Section 7 Column-stabilized Units

701. Application

The requirements in this Section apply to the column-stabilized units.

702. Overall strength

1. The overall strength of the unit is to be in accordance with the requirements in 412.

2. For units of this type, the highest stresses may be associated with less severe environmental con- ditions than the maxima specified by the owner or designer. Particular attention is to be given to such a case.

703. Strength of structure in way of position mooring system

Local structure in way of fairleads, winches, etc., forming a part of the position mooring system, is to be designed to the breaking strength of the mooring line or chain.

704. Upper structure

1. The scantlings of the upper structure are not to be less than those required by Rules for the Classification of Steel Ships in association with the loadings indicated on the deck loading plan. These loadings are not to be less than the requirements specified in 306.

2. When the upper structure is considered to be an effective member of the overall structural frame of the unit, the scantlings are to be sufficient to withstand actual local loadings plus any additional loadings superimposed due to frame action, within the stress limitations of Sec 4.

3. When upper structure is designed to be floating in any mode of operation or damaged condition, or to meet stability requirements, it is to be subject to special consideration.

4. Deckhouses fitted to the upper structure are to be

Pt 3, Ch 17 of Rules for the Classification of

designed in accordance with the requirements in

Steel Ships. The construction and scantlings of

deckhouses provided on the hull of the unit are to be determined, taking account of their location and the environmental conditions in which the unit will operate.

5. The upper structure, including the parts of the well, etc., is to be good in the continuity of longi- tudinal strength and transverse strength.

6. Unless deck structures are designed for wave impact, a clearance acceptable to the Society should be maintained between passing wave crests and the deck structure. The Administration should be provided with model test data, reports on past operating experience with similar configurations or by calculations showing that adequate provision is made to maintain this clearance.

7. The structural arrangement of the upper hull is to be considered with regard to the structural in- tegrity of the unit after the assumed failure of any primary girder. The Society may require a

structural analysis showing satisfactory protection against overall collapse of assumed failure when exposed to environmental loading corresponding to a

the unit after such an one-year return period

for the intended area of operation.

705. Column, lower hulls and footings

1. Where columns, lower hulls or footings are of stiffened shell construction, the scantlings of plating, stiffeners, girders, etc. are not to be less than determined by the requirements in 413. 2 and Par

3. In this case, and are to be in accordance with the requirements in the followings.

(1) Where an internal space is loaded with liquid, is the vertical distance in metres

the

from load line to the tank top and is the vertical distance in metres from the tank top top to the of overflow pipes. Where, however, the specific gravity of the liquid is greater than sea

that of water, and are to be modified taking account of specific gravity.

(2) Where an internal space is a void compartment, the top of is at the of is 0.6 times the design wave height in the severe storm condition at the design water depth.

(3) The minimum values of and are not to be less than 6 metres for immersion and 3.4 metres for other areas.

load line and the top above the water level

areas subject to wave

2. Where columns, lower hulls or footings are designed as shells either unstiffened or ring stiffened, the scantlings of shell plating and ring stiffeners are to be determined to satisfy the strength re- quirements in Sec 4 in response to the design heads, and , specified in Par 1.

3. The scantlings of deep tank bulkheads and their stiffeners provided in columns, lower hulls or foot-

ings are not to be less than determined by the requirements in Pt 3, Ch 15 of

Classification of Steel Ships.

Rules for the

4. When column, lower hull or footing is an effective member for the overall strength of the unit, the stress resulting from the overall strength is to be added by the stress determined by the require- ments in Par 1.

5. Where a column, lower hull or footing is a part of the overall structural frame of

a unit, consid-

eration should also be given to stresses resulting from deflections due to the applicable combined loading.

6. Particular consideration is to be given to structural details, reinforcement, etc., in areas subject to high local loadings indicated in the followings ;

(1) Areas subject to bottom bearing loads, where applicable,

(2) Bulkheads of partially filled tanks, etc.,

(3) Areas liable to sustain external damages,

(4) Jointed parts between columns and footings or lower hulls,

(5) Areas subject to wave impact.

7. Where a unit is designed for operations while supported by the sea bed, the effects of scouring are to be considered. The effects of skirt plates, where provided, are to be specially considered.

706. Bracing members

1. Bracing members are to be designed to transmit loadings and to make the hull structure effective against environmental forces, and when the unit is supported by the sea bed, against the possibility of uneven bearing loads.

2. Bracing members are to have sufficient strength for buoyancy, wave and current forces and wave impact.

3. When bracing members are of tubular section, ring frames may be required to maintain stiffness and roundness of shape.

4. When bracings are watertight, they are to be suitably designed to prevent collapse from external hydrostatic pressure.

5. When any one slender bracing member are lost, overall strength of unit is compiled with the fol- lowing requirements where overall structure analysis are carried out based upon the design loads specified in Sec 3.

(1) For determining the design loads, environmental loads such as wind force, wave force, etc,. are

to be obtained from not less than 1 year return period.

(2) Notwithstanding the kind of stress, the allowable stress at the combined loads condition is to be following formulae.

: allowable stress (N mm )

: specified yield stress of materials (N mm

(3) In case of taking into consideration of combined compression stress, 1.0

specified 412. 3 may be

6. Underwater bracing members are to be provided with a leak detection system make it possible to detect fatigue cracks at an early stage.

7. The unit’s structure should be able to withstand the loss of any slender bracing member without causing overall collapse when exposed to environmental loading corresponding to a one-year return period for the intended area of operation.

8. Consideration should be given to the need for ring frames to maintain stiffness and shape in tubu- lar bracing members.

707. Wave clearance

1. Unless deck structures are designed either in accordance with the requirements in 704. 3 or by considering wave impact, to the satisfaction of the Society, reasonable clearance between the lower surface of deck structure and the wave crest is to be ensured for all afloat modes of operation, taking into account the predicted motion of the unit relative to the surface of the sea.

2. For on-bottom modes of operation, clearances are to be in accordance with those specified in 609.