Section 5 Drag embedment anchors – General

5.1.1 All drag embedment anchors are required to be test loaded at installation to eliminate slack from the grounded section of the mooring lines and to ensure that the mooring line’s inverse catenary is set; limiting unacceptable mooring line slack in-service; to detect damage to mooring components and to provide assurance of the anchors holding capacity. Anchor installation test load procedures are set out in Pt 3, Ch 14, 5.6 Specific Installation Test Load Requirements.

5.1.2 Appropriate fluke-shank angle should be selected for the soils at the anchor location during the anchor point design phase. It should be ensured that any ballast material included in hollow anchor flukes does not impact on anchors ability to penetrate the seabed.

5.1.3 It should be noted that limited uplift resistance can be provided by some drag anchors, particularly at shallow seabed penetrations. Mooring systems should be designed to prevent uplift occurring at the anchor or it should be demonstrated that the anchor can provide sufficient vertical load resistance at the intended location.

5.2.1 This sub-Section, and Pt 3, Ch 14, 5.3 Drag embedment anchors – Holding capacity, apply to drag embedment anchors of high holding power type. Proposals for the use of other anchor types will be specially considered.

5.2.2 Anchors are to be of an approved type.

5.2.3 Material selection for drag embedment anchors are, generally, to be in accordance with Pt 4, Ch 2, 4 Steel grades, taking the structural category as ‘Primary’.

5.2.4 Supporting calculations to verify the structural strength of the anchor for design service loads and for proof test loads are to be submitted.

5.2.5 The anchors are to be manufactured in accordance with the requirements of Ch 10 Equipment for Mooring and Anchoring of the Rules for Materials.

5.2.6 Anchors are to be subject to proof test loading in the manner laid down in the Rules. The level of proof test loading for positional mooring anchors is 50 per cent of the minimum rated breaking strength of the attached anchor line.

5.2.8 Not with standing the above, attention is drawn to the separate requirement of some National Authorities for proof load testing of anchors.

5.3.1 The requirements of Pt 3, Ch 14, 5.2 Drag embedment anchors – Structural aspects are also to be considered, in addition to this sub-Section.

5.3.2 Factors of safety for anchor holding capacity for floating offshore installations at a fixed location are not to be less than the values given in Pt 3, Ch 14, 5.3 Drag embedment anchors – Holding capacity 5.3.2. Anchors for positional mooring of mobile offshore units are to be sufficient in number and holding capacity for the intended service. It is the Owner's/Operator's responsibility to ensure adequate anchor holding capacity for each location or holding ground.

5.4.1 For drag anchors, the ultimate holding capacity, penetration and drag are to be based on empirical design data for the specific type of anchor under consideration. The soil conditions at the anchor location and previous experience in similar soil conditions with the specific type of anchor are to be considered.

5.4.2 Particular consideration is to be given to locations with layered soil conditions and their effect on ultimate holding capacity, penetration and drag.

5.4.3 The anchor points are to be so designed that their movements remain within tolerable limits. Estimated values of anchor point movements are to be submitted together with the basis and details of the calculations made.

5.4.4 Anchor sizing charts should be used with caution. Special care should be taken where soil conditions at the intended anchor location differ from those presented in available sizing charts, for example many sizing charts contain no provision for a layered seabed or calcareous soils.

5.4.5 The use of analytical design methods or numerical analysis to predict drag anchor holding capacity, penetration and drag will be considered as a supplement to Pt 3, Ch 14, 5.4 Drag Anchors - Acceptable Design Methods, provided that the methods have been calibrated to actual anchor behaviour.

5.5.1 Expected drag anchor trajectory during installation should be submitted to LR for review. This should include expected depth of anchor fluke tip and attitude at the completion of installation activities. This prediction is of particular importance when considering the effects of scour or layered soils.

5.5.2 Final anchor position after completion of test load should be inspected and recorded/estimated based on expected chain catenary and submitted to LR. Alternatively anchor position may be directly measured using a monitoring device.

5.6.1 All drag anchors are required to be test loaded at installation to the satisfaction of the LR Surveyor. The methodology for monitoring and accepting load applied and anchor drag during the anchor preload period shall be submitted to LR prior to installation. Records of load and drag versus time shall be submitted to LR once anchor installation is complete.

5.6.2 Installation test load magnitude is to be agreed with LR prior to installation, and should take account of cyclic loading, potential for scour, unconventional soil types and any other factors which may adversely affect in service anchor movement. Where the above are not deemed to have a significant effect a test load of not less than 80% of the maximum intact load may be appropriate; however in some circumstances a test load of 100% or more of the intact load may be required to demonstrate required anchor performance. The test load is not, however, to exceed 50% of the minimum breaking strength of the mooring line.

5.6.3 Acceptance of installation test load by attending LR surveyor will be based on assessment of supplied methodology, load achieved, anchor drag and hold period.

5.6.4 The test load is to be held for a period of not less than 20 minutes. During this time no anchor drag should be observed.
Where it is not possible to monitor anchor position directly during the hold period this should be estimated from vessel position and checked post hold period. Visual ROV inspection of expected anchor position is also recommended.