POSITION MOORING (POSMOOR) - Rules and standards - … · 2014-09-18 · POSITION MOORING (POSMOOR)...

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PART 6 CHAPTER 2

RULES FOR CLASSIFICATION OF

MOBILE OFFSHORE UNITS

SPECIAL EQUIPMENT AND SYSTEMS ADDITIONAL CLASS

POSITION MOORING (POSMOOR) JANUARY 1996

SECTIONS PAGE

1 General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Environmental Conditions and Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 Mooring System Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4 Thruster Assisted Mooring System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5 Mooring Equipment ........................................................................................... 16 6 Tests .............................................................................................................. 19

APPENDICES

A Supplementary Requirements of the Norwegian Maritime Directorate (NMD) and the Norwegian Petroleum Directorate (NPD) ............................................................... 20

DET NORSKE VERITAS Veritasveien 1. N-1322 Hovik. Norway Tel.: +47 67 57 99 00 Fax: +47 67 57 99 11

CHANGES IN THE RULES

General

The present edition of the Rules includes additions and amendments decided by the Board as of March 1996, and supersedes the July 1989 edition of the same chapter.

The Rule changes come into force on 1st of July 1996.

This chapter is valid until superseded by a revised chapter. ~upplements will not be issued except for an updated list of minor amendments and corrections presented in the introduction booklet. The introduction booklet is normally revised in January and July each year.

Revised chapters will be forwarded to all subscribers to the Rules. Buyers of reprints are advised to check the updated list of Rule chapters printed in Pt.O Ch.I Sec.I to ensure that the chapter is current.

Main changes

• Sec.I General Requirements

- A new item A103 on reliability analyses has been added. - A new item A200 on definitions has been added. - A new item A400 on national regulations has been added. New

class notations POSMOOR (N) and POSMOOR V (N) have been introduced.

- New items B105-B107 on requirements for mooring chain, steel wire ropes and synthetic wire ropes have been added.

- Item B300 on anchor line record has been amended. - A new item B400 on additional documentation for long term

mooring has been added.

• Sec.2 Environmental Conditions and Loads

Items A101-A102 on environmental conditions have been amended. The guidance note on wind and gust spectra has been extended. Item B203 on high wave frequency calculations has been extended.

- Item B300 on low frequency wind and wave induced motions has been amended. A new guidance note on low frequency calculations has been added.

- Item B400 on model testing has been added.

• Sec.3 Mooring System Analysis

- Item A 102 has been amended. - Item A103 on load cases has been amended. - Item A104 on shallow water effects has been extended and a

guidance note on amplification factors has been added. - A new item A 105 on taut leg mooring system has been added. - Items A106-A107 have been amended. - A new item A108 on effective elastic modulus has been added,

including a guidance note on modulus calculation.

© Det Norske Veritas AS

- Item A203 on quasistatic analysis and loading combinations has been extended.

- A new item A206 on optimisation has been added. - Items A301-A302 have been amended. - The guidance note in A303 has been amended. - A new item A304 on line tension calculations has been added. - Item A400 on transient motion has been amended. - A new item A500 on relative direction of wind, waves and cur-

rent has been added. A new item A600 on long term mooring for production and storage units has been added, including fatigue calculations, marine growth and corrosion allowance.

- Items BlOO and B200 on operation conditions I and II have been updated.

- Item ClOl has been amended. - A new item C202 on safety factors for synthetic fibre ropes. - Items C302 and C303 have been amended. - Item C401 has been amended. - Item C501 on minimum distances has been amended. - Table Cl on safety factors has been amended. - A new item C503 has been added. - Figures 1-3 on anchor patterns have been amended.

• Sec.4 Thruster Assisted Mooring System

- Table A 1 has been amended. - A new item A105 on turret moored ship shaped units has been

added. - Item B701 has been amended.

A new item B905 regarding consequence analysis has been added.

• Sec.5 Mooring Equipment

- Items AlOl, A102 and A104 have been amended. - A new item A105 on suction anchors has been added. - A new item A203 on proof testing of anchors above 20 tonnes

has been added. - A new item A304 on proof load test.ing of anchors above 20

tonnes has been added. - New items A400 and A500 on holding power requirements have

been added. - Subsection B on anchor chain cables, steel wire ropes and syn

thetic fibres has been amended. - Subsection Con fairleads has been amended.

Subsection D on windlasses, winches and stoppers has been amended and extended.

• Sec.6 Tests

- Item B102 on test of simulated failures has been amended.

• Appendix A

A new appendix on supplementary requirements of the Norwegian Maritime Directorate and the Norwegian Petroleum Directorate not covered by the Rules.

Computer Typesetting by Division Technology and Products, Det Norske Veritas AS Printed in Norway by Dct Norske Veritas AS January 1996

1.96.2000

It is agreed that save as provided below Del Norske Veritas, its subsidiaries, bodies, officers, directors, employees and agents shall have no liability for any loss, damage or expense

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CONTENTS

SEC. 1 GENERAL REQUIREMENTS ..••.....••....•...... 1

A. Classification . . . . . •• . . . . • . •• . . . . •• . . . . . . . • . . . . .. . . . . . . .•. . . . . .. . . . . 1 A JOO Scope... I A 200 Definitions . . . . .. . . . . . . . . . . . . . . . . 1 A 300 Class notations . . . . . 2 A 400 National regulations 2 A 500 Basic assumptions . 2

B. Documentation . ..••.. .. ..••.. ..••.. .. ...•.. ...... .. ..... .. ...... .. 2 B 100 Plans, particulars and certificates . . . . . . . . . . . . . . . . . .. . . . 2 B 200 Additional documentation for TA and ATA notations 3 B 300 Anchor line record .. . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . 3 B 400 Additional documentation for long term mooring . . . 3

C. Structural Arrangement for Mooring Equipment .. ..... 3 C I 00 General .. . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 3

SEC. 2 ENVIRONMENTAL CONDITIONS AND LOADS ......................................•.....••....•• 4

A. Environmental Conditions ... ......... .. ...... ...... .... .. ..... 4 A 100 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

B. Environmental Loads . .. ....... ...... .. ...... ...... ..••.. .. ..... 4 B 100 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 B 200 High frequency wave induced motions . . . . . . . . . . . . 5 B 300 Low frequency wind and wave induced motions . 5 B 400 Model testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

SEC. 3 MOORING SYSTEM ANALYSIS ............••... 6

A. Method ............................................................. 6 A 100 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 A 200 Quasistatic analysis . . . . . . . . . . . . . . . . . . . . . . 7 A 300 Dynamic analysis . . . . . . . . . . . . 8 A 400 Transient motion . . . . . . . . . . . . . . . . . . . . . . . . 8 A 500 Direction of wind, waves and current relative to the

unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 A 600 Long term mooring for production and storage units 8

B. Operation Conditions ........................................... 9 B 100 Operation condition I . . .. . . . . .. . . . . . . . . . . . . 9 B 200 Operation condition II ................. , . . . 10 B 300 Redundancy for operation condition II . 10

C. Safety Factors and Premises ....... ...... ..••.. ..•... ..•... .. 10 C 100 General .. .. .. .. .. .... .. .. .. .. .. .. 10 C 200 Safety factors . . . . . 10 C 300 Permissible horizontal offset 10 C 400 Permissible line length . .. .. 10 C 500 Anchor patterns . . . . . . . . . . . . . . 11

SEC. 4 THRUSTER ASSISTED MOORING SYSTEM 13

A. Classification •. • . . . . .• . . . . . . . • . . . . •• . . . . . . •. . . . . .. . . . . ... . . . . . . . . . 13 A I 00 General . . . . . . . . . . . . . . . . . 13 A 200 Definitions . . . . . .. . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . 13

B. System Requirements •.. .. ..•..•.. .......... .. .... .. ..•.... .. .. 14 B 100 Thruster systems . . . . . . . . .. . . . . . . 14 B 200 Power systems . . . .. . . . . . . .. . . . . . 14 B 300 Control systems .. .. ... .. .. . .. .. .. . ... .. .. .... .. .. .. .. .. . 14 B 400 Manual thruster control . . . . . . . . . .. . . . . . . .. . . . . . . 14 B 500 Remote thrust control, joystick system 14 B 600 Automatic thruster control .. .. .. .. .. .. .. .. 14 B 700 Automatic control .. ... .. ... . .. .. .. .. .. .. .. 14 B 800 Monitoring . . . . . . . . . . . . . . . . . . . . 15 B 900 Consequence analysis . . 15 B 1000 Simulation . ... .. . .. .. .. .. .. .. .. 15 B 1100 Logging . . . .. .. .. . . . . .. .. .. . . ... .. . . .. .. .. 15 B 1200 Self-monitoring . ... .. 15

SEC. 5 MOORING EQUIPMENT ........................... 16

A. Anchors ...... .. ........ ........ .. ...... .. ....... .. ..••.. .. .. .... .. 16 A 100 General . .. .. .. . . . . .. .. ... . .. .. .. . . .. .. .. . .. ... .. . ... .. .. . 16 A 200 Proof testing of anchor strength of embedment type 16 A 300 Type approval of embedment anchors .. .. 16 A 400 Holding power requirements . .. . .. .. ... .. .. .. .. .. 16 A 500 Holding power requirements - long term mooring 16

B. Anchor Lines ..................................................... 17 B 100 General .................................................... 17

C. Fair leads . .. .. ........ ........ .. ...... .. ...... .. ... .... .. .. .•.. .. .. 17 C 100 General ............................... . ....... 17

D. Windlasses, Winches and Stoppers ... ... ..•... .. .. .... .. ... 17 D 100 General ... ................. . .......... 17 D 200 Materials and certification . . . . . . . . . . . . . . . . . 17 D 300 Capacity and system requirements . . . . . . . . . . . . . . . . . . . 17 D 400 Stoppers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 D 500 Strength and design load . ... .. .. . . .. .. .. .. .. .. .. .... .. . 18

E. Tension Measuring Equipment .............................. 18 E 100 General .................... . 18

SEC. 6 TESTS .. .. ... ..••.. .. ..•..•.. ..•.••.. .. ..•••. .. ....... .. .. 19

A. Test of Windlass/Winch . ..•••.. .. ..••.•.. .. ..••.. ..•••. .. ..•.. 19 A 100 Tests before assembly. ........... 19 A 200 Functional test . . . . .. . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . 19

B. Test of TA and ATA Systems ............................... 19 B 100 General . .. . . . . .. .. .. . . .. .. .. . .. . .. .. . . . . .. .. ... . . .. .. .... .. 19

APP. A SUPPLEMENTARY REQUIREMENTS OF THE NORWEGIAN MARITIME DIRECTORATE (NMD) AND THE NORWEGIAN PETROLEUM DIRECTORATE (NPD) .............................. 20

A. General . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . 20 A 100 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

B. Main Part .. .. ...... .. ........ .. ...•.. .. ..•... .. ... .... .. .. .... .. .. 20 B 100 Requirements . . . . . . . . .. . . . . . . 20

Rules for Mobile Offshore Units , January 1996 Pt.6 Ch.2 Sec.1 ~ Page 1

SECTION 1 GENERAL REQUIREMENTS

Contents

A. Classification A 100 Scope A 200 Definitions A 300 Class notations A 400 National regulations A 500 Basic assumptions

B. Documentation B 100 Plans, particulars and certificates B 200 Additional documentation for TA and AT A notations B 300 Anchor line record 8 400 Additional documentation for long term mooring

C. Structural Arrangement for Mooring Equipment C 100 General

A. Classification

A 100 Scope

101 The requirements in this chapter apply to single point or spread point mooring systems which may be installed on units. The term •unit> in this chapter is defined as ship-shaped vessels, column stabilized units, offshore loading buoys, or other floating bodies relying on a catenary mooring system for station keeping.

102 These rules are made to cover reliability of the mooring system and equipment on units for the purpose of safe positioning mooring.

103 Design based on reliability analyses is subject to acceptance by the Society in each separate case. A mooring system design based on reliability analyses has to document that the safety level for the system is at least as high as obtained by using the safety factor approach.

A 200 Definitions

201

Co

Co

Dnom

k

h

H, m

N = t/Tz

Pb

Tmax

= stud chain: 2,6 studless chain: 2,4 with respect to chain diameter

= steel wire rope: 1,8

= nominal chain or wire diameter

= mooring system stiffness

= water depth

= significant wave height

= unit mass including added mass

= t is the specified storm duration in seconds

minimum breaking strength of the anchor line adjusted for corrosion allowance

maximum tension in the most heavily loaded line

= resonance period for low frequency motion

= zero up-crossing wave period

= peak period

U(lom, lhour) = 1 hour average wind speed at 10 m above sea water level

Vcwind

Xmean

xmax LF

xsign LF

xsign WF

xmax WF

liT grnwth

µ

Pseawater

"LF

"WF

202

= wind driven current velocity

= tidal driven current velocity

= quasi static offset at which the dynamic line tensions are calculated

mean offset due to static loads caused by wind, wave and current

maximum low frequency motion from wind and waves

= significant low frequency motion from wind and waves

quasi-static position of the unit at which the quasi-static line tensions are calculated

significant wave frequency motion

the probable largest wave frequency motion response

marine growth surface thickness

2,0 for chain, 1,0 for wire rope

density of sea water

standard deviation of low frequency motion

standard deviation of wavefrequency motion.

Co-linear Environment: Wind, waves and current are acting in the same direction

Equilibrium position: The position where the static environmental force is equal to the restoring force of the mooring system.

HSE: Health & Safety Executive

Long term mooring: Mooring at the same location for more than 5 years

Low frequency motion: Slow-drift horizontal motion of a moored structure caused by fluctuating wind forces due to gust and resonance oscillations excited by non-linear interaction effects between waves and the body motion. Slow-<lrift excitation loads is large when the mean wave load is large. This means slowdrift motions are most important for large volume structures.

Marine growth: caused by soft (bacteria, algae, sponges, sea quirts and hydroids) and hard fouling (goose barnacles, mussels, barnacles, tube worms)

Net thrust capacity Thrust capacity after all type of loss in thrust capacity is considered.

NMD: Norwegian Maritime Directorate

Non-co-linear environment: Wind, waves and current are acting in different direction

U(!Om, 10 min) = 10 minutes average wind speed at 10 m NPD above sea water level Norwegian Petroleum Directorate

DET NORSKE VERITAS

Rules for Mobile Offshore Units , January 1996 Page 2 - Pt.6 Ch.2 Sec.1

Operation condition /: Position mooring where a single failure of positioning system will not lead to a critical situation for the overall safety of the unit and those onboard. Typical; standby (The unit has terminated drilling and disconnected), oil production is terminated and pressure sources are isolated, accommodation unit with gangway lifted and new position more than 50m away from the fixed installation.

Operation condition II: Position mooring which will lead to a critical situation for the overall safety of the unit and those on board, if position limitations are exceeded. Typical; drilling, production of oil, accommodation unit with gangway connected and crane operations

Splash zone: Defined as the wave amplitude above and below mean water level. The wave is to have a return period of 100 year at the specified location

Taut leg mooring: A mooring system where all mooring lines including the leeward lines) are taut under all conditions. Such system is causing large vertical forces on the anchors.

Transient motion: The units motion form intact equilibrium position after a line failure to it's new equilibrium position

Wave frequency motion: Mainly linearly-excited motion in the wave frequency range of significant wave energy

A 300 Class notations

301 For Mobile Offshore Units (MOU) the requirements are to be regarded as supplementary to those given for the assignment of main class, see Pt.3 Ch.2 Sec.5.

302 For units classified as ships the requirements are regarded as supplementary to the assignment of main class, see Pt.3 Ch.3 Sec.3.

303 Units with equipment complying with the requirements of this chapter will be assigned the class notation POSMOOR or POSMOOR V.

304 The additional letter V refers to a mooring system which is designed for positioning of the unit in the vicinity of other structures.

Guidance note: For semisubmersibles with a conventional mooring systems, the class notation POSMOOR V applies when the distance between the unit and other floating or fixed structures is less than 300 m. The safety factors of the anchor lines are dependant of the collision hazard and consequences of failure, see Sec.3 C. For semisubmersibles with an unconventional anchoring systems, and for all type of moored ships, the limiting distance between the unit and other structures to avoid collision hazard will be subject to special consideration by the Society.

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305 If the unit's mooring system is designed for thruster assisted mooring, the system notation letters TA or AT A may be added to POSMOOR.

A 400 National regulations

401 Units, which in addition to the requirements in these rules, comply with the applicable requirements of the Norwegian Maritime Directorate (NMD) stated in Appendix A, will if requested by client, be assigned one of the following additional class notations:

POSMOOR (N)

POSMOOR V (NJ

The class notation letters TA and A TA may be added if the unit's mooring system is designed for thruster assistance.

The Society interprets the requirements contained in these rules including Appendix A as being concurrent with the NMD regulations for the areas covered by these rules. These rules including Appendix A are also concurrent with the Norwegian Petroleum Directorate (NPD) regulations concerning mooring system for floating production units.

402 The requirements in these rules are considered to be concurrent with requirements of Health & Safety Executive (HSE) laid down in the HSE Guidance for areas covered by these rules.

A 500 Basic assumptions

501 It is the intention of these mies that by specifying an environmental condition for upper and lower water depth, all intermediate water depths are covered

502 The classification is based on the condition that an up to date anchor line record (see B300) is kept available for presentation to the Society's surveyors.

B. Documentation

B 100 Plans, particulars and certificates

101 Plans, particulars and certificates of the mooring equipment to be submitted as basis for approval/ documentation are specified in Pt.3 Ch.2 Sec.5 A300.

102 In addition to 101 the following is to be submitted for approval, as applicable:

- windlass/winch and stopper design - anchor design including anchor weight and anchor size.

Material specification - anchor line type including total line length and dimen

sion. Material specification - buoyancy /weight elements.

103 In addition to 102 the following is to be submitted for information, as applicable:

- windlass/winch lifting capacity and static and dynamic braking capacity. Strength calculation of main components of windlass/winch, i.e. cable lifter/drum, couplings, shafts, brakes, gear and frame base

- strength calculation of anchor unless type approval has been given. Holding power calculations for anchors designed to take vertical forces

- environmental induced loads and motions of the unit in all design conditions

- restoring forces and maximum line tension in all design conditions

- horizontal in-plane transient motions of the unit after single failure of the mooring system, as defined in Sec.3 A400. New equilibrium positions to be given

- anchor pattern used in the mooring system analysis - waterdepth range at which the unit is intended to be op-

erated.

104 The items for which DNV-certification/inspection report is required are specified in Pt.3 Ch.2 Sec.5 Table A2. In addition to the requirements listed there, DNV-certification will be required for:

- synthetic fibre rope - synthetic fibre rope end attachments.

DET NORSKE VERITAS

105 Mooring chain (with or without stud) and accessories are to be made by manufacturers approved by the Society for the pertinent type of anchor chain, size and method of manufacture. Approval and production testing are to be carried out in accordance with DNV Certification Note No. 2.6 "Certification of Offshore Mooring Chain" and Pt.3 Ch.2 Sec.5 E Mooring Chain and Accessories.

106 Steel wire rope is to be made by a manufacturer approved by the Society. Approval and testing are to be carried out in accordance with DNV Certification Note No. 2.5 "Certification of Offshore Mooring Steel Wire Rope".

107 Synthetic fibre rope with attachments are to be approved on a case to case basis. The following properties are to be documented:

Basic rope information:

- rope fibre type - rope diameter - weight in air and water - reel/rope diameter ratio - rope length - sheathing type - end termination.

Rope properties:

- static breaking strength - fatigue strength - residual breaking strength - creep properties - axial stiffuess under static and dynamic load - heat build-up under dynamic loading - torque/twist behaviour - resistance to chemical attacks in the offshore environ-

ment.

B 200 Additional documentation for TA and AT A notations

201 Documentation for thruster systems and power system is to be submitted according to Main Class requirements.

202 In addition to documents required by 201 the following is to be submitted for approval:

- system schematics for remote thrust control system - systems schematics for automatic thrust control system - power distribution schematics for thrust control systems - test program for sea trials for thruster assistance.

203 In addition to documents required by 201 the following is to be submitted for information:

- net available thrust output of each thruster showing which effects have been considered to derive the net thrust relative to nominal thrust output

Rules for Mobile Offshore Units , January 1996 Pt.6 Ch.2 Sec.1 - Page 3

- layout of remote thrust control operator panels - operating manual for thruster assistance.

204 DNV certificate for automatic thruster control system is to be submitted.

205 If the thruster assistance is subject to redundancy requirements, see Sec.3 B300, the redundancy is to be documented by either

- a Failure Mode and Effect Analysis, covering all relevant sub-systems, or

- a test program covering failure situations and thereby demonstrating redundancy, which is to be carried out during thruster assistance sea trials.

B 300 Anchor line record

301 An anchor line record is to be kept onboard, giving the following particulars (and spaces for updating, see A300) for every mooring line:

- position of the unit - type of mooring lines - maker and time of production - position of joining shackles - type and position of weight/buoyancy elements - record and position of breakage - any combination of different types/age of chain/wire rope

/fibre rope in one line - record of periodical surveys.

B 400 Additional documentation for long term mooring

401 The following documentation is required:

- fatigue calculations of mooring lines and connection elements, see Sec. 3 A600

- marine growth applied in the design of the mooring system, see Sec. 3 A600

- corrosion allowance, see Sec.2 A600 - anchor design including calculation of anchor holding

power.

C. Structural Arrangement for Mooring Equipment

C 100 General

101 The structural arrangements is to comply with requirements given in Pt.3 Ch.2 Sec.5 B, as applicable.

102 During normal operation of the mooring system, any hull protection systems or other obstructions are not to interface the anchor lines.

DET NORSKE VERITAS

Rules for Mobile Offshore Units, January 1996 Page 4 - Pt.6 Ch.2 Sec.2

SECTION 2 ENVIRONMENTAL CONDITIONS AND LOADS

Contents

A. Environment.al Conditions A 100 General

B. Environmental Loads B 100 General B 200 High frequency wave induced motions B 300 Low frequency wind and wave induced motions B 400 Model testing

A. Environmental Conditions

A 100 General

101 The class notation POSMOOR and POSMOOR V will be related to the limiting environmental condition(s) as specified to the Society.

102 The environmental conditions are to include the fol-lowing:

U(I0m,1omin)= 10 minutes average wind speed at 10 m above sea water level

or, if low frequency motion has to be considered:

1 hour average wind speed at 10 m above sea water level plus a time varying component calculated from a wind gust spectrum

Table Al Extreme weather condition Location

h Field Position

(m)

Beryl 59°36'N 120 1 °30'E

Brent 61° 4'N 145 1°21 'E

Ekofisk 56°32'N 70 3°15'E

Frigg 59°54'N

105 2°8'E

Maureen 58°5'N 95 1 °45'E

Statfjord 61°13'N

145 1°55'E

Valhall 59°l6'N 70 3°13'E

Viking 53°30'N

30 2°20'E

Haltenbanken 65°7'N 250 8°E

Troms0flaket 71°19'N 240 20°E

Vcwind Vtc H,

= wind driven current velocity = tidal driven current velocity = significant wave height

Tz Tp h

= zero up-crossing wave period or = peak period = water depth.

The connection between Tz and Tp is given in Classification Note No. 30.5.

The environmental conditions as specified to the Society will be included in the Appendix to Classification Certificate.

Guidance note: For ship shaped units or any moored structure sensitive to slowly varying wave induced motions, the 1 hour average wind speed should be applied in connection with a wind gust spectrum for calculation of slowly varying wind induced motions.

One of the following wind gust spectra may be applied:

- Harris - Sletrin_gen - NPD 1l.

1) To be applied for production and storage units which are to comply with NPD requirements.

For extreme wind~ conditions at North Sea/North Atlantic locations the Sletringen wind gust spectrum is recommended.

For North Sea/North Atlantic the following values may be used for extreme conditions, see Table Al.

Waves Wind Current

Hs Tz - range U1om,lOmin v"' vwind

(m) (s) (mis) (mis) (mis)

16 11,0-14,5

16 11,0-14,5

14 10,0-13,5 41 0,5 0,8

16 11,0-14,5

15 10,5-14,0

16 11,0-14,5

14 10,0-13,5 0,5

10 8,5-12,5 41 0,8

17 11,0-15,0 0,8

17 11,0-15,0

For units where low frequency motion is an important part of B. Enviromnental Loads the total moti~n, a range ~f Hs and wave periods <!z or TP)' each combination representing a 100 year return period, should B 100 General be considered in the mooring analyses.

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103 It is the intention of these rules that by specifying environmental conditions for upper and lower water depth limits all intermediate water depths may normally be covered.

101 Wind and current forces and moments acting on the unit may be calculated according to Pt.3 Ch.! Sec.4, Classification Note No. 30.5 or other recognized method or based on model testing.

DET NORSKE VERITAS

102 Wave drift forces may be calculated according to recognized theory (e.g. diffraction theory) or based on model testing.

B 200 High frequency wave induced motions

201 High-frequency motions of the unit arising from first order wave induced forces may be calculated according to recognized theory or based on model testing. The waterdepth to be taken into account.

202 The motion of the unit in irregular sea may be calculated taking short-crested sea into account.

203 The most probable largest wave frequency motion amplitude may be calculated assuming that the extreme motion response fits a Rayleigh distribution

where

xmax HF

N = t/Tz

max / XHF = "WF-v2 In N

the probable largest wave frequency motion response standard deviation of wave frequency motion response t is the specified storm duration in seconds and is be at least 2 hours. T z is the zero up-crossing wave period.

B 300 Low frequency wind and wave induced motions

301 Low frequency wave induced motion may be calculated according to recognised theory or based on model testing. The test/simulation duration time is to be adequate to provide adequate statistics, but is not to be taken less than 3 hours.

302 Low frequency wind induced motion may be calculated according to recognised theory or based on model testing, see A102. The test/simulation duration time is to be sufficient to provide adequate statistics, but is not to be taken less than 3 hours.

Guidance note: Low frequency motions are induced by the low frequency component of the second order wave force which in general are quite small compared to the first order forces. Because of this, the low frequency forces do not play a significant role in the motions in the vertical plane (i.e. roll, pitch and heave motions) where large hydrostatic restoring forces are present. However, in the horizontal plane (i.e. surge, sway and yaw motions), where the only restoring forces present are due to mooring or dynamic positioning systems and production risers, the motions produced by the low frequency force can be substantial. This is in particular true in frequencies near the natural frequency of the mooring system. Therefore in general, only low frequency surge, sway and yaw motions are included in a mooring analysis.

Low frequency motion of a moored unit is narrow banded in frequency since it is dominated by the resonance at the natural frequency of the moored unit. The motion amplitude is highly dependent on the stiffness of the mooring system, and on the system damping. A good estimate of damping is critical in


computing low frequency motions. There are four main sources of damping:

- viscous damping .of the unit - wave drift damping - mooring and riser system damping - thruster damping (only applicable for thruster assisted moor-

ing).

The wave drift damping and the mooring system damping are often the most important parts of the total damping.

The calculation of the most probable largest LF motion may be performed by using a method called "Stansberg's distribution" which is given as:

A

B

N = t/TLF

where

standard variation of low frequency motion response

2.jM M+ 1

= ( ~~ ~ :~ ) (0,3 M - 0,5)

= the specified storm duration in seconds TLF resonance period of the low frequency motion

= 27r fl

m = unit mass including added mass in kg k = mooring system stiffness in Nim taken at the vessel's

mean position.

For a system with a resonance period in the range of 100-150 seconds, the value M is as follows with respect to critical damping:

M % critical damping

13 9

7,5 14

4,5 21

A conservative approach will always be to apply a logarithmic distribution:

xL:fx = crLF In N

However, for a weakly damped system (damping _coefficient less than 5% of critical damping), the extreme value behaviour is expected to approach the Rayleigh model asymptotically.


B 400 Model testing

401 Model test program and test facilities are to be to the satisfaction of the Society.

402 The model is to be at an adequate scale and is to fully represent the moored installation.

DET NORSKE VERITAS

Rules for Mobile Offshore Units, January 1996 Page 6 - Pt.6 Ch.2 Sec.3

SECTION 3 MOORING SYSTEM ANALYSIS

Contents

A. Method A 100 General A 200 Quasistatic analysis A 300 Dynamic analysis A 400 Transient motion A 500 Direction of wind, waves and current relative to the unit A 600 Long term mooring for production and storage units

B. Operation Conditions B 100 Operation condition I B 200 Operation condition II B 300 Redundancy for operation condition II

C. Safety Factors and Premises C 100 General C 200 Safety factors C 300 Permissible horizontal offset C 400 Permissible line length C 500 Anchor patterns

A. Method

A 100 General

101 The mooring system is to be analysed for the operation condition(s) given in B, as applicable for the function(s) of the unit.

102 The most unfavourable of the following load cases is to be considered for extreme condition for units not sensitive to low frequency motion, and to be used for the mooring system analysis:

1) 10 minutes average wind speed and sea state, each corresponding to a 100 year return period, combined with a 10 year return period current.

2) 10 minutes average wind speed corresponding to a 10 year return period, combined with a 100 year return period seastate and current.

Guidance note: Unless more detailed information on environmental data is available, the following correlation may be applied:

- the 10-year-windspeed to be taken as 90 percent of 100-year-windspeed

- the 10-year-current velocity corresponds to the 100-year-current with the wind generated current reduced by 10 percent.


103 The most unfavourable of the fo1Iowing load cases is to be considered for extreme condition for units sensitive to low frequency motion, and to be used for the mooring system analysis:

I) I hour average wind speed in combination with a gust spectrum and sea state, each corresponding to a 100 year return period, combined with a 10 year return period current.

2) I hour average wind speed in combination with a gust spectrum and sea state, each corresponding to a 10 year return period, combined with a I 00 year return period current.

104 When anchoring takes place in shallow water, the following has to be included in the calculation of wave frequency motion:

- the stiffiiess effect of the mooring system has to be included when the water depth is less than 70 m

- when the water depth is less than 100 m, the shallow water effect is to be included in the horizontal wave frequency motions, see Figure 1, 3 and 4

- for ship shaped units the influence of shallow water on the current force has to be considered when the waterdepth/draught ratio is less than 2,5, see Figure 2.

Guidance note: For column stabilised units the amplification factors for the wave frequency motion may be taken according to Fig.1.

1.9 -,.----~-~~~-1 ----T~sec 1.8 - Tz = 11 sec i

- - - - . Tz = 12 sec · ___ Tz = 13 sec I

.. Tz = 14 sec 1

~ 1.7 g u 1.6 .. ... c 1.5 0

~ 1.4 u !!: 1.3 ii E 1.2 <

1.1

30 40 50 60 70 80 90 100 Water Depth (m)

Fig.1 Amplification factors for surge and sway wave frequency motion

The water depth influence on current drag coefficients for a ship shaped unit is given in Fig.2

3.5

3

"" 2.5 '\ ~

15. ~

• .! 2 .,, ~ '\.

-------·-. ·-·-----~ i1.5 .! ~ ~ c • ....... --"' .,, 1 ~ .5 c .,,

0.5 "' !,!. .,, !,!. 0

2 3 4

Water depth/Draught

Fig. 2 Water depth influence on current drag coefficient for ship shaped units

For ship shaped units the amplification factors to be applied for wave frequency motion may be taken according to Fig.3 and Fig.4

DET NORSKE VERITAS

1.8

1.7

1.6 :; 0 .5 ~ ~

= .2 1.4 -;; u

"' 1.3 ~ ~ 1. 2

1.1

--------~ --------TZ= 11See I

30 40

___ Tz=12 sec

____ .Tz=13secl ___ Tz=14 sec

----Tz=15sec ____ Tz=16 sec _____ Tz=17 sec

50 60 70 80 90 100 Water depth. (m)

Fig. 3 Amplific.ation factor for surge/wave frequency motion

2.2 ·--Tzi::1Q s·~~

- - - Tz=11 sec 2.0 - - - - . Tz=12 sec

!!! ' I 11- --""'"" ~ 1.8 - - - - Tz= 14 sec I

.e Tz=15 sec ~ _____ Tz=16 sec 0 1.6

,_ I -·-·-· Tz=17sec1 :;

" --,··-- -· I I I -"' 1.4 :a. E

<( 1.2

1 . o _l___l__j_--'--_ _J__J___j_

30 40 50 60 70 80 90 100 Water depth (m)

Fig. 4 Amplification factors for sway/wave frequency motion

---e-n-d---o-f---G-u-i-d-a-n-c-e---n~o-t-e---

105 For taut leg mooring system the stiffness of the mooring system is to be included in the calculation of wave frequency motion regardless of water depth.

106 When detailed field measurements are not available, the variation in current velocity is to be taken as described in Classification Note No. 30.5 "Environmental Conditions and Environmental Loads".

107 When detailed field measurements are not available, the extreme wind velocity variation as a function of averaging time and height above mean water level is to be taken as described in Classification Note No. 30.5 "Environmental Conditions and Environmental Loads".

108 Effective elastic modulus are to be obtained from the manufacturer.

Guidance note: For preliminary design the effective elastic modulus applied in the mooring analysis may be taken as:

- stud chain NV R3 (12,028 - 0,053 d) 1010 N/m2

- stud chain NV R4 (8,208 - 0,029 d) 1010 N/m2

- studlees chain NV R3 (8,37 - 0,0305 d) 1010 N/m2

- studlees chain NV R4 (7,558 - 0,0267 d) 1010 Nimz

where d is the chain diameter in mm


- six strand wire rope: E = 7,0 x 1010 N/m2 corresponding to nominal diameter of the wire

- spiral strand wire rope: E = 1,13 1011 N/m2.


109 Unless otherwise documented, a friction coefficient of 1,0 is to be applied in the line direction between the anchor and the sea bed.

110 The stiffness characteristics of the mooring system are to be determined from recognised theory, e.g. based on catenary equations.

111 The analysis of the mooring system behaviour may be based on quasi-static or dynamic approach. For deeper water than 450 m, dynamic analysis according to 300 has to be carried out. Upon special consideration the Society may require a dynamic approach for water depths less than 450 m.

Dynamic analyses are recommended for floating production and/or storage units when the water depth is more than 200 m.

112 The maximum allowable deviation for each anchor line in the anchor pattern, as basis for assignment of POSMOOR is ±10°.

A 200 Quasistatic analysis

201 In a quasi-static analysis the direction of wind, wave and current is to be according to 500.

202 For column stabilised units not sensitive to low frequency motions; wind, current and wave drift forces are to be assumed to be coincident and the maximum force to occur at the same time, see 501. The loading combination for the extreme condition is to be taken according to 102.

203 For other moored units than described in 202, sensitive to low frequency wave and/or wind induced motions, the loading combination for the extreme condition is to be taken according to 103. Directions of wind, waves and current relative to the unit are described in 500.

The maximum excursion may be obtained by combination of second order rr1otion with the first order wave induced motion as follows:

max Xs_ig~ h XmLFax > XWFmax XTOT = Xmean + XLF + WF w en

X X Xmax xsign h xmax < xmax TOT= mean+ WF + LF wen LF WF

where

Xmean

xmax LF

xmax WF

xsign LF

xsign WF

quasi-static position of the unit at which the line tensions are calculated mean offset (equilibrium position) due to static loads caused by wind, wave and current maximum low frequency motion from wind and waves maximum wave frequency motion

significant low frequency motion from wind and waves significant wave frequency motion.

Guidance note: The Society may accept other methods provided the method is satisfactorily documented, and the safety level is maintained.

The required probability level of the maximum combined waveand low frequency motion is the most probable maximum value in a storm with a duration of 3 hours.


DET NORSKE VERITAS


204 The total quasistatic displacements and the associated tension in the most heavily loaded line are to be determined based on the stiffness characteristics of the mooring system.

205 For equilibrium position after line failure, the tension distribution prior to line failure is not to be adjusted to optimize for the usage factor requirements in these conditions.

206 The pretension distribution applied in the mooring calculations is not to be optimised beyond what is easily achievable during operation on board. If optimisation of the mooring system is considered in the calculation the procedure is to be included in the operation manual.

A 300 Dynamic analysis

301 In a dynamic analysis the following is normally to be taken into account:

- time varying effects of exciting forces and moments - inertia and damping effects of the moored unit and its

mooring system - non-linear structural dynamics of the mooring line.

302 Dynamic analyses carried out in both time and frequency domain will be accepted.

303 The maximum path of excursion and the corresponding tension in the most heavily loaded line within a time period of at least 3 hours to be documented. The documentation is to include the instant at which the maximum anchor line response occurs.

Guidance note: The dynamic analysis should include the low frequency effects of the slowly varying wave drift forces and, if relevant, the time dependent nature of the windloading. Current may normally be taken as constant. In case a simplified frequency-domain analysis is chosen, the non-linear damping and restoring forces may be approximated by a stochastic linearization method as follows:

- calculate the natural frequency of the mooring system - calculate spectral density of low-frequency excitation due to

wind and waves at the natural frequency - estimate the damping ratio for the unit including the contrib

ution from the mooring system calculate standard deviation of the low-frequency response using a constant excitation spectrum given by the spectral density at the natural frequency.

The Society will accept an analysis method whereby the low frequency dynamic behaviour of the moored unit is analysed separately from the high frequency behaviour of local anchor lines by superimposing the values from the global response analysis to arrive to the total tension. Estimation of short-term extreme values for dynamic anchor line forces may be determined by using one of the following methods:

1) Statistical estimation of extreme motion of the upper-end mooring points as basis for harmonic excitation associated with representative period and phase angle relation between vertical and horizontal motion.

2) Time serie generation of wave surface elevation for a certain duration, e.g. 3 hours, for location of highest wave. Dynamic tension in the upper-end mooring points to be simulated fo1 a period of 2 to 3 minutes containing the highest wave.

3) For long term mooring it is rccOmmended to generate time series of motion in six degrees of freedom to be applied in the dynamic line tension calculation. Simulation is to have a duration of at least 3 hours.


304 XLF is the quasi-static offset at which the dynamic line tensions are calculated.

Total line tension is the sum of dynamic line tension and quasi-static line tension at the offset:

XLF = XToT - xri~x

A 400 Transient motion

401 The transient motion of the unit following loss of holding power in one mooring line or a single control or thrust failure in the thruster system is to be documented.

Guidance note: Transient motion after a single failure in the thruster control system, or the event that one or all thrusters are lost, are handled in the same way as for a single failure. It is the difference in the restoring forces for the intact system and the system after a failure, which is the force that generate the transient motion.

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

402 The documentation is to include information of the unit's excursion path, unit orientation and maximum line tension during transient mode.

403 For calculation of maximum tension, the transient motion as described in 401 and 402 is to be combined with the significant wave frequency motion amplitude in the weather direction for the present sea state. Normally low frequency motion is not considered. However, if the standard deviation of the low frequency motion is twice the standard deviation of the wave frequency motion, the sum of the significant wave and low frequency motion is to be combined with the transient motion described in 40 I and 402.

A 500 Direction of wind, waves and current relative to the unit

501 For column stabilised units and ships which are directionally fixed, the loads from wind, waves and current are assumed ac;ting in the same direction. For units with symmetrical anchor pattern, at least head, quartering and beam load directions are to be analysed. For units with non-symmetrical anchor pattern, all directions from 0° to 360°, with a spacing of 45°, are to be investigated. Directional distribution of wind, waves and current may be applied if available.

502 For weather vaning units such as turret moored production/storage vessels, the following two combinations of wind, wave and current are to be applied if no information is obtainable from the location in question:

1) Co-linear environment Wind, waves and current acting in the same direction. The direction is to be relative to the unit's bow, equal to the significant value of the unit's yaw motion.

2) Non-co-linear environment

- wave towards the units bow ( 0 °) - wind 30 ° relative to the waves - current 45 ° relative to the waves.

Wind and current are approaching the unit from the same side of the unit.

A 600 Long term mooring for production and storage units

601 Fatigue calculations are to be carried out for mooring lines and connection elements if the unit is to stay at the same location for more than 5 years. Fatigue calculations may be carried out according to AP! RP 2SK "Design and Analysis of Station keeping System for Floating Structures", or by applying probabilistic methods. The number of load cycles are to be multiplied with the factors given in Table Al.

DET NORSKE VERITAS

Table Al Deiign life factors Access for inspection Design load factors

Regular inspection 3

Inspection not possible 10

602 Marine growth is to be included in the analysis of long term mooring system for production/storage vessels. The thickness of the marine growth is to be in accordance with the specification for the actual location.

Guidance note: Marine growth is dependent on the location. If no data is available the following is to used:

Water depth Thickness (m) (mm)

+2 to -40 100

below -40 20

The specific weight of marine growth may be set to 13 kN/m3.

The effect of marine growth has to be included in the analysis by an increase in weight and drag coefficients.

Mass of marine growth:


The breaking strength of anchor lines which forms the basis of the mooring calculations is to be adjusted for reduction in strength due to corrosion, wear etc. according to the values given above.

604 The life time of a steel wire rope is dependent on construction and degree of protection. Guidance for choice of steel wire rope construction depending on wanted design life is given in Certification Note 2.5.

B. Operation Conditions

B 100 Operation condition I

101 Operation condition I is defined as follows:

- position mooring where a single failure of positioning system will not lead to a critical situation for the overall safety of the unit and those onboard.

102 Single failure in this condition is defined as anyone of the following failure modes:

a) Loss of holding power of any-one single mooring line.

Mgrnwth = ~ ((Dnom + 2 il.Tgrnwth)2- D~om) Pgrnwth µ (kg/m). b) Loss of thrust of any single thruster, if thrusters are in

stalled. Submerged weight of marine growth:

Wgrowth = Mgrowth ( 1 -

where

Pseawater Pgrowth )

9,81 1000

Pgrowth Pseawatcr Dnom .1Tgrowth µ

= density of marine growth = density of sea water = nominal chain or wire diameter = marine growth surface thickness = 2,0 for chain, 1,0 for wire rope.

Increase of drag coefficients due to marine growth:

_ ( Dnom + 2 il.T grnwth ) Cogrowth - Co D

nom

(kN/m)

Cn = stud chain: 2,6, Studless chain: 2,4 with respect to chain diameter

Cn = steel wire rope: 1,8.


603 Corrosion allowance, including wear and tear, of chain and connection elements is to be included in design. The corrosion allowance is at least to be as given in Table A2.

TableA2 Corrosion allowance Corrosion all.owance referred

to the chain diameter Part of mooring line

No inspection Regular inspection l) (mm/year) (mm/year)

Splash zone 2) 0,4 0,2

Catenery 0,3 0,2

Bottom 2) 0,4 0,3

1) Survey extent according to Pt.7 Ch.2 Sec.4, with necessary adjust-ments. Acceptance criteria is 2% reduction in the diameter of the chain with the breaking strength used in design of the mooring sys-tern.

2) Defined as the wave amplitude above and below mean water level. The wave is to have a return period of 100 year at the specified lo-cation.

3) Corrosion allowance for the bottom segment has to be considered for each location depending on the soil conditions.

c) Single failure in thruster control or power system leading to stop of one or several thrusters, if thrusters are installed.

103 Operation condition I may be accepted for survival condition of drilling units with the riser disconnected, and the unit located at a distance of at least 300 m from other structures, see Table Cl, Fig.5 and Sec.! A304. If the unit is between 50 m and 300 m from other structures, POSMOOR V requirements are to be applied, see Fig.6.

104 Operation condition I may be accepted for survival condition of support units such as accommodation and crane units at a distance of at least 300 m away from other structures, see Table Cl, Fig.5 and Sec.! A304. If the unit is between 50 m and 300 m from other structures, POSMOOR V requirements are to be applied, see Fig.6.

105 Operation condition I may be accepted for units designed for production and/or injection of oil, :water or gas through a system consisting of one flexible riser, and associated well control umbilical. The unit is to be positioned at a safe distance from other structures, where failure due to loss of position is not critical for the overall safety and those on board, see Sec. I A304. It is also to be documented that the riser and umbilical can withstand the drift-off following a single failure, see 102.

Requirements for risers, see C302 and Ch.6 Sec. 7.

106 Operation condition I may be accepted for units designed for production and/or injection of oil, water or gas through a system consisting of several flexible risers and associated control umbilicals. The riser/umbilical system is to be either disconnected from the unit, or at least the production has to be terminated and pressure sources isolated to be able to maintain operation condition I. Documentation is required showing that the risers can withstand the drift-off following a single failure, see 102.

Fixed risers have to be disconnected if the unit is to be in operation condition I.

Requirements for risers, see C302 and Ch.6 Sec. 7.

DET NORSKE VERIT AS


B 200 Operation condition n 201 Operation condition II is defined as follows: Positioning mooring where exceedance of position limitations will lead to a critical situation for the overall safety of the unit and those onboard.

202 Single failure in this condition is defined as any-one of the following failure modes:

a) Loss of holding power of any-one single mooring line.

b) Loss of thrust of any single thruster.

c) Single failure in thruster control or power system leading to stop of one or several thrusters.

203 Operation condition II applies to operation of drilling units with rigid riser connected, see, Table Cl and C302.

204 Operation condition II applies to operation of support units such as accommodation and crane units operating at a distance less than 50 m from other installations. The distance is not to be less than described in C304 for a unit with gangway connected, see Table Cl, Fig.7 and Sec.I A304 for operation in POSMOOR V mode.

205 Except as provided for in 105 and 106, operation condition II applies to units designed for production and/or injection of oil, water or gas through a system consisting of one or several rigid or flexible risers, and associated well control umbilicals, when the units are in production mode. It is to be documented that the risers and associated umbilicals can withstand the drift-off following a single failure and that safety factors are not less than defined for POSMOOR operation condition II

206 Requirements for risers, see C302 and Ch.6 Sec.7.

207 Operation of a unit at a distance less than 50 m from other installations, see Table Cl, Fig.7 and Sec.I A304.

B 300 Redundancy for operation condition II

301 If a unit is designed to remain in operation condition II after the occurrence of a single failure as defined in 202, the system is to include redundant components or systems so that the single failure will not cause critical loss of position or exceedance of anchor line tension.

302 If a unit is designed to transfer from operation condition II to operation condition I after the occurrence of a single failure, e.g. by disconnection of riser or gangway, the system design is to ensure adequate position keeping for the duration of the transfer between the two conditions.

C. Safety Factors and Premises

C 100 General

101 The permissible safety factor is defined as:

SF= Pb/Tmax

Pb minimum breaking strength of the anchor line adjusted for corrosion allowance

T max = maximum tension in the most heavily loaded line.

The safety factor is to be calculated for each segment in. a multi-segment line.

102 The permissible safety factor is dependent on:

- operation condition

- class notation, i.e. intended service - type of analysis - type of anchor line.

C 200 Safety factors

201 The permissible safety factors for chain cable and steel wire rope depending on the intended operation( s) are given in Table Cl.

202 Synthetic fibre ropes are at least to have safety factors which are the safety factors given in Table CI multiplied with 1,10.

C 300 Permissible horizontal offset

301 The horizontal offset from a given reference point is to be within the operational service limitations.

302 When the unit is connected to a rigid riser, the maximum horizontal offset during transient motion including significant motion or the offset during temporary mooring after a single failure is given by the maximum allowable riser angle at the BOP ball joint and a safety margin of2,5% of the water depth to be deducted.

Maximum offset of a flexible riser is not to exceed the manufactures specification. with a safety margin of 5 % of the water depth to be deducted.

303 Maximum environmental loads for drilling operation are also to take the heave compensating capacity into consideration.

304 When the unit is connected by a gangway to another structure, the positioning system design and gangway structure is to meet the following:

- the distance between the structures (except for the gangway) is not to be less than 10 m at any point during transient motion (significant high frequency induced wave motion included)

- during normal operation an excursion reserve of 1,5 m on the specified maximum excursion of th~ gangway is to be included -.

- the gangway is to be equipped with an alarm in the control room which is to be activated when the maximum excursion is exceeded

- the gangway is to be positioned so that it will not collide with any other structure after a single failure.

C 400 Permissible line length

401 For anchors not specially designed to take uplift forces, the following generally applies:

- the mooring lines are to have sufficient length as to avoid that the embedded part of the anchor line intersects the seabed under an uplift angle, for all relevant design conditions in operation condition II. In operation condition I vertical forces on the anchors during transient motion (significant motion included), and other single failure may be accepted if it is documented that the vertical forces experienced will not destroy the holding capacity of the anchors.

402 On a case by case basis an uplift angle is accepted also for design conditions in operation II, see Sec.5 A400.

Maximum deployed line length allowed to be taken into account in the calculations is limited to suspended length at line tension equal to breaking strength of the line plus 500 m.

DET NORSKE VERITAS

403 Anchors designed to withstand vertical forces will be accepted in both operation conditions, see Sec.5 A400.

C 500 Anchor patterns

501 The anchor pattern is not to interfere the safety of bottom pipelines, flowlines or other petroleum systems. The minimum clearance between mooring lines and all type of

Table Cl Permissible safety factors


subsea equipment is 10 m for intact condition. In damage condition contact is not permitted. Contact between mooring lines and risers is not permitted.

502 Crossing of anchor lines is normally not accepted.

503 For vicinity operations positive clearance is to be obtained between the anchor lines and e.g. fixed structures.

Operation condition Quasistatic analysis Dynamic analysis

POSMOOR 2) POSMOOR V 1) 2) POSMOOR 2) POSMOOR V 1) 2)

Intact system 1,80 2,00 1,50 1,65

I Transient motion 1,10 1,10 1,00 1,00 Temporary mooring after single line failure 1,25 1,40 1,10 1,25

Intact system 2,70 3,00 2,30 2,50

II Transient motion 1,40 1,40 1,20 1,20

Temporary mooring after single line failure 1,80 2,00 1,50 1,65

1) Applies for anchor lines which are located within a critical sector, normally in a 180 degrees sector facing away from the installation, see figures 6 and 7.

For anchoring less than 50 m from a structure, the anchor lines outside the critical sector are to be designed according to operation condition II, POSMOOR, while the lines within the critical sector are to be designed according to operation condition II POSMOOR V.

Depending on type of operation, anchoring within 50 m and 300 m from a structure, the anchor lines outside the critical sector are to be designed according to operation condition I or II, POSMOOR, while the lines within the critical sector are to be designed according to operation condition I or II POSM OOR V.

2) The safety factors are to be multiplied with a factor 1,10 to be applicable for fibre ropes.

3DOm 300m

300m 5Dm 300 m

Fix:ad <'.; _o_o_ installation

Lines in Crttical Sector

Fig. 5 Fig. 6

A Unit 300 m or more away from an installation. Safety factors according to POSMOOR, operation condition I or II, depending on type of operation.

Depending on type of operation, anchoring within 50 m and 300 m from a structure, the anchor lines outside the critical sector are to be designed according to operation condition I or II, POSMOOR, while the lines within the critical sector are to be designed according to operation condition I or II POSMOOR V.

DET NORSKE VERITAS


5Dm

Fixed <( _ so _ Installation

~ - - - - - - - - - - - - - - ~- -r, ~,,

' '

Lines in Critical Sector

Fig. 7

For anchoring less than 50 m from a structure, the anchor lines outside the critical sector are to be designed according to operation condition II, POSMOOR, while the lines within the critical sector are to be designed according to operation condition II POSMOOR V.

DET NORSKE VERITAS


SECTION 4 THRUSTER ASSISTED MOORING SYSTEM

A. Classification A l 00 General A 200 Definitions

B. System Requirements B 100 Thruster systems B 200 Power systems B 300 Control systems

Contents

B 400 Manual thruster control B 500 Remote thrust control, joystick system B 600 Automatic thruster control B 700 Automatic control B 800 Monitoring B 900 Consequence analysis B 1000 Simulation B 1100 Logging B 1200 Self-monitoring

A. Classification

A 100 General

101 For units equipped with thrusters, a part or full net thrust effect may be taken iuto account iu all design conditions. This effect is depending on the layout of the thrust control system and the design conditions. The permissible use of thrusters and the effects are given iu Table Al and is subjected to the system notation letters as follows:

Letters

The vessel is provided with a thruster assisted TA mooring system which is dependent on a

manual remote thrust control system.

The vessel is provided with a thruster assisted ATA mooring system which is dependent on an

automatic remote thrust control system.

102 The net thrust referred to iu Table Al is to be based on the following conditions:

- fixed thrusters may be considered only if the thrust produced contributes to the force or moment balance

- azimuthing thrusters will be considered to provide thrust in all directions, unless specific restrictions are defined

- thruster induced moment is to be taken into account when thruster assistance is analysed.

103 When thrusters are !'sed, failures leading to stop of thrusters are to be considered equivalent to liue failure as defined in Sec.3 B!OO to B200, and the corresponding safety factors will apply. See Sec.3 Table Cl.

Guidance note: The maximum effect of single failure should not cause lower safety factors than those in Sec.3 Table Cl. Black-out will be one typical maximum effect single failure. If black-out leads to lower safety factors than those permitted for damage condition, the power system has to be arranged with redundancy. Alternatively, the proportion of thrust to be used has to be reduced to a level where black-out still may be acceptable in view of safety factors.


104 Manual thruster control is intended only for limited time periods, and the arrangement assumes the continuous attention of an operator.

105 Turret moored ship shaped units which are not naturally weather vaning and hence dependent of headiug control, are to be equipped with an automatic remote control system according to A TA requirements.

A 200 Definitions

201 TA, thruster assistance, signifies a system comprising:

- thruster system - power system - control system - reference systems

where the thrusters are controlled manually to produce a thrust which will assist the mooriug system of the unit.

202 AT A, automatic thruster assistance, signifies a system similar to TA with the addition of an automatic control mode.

203 The thruster system comprises the thruster units, included gear drives and control hardware for control of thruster speed/pitch and azimuth.

Guidance note: Classification according to TA or ATA does not imply specific requirements regarding number of thrusters or capacity of these. The effect of thrusters will be determined and incorporated in the mooring analysis.


Table Al Permissible use of thrust effect in thruster assisted mooring system

Operation Unit's layout of thrust control system

condition Manual remote control Automatic remote control Letter: TA Letter: ATA

I 70% of net thrust effect from all thruster l) The net thrust effect from all thruster

II Is not to be accounted for The net thrust effect from all thrusters

I 70% of net thrust effect from all thrusters I) The net thrust effect from all thrusters 2> Single failure

II Is not to be accounted for The net thrust effect from all thrusters 2> Single failure 1) Provided continuous watch at joystick.

2) A failure leading to stop of thrusters is to be considered equivalent to a line failure. as a single failure and the safety factors in Sec.3 Table Cl arc satisfied.

Redundancy equipment is not required if blackout is considered

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204 The power system comprises all units necessary to supply the thruster assist system with power.

205 The control system comprises all central control hardware and software exercising pitch/speed and azimuth control of individual thrusters or groups of thrusters.

B. System Requirements

B 100 Thruster systems

101 Thrusters are to comply, as applicable, with Pt.4 Ch.2 Sec.6, or Rules for Classification of Ships Pt.4 Ch.2 Sec.8, in accordance with the main class of the unit.

102 The thruster configuration may consist of both fixed and rotatable thrusters. Thrust output may be controlled by variable pitch, or variable speed, or other means which may be approved upon special consideration. The thruster configuration will be evaluated on basis of the mooring analysis.

B 200 Power systems

201 Electrical installations are to comply with Pt.4 Ch.4 or Rules for Classification of Ships Pt.4 Ch.4, in accordance with main class of the unit.

202 An automatic power management system is to be provided which will ensure adequate running generator capacity relative to power demand, i.e. available power reserve, and will execute immediate limitations in power consumption to prevent black-out due to overload caused by sudden shortage of available power.

203 The capacity of the power system is to be evaluated on the principle that single failure in the power system is to be considered equivalent to anchor line failure. The limiting requirements for tensions and motions as specified for the type of operation are to be applied.

204 If the permissible safety factors depend on certain thrusters to remain intact after failure as in 203, the power system is to be designed with redundancy to ensure operation of these thrusters. Rules for Dynamic Positioning Systems for Ships and Mobile Offshore Units (DYNPOS) may be used as guidance.

B 300 Control systems

301 Notation TA is to include:

- manual control of each thruster - remote thrust control, joystick system.

Notation A TA is to include:

- manual control of each thruster - automatic control of all thrusters.

302 A mode selector is to be arranged in the thruster assistance control area to enable switching between remote thrust control, or automatic control, and manual control.

B 400 Manual thruster control

401 Manual operation of each thruster, start, stop, azimuth, and pitch/speed control are to be arranged. Displays are to be provided for all information necessary for safe and practical operation.

402 Individual stop (emergency stop) of each thruster is to be possible from the thruster assistance control area.

403 The location of the thruster assistance control stand is to be chosen with consideration of the operation. Units operating at safe distance from other stationary structures may

have the control stand in a control room with no direct view of the vessel surroundings. Units operating in the vicinity of other structures, i.e. with notation POSMOOR V, are to have a control stand wherefrom there is good view of the unit surroundings.

404 The thruster assistance control stand is to be equipped with displays for line tension and line length measurements.

B 500 Remote thrust control, joystick system

501 The remote thrust control system is to be located in the control area together with the manual thruster controls and with the same access to thruster and mooring displays.

502 The remote thrust control system is to be a joystick system with integrated control of all thrusters. Automatic heading control is to be included.

503 At least one gyro compass is to be interfaced to the joystick system.

B 600 Automatic thruster control

601 The automatic control mode is to include the following main functions:

a) Automatic control for optimal use of available thrust in cooperation with the mooring system forces, and automatic compensation of the effects of mooring line failure, thruster failure and thruster power failure. Detailed requirements are g:Wen in 700.

b) Monitoring of position and mooring line tensions and alarm for excursions of limits. Detailed requirements are given in 800.

c) Consequence analysis consisting of prediction of line tensions and vessel position, both transient and final, in the event of single line failure, or thruster failure, under the prevailing environmental conditions. Detailed requirements are given in 900.

d) Simulation of motion and tensions during maneouvres, changes of anchor patterns, effects of changing weather conditions, and effects of failures in thrusters or mooring. Detailed requirements are given in 1000.

e) Logging of relevant parameters for display or hard copy on operator request. Detailed requirements are given in 1100.

f) Seif diagnostics with alarms for faults within the automatic control system or in data received from interfaced equipment. Detailed requirements are given in 1200.

602 The automatic control system is to be powered from an uninterruptible power source, UPS. The battery power reserve in the UPS is to be sufficient for 15 minutes operation.

603 Redundant automatic thruster control is required when the mooring analysis is based on thruster assistance to meet limiting requirements.

B 700 Automatic control

701 Low frequency motion is to be effectively damped by use of thrusters. Typically, the excursion amplitude is to be about halved within 5 cycles after introduction of thruster assistance.

702 The thrusters are to be controlled to produce thrust to counteract the environmental forces. The thrust is to be proportionate to the magnitude of line tension and position offset. Thrusters may be deactivated when line tensions and position offset are within acceptable limits.

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703 The thrusters are to be controlled to produce thrust to compensate for the effects of mooring line failure.

704 The thruster control system is to be able to reallocate thrust when failure of a thruster is detected, or the operator deselects a thruster.

705 When the power demand for use of thrusters exceeds available power, the control system is to use the available power in an optimal manner and introduce thrust limitations to avoid overloads and black-out situations. The method of thrust limitation is to be quick enough to avoid black-out due to the sudden overload caused by stop of one or more generators.

B 800 Monitoring

801 Continuous monitoring is to be provided of all important parameters, which at least will include position, heading, line tensions and available electrical power.

802 Deviations from desired position and heading are to be compared with 2 adjustable limits. An alarm is to be released when passing either limit. When passing the first limit, the alarm may be considered as a warning and is to be distinguishable from the alarm released at the second, more severe limit.

803 Line tensions are to be monitored and compared to both high and low limits.

804 Low line tension alarm may be interpreted as line failure if the line tension measurement system has- self-check facilities, and these have not detected a measurement failure. Otherwise, the low tension alarm is not to be interpreted as line failure and used for thruster control unless one more parameter e.g. position or heading indicates line failure.

805 Monitoring of position is to be based on position measurements from at least one position reference system. The position being calculated from mooring system data may be used to check the direct position measurement, and may be used in the event of failure of the position reference system.

806 The position measurement is to have an accuracy of 2 % of water depth, obtained either directly by one source of reference, or by pooling the results of several.

807 The position reference system is to be installed in a location and manner most suitable for its type.

808 The position measurements are to be transformed to represent the position of any critical point on the vessel as determined by its application.

809 The A TA control panel is to be equipped with alarm display for thrusters, which may be relayed from the thruster alarm panel or general alarm system.

810 There is to be alarm display for failure of external devices interfaced to the ATA system, e.g. gyro compass, wind sensor, UPS.

811 All alarms are to be acknowledged by the operator at the A TA control panel. For alarms relayed from general alarm system or other common source, the acknowledgement is to have only local effect.


B 900 Consequence analysis

901 Concurrent with control and monitoring, there is to be performed an ana~ysis of the consequences of certain defined failures under prevailing operating conditions. The consequences are defined as line tensions and position deviations in excess of accepted limits.

902 The failures to be considered are to include failure of any mooring line, failure of any single thruster, or stop of thrusters as will occur in the event of the most serious failure in the power system.

903 The consequence analysis is to check the consequence criteria against all defined faults in sequence, and the repetition rate is not to be less than once per 5 minutes.

904 All computed consequences are to release an alarm, or a warning. The consequence and reason are to be suitably identified. The warning/alarm is to be acknowledged.

905 The software/hardware used for preparing the consequence analysis is exempted from the redundancy requirements for automatic control system, see 603.

If the consequence analysis function is carried out by nonredundant equipment, failure of this is to cause alarm.

B 1000 Simulation

1001 The simulation functions may be executed in an offline computer system with access to process data. If the control system is used for simulation, the priority is to be next to control, monitoring and consequence analysis.

1002 The simulation facility may use the display system of the control system, but is not to obstruct the presentation of alarms.

1003 The simulation facility should at least provide for:

- simulation of mooring conditions on input of proposed anchor pattern and line tensions

- simulation of effects of changing weather conditions - simulation of line tensions and transit motion and final

position caused by line failure. The effects are to be displayed in true time scale

- simulate relevant functions both with and without thruster assistance.

B 1100 Logging

1101 Automatic logging is to be done of important parameters. This will at least include all line tensions, position and heading deviations, power consumption, thru~t resultant in magnitude and direction, wind speed and direction.

1102 The frequency of data recording is to be high enough to give reasonable presentations of transient behaviour.

1103 The data is to be presented in graphical form, covering at least one hour back in time.

B 1200 Self-monitoring

1201 There is to be automatic self-monitoring of the automatic control system, which is to detect computer stop, software hang-ups, power failures, and false operation of interfaced equipment as far as this can be determined from the central system.

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SECTION 5 MOORING EQUIPMENT

Contents

A. Anchors A 100 General A 200 Proof testing of anchor strength of embedment type A 300 Type approval of embedment anchors A 400 Holding power requirements A 500 Holding power requirements - long term mooring

B. Anchor Lines B 100 General

C. Fairleads C 100 General

D. Windlasses, Winches and Stoppers D 100 General D 200 Materials and certification D 300 Capacity and system requirements D 400 Stoppers D 500 Strength and design load

E. Tension Me.asuring Equipment E 100 General

A. Anchors

A 100 General

101 The anchors are normally to be of embedment, pile or suction type. Other anchor types may be accepted upon special consideration.

102 For drilling, accommodation and crane vessels, the anchors of embedment type are to be designed in such a way that additional anchors can be attached.

103 Relevant rule requirements for embedment type anchors given in Pt.3 Ch.2 Sec.5 are applicable.

104 Anchor piles are to account for pile bending stresses as well as ultimate lateral pile capacity. Pile embedment is also to be sufficient to develop the axial capacity to resist vertical loads with an appropriate factor of safety. The design is to be based on recognised codes and standards.

Guidance note: Design criteria for anchors piles may be taken according to Rules for Fixed Offshore Installation, Pt.3 Ch.1 Sec.9.

For pile anchors designed to resist vertical loads containing significant cyclic component, the factor of safety applies to the cyclically degraded axial pile capacity, see Classification Note No. 30.4 Sec. 2.7.

An analysis method capable of accounting for both aspects of behaviour is to model the pile as a beam column on an inelastic foundation. The inelastic foundation can be modelled using a soil resistance-deflection (d-y) curves, which are described for various soils in API RP 2A.


105 Suction anchors without significant vertical load component may be designed according to the principles outlined for gravity base foundations in Rules for Classification of Fixed Offshore Installations Pt.3 Ch.5, in Classification Note No. 30.4 "Foundations", or other recognised standards. For suction anchors designed to resist vertical loads e.g. in taut leg mooring systems, the design basis is subject to approval on a case by case basis. An important load case for suction anchors is buckling due to the difference between outside and inside pressure.

A 200 Proof testing of anchor strength of embedment type

201 The strength of anchor and shackle is not to be inferior to that of the anchor line. For dimension of anchor shackle, see Pt.3 Ch.2 Sec.5.

202 Proof testing of the anchor is to be carried out according to Pt.3 Ch.2 Sec.5, with the exception of the proof load which is to be 50 % of the minimum breaking strength of the anchor line.

203 Proof testing of anchors with a weight above 20 tonnes may be omitted. The safety factor required for documentation of the ability of the anchor to withstand the required proof load by calculation methods instead of testing to be equivalent to 0,9 of the material yield stress in each case. In the case of long term mooring, omitting the proof load testing has to be based on an acceptance from the owner and the National Authorities in question. The corresponding anchor shackles are supposed to be tested according to Pt.3 Ch.2 Sec.5 E200.

A 300 Type approval of embedment anchors

301 Design of embedment type anchors may be given Type Approval.

302 The anchor and shackle are to be designed to withstand a load equivalent to the minimum breaking strength of the strongest anchor line assumed applied in connection with the anchor in question, without exceeding the breaking strength. For anchor shackles, see also Pt.3 Ch.2 Sec.5.

303 Testing of the anchor is to be carried out according to Pt.3 Ch.2 Sec.5, with the exception of proof load which is to be 50 % of the minimum breaking strength of the strongest anchor line assumed applied in connection with the anchor in question.

304 Proof load testing of anchors with a weight above 20 tonnes may be omitted, see 203.

A 400 Holding power requirements

401 To ensure sufficient holding power of each drag anchor during survival condition (e.g. drilling, accommodation and crane vessels operations), the holding capacity at each new location is to be tested to the maximum line tension expected at the location, or to a load which experience has proven necessary to ensure anchor holding force under extreme weather conditions. This tension is to be maintained until the anchor holding capacity is ensured, normally at least 15 minutes. The testing tension is, however, not to exceed 40 % of the breaking strength of the anchor lines.

A 500 Holding power requirements - long term mooring

501 The long term holding capacity of drag anchor designed for long term mooring of a floating production/ storage system is to be equal to the maximum calculated load in the mooring system (anchor with embedded part of mooring line) multiplied by the factors of safety given in Table Al.

Table Al Factors of safety for drag anchors Cohdition Quasi-static Dynamic

Intact system 1,8 1,5

Mooring after a single failure 1,2 1,0

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502 The basis for assessment of the long term holding capacity and the requirement to pretensioning load during setting of the anchors are to be documented and submitted for approval.

503 The Society may accept, on a case by case basis, that the embedded part of the mooring line intersects the seabed under a certain uplift angle.

504 The pretensioning load is to be maintained until the anchor holding capacity is ensured, normally at least 20 minutes.

505 For drag anchors subject to significant uplift loads, the factors of safety in Table Al may have to be increased, or else the anchors have to be designed to resist uplift loads, see Sec.3 C401.

B. Anchor Lines

B 100 General

101 The chain cable anchor lines used in the position mooring system may be of stud or studless type. The steel grades are NV R3, NV R3S or NV R4. The chain cable may be substituted, partly or completely, by steel wire rope or by synthetic fibre rope.

Guidance note: Upon special consideration other chain grades may be accepted. Notice is to be paid to possible regulations concerning anchor lines for mobile offshore units given by the National Authorities for the location in question.


102 The diameters of the stud or studless chain cable and/or steel wire ropes to be used in the position mooring system are related to the required minimum breaking strength of the anchor lines, as analysed and calculated according to Sec.3.

103 Requirements concerning materials, manufacture, testing and tolerances, and other relevant requirements for anchor chain cables and accessories are specified in Certification Note 2.6 and in Pt.3 Ch.2 Sec.5.

104 Relevant requirements concerning steel wire ropes are specified in Certification Note 2.5 and in Pt.3 Ch.2 Sec.5.

105 Relevant requirements for synthetic fibre ropes are to be decided upon for each separate case, see Sec. I B I 07.

C. Fairleads

C 100 General

101 Fairleads are to be designed in accordance with Pt.3 Ch.2 Sec.5 H. For fairleads which are close to e.g. less than 5 chain links from a windlass, it is advisable to use more than 5 pockets due to high stiffness of the system.

102 Increasing number of pockets in fairleads above the rule requirement will generally lead to lower stress and reduced wear.

Guidance note: It is recommended to have 7 pockets in the lower fairlead for long term mooring systems



D. Windlasses, Winches and Stoppers

D 100 General

101 The windlass/winch is normally to have :

- one cable lifter/drum for each anchor - coupling for release of each cable lifter/drum from the

driving shaft - static brakes for each cable lifter/drum - dynamic braking device - quick release system.

102 The number of pockets in the cable lifter is not to be less than 5, see Pt.3 Ch.2 Gl02.

103 The ratio between winch drum diameter and wire diameter is normally to be in accordance with the recommendations of the wire manufacturer, see Pt.3 Ch.2 Sec.5 G106

104 As far as practicable and suitable for the arrangement, the drums are to be designed with a length according to Pt.3 Ch.2 Sec.5 G!06.

105 When all the rope is reeled on the drum, the distance between the top layer of the wire rope and the outer edge of the drum flange is to be in accordance with Pt.3 Ch.2 Sec.5 G107.

106 The strength of the drum is to be calculated according to Pt.3 Ch.2 Sec.5 G 109 and G 110.

D 200 Materials and certification

201 Material requirements for the main components in windlasses/winches are to comply with relevant specifications given in Pt.2 and Pt.3 Ch.2 Sec.5 G200.

202 Drums are either to be fabricated from steel plates or be casted, see Pt.3 Ch.2 Sec.5 GIOS.

D 300 Capacity and system requirements

301 The lifting force of the windlass/winch in stalling is not to be less than 40 % of the minimum breaking strength of the relevant anchor line. The windlass/winch is to be able to maintain the stalling condition until the brakes are activated.

302 For windlasses/winches not fitted with stoppers, the braking system is to be separated into two independent systems, each able to hold a minimum static load corresponding to 50% of the minimum breaking strength of the anchor line. The brakes are to work directly on the wildcat/drum or wildcat/drum shaft.

303 For windlasses/winches not fitted with stoppers, the brakes, when engaged, are not to be affected by failure in the normal power supply. In event of failure in the power supply, a remaining braking force of minimum 50 % of the windlass' /winch's braking force is to be instantly and automatically engaged. Means are also to be provided for regaining maximum braking capacity in event of power failure.

304 Windlasses/winches fitted with a stopper device, the capacity of the stopper device is not to be less than the minimum breaking strength of the anchor line. The windlasses/ winches are also to be fitted with an independent brake, with static braking capacity of minimum 50 % of the breaking strength of the anchor line.

305 The windlasses/winches are in addition to the static brakes also to be fitted with a dynamic brake. The characteristics of speed/load to which the dynamic brake system can be exposed during setting of the anchor without dam-

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aging overheating occurring, is to be documented and included in the operation manual. These characteristics are also to be reported to the Society and will be included in the "appendix to classification certificate".

306 For pre-installed passive mooring system applicable for long term mooring, stalling capacity less than 40 % of mooring line minimum breaking strength is to be considered on a case to case basis. Deviation with respect to the braking capacity may be acceptable, provided acceptance from the owner and national authorities in question.

307 It is to be possible to carry out a controlled lowering of the anchor lines in case of an emergency. Individually or in convenient groups it is to be possible to release the brakes/stoppers from a well protected area by the winch itself, and from a manned control room or bridge. This is normally to be possible without special preparations and by means of stored energy within 15 seconds and up to a tension corresponding to the breaking strength of the anchor line. During the emergency release it is to be possible to apply the brakes once in order to halt the lowering and thereafter releasing them again. No single error, including operator's error, is to lead to release of more than one anchor line.

308 A manually operated back-up system for emergency lowering of the anchor line is to be provided in the vicinity of the winch/stopper. This is provided in order to make it possible to lower the anchor line in case the stored energy, see 306, should fail to release the brake/stopper.

309 If a riser disconnect system is fitted, it is not to be possible to release the anchor lines while risers are connected to the unit. A special safety system preventing this is to be provided. Emergency release is nevertheless to be possible with risers connected after a manual cancellation of the above system.

310 An audible alarm system is to be fitted by each windlass/winch in order to warn that remote operation of the windlasses/winches is to take place.

At locations where remote operation of the windlasses/winches can be carried out, signboard is to state that the alarm system is to be engaged prior to remote operation of the windlasses/winches.

311 For long term mooring with pre-installed passive mooring systems, deviations from the rules may be acceptable, provided agreed upon in advance and accepted by the owner and national authorities in question.

D 400 Stoppers

401 The chain stoppers may be of two different types:

- a stopper device fitted on the cable lifter/drum shaft preventing the cable lifter/drum to rotate (pawl stopper)

- a stopper preventing the anchor line to run out by direct contact between the stopper and the anchor line.

The latter type is to be of such design that the anchor line is not damaged at a load equivalent to the minimum breaking strength of the anchor line.

402 The material requirements are given in Pt.3 Ch.2 Sec.5 G200.

D 500 Strength and design load

501 For the structural parts of the windlass/winch and stopper, the strength requirements are given in Table DI.

E. Tension Measuring Equipment

E 100 General

101 If tensioning measurement is installed, instrumentation is to comply with Pt.4 Ch.5 Instrumentation and Automation, Sec. 3 Component Design and Installation.

Table DI Design load and strength requirements for winch/windlass Case Load on line Maximum equivalent stress, a e to be the smaller of

the following values:

Stopper engaged Pa 0,7~ ab or 0,9 af 10 stopper

Brake( s) engaged 0,5 PB for each brake 0,73 ah or 0,9 ur

Pulling 0,4Pa 0,5 "b or 0,60 er f ere= defined in Pt.3 Ch.1 Sec.3 C300.

crf= specified minimum upper yield stress of the material.

crb =specified minimum tensile strength of the material.

PB= minimum breaking strength of anchor line.

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SECTION 6 TESTS

Contents

A. Test of Windlass/Winch A 100 Tests before assembly A 200 Functional test

B. Test of TA and ATA Systems B I 00 General

A. Test of Windlass/Winch

A 100 Tests before assembly

101 Before assembly the parts mentioned in Pt.3 Ch.2 Sec.5 G300 are to be pressure tested in the presence of the surveyor.

A 200 Functional test

201 After completion at least one windlass/winch of a delivery to one unit is to be shop tested in the presence of a surveyor to verify that the required lifting capacity, static/dynamic braking capacity can be attained. Alternatively, it may be accepted that only the prime mover of one windlass/winch is tested. In such cases calculations are to be submitted for verification ofresulting lifting forces as well as the braking force.

202 After installation onboard, functional tests are to be carried out in the presence of a surveyor. These tests are to verify that all windlasses/winches with brakes, stoppers, release mechanisms, etc. function satisfactorily.

A test program is to be prepared for the surveyor's approval.

203 At least one of the windlasses/winches is to be tested for its maximum continuous lifting capacity.

204 The dynamic brake is to be tested on at least one of the windlasses/winches.

B. Test of TA and ATA Systems

B 100 General Tests of thruster assisted mooring are to be carried out in a realistic mooring situation according to an approved test procedure in the presence of a surveyor.

101 All control, monitoring, alarm and simulation functions of the thruster control systems are to be tested.

102 In addition to 101, tests of simulated failures are to be carried out to verify redundant system in thruster and power installations. Approval of alternative means of demonstrating these functions may be granted.

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Rules for Mobile Offshore Units , January 1996 Page 20 - Pt.6 Ch.2 App.A

APPENDIX A SUPPLEMENTARY REQUIREMENTS OF THE NORWEGIAN MARITIME

DIRECTORATE (NMD) AND THE NORWEGIAN PETROLEUM DIRECTORATE (NPD)

Contents

A, General A JOO Application

B. Main Part B JOO Requirements

A. General

A 100 Application

lOI Units complying with the requirements of the rules and the supplemental requirements of the Appendix will, after completion of necessary design surveys be assigned one of the following additional class notations:

POSMOOR IN) POSMOOR V (N)

102 The supplementary requirements contained in this Appendix have been transferred from the following Norwegian Maritime Directorate (NMD) regulations:

a) Regulations of 4 September 1987 concerning anchoring/positioning systems on mobile offshore units.

b) Regulation of 10 February 1994 for mobile offshore units with production plants.

c) Guidelines for positioning system on mobile offshore units, dated 21. August 1986.

NPD has accepted that mooring systems are designed according to NMD regulations.

B. Main Part

B 100 Requirements

lOI The supplementary requirements given in the following are arranged in sections corresponding to the sections in this rule chapter (Pt.6 Ch.2).

Reference to the NMD Regulation are given for each item.

Sections

A 1. 0 General requirements A 2.0 Environmental loads A 3.0 Mooring system analysis A 4.0 Thruster assisted mooring system A 5.0 Mooring equipment A 6.0 Test and inspections

A 1.0 General Requirements

A.I.I General requirement

I.I.I The general requirements are covered by section 1 of these rules.

A 1.2 Definitions

Consequence Class I: Operation where damage or pollution of small consequence may occur in case of failure of the positioning capability (equivalent with operation condition 1).

Consequence Class 2: Operations where failure of positioning capability may cause pollution and damage with large economic consequences, or personnel injury.

Consequence Class 3: Operations where fatal accidents, or severe pollution and damage with large economic consequences, are probable results of loss of position (equivalent with operation condition II)

Operation Condition: When the unit is performing its intended work at a given location, for instance any unit condition when risers are connected and production is in progress (i.e. processing of oil/gas) or when the production systems contain hydrocarbons under pressure.

Single failure: For units/operations in Consequence Class 3, the definition of single failure has no exceptions, and is to include incidents of fire, and associated effects of smoke and fire-fighting agents, flooding from sea, technical breakdown of systems and components including all electrical and mechanical parts. A single act of maloperation is defined as a single failure. For units/operations in Consequence Class 2, certain exceptions in relation to requirements for Class 3 will be allowed in the definition of single failure. Flood and fire are not to be considered beyond general requirements. Failure of non-moving components, e.g. pipes, manual valves, cables etc. may not to be considered if adequate reliability of a single component can be documented and the part is protected from mechanical damage. A single act of maloperation is defined as a single failure. For Class 3, 2 and 1, fracture of any anchor line is to be considered as a single failure.

Stand by: A unit condition when risers are connected and the production has been shut down.

Survival Condition: Any unit condition when risers have been disconnected due to weather condition etc.

A 2.0 Environmental Loads

2.1. I Regulation of 10 February, § I6, 6 For mobile offshore units with production plants, the positioning system is to be checked and the system dimensioned for double failure based on a storm with a return period of 10 years. The calculations are to be carried out for stand by and operation condition.

A 3.0 Mooring System Analysis

A 3. I Maximum deviation of mooring pattern. Guidelines, Section 1.4.2 Allowable deviation from previously approved anchor pattern is a change in angle for any line of maximum ±5 °.

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A 3.2 Operations in vicinity of other structures

3.2.1 Guidelines, Section Cl

For column stabilised units such as drilling, accommodation and crane vessels close to fixed installations and in accepted stand off position; anchor lines which are located within a critical sector and therefore endangers the fixed installation if broken, are to satisfy Consequence Class 2 requirements. The other lines are to satisfy Consequence Class 1 requirements. The critical zone is dependent on the distance to the installation and is defined by tangents from the extreme

Table A 3.1 Consequence Classes

Location in relation - not in the vicinity -in the vicinity but to other installations greater than X

Direction of anchor line in relation to - facing other installations

Condition

Operation 3 3

Stand by 2 2

Survival 1 1

Rules for Mobile Offshore Units , January 1996 Pt. 6 Ch. 2 App.A - Page 21

comers of the mobile units to circles of 50 m radius from the critical points on the fixed installation. To avoid a too sensitive optimisation, the safety factor of 2,0 (quasi-static analysis) will be evaluated in relation to the critical sector on a case to case basis.

3.2.2 Regulation of 10 February, §16, 7

For mobile offshore units with production plants the Table A 3. 1 contains consequence classes for various operational phases which are to be the basis for calculations of safety factors, see table A 3.3.

- in the vicinity but - in the vicinity but - in the viciniry but greater than X less than X less than X

facing away facing facing away

3 3 3

2 2 3

2 The distance X is given by the distance between the unit and the other installation being so great that the unit, following a possible multiple line failure (3 or more) will turn clear of the other installation with a smallest distance not less than 10 metres in the most adverse phases.

A 3.3 Quasi-static versus dynamic analysis

3.3.1 Guidelines, Section 2.4.2

For column stabilised units such as drilling, accommodation and crane vessels, line dynamics and low frequency motions are normally neglected. Wave drift forces are applied stat-

. ically.

Quasi-static calculations are generally applicable for water depths up to 200 m. The method can also be used for water depths between 200 m and 450 m when the same mooring system is approved for small water depths (70 m or less). This is unless particularly high stiffness and line tension is specified - in which case line dynamics becomes important.

3.3.2 Guidelines, Section 2.3.1

The wave frequency motion will be estimated maximum in 2 hours using a short crested sea with cos2 energy distribution. If long crested sea is used, the maximum motion can be reduced with 10 % .

3.3.3 Regulation of 10 February, §16, 14

For units with production plants the effect of dynamic forces on the mooring system is to be taken into consideration when calculating the total tension in the anchor lines for operation in water depths of more than 200 m.

A 3.4 Required safety factor

3.4.1 Column stabilised units such as drilling, modation and crane vessels. Regulation of 4 September 1987, §6 5.5

accom-

For water depths larger than 450 m, the NMD is to be especially contacted to discuss calculation method and safety

factors. The required safety factors for quasi-static analysis are given in Table A 3 .2

Table A 3.2 Safety factors Consequence Class Quasi-static Dynamic

Intact system 2,0

1 Transient motion 1,0 No formal safety

Temporary mooring 1,4 factors Special after single failure contact should Intact system 3,0 be made to

Transient motion 1,4 NMD on a case

3 to case basis Temporary mooring after single failure 2,0

Safety factors to be applied for lines within the critical sector upon a line failure

Transient motion 1,4 2 Temporary mooring

after single failure 2,0

For units with their own propulsion machinery operating far away from other installations and whose operating experience has been adequately documented, somewhat lower safety factors can be accepted in the extreme condition. It is a condition that consequences of possible anchor line break are sufficiently clarified and found acceptable.

3.4.2 Mobile offshore units with production plants. Regulation of 10 February 1994, § 16, 7

The required safety factors are given in Table A 3.3

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Rules for Mobile Offshore Units , January 1996 Page 22 - Pt.6 Ch.2 App.A

Table A3.3 Safety factors CONSEQUENCE CLASS

CONDITION Class 3 Class 2 Class 1

QS DY QS DY QS DY Equilibrium position, 3,0 2,5 2,7 2,3 2,0 1,65 Intact

Equilibrium after sin-2,0 1,65 1,8 1,5 1,4 1,25 gle failure

Transient motion after 1,4 1,2 1,4 1,2 1, 1 1,0 single failure

Equilibrium position 2,0 1,65 1,8 1,5 after double failure 1)

Transient motion after 1,4 1,2 1,4 1,2 double failure 1)

I) For a double failure the 10 year weather condition is used, which is defined as 10 year wind and waves and I year current.

If a unit is designed to shut down production and move from operation to standby condition (from Consequence Class 3 to 2) at a weather condition less than 10 year level, the requirement to mooring line safety factors for double failure in Consequence Class 3 will be based on that particular weather condition.

3.4.3 Corrosion allowance. Regulation of 10 February 1994, § 16, 9

NPD requirements for corrosion allowance in the splash zone is 0, 8 mm/year referred to the chain diameter, when no inspection is carried out. Corrosion allowance for the rest of the chain is also required, but the rate may be smaller. Corrosion allowance for bottom segments are to be evaluated on a case to case basis. The breaking strength of mooring line which forms the basis for the mooring calculations is to be adjusted for the reduction in strength due to corrosion, wear etc.

3.4.4 Line tension optimisation. Regulation of 10 February 1994, § 16, 11

If it is intended that the length of the anchor lines is to be adjustable depending on weather conditions (active anchoring system), this operation is to be tested and documented for relevant weather conditions. For such systems it is to be possible to plan the adjustment on board by means of simulation. Winches are normally to be capable of being operated without causing shut-down of other critical systems. The owner is to prepare a plan and instructions for such active winch operation, to be included in the operation manual.

3.4.5 Drift/collision. Regulation of 10 February 1994, §16, 3

Calculation of motion is to be made for the unit and the loading tanker - or for the unit against a fixed installation -with regard to drift/collision. The calculations are to be made for relevant weather conditions including the most severe single failure for the unit (anchor line damage, thruster stop or similar), or the most severe single failure for the tanker (main engine or thruster stop).

3.4.6 Riser. Regulation of 10 February 1994, §9, 6

A risk analysis in accordance with NMD' s regulation currently in force concerning risk analysis is to determine whether there is a need for a riser disconnect mechanism, and for what design accidental events such disconnection is necessary.

A 4.0 Thruster Assisted Mooring System No additional requirements.

A 5.0 Mooring Equipment

A 5.1 Fairleads

5.1.1 Regulation of 4 September 1987, §6, 4.1

The nominal tension in the fairlead as well as fairlead attachment to the unit is not to exceed 0,8 times the minimum specified yield tension, nevertheless a maximum of 80 % of the breaking strength of the material, with anchor chain and/or steel wire rope in the most unfavourable direction to the breaking point. By most unfavourable direction is understood unfavourable angle of incidence according to the operating manual ± 10 % in the most unfavourable direction, or in horizontal direction. Attachment of fairlead is to be designed and dimensioned in such a way that in case of a possible overload of the fairlead no hull damage occurs.


The fairlead is to have a mounting as free of maintenance as possible and have such a design that unnecessary break in steel wire rope and wear of chain are avoided.

5.1.3 Regulation of 4 September 1987, §6, 4.4 and 4.5

The fairleads are to_ have a minimum of 7 pockets and the groove width is not to exceed 1, 7 times the chain diameter. Other constructions which provide similar or better support for the chain may also be accepted

Steel wire rope/fairleads are to have a diameter and groove construction according to the recommendation of the steel rope manufacturer.

5.2 Winches


The anchor winch is to have at least two independent holding brake systems engaged at any time. The total static holding force is at least to correspond to the breaking strength of the relevant anchor chain/steel rope (first steel rope layer). The weaker brake is to be able to hold at least 50 % of the said breaking strength. In addition the winch is to have a dynamic brake system. The winch brakes are to be able to stop possible combined loads from anchor, anchor chains/steel rope and anchor handling unit during setting of the anchor at maximum speed (see 5.2.2 below). Nominal tension in the brake system (i.e. in all parts that are being exposed to loads during the braking) is during such loads not to exceed 0,85 times of the minimum specified yield point of the material though a maximum of 80 % of the breaking strength of the material. On winches without chain/steel rope stoppers the holding brakes are to work directly on the wildcat or wildcat axle.

5.2.2 Regulation of 4 September 1987, §6, 2.3 The characteristics of speed/load to which the dynamic brake system can be exposed during setting of anchor without damaging overheating occurring are to be documented and included in the operation manual.


The anchor winch with its associated parts is to be designed so as to permit releasing the entire chain/steel rope in a safe manner at a heeling angle corresponding to the maximum angle after assumed damage.


There is to be a system which efficiently prevents the possibilities of sparks resulting from lowering of anchor chain

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from igniting gas: This may be done by e.g. a permanent sprinkler or a system for slow lowering of the anchor chain.


Anchor winches are to be approved. Every winch is to be delivered with workshop certificate which includes the following information:

- breaking capacity during setting of the anchor, as specified in NMD 8, 2.3. Maximum setting speed as a function of anchor line tension (should be given in the form of a table or curve

- "break-in" capacity. Maximum break-in capacity as function of anchor line tension should be given in form of a table or curve.


Units which have to pretension the anchors itself are to have anchor winches which have a pulling force that after anchoring makes it possible to test the holding force of the anchor system statically up to the maximum load to be expected in accordance with calculations or which experience has proven necessary to secure anchor holding force under extreme weather conditions, nevertheless a minimum of 0,_35 - 0,4 times the breaking load of the anchor chain/steel wire rope.

5.3 Anchors

5.3.1 Regulation of 10 February 1994, §16, 10

For long term mooring systems used by production units, the anchors (including pile anchors) are to have a holding force at least 25 % greater than the maximum force to which the anchor may be exposed according to the anchoring calculations. The strength of the anchors and their holding force is to be documented on the basis of calculations or tests. The calculations are to take seabed conditions and _any previous tests into consideration.

5.4 Operation and instrumentation


It is to be possible to operate the anchor winches from well protected separate operating house by the winch itself. From the operating house it is to be possible to survey the anchor handling unit, chain/steel rope, anchor winch, anchor chain/steel rope stoppers to such an extent that safe laying out and heaving in can be performed. The house is to be located so that it will not be hit by the end of the anchor line in case of release of the whole length.


A satisfactory communication system is to be installed between the manned control room or bridge and the operating house by the winch. The sound level in the operating house is to be such that communication can take place without problem (recommended sound level not higher than 75 dB). It is not to be necessary to let go of operating handles in order to work the commuoication system (e.g. headphones with microphones).

Rules for Mobile Offshore Units , January 1996 Pt.6 Ch.2 App.A - Page 23

5.4.3 Regulation of 4 September 1987, §6, 7 .3

Instruments for reading anchor chain/steel rope velocity and of tension in anchor chain/steel rope, as well as the length laid out are to be installed on the operating panel locally for the winch. Control room or bridge where remote release can be performed are to have instrumentation for continuous reading of tension in anchor chain/steel rope. The instrumentation is to have the relevant danger limits marked. In addition NMD may require that the unit is equipped for registration of anchor line tension on magnetic tape.


In addition, other instruments, alarms which are essential to the safe and correct use of the system are to be installed at all locations from which the anchoring system can be operated.

6.0 Test and Inspection

6.1 Regulation of 4 September 1987, §7, 1.3 Setting of all anchors at top speed and tension in the anchor line for testing of the dynamic and static braking capacity of the winch.

6.2 Regulation of 4 September 1987, §7, 1.4 Testing of emergency release of all chain/steel rope stoppers and brakes under stress.

6.3 Regulation of 4 September 1987, §7, 1.7 Control of remainder braking force after failure in the power supply.

6.4 Regulation of 4 September 1987, §7, 1.8 Tension measurement in the anchor chain where it runs over wildcat and fairleads, can be required.

6.5 Regulation of 4 September 1987, §7, 1.9 Inspection of possible thruster capacity, power supply and steering system for such according to a testing program further agreed upon.

6.6 Regulation of 10 February 1994, §16, 8 For mobile offshore units with production plants a plan is to be drawn up for production control, shipyard control and follow-up control during operation of anchor lines and other components exposed to tension and/or wear, such as fairleads and wildcats in the anchoring system. The extent of the follow-up control will depend on the production and shipyard control carried out and component design and dimensioning and material used. The plan is to cover the life span of the components and is to, in addition to standard control and maintenance, take special account of the following:

- components exposed to repeated high loads and/or load variation (fatigue)

- components exposed to substantial wear, corrosion, ero-sion, fouling, etc.

- special seabed conditions - other operational condition calling for special control.

The plan is to be included in the operational manual/ maintenance system.

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