Classification: Statoil Internal Status: Draft

Summary of lectures 2007SH-part of Design of Offshore Structures

Sverre Haver, Statoil,

Trondheim, April 30

2

Lecture February 19:

Motivation and Rules and Regulations on the Norwegian Continental shelf

Limit states:FLS (Fatigue limit state)

ULS (Ultimate Load Limit state): Loads corresponding to an annual excedance probability of 10-2.

ALS (Accidental Load Limit State): Loads cooresponding to an annual exceedance probability of 10-4.

Hierarchy between regulations:Top regulation: Framework regulation

Thereafter: Management regulation, Information regulation, Facilities Regulation, Operation Regulation

Next levels: Guidelines to regulation, national and international standards, e.g NORSOK standards, ISO standards

Last level: Company requirements

3

Lecture February 19

Purpose of regulations:Ensure a high level of health, environment and safety in the petroleum activity.

Principles of risk reduction:ALARP: After fulfilling requirements, the level shall be improved as much as isrealistically possible.

BAT: Best availale technology shall be used

Precautionary principle: Use the solution with the smallest uncertainties relatedto the solution.

Substitution principle: Factors which may cause harm to environment and safety shall be replaced with factors with less potential for harms.

Barriers (Management regulation): A barrier is anything that can stop a hazardous event between the initian of a hazord and the victim.

4

Lecture February 19

Self-control principle:

* The operator is responsible for for ensuring that his petroleum activities are inagreement with the PSA regulations!

* A high level of safety is the intention of the regulation.

* The safety level shall be continuously improved.

* In connection with major modifications the agreement with the lates versionsof the rules and regualtions shall be ensured by the operator. In this connectionthe best available informations are to be used.

Limit state control (N-003) (see e.g Ch. 3 of Torgeir Moan kompendium for examples of various load types):

m

ceevvpp

yxxx

5

Lecture February 19

Curiculum:

•Ch. 1 and Ch. 2, SH_lectures week 8 – 2007

•Lecture notes: SH-lecture_February 19th

•T. Moan: ”Design of Offshore structures”,

Ch. 3.1, Ch. 3.2, Ch. 3.3 and 3.4

6

Lecture February 20

Classification of structural problems

•First level of classification

Linear mechanical system versus non-linear mechanical system.

•Linear response problem

= linear mechanical system plus linear loading and Gaussian sea, ( i.e.

linear wave theory)

The deterministic structure is described by the transfer function.

Stochastic response analysis is conveniently done in the frequency domain,

sresp() = RAO()2 * swave().

Long term response analysis for obtaining 10-2 and 10-4 loads is more or less

straight forward.

7

Lecture February 20

•Drag load governed response (jacket and jack-up structures)

a) Quasistatic analysis (inertial and damping force can be neglected)

OK if natural period is lower than – say – 2s. Design wave analysis

is convenient. Statistical methods used to find Hq, q=10-2 or 10-4.

•Usually Stoke 5th order wave is used in practical work when design

wave method is used for drag governed structures.

8

Lecture February 20

Curiculum

•Chs.3 and 4, SH_lectures week 8 – 2007

•Lecture notes.

•T. Moan: ”Design of Offshore structures”,

Chs. 3.6.3 and 3.64, Chs. 5.1.7 and 5.1.8

9

Lecture March 12Time domain solutions, higher order forcing and design wave method

•Dynamically sensitive drag governed structures:

Short term solution by solving equation of motion in time domain.

The various steps of a time domain analysis were pointed out. The aimed

results of the time domain approach were discussed.

•Higher order forcing:

Example showing the forces experienced by a fixed pile was reviewed for a

sinusoidal wave and a modulated wave. For the modulated wave we saw that

the drift force varied slowly as compared to the wave frequency.

•Design waves – linear or non-linear waves?

10

Design wavesCalculation of design response using Stokes

x

z

Procedure (100-year response):

0) Determine 100-year waveprofile.

i) Response is calculatededfor the given wave position.

ii) Wave is stepped somemeters forward, and theresponse is calculated forthe new position.

iii) Step ii) is repeated until wave has passed structure.

iv) Highest response is takenas characteristic designresponse.

H100

T(H100)

11

Lecture March 12

•Slamming type load

Time

Response

Load

T1

T0

Free vibration

Forcedvibration

12

Lecture March 12

•Results – dynamic amplification of impulse type loading.

(Useful when giving guidance without calculations.)

13

Lecture March 12

Curiculum

•Ch. 5 and 6, SH-lectures_week 11 -2007

•SH_lecture TMR 4195, Monday March 12 2007

•Lecture notes

•Supporting litearture:

T. Moan: ”Design of Offshore structures”,

Ch. 3.6.4, Chs. 5.1.1 – 5.1.5

14

Lecture March 13

Long term response analysis

•Ocean waves as a stochastic process. (Linear waves, second order

waves and freak waves).

•Long term analysis – all sea state approach

• Long term analysis – random storm approach

h t

THTHYX

XY dhdtthfthyFtht

tyFpspsXX

),(),|'(),(

1)'( |

)(1exp)())(()(1

yFrpyFyFssy Y

rR

rYY

15

Lecture March 13

Curiculum

•Ch. 7.1 (except 7.1.4.3) and 7.2, SH-lectures week 12 – 2007

•Lecture notes

•Support litterature:

T. Moan: ”Design of Offshore structures”,

Ch. 3.6

16

Lecture March 19Closing remarks on random storm approach, example random storm, reliability methods for long term analysis, environmental contours

•How one could account for non-observed storms was shown.

•Example of random storm approach was presented. Importance of

including the short term variability of the storm maximum was

demonstrated. Important to understand.

•Long term analysis using reliability methods was presented by using

these methods for obtaining an environmental contour. Again the

importance of including the short term variability was demonstrated.

17

Lecture March 19

Curiculum

•Appendix 7A_Week 12 (Meant to be appendic SH_lectures_week 12 –

2007)

•Lecture notes

•Ch. 7.1.4.3, SH_lectures_week 12 - 2007

18

Lecture March 20

Environmental contour lines, Mooring and Model Testing

• Environmental contour lines were introduced for hs, tp and 3-hour maximum

response. Sources of variability were pointed out. The importance and

consequences of including short term variability was demonstrated.

• Methods involved in designing mooring line loads were briefly discussed.

• Model testing was mentioned as a good alternative for complex systems. Some

advice was given how to use model tests for direct design application (contour

lines and target percentile of 3-hour extreme value distribution for the wors

sea states)

19

Lecture March 20

Curiculum

•Ch. 7.1.4.3, 7.3, 7.4,and 5.4, SH_lectures_week 12-2007

•Motions and Mooring Lines of Floaters

•Lecture notes