Section 3 Self-elevating units

3.1.1 This Section outlines the structural design requirements of self-elevating units. Additional requirements for particular unit types related to the design function of the unit are given in Pt 3 FUNCTIONAL UNIT TYPES AND SPECIAL FEATURES.

3.1.2 A self-elevating unit is a floating unit which is designed to operate as a sea bed-stabilised unit in an elevated mode, see Pt 1, Ch 2, 2 Definitions, character of classification and class notations.

3.1.3 Production units are to comply with the requirements of Pt 3, Ch 3 Production and Storage Units as applicable.

3.1.4 The structural analysis and determination of primary scantlings are to be on the basis of the distribution of loadings expected in all modes of operation.

3.2.1 When in the elevated position, the unit is to be designed to have a clearance air gap between the underside of the hull structure and the highest predicted design wave crest superimposed on the maximum surge height over the maximum mean astronomical tide. The minimum clearance is not to be less than 1,5 m. Calculations, model test results or prototype reports are to be submitted for consideration.

3.3.1 The structure is to be designed to withstand the static and dynamic loads imposed upon it in transit, installation and elevated conditions. All relevant distributions of gravity and variable loads are to be considered, as are stresses due to the overall and local effects, see Pt 4, Ch 3, 4 Structural design loads.

3.3.2 The permissible stresses are to be in accordance with Pt 4, Ch 5 Primary Hull Strength and the minimum local scantlings of the unit are to comply with Pt 4, Ch 6 Local Strength.

3.3.4 The general requirements for investigating accidental loads are defined in Pt 4, Ch 3, 4.16 Accidental loads. In transit conditions, collision loads against the hull structure will normally only cause local damage to the hull structure and consequently loading condition (c) in Pt 4, Ch 4, 3.3 Structural design 3.3.3 need not be investigated from the overall strength aspects. When in the elevated position, accidental damage to the legs is to be considered in the design and the unit is to be capable of absorbing the energy of impact in association with environmental loads corresponding to the appropriate one year storm condition.

3.3.5 In general, for loading condition (c) in Pt 4, Ch 4, 3.3 Structural design 3.3.3, the level of impact energy absorbed by the local leg structure is not to be taken less than 2 MJ. If the unit is only to operate in protected waters, as defined in Pt 1, Ch 2, 2.4 Class notations (hull/structure), the level of impact energy absorbed by the local leg structure may be reduced but should not be less than 0,5 MJ. Collision loads will, in general, only cause local damage to one leg, but the possibility of progressive collapse and overturning should be considered in the design calculations which should be submitted for consideration.

3.3.6 The permissible stress levels after credible failures or accidents are to be in accordance with Pt 4, Ch 5 Primary Hull Strength.

3.3.7 Fatigue damage due to cyclic loading is to be considered in the design of the legs of the unit for transit and elevated conditions. Fatigue damage is considered accumulative throughout the unit’s design life. The extent of the fatigue analysis will be dependent on the mode and area of operations, see Pt 4, Ch 5, 5 Fatigue design.

3.4.1 The hull is to be considered as a complete structure having sufficient strength to resist all induced stresses while in the elevated position and supported by its legs. All fixed and variable loads are to be distributed, by an accepted method of rational analysis, from the various points of application to the supporting legs. The scantlings of the hull are then to be determined consistent with this load distribution.

3.4.2 Due account must be taken of loadings induced in the transit condition from external sea heads, variable deck loads and legs.

3.5.1 Deckhouses are to have sufficient strength for their size, function and location. Requirements for scantlings are given in Pt 4, Ch 6, 9 Superstructures and deckhouses.

3.5.2 Special consideration is to be given to the scantlings of deckhouses and deck modules which will not be subjected to wave loading in any operating condition such as units which are ‘dry-towed’ to the operating location.

3.6.1 Load carrying members in the jackhouses and frames which transmit loads between the legs and the hull are to be designed for the maximum design loads and are to be so arranged that loads transmitted from the legs are properly diffused into the hull structure. The scantlings of jackhouses are not to be less than required for deckhouses in accordance with Pt 4, Ch 6, 9 Superstructures and deckhouses.

3.7.1 The scantlings and arrangements of the boundaries of leg wells are to be specially considered and the structure is to be suitably reinforced in way of leg guides, taking into account the maximum forces imposed on the structure. The minimum scantlings of leg wells are to comply with Pt 4, Ch 6, 3.3 Self-elevating units.

3.8.1 Legs may be either shell type or lattice type. Independent footings may be fitted to the legs or legs may be permanently attached to a bottom mat. Shell type legs may be designed as either stiffened or unstiffened shells.

3.8.2 Where legs are fitted with independent footings, proper consideration is to be given to the leg penetration of the sea bed and the end fixity of the leg.

3.8.3 Leg scantlings are to be determined in accordance with a method of rational analysis and calculations submitted for consideration, see Pt 4, Ch 3, 3 Structural idealisation.

3.8.4 For lattice legs, the slenderness ratio of the main chord members between joints is not to exceed 40, or two thirds of the slenderness ratio of the leg column as a whole, whichever is the lesser, unless it can be shown that a calculation taking into account beam-column effect, joint rigidity and joint eccentricity justifies a higher figure.

3.10.1 In field transit conditions within the same geographical area, legs are to be designed for acceleration forces caused by a 6° single amplitude of roll or pitch at the natural period of the unit, plus, 120 per cent of the gravity forces caused by the legs’ angle of inclination, unless otherwise verified by appropriate model tests or calculations. The legs are to be investigated for any proposed leg arrangement with respect to vertical position during field transit moves, and the approved positions are to be specified in the Operations Manual. Such investigation is to include strength and stability aspects. Field transit moves may only be undertaken when the predicted weather is such that the anticipated motions of the unit will not exceed the design condition.

3.10.2 The duration of a field transit move may be for a considerable period of time and should be related to the accuracy of weather forecasting in the area concerned. It is recommended that such a move should not normally exceed a twelve hour voyage between protected locations or locations where the unit may be safely elevated. However, during any portion of the move, the unit should not normally to be more than a six hour voyage to a protected location or a location where the unit may be safely elevated. Suitable instructions are to be included in the Operations Manual. Where a special leg position is required for field moves, this position is to be specified in the Operations Manual.

3.11.1 In ocean transit conditions involving a move to a new geographical area, legs are to be designed for acceleration and gravity loadings resulting from the motions in the most severe anticipated environmental transit conditions, together with corresponding wind moments. Calculation or model test methods may be used to determine the motions. Alternatively, legs may be designed for the acceleration and gravity forces caused by a design criterion of 20° single amplitude of roll or pitch at a 10 second period. For ocean transit conditions, it may be necessary to reinforce or support the legs, or to remove sections of them. The approved condition is to be included in the Operations Manual.

3.12.1 When lowering the legs to the sea bed, the legs are to be designed to withstand the dynamic loads which may be encountered by their unsupported length just prior to touching the sea bed and also to withstand the shock of touching bottom while the unit is afloat and subject to wave motions.

3.12.2 Instructions for lowering the legs are to be clearly indicated in the Operations Manual. The maximum design motions, bottom conditions and sea state while lowering the legs are to be clearly stated. The legs are not to be lowered in conditions which may exceed the design criteria.

3.12.3 For units without bottom mats, all legs are to have the capability of being preloaded to the maximum applicable combined gravity plus overturning load. The approved preload procedure should be included in the Operations Manual.

3.12.4 Consideration is to be given to the loads caused by a sudden penetration of one or more legs during preloading.

3.13.1 When the legs are resting on the sea bed, the unit is to have sufficient positive downward gravity loadings on the support footings or mat to withstand the overturning moment of the combined environmental forces from any direction, with a reserve against the loss of positive bearing of any footing or segment of the area, for each design loading condition. The most critical minimum variable load condition is to be considered for each loading direction and in no case is the variable load to be taken greater than 50 per cent of the maximum and using the least favourable location of the centre of gravity.

3.13.2 The safety factor against overturning is to be at least 1,25 with respect to the rotational axis through the centres of the independent footings at the sea bed. For a unit with a mat type footing, the rotational axis is to be taken at the maximum stressed edge of the mat.

3.13.3 For independent footings, the safety factor against sliding at the sea bed is to be related to the soil condition, but in no case is the safety factor to be taken as less than 1,0.

3.14.1 Classification will be based upon the designer’s assumptions regarding the sea bed conditions. These assumptions are to be recorded in the Operations Manual.

3.14.2 Full details of the sea bed at the operating location are to be submitted to LR for review at the design stage. The effects of scouring on bottom mat bearing surfaces and footings is to be considered, see Pt 4, Ch 4, 3.16 Bottom mat 3.16.3.

3.15.1 For units with independent legs, foundation fixity should not normally be considered for in-place strength analysis of the upper parts of the leg in way of the lower guides unless justified by proper investigation of the footing and soil conditions.

3.15.2 For in-place analysis, the lower parts of the leg with independent footings are to be designed for a leg moment no less than 50 per cent of the maximum leg moment at the lower guides, together with the associated horizontal and vertical loads.

3.16.1 When the legs are attached to a bottom mat, the scantlings of the mat are to be specially considered, but the permissible stress levels are to be in accordance with Pt 4, Ch 5 Primary Hull Strength. Particular attention is to be given to the attachment, framing and bracing of the mat in order that the loads from the legs are effectively distributed into the mat structure.

3.16.2 Mats and their attachments to the bottom ends of the legs are to be of robust construction to withstand the shock load on touching the sea bed while the unit is afloat and subject to wave motions.

3.16.3 The effects of scouring on the bottom bearing surfaces should be considered by the designer, with a stated design figure for loss of bearing area. The effects of skirt plates, where provided, may be taken into account, see also Pt 4, Ch 4, 3.14 Sea bed conditions 3.14.1.

3.16.4 The minimum local scantlings of the mat structures are to comply with Pt 4, Ch 4, 3.17 Independent footings 3.17.5 and Pt 4, Ch 4, 3.17 Independent footings 3.17.6.

3.17.1 Independent footings are to be designed to withstand the most severe combination of overall and local loadings to which they may be subjected, see also Pt 4, Ch 4, 3.16 Bottom mat 3.16.3. In general, the primary structure is to be analysed by a three dimensional finite element method.

3.17.2 The complexity of the mathematical model together with the associated element types is to be sufficiently representative of all parts of the primary structure to enable internal stress distributions to be established.

3.17.4 The permissible stresses are to be based on the safety factors for yield and buckling as defined in Pt 4, Ch 5, 2 Permissible stresses. The preload cases may be considered as load case (a) in Pt 4, Ch 5, 2 Permissible stresses while the loadings associated with the maximum storm cases may be taken as load case (b) in Pt 4, Ch 5, 2 Permissible stresses.

3.17.5 The minimum local scantlings of the bottom shell and stiffening and other areas subjected to pressure loading are to be determined from the formulae for tank bulkheads given in Pt 4, Ch 6, 7 Bulkheads. The loadhead should be consistent with the maximum bearing pressure, determined in accordance with Pt 4, Ch 4, 3.17 Independent footings 3.17.3, and the wastage allowance of the plating should be not less than 3,5 mm, see also Pt 4, Ch 4, 3.17 Independent footings 3.17.6.

3.17.6 Where it is intended to operate at a fixed location for the design life of the unit, the footing/leg structure which is below the mud line or internal areas of the footings which cannot be inspected are to have their structure designed with adequate corrosion margins and protection. The corrosion allowance for wastage and the means of protection are to be to the satisfaction of LR and are to be agreed at the design stage.

3.17.7 When the structure consists of compartments which are not vented freely to the sea, the scantlings of the shell boundaries and stiffening are not to be less than required for tank boundaries in Pt 4, Ch 6, 7 Bulkheads using the load head not less than 1,4m, where is defined in Pt 4, Ch 1, 5 Definitions.

3.17.8 Where the legs of the unit are made from steel with extra high tensile strength, special consideration is to be given to the weld procedures for the leg to footing connections. Adequate preheat should be used and the cooling rate should be controlled. Any non-destructive examination of the welds should be carried out after a minimum of 48 hours have elapsed after the completion of welding.

3.18.1 When self-elevating units are fitted with cantilevered lifeboat platforms, the strength of the platforms is to comply with Pt 4, Ch 4, 1.9 Lifeboat platforms. If the lifeboat platform can be subjected to wave impact forces in transit conditions, the scantlings are to be specially considered and details are to be submitted for consideration by LR.

3.19.1 General requirements for topside structure are given in Pt 4, Ch 4, 1.10 Topside structure.