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NTNU Faculty of Engineering Science and Technology Department of Marine Technology

EXERCISE 2

TMR 4195 DESIGN OF OFFSHORE STRUCTURES Due date: 31.01.11 Distributed: 17.01.11 Sign: V.L.

PROBLEM 1 Overturning Stability of Jack-up

Figure 1 shows a jack-up platform with four legs and a square deck. The legs rest on the sea

floor without penetration. The deck, including fixed equipment, has a mass of 2.0E+06 kg.

The payload varies between 1.0E+06 and 2.0E+06 kg, which is also assumed to be uniformly

distributed over the deck. Each column has a mass of 6.0E+05 kg and is flooded. Suppose that

the platform is subjected to a 100-year sea load. This load is for simplicity assumed to vary

linearly on each leg, with a maximum value q0, equal to 4.0E+04 N/m.

Check the overturning stability according to the DNV-Rules (Appendix I).

FIGURE 1 Jack-up Platform

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PROBLEM 2 Damage Stability of Semi-submersible

This exercise is concerned with the damage stability of a semi-submersible drilling platform

(See Figure 2). Relevant DNV-Rules in Appendix I are to be used.

a) Based on information from Figure 2, sketch the effect of damage on:

1. Neutral axis for the waterline plane

2. Center of buoyancy and gravity

3. GZ-curves

FIGURE 2 Semi-submersible platform with damage

b) Briefly review the stability criteria used for semi-submersible mobile drilling units for

1. Intact condition

2. Damaged condition

3. Comment on differences of the above two.

c) How are wind forces to be determined for the stability of mobile units?

1. Comment on the accuracy of this calculation procedure.

2. How are the effects of wave motions considered?

d) Figure 3 shows the GZ-curves for intact and damaged conditions for this platform.

1. Explain the characteristic features of the curves in Fig 3b.

2. Assume that the ‘heeling arm’ is 0.5m for all angles of heeling, check whether the intact

stability criteria are fulfilled. Does the height of the deck have any influence on the

compliance with the stability requirements? If the criteria are not fulfilled, what can be

done to fulfill them?

3. Assume that the ‘heeling arm’ is 0.25m for all angles of heeling in damaged condition.

Check whether the damage stability criteria are fulfilled. If the criteria are not fulfilled,

what can be done to fulfill them?

4. What is the most critical case with respect to complying with damage stability criteria:

total loss of a column or complete flooding of the column? And why?

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FIGURE 3a GZ-curves for semi-submersible (Intact)

FIGURE 3b GZ-curves for semi-submersible (Damage)

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DNV

Stability-related Rules Mobile Offshore Units Pt.3 Ch.2

Appendix I TMR 4195 Design of Offshore Structures Exercise 2

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