1. The arrangement of the anchoring system shall be such that
– interference with the risers is precluded, – mooring operations of attending vessels will
not be impaired, and
– the product transfer facilities (hoses, etc.), where provided, not endangered.
2. Inspection/monitoring and maintenance 2.1 The anchoring system should be designed having in mind also the possibility of/need for inspection and repair, particularly of elements prone to corrosion and abrasion.
2.2 Corrosion protection and/or regular surveys are to be combined in a maintenance procedure, subject to approval by BKI in case of classification, and incorporated in the Operation Manual. See also Section 2.
2.3 For mooring systems of floating production units, particularly in cases where other - fixed or mobile - installations are stationed nearby, monitoring of the forces in representative mooring lines is recommended, e.g., by strain measurement. These measurements should be supplemented by and correlated with a position monitoring system for the unit (e.g. satellite reference navigation system).
3. Anchor lines
3.1 For installations complying with all rule and classification requirements regarding design calculations (mooring system analysis), materials, control of corrosion and wear, etc., the safety factors γa shown in Table 7.1 may be used.
For all load conditions investigated also the case “one line failed/missing” shall be calculated (see Table 7.1).
Under unfavorable conditions, e.g., where a reliable assessment of environment conditions is not possible or regular surveys cannot be guaranteed, the safety factors should be increased.
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Table 7.1 Safety factors γa
All lines intact 2,0 1,7
One line missing 1,4 1,3
1 See 6., below
(γa = minimum breaking load/maximum calculated force in anchor line)
3.2 In anchor lines consisting of different components, all connecting elements shall be designed and tested for at least the same ultimate load as the main component (e.g. chain), except for a ‘weak link’
incorporated intentionally in particular cases. Thisalso applies to clump weights and buoys incorporatedin the anchor line to influence its restoring characteristics. An additional safety factor of 1,1 is recommended for the connecting elements.
3.3 Steel wire or fiber rope used for anchoring lines shall be of special make, to be agreed upon with BKI.
For synthetic fiber ropes, the long-term mechanicaland chemical resistance/adequacy shall be proven;relevant test results and investigations shall be presented for information. The elasticity of the ropematerial shall be taken into account in the calculations of the anchoring system.
Steel wire should not be used in portions of the anchor line subject to varying contact with the sea bottom (abrasion/corrosion).
3.4 All mooring lines shall be subjected to a load test after installation. The test load should correspond to the maximum load resulting from the mooring analysis (see 6.), where practicable.
4. Anchor points
4.1 Evidence shall be provided of the adequacy and the holding capacity of the anchor type selected,in relation to the seafloor properties prevailing at the anchoring site. A proof load equal to the maximum dynamic (design) load should be applied for at least15 minutes.
4.2 The accuracy of positioning of the anchors shall be established in the design procedure, depending on length of anchor line, water depth etc. The actual anchor points have to be monitored and documented during installation.
4.3 For anchors of special design - high holding capacity anchors - tests will be required, proving the values aimed at with a view to the soil conditions to be expected at the installation site.
4.4 Where anchors are provided which are not capable of resisting vertical (lifting) forces, the analysis of the system shall prove that under all loading conditions only horizontal forces will occur at the anchoring points.
4.5 For the pull-out safety, i.e., holding capacity divided by maximum (calculated) anchor line force, a minimum value of 2 is recommended. The safetyfactor may have to be adjusted, depending on anchortype and soil conditions.
4.6 For anchoring elements suitable for vertical traction loads, i.e., piles or gravity-type anchors, see Rules for Structures, Volume 2, Section 7.
4.7 Load testing after installation: See 3.4. The exposure to the test load shall be long enough tomake sure that the anchor has settled.
5. Auxiliary equipment
5.1 Anchor and anchor line handling equipment, such as windlasses and winches, stoppers, fairleads, shall be designed in accordance with good shipbuilding practice and dimensioned in relation to the breaking strength of the anchor lines. See Rules for Specific Type of Units and Equipment, Volume 3, Section 5.C.
and Rules for Machinery Installations Volume 4, Section 8. See also Section 13, G.
5.2 Quick-release connections shall be of a proven design and are to be tested in presence of aBKI Surveyor. Failure shall not occur under loads less than 1,5 times the breaking load of the attachedanchor line.
Safe release must be possible undera load equal to the line load and under all predictableangles of application of the load.
5.3 In cases where disconnection of the riser and anchoring systems is provided (see Section 3, C. and 17, G.), details of the necessary equipment - hoists, actuators, storing facilities, etc. - are to be presented for approval. The parts of the anchoring system remaining on location, including, e.g., buoyancy aids and the related control equipment, will be included in the approval procedure upon special agreement, but should be covered by the submitted documentation at least for information.
5.4 For ("active") turret turning installations, see F below.
Section 7 - Anchoring/Positioning Systems C 7 - 3
6. Motion and strength analysis of the anchoring system
6.1 Cases to be considered
6.1.1 A 3-dimensional analysis of the system, consisting of the moored unit and all anchoring elements contributing to the restriction of motions/displacements (excursions), has to be carried out, considering
– different load conditions and load cases as defined in Section 3, C., including possible variations of draught (empty, ballast, fully loaded unit),
– a sufficient number of sea states and wind/wave/current directions (see below), – constraints imposed by the riser system as
well as any other subsea equipment to be installed,
– attending vessels moored to the unit,
– the case one anchor line failed or not installed (see 3.1 and 6.2 below),
6.1.2 The number of cases investigated must be sufficient to identify maximum forces in the anchor lines and connecting elements, and any other situations considered as critical.
Regarding allowed direction of line pull at the anchoring points, see 4. above.
6.1.3 The load condition "Extreme environmental loads" (see Section 3, C.), applied to the investigation of the positioning system, shall usually include two cases:
– 100 year return period waves, with associated wind and currents
– 100 year wind, with associated waves and currents.
(See also Section 3, D.4.).
The effect of sea swell may have to be considered.
Note:
Steady forces due to wind and current can be computed analytically or obtained from model tests (see 6.6). For tanker based production units, wind and current coefficients documented in "Prediction of Wind and Current Loads on VLCCs, " public domain data compiled by OCIMF, can be used. Additional wind forces due to the presence of process and other equipment can be significant and must be separately determined.
6.2 The anchoring system should be designed in
such a way that a failure of any single anchor line will not cause progressive failure of the remaining lines, or riser damage/failure under load conditions where production continues.
6.3 Motion analysis methods
Wave forces acting on a floating vessel positioned by catenary mooring comprise the following three components:
– first-order forces at wave frequencies – second-order forces at frequencies lower than
wave frequencies
– steady component of the second-order force, the so-called mean drift force.
To take account of the forces mentioned above, both quasi-static and dynamic approaches are generally used.
However, quasi-static analysis methods should be applied only in the early stages of mooring design as approximation, and a dynamic analysis method is generally necessary for final design confirmation and for the fatigue assessment.
The method(s) used in the particular case shall be specified in the approval documentation.
Note:
In a quasi-static analysis, the dynamic wave loads are accounted for by statically offsetting the vessel by an appropriately defined wave induced dislocation. Loads induced by vertical fairlead motions and the dynamics of the mooring system itself are neglected.
A dynamic analysis accounts for the dynamic response of the mooring system itself. Time-varying effects due to mooring line inertia and damping are included. The time-varying fairlead motions are calculated from the vessel's surge, sway, heave, roll, pitch and yaw motions.
Generally, to predict mooring line response, it suffices to account for only the vertical and horizontal fairlead motions in the plane of the mooring line catenary.
A dynamic mooring analysis for a ship-shaped production unit must account for the dominant effect of low -frequency motions. Specifically, the analysis needs to consider the following aspects:
– coupled motion equations for surge, sway and yaw
– adequate formulation of slowly varying wave drift forces
– low-frequency damping – mooring line dynamics – riser effects.
6.4 The safety factors to be used for dimensioning of the mooring line components are given inTable 7.1, see C.3.
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6.5 Fatigue analysis: For the important load bearing components of the mooring system, generally a fatigue investigation will be required, based on the load history derived from the calculations mentioned above.
For fatigue analysis, see Section 4, B.2.5.
6.6 Model tests may be incorporated in the design procedure to supplement or confirm data previously obtained by calculation (e.g., model towing basin experiments, wind tunnel testing).
The results of such tests may be accepted by BKI as part of the design documentation, if the tests are carried out in an acknowledged laboratory, and evidence is provided of the validity of the modeling assumptions and the accuracy of measurements and data evaluation.
6.7 Special consideration may be given to an arrangement where the anchoring system is used in conjunction with thrusters to maintain the unit on position (see E.). Dimensioning of the anchoring elements will be considered from case to case.
3. The calculation model used for the final design shall be suitable for revealing :
– the maximum forces occurring in the system elements as well as the force variation needed for a fatigue analysis, and.
– the maximum forces occurring in the system elements as well as the force variation needed for a fatigue analysis, and
– the maximum excursions and rotational movements (in articulations, swivels - control of admissible relative movements).
4. For the transfer and installation phase, special investigations may be necessary, taking account of the environment conditions to be expected (limiting conditions). See also Rules for Structures, Volume 2, Section 8.
5. Articulations
1. For single point mooring systems (e.g., SALM (Single Anchor Leg Mooring)), a dynamic analysis is required, taking account of the following influences/effects, where relevant:
– environment and hydrodynamic forces acting on both, the moored unit and the SPM structure 1
– interactions between the structures involved (eventually including flexible connecting ele-ments)
– non-linear effects, e.g., due to buoyancy changes
– friction effects (articulations).
Depending on the configuration of the system and on the state of design progress (preliminary to final), different calculation methods (frequency domain, time domain) and mathematical models (varying degree of simplification) may be used. The method(s) and model(s) used shall be agreed upon with BKI.
2. For SPM mooring systems the
requirements/statements of C.6.1, 6.5 and 6.6 are applicable in principle.
Note:
Due to the non-redundant configuration, the system integrity depends on the performance of every single element in the mooring force transmission line. Detail design and maintenance/inspections are therefore still more important than in the case of conventionalspread mooring.
5.1 Articulated connections shall be designed with a view to maintenance, inspection and repair.
Lubrication should be reliable and automatic, as far as possible.
5.2 Materials for contact surfaces shall be carefully selected with a view to corrosion and abrasion properties, and experience gained.
5.3 Safety factors for design contact pressure and allowances for motion range limitation will be considered from case to case.
6. For buoyant structures forming part of the SPM mooring, damage cases (one/two compartments flooded) will generally have to be investigated. See also Rules for Mooring and Loading Installations.
7. Local hydrodynamic loads (slamming) may have to be accounted for in the structural design.
E. Dynamic Positioning (DP)