Section 6 Position Keeping Systems and Components

601. General

Units provided with position keeping systems equipment in accordance with this Section may have a Additional Installation Notation, "PKS" after the class notation.

602. Anchoring systems

1. General

Plans showing the arrangement and completed details of the anchoring system, including anchors, shackles, anchor lines consisting of chain, wire or rope, together with details of fairleads, wind- lasses, winches, and any other components of the anchoring system and their foundations are to be submitted to the Society.

2. Design

(1) An analysis of the anchoring arrangements expected to be utilized in the unit's operation is to be submitted to the Society. Among the items to be addressed are:

(A) Design environmental conditions of waves, winds, currents, tides and ranges of water depth

(B) Air and sea temperature

(C) Ice condition (if applicable)

(D) Description of analysis methodology

(2) The anchoring system should be designed so that a sudden failure of any single anchor line will not cause progressive failure of remaining lines in the anchoring arrangement.

(3) Anchoring system components should be designed utilizing adequate factors of safety and a de-

sign methodology suitable to identify the most severe loading condition for each component. In particular, sufficient numbers of heading angles together with the most severe combination of wind, current and wave are to be considered, usually from the same direction, to determine the maximum tension in each mooring line. When a particular site is being considered, any appli- cable cross sea conditions are also to be considered in the event that they might induce higher mooring loads.

(A) When the Quasi Static Method is applied, the tension in each anchor line is to be calcu- lated at the maximum excursion for each design condition defined in (B) below, combining the following steady state and dynamic responses of the Unit:

(a) steady mean offset due to the defined wind, current, and steady wave forces

(b) most probable maximum wave induced motions of the moored unit due to wave ex- citation

For relatively deep water, the effect from damping and inertia forces in the anchor lines is to be considered in the analysis. The effects of slowly varying motions are to be included when

the magnitudes of such motions are considered to be significant.

(B) Factors of safety (FOS) are dependent on the design conditions of the system (intact, dam- aged, or transient), as well as the level of analyses (Quasi static or dynamic analysis). The minimum Quasi Static FOS, specified in the table below, at the maximum excursion of the unit for a range of headings should be satisfied if the quasi static method outlined in (A) is applied. Otherwise, the minimum Dynamic Analysis FOS in the table below should be sat- isfied, including the effects of line dynamics when these effects are considered significant.

Ch 4 Design Condition Ch 4

The defined 'Operating' and 'Severe Storm' are to be the same as those identified for the design of the unit, unless the Society is satisfied that lesser conditions may be applicable to specific sites.

(C) In general, the maximum wave induced motions of the moored unit about the steady mean offset should be obtained by means of model tests. The Society may accept analytical calcu-

lations provided that the proposed method is based on a sound methodology which has been

validated by model tests.

In the consideration of column-stabilized units, the value of Ǽᾯ and Ǽᾜ , as indicated in Ch 4, 102. 2, may be introduced in the analysis for position keeping mooring systems. The in- tent of Ch 7, 203.(Wind tunnel test) and of Ch 7, 204.(Other stability requirements) may also be considered by the Society.

(D) The Society may accept different analysis methodologies provided that it is satisfied that a level of safety equivalent to the one obtained by (A) and (B) above.

(E) The Society may give special consideration to an arrangement where the anchoring systems

are used in conjunction with thrusters to maintain the unit on station.

603. Equipment

1. Windlass

(1) The design of the windlass is to provide for adequate dynamic braking capacity to control nor- mal combinations of loads from the anchor, anchor line and anchor handling vessel during the deployment of the anchors at the maximum design payout speed of the windlass. The attach- ment of the windlass to the hull structure is to be designed to withstand the breaking strength

Ch 4 Design Condition Ch 4

of the anchor line.

(2) Each windlass is to be provided with two independent power operated brakes and each brake is to be capable of holding against a static load in the anchor lines of at least 50 percent of its breaking strength. Where the Society so allows, one of the brakes may be replaced by a man- ually operated brake.

(3) On loss of power to the windlasses, the power operated braking system should be automatically

applied and be capable of holding against 50 percent of the total static braking capacity of the windlass.

2. Fairleads and sheaves

(1) Fairleads and sheaves should be designed to prevent excessive bending and wear of the anchor lines. The attachments to the hull or structure are to be such as to withstand the stresses im- posed when an anchor line is loaded to its breaking strength.

604. Anchor line

1. The Society is to be ensured that the anchor lines are of a type that will satisfy the design con- ditions of the anchoring system.

2. Means are to be provided to enable the anchor lines to be released from the unit after loss of main power.

3. Means are to be provided for measuring anchor line tensions.

4. Anchor lines are to be of adequate length to prevent uplift of the anchors under the maximum de- sign condition for the anticipated area(s) of operation. However, only steady wind, wave and cur- rent forces need to be applied in evaluating anchor uplift forces in transient conditions.

605. Anchors

1. Type and design of anchors are to be to the satisfaction of the Society.

2. All anchors are to be stowed to prevent movement during transit.

606. Quality control

1. Details of the quality control of the manufacturing process of the individual anchoring system com- ponents are to be submitted. Components should be designed, manufactured and tested in accord- ance with recognized standards insofar as possible and practical. Equipment so tested should, insofar

as practical, be legibly and permanently marked with the Society's stamp and delivered umentation which records the results of the tests.

with doc-

607. Control stations

1. A manned control station is to be provided with means to indicate anchor line tensions dividual windlass control positions and to indicate wind speed and direction.

2. Reliable means are to be provided to communicate between locations critical to the operation.

at the in- anchoring

3. Means are to be provided at the individual windlass control positions to monitor anchor line ten- sion, windlass power load and to indicate amount of anchor line payed out.

608. Dynamic positioning systems

1. Thrusters used as a sloe means of position that provided for anchoring arrangements to

keeping should provided a level of safety equivalent to the satisfaction of the Society.

Ch 5 Type of Units Ch 5