API RP 2T-2010 Planning, Designing, and Constructing Tension Leg Platforms

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PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 121

9.6.2 Tendon Pipe

9.6.2.1 General

The tendon pipe cross-section should be designed to safely resist the combined effect of global static loads

arising from pre-tension, tension and bending, which react to the environmental loads acting on the hull, and

external hydrostatic pressure. When local discontinuities are present in an otherwise straight and continuous

pipe body, local secondary bending stresses through the thickness of the pipe section are introduced and the

tendon pipe should also be designed to locally resist the resulting membrane and bending stresses.

The tendon pipe body is not as susceptible to cyclic loads as compared to pipe-to-pipe or pipe-to-connector

girth welds, provided that no damage is inflicted to inner or outer diameter pipe surfaces (ID or OD). One

means of protecting the OD is by using robust protecting coatings. Corrosion (pitting) on the ID should be

prevented during storage, fabrication, and service or allowed for in the design.

9.6.2.2 Pipe Acting Stresses

9.6.2.2.1 General

The global loads obtained according to the safety criteria defined in 5.2 should be used to generate global net

section stresses. If changes in diameter or wall thickness are present, then local through-thickness bending

stresses will also act on the pipe.

9.6.2.2.2 Global Net Section Stress

The net section stress at a cross-section results from axial load and general bending moments. The global

axial stress should be calculated as the normal stress acting on the gross cross-sectional area. Global

bending stress should be obtained as the extreme-fiber normal stress due to the bending moment acting on

the pipe cross section.

9.6.2.2.3 Local Stresses

Local stresses developed in tendons at diameter and thickness transitions should be quantified as described

in 9.6.2.2.4 and 9.6.2.2.5.

9.6.2.2.4 Diameter Transitions

Sectional transitions may be included in the pipe body where a reduction in the tendon diameter-to-thickness

ratio is required. However, the presence of sectional transitions in the tendon pipe body in conjunction will

result in local through-thickness bending stresses due to eccentricity of the global section load path, as well

as hoop stresses due to unbalanced radial loads at the junctions between the transition piece and the pipes.

The magnitude of these stresses depends on the transition piece configuration. Bell-shaped forgings, as

depicted in Figure 11, minimize the local additional stresses. Care should be taken to size the length of the

“straight sections” at the top and bottom such that the local stress levels have essentially returned to the

nominal pipe body stress at the locations where circumferential girth welds are to be made. Otherwise an

additional secondary local stress will be present at the weld, adversely affecting the fatigue life of the girth

weld.

The additional bending (f

') and hoop (f

') stresses may be calculated via finite element analysis (FEA) or

conservatively estimated according to Equation (36) and Equation (37), per API 2A-WSD, Section 3.4.

bab

0.6 ( )

()tan

ttt

fff

=+α

(36)

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122 API RECOMMENDED PRACTICE 2T

hab

0.45 ( ) tan

fff

=+α

(37)

where

t is the tendon thickness;

is the cone thickness;

is the mean tensile stress, T/A

;

is the outer fiber bending stress, M D

/2I ;

α is one-half of the cone apex angle;

T is the tendon wall tension, T

− pA

;

is the tendon effective tension;

p is the hydrostatic pressure outside the tendon at bottom elevation (positive);

is the tendon outside diameter;

is the tendon inside diameter;

is the tendon outside cross-sectional area, π D

/ 4 ;

is the tendon steel cross-sectional area, π (D

− D

)/4 ;

I is the tendon steel moment of inertia, π (D

− D

)/64.

When (f

+ f

) is tensile, the hoop stress f

' is tensile at the smaller-diameter junction and compressive at the

larger-diameter junction.

Referring to Figure 11, the acting radial, longitudinal and hoop stresses to enter the design criteria should be

taken as:

==−

=++

(38)

9.6.2.2.5 Thickness Transitions

Figure 12 depicts a typical thickness transition where local bending and membrane, or net section stress,

developed locally through thickness. For strength checks, these stresses should be obtained from

linearization, as detailed in C.5.1.

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PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 123

Figure 11—Local Stress Check at Tendon Section Transitions

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124 API RECOMMENDED PRACTICE 2T

Figure 12—Typical Section, Applied Loads, and Stress Distributions Through-thickness

9.6.2.3 Pipe Strength Criteria

9.6.2.3.1 General

The tendon pipe should be designed to meet both the global and local strength criteria set forth below.

The industry practice includes use of tension/collapse interaction equations developed for API 2A-LRFD,

D.3.3, but with either WSD or LRFD safety factors. See C.3 for additional information.

For designs using the working stress designs (WSD) and load and resistance factor design (LRFD)

approaches, the recommended criteria are described in 9.6.2.3.2 and 9.6.2.3.3 respectively.

9.6.2.3.2 Global Tension-collapse Strength Criteria with WSD Safety Factors

The tendon pipe subjected to longitudinal tensile stresses arising from the combined action of tension and

bending acting in conjunction with hydrostatic pressure should satisfy the following interaction equations.

0.6 1.0AB AB

++ ≤

(39)

tbt

()

fSF

(40)

(41)

=−

(42)

Differential

Stress

Average

Stress

Linear

Distribution

Peak stress

Elastic

Distribution

()r

(

)

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PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 125

where

is the axial tensile stress due to wall tension;

is the outer fiber bending stress due to bending moment;

is the hoop stress due to hydrostatic pressure;

is the elastic hoop buckling stress;

is the critical hoop buckling stress.

The elastic and critical hoop buckling stresses, F

and F

respectively, are defined as follows:

= 0.88E (t/D)

(43)

where

D is the tendon pipe outer diameter;

t is the tendon pipe wall thickness;

E is the modulus of elasticity;

If F

< 0.55F

(elastic buckling),

= F

(44)

If F

> 0.55F

(inelastic buckling),

0.4

hc y y

0.7

⎡⎤

=≤

⎢⎥

⎣⎦

(45)

The safety factors for tension, SF

, and for hydrostatic collapse, SF

, used in the interaction check are listed in

Table 8 (see C.3).

Table 8—Safety Factors for Tension-collapse Check

Safety Criteria

Tensile

Hoop

A (operating) 1.67 1.63

B (extreme) 1.25 1.38

S (survival) 1.05 1.25

Tendon pipe subjected to compressive wall stresses and hydrostatic pressure should use the appropriate

analysis methods and safety factors provided in API 2A-WSD or API 2A-LRFD.

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126 API RECOMMENDED PRACTICE 2T

9.6.2.3.3 Global Tension-collapse Strength Criteria with Load and Resistance Factors

The tendon pipe subjected to longitudinal tensile stresses arising from the combined action of tension and

bending acting in conjunction with hydrostatic pressure should satisfy the following interaction equation, which

is repeated from Equation (39):

0.6 1.0AB AB

++ ≤

(46)

In the LRFD approach, the terms are defined as:

()

(47)

hhc

(48)

η= −

(49)

where, in the LRFD approach

is the axial tensile stress due to the factored wall tension;

is the outer fiber bending stress due to the factored bending moment;

is the hoop stress due to the factored hydrostatic pressure;

is the elastic hoop buckling stress;

is the critical hoop buckling stress.

The elastic and critical hoop buckling stresses, F

and F

respectively, are defined as follows:

= 0.88E(t/D)

(50)

where

E is the modulus of elasticity;

t is the thickness of the unstiffened tendon pipe;

D is the thickness of the unstiffened tendon pipe.

If F

< 0.55 F

(elastic buckling) F

= F

(51)

If F

> 0.55F

(inelastic buckling)

0.4

hc Y Y

0.7

⎡⎤

=≤

⎢⎥

⎣⎦

(52)

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PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 127

The load factors for TLP tension response are defined relative to static loads (pretension and hydrostatic

pressure), environmental tension effects, and tension margins. The load factors L

, L

and L

and resistance

factors

and φ

used in the interaction code check are listed in Table 9.

Table 9—Load and Resistance Factors for Tension-collapse Check

Design

Condition

Load Factors Resistance Factors

Operating 1.00 1.30 1.50 0.95 0.8

Extreme 1.00 1.10 1.35 0.95 0.8

Survival 1.00 1.00 1.00 0.95 0.8

where

is the load factor for the design margin;

is the load factor for the static pretension;

is the load factor for the environmental and inertia loads;

is the resistance factor for the axial, bending and shear strength;

is the resistance factor for the hoop buckling strength.

In the analysis, the inertia loads are not factored separately from environmental loads because tendon tension

responses are part of an overall dynamic system response, which includes both environmental and inertial

components. It is not feasible to separate applied environmental loads from dynamic inertia loads in a general

response model.

The level of uncertainty of inertia loads is considered similar to that of applied environmental loads in TLP

analysis.

The factored loads are defined in Equation (53) through Equation (55).

The factored wall tension is defined as:

































 (53)

where

margin

is the total of tension margins added external to the response simulation;

pre

is the pretension;

tide

is the tendon tension increase due to the tide and storm surge;

mean

is the mean tension due to the mean environmental loads (including setdown effects);

dyn

is the dynamic tension response induced by the environmental, including the inertia portion of

the response.

The factored bending moment is defined as:

3 mean dyn

()MLM M=+

(54)

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128 API RECOMMENDED PRACTICE 2T

where

mean

is the mean bending moment;

dyn

is the dynamic bending moment.

The factored hydrostatic pressure is defined as:

2w z

LH=γ

(55)

where

is the density of seawater;

is the submerged depth of the tendon section.

In the LRFD factor approach, tendon pipe net section stress utilization ratio shall be checked separately using

Equation (39). The A ratio in the equation shall be no greater than 1.0 based on the factors of safety defined

in Table 8.

9.6.2.3.4 Local Stresses

Local stresses developed due to diameter or thickness transition in the tendon should be checked according

to the following criteria.

a) Diameter transitions—When the hoop stress f

' is tensile, a local check based on a von Mises equivalent

stress at the outer fiber should be made according to the following stress criteria:

r θθz

mem+bend

707[ ]

σσ σσ

σσ

.((

() ) )

=++≤

−−−

(56)

is the nominal strength of the material and SF is the safety factor given in the Table 10 according to the

safety criteria.

When the hoop stress f

' is compressive, f

' should meet the criteria for F

given in 9.6.2.3.2 or 9.6.2.3.3,

except that, in Equation (40) and Equation (47), (f

+ f

) becomes (f

+ f

′), and in Equation (41) and

Equation (48), f

becomes (f

+ f

′).

b) Thickness transitions—The radial and hoop components of stress approach zero such that the von Mises

equation reduces to the following checks for local membrane or net section stress

(σ

mem

) and combined

membrane plus bending stress (σ

mem+bend

mem

σ≤

(57)

mem+bend

σ≤

(58)

The safety factors for local pipe strength may be taken according to Table 10.

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PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 129

Table 10—Local Pipe Strength Safety Factors

Safety Criteria

Safety Factors

Membrane

Membrane +

Local Bending

A (operating) 1.67 1.11

B (extreme) 1.25 0.83

S (survival) 1.05 0.83

9.6.3 Tendon Girth Welds

Satisfying the strength criteria stipulated in 9.6.2 for the tendon pipe body ensures the strength performance

of the girth weld as long as the welds are overmatched in strength relative to the actual maximum pipe

material properties. Loss of pipe wall thickness resulting from dressing the weld reinforcements should be

considered in the design for local fatigue. Girth welds, however, are especially susceptible to cycling loads

due to the presence of flaws inherent to the welding process.

9.6.3.1 Girth Weld Acting Stresses

The cyclic loading acting on the welds to be considered here are the same as those acting on the pipe body

generated via the load analysis methods discussed in 9.4. Additionally, secondary local stress at the weld

resulting from the attachment of mechanical connectors, section transitions, pipe misalignment, or local thin

spots resulting from dressing of the weld reinforcement should be considered.

9.6.3.2 Girth Weld Fatigue Design

9.6.3.2.1 General

The reliable fatigue design of the tendon girth welds requires the interaction between analysis, fabrication

considerations, and qualification and/or confirmation testing as needed. Figure 13 shows an example of

elements to be considered in the fatigue design process for fracture-critical tendon girth welds, which is

complementary to that described in 9.2.5 for the overall design process.

9.6.3.2.2 Girth Weld Fatigue Damage Analysis

Based on the stress histories acting on the tendon girth welds, conventional Miner’s rule cumulative fatigue

damage should be conducted. The resistance side of the damage calculation should use a design S-N curve

consistent with the type and quality of weld sought.

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130 API RECOMMENDED PRACTICE 2T

Figure 13—Design, Fabrication, and Verification Process for Fracture-critical Tendon Welds

For tendon welds designed not to be inspected for fatigue cracks in-service, the calculated S-N fatigue life

should be at least 10 times the planned life of the facility. A lower life design factor may be used when:

a) a proven, reliable in-situ inspection/crack detection method and an expedient replacement or repair plan

are to be employed, or

b) the uncertainties of the variables entering the damage calculations are reduced (Wirsching, 1986

[250]

). In

no case, however, should the fatigue life of the weld be less than three times the planned life of the

facility.

S-N design curves given by BS 7608, Table 7, for circumferential butt welds in tubes are recommended in

accordance with the manufacturing requirements, special inspection requirements and notes stipulated.

Welds made from one side but which otherwise meet all other requirements stipulated for Class C can also be

assessed with the Class C curve. Any other pertinent curve may be used, provided it is relevantly justified on

the basis of pertinent data. Invoking an endurance limit or change in slope, such as in BS 7608, should be

based on data that includes variable amplitude loading effects. If welds are ground properly (see 9.6.3.3) a

correction for thickness effect on fatigue may be waived. A thickness effect may also be waived if the S-N is

based on actual component size data (see C.4.1).

Require

-Tolerances

-Properties

Select

-Pipe Size

-Pipe Type

Analyze

Dynamic

Response

OK ?

Data

da/dN

Calculate

FM for Life

& Fracture

Yes

Assume

Flaw Size,

Local & Type

Require

-Life

-Max. Load

Select

Mech. Prop.

Set

Flaw Size

& type

Fatigue

Damage

Calculate Input

- SN Curve*

- SCF

Select

Inspection

Method

Determine

PoD and

Accuracy

Yes

OK ?

Make

Weld

Samples

Qualify

-Mech. Prop.

-Residual σ

-Hardness

Develop

Welding

Procedure

Inspect

Qualified

Samples

OK?

Yes

Material

Welding

LIFE

Select

Welding

Variables

Select

Yes

Analysis

Inspection

Start

Test

Fatigue &

Environment

Fatigue

Specimens

Design & Fab

tests

design

End

Inspect

Qualified

Samples

OK?

Testing

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