Norwegian Marine Technology Research Institute

Ankerlinebrudd – bølgekrefter på flytende innretninger i ekstreme sjøtilstander

Carl Trygve Stansberg, dr.ing. sjefsforsker, MARINTEK avd. Offshore hydrodynamikk,

Trondheim

Konstruksjonsdagen, PTIL, Stavanger 27 august 2014

2

Moored semisubmersible model test in steep irregular waves

3

Wave and current forces on floating bodies in high sea states

4

Wave and current forces on floating bodies in high sea states

Innhold:

- Bakgrunn: Økende fokus på forebygging mot linebrudd, bl.a. p.g.a.

nylige hendelser i grov sjø, både i Nordsjøen og andres steder.

- Saktevarierende (LF) bølgekrefter i høy sjø og med effekter av strøm

er sannsynliggjort å være en av faktorene. Viskøse og ikke-lineære

effekter anses å være viktige faktorer

- Dagens ingeniør-verktøy er basert på lineære analyser for små bølger,

og vannligvis uten strøm eller viskøse bidrag til LF krefter I noen grad

med empiriske "ad-hoc" korreksjonsmetoder - i deler av industrien

Innhold (forts):

- Erfaringer fra modellforsøk gjennom de siste 10-20 år viser viktig

info for fysisk forståelse og kvantifiseringen av tilleggs-effektene.

- Teoretiske og numeriske studier med avanserte verktøy hjelper på

forståelsen og hva som bør forbedres

- Behov for konsensus og alment aksepterte forbedrede prosedyrer

og metoder. Et foreløpig forslag til umiddelbar forbedring for semi er

lagt fram (studie for Statoil). En 2-års JIP for mer permanent forbedring

er i oppstarten (EXWAVE JIP), v/ MARINTEK og DNV GL

Norwegian Petroleum Safety Authority (PSA): Number of mooring line incidents in the Norwegian sector 2000 – 2012.

0 1000 2000 3000 40000

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Time(s)

Effe

ctiv

e te

nsio

n(k

N)

Top tension

3160 3180 3200 3220 3240 3260 3280 3300

1000

2000

3000

4000

5000

6000

7000

8000

9000

Time(s)

Effe

ctiv

e te

nsio

n(k

N)

Top tension

Simulated

Measured

(MARINTEK simulations based on measured fairlead motions)

Example real incident: Drilling semi line break on 26 January 2012 (Kaasen 2013)

Reconstructed wave group around time of break

9

Blue = wave Green = vessel heave

3200 3220 3240 3260 3280 3300 3320

-10

-5

0

5

10

Estimated wave

Time (s)

(m)

12.2 s

The mean period of the wave group is about 12.5 seconds Peak period of sea spectrum is 14 seconds High steep waves

Platform trajectory

10

20 25 30 35 40

-40

-38

-36

-34

-32

-30

-28

-26

-24

-22

-20

X (north) (m)

Y (

west)

(m)

Motion in XY plane

Mean position

prior to break

Break

happened

here

Why wave conditions are believed to be one important factor leading to critical offset and overload

- Several incidents have occurred in storm sea states

- Several incidents have happened during the same storm(s)

- Some real-time measurements indicate this (but data are scarce)

- Model test experiences confirm deviations from standard industry tools and practice

- Various theoretical/numerical analyses also confirm such effects

- Procedures and practice vary between different parts of the industry

Why wave conditions are believed to be one important factor leading to critical offset and overload)

- Several incidents have occurred in storm sea states

- Several incidents have happened during the same storm(s)

- Some real-time measurements indicate this (but data are scarce)

- Model test experiences confirm deviations from standard industry tools and practice

- Various theoretical/numerical analyses also confirm such effects

- Procedures and practice vary between different parts of the industry

- In combination with other causes (corrosion, fatigue, operational, . . )

Today's methods and tools - 3D potential flow radiation-diffraction panel method. Usually the wave loads are calculated in the frequency domain.

- Motion response: time-domain where frequency dependent added mass, wave damping and excitation forces are converted to retardation function and excitation force time histories. Alternatively the force time histories can be calculated directly in time domain. Viscous forces may be added.

- First-order potential theory is mostly applied- small waves and small vessel motions assumed.

- LF wave forces: full second-order modelling (Quadratic Transfer Functions – QTF's) is possible, while in practice the much simpler "Newman's approximation" is mostly used . OK in many cases for horizontal slow-drift motions in deep water, especially because of the long natural floater periods. Not receommended in shallow water

Standard industry software at MARINTEK and DNVGL /SESAM:

MULDIF – WAMIT – WADAM – SIMO – RIFLEX – MIMOSA

In addition: Various other similar tools (HtdroStar, Aqua, etc.)

Model tests - snapshots in steep waves

FPSO's & Semi

LF surge

Wave group induced LF motions in irregular wave FPSO model test

Wave elevation

LF surge

Line tension

Showing longer record, also incuding resulting line tension

Effects from current on mooring force

Model test calibration of numerical model of FPSO in 100-yr sea state

• Wind, waves and current

• Adjust drift coeff., damping and wave-current interaction until the model "behaves well"

After calibration Model test

Before calibration Model test

Lin

e te

nsi

on

Li

ne

ten

sio

n

Wave-current interaction for FPSO, numerical prediction (new MARINTEK MULDIF software 2013):

Mean wave drift force coefficients, for current=0 and =1m/s.

Moored semi model test example, with and without current

Viscous drift forces and wave-current interaction are important: Standard modelling underpredicts measured wave drift loads.

Dots: Standard calculation (WAMIT)

Storm waves

Semi wave drift coefficients: WAMIT vs. model tests in various sea states

There are also advanced tools that are not yet industry standards, but can be

used for future particular investigations:

- Higher-order potential theory methods

- Fully nonlinear --------" --------------

- CFD

- Other

From observations: Items related with increased LF forces:

- Wave-current interaction

- Viscous effects on low-frequency wave forces (on semis, TLPs, Spars),

- Large amplitude wave-frequency (WF) vessel motions, including also

non-linear effects from flared vessel geometry at SWL and discontinuous

hydrostatic stiffness in high waves

- Second- and higher-order wave load effects including deviations from

Newman's approximation

- Consistency issues related to combination of frequency-domain WF and

time-domain LF modelling

- Effects of high wave groups and kinematics of nonlinear steep waves

Pilot study for Statoil (2013-2014), by MARINTEK & DNV GL:

EXWAVE – Pilot Study: Wave Forces on Floating Units in Extreme Waves

and Current.

Exploring possible short-comings in prediction of LF wave forces in high

wave conditions, through case studies and recommendations. Emphasis

on semi's, but general floaters also addressed

- Task 1. Review of theoretical models

(State-of-the-art of methods in industry tools)

- Task 2a. Case studies.

(Sensitivity analysis of 2 semi cases: A drilling semi, and a large

production semi, using state-of-the-art tools (SIMO, WAMIT))

- Task 2b. Recommendations for actions and further work.

(Short-term/preliminary actions, and proposal of a JIP - EXWAVE)

From pilot study: A preliminary, simple correction formula for semi

wave drift coefficients (subject to further improvements/

adjustments in EXWAVE JIP:

fD = Fd(w,U,Hs)/ A2 = [fd,pot *(1+Cp*U) + B(G*U + Hs)]

where: fd,pot = second-order wave drift coefficients from first-order potential theory (e.g. WAMIT, WADAM or MULDIF)

A = wave amplitude

Cp = potential-flow wave-current interaction coefficient, chosen to be 0.25 (Aranha's formula

B = B' * dsum (viscous drift) , B' = k * p ; dsum = S di (column diameters)

k= the wave number 2p / l l = wave length

p = exp[-1.25(kD0)2], [kN/( m3) ] D0 = diameter of main columns

G=10 (with dimension time, [s]) viscous wave-current interaction part; empirical

Objectives:

Identify, explore and describe technology gaps that can explain the occurrence of extreme wave-induced forces higher than expected, which may result in overload and line breakages in steep storm sea states.

Emphasis shall be on wave forces leading to slowly varying vessel motions resulting in high mooring line (and DP) loads, with possible deviations from "standard" 2nd order drift forces

Recommend improvements with guidelines in industry practice, taking into account proper modeling of these wave-induced forces

EXWAVE JIP plan 2014-2016

MARINTEK-DNV GL

Application areas:

Positioning of drilling vessels (MODUs) as well as floating production platforms like FPSO's and semis. Moored and DP-controlled.

Outcome

Proposed improvements to today's tools and procedures Proposed improvements to generally accepted industry practice A handbook / guideline / background documentation for 'RP'

documents and similar

EXWAVE JIP plan 2014-2016

The EXWAVE JIP - Five Work Packages:

WP1 – Review of methods and tools to identify possible weaknesses

WP2 – New case studies with numerical simulations and model tests

WP3 – Detailed investigation of critical effects and phenomena

WP4 – New improved formulation, implementation and testing within

existing approaches

WP5 – Recommendations and guidelines

(WP2-WP3-WP4 constitute the major amount of work to be carried out in

the project.)

JIP contact persons: CarlTrygve.Stansberg@marintek.sintef.no

Arne.Nestegard@dnvgl.com

Current overall status:

JIP idea was initiated after several incidents in the North Sea during recent years

Industry feedbacks so far: Significant interest, many companies intend to join the JIP.

Kick-off planned 17th Oct 2014 (33rd FPSO JIP Week, Korea)

"Pre-kick-off" – Miniseminar to be held in Stavanger, Norway, 9th September (Rica Airport Hotel), particularly planned for those involved in the Norwegian Sector

JIP contact persons:

CarlTrygve.Stansberg@marintek.sintef.no Arne.Nestegard@dnvgl.com

EXWAVE JIP plan 2014-2016