Section 3 Hull Structures

301.

1.

2.

3.

General

Hull structures are to be in accordance with this section in addition to relevant requirements in Ch 3, Sec 2.

Structural analysis is to be carried out in accordance with the requirements in Ch 4, Sec 2, Sec

3 and Sec 4 of Rules for Mobile Offshore Units.

Self-elevating ships are to be in accordance with the followings in addition to the requirements in the above 1. and 2.

(1) Structural analysis for overall strength is to be carried out in accordance with the requirements in the above 2. In addition, an unbalanced supported condition by the legs, if necessary, is to be considered.

(2) The hull are to be in consideration with a structure having sufficient strengths to endure the load that occurs while supported by all legs.

(3)

(4)

The scantlings of the respective hull structural members are to comply with the requirements in Ch 4, Sec 2, 3 and 4 of Rules for Mobile Offshore Units in accordance with the reference to the loads prescribed in Ch 4, Sec 1 in addition to the requirements of (1).

The hull structure, including the parts of the well, etc., is to be good in the continuity of lon- gitudinal strength and transverse strength.

(5) The ship is to be designed with for a crest clearance of either 1.2 m , or 10 % of the combined

storm tide, astronomical tide and height of the maximum wave crest above the mean low water

level, whichever is the smaller, between the underside of the crest of the design wave. This crest elevation is to combined astronomical and storm tides.

the unit in the elevated be measured above the

position and level of the

302. Legs

Legs of the self-elevating ships are to be in accordance with the following requirements in addition to the requirements in 301. However, the measurements can be decided according to the model ex- periment and analytical method that has been approved appropriate by the Society regarding the construction and the motions of the legs.

(1) Legs are to be either shell type or truss type and, as a guidance, footings or bottom mats are

to be fitted. Where footings or bottom mats are not fitted, proper consideration is to be given to the leg penetration of the sea bed and the end fixity of the leg. In strength calculation of such a leg, the leg is to be assumed as pin-supported at a position at least 3 meters below the sea bed.

(2) Legs in the field transit condition are to be in accordance with the followings. The field transit

condition means the condition which does not exceed a 24-hours voyage protected locations or locations where the unit may be safely elevated.

portion of the move, the unit is to be capable of arriving at a protected

where the unit may be safely elevated within six hours.

between two areas in However, during any location or a location

(A) The legs are to have sufficient strength for the bending moment obtained from the follow- ing formulae :

ŶË ĞËĦËŶË FkN ­ m F

ŶË : Dynamic bending moment caused by a 10° single amplitude of roll or pitch at the natural period of the unit (kN ­ m ).

ŶË : Static bending moment due to gravity caused by a 10° legs' angle of inclination (kN ­ m ).

(B) The legs are to be investigated for any proposed leg arrangement with respect to vertical position. Such investigations are to include strength and stability aspects.

(3) Legs in the ocean transit condition are to be designed in accordance with the followings :

(A) The legs are to be designed for acceleration and gravity moments resulting from the mo- tions in the severest anticipated environmental transit condition, together with corresponding

wind moments.

(B) The legs are to have sufficient strength for the bending moment obtained from the follow- ing formulae :

ŶĖ ĞËĦĖŶĖ FkN ­ m F

ŶĖ : Dynamic bending moment caused by a 20° single amplitude of roll or pitch at a 10-second period (kN ­m ).

ŶĖ : Static bending moment due to gravity caused by a 20° legs' angle of inclination (kN ­m ).

(C) For ocean transit condition, it may be necessary to reinforce or support the legs, or to re- move sections of them.

(D) The approved condition is to be included in Operating Booklet.

(4) Legs are to be designed to withstand the dynamic loads which may be encountered by their unsupported length just prior to touching bottom, and also to withstand the shock of touching

the seabed while the ship is afloat and subject to wave motions.

(5) The maximum design motions, bottom conditions and sea state while lowering legs and the sea state while raising the legs are to be clearly indicated in the operating manual.

(6) When computing leg stresses, while in the elevated position, the maximum overturning load on

the unit, using the most

adverse combination of applicable variable loadings together with the

loadings as specified in Ch 4 of Rules for Mobile Offshore Units, is to be considered.

Forces and moments due

to lateral frame deflections of the legs are to be taken into account .

(7) Leg scantlings are to be determined in accordance with a method of rational analysis, to the satisfaction of the Society.

(8) Except for self-elevating ships utilizing a bottom mat, each leg is to have the capability of be-

38 Guidance for Offshore Support Vessels 2015

ing pre-loaded to the maximum applicable combined load after initial positioning at a site. The pre-loading procedures are to be included in the operation manual.

303. Bottom mats

In cases where the bottom mats are installed to the legs of self-elevating ships, such bottom mats are to be in accordance with the following requirements:

(1) The construction of bottom mats is to be designed so that loads transmitted from the legs may

be evenly distributed to the respective parts of the mats.

(2) The thickness of shell plating of shell stiffeners are not to be less

the bottom mats without opening to the sea and scantlings of than determined by the requirements in Ch 4, 402. and 403.

In this case, the top of YZ is at the water level at flood tide, and the top of YẄ is 0.6 times the design wave height in the severe storm condition above the water level at the design water

depth.

(3) The scantlings of watertight bulkheads and their stiffeners provided in the bottom mats are not to be less than determined by the requirements in Pt 3, Ch 14 of Rules for the

Classification of Steel Ships. In this case, the top of Y is to be substituted to the top of YẄ

specified in 2.

(4) Where the unit is resting on the sea bed, the effects of scouring are also to be considered. The effects of skirt plates, where provided, are to be specially considered.

(5) Mats are to be designed to withstand the shock of touching bottom while the unit is afloat and

subject to wave motions.