Section 11 Topside to hull structural sliding bearings

11.1.1 This Section covers the minimum technical requirements for the design, engineering, fabrication, assembly, inspection and testing of resilient bearing pads used as support interface between topside modules and the floating offshore installation.

11.1.2 Module bearing support arrangements are to be designed to ensure the effects of vessel deformations due to global hogging, sagging and torsion on the topside structure are minimised while moment transfer from the topside modules to the hull structure is kept to a minimum. In general this needs only to be considered for topsides modules where the support spacing is greater than three or more transverse frames.

11.2.1 For definitions, symbols and nomenclatures, see EN 1337 parts 1, 2, 3, 5 and 8 to 11.

11.4.1  Function and types. The bearings are located at the interface between the topside modules and the hull, their function being to minimise the structural interactions of the two bodies. Particularly, they shall reduce the bending moments in the hull module support frames as well as the tension, compression and torsion in the module primary girders. Additionally, fatigue effects will be significantly reduced on both module support frames and modules.

11.4.2 The focus of this Section is on elastomeric bearing pads which are extensively used in floating offshore installations. The bearings covered in this Section are shown in cases 1.1 to 1.8 of Table 1 of EN 1337-1.

11.5.2  The effect of displacement on bearings.Horizontal displacements will induce rubber strain in elastomeric bearings, and will induce sliding upon PTFE/steel surfaces for pot bearings, while vertical displacements will induce compression or tension in both types. These effects must be considered in line with the bearing material’s shear, tension and compression properties.

11.5.3  Rotations for bearing design. In the absence of detailed analysis, the bearings are to be designed for a minimum rotation of +/-0,5 degrees about both horizontal axes to ensure topside members satisfy the allowable deflection criterion of 1:300.

11.6.1 Bearings under topside structures may be exposed to dirt, debris, oil and moisture that promote corrosion and deterioration. As a result, these bearings should be designed and installed to minimise environmental damage and to allow easy access for inspection. The service demands on bearings are very severe and result in a service life that is typically shorter than that of other structural elements. Therefore, allowance for bearing replacement should be given consideration in the design process and, where possible, lifting locations should be provided to facilitate removal and re-installation of bearings without damaging the structure. See EN 1337-9, 10 and 11 for specifications.

11.7.1  Design life. The module bearings are required to be designed for the same service life as the module structures. The supplier of bearing material is to provide adequate evidence to support the design life of the bearings under the specified project’s conditions.

The bearings could come into contact with miscellaneous hydrocarbons due to leakages occurring on the process equipments located on the modules. The supplier shall consider this potential event and ensure the proposed solution and supplied products do not jeopardise structural integrity or satisfactory system performance over the design life, in the event that this potential condition occurs.

However, bearing pads are not designed for blast, fire or cryogenic spills events. If necessary, a protection of bearing pads will be designed to ensure their integrity.

Passive fire protection of the bearings may be considered to protect pads against fire events.

11.7.3 Modules are to be constrained against excessive movement with lateral restraints, for example, horizontal stoppers for sliding bearings. Modules are also to be constrained against uplift unless it can be confirmed that uplift cannot occur. Consideration should be given to restricting the number of longitudinal supports to two to prevent transfer of vertical displacement of the hull to the module.

11.8.1 Bearing selection is influenced by many factors, including loads, geometry, maintenance, available clearance, displacement, rotation, deflection, availability, policy, designer preference, construction tolerances and cost. In general, vertical displacements are restrained, rotations are allowed to occur as freely as possible, see Pt 4, Ch 6, 11.5 Displacements 11.5.3, and horizontal displacements may be either accommodated or restrained. The reaction loads on each bearing are to be in accordance with the topside structural analysis and are to account for the worst scenario loading condition, taking the relative stiffness between the topsides and hull structure into account in the analysis, as appropriate.

11.8.2 Typically, steel stoppers are used with elastomeric bearings to transfer horizontal forces from topside to the substructure. The load transfer system between bearing plates and stoppers shall be carefully designed in order to minimise impact effects.

11.9.1 The shear stiffness of the bearing is its most important property because it affects the forces transmitted between the superstructure and substructure. Elastomers are flexible under shear and uniaxial deformation, but they are very stiff against volume changes. This feature makes possible the design of a bearing that is flexible in shear but stiff in compression.

11.9.2 Only neoprene for plain elastomeric bearing pads and steel-reinforced elastomeric bearings is recommended. All elastomers are visco-elastic, non-linear materials and, therefore, their properties vary with strain level, rate of loading and temperature. Bearing manufacturers evaluate the materials on the basis of international rubber hardness degrees (IRHD). However, this parameter is not considered to be a good indicator of the shear modulus ‘G’. The shear modulus ‘G’ should not be taken less than 0,7 MPa (an IRHD not less than 50 or 55).

11.10.1 EN 1337 provides only test and design methods for repeated compression loadings. These should be followed in detail.

11.11.1 Care should be taken for design of load transfer in fixed and sliding bearings. Sliding bearings should be designed according to EN1337-2. Maximum deflections under each loading case should be calculated considering non-linear behaviour. No gaps between bearing plates and stoppers are allowed. For common details, see Steel Bridge Bearing Design and Detailing Guidelines, AASHTO/NSBA G9.1 – 2004.