried out according to recognised methods, see DNV-RP-H103 Sec.6.

DNV-OS-H206: Loadout, transport and installation of subsea objects ...

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