Section 5 Single Point Mooring Systems

501. Design loads for structures

1. The design of the structure and equipment of single point mooring systems is to consider the se- verest combination of various loads including at least the following. A detailed report about such designs is to be submitted to the Society for reference.

(1) Dead loads

(2) Dynamic loads due to motion (including rotating motion around turn tables)

(3) Mooring loads

(4) Fatigue loads

2. In order to consider the design loads acting on turret systems, the loads from mooring lines or ris- ers due to gravity, buoyancy, inertia, and hydraulic forces, etc. are to be taken into account.

502. Structural components

1. Structural components are, in principle, to be in compliance with the codes or standards deemed appropriate by the Society and structural strength is to be evaluated by suitable methods such as FEM, etc.

2. When performing the analysis mentioned in Par 1 above, the allowable stress for von Mises stress is to be 60% of the specified yield strength (not to exceed 72% of the specified tensile strength) of the material used for the part in concern. In the case of transient conditions in the one broken mooring line condition, however, the value of allowable stress may be increased up to but not ex- ceeding 80% of specified yield strength.

3. Structural components are to have sufficient strength against buckling in consideration of their shape, size, surrounding conditions, etc.

4. A fatigue life evaluation is to be carried out for those parts among essential components designated by the Society, such as turret systems, yokes, etc. In such cases, a usage factor of 0.33(0.1 for in-

accessible areas) is to be used for such evaluations.

5. The structures of the periphery facilities for positioning, the connections between such periphery fa- cilities for positioning and mooring systems and the connections between such periphery facilities for positioning and seabed mooring points are to be as deemed appropriate by the Society.

6. The parts of the hull structures of Units which transmit and dissipate the loads from turrets and yokes (turret bearing parts, etc.) are to be capable of withstanding such loads and are to be suit- ably reinforced.

503. Mechanical components

1. The mechanical components of single point mooring systems(turret bearings, driving mechanisms, various connecting attachments, etc.) are to be in accordance with standards/codes deemed appro- priate by the Society in addition to relevant requirements given in Guidance for single point mooring.

2. The bearings which carry the loads from rotation structures and mooring lines(turret bearings, etc.) are to be designed with a safety factor of not less than 2.0 against the destructive yielding of the bearing surface.

3. Notwithstanding Par 2 above, bearings which do not carry loads are to be as deemed appropriate by the Society.

504. Turret mooring

1. A turret mooring system is one type of station keeping system for a floating installation and can either be installed internally or externally.

2. Both internal and external turret mooring systems will allow the installation to weathervane around the turret.

3. The mooring lines are fixed to the sea bottom by anchors or piles.

4. For an internal turret system, the turret is supported in the installation by a system of bearings.

5. The loads acting on the turret will pass through the bearing system into the installation.

6. Typically, a roller bearing is located near the installation deck level, and radial sliding bearing is located near the keel of the installation. For an external turret mooring system, the installation is extended to attach the turret mooring system at the end of the installation.

7. The loads acting on an internal turret system include those basic loads induced by the mooring lines, risers, gravity, buoyancy, inertia and hydrostatic pressure.

8. Other loads, such as wave slamming and loads resulting from misalignment and tolerance, that may have effect on the turret should also be considered in the design. In establishing the controlling turret design loads, various combinations of installation loading conditions ranging from the full to minimum storage load conditions, wave directions, and both collinear and non-collinear environ- ments are to be considered. The mooring loads and loads applied to the external turret structure are transferred through its bearing system into the installation. The load range and combinations to be considered and analysis methods are similar to those stated for an internal turret mooring system, with additional consideration of environmentally-induced loads on the turret structure.

9. A structural analysis using finite element method is required to verify the sufficient strength of turret structure.

10. The allowable von Mises stress of the turret structure is to be 0.7 of the yield strength for operational intact mooring design conditions.

11. The von Mises stress allowed for the design storm intact mooring design conditions and for design storm one-line broken mooring condition are 90% and 100% of the yield strength, spectively, to verify the turret structure mooring attachment locations and supporting structure.

the the

the re-

12. A fatigue evaluation of the turret system using a spectral method or other proven approaches is needed to determine the fatigue lives for the turret components.