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Platform, P ipeline and Subsea Technology 1

FATIGUEANALYSIS

A conceptual design study is being prepared for the braced monotower illustrated above.The design water depth is 40 metres and the topsides weight is 150 tonnes. The design lifeof the structure is 15 years.

The upper brace to caisson joint shown is continually submerged when the structure is inplace, and the structure has a passive cathodic protection system.

The oceanographic consultants have determined that the majority of the waves come fromthe North-West and have provided the following simplified annual wave occurrence table:

Wave Height (m) Period, Tz (sec) Number of Waves

(from North-West)1.5 6.8 2,647,100

4.5 9.0 98,850

7.5 10.5 7,180

11.0 11.2 620

17.0 12.0 60

An analysis has been carried out to determine the brace loads under the action of each ofthe waves in the above occurrence table. The effect of dynamics has been accounted for bytreating the structure as a single degree of freedom system, and applying a Dynamic

Amplification Factor (DAF) to the loads. The results of the analysis (including dynamics) aregiven below:

Brace:1000mm OD

22mm wtChord:2400mm OD

50mm wt50

90

45

0

36m

7/29/2019 Fatigue Assignment 2010

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Platform, P ipeline and Subsea Technology 2

Brace Load Ranges

Wave Height (m) Axial Force(compression)

(kN)

In-Plane

Bending Moment(kNm)

Out-of-PlaneBending Moment

(kNm)

1.5 100 70 20

4.5 370 250 75

7.5 685 460 135

11.0 1080 730 215

17.0 1830 1225 360

Stress Concentration Factors for the joint can be calculated using Table 1 of the Efthymiouparametric equations. For the purpose of your analysis, neglect the influence of the nearbybraces on the SCFs. The main caisson can be assumed to be equivalent to a pinnedmember for the purpose of this assignment.

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1. Use the simplified wave data and HSE S-N curves to make a preliminary estimate of thefatigue life of the brace, considering the hot-spot location at 45 . It may be assumed thatall the loads given in the above table add at this hot-spot location. Only consider the in-place loads in the analysis. (10 marks)

2. Which of the wave height populations contributes most to the fatigue damage at thislocation? (2 marks)

3. What methods could be used to inspect this joint for fatigue damage? (3 marks)

4. The operator is interested in increasing the topsides weight from 150 to 200 tonnes.What implications would this have on the fatigue performance of the joint assuming thatthe member sizes dont change? (5 marks)