1
Revision of
Norsok Standard N-003
on
Actions and Action Effects
with 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 Introduction
Framework for regulatory requirements
PSA Regulations (e.g. Framework -; Facilities - )
ISO
ISO 2394
ISO 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
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 loadbearing structures
NORSOK M-; R-002; S-001; Z-013
Other standards & guidelines
Classification societies: DNVGL
Eurocodes
Apparent safety regimes:
Prod.platforms –
MODUs - Jack-ups
4 Introduction
NORSOK Standardization
The industry’s strategy with respect to
NORSOK standards
•
•
•
Until about 1997
Regulations, Design standard
and guidelines were managed
by NPD (PSA)
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-003
General 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
5 Introduction
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)
6 Introduction
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
7 Introduction
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, runup
11.5 Metocean AE applicable to all types of
structures
- wave slamming and impact by breaking
waves
- air gap and wave in deck analysis
- Green water
Commentary
8 Introduction
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
9 Introduction
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
10 Introduction
N-001 vs. N-003
The challenge is the balance between
safety and costs - cfr. ALARP
• N-001 (and N-006 for existing structures shall specify
Design 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 guidelines
relating to airgap, freeboard, green water and wave in deck
-
ISO 19902 (for jackets)
DNV(GL) RP-C205
DNVGL OTG-13
DNVGL OTG-14
11 Introduction
Hazardous scanarios to be considered
relating to airgap, freeboard, green water and wave in deck
SEMI Thunder Horse
• 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
12 Introduction
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
13 Introduction
Consistent ULS requirements
ULS 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.
The design format: R /γ  γ S S  BSSC , BS  1;
c
R
S c
Reliability basis (design value format):  B R , B  1; V  0.1
R
R C
R
R

Sd  S exp  S VS    S Sc
hence
 S  S exp  S VS  / Sc
 S 
VS
VR2  VS2
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);
Conclusion: Then adequate safety needs to be achieved by
using a properly conservative approach
14
N-003 standard
Metocean data
Note: The revised N-003
is not finalized
• 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
15
N-003 standard
Characteristic action (effects)
«Modified
contour
approach»?
Background: Wave theories
0.4
Wheeler
stretching
0.2
0
z (m)
16
Second order
formulation
by Sharma and
Dean, 1981)
-0.2
- second order methods :
Johannessen (blue line);
Stansberg (green line)
-0.4
-0.6
-0.8
-1
0
0.2
0.4
0.6
0.8
1
Horizontal Velocity (m/s)
1.2
1.4
BEM –a fully
nonlinear
solution
Comparison of calculated
crest kinematics (CresT JIP, Marin)
Wave particle kinematics beneath two large wave
events recorded in a 10-4 sea state;
(a) spilling and (b) over-turning (Swan, 2016)
17
N-003 standard
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 𝑐𝑜𝑠 𝑛 θ 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
18
N-003 standard
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).
19
N-003 standard
Wave-in-deck actions
Unless the air gap is sufficient to avoid wave in deck impact,
relevant deck impact analysis shall be performed.
• 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
20
N-003 standard
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.
21
Crest height distribution (Swan et al., OSRC 2016)
Lab.
Hs=12.5m
Crest height
observations
in the field (Hs≥12m)
breaking
(by both spilling
and over-turning)
Lab.
Hs=17.5m
Lab.
Hs=15.0m
Crest height distributions recorded in a laboratory, Tp=16.0s
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 N-003 standard
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)
24
N-003 standard
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 deck
sea-states; real irregular vs. regular waves; hydrodynamic models
(semi- emprical vs. CFD vs. experimental methods)
25
N-003 standard
Local wave impact
Local 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.
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
N-003 standard
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.
28
N-003 standard
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.
29
N-003 standard
Model tests
Hydrodynamic 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.
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.