Section 3 Structural Design and Stability

301. General

1. Floating SPM structure

A floating SPM structure consists of a buoyant hull held in position by anchor leg(s) that transmit mooring forces to the seabed, the equipment and piping used to carry fluid cargo or products, and provides a platform for hawser mooring attachment points.

2 Fixed SPM structure

Fixed SPM structures, such as a SALM or a tower mooring, are typically supported at the seabed by piles or a gravity based foundation. A SALM is often designed as buoyant structure, while a tower mooring may be designed with tubular members. The structure supports the equipment and piping used to carry fluid cargo or products, and provides a platform for hawser (or rigid) mooring attachment points.

302. General design criteria

1. Strength of structure

The structure and framing members are to be of adequate size and strength to withstand the operat- ing and storm loads established in Sec 2. Each mooring attachment point between vessel and the SPM is to be designed to withstand an appropriate portion of the total operating mooring load on the connecting structure (hawser or rigid yoke). Each anchor attachment point or pile foundation is to be designed to withstand the operating load or the storm load, whichever is greater. Stress levels due to loads as determined from Sec 2 are to be within the requirement given in 303. and 304.

2. Pile foundation

For an SPM structure intended to be anchored by piles, the pile design is to be in accordance with the appropriate sections of API RP 2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms."

3. Corrosion control

Where deemed necessary to suit the particular type and service of the structure, a reduction in scantlings in association with protective coatings with or without sacrificial anodes may be consid- ered from those determined by the requirements in 305. 1 to 3. The maximum reduction that will be allowed is 10 % of the shell plating, but not more than 3 , provided that the section mod- ulus reduction is no more than 10 %. In such instances, the justification for the reduction is to be submitted for review together with the particulars of the coatings with or without sacrifical anodes including the program for maintenance. The plans are to show the required and proposed

both suitably identified. Where any of the proposed reductions are approved, a notation will be made in the Record that such reductions have been taken.

Where scantlings and structural design are determined by the requirements of 303. and 304. or by alternative structural design methods other than the requirements in 305. the following apply :

(1) Where effective methods of corrosion needed. The particulars of the coatings for maintenance are to be submitted.

control with or

are provided additional scantlings may not be without sacrificial anodes including the program

(2) Where effective methods of corrosion

thicknesses are to be suitably increased

control are not provided, the scantlings and structural

by a margin based on expected rates of corrosion partic-

ular to the SPM's location and the type of

corrosive environment in contact with the structure.

The scantling increases are to be submitted to the Society for review.

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303. Stresses

1. Structural analysis

The overall structure of the SPM buoy is to be analyzed using appropriate methods, such as frame analysis or finite element methods to determine the resultant stresses for each member, under the loadings stipulated herein. A complete analysis is to be submitted for each of the structural frames for review. Full consideration is to be taken of secondary stresses, carry over moment, etc., and of three dimensional aspects such as direction of applied forces or reactions. Consideration is to be given to the need of analysis for each loading condition including the following:

(1) Transmission of the operating hawser (or yoke) load from the hawser (or yoke) attachment point(s) to the anchor leg attachment point(s) or to the foundation,

(2) Application of the maximum anchor load to the anchor leg attachment point including applica- tion of appropriate wave and hydrostatic loads, in the case of a floating structure,

(3) Application of the maximum wave, maximum wind and maximum current loads in the case of a fixed structure.

2. Bending stresses

(1) Provisions against local buckling

When computing bending stresses, the effective flange areas are to be reduced in accordance with accepted "shear lag" and local buckling theories. Local stiffeners are to be of sufficient size to prevent local buckling or the allowable stress is to be reduced proportionately.

(2) Consideration of eccentric axial loading

In the consideration of bending stresses, elastic deflections are to be taken into account when determining the effects of eccentricity of axial loading and the resulting bending moments super- imposed on the bending moment computed for other types of loadings.

3. Buckling stresses

The possibility of buckling of structural elements is to be specially considered in accordance with

304. 3 For a fixed SPM structure, the buckling of tubular members is to be evaluated in accord- ance with Pt 3 of "Rules for Fixed Offshore Structures."

4. Shear stresses

When computing shear stresses in bulkheads, plate girder webs, or shell plating, only the area of the web is to be considered effective. The total depth of the girder may be considered as the web depth.

304. Allowable stresses

1. General

The members of effective structural elements of the SPM structure are to be analyzed using the loading conditions stipulated below and the resultant stresses are to be determined.

For each loading condition considered, the following stresses are to be determined, and are not to exceed the allowable stresses in 2.

(1) Stresses due to combined gravity, environmental and mooring loading at operating design con- dition as described in 104. 1.

(2) Stresses due to combined gravity, environmental and mooring loading at design storm condition as described in 104. 1.

2. Member stresses

Individual stress components and as applicable, direct combinations of such stresses, are not to ex- ceed the allowable stress , as obtained from the following equation.

: specified minimum yield point or yield strength as defined in Pt 2, Sec 1.

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: factor of safety

for design operational loadings as defined in 304. 1. (1) for axial or bending stress : 1.67

for shear stress : 2.5

for design storm loadings as defined in 304. 1. (2) for axial or bending stress : 1.25

for shear stress : 1.88

3. Buckling considerations

Where buckling of a structural element due to compressive or shear stresses, or both is a consid- eration, the compressive or shear stress is not to exceed the corresponding allowable stress as obtained from the following equation.

: critical compressive or shear buckling stress of the structural element, appropriate to its di-

mensional configuration, boundary conditions, loading pattem, material, etc.

: factor of safety

1.67 for design operational loadings as defined in 304. 2.

1.25 for design storm loadings as defined in 304. 2.

4. Members subjected to combined axial load and bending

due to bending, the computed stresses ale to comply with the following requirements

≤ ≤

and in addition, at ends of members :

for design operational loadings as defined 304. 2.

≤

for design storm loadings as defined in 304. 2.

≤

(2) When structural members are subjected to axial tension in combination with tension bending, the computed stresses are to comply with the following requirements : However, the computed bending compressive stress, taken alone shall not exceed

due to

for design operational loadings as defined 304. 2.

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≤

for design storm loadings as defined in 304. 2.

≤

where,

: computed axial compressive or tensile stress

: computed compressive or tensile stress due to bending

: allowable axial compressive stress, which is to be the least of the

(A)

(B)

(C)

Yield stress divided by factor of safety for axial stress specified in 304. 2. Overall buckling stress divided by factor of safety specified in 304. 5. (1) Local buckling stress divided by factor of safety for axial stress specified in 304. 5. (2).

: allowable axial compressive stress due to bending, determined by dividing the yield

stress or local buckling stress, whichever is less, by the factor of safety specified in

Euler buckling stress, may be increased 1/3 for combined loadings as defined in 304.

2.

:

:

:

:

Modulus of elasticity unsupported length of column

effective length factor which accounts for support conditions at ends of length l. For cases where lateral defections of end supports may exist K is not be considered less than 1.0.

radius of gyration

is a coefficient as follows

(A) For compression members in frames subject to joint translation (sideways),

(B) For restrained compression members in frames braced against joint translation and not subject to transverse loading between their supports, in the plane of bending.

but not less than 0.4, where ( ) is the ratio of the smaller to larger mo-

ments at the ends of that portion of the member un-braced in the plane of bend- ing under consideration. The ratio ( ) is positive when the member is

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(C) For compressive members in frames braced against joint translation in the plane of loading and subject to transverse loading between their supports, the value of

may be determined by rational analysis. However, in lieu of such analysis the following values may be used :

(a) for members whose ends are restrained,

(b) for members whose ends are unrestrained,

5. Column buckling stresses

(1) Overall buckling

For compression members which are subject to overall column buckling, stress is to be obtained from the following equations.

the critical buckling

, ·

,

≥

: critical oveall bucking stress

, ,

are defined in 4. (2)

(2) The factor of safety for overall column buckling is to be as follows

(A) For gravity and mooring loading as defined in 2.

,

≥

,

(B) For combined loading as defined in 2.

,

≥

,

(3) Local buckling

Local bucking members which are subjected to axial compression or compression due to bend- ing are to be investigated for local buckling, as appropriate, in addition to overall buckling, as specified in 5. In the case of unstiffened or ring-stiffened cylindrical shells, local buckling is to be investigated if the proportions of the shell conform to the following relationship.

: mean diameter of cylindrical shell

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: thickness of cylindrical shell (expressed in the same units as ),

and are as defined in 5. (1)

6. Equivalent stress criteria for plated structures

For plated structures considered on the basis of the equivalent stress criterion and analyzed in ac- cordance with the loading given in 1, the factors of safety will be specially considered.

305. Structural design

The hull and frame(s) which are part of the floating structure are to be designed in accordance with the requirements of 303. and 304. In addition to those requirements, the scantlings of plating, stiffeners, and beams are to meet the requirements of 1 to 3. Alternatively the hull and frame de- sign is to be based on a systematic analysis based on sound engineering principles and accounting for the external static and dynamic pressures imposed by the marine environment and the internal pressure of the contents of tanks and floodable compartment.

1. Plating

(1) Hull and watertight bulkhead plating

Hull plating is to be of the thickness derived from the following equation. but not less then

6.5 or +2.5 whichever is greater

: thickness in

: stiffener spacing in

: (3.075 - 2.077) ( + 0.272) (l

≤ 2) = 1( >

: aspect ratio of the panel (longer edge/shorter edge)

: 235/ ( )

: specified minimum yield point or yield strength in or 72 % of the speci- fied minimum tensile strength, whichever is the lesser

for plating, the greatest distance in from the lower edge of the plate to the highest wave crest level during the most unfavorable design situation, or 1.0 , whichever is greater.

(2) Tank plating

Where the internal space is a tank, the design head , in association with the equation given in (1), is to be taken from the lower edge of the plate to a point located at two thirds of the dis-

tance from the top of the tank to the top of the overflow or 1.0 , whichever is greater.

Where the specific gravity of the liquid exceeds 1.05, the design head , in this section is to

2. be increased by the ratio of the specific gravity to 1.05.

Stiffeners and beams

The section modulus of each bulkhead stiffener or beam in association with the plating to which it

7.8

: 0.9 for stiffeners having clip attachment to decks or flats at the ends or tachments at one end with the other end supported by girders

1.0 for stiffeners supported at both ends by girders

having such at-

vertical distance, in , from the middle of length to the same to which

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plating is measured 1. (1)

: spacing of stiffeners, in

: length, in between supports. Where brackets are fitted at shell, deck, or bulkhead sup- ports, and the brackets arc in accordance with Table 3.l and have a slope of approx- imately 45 degrees, the length may be measured to a point on the bracket located at a distance from the toe equal to 25 % of the length of the bracket.

3. Girders and webs

(1) Strength requirements each girder or web which supports a frame, beam, or stiffener is to have a section modulus not less than obtained from the following equation.

: 4.74

: 1.5

vertical distance, in , from the middle of in the case of girders or from the mid- dle of in the case of webs, to the same heights to which for plating is measured 1.(1)

: sum of half lengths, in (on each side of girder or web) of the stiffeners or beams supported

: length, in , between supports; where brackets are fitted at shell, deck or bulkhead supports, and the brackets are in accordance with Table 3.l and have a slope of ap- proximately 45 degrees, the length may be measured to a point on the bracket lo- cated at a distance from the toe equal to 25 % of the length of the bracket

Where efficient struts are fitted connecting girders or webs on each side of the tanks and spaced not more than four (4) times the depth of the girder or web, the section modulus for each girder or web may be one-half that obtained from the above.

(2) Proportions Girders and webs to have a depth not less than 0.125 where no struts or ties

are fitted and 0.0833 where struts are fitted. The thickness is not to be less than l % of depth plus 3 , but need not exceed 11 . In general, the depth is not to be less than 2.5 times the depth of cutouts.

(3) Tripping Brackets Girders and webs are to be supported by tripping brackets at intervals of

about

3 near the change of the section. Where the width of the unsupported face plate ex-

ceeds 200 , tripping brackets are to support the face plate.

Table 3.1 Thickness and Flange of brackets and kness

306. Stability

The hull is to be divided by bulkheads into watertight compartments. be provided for access to all main floodable compartment.

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Watertight manholes are to

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1. Intact stability

(1) The hull is to be stable under the following conditions.

(A) In calm water without mooring leg(s) in place

(B) During installation

(C) In the operating environment with all mooring legs in place and pretensioned under the op- erating hawser load

(D) Under tow, if the buoy is towed

(2) The designer is also to verify the following:

(A) Positive GM providing adequate initial stability in the calm water condition without any mooring leg(s) in place.

(B) Sufficient reserve stability to withstand the overturning moments caused by the environ-

mental and operational loads during towout installation and operation. The waterline at any equilibrium condition is to be below the first downflooding point.

(C) The compartment are to be arranged so that the hull or buoy will not capsize or sink due to the pull of the anchor legs under pretension and of the underbuoy hoses/flexible risers under the design storm condition.

2. Damage stability

The designer is to verify that the buoy has enough reserve buoyancy to stay afloat in a condition with one compartment (adjacent to the sea) damaged. It is also required to verify that the waterline is below the first downflooding point in a damage equilibrium condition with one compartment damaged under the design operating condition.

307. Fixed mooring structure

The fixed mooring structure is to be analyzed as a space frame taking into account the gravity, functional, environmental and mooring loads. The analysis is to take into account operating and maximum conditions. For SALM type of mooring structure, the analysis is be in accordance with the requirements of 303. and 304. The connections between vessel and fixed mooring platform other than those stated in 405. should be adequately designed. The design of the fixed mooring platform is to withstand the operating and storm condition as described in 304. 1 structure with buoyant structural elements is to meet requirements of 303. and 304., while a tower mooring structure designed as a gravity based fixed structure with tubular members is to be in accordance with Pt 3 of "Rules for Fixed offshore stucturos."

308. Additional structural requirements

An appropriate system is to be designed to prevent damage to the cargo transfer system due to im- pact from attendant vessels.

309. Buoyancy tanks for hoses/flexible risers

The buoyancy tank provides buoyancy to support the weight of hoses and flexible risers belonging to the single point mooring system. The average shell membrane stress at the test pressure is to be limited to 90 % of the minimum specified yield strength when subject to hydrostatic testing, and to 80 % of the yield strength under pneumatic testing. The combination of average shell membrane stress and bending stress at design operating pressure is to be limited to 50 % of the ultimate strength or the minimum specified yield strength whichever is less. When the external pressure is not compensated by internal pressure the stress values are also to be checked against critical buckling.