1

Revision of

Norsok Standard N-003

on

Actions and Action Effectswith a focus on

airgap, freeboard and wave actions

in relation to the deck structure

by

Torgeir Moan, NTNU

2

Outline• Introduction (relating to airgap, freeboard, green water and wave in deck)

- Framework for regulatory requirements; i.e. NORSOK and other standards

- N-001 vs. N-003 and other standards & guidelines

- Hazardous scenarios to be considered

- Basic principles for standardization

- Consistent ULS requirements – relating to wave in deck

• N-003 standard relating to airgap, freeboard, green water and wave in deck

- Metocean data

- Wave theories

- Airgap criterion

- Wave in deck action

- Air gap and wave in deck analysis for transparent fixed structures

- Local wave impact

- Airgap and wave in deck analysis for floating structures

- Freeboard exceedance and green water relating to ships

- Green water analysis

- Model tests

- Concluding remarks

3

Framework for regulatory requirements

NORSOK (from 1997)

N-001 Integrity of offshore structures;

1996 (Rev. 8, September 2012)

N-003 Actions and action effects; (Ed.

2, Sept. 2007, Ed.3 2016)

N-004 Design of steel structures; (Rev.

3, February 2013)

N-005 Condition monitoring of

loadbearing structures

N-006 Assessment of structural

integrity for existing offshore load-

bearing structures

NORSOK M-; R-002; S-001; Z-013

ISO

ISO 2394ISO 19900 General principles

(2013)

ISO 19901-1 Metocean

ISO 19902 Fixed steel

structures

ISO 19903 Fixed concrete

structures

ISO 19904 Floating structures

ISO 19905-1-3 MODU

ISO 19906 Arctic structures

Other standards & guidelines

Classification societies: DNVGL

Eurocodes

Apparent safety regimes:

Prod.platforms –

MODUs - Jack-ups

PSA Regulations (e.g. Framework -; Facilities - )

Introduction

4

NORSOK StandardizationThe industry’s strategy with respect to

NORSOK standards

• In general use international standards; but in some

NORSOK standards are necessary

• Withdraw national and industry standards when equivalent

international standards are available

• Contribute to the development of international standards that

serve the needs of the industry (and the PSA) – based on

«home made standards»

• Devote efforts to develop and maintain NORSOK standards

Scope of N-003General principles and guidelines for determination of

characteristic actions and action effects for

design, (re-)assessment and verification of structures

• Standards

should specify

best practice

• If experiences

are limited,

the formulation

will be general

Until about 1997

Regulations, Design standard

and guidelines were managed

by NPD (PSA)

Introduction

5

N-003 Committee

Members

• Gerhard Ersdal, Ptil

• Ove Tobias Gudmestad, UiS

• Sverre Haver, Statoil (til 01.05.2014), UIS/NTNU

• Torgeir Moan, NTNU

• Arne Nestegård, DNVGL

• Finn Gunnar Nielsen, Statoil (til 01.07.2016), UiB

• Tone M.G. Vestbøstad, Statoil

• Ole David Økland, Marintek

Supporting members

• Knut Arnesen, DNVGL (seismic actions and - effects)

• Kenneth Eik Johannesen, Statoil, Mosleth DNVGL (ice, icing, snow)

• Jon Kristian Haugland, Øyvind Fjukmoen, DNVGL (marine growth)

• Einar Nygaard, Statoil (metocean data)

Introduction

6

Background for the revision of N-003

• Existing version of 02.09.2007

• Ongoing revision – focus areas:

- cold climate (Barents) activities (sea ice, icebergs, level ice; icing; snow)

- accidental actions - ship collision risk

- metocean data - e.g. introduction of important issues in N-002,

hindcasting data basis; e.g. NORA-10 and

- climate change

- design waves, sea state (contour method), longterm analysis

- requirements to numerical, experimental and in-service obs.

- wave load effects on jackets etc ( Kinematics; Drag and inertia coefficients)

- airgap (water level, wave kinematics (crest)…)

- wave impacts, green water; ringing/springing/whipping

- CFD methods in wave load calculation

- marine growth

- action combinations (ULS, ALS – also for damaged condition)

- editorial – direct it to users involved in fixed versus floating facilities

reorganized to separate the treatment of action effects on fixed

and floating platforms

Introduction

7

Relevant sections for airgap; freeboard and

green water and wave-in-deck

6.1 General

- data, climate

- Sect. 6.1.3 Determination of char.

actions

6.2 Modelling of metocean conditions

-including wave kinematics

6.3 Hydrodynamic actions

- nonlinear actions (use of CFD)

- slamming, breaking wave impacts, run-

up11.5 Metocean AE applicable to all types of

structures- wave slamming and impact by breakingwaves

- air gap and wave in deck analysis- Green water

Commentary

Introduction

8

Brief historical notes- on experiences in the GoM and North Sea

• GoM Camille (1969) – the strongest hurricane since 1935

in the GoM; Andrew (1992); Lilli (1996); Katrina (2005) –

the next largest

• New metocean guideline API (2007) 1000 years

airgap/robustness check

• NPD (1977) 1.5 m (but with no clear spec. of ref. crest)

- sometimes a larger airgap was specified

• NPD (1984) PLS (ALS) req. and «the 10-4 probability»

• In the mid-1980s the Ekofisk field was found to be suffering

from an unexpected degree of subsidence

Introduction

9

Relevant regulatory requirements• The PSA Facilities Regulations state that accidental loads/actions and

environmental loads/actions corresponding to 10 000 year return-period

shall not result in loss of a main safety function. Furthermore, in

Guidelines regarding the facilities regulation it is referred to the

NORSOK-standards N-001, N-003 and N-004.

• According to NORSOK N-001 a facility shall be checked for different

safety limit states (ULS, ALS, FLS)

• According to NORSOK N-001 Section 6.4.1 Facilities with insufficient

deck clearance shall be designed for actions caused by waves and

current and impact actions should be verified by properly defined model

tests.

• Design criteria depend on whether the facility is

- manned or not,

- operating or shut-down

Introduction

10

N-001 vs. N-003

• N-001 (and N-006 for existing structures shall specifyDesign Criteria in view of acceptable safety level- characteristic values ( second step in ALS)

- (possible) explicit req. to air gap and green water versus

direct design check

- safety factors or intentionally conservative methods

(e.g. reflecting increased uncertainty in

hydrodynamic actions on platform decks; use of

nonlinear load effect analysis of complex structures

(vs. Limit states)

N-003 vs. Other standards and guidelinesrelating to airgap, freeboard, green water and wave in deck- ISO 19902 (for jackets)

- DNV(GL) RP-C205

- DNVGL OTG-13

- DNVGL OTG-14

The challenge is the balance between

safety and costs - cfr. ALARP

Introduction

11

Hazardous scanarios to be consideredrelating to airgap, freeboard, green water and wave in deck

• Overall failure modes:

- global structural failure

(possible imposed forced

deformations on risers and conductors

- excessive heeling or capsizing of floaters

• Water impact on the topside

• Water on deck (even lower deck for topsides

with truss girders and light cladding cover

- implication on strength and stability

(of floaters) - in case of progressive flooding

• Water impact on life boats and other safety

critical equipment

• Direct effect on personnel

SEMI Thunder Horse

Introduction

12

Basic principles for standardization

- Standards should represent best practice

(air gap, wave in deck; green water are complex problems

and with limited support from field experiences and

research)

- Requirements should be such that compliance can be

demonstrated

- The standard should be consistent; i.e. with respect to

safety level.

- Various methods for prediction of actions are envisaged.

Moreover, they are described by a «procedure» with

opportunities of interpretation – as opposed to strength

formulations expressed by formulae

Introduction

13

Consistent ULS requirementsULS requirements to the structure are ensured by

- the definition of characteristic action effects and resistances

and load and resistance factors

- different methods are applied to determine actions and action

effects, with different uncertainty; but the action factor is

normally not differentiated.

Conclusion: Then adequate safety needs to be achieved by

using a properly conservative approach

c R S cR /γ γ S S S C S

R R C R R

B S ,B 1;

B R ,B 1; V 0.1

The design format:

Reliability basis (design value format):

hence expd S S S S cS V S

22

SR

S

S

VV

V

exp /S S S S cV S

Action model uncertainties estimated by comparison with model tests used

in «Ekofisk subsidence» reliability assessment (OTC13187, 2001)

CoV: 0.18 (wave height); 0.25 (jacket hydrodynamic load)

0.35 (Kaplan’s method for wave action);

Introduction

14

Metocean data

• Regarding wind and waves high quality and verified

hindcast data base (e.g. NORA10).

• For permanent facilities with a planned service life of more

than 50 years, climate change shall be accounted for.

Commentary

- In lack of more detailed documentation the following increase in

metocean values 50 years ahead shall be used:

- extreme significant wave height & wind speed: 4% increase.

- sea level: 0.25 m

• Measurement of current and joint wave, wind and current

conditions

Note: The revised N-003

is not finalized

N-003 standard

15

Characteristic action (effects)

N-003 standard

«Modified

contour

approach»?

16 Background: Wave theories

0 0.2 0.4 0.6 0.8 1 1.2 1.4-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

z (

m)

Horizontal Velocity (m/s)

- second order methods :

Johannessen (blue line);

Stansberg (green line)

Wheeler

stretching

Wave particle kinematics beneath two large wave

events recorded in a 10-4 sea state;

(a) spilling and (b) over-turning (Swan, 2016)

Comparison of calculated

crest kinematics (CresT JIP, Marin)

Second order

formulation

by Sharma and

Dean, 1981)

BEM –a fully

nonlinear

solution

17

Wave theories

• Irregular waves

- problems insensitive to crest elevation and kinematics

in the crest: linear wave theory; Gaussian waves

(FD analysis)

- problems sensitive to crest elevation and kinematics

in the crest: second order wave elevation process &

consistent kinematics (see e.g. DNV RP C205)

(linear theory with Wheeler stretching in exceptional cases)

- If short crested sea is introduced in connection with estimating extremes,

the exponent, n, in the wave directional function 𝑐𝑜𝑠𝑛θ should not be

taken to be lower than 10 without a more detailed documentation.

- Special consideration should be made for crest kinematics

in sea states with near breaking waves or breaking waves

N-003 standard

18

Airgap criterion (commentary)

• Basic principle: avoid wave in deck or design for wave in

deck

• Relevant to consider 10-2 and 10-4 events

since such characteristic values are used

in ULS and ALS design checks (for facilities unmanned in

storms NORSOK N-001 specify ALS check with annual

probability of exceedance of 10-3 )

• Due to the complexity and uncertainty associated with

determining wave-in-deck actions; designing for a

positive airgap above a wave crest with an annual

probability of exceedance of 10-4 is recommended

(implying 30% increase in 10-2 crest).

N-003 standard

19

Wave-in-deck actions

• Inertia and drag, slamming, and buoyancy actions

• Consider water entry and exit; horizontal and vertical actions

• Express the time variation of the transient action

• Methods

- semi-empirical ( Kaplan (1992, 1995), API, DHI, DNV,

in-house methods in various oil companies

- component models: wave loading on each deck component is

determined separately. Validate by CFD and/or experiments

-silhouette method; Horizontal drag force

where where ρ is sea water density 𝑢𝑤 is the water particle

velocity, A = sd · b is the exposed area, 𝑠𝑑 is the inundation

(height) and b the width of the inundated area. Validation!

- CFD

- laboratory experiments

Unless the air gap is sufficient to avoid wave in deck impact,

relevant deck impact analysis shall be performed.

N-003 standard

20

Wave in deck actions

Methods

- Simplified method by Kaplan et al. 1995

- CFD methods

- laboratory tests

The approach applied should be validated

by high quality model tests with

due consideration of

- the large inherent stochastic variability and

- uncertainty in predicting deck impact actions.

N-003 standard

21 Crest height distribution (Swan et al., OSRC 2016)

Crest height distributions recorded in a laboratory, Tp=16.0s

Crest height

observations

in the field (Hs≥12m)

Lab.

Hs=12.5m

Lab.

Hs=15.0m

Lab.

Hs=17.5m

breaking

(by both spilling

and over-turning)

Norsok N-003: Forristall crest height distribution. in agreement with a

second-order surface process. (see e.g. DNV RP C205)

22 N-003 standard

23

Wave in deck actions• A wave event can either be an extreme crest event in an irregular design

seastate or a regular design wave. A regular design wave approach must

be shown to be conservative.

•The wave event approach should include the following:

- Determine design crest height Cq (corresponding to annual probability q)

from Forristall (long) crest height distribution and long term statistics.

- Increase Cq by a factor 1.1.

- Adjust height of top of crest to include storm surge and tidal level

- Construct a regular wave with the relevant crest

• Determine the flow and actions in the design event by using a CFD with

- due consideration of spatial and temporal convergence of the model.

- due account of possible effect of compressed air

- due consideration of the geometric modelling of the deck

- the effect of possible breaking waves shall be assessed.

- model the action as time variant (due to possible dynamic action effects)

N-003 standard

24

Wave in deck loads for fixed structures- The enhanced action effect on the substructures below deck due to

disturbed kinematics from wave impact on deck shall be considered.

- For large volume fixed structures wave diffraction effects shall be

included in the airgap analysis. For such structures higher order

diffraction e.g. due to caisson effects may be important.

- High quality model tests are recommended to validate the airgap

analysis

Uncertainties - airgap prediction

- wave action in decksea-states; real irregular vs. regular waves; hydrodynamic models

(semi- emprical vs. CFD vs. experimental methods)

N-003 standard

25

Local wave impactLocal wave impact above maximum crest due to effects not accounted

for in the above requirement to positive airgap, may be permitted to

occur on any part of a deck structure provided

- they can be designed for

- the water does not threaten personnel's life, or damage pipes and

other equipment which may lead to environmental damage in ULS

and ALS . Commentary: - guideance on pressure levels /

pressure area is given

- For a fixed platform the affected

area can reach a maximum

height of 1.4 times the ULS

wave crest height for the relevant sector.

- the deck structure adjacent to platform columns shall be designed to

resist the possible pressure actions due to run-up along columns.

N-003 standard

26

Airgap and wave in deck analysis for

floating structures- The platform motions contribute relative velocity and acceleration

and the deck (impact) contact with the waves influence to some

extent the motions

- The deck height varies in time and space

- reserve buoyancy for stability wold normally imply that the deck

needs a certain strength due to the hydrostatic and - dynamic

pressure- In wave in deck analysis of large volume

floating structures, higher order wave

diffraction effects may be accounted for by

- using a factor 1.2 on the first order

wave response based on Gaussian sea

states, or

- a higher order wave theory which is

documented to yield reliable predictions.

The analysis of wave/structure interaction

effects should be made. The analysis results

should be validated by model testing.

27

Freeboard exceedance and green water

relating to ships

- Green water occurs when the wave elevation exceeds the

ship freeboard

- Areas occupied by personnel, or where safety-related

equipment is located, shall not be exposed to waves with an

annual probability greater than 10-2.

- The freeboard exceedance and actions shall be assessed by

validated calculations or model tests.

- Green water typically induces pressure actions and local

slamming actions on the exposed structures.

- The influence of green water on the stability or global motions

of a floating facility shall be evaluated.

N-003 standard

28

Green water analysis

- For ships, relative wave elevation exceeding the freeboard

can be determined using potential theory by computing the

relative motion

- The flow of water on deck and resulting action effects on

deck structures should be determined by using advanced

nonlinear methods and/or model tests.

- The effect of wave slamming, run-up or green water actions

should be appropriately combined with the other wave action

effects.

N-003 standard

29

Model testsHydrodynamic model tests should be carried out to:

• confirm that no important hydrodynamic action has been overlooked (for

new types of facilities, metocean conditions, adjacent structure),

• support theoretical calculations (due their large uncertainties),

• verify theoretical methods on a general basis.

• direct determination of action/action effects for complex problems where

numerical methods are insufficient or not available.

Implement experimental test results by carefully considering

• scaling effects,

• model simplifications (e.g. related to damping),

• limitations in testing facilities (e.g. finite dimensions; quality of waves, .)

• uncertainties regarding the data recording and processing,

• uncertainties with regard to long-term variability,

• statistical uncertainties with respect to limited samples (suggesting

e.g. that extreme values should be determined by extrapolation

• The model test shall be executed and documented in such a manner

that they are repeatable.

• Interpretation of experimental results should be supported by numerical

analyses both in model and full scale.

N-003 standard

30

Concluding remarks

• Experience shows that airgap, freeboard, wave in

deck or water on deck are important considerations

in design

• These problems, are however, complex - especially

to deal with by theoretical analysis and limited

experiences are available. Current design standards

are in their first stage.

• Unless a proven conservative approach can be used,

experimental evidence for the relevant problem is

crucial.