Section 2 Mooring Analysis

201. General

1. Mooring analysis is to be conducted based on the environmental conditions as specified in Ch 3, Sec 3. Such analysis is to include the evaluations of the mean environmental forces, the extreme response of the Units, and the corresponding mooring line tension.

2. Mooring system analysis as deemed appropriate by the Society is to be carried out for the all pro- spective mooring conditions. The effects due to the draught changes of the Units are to be taken into consideration. In the case of Units mooring to individual periphery facilities, such as CALM, separate from the Units, mooring analysis for the total system, including any periphery facilities, is to be carried out.

3. In case of mooring systems using mooring lines, analysis is to be carried out under the awareness that there is no harmful excessive bend of any lines in way of the contact points between mooring lines and mooring equipment (fairleaders, etc.) fitted on board Units.

4. The mooring systems of Units and the seabed mooring points (anchors, sinkers, piles, etc.) of any periphery facilities for positioning are not to be slid, uplifted, overturned, etc. against any envi- sioned force from the mooring lines. In cases where scouring effects are not considered to be neg- ligible, appropriate consideration is to be taken such as the modification of burial depth, protection against the flow around seabed mooring points, etc.

5. Mooring analysis is to be made under the awareness that the equipment for mooring systems is subjected to steady forces of wind, current and mean wave drift force as well as wind and wave induced dynamic forces. Maximum line tension is to be calculated considering that wind, wave, and current come from unrestricted directions. However, in cases where the data for the specific positioning area of a Units prove a restricted direction of wind, wave and current in that area, cal- culations under such specific directions may be accepted in cases where deemed appropriate by the Society.

6. The maximum offset of a Units and maximum tension of a mooring line is to be calculated.

Depending on the analysis objectives, a quasi-static analytical method, or dynamic analytical method

as deemed appropriate by the Society may be used for calculations.

7. In the case of deep water operations with large numbers of production risers, mooring system anal- ysis is to take into account riser loads, stiffness, damping, etc. in case where the interaction be- tween Units/mooring systems and riser systems are significant.

202. Mean environmental forces, etc.

1. The calculation of steady forces due to wind and current are to be in accordance with Ch 3, Sec 3.

2. Mean and oscillatory low frequency drift forces may be determined by model tests or using hydro- dynamic computer programs verified against model test results or other data. Mean drift forces to be as deemed appropriate by the Society.

3. Load information is to be prepared based on appropriate analysis, model tests, etc., and such in- formation is to be provided on board.

203. Maximum offset and yaw angle of the installation

1. Maximum offset may be calculated as the sum of the offset due to steady components such as wind, current, and wave (steady drift), and dynamic motion offset due to the dynamic components of forces induced by waves (high and low frequency).

2. The following formula is to be adopted as the standard for calculating maximum offset. In the fol- lowing formula, mean offset and significant single amplitude or maximum amplitude of the max- imum offset obtained from model tests or analysis methods deemed appropriate by the Society are used.

Åmax G ÅŶẀ ŴŸ Ğ ÅŶX Fmax F Ğ ÅŽX FZYX F

or

Åmax G ÅŶẀ ŴŸ Ğ ÅŶX FZYX F Ğ ÅŽX Fmax F

whichever is greater.

where ÅŶẀŴŸ ÅŶX FZYX F ÅŽX FZYX F

: Mean offset of the Units due to wind, current and

: Significant single amplitude low frequency motion

: Significant single amplitude wave frequency motion

mean drift

3. The maximum values of low frequency motion ÅŶX Fmax F and wave frequency motion ÅŽX Fmax F may be calculated by multiplying their corresponding significant single amplitude values by the factor Æ,

which is to be calculated as follows:

Ë

Æ G JË ĬJË∈À

J

À G Å ÅŴ

Å : Hypothetical storm duration (seconds), minimum 10,800 (i.e. 3 hours). In the case of areas with longer storm durations (monsoon areas), Å needs to be a higher value.

ÅŴ : Average response zero up-crossing period (seconds)

4. In the case of low frequency components, ÅŴ may be taken as the natural period ÅŸ of a Units with a mooring system. ÅŸ can be calculated as follows using the mass of the Units Ŷ (including added mass, etc.) and the stiffness of the mooring system Ý for horizontal motion (port-starboard, fwd-aft, yaw motion) at the Units s mean position and equilibrium heading as follows:

Ĭ

JŶ

ÅŸ G ËŘ JÝ

In such cases, information about the stiffness of mooring systems, damping forces, and other pa- rameters which may affect the maximum values of low frequency motion are to be submitted to the Society for reference.

5. In order to assess the motion of Units in waves in relatively shallow water, shallow water effects are to be taken into account. In cases where the changes in tidal levels in shallow waters are rela- tively large, the tidal difference affecting Units motion and the tension acting on mooring lines is to be considered.

6. In the case of single point mooring systems, the maximum offset for motion in waves is to be calculated using a non-linear time history domain method or model tests. In such cases, wave ir- regularities and wind variances are to be considered as well.

204. Calculation of mooring line tensions, etc.

1. In order to calculate the maximum tension acting on the mooring lines, the severest combination of wind, waves and current is to be considered together with a sufficient number of angles of incidence. Although this severest condition generally corresponds to cases where all of the wind, wave and current directions are consistent, in the case of specific sea areas, the combination of wind, waves and current in different directions which are likely to create a higher tension are to be taken into account as needed.

2. In calculating the tension acting on mooring lines, at low are to be considered. Sub-paragraph (4) may be

least Sub-paragraph (1) to (3) mentioned be- assessed as necessary. This analytical proce-

dure can be called a quasi-static analytical procedure and is to be adopted as the standard for cal- culating the tensions acting on mooring lines. The maximum tension of mooring lines calculated by this quasi-static analytical procedure has to have, in principle, a suitable safety factor specified in Table 6.1 corresponding to specific breaking tension.

(1) Static tension of mooring lines due to net weight and buoyancy.

(2) Steady tension of mooring lines due to a steady horizontal offset of Units induced by wind, waves and current.

(3) Quasi-static varying tension of mooring lines due to Units motion induced by waves.

(4) Tension of mooring lines in consideration of their elastic elongation in cases where they are

used in a moderately taut condition (generally in shallow waters), or in cases where mooring lines with low rigidity such as fibre ropes are used.

Table 6.1 Safety Factors for Mooring Lines

3. The maximum tension in a mooring line Åmax is to be determined as follows:

Åmax G ÅŶẀŴŸ Ğ ÅŶX Fmax F Ğ ÅŽX FZYX F

or

Åmax G ÅŶẀŴŸ Ğ ÅŶX FZYX F Ğ ÅŽX Fmax F

whichever is greater where

ÅŶẀŴŸ ÅŶX FZYX F ÅŽX FZYX F

: Mean mooring line tension due to wind, current and mean steady

: Significant single amplitude low frequency tension

: Significant single amplitude wave frequency tension

drift

The maximum values of low frequency tension ÅŶX Fmax F and wave frequency tension ÅŽX Fmax F are to be calculated by the same procedure as that used for obtaining the motions at low frequency and wave frequency described in 203. 2 above.

4. Mooring systems are to be designed so that the failure of any one mooring line does not cause the progressive failure of the remaining mooring lines. The tension acting on the remaining mooring lines is to be calculated using the quasi-static analytical procedure. The safety factors for the ten-

sion of such mooring lines are, in principle, not to be less than those specified in Table 6.1 cor- responding to their respective specific breaking tension. The period of recurrence of environmental loads such as wind and wave loads, however, may be taken as one year.

5. In the analysis of the one broken mooring line condition given in Par 4 above, in the case of a Units which is moored in the proximity of other Units, the safety factors for any mooring lines ar- ranged on the opposite side of the other Units are to be taken as 1.5 times of those indicated in Table 6.1.

6. In cases where the following Sub-paragraph (1) and (2) are taken into account in addition to Par

2 above, the safety factors required in cases where quasi-static analytical procedures are adopted

may be modified to values deemed appropriate by the Society.

(1) Dynamic tension in mooring lines due to damping forces and inertia forces acting on each mooring line in cases where they are generally used in deep water.

(2) Quasi-static low-frequency varying tension of mooring lines due to the low-frequency motion of

Units in irregular waves in cases where they are used in a sufficiently slack condition. (in cas- es where the natural period of motion of a Units in a horizontal plane is sufficiently longer than the period of ordinary waves)

7. In the case of Taut Mooring systems, the following are to be complied with in addition to Par 1

to Par 5 above:

(1) Such systems are to be designed so that no slack is caused in any mooring line due to changes in line tension.

(2)

(3)

(4)

Changes in the tension of mooring lines due to tidal difference including astronomic tides and

meteorological tides are to be considered.

The effects of any changes in the weight and displacements of heavy items carried on board upon the tension of mooring lines are to be sufficiently taken into account.

In cases where the effects of the non-linear behavior of mooring lines on their tension are not negligible, tension due to non-linear behavior is to be considered.

205. Fatigue analysis

1. The fatigue life of mooring lines is to be assessed in consideration of the changing tension range, Å and the number of cycles, Ÿ. The fatigue life of mooring lines is to be evaluated by estimating the fatigue damage ratio, ǼY in accordance with Miner's law using a curve relating the changing tension range to the number of cycles to failure.

ŸY

ǼY G JÀ Y

ŸY : Number of cycles within the tension range interval, Y , for a given sea state.

ÀY : Number of cycles to failure at changing tension range, The cumulative fatigue damage, Ǽ for all expected number of wave scatter diagram) is to be calculated as follows:

ÀÀ

Ī

Ǽ G ǼY

Y G Ë

ÅY .

sea states ÀÀ (identified in a

The value of Ǽ divided by the usage factor (ɳ) specified in Table 6.2 is not to be greater than

1. In such cases, the usage factors for the underwater parts of the mooring lines are, in principle,

to be taken to be that of an inaccessible and critical area.

2. The fatigue life of each mooring line component is to be considered. Å G À curves for various line components are to be based on fatigue test data and regression analysis.

3. Special consideration is to be given to the fatigue strength of the connections between the mooring lines and hull structures of Units, the connections between the mooring lines and seabed mooring points, and the connections between the mooring lines and other mooring lines.

Table 6.2 Usage Factor, ɳ