Section 2 Design Loads

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201. Design loads

The design conditions are to be established by varying vessel size and loading conditions to de- termine the critical loading conditions under the enviornmental conditions described in 104. The de- signer is to submit calculations for the design condition. The following loads are to be considered in the design:

(1) Dead loads and buoyancy

(2) Environmental loads

(3) Mooring loads

(4) Fatigue loads

202. Dead loads and buoyancy

Dead loads are the weight of the SPM structure and associated structural appendages, and equip- ment which are permanently attached to the structure.

The buoyancy of the SPM structure result in upward forces, the distribution of which depends on the distribution of the submergence of the structure.

203. Environmental loads

The environmental loads due to the following environmental parameters are to be considered in the design :

(1) Waves

(2) Wind

(3) Currents

(4) Tides and storm surges

(5) Ice and snow

(6) Marine growth

(7) Air and sea temperatures

(8) Other phenomena, such as tsunamis, submarine slides, seiche, abnormal composition of air and water, air humidity, salinity, ice drift, icebergs, etc. may require special consideration.

1. Wave loadings

The wave loads on the SPM structure and the moored vessel are to be determined by suitable methods such as strip theory, diffraction theory, Morison's equation, etc. The wave loading on a tower mooring is to be in accordance with the Pt 3 of "Rules for Fixed Offshore Structures."

The wave induced responses of a vessel consist of three categories of response, e.g., first order (wave frequency)motions, low frequency or slowly varying motions, and steady drift are to be taken into account for designing the SPM structure including the mooring line, anchors, piles, etc. as applicable.

2. Wind forces

For a moored vessel, wind forces on the vessel may be calculated using the coefficients presented in the document "Prediction of Wind and Current Loads on VLCCs", Oil Companies International Marine Forum (2nd Edition, OCIMF), 1994. For equipment onboard with unusual shape and ar- rangement, wind forces on such equipment may be calculated as drag forces and are to be added as necessary. Wind tunnel tests may be required in some design to determine the wind loads.

The wind force on the SPM structure and the moored vessel is considered as a constant (steady) force due to the one-minute wind. Alternatively, the designer may use a one-hour wind with appro- priate wind spectrum.

The wind force on the SPM structure and wind exposed appendages and unusual items onboard the

vessel may be calculated as drag force. The wind pressure calculated using the following equations :

on any particular windage may be

× ( )

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: Wind velocity in

:

:

Shape coefficient (dimensionless) in Table 2.1 Height coefficient (dimensionless) as follows

where, the velocity of wind at a height

above water line is to be calculated as fol-

×

is to be taken as equal

at elevations below the reference elevation

: Velocity of wind at an reference elevation

of 10

: 0.10 typically for one-minute average wind, other values supported by site spe- cific data will be specially considered.

The corresponding wind force

on the windage is:

×

( )

: Projected area of windage on a plane normal to the direction of the wind, in

The total wind force is then obtained by summing up the wind forces on each windage. The shape coefficients for typical structural shapes are presented (for reference only) in Table 2.l. The height

coefficients to represent the wind velocity profile(corresponding to value of 0.10) are in

presented

Table 2.1 Shape coefficient

for windages Table 2.2 Height coefficient

3. Current forces

For a moored vessel, current forces on the vessel alone may be calculated by using coefficients based on model test data as presented in "Prediction of Wind and Current Loads on VLCCs", pub- lished by OCIMF (2nd Edition, 1994). For underwater bodies of unusual shape and arrangement model tests may be required to determine the current forces.

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The current forces on the submerged buoy and/or mooring structure, hull of the moored vessel, mooring lines, rises or any other submerged objects associated with the system are to be calculated using the appropriate current profile. The basis of the current profile depends on the environmental conditions described in 104. 1.

Current force on the submerged part of the mooring structure, mooring lines, rises, etc. are to be calculated as the drag force as shown below :

(

: mass density of water, 1.025

:

drag coefficient, in steady flow (dimensionless).

current velocity vector normal to the plane of projected area in

: projected area exposed to current in

204. Mooring loads

The design loads of mooring legs, and mooring elements(flexible hawsers or rigid mooring element such as arm and yoke) between the vessel and SPM may be calculated based on physical model testing of the system, or by analytical methods verified by physical model testing of a similar system. The calculation to determine the mooring load is to include high frequency, low frequency, and mooring line dynamics. The most probable extreme values are to be obtained by time domain analysis for the design storm described in 104. 2 using a storm duration of three hours, unless specific site data supports other durations.

1. Operating mooring loads

Operating mooring loads are the loads on the SPM structure and foundation with the vessel moored

to it. The loads are to be dition as indicated in Sec

determined in the operating environment for the established design con-

1. Operating mooring loads are to be established and submitted for the

hawser, rigid connection between the vessel and the SPM as applicable, and the SPM anchor leg

loads.

(1) Operating mooring load between vessel and SPM

The operating mooring load between vessel and SPM is to be established for the SPM system. The operating mooring load is defined as the maximum load imposed on the mooring element (e.g., hawser or rigid ann and yoke)for the maximum size vessel for the operating environmental condition described in 104. 1, unless a smaller moored vessel is apt to impose higher loads un- der the influence of the operating wind, wave, current and tides as established in Sec 1. Data and calculations are to be submitted to establish the validity of this operating mooring load. The operating mooring load may be statistically determined from model testing and/or analysis. The model testing and analysis on which the operating mooring load is based is to reflect the com- bined effect of wind, waves, current and tides on the loaded and unloaded vessel. The model testing is to model the mooring system appropriately in regard to load-displacement character- istics, and pretensioning of mooring legs as applicable.

(2) Operating anchor leg loads

The anchor leg loads in the operating environmental condition are to be established for the an-

chor leg or legs with the vessel at the mooring. The operating anchor leg load is defined as the maximum load in the most highly loaded anchor leg for the maximum size vessel for which the SPM is designed, or other vessel of a smaller size if the smaller vessel is apt to im- pose higher loads. For a mooring system with several anchor legs of different size or con- struction, an operating anchor load is to be established for each anchor leg. Model test data and/or calculations are to be submitted to establish the validity of the operating anchor load.

2. Design storm loads

Design storm loads are to be established for the SPM structure, each anchor leg, and the founda- tion as applicable for the storm condition as described in 104. 1. Model test data and/or calcu- lations are to be submitted to establish the validity of these loads.

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205. Fatigue loading

For tower mooring system, fatigue analysis of the structure is to be performed in accordance with

Part 3 of "Rules for Fixed Offshore Structures "