DNV GL © 2016

Ungraded

30 August 2016 SAFER, SMARTER, GREENERDNV GL © 2016

Ungraded

30 August 2016

Arne Nestegård

DNV GL regelverk om bølger i dekk på semier

1

Ptil - Konstruksjonsdagen

DNV GL © 2016

Ungraded

30 August 2016

Relevant DNV GL Standards, Recommended Practices / Guidelines

� DNVGL-OS-C101 Design of offshore steel structures, general – LFRD Method - April 2016

� DNVGL-OS-C103 Structural design of column stabilised units – LFRD Method - July 2015

� DNVGL-RP-C103 Column-stabilised units – July 2015

� DNV-RP-C205 Environmental Conditions and Environmental Loads – April 2014

� OTG-13 Prediction of air gap for column stabilised units (Draft) – July 2016

� OTG-14 Horizontal wave impact loads for column stabilised units (Not issued)

2

DNV GL © 2016

Ungraded

30 August 2016

DNVGL-OS-C101 Design of offshore steel structures, general

3

For units with N-notation operating on the NCS, the structural integrity of the unit should be documented also for the 10-4 minimum air gap case in an ALS condition

Note:

DNV GL © 2016

Ungraded

30 August 2016

DNVGL-OS-C103 Structural design of column stabilised units

4

DNV GL © 2016

Ungraded

30 August 2016

DNVGL-OS-C103 Structural design of column stabilised units

5

Ch. 2 Sec. 3

DNV GL © 2016

Ungraded

30 August 2016

DNVGL-RP-C103 Column-stabilised units

6

DNV GL © 2016

Ungraded

30 August 2016

DNVGL-RP-C103 Column-stabilised units

7

Misstatement introduced in DNVGL-RP-C103

DNV GL © 2016

Ungraded

30 August 2016

DNV-RP-C205 Environmental Conditions and Environmental Loads

8.2.6 Simplified analysis

� A simplified method to investigate air gap is to employ linear radiation-diffraction

analysis to determine the diffracted wave field and the linearized platform motion.

The surface elevation is then modified by a coefficient to account for the

asymmetry of crests and troughs.

� where α is an asymmetry factor, η(1) is the linear local surface elevation. η is then

treated as an RAO for each location and for each frequency and each direction.

� The use of an asymmetry factor α = 1.2 is generally found to yield conservative

results for standard floater concepts like TLP and semisubmersibles. α varies

along the Hs(Tp) contour, generally decreasing as Tp increases.

8

)1()2()1( αηηηη =+=

DNV GL © 2016

Ungraded

30 August 2016

Guideline for air gap predictions

9

The objective of OTG-13 is to

define a recommended

procedure for estimating air

gap for column-stabilized units.

The procedure can be applied

to predict air gap for operating

conditions as well as design air

gap with annual probability of

exceedance q = 10-2 (ULS) and

q = 10-4 (ALS).

DNV GL © 2016

Ungraded

30 August 2016

Definition of upwell and air gap

10

a0

z

η

a

[ ] ),,(),,(),,(),,( 00 tyxatyxtyxzatyxa χη −=−+=

Air gap:

UpwellStill water air gap

DNV GL © 2016

Ungraded

30 August 2016

Contributions to upwell and air gap

� Contributions to upwell:

– Wave frequency (WF) upwell

– Low frequency (LF) upwell

– Mean upwell due to mean inclination of floater

� Vertical displacement of floater

� Wave surface elevation (simplified analysis)

asymmetry factor

11

WFχ

LFχ

meanχ

),,(),,(),(),,( tyxztyxzyxztyxz LFWFmean ++=

)()()( LNLL αηηηη ≈+==α

- Wave asymmetry- Non-linear diffraction effects

DNV GL © 2016

Ungraded

30 August 2016

Asymmetry factor

� In lack of a more complete numerical analysis or model tests, an asymmetry

factor α = 1.2 may be applied for all horizontal positions underneath the deck box

for ordinary catamaran type column-stabilized units, excluding run-up areas close

to columns. An enhanced asymmetry factor may be considered along the outer

edge of the deck box up-wave of columns.

� Asymmetry factors derived from model tests shall be extracted at the 90%

percentile level in the governing sea state, for both ULS and ALS. The asymmetry

factor for each position is defined as the ratio between the extreme value η90 from

the model test and the extreme linear surface elevation from the numerical

analysis, also taken as the 90% percentile.

� The extreme value from the model tests can be obtained assuming Gumbel

distributed maxima while the linear extreme value from the analysis is obtained

from a Rayleigh distribution

12

)(90

90Lη

ηα =

DNV GL © 2016

Ungraded

30 August 2016

Contributions to upwell

13

� Wave frequency upwell 90% percentile in design sea state

� Low frequency upwell MPM in design sea state

� Mean upwell

� Total upwell

� In lack of available model tests or numerical prediction of LF motions, each of the maximum LF roll and LF pitch angle can be taken as 5 deg. For oblique sea the rotation can be assumed to be in-line.

� Irrespective of intended mean inclination, an additional inclination of 1 deg in the most critical wave direction may be applied in ULS to account for uncertainty in ballasting.

WFL

WF z−= )(αηχ

LFLF z=χ

meanmean z−=χ

22LFWFmean χχχχ ++=

DNV GL © 2016

Ungraded

30 August 2016

Low frequency upwell

� Low frequency (LF) contributions from roll and pitch

� LF heave motion is neglected

� LF motions excited by wind and waves

� Both contributions estimated in frequency domain (wind moment spectrum &

difference frequency wave induced moment spectrum (from QTFs))

� The maximum low frequency roll and pitch angles are taken as the MPM values

� Assume contributions are not correlated

14

( ) 2/1)( ln2)( NLFi

LFi ξσξ =

)sin()sin( )(4

)(5

LFLFLF yxz ξξ +−=

2,

2, windLFwaveLFLF zzz +=

DNV GL © 2016

Ungraded

30 August 2016

Design sea states –short term conditions

� For worldwide operation based on North Atlantic wave conditions the short term

wave conditions shall be modelled by application of the Jonswap wave spectrum.

� For site specific design the relevant two-peak wave spectrum for combined wind

sea and swell should be applied. For the Norwegian Continental Shelf the

Torsethaugen wave spectrum may be applied.

� The sea state can be taken as short crested with a directional spectrum

where n = 6 for operational conditions ( < 8 m) and n = 10 for extreme

conditions i.e. > 8 m (for ULS and ALS).

15

θncos

sH

sH

DNV GL © 2016

Ungraded

30 August 2016

Design sea states – long term conditions

� For worldwide operation, the North Atlantic wave conditions as described in DNV-

RP-C205 shall be applied for ULS.

� For restricted operation site specific conditions may be used for ULS. For ALS

relevant site specific wave conditions may be applied.

� The ULS and ALS, air gap may be estimated by a contour line method where the

steepness criterion given in DNV-RP-C205 can be used to limit the steepness of

the sea states.

� The design sea state shall be selected as the less steep sea state either along the

steepness criterion curve or the q annual probability contour which is the most

critical wrt air gap.

� For ALS the design sea state shall be sought along the site specific q = 10-4

annual probability contour.

16

DNV GL © 2016

Ungraded

30 August 2016

Design sea states

17

North Atlantic

NCS site specific (10-2 & 10-4)

DNV GL © 2016

Ungraded

30 August 2016

Special effects to consider for air gap predictions

� Effect of current

� Non-linear effects

– Trapped waves

– Shallow pontoons

– Non-linear motion effects

– Wave run-up along columns

18

Non-linear effects may give a shift in phase

of heave and pitch motion resulting in

negative air gap.

DNV GL © 2016

Ungraded

30 August 2016

Guideline on horizontal wave impact loads (not issued)

19

The object of OTG-14 is to

provide a guideline for the

loads to be used to

document structural and

floating integrity for MOUs

which are subject to

horizontal wave impact with

the deck in the design

conditions.

DNV GL © 2016

Ungraded

30 August 2016

Basis for guideline

� The database consists of more than 300 realisations of three hour sea states with

return periods of 1 year to 10 000 year in a typical NCS environment.

� Based on these results, a model for slamming loads has been developed and long

term analyses of wave impact loads for several joint Hs, Tp distributions and

several typical MOUs have been carried out in order to estimate 100 year and

10 000 year load levels conditional on freeboard exceedance (negative air gap).

� The database consists of normal wave impact due to long crested waves, the

results are conservative and may be reduced in subsequent revisions of this OTG.

20

DNV GL © 2016

Ungraded

30 August 2016

Statistical model

21

Freeboard exceedance

DNV GL © 2016

Ungraded

30 August 2016

Design slamming pressure impulse

22

Ppeak

P1,sustained

P2,sustained

P3,sustained

Ppeak = Ppeak(∆z)

P1,sustained = P1,sustained(∆z)

P2,sustained = P2,sustained(∆z)

P3,sustained = P3,sustained(∆z)

∆z = distance from pressurepanel up to maximum crestlevel

Guidance also given for pressure p(z) above maximum crest level

Curves given for each

of 10-2 and 10-4.

Design pressure impulse is a function of distance to q-probability crest level

DNV GL © 2016

Ungraded

30 August 2016

Global integrity evaluations

� Significant exceedance of available freeboard in the ULS and ALS conditions may

threaten the global integrity of the structure. Possible failure mechanisms must be

evaluated for each individual unit.

Examples:

– Loss of floating integrity due to excessive deck loading

– Loss of floating integrity due to excessive water on deck

– Loss of structural integrity due to deck loading

– Uncontrolled collapse of deck members due to excessive local deck loading

– Progressive flooding of the unit due to wave impact damage

– Loss of lifeboats or other main safety functions due to wave impact loading

23

DNV GL © 2016

Ungraded

30 August 2016

Model tests for air gap and wave impact loads

� For structures where air gap calculations indicate that the available freeboard in

front of the deck box may be significantly exceeded (more than 2-3 m), DNV GL

strongly recommend to carry out model tests to verify the air gap calculations and

the local and global wave induced deck loads.

� Model testing of floating structures subject to wave impact loads are complex and

requires experienced model test contractors and experienced follow up teams.

Particular attention should be given to:

– Ensuring that the structure and structural components are sufficiently stiff so

that global and local loads may be accurately measured

– Positioning the air gap probes and load measurement units at critical locations

– Selecting the critical sea states

– Ensuring that sufficient tests are carried out to yield reliable load statistics in

the governing sea states.

– Panel dimensions of more than 3x3 m should not be employed unless it is

demonstrated that a larger panel size is relevant for local structural design.

24

DNV GL © 2016

Ungraded

30 August 2016

Model tests for air gap and wave impact loads (cont)

� Wave impact loads in particular are intermittent and highly variable and it is great

challenge to estimate the loads with the appropriate return period.

� It is known that the ULS and ALS design loads can occur at very high percentile in

the 100 year and 10 000 year sea states. Nevertheless, a percentile level for the

ULS and ALS load level of 90% may be acceptable in the three hour 100 year and

10 000 year governing sea states respectively, provided that a carefully controlled

model test is carried out.

� Balance should be sought between conservative and non-conservative effects

– Conservative effects :

– Long-crested sea

– Scale effects

– Non-conservative effects:

– 90% percentile for slamming pressure

– Estimate of extreme on each panel (point statistics)

25

DNV GL © 2016

Ungraded

30 August 2016

SAFER, SMARTER, GREENER

www.dnvgl.com

QUESTIONS?

26

Arne Nestegård

arne.nestegard@dnvgl.com

(+47) 414 19 215