Section 3 Offshore cranes

3.1.1 This Section applies to cranes which are designed to operate in offshore conditions. These are defined as open sea environment in which there is significant movement of the ship or installation (due to wave action) on which the crane is mounted or from which the crane is offloading. The sea state will, generally, be in excess of a significant wave height of 0,6 m.

3.1.2 Cranes mounted on fixed installations used solely for lifting operations on the installation itself may be considered as shipboard cranes as defined in Ch 4, 2 Shipboard cranes. Where cranes are mounted on floating installations, horizontal and vertical accelerations, see Ch 4, 3.4 Offlead/sidelead displacement, heel/trim angles and base accelerations 3.4.4, are to be applied in addition to the requirements of Ch 4, 2 Shipboard cranes.

3.1.3 The requirements of Ch 4, 2 Shipboard cranes are to apply to offshore crane design except where specific requirements are defined in this Section.

3.1.4 The scope of this Section covers jib cranes and fixed structures used for lifting operations. Other lifting appliances will be specially considered on the general basis of these requirements if considered applicable and appropriate.

3.2.1 The specified service category as per Ch 1, 2.3 Service category 2.3.2 is to be applied. Due to the severe nature of the offshore environment the duty factor F _d is defined as 1,20 for all offshore cranes and other lifting equipment being used in open sea/offshore conditions. Alternative proposals to use lower duty factors shall be based on the principles as outlined in Ch 4, 2.3 Duty factor 2.3.2 and will be specially considered.

3.3.1 The dynamic force due to hoisting for offshore cranes is to include the effect of relative movement of the crane and load in addition to normal hoisting shock and dynamic effects.

3.3.2 The hoisting factor is considered to be dependent on the design operational sea conditions and which is to be defined by the significant wave height and is to be calculated from the following expression:

where

where

The load supporting deck velocity v _D and crane jib tip velocity v _C are to be taken from a recognised National or International Standard (e.g. EN 13852) in the absence of project-specific data. The proposed velocities are to be agreed with LR.

The hoisting factor shall not be taken less than that defined in Ch 4, 2.5 Dynamic forces 2.5.2, but in no case shall the product of hoisting and duty factor be less than 1,30 for offshore cranes carrying out offboard lifts.

3.3.3 To calculate the crane system stiffness the following combination of structural elements are to be considered:

1. hoist rope system;

2. luffing rope system;

3. pedestal;

4. crane house;

5. crane jib.

Other items maybe considered additionally only in the case where they are permanently installed on the crane. The stiffness of the wire rope is to be taken into account as per rope manufacturer’s recommendations using the Young’s modulus and the associated area of the wire rope.

3.3.4 When a motion compensator, shock absorber, or similar device is fitted, proposals to use lesser hoist factors will be specially considered. Means are to be foreseen which provide the crane driver with the operational status of the fitted device.

3.3.5 As an alternative to the method of determining the dynamic forces indicated in Ch 4, 3.3 Dynamic forces 3.3.1, consideration will be given to submissions based on a dynamic analysis of the crane and associated structure (e.g. by means of a motion response analysis).

3.4.1 The design offlead and sidelead displacements are related to the significant wave height and are defined as follows:

1. Offlead displacement, in metres:

   O

   =

   2,5 + 1,5H 1/3

   where

   H 1/3

   =

   significant wave height, in metres

2. Sidelead displacement, in metres:

   S

   =

   0,5O

   The offlead displacement is defined as being in the jib luffing plane while the sidelead displacement is defined as transverse to the jib luffing plane. The offlead and sidelead displacements provided are to be considered at the load supporting deck location (e.g. offshore supply vessel load supporting deck). Proposals to use other values will be specially considered.

3.4.2 In addition to the operating conditions the crane and its stowage arrangements are to be designed to withstand the most severe combination of motions which can occur when the crane is stowed. In the case of ship mounted cranes, see Ch 4, 2 Shipboard cranes.

3.4.3 The inclination of the crane base usually referred to as heel and trim angles shall be taken from a recognised National or International Standard (e.g. EN 13852) in the absence of project-specific data. The heel and trim angles are to be agreed with LR and consideration will be given to arrangements and methods to reduce the heel and trim angle (e.g. vessel ballasting, etc.).

3.4.4 The horizontal and vertical accelerations (which need to include dynamic roll and pitch) of the crane base shall be taken from a recognised National or International Standard (e.g. EN 13852) in the absence of project-specific data and are to be applied in the most unfavourable direction. The proposed horizontal and vertical accelerations are to be agreed with LR.

3.5.1 The basic loadcases are to be considered as per Ch 4, 2.15 Load combinations 2.15.1. Deviations from those loadcase definitions are provided in Ch 4, 3.5 Load combinations 3.5.2 and Ch 4, 3.5 Load combinations 3.5.3.

3.5.2  Case 1. For the condition of the crane operating without wind the design is to be considered with respect to a combination of dead load, live load and horizontal forces defined in Ch 4, 2.6 Dynamic forces due to crane movements to Ch 4, 2.11 Forces due to ship motion, as given by the following expression:

• F _d [L _g + F _h (L _l +L _h1) + L _h2 + L ₁]
• where

3.5.3  Case 2. For the condition of the crane operating with wind the design is to be considered with respect to a combination of dead load, live load and horizontal forces defined in Ch 4, 2.6 Dynamic forces due to crane movements to Ch 4, 2.11 Forces due to ship motion, together with the most unfavourable wind load. This is given by the following expression:

• F _d [L _g + F _h (L _l +L _h1) + L _h2 + L ₁ + L ₂] + L _w
• where

3.5.4 The effects of offlead and sidelead are to be combined with the crane supporting deck inclinations (e.g. heel/trim) for the live load as defined by the following symbolic expressions:

For the dead load only the crane supporting deck inclination (e.g. heel/trim) is to be considered.

3.6.1 When a load is lifted from a ship, the load hoist speed is to be high enough to ensure that after the load is lifted a second wave does not cause the ship to re-contact the load.

3.6.2 The minimum hoisting speed is to be obtained from the following expression:

where

3.6.3 Special consideration will be given to heavy lift cranes as the minimum hoisting speeds defined in Ch 4, 3.6 Hoisting speed 3.6.2 might not be achievable.

3.7.1 In general, the ring is to be manufactured from a steel forging having an ultimate tensile strength of range 820 to 1100 N/mm² and an elongation, based on a gauge length of five diameters, of not less than 15 per cent.

3.7.2 For offshore installations with design temperatures down to –20°C, slew bearings require Charpy V-notch impact tests to be carried out at –20°C and achieve a minimum average energy of 42J. Special consideration will be applied to for design temperatures below –20°C.

3.7.3 Type testing, whereby a sample of the critical part of the ring is tested statically and with respect to fatigue to prove the adequacy of other rings manufactured to the same specification, standard and methods, is to be carried out for three roller type bearings and other designs where provision of fillet radii of the bearing surfaces are considered to introduce stress concentrations in a failure path which would result in the loss of the crane.

3.7.4 The ring is to be considered with respect to static loads resulting from the worst load combination of Ch 4, 2.15 Load combinations 2.15.1 and associated with an allowable stress obtained from test in accordance with Ch 4, 3.7 Slew rings 3.7.3 multiplied by a stress factor = 0,4.

3.7.5 The ring is also to be considered with respect to fatigue loading based on load combination Case 2 of Ch 4, 2.15 Load combinations 2.15.1, multiplied by a load spectrum factor of 0,7 and associated with an allowable stress determined from S-N curves obtained from the type testing of Ch 4, 3.7 Slew rings 3.7.3 on the basis of 2 x 10⁶ cycles and multiplied by a stress factor = 0,67.

3.7.6 Slewing ring bolts are to comply with ISO 898-1 and in general, are not to exceed Grade 10.9. Threads of all bolt grades should be rolled after heat treatment to improve fatigue strength.

3.7.7 The bolts are to be pre-tensioned in accordance with Ch 4, 2.24 Slewing ring and slewing ring bolting 2.24.2 and are to be considered with respect to the static and fatigue design conditions of Ch 4, 3.7 Slew rings 3.7.4 and Ch 4, 3.7 Slew rings 3.7.5 taking due account of pre-tension.

3.7.8 Slew rings are to be manufactured at works approved by LR under LR survey with the following additional requirements:

1. Slew ring material in the forged state and final bulk heat treated condition is to be tested to ensure compliance with the requirements of Ch 4, 2.25 Materials 2.25.3, Ch 4, 3.7 Slew rings 3.7.2 and Ch 11, 1.2 General material requirements 1.2.3.

2. Magnetic particle examination of the machine finished components of the ring to ensure that they are free from cracks, etc.

3.8.1 The requirements for selection of materials and associated impact toughness requirements are given in Ch 4, 2.25 Materials and Ch 11 Materials and Fabrication.

3.8.2 For minimum design temperatures below –40°C, methods of demonstrating acceptable toughness will be specially considered.

3.9.1 The rope safety factor SF for offshore cranes is to be determined from the following expression:

The factor is not to be taken less than 1,0.

3.9.2 The required breaking load of the rope is given by:

3.10.1 Where it is proposed to install a motion compensator or shock absorber device to reduce the impact load applied to the crane with a view to improving its rating, this will be specially considered and, in general, subject to the following procedure:

1. Plans, calculations and proposed test procedures are to be submitted for approval.

2. Units are to be manufactured and tested under survey.

3. Testing to include ‘in factory’ tests under simulated design offshore conditions together with normal proof test requirements.

4. For initial approval of new devices, the crane installation is to be instrumented to enable maximum load in hoist system to be monitored for various sea conditions and SWL and the result submitted for consideration.

3.11.1 Automatic Overload Protection Systems (AOPS) and Manual Overload Protection Systems (MOPS) in accordance with a recognised National or International Standard (e.g. EN 13852-1) shall be fitted. Alternative systems, methods or concepts to prevent overload of the crane will be specially considered. Proposals for heavy lift cranes, see Ch 4, 1.2 Lifting appliances and crane types 1.2.1.(k), will also be specially considered.