gl-structural design.pdf

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Rules for Classification and Construction

IV Industrial Services

6 Offshore Technology

4 Structural Design

Edition 2007


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The following Rules come into force on December 1st , 2007

Germanischer Lloyd Aktiengesellschaft

Head OfficeVorsetzen 35, 20459 Hamburg, Germany

Phone: +49 40 36149-0

Fax: +49 40 36149-200

[email protected]

www.gl-group.com

"General Terms and Conditions" of the respective latest edition will be applicable

(see Rules for Classification and Construction, I - Ship Technology, Part 0 - Classification and Surveys).

Reproduction by printing or photostatic means is only permissible with the consent of

Germanischer Lloyd Aktiengesellschaft.

Published by: Germanischer Lloyd Aktiengesellschaft, Hamburg

Printed by: Gebrüder Braasch GmbH, Hamburg


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Table of Contents

Section 1 Environmental Conditions

A. Basic Considerations .......................................................... ........................................................ 1- 1

B. Wind ........................................................... ........................................................... ..................... 1- 1

C. Sea Currents ................................................................. ......................................................... ..... 1- 2

D. Sea Waves .................................................................... ......................................................... ..... 1- 4

E. Sea Level ................................................................. ............................................................. ...... 1- 10

F. Climatic Conditions, Temperature and Marine Growth ............................................................. 1- 10

G. Sea Ice and Icebergs ................................................................ ................................................... 1- 11

H. Sea Bed .......................................................... ............................................................... ............. 1- 11

Section 2 Design Loads

A. Basic Considerations .......................................................... ........................................................ 2- 1

B. Environmental Loads ............................................................. .................................................... 2- 1

C. Permanent Loads ....................................................................... ................................................. 2- 9

D. Functional Loads .............................................................. .......................................................... 2- 10

E. Accidental Loads .................................................................... .................................................... 2- 10

F. Transportation and Installation Loads ........................................................................................ 2- 11

G. Earthquake Loads ............................................................... ........................................................ 2- 12

Section 3 Principles for Structural Design

A. General ............................................................ .............................................................. ............. 3- 1

B. Design Methods and Criteria ...................................................................... ................................ 3- 1

C. Loading Conditions ............................................................ ........................................................ 3- 3

D. Allowable Stress Design ........................................................................... ................................. 3- 5

E. Tubular Joint Design ..................................................................... ............................................. 3- 6

F. Effective Width of Plating ....................................................................... ................................... 3- 8

G. Buckling ................................................................ ................................................................ ..... 3- 8

H. Fatigue Strength ......................................................... ........................................................... ..... 3- 23

I. Dynamic Analysis ................................................................ ...................................................... 3- 32

Section 4 Steel Structures

A. Materials ............................................................... ................................................................ ..... 4- 1

B. Fabrication ................................................................. .......................................................... ...... 4- 12

C. Welding ........................................................... .............................................................. ............. 4- 19

D. Inspection and Testing .......................................................................................................... ..... 4- 42

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Section 5 Concrete Structures

A. General ................................................................... ........................................................... .......... 5- 1

B. Materials .............................................................. ............................................................. .......... 5- 2

C. Verification of Quality .................................................................... ............................................ 5- 3D. Placing and Curing the Concrete ................................................................... ............................. 5- 4

E. Formwork and Falsework ......................................................... .................................................. 5- 5

F. Reinforcement .................................................................. ......................................................... .. 5- 5

G. Principles of Calculation .................................................................... ......................................... 5- 7

H. Calculations for Pre-stressed Structures ................................................................. ..................... 5- 9

Section 6 Corrosion Protection

A. General ................................................................... ........................................................... .......... 6- 1

B. Material Selection .................................................................... ................................................... 6- 4

C. Coating Selection ..................................................................... ................................................... 6- 8D. Cathodic Protection ................................................................ .................................................... 6- 11

Section 7 Foundations

A. General ................................................................... ........................................................... .......... 7- 1

B. Geotechnical Investigations ............................................................................ ............................ 7- 1

C. General Design Considerations ......................................................................... .......................... 7- 1

D. Pile Foundations .......................................................... .............................................................. . 7- 2

E. Gravity Type Foundations ...................................................................... .................................... 7- 6

Section 8 Cranes and Crane Support Structures

A. Scope of Application ............................................................. ..................................................... 8- 1

B. Environmental Conditions ...................................................................... .................................... 8- 2

C. Cranes ....................................................... ................................................................ .................. 8- 3

D. Crane Support Structures ................................................................ ............................................ 8- 4

E. Tests and Examinations on Site ... ....................................................................... ........................ 8- 4

F. Documentation .............................................................. ............................................................. . 8- 4

Section 9 Helicopter Facilities

A. General ................................................................... ........................................................... .......... 9- 1

B. Structure of the Helicopter Deck ............................................................................ .................... 9- 2

C. Helicopter Deck Equipment .............................................................. .......................................... 9- 4

D. Fire Protection and Fire Extinguishing Systems ......................................................................... 9- 5

E. Aviation Fuel System ................................................................... ............................................... 9- 6

F. Requirements for Winching .................................................................. ...................................... 9- 8

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Section 10 Marine Operations

A. General ............................................................ ............................................................. .............. 10- 1

B. Standards and Guidelines ............................................................... ............................................ 10- 1

C. Loadout ............................................................. ............................................................ ............. 10- 1D. Transportation .............................................................. ........................................................ ...... 10- 3

E. Lifting Operations ............................................................. ......................................................... 10- 4

F. Installation Offshore ...................................................................... ............................................. 10- 11

G. Other Marine Operations for Marine Warranty Survey .......................................................... .... 10- 13

Annex A Welding and Test Piece Positions

Annex B List of Standards, Codes, etc. Quoted

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Index

A Accidental loads ................................................................... ......................................................... ................ 2-10, 3-4

Admixtures ................................................................ .............................................................. ............................... 5-2

Aggregates ................................................................. ............................................................. ............................... 5-2

Air gap ........................................................... ................................................................. ....................................... 2-7

Allowable stress design ................................................................ .............................................................. ............ 3-5

Anchor handling ........................................................................... ............................................................ .......... 10-13

Anode materials .................................................................... ........................................................ .............. 6-14, 6-24

Anodes ................................................................. .......................................................... ............................. 6-15, 6-25

Aviation fuel system ................................................................ .................................................................. ............ 9-6

B

Bearing capacity ................................................................. .............................................................. ..................... 7-8

Buckling .......................................................................................................................................................... 3-8, 5-8

bar elements ................................................................... ............................................................. ..................... 3-9

lateral .............................................................. ................................................................... ............................. 3-18

plane and curved plate panels ............................................................... ....................................................... ... 3-13

shell elements ......... ...................................................................... ...................................................... ............ 3-21

torsional ............................................................. ................................................................. ............................ 3-20

C

Cable laying .... ...................................................................... .......................................................... ................... 10-14

Cargo barge ...................................................... ............................................................... ..................................... 10-3

Cargo handling ........................................................ ................................................................. .............................. 8-3

Catalogue of details ..................................................................... .............................................................. ........... 3-34

Categories of structural members ..................................................................... ...................................................... 4-1

Cathodic protection ........................................ .................................................................. .................................... 6-11anodes ........................................................... ............................................................. ..................................... 6-14

calculation and design procedure .............................................................. ..................................................... 6-17

design current densities ............................................................. ........................................................... .......... 6-11

galvanic anodes system ............................................................. ........................................................... .......... 6-12

impressed current system ............................ ................................................................. ................................... 6-23

location of anodes .......................................................... ........................................................... ............ 6-22, 6-25

safety ....................................................................... ......................................................... ..................... 6-23, 6-27

systems ................................................................ ............................................................... ............................ 6-12

Cements .......................................................... ................................................................ ....................................... 5-2

Certification of cranes .............................................................. ............................................................... ........ 8-1, 8-5

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Classification of cranes .............................................................. .......................................................... ........... 8-1, 8-5

Climatic conditions ........................................................ ............................................................... ....................... 1-10

Coating selection .. ...................................................................... .......................................................... ................. 6-8

Collision .............................................................................................................................................................. 2-10Concrete ................................................................................................................................................................ 5-2

curing ............................................................................................................................................................... 5-4

placing and working ....................................................... ................................................................... ............... 5-4

pre-stressed structures ............................................................ ........................................................ ........... 5-6, 5-9

principles of calculation ........................................................................... ........................................................ 5-7

quality .............................................................................................................................................................. 5-3

reinforcement ................................................................................................................................................... 5-5

standards and safety concepts ........................................................... ........................................................ ....... 5-1

structures .......................................................................................................................................................... 5-1

water ................................................................................................................................................................ 5-2

Concreting under water ............................................... .................................................................. ......................... 5-5

Conductor shielding ...................................................................................................... ......................................... 1-4

Conveyance of persons ..................................................... ............................................................. ........................ 8-3

Corrosion ............................................................................................................................................................... 3-1

Corrosion protection ....................................................... .............................................................. ......................... 6-1

cathodic protection ...................................................................................... ................................................... 6-11

coating selection ..................................................................... .................................................................. ....... 6-8

coatings and coating breakdown factors .......................................................... ................................................ 6-9

high strength steel and corrosion resistant alloys (CRAs) ....................................................... ......................... 6-6

material selection ... ....................................................................... ............................................................ ....... 6-4

metal sheathing .... ................................................................. ........................................................... ................ 6-5

structural design ........................................ ................................................................. ...................................... 6-4

Crane

davits for survival craft/rescue boats ........................................................................... ..................................... 8-3

duties ................................................................................................................................................................ 8-1manual .............................................................................................................................................................. 8-4

sea lashing systems ...................................................................... ............................................................. ....... 8-3

structures .......................................................................................................................................................... 8-1

support structures ............................................... ............................................................. ................................. 8-4

Crane vessels ......... ....................................................................... ....................................................... .............. 10-10

Cranes ....................................................... ................................................................. .......................... 8-1, 8-3, 10-10

Cumulative damage ratio ........................................................ ............................................................... .............. 3-28

Current blockage ............................................................... ........................................................... ......................... 1-3

D

Derrick systems .... ....................................................................... ......................................................... ................. 8-3

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Design criteria ............................. ...................................................................... ..................................................... 3-2

Detail categories ........................................................ .............................................................. ............................ 3-27

Documentation ........................ ...................................................................... .......................................... 5-1, 8-4, 9-2

Ductility level earthquake .................................................................. ....................................................... ........... 2-12Dye penetrant inspection .............................................................. ............................................................. ........... 4-52

Dynamic analysis ..................................................................... ......................................................... ................... 3-32

Dynamic positioning ................................................................... ............................................................ ........... 10-14

E

Earthquake

analysis .............................................................. ................................................................. ............................ 3-33

ductility level .............................................. ................................................................ ...................................... 3-5

loads ........................................................... ............................................................... ............................ 2-12, 3-32

Effective width of plating .......................................................................... .................................................... 3-8, 3-17

Environmental design conditions ....................................................................... ............................................. 1-1, 8-2

Environmental loads ............................................................... .......................................................... ..................... 2-1

Existing cranes .................. ....................................................................... .......................................................... .... 8-4

Extreme environmental loads ................................................................... ........................................................... ... 3-4

F

Fabrication ............................................................. ................................................................. ............................. 4-12

cold and hot forming ............................................................... ............................................................. .......... 4-14

deviations, defects and repair work ............................................................ .................................................... 4-13

fitting and assembly ........................ ................................................................. ............................................... 4-14

heat treatment ........... ....................................................................... ............................................................ ... 4-15

identification and storage of materials ............................................................................. ............................... 4-13

quality assurance and control ........................................................ ....................................................... .......... 4-12

surface and edge preparation ....................................................... ......................................................... .......... 4-13

tolerances ............................................................................................... ...................................................... .. 4-15

Failure Mode Analysis (FMA) ............................................................................. .................................................. 8-3Falsework .................................................................... ............................................................. .............................. 5-5

Fatigue assessment procedure ........................................................................ .................................................... .. 3-25

Fatigue strength ............................................................... ................................................................. .................... 3-23

Fire extinguishing ................................................................. ............................................................ ..................... 9-6

Fire protection ........................................................ ................................................................. ............................... 9-5

Formwork ......................................................... ............................................................... ....................................... 5-5

Foundations ................................................................ ............................................................. ............................... 7-1

design considerations .............................................................. .............................................................. ........... 7-1

geotechnical investigations ........................................... ................................................................. ................... 7-1

gravity type ............................................................. ............................................................ .............................. 7-6

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pile type ..................................................................... ............................................................. ......................... 7-2

Fuel transfer system ........................................................ .............................................................. ......................... 9-7

Functional loads .............................................................................................. ..................................................... 2-10

G

Geotechnical investigations ............................. ..................................................................... ................................. 7-1

Global design .............................................................................................. ................................................... ...... 3-32

Global safety factors ................................................................ .............................................................. ................ 5-8

Gravity type foundations ............................................................ ........................................................... ................ 7-6

dynamic behaviour ............................................ ............................................................ ................................. 7-11

hydraulic instability ............................................................................................... ......................................... 7-11

installation .................................................................................................................................................... 10-12

installation and removal ....................................................... ............................................................ .............. 7-11

stability ............................................................................................................................................................ 7-7

static deformations ............................................... .................................................................... ...................... 7-11

Grout .......................................................................................... ......................................................... ............ 5-3, 7-6

Gust wind ............................................................. ................................................................ .................................. 1-2

H

Helicopter data ....................................................................... .............................................................. ................. 9-1

Helicopter deck

arrangement ..................................................................................................................................................... 9-1

equipment ........................................................ ............................................................... .................................. 9-4

structure ........................................................................................................................................................... 9-2

Helicopter facilities ......................................................... ............................................................. ......................... 9-1

Hot spot stress ................................................................. ............................................................ ........................ 3-26

I

Ice accretion ................................................................... ........................................................... ..................... 2-8, 8-2

Icebergs ............................................................................................................................................................... 1-11

Impact .................................................................................................................................................................. 2-10

Inspection ............................................................................................................................................................ 4-42

methods and extent ............................................................. ............................................................. .............. 4-44

personnel, supervisors ................................................................ ...................................................... .............. 4-43

standards, codes ......... ............................................................ ......................................................... ............... 4-42

testing equipment ................................................. ................................................................... ....................... 4-43

Installation loads ............................................................................................. ..................................................... 2-11

Installation offshore .................................................................. ........................................................... .............. 10-11

J

Jacket installation ................................................................. ............................................................... .............. 10-11

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JONSWAP spectrum ................................................................ .................................................................. ........... 1-6

L

Launch ............................................................ ............................................................... .................................... 10-11

Layout plan for loads ............................................................ ........................................................... .................... 2-10

Lift points .................................................................... ............................................................ ............................. 10-6

Lifting equipment ............................................................................ ......................................................... .......... 10-10

Lifting operations .......................................... .................................................................. ..................................... 10-4

Limit damage ratio ............................................................ ................................................................ ................... 3-28

Limit states ...................................................................... ............................................................... ................. 3-2, 5-8

Loading conditions .......................................................................... .......................................................... ............. 3-3

Loadout ............................................................ .............................................................. ...................................... 10-1

Loads ................................................................................................ ......................................................... ............ 2-1

Local design ...................................... ................................................................. .................................................. 3-32

Local imperfections ................................................................... .............................................................. ............ 3-21

M

Magnetic particle inspection ................................................................... .......................................................... ... 4-51

Marine growth .. ................................................................. ........................................................... ......... 1-4, 1-10, 2-9

Marine operations .............................................................. ............................................................ ............... 3-5, 10-1

Marine Warranty Survey ..................................................................... .......................................... 10-3, 10-10, 10-13

Materials .............................................................. .................................................................. ......................... 4-1, 5-2

Mating operation .......................................... ................................................................... ................................... 10-13

Misalignment of butt joints ........................................................................... .................................................... ... 3-22

Modelling of the structure ...................................................................... ............................................................. ... 3-3

Mooring system ........................................................... ............................................................ .............................. 2-6

N

Natural seas .............................................................. ................................................................ .............................. 1-8

Nominal stress ............................................................... .......................................................... ............................. 3-27

O

Operating loads ........... ....................................................................... ....................................................... ............. 3-4

Out-of-roundness ............................................................. ........................................................ ............................ 3-21

Overlapping joints .................................................... ................................................................ .............................. 3-7

Overturning ............................................................... .............................................................. ............................... 7-7

P

Padeyes ............................................................. ............................................................. ...................................... 10-6

Permanent loads ....................................................................... ....................................................... ................ 2-9, 3-3

Pierson-Moskowitz spectrum ............................... ................................................................. ................................. 1-6

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Pile

axially loaded piles .......................................................... ................................................................. ............... 7-3

foundations ............................................................ .................................................................. ......................... 7-2

grouted pile to structure connection ................................................................ ................................................. 7-6installation .................................................................................................................................................... 10-12

laterally loaded piles .......................................................... ............................................................... ............... 7-5

pile design ............................................................... ................................................................ ......................... 7-2

pile groups ............................................................ .................................................................. ......................... 7-5

pile structure design ............................................................ .............................................................. ............... 7-5

Pipe laying .................................................................... .............................................................. ....................... 10-14

Plastic design .................................................................. ............................................................. .......................... 3-3

Pre-stressing steel ............................................................ ........................................................ ......... 5-3, 5-4, 5-6, 5-9

R

Radiographic inspection ...................................................................... ......................................................... ....... 4-45

Reinforcing steel ................................................................... .............................................................. ............ 5-3, 5-4

S

Safety factors ............................................................... ........................................................... ...... 3-5, 3-25, 8-3, 10-6

Safety format ............................................................. .................................................................. .......................... 3-2

Scour ......................................................... ................................................................. ............................................ 7-2

Sea

bed ....................................................... ............................................................ .............................................. 1-11

current loads ................................................................................................. .................................................... 2-2

currents ............................................................................................................................................................ 1-2

ice ................................................................................................................................................................... 1-11

level ............................................................................................................................................................... 1-10

loads ................................................................................................................................................................. 2-9

Seafastening ......................................................................................................................................................... 10-3

Seismic activities ............................................................. .......................................................... ................. 1-11, 2-12Shackles ............................................................................................................................................................. 10-10

Shear stresses ..................................................................... ........................................................ ................... 3-6, 3-27

Shielding ................................................................................................................................................................ 2-2

Skirts ...................................................................................................................................................................... 7-6

Slew rings ......... ....................................................................... .............................................................. ................ 8-4

S-N curves ........................ ....................................................................... ...................................................... ...... 3-28

Snow accretion .............................................................. ............................................................... ......................... 2-8

Spectral moments ........ ....................................................................... ............................................................ ....... 1-6

Spreader frames ............................................................. ............................................................... ....................... 10-6

Steel

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chemical composition and suitability for welding .................................................................. .......................... 4-2

forgings and castings ............................................................... ............................................................ ........... 4-12

identification and marking ........... .............................................................................. ..................................... 4-11

impact energy after strain ageing .................................................................... ................................................ 4-11inspections .................................................................... ............................................................... ................... 4-11

manufacturing procedures ....................................................... .............................................................. ........... 4-2

mechanical properties .......................... ...................................................................... ....................................... 4-3

non-destructive testing ..................................................................... ............................................................. .. 4-11

selection criteria .................................................................... ............................................................... ............ 4-2

stress relieving treatment .............................................................. .................................................................. .. 4-3

supply condition and heat treatment ..................................................................... ............................................ 4-2

Strength level earthquake ....................................................................... ........................................................... ... 2-12

Structural analysis ............................ ....................................................................... ............................................. 3-32

Structural design ............................................................ .................................................................. ...................... 3-1

Structural members ...... ....................................................................... ............................................................... .... 4-1

Sustained wind ....................... ....................................................................... ..................................................... .... 1-2

T

Temperature ............................................................... .............................................................. ............................ 1-10

Test reports ........................................................ ............................................................. ..................................... 4-52

Tests ................................................................ ............................................................... ........................................ 8-4

Topside installation ............................................ ..................................................................... ........................... 10-13

Towing ............................................................ ............................................................... ...................................... 10-3

Transportation ......................................................... ................................................................. ............................ 10-3

Transportation loads ........................................................... ........................................................... ............... 2-11, 3-5

Trunnions ......... ...................................................................... ...................................................... ............... 10-8, 10-9

Tsunamis ......................................................... ............................................................... ...................................... 1-10

Tubular joints ....................................................................... .......................................................... ............... 3-6, 4-35

U Ultimate limit states .............................................................. .......................................................... ................ 3-2, 5-7

Ultrasonic testing ............................................................. ................................................................ .................... 4-46

Upending ........................................................ ................................................................ .................................... 10-12

Utilization factor ................................................................. ............................................................. .................... 6-16

V

Vibratory loads ......................................................... ............................................................... .............................. 2-8

Vortex shedding .................................. ................................................................. .................................................. 2-8

W

Wave

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impact ............................................................. ............................................................... ........................... 2-7, 3-1

loads ................................................................................................................................................................. 2-3

parameters ................................................................. .............................................................. ......................... 1-8

period ................................................... ............................................................ ................................................ 1-7 pressure .............................................................. ............................................................ .................................. 1-9

Wave design .................. ................................................................. ...................................................... ................. 1-5

Waves .................................................................................................................................................................... 1-4

Wear ...................................................................................................................................................................... 3-1

Weibull distribution .............................................................................. ......................................................... ...... 3-29

Welding ............................................................................................................................................................... 4-19

calculation of welded joints .................................................................... ....................................................... 4-35

consumables ................................................................................................................................................... 4-19

design of weld connections ............................................................. ......................................................... ...... 4-29

performance ........................................................ ............................................................ ............................... 4-36

preheating and heat input ............................................................................. .................................................. 4-37

procedure specification and qualification ...................................................................... ................................ 4-20

qualification of welders ........................... ...................................................................... ................................. 4-27

repairs ............................................................................................................................................................ 4-41

underwater .............................................................. .............................................................. ................ 4-25, 4-42

weather protection ........................................................................................ .................................................. 4-37

workmanship .................................................................................................................................................. 4-36

Winching ............................................................................................................................................................... 9-8

Wind

direction ........................................................................................................................................................... 1-2

loads ................................................................................................................................................................. 2-1

properties .......................................................... ............................................................. .................................. 1-1

speed ................................................................................................................................................................ 1-2

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Section 1

Environmental Conditions

A. Basic Considerations

Environmental conditions give rise to loads imposedon the structure by natural phenomena including wind,current, waves, earthquake, snow, ice, and earthmovement. Environmental loads also include thevariation in hydrostatic pressure and buoyancy onstructural components caused by changes in the waterlevel due to waves and tides. Environmental loadsshall be anticipated from any direction unless knowl-edge of specific conditions makes a different assump-

tion more reasonable.

The unit shall be designed for the appropriate loadingconditions that will produce the most severe effects onthe structure. Environmental loads, with the exceptionof earthquake load, shall be combined in a mannerconsistent with the probability of their simultaneousoccurrence.

1. Determination

Pertinent meteorological and oceanographic condi-tions affecting a unit’s operating site shall be defined.The corresponding environmental design data should

be prepared to develop the descriptions of normal andextreme environmental conditions. Environmental con-ditions may be determined by wind, sea currents, seawaves, sea level, climatic conditions, temperature andfouling, sea ice, sea bed conditions, and other influ-ences as applicable.

2. Normal environmental design conditions

Normal environmental conditions are those conditionsthat are expected to occur frequently during the life ofthe unit. Normal environmental conditions, important

both during the construction and the service life of aunit, consider the most adverse possible effect duringthe installation and the operation of the unit.

3. Extreme environmental design conditions

3.1 Extreme environmental conditions occurrarely during the life of the unit. Extreme design con-ditions are important in formulating design loads forthe unit when out of operation, while resting on its sitewith all equipment secured in a seaworthy condition.Design loads may be specified on the basis of statisti-cal observations if available. Probabilistic estimations,if not specified in the Rules, are to be approved byGermanischer Lloyd.

3.2 Fixed marine installations shall be designedto meet limiting environmental conditions valid for thedesignated operating area.

3.3 Mobile units shall be designed and operatedto meet limiting environmental conditions as specifiedin the operating manual.

4. Estimation of design parameters

Estimation of design parameters on the basis of envi-ronmental design conditions, e.g., estimation of wave

particle velocity or acceleration on the basis of waveheight and period, if not carried out as specified in the

Rules, is to be approved by Germanischer Lloyd.

5. Superposition of parameters

Superposition of different environmental design pa-rameters is to be based on physical relations or onstochastic correlations between these parameters or

between the environmental phenomena to which they belong.

6. Simplifying assumptions

Simplifying assumptions in these Rules are made inaccordance with the specification of related safety

factors for structural design. If estimates of environ-mental design parameters are based on assumptionsthat are more appropriate for the design case thanthose specified in the Rules, a reduction of the safetyfactor may be approved by Germanischer Lloyd, seealso Section 3, D.

B. Wind

Although wind loads are dynamic in nature, offshorestructures respond to them in a nearly static fashion.However, a dynamic analysis of the unit/installation is

indicated when the wind field contains energy at fre-quencies near the natural frequencies of the unit/instal-lation (be it fixed or moored to the ocean bottom).Sustained wind speeds should be used to determineglobal loads acting on the unit/installation, and gustspeeds should be used for the design of individualstructural elements.

1. Wind properties

Wind speed changes with both time and height abovesea level. Therefore, the averaging time and heightshall be specified. Common reference times are oneminute, ten minutes, or one hour. The common refer-

ence height is ten (10) meters. Wind forces should becomputed using the one (1) minute mean wind speed,and appropriate formulas and coefficients may bederived from applicable wind tunnel tests.

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2. Wind force direction

Wind forces shall be considered from any directionrelative to the structure.

3. Mean wind speed

The mean wind speed at the reference height of 10 m,averaged over time t , may be estimated by the formula

u(t) = Ct u (tr )

where

u(t) = mean wind speed at a reference height of10 m [m/s]

Ct = wind speed averaging time factor

= [1 - 0,047 ln (t / tr )]

u(tr ) = reference wind speed [m/s]

t = averaging time [minutes]

tr = reference time

= 10 [minutes]

Wind speed averaging time factors Ct for selected

averaging times t are given in Table 1.1:

Table 1.1 Wind speed averaging time factors Ct

Averaging time t Ct

3 seconds 1,249

5 seconds 1,225

15 seconds 1,173

1 minute 1,108

10 minutes 1,000

1 hour 0,916

4. Minimum wind speed for stability calcula-

tions of mobile offshore units

For the requirements for stability calculations of mobile offshore units please refer to Chapter 2, Section 7,B.4.

5. Sustained wind speed

The greatest one (1) minute mean wind speed, expected to occur over a return period of 100 years andrelated to a reference level of 10 m above sea level, isgenerally referred to as the sustained wind speed, uS.This sustained wind speed is to be used for the deter-mination of global loads.

6. Gust wind speedThe greatest three (3) second mean wind speed, ex-

pected to occur over a return period of 100 years, is

referred to as the gust wind speed, uG. This gust wind

speed is to be used for the determination of local

loads. The gust wind speed is related to the sustainedwind speed as follows:

uG = 1,137 uS

The gust wind component may be considered as a zeromean random wind component which, when superim-

posed on the constant, average wind component yieldsthe short-term wind speed.

The wind in a 3 second gust is appropriate to deter-mine the maximum static wind load on individualmembers; 5 second gusts are appropriate for maxi-mum total loads on structures whose maximum hori-zontal dimension is less than 50 m; and 15 secondgusts are appropriate for the maximum total staticwind load on larger structures. The one minute sus-tained wind is appropriate for total static superstruc-

ture wind loads associated with maximum wave forcesfor structures that respond dynamically to wind excita-tion, but which do not require a full dynamic windanalysis. For structures with negligible dynamic re-sponse to winds, the one-hour sustained wind is ap-

propriate for total static superstructure wind forcesassociated with maximum wave forces.

7. Statistics of wind speed

The Weibull distribution may be used to describe thestatistical behavior of the average wind speed u(z, t),referred to a fixed height and an averaging time, as

follows:

Pr (u) = 1 – exp [ – (u / u0)c]

where

Pr (u) = cumulative probability of u

u = u(z, t)

= wind speed [m/s]

u 0 = Weibull scale parameter

c = Weibull slope parameter

Gust wind speed may be assumed to follow theWeibull distribution.

8. Wind direction

The wind direction shall generally be assumed to beidentical with the dominant direction of wave propa-gation.

C. Sea Currents

1. Sea currents are characterized as:

– near-surface currents, i.e., wind/wave generatedcurrents, see 2.

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– sub-surface currents, i.e., tidal currents and ther-mosaline currents, see 3.

– near-shore currents, i.e., wave induced surfcurrents, see 4.

2. Near-surface currents

For near-surface currents, the design velocity may beestimated as follows:

uw(z) = k(z) uS

where

uw(z) = near surface current velocity [m/s]

k(z) = factor depending linearly on the vertical co-ordinate z

= 0,01 for z = 0 m

= 0 for z ≥ –15 m

= to be obtained by linear interpolation

for 0 > z > –15 m

z = vertical coordinate axis above mean sea level[m]

uS = sustained wind speed used for design [m/s]

= one (1) minute mean at z = 10 m as definedunder B.5. [m/s]

3. Sub-surface currents

For sub-surface currents the design velocity is to be based on the current velocity at sea level (z = 0),which shall be based on observed values provided bycompetent institutions. The data are subject to ap-

proval by Germanischer Lloyd. The vertical velocity

distribution for 0 ≥ z ≥ – d may be determined as fol-lows:

uSS(z) = [(z + d) / d] 1/7 uS0(0)

where

uSS(z) = sub-surface current velocity [m/s]

d = water depth [m]

z = vertical coordinate axis [m]

uS0 = current velocity at sea level [m/s]

Linear superposition of uSS(z) and uw(z) is applicable.

4. Near-shore currents

For near-shore currents, which have a direction paral-lel to the shore line, the design velocity at the locationof breaking waves may be estimated as follows:

unS

= 2 sB

g H⋅

where

unS = near-shore current velocity [m/s]

s = beach slope

= tan α

α = inclination of beach

g = 9,81 m/s2

HB = breaking wave height [m]

= b / [1 / dB + a / (g TB2)]

a = 44 [1 – exp(–19 s)]

b = 1,6 / [1 + exp(–19 s)]

dB = water depth at the location of the breakingwave [m]

TB = period of the breaking wave [s]

For very small beach slopes, HB may be estimatedfrom

HB = 0,8 dB [m]

5. Current blockage

For structures that are more or less transparent, thecurrent velocity in the vicinity of a structure is reduced

by blockage. The presence of the structure causes theincident flow to diverge, with some of the incidentflow going around the structure rather than through it.

The degree of blockage depends on the kind of struc-ture. For dense fixed space frame structures it will belarge, while for some kinds of transparent floaters itwill be small. Approximate current blockage factorsfor typical jacket-type structures are given in Table1.2.

For structures with other configurations, a currentfactor can be calculated according to C.2.3.1 b4 ofAPI RP 2A-WSD. Factors less than 0,7 should not beused unless empirical evidence supports them.

Table 1.2 Current blockage factors

Number of

legs

Current

heading

Blockage

factor

3 All 0,90

4 End-on 0,80

Diagonal 0,85

Broadside 0,80

6 End-on 0,75

Diagonal 0,85

Broadside 0,80

8 End-on 0,70

Diagonal 0,85

Broadside 0,80

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6. Combined wave and current kinematics

Wave kinematics, adjusted for directional spreadingand irregularity, should be combined vectorially withthe current profile, adjusted for blockage. As the cur-

rent profile is specified only to storm mean waterlevel, it shall be stretched or compressed to the localwave elevation. For slab current profiles, simple verti-cal extension of the current profile from storm meanwater level to the wave elevation is acceptable. Forother current profiles, linear stretching is acceptable.In linear stretching, the current at a point with eleva-tion z, above which the wave surface elevation is η, isobtained from the specified current profile at elevationz’. The elevations z and z’ are related as follows:

(z z ') d(z z ')

(d )

+ ⋅+ =

+ η

where d is the storm water depth. Points z and η are both positive above and negative below storm meanwater level.

7. Marine growth

All structural members, conductors, risers, and appur-tenances shall be increased in cross-sectional area toaccount for marine growth thickness. Also, elementswith circular cross-sections shall be classified as eithersmooth or rough, depending on the amount of marinegrowth expected to have accumulated at the time of

loading.

8. Conductor shielding

Depending on the configuration of the structure andthe number of well conductors, forces caused by asteady current with negligible waves may be reducedif the conductors are closely spaced. A shielding fac-tor, to be applied to the current force for the conductorarray, can be estimated according to Table 1.3, inwhich S is the center-to-center spacing of the conduc-tors in the current direction and D is the diameter ofthe conductors, including marine growth:

Table 1.3 Shielding factors

S/D Shielding factor

1,75 0,450

2,00 0,500

2,50 0,625

3,00 0,500

3,50 0,875

4,00 and larger 1,000

For other values of S/D, linear interpolation may beapplied.

D. Sea Waves

1. Design criteria

The design environmental criteria shall be developed

from the environmental information. These criteriamay be based on risk analysis where prior experienceis limited. For new and relocated structures that aremanned during the design event, or where the loss ofor severe damage to the structure could result in se-vere damage, a 100-year recurrence interval should beused for oceanographic design criteria. Considerationmay be given to reduced design requirements for thedesign or relocation of other structures that are un-manned or evacuated during the design event, thathave either a shorter design life than the typical 20years, or where the loss of or severe damage to thestructure would not result in a high consequence offailure.

2. Design conditions

Two alternative methods may be used to specify de-sign conditions for sea waves, namely, the determinis-tic method based on the use of an equivalent designwave or the stochastic method based on the applica-tion of wave spectra.

The deterministic method describes the seaway asregular periodic waves, characterized by wave period(or wave length), wave height, wave direction, and

possible shape parameters.

The deterministic wave parameters may be based onstatistical methods.

3. Two-dimensional wave kinematics of regu-lar waves

Analytical or numerical wave theories may describethe wave kinematics. For a specified wave period T,wave height h, and storm water depth d, two-dimensional regular wave kinematics can be calcu-lated using appropriate wave theory. The followingtheories are mentioned:

– linear (Airy) wave theory, where a sine functiondescribes the wave profile

– stokes 5th order wave theory, appropriate forhigh waves

– stream function theory, where wave kinematicsare accurately described over a broad range ofwater depths

– solitary wave theory for shallow waters

Other wave theories, such as Chappelear, may beused, provided an adequate order of the solution isselected.

In the (H/gT2, d/gT2) plane, regions of applicability ofvarious theories are shown in Fig. 1.1 as functions ofwave period T, wave height H, and storm water depthd (g = acceleration of gravity).

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Fig. 1.1 Regions of applicability of wave theories

3.1 Equivalent design wave

Wave loads on a platform are dynamic in nature. Formost design water depths presently encountered, theseloads may be adequately represented by their staticequivalents. For deeper waters or where platformstend to be more flexible, a load analysis involving thedynamic action of the structure is required.

3.2 Design wave period and design waveheight

The design wave period TD, is associated with thehighest wave. In any given storm, there is only onemaximum wave, and the associated period is the most

probable period of the maximum wave. For this reasonit is common to give a range of wave periods for TD that are independent of the peak period T p. The peak

period is the value associated with the “peak” of the

wave spectrum. A range of design wave periods isspecified as follows:

1/3 D 1/33,5 H T 4,5 H⋅ < < ⋅

where

TD = design wave period [s]

H1/3 = significant wave height [m]

The design wave period TD shall not exceed 25 s.The design wave height HD, may be estimated as fol-

lows:

D 1/3D

3600H 0,5 ln H

T

⎛ ⎞= ⋅ ⎜ ⎟

⎝ ⎠

This method generally applies to deep water waves,i.e. waves of periods T that satisfy the following con-dition:

( )2d / g T 0,06⋅ >

where

d = water depth [m]

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Design wave parameters for wind-induced transitionalwater waves, i.e. water waves of periods T that satisfythe condition

( )20,002 d / g T 0,06< ⋅ <

may be defined from information on extreme windspeed and fetch, using relevant theories subject toapproval by Germanischer Lloyd.

4. Short-term wave conditions

Short-term stationary natural sea states may be de-scribed by a wave spectrum, i.e. by the power spectraldensity function of the vertical sea surface elevation.Short-term wave conditions are defined as seawayswhose representative spectrum does not change for a

brief but not closely specified duration. Wave spectramay be given in tabular form, as measured data, or inanalytic form.

The two parameter modified Pierson-Moskowitz spec-trum and the JONSWAP spectrum are most frequentlyapplied. Both spectra describe sea conditions relevantfor the most severe sea states. Application of otherwave spectrum formulations is to be approved byGermanischer Lloyd.

4.1 The Pierson-Moskowitz spectrum

The principal natural sea state parameters are as fol-lows:

– The significant wave height H1/3 is defined as

the average of the one-third highest waveheights in a record of stationary sea surface ele-vations.

– The characteristic wave period T1 is defined asthe average observed wave period in a record ofstationary sea surface elevations.

The visually observed wave height Hv and the visuallyobserved wave period Tv may be considered approxi-mately equal to H1/3 and T1, respectively.

The following expression describes the wave energyspectral density:

21/ 3

4 5 4 41 1

42 41/ 3 p p

5

H 691S( ) 173 expT T

H 50,313 exp

4

⎡ ⎤ω = −⎢ ⎥⋅ω ⋅ω⎢ ⎥⎣ ⎦

⎡ ⎤⋅ω ω⎛ ⎞⎢ ⎥= − ⎜ ⎟

ω⎢ ⎥ω ⎝ ⎠⎣ ⎦

where

S(ω) = spectral density [m2s]

ω = circular wave frequency [s1]

= 2π/T

T = wave period [s]

ω p = circular spectral peak frequency [s1]

= 2π/T p

T p = modal period [s]

= period at which the spectrum is a maximum

4.2 The JONSWAP spectrum

The JONSWAP spectrum is also frequently applied.In addition to the significant wave height H1/3 parame-ters are needed to describe the spectral shape, namely,the peakedness parameter γ and the spectral shape

parameter σ.

The following expression describes the wave energyspectral density:

( )

21

2

p2 2

p

0,191 T 12 exp

21/ 3

4 5 4 41 1

exp42 42

1/3 p p

5

H 944S( ) 155 exp

T T

H 50,205 exp

4

⎡ ⎤ω −−⎢ ⎥

σ⎣ ⎦

⎡ ⎤ω−ω⎢ ⎥−⎢ ⎥

σ ω⎢ ⎥⎣ ⎦

⎡ ⎤ω = − γ⎢ ⎥

ω ω⎢ ⎥⎣ ⎦

⎡ ⎤ω ω⎛ ⎞⎢ ⎥= − γ⎜ ⎟

ω⎢ ⎥ω ⎝ ⎠⎣ ⎦

where

S(ω) = spectral density [m2s]

γ = peakedness parameter

= 3,30 for a mean spectrum

σ = spectral shape parameter

= 0,07 if ω ≤ 5,24/T1

= 0,09 if ω > 5,24/T1

T1 = average observed or mean period [s]

4.3 Spectral moments

The spectral moments, mn, of order n are defined as

follows:

nn 0m S( ) d

∞= ω ω ω∫ for n = 0, 1, 2, …

where

S(ω) = wave energy spectral density [m2

s]ω = circular wave frequency [s1]

= 2π/T

= wave period [s]

The following quantities may be defined in terms ofthe spectral moments:

– Significant wave height:

03/1 mH = [m]

– Average observed wave period (center of grav-

ity of the wave spectrum):

1

01

m

m2T π= [s]

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– Zero-crossing period (average period betweensuccessive crossings):

2

00

m

m2T π= [s]

– Crest period (average period between successivewave crests):

2C

4

mT 2

m= π [s]

– Significant wave slope:

2

0

m2s

g m

γ= ⋅ ≈

π ⋅ π

where

γ = peakedness parameter

= 3,30 for a mean spectrum

4.4 Spectral density and moments in terms ofwave frequency

The wave spectral density may also be given in termsof wave frequency f in [Hz]. The relationship is

S(f ) 2 S( )= π⋅ ω

The moments of the wave energy spectral density mayalso be given in terms of the wave frequency f in [Hz].

The relationship is

n nn n0m (f ) f S(f ) df (2 ) m

∞= ⋅ ⋅ = π ⋅∫

4.5 Other wave periods

The following wave periods are also in use:

– The significant wave period, T1/3, is the average

period of the 1/3 highest wave periods.

– The maximum wave period, Tmax, is the period

of the wave with maximum height.

4.6 Wave spreading and short-crested seastates

The wave spectra given above are uni-directional.However, real waves travel in many different direc-tions. Directional short-crested wave spectra may bederived from a uni-directional wave spectrum as fol-lows:

)(W).(S),(S αω=αω

where

S(ω,γ) = directional short-crested wave energy spec-tral density

α = angle between direction of elementary wavetrains and primary wave direction

W(α) = directionality function

= const coss α

The value of the power constant s shall reflect an

accurate correlation to the actual sea state. For typi-cally occurring conditions in the open ocean s = 2 isappropriate.

For design purposes it is usual to assume that the sec-

ondary wave directions are spread over –90° < α <+ 90°. Energy conservation requires that the direction-

ality function fulfils the requirement

1)(Wmax

min

=α∫α

α

4.7 Wave period relationships

For the Pierson-Moskowitz spectrum, wave periodsare related to each other by the factors given in Ta-

ble 1.4:

Table 1.4 Factors relating wave periods for thePierson-Moskowitz spectrum

T1 T0 T1/3 Tp Tmax

T1 1 1,086 0,88 0,77 0,74

T0 0,92 1 0,81 0,71 0,68

T1/3 1,14 1,24 1 0,88 0,85

Tp 1,30 1,41 1,14 1 0,96

Tmax 1,35 1,46 1,18 1,04 1

For example:

T1 = 0,77 T p = 1,09 T0 and

T0 = 0,71 T p

T p = 1,30 T1

For the JONSWAP spectrum, wave periods are relatedto each other as follows:

T1 = 0,83 T p = 1,07 T0 and

T0 = 0,78 T p

Note that the modal period of the Pierson-Moskowitzspectrum differs from the modal period of theJONSWAP spectrum:

For the Pierson-Moskowitz spectrum, T p =1,30 T1.

For the JONSWAP spectrum, T p =1,20 T1.

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5. Response in natural seas

5.1 Response from different wave components

A useful consequence of linearly predicted response isthat results from regular waves of different ampli-tudes, wave lengths and propagation directions can besuperimposed to obtain the response in natural sea-ways made up of a large number N of regular waves ofdifferent length and height. Thus, the response fromdifferent wave components can be written as

N2

an n n nn 1

H ( ) sin [ t ( ) ]=

ς ⋅ ω ⋅ ω ⋅ + δ ω + ε∑

where

⎟H(ωn)⎜ = response amplitude per unit wave ampli-tude (transfer function)

δ(ωn) = a phase angle associated with the response

Both⎟H(ωn)⎜and δ(ωn) are functions of the frequencyof oscillation ωn. The response can be any linearwave-induced motion of or load on the structure.

5.2 Variance of the response

In the limit, as N approaches infinity, the variance ofthe response, σr , is found as follows:

2 2r

0

S( ) H ( ) d∞

σ = ω ⋅ ω ⋅ ω∫

The Rayleigh probability function may be used toapproximate the probability density function for themaxima (peak values) of the response, p(R):

2 2r 2

r

R p(R) exp ( 0,5 R / )= − ⋅ σ

σ

5.3 Most probable largest value during ashort-term seastate

During a short-term seastate of significant wave heightH1/3 and mean wave period of the sea spectrum T1 the

most probable largest value R max over time t is then

)T/tln(..2R 12r max σ=

Strictly speaking, the mean period of the responsevariable should be used instead of the average ob-served (mean) wave period T 1. However, for linearwave-induced motions and loads the difference whenestimating Rmax is small and can be neglected.

5.4 Long-term probability of the response

Short-term response predictions are sometimes con-

sidered inadequate for the design and reliability analy-sis of offshore structures, because the assumption ofstationarity (of a statistically invariant record) restrictsthe validity of the analysis. A long-term probability

defines events and extreme value statistics for a periodon the order of 20 to 100 years, as opposed to a fewhours for the short-term probability.

The long-term prediction considers all sea states the

structure is expected to encounter during its designlifetime. This may be accomplished in the form of the

frequency of occurrences of all possible sea states or

the long-term wave data over the entire life of the

structure, i.e. in the form of a wave scatter diagram

that conveniently describes a family of sea states, each

characterized by the wave spectrum parameters (H1/3,T1) or (H1/3, T1, γ) as defined in 4.1 and 4.2, respec-tively.

The long-term method provides a rational criterion ofacceptability in the form of a uniform likelihood that agiven response level will be exceeded during the life-time of the structure.

The long-term prediction can be performed in thefrequency domain for a linear system or in the timedomain for a nonlinear system.

Combining the Rayleigh distribution and a joint prob-ability (H1/3,T1) from a wave scatter diagram listing M

significant wave height intervals and K spectral peak periods yields the long-term probability of the re-sponse:

2M K

jk 2 j 1k 1 rjk

R P(R) 1 exp p

2= =

⎛ ⎞⎜ ⎟= − − ⋅∑ ∑⎜ ⎟⋅σ⎝ ⎠

where

M = number of significant wave height intervals

K = number of spectral peak periods listed in thewave scatter diagram

P(R) = long-term probability that the peak value ofthe response does not exceed R

σrjk = standard deviation of the response in theseastate (H1/3,T1) of interval j and mean pe-

riod k

p jk = probability of occurrence of seastate (H1/3,T1)

of interval numbers j and k

6. Derived wave parameters

Derived wave parameters, such as wave particle ve-locities and accelerations, may be used to determinedrag and inertial forces on underwater portions ofoffshore drilling units or structures that are operatingor situated in locations where the water depth is con-sidered deep. Subject to approval by GermanischerLloyd, other appropriate methods to determine theseforces will be considered, provided they are refer-

enced and supported by calculations. Linear (Airy)wave theory is applicable to define these parameters if

specific limitations of H/(gT2) are observed, see 3.1 and 3.2.

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6.1 Surface wave profile

The wave profile is defined as follows, see Fig. 1.2:

wz 0,5 h cos (k x t)= ⋅ ⋅ ⋅ − ω⋅ when d > λ/2

where

z = vertical coordinate of wave surface

k = 2π/λ

ω = 2π/T

λ = wave length [m]

T = wave period [s]

t = time [s]

hw = wave height, crest to trough [m]

h = distance below surface [m]x = distance from the origin [m]

d = depth from still-water level to bottom [m]

6.2 Water particle velocity for deep waterwaves

Using a right handed coordinate system x-y-z with x-and y-axes lying in the undisturbed sea surface and thevertical z-axis directed vertically upward (Fig. 1.2),the horizontal and vertical particle velocities at time tare defined as follows:

Horizontal:

khx wv 0,5 h e cos (k x t)

−= ⋅ω⋅ ⋅ ⋅ ⋅ − ω⋅ [m/s]

Vertical:

khz wv 0,5 h

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