Section 6 Tension-leg units

6.1.2 The requirements of Pt 4, Ch 4, 1 Column-stabilised units for semi-submersible units are to be complied with as applicable.

6.1.3 The term ‘tension-leg’ used in this Section includes all the component parts of the pre-tensioned mooring system in one group and includes the top connections to the unit and the bottom connections to the sea bed foundation. Each unit will have a number of tension legs. Each tension leg may be made up of individual tensioned cables or members which are referred to in this Section as ‘tethers’.

6.2.1 Unless the upper hull structure is designed for wave impact, a clearance ‘air gap’ of 1,5 metres between the underside of the upper hull deck structure and the highest predicted design wave crest is to be maintained during operation on station. Calculations, model test results or prototype reports are to be submitted for consideration.

6.2.2 In cases where the unit is designed without an adequate air gap in accordance with Pt 4, Ch 4, 6.2 Air gap 6.2.1, the scantlings of the upper hull deck structure are to be designed for wave impact forces. If the whole hull structure is waterborne, the scantlings are to be specially considered but they are not to be less than would be required for a semi-submersible unit.

6.3.1 The Owner or designer is to specify the environmental criteria for which the installation is to be approved. The extreme environmental conditions applicable to the location are to be defined, together with all relevant operating environmental limits. Full particulars are to be submitted with sufficient supporting information to demonstrate the validity of the environmental parameters, see Pt 4, Ch 3, 4 Structural design loads.

6.3.2 The environmental loading on the installation and its motion responses are to be determined and the dynamic effects are to be considered, see Pt 4, Ch 3, 4 Structural design loads.

6.3.3 When determining the critical design loadings on tethers, realistic combinations of environmental loadings and unit response are to be taken into account. All loadings and unit motions are to be agreed with LR and the full range of operating draughts are to be considered.

6.3.4 Motions may be determined by a suitable combination of model tests and calculation methods.

6.3.5 The possibility of resonant motions is to be fully investigated, taking a second order wave and wind forces into account. The likelihood of the occurrence of rotational and vertical oscillations is to be particularly considered.

6.3.6 In determining environmental loads, account is to be taken of the effect of marine growth. Both an increase in the dimensions of submerged members and the change in surface characteristics are to be considered.

6.4.1 The general requirements for structural design are given in Pt 4, Ch 3 Structural Design, and the requirements of Pt 4, Ch 4, 1 Column-stabilised units for semi-submersible units are to be complied with, except where modified by this Section.

6.4.3 All modes of operation are to be investigated. The design load cases defined in API Recommended Practice 2T Third Edition, July 2010 Planning, Designing, and Constructing Tension Leg Platforms (hereinafter referred to as API RP 2T) are to be complied with. The permissible stresses applicable for these load cases are shown in Pt 4, Ch 4, 6.4 Structural design 6.4.6. In all cases, platform configuration should consider both minimum weight and maximum weight variations.

6.4.4 The minimum local scantlings of the unit are to comply with Pt 4, Ch 6 Local Strength.

6.4.5 Although a tension-leg unit will not be classed in the transit condition and during site installation, the transit condition and the site-specific installation condition are to be investigated and submitted to LR.

6.5.2 Adequate test data is to be submitted to LR to demonstrate that the materials and fittings used for tension legs will have adequate service life. The design philosophy relating to the life and replacement of tension legs and their fittings is to be clearly stated at the design stage.

6.6.1 When reference to tension legs is made in this sub-Section, the Rules apply to tethers constructed of wire ropes, tubes or any other equivalent section.

6.6.2 The design of tension legs is to comply with API RP 2T in addition to the requirements in this Section.

6.6.3 The leg system is to be fail-safe, in that failure of a single tension-leg member at any time during the life of the installation will not induce stress levels in any other tension-leg member that will produce fatigue failure in that member or its associated fittings in less than one year, assuming average winter conditions, or induce increased accumulated fatigue damage to reduce significantly the overall fatigue life of the system.

6.6.4 In general, each tension leg is to be assembled from tether members of only one type and size. The cross-section of tension leg members may vary in a consistent manner over depth. The fitting of materials having different elastic constants in parallel load-carrying components of a tension leg will not normally be accepted.

6.6.5 All leg tether members forming any one tension leg are to be set to an approximate common tension. Suitable means of adjusting the tensions of the individual components of each leg are to be provided at the upper end of each individual tether, consistent with tension adjustments or tolerances required by design to keep the tendons system within design envelopes.

6.6.6 Means are to be provided for monitoring the tensions in tension-leg components.

6.6.7 The design is to be such that, with suitable ballasting, the minimum tension in any tether can be adjusted to be not less than five per cent of the normal pre-tension. A lesser tension is not normally permitted. Where the Owner requests a relaxation of this requirement, appropriate dynamic analysis is to be carried out to evaluate the tether design.

6.6.8 No end terminal or other fitting associated with the tension legs is to be dependent upon the maintenance of the leg tension to retain it in place.

6.6.9 In general, all leg connections including pins, bearings, locks, etc., are to be arranged by positively activated wedging systems, or otherwise, so that there are no slack fits or non-essential clearances. Screwed and bolted fittings are to be provided with positive locking arrangements.

6.6.10 Arrangements are to be made to prevent kinking and sharp bends in tether members in way of the end fittings. In determining the maximum angles that may be assumed by the leg members in way of end fittings, account is to be taken of the maximum extent of snaking or other dynamic distortions of the legs that could occur in extreme environmental conditions.

6.6.11 The effects of scuffing and wear of tethers within rope guides, bell mouths and other systems due to the movement of leg components caused by motions of the unit are to be taken into account in the design.

6.6.12 The extreme maximum and minimum tether loads, which determine the tether design requirements, are to be calculated.

6.6.13 Tether misalignment where tethers are not completely vertical and parallel are to be taken into account.

6.6.14 The maximum tether load is to be determined at the top of the tether with the unit at its minimum design storm weight and with the highest water level. The calculation is to include the effects of the worst combination of the horizontal centre of gravity position, wave loading, wind and current loading, tether misalignment and dynamic response and platform motions.

6.6.15 The minimum tether load is to be determined at the bottom of the tether with the unit at its maximum design storm weight and at the lowest water level. The calculation is to include the effects of the worst combination of the horizontal centre of gravity position, wave loading, wind and current loadings, tether misalignment and dynamic response, platform motions, catenary effects of tethers and the design margin.

6.6.16 When calculating the minimum tether load, a design margin of five per cent of the nominal pre-tension is to be applied.

6.6.17 The unit with the most unfavourable combination of weight, centre of gravity and buoyancy is to be capable of surviving the worst design damage condition. The requirements for watertight and weathertight integrity are to comply with Pt 4, Ch 6 Local Strength.

6.6.18 After flooding of any compartment as required to satisfy Pt 4, Ch 4, 6.6 Tension-leg design 6.6.16, the requirements of Pt 4, Ch 4, 6.6 Tension-leg design 6.6.15 are to be complied with.

6.6.19 Within a period of 12 hours from commencement of any accidental flooding, the loading of the unit is to be adjusted, as necessary, so that the tensions of all tethers at their lower ends remain positive under the most unfavourable environmental conditions which could be expected to occur at the location within a return period of not less than one year. The loading adjustment may be means of deballasting, and/or removal, dumping or horizontal movement of deck loads.

6.8.1 In the design of tether components, consideration is to be given to the fatigue damage that will result from cyclic stresses. A detailed fatigue analysis is to be performed. The combined axial and bending stress is to be determined by dynamic analysis and is to consider variations around the tether circumference.

6.8.2 Where the tethers are built up of various components such as screwed sections or chain link, the effect of many tether components being connected in series is to be adequately accounted for in the design fatigue life.

6.8.3 The fatigue life of tethers and their end connections and the factors of safety on the calculated design fatigue life are to comply with the requirements of Pt 4, Ch 5, 5 Fatigue design.

6.9.1 Tension-leg foundation design requirements are defined in Pt 3, Ch 14 Foundations.

6.10.1 Essential mechanical components are to be designed such that the components are capable of being condition monitored, repaired and/or replaced. Prototype testing may be required for specialised components or novel design arrangements.

6.11.1 The tether system is to be suitably instrumented and monitored in service to ensure that the system is performing within design limitations.

6.11.2 Provision is to be made to monitor tether top tensions. In addition, it is recommended that the platform mean offset position and the upper and/or lower flexible joint angles of tethers are monitored.

6.12.1 Tethers are to be inspected at Periodical Surveys and the Owner/designer is to prepare a planned procedure for inspection, retrieval and replacement of tethers in the event of damage or as part of a planned schedule.

6.12.2 The replacement procedures involved are to be clearly documented with regard to the retrieval method, equipment required and unit operations. The procedures are to be included in the unit’s Operations Manual.

6.12.3 It is recommended that an adequate number of spare parts of tethers and mechanical fittings are supplied to the unit and made available during its service life.