1

A STUDY ON BEHAVIOR OF TENDONS ON STABILITY OF TLP

BYK. DINESH REDDY

(M120215CE)

GUIDED BYDr. T. P. SOMASUNDARAN

2

Contents

• Introduction

• TLP Mechanics

• Assumptions

• Influence of tension, weight and hydrostatic

pressure on TLP tendons

• Effect of static offset on TLP modelling

• Effects of tendon disconnection on TLP

• Summary

• References

3

Introduction

• What is TLP ?

• What is tendon ?

• What is tendon function ?

• On what factors the tendon influenced ?

• What is an offset and setdown ?

• Why pretension for tendon ?

4

TLP Mechanics

5

Cont ….

6

Cont ….

7

Gravity/Buoyancy loading

8

Gravity/Buoyancy loading

9

Node centred wave loading

10

Node centred wave loading

11

DOF

12

Assumptions

• Initial pretension in all tethers is equal and

remains unaltered over time.

• Wave diffraction effects are neglected.

• Change in the pretension is calculated at every

time step and the equations of equilibrium at each

time step modify elements of the stiffness matrix.

• Wave forces on the tethers are assumed to be

negligible.

13

Influence of tension …..

• For small angles

• Fz is about 7% of

total buoyancy and

Fx is 25% for an

angle of 150.

14

Cont …..

15

Cont ….

• From the above equation, top tension can be reduced by

reducing the tendon weight in air or reducing the bottom

tension.

• Reducing the top tension permits the flexibility to

reduce the size of the platform or to increase the deck

playload.

16

Cont ….

• Reaction on the hull R1 can be

minimised by reducing the

tendon weight in air and

reactions at the bottom anchor

connector R0.

• Increasing the buoyant force

by increasing the D/t ratio.

17

Cont ….

• Minimum tension are governed by two different

requirements.

1. to provide the required contact in the anchor

connector.

2. not to exceed the global and local buckling

loads.

• The minimum tension requirement is normally specified

to be greater than zero.

18

Composite Materials

• According to author (Shaddy et al., 1989) two composite tendons

were examined, one with the same cross-section area as the steel

tendon (60 sq in.) and other with sufficient cross-sectional area to

provide the same vertical stiffness (85 sq in.) as the steel tendon.

For neutrally buoyant tendons (zero weight in water), both

materials will provide the same hull reactions.

• In this case, the composite tendon provides an advantage since it

has approximately one-half the diameter required by steel tendon

and small diameter translates into less drag force.

• To avoid hydrostatic collapse in very deep waters, steel tendons

are limited to D/t ratios of about 15.

19

Cont ….

20

Cont ….

• In case of zero current, steel and composite neutrally

buoyant tendons will both provide a restoring force ratio

of one.

• If the steel tendon D/t ratio is limited to 15, the

composite tendon will provide 19% higher restoring

force.

• In the presence of a 3 ft/sec current, the restoring force

of the composite tendon is approximately 30% higher

than for the steel tendon with D/t = 15.

21

Cont ….

• The outer diameter is 0.6096 m, wall thickness is

20.6248 mm, pretension is 3337.5 kN, maximum

top tension, is 6675 kN, minimum top tension, is

1468.5 kN, water depth, is 549 m, tendon top

depth, is 18.3 m and solid diameter 220.4212

mm.

22

Cont ….

Tensions and Reactions (kN) Tubular tendon Solid tendon

Minimum top tension, T1 1468.5 1468.5

Maximum top tension, T1 6675 6675

Maximum top rection, R1 6728.7 6682.02

Maximum bottom tension, T0 5117.5 5117.5

Minimum bottom tension, T0 -89 -89

Minimum bottom reaction, R0 1521.9 121.485

Maximum bottom reaction, R0 6728.4 5327.985

23

Effect of Static Offset on TLP

• The dynamic characteristics of a TLP are functions of

the magnitude of a static offset that the TLP may have

experienced under the action of wind and current loads

acting on the platform.

• This dependence, which becomes more pronounced as

the water depth increases.

24

Stiffness formulation

Deformation of Tendon or Riser under own weight

25

Cont ….

K = f -1 This gives stiffnes matrix of individual tendon

26

Cont ….

27

Cont ….Undeformed

28

Effects of tendon disconnection

• TLP is one of the proven technologies to support the

risers in the severe environment by allowing negligible

vertical-plane motions, as heave, roll, and pitch.

• The vertical-motion characteristics of the TLP are

mainly determined by the tendon configuration and

properties, while those of other floaters are mostly

affected by the hull geometry.

29

Cont ….

• Thus, damaged or broken tendons may result in

catastrophic impact on the TLP hull and risers.

• The TLP tendons may break at the top or unlatch at the

bottom during the harsh environment.

• The break at the top may occur when the tension

exceeds the breaking strength.

• The unlatch at the bottom may happen when the bottom

tension becomes negative and experiences the down

stroke.

30

Cont ….

31

Cont ….

• Numerical Model

• Natural periods and damping factors

32

Cont ….

Heave – free decay

33

Cont ….

Comparison of transient effects after down-wave tendons unlatched at the bottom-top tension of the unlatched tendons

34

Cont ….

Comparison of transient effects after upwave tendons broken at the top-top tension of the most neighboring tendon

35

Cont ….

Transient effect on tension when #2 tendon is broken in a regular wave (T = 12s and H=7.6m)

36

Summary

• For the uniform straight tendons and for inclination angles up to

150, most of the upthrust buoyancy forces are generated from

pressures acting on the tendon ends and not on the tendon sides.

• Reducing the tendon top tension can be accomplished by reducing

the bottom tension or reducing the tendon weight in air.

Lightweight composite materials, therefore, can provide a

significant advantage.

37

Cont ….

• Minimum tension requirements are established by two different

criteria. The first criterion is to maintain a minimum compressive

reaction on the elastomer joint. The second criterion is to maintain

minimum tension or maximum compression sufficient to prevent

global buckling and unacceptable stresses.

• For deepwater slender steel tendons, preliminary results indicate

that the hydrostatic collapse crieteria is a more active constraint

than the strength criteria. For very deep waters, the allowable D/t

ratio is limited to about 15.

38

Cont ….

• The presence of a large static offset does lead to significant

changes in the stiffness matrix of the platform, but does not

necessarily lead to significant changes in all 6 fundamental natural

periods of the TLP.

• The periods of the so called horizontal modes (surge, sway, yaw)

do decrease somewhat and do so in manner consistent with the

increase in the total tendon force.

• As the static offset increases, greater coupling occurs among the

various natural modes.

39

Cont ….

• The heave and pitch natural periods are appreciably affected by

the tendon breakage and unlatch.

• The up-wave tendon breakage increases the maximum pitch and

tension more than the down-wave tendon unlatch because the mean

heel angle of the latter is smaller due to the weight of hanging

tendons.

• The inclusion of nonlinear terms in the wave excitations is

important to find the dynamic and maximum tendon tension in

reliable manner.

40

Cont ….

• The transient effect generally increases as the number of broken

lines increases.

• However, in the unlatched case, the maximum tension is less

affected because the unlatch happens when the tension is

minimum.

41

References

• Chandrasekaran, S., Jain, A. K., Chandak, N., R 2007. “Response Behavior of Triangular

Tension Leg Platforms under Regular Waves Using Stokes Nonlinear Wave Theory”. Journal

of Waterway, Port, Coastal, and Ocean Engineering 133, 3.

• Murray, J., Yang, C.K., Yang, W., Krishnaswamy, P., Zou, J., 2008b. “An extended tension

leg platform design for post-Katrina Gulf of Mexico”. In: Proceedings of the international

Offshore and Polar Engineering Conference (ISOPE08) #287, Vancouver, Canada.

• Oran, C., 1992. “Effect of static offset on TLP modeling”. Journal of Engineering Mechanics

118, 74-91.

• Shaddy, Y.H., William, H.T., Jerry, G.W., 1989. “Influence of Tension, weight and

hydrostatic pressure on TLP tendons”. Journal of Waterway, Port, Coastal, and Ocean

Engineering 115, 172-189.

• Yang, C.K., Kim, M.H., 2010. “Transient effects of tendon disconnection of TLP by hull-

tension-riser coupled dynamic analysis”. Ocean Engineering 37, 667-677.

42

THANK ‘U’