CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH
4.1 Design Loads
CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH
4.1 Design Loads
4.1.1 General
(a) The loads to be considered in the analysis of structures are divided into:
(i) principal loads (see 4.1.2)
(ii) vertical loads due to operational motions (see 4.1.3) (iii) horizontal loads due to operational motions (see 4.1.4) (iv) loads due to climatic effects (see 4.1.5)
(v) loads due to motion of the vessel on which the gangway is mounted (see 4.1.6).
(b) The determination of the loads specified by the designer is to be documented with enclosed calculations, references to standards, or other justification.
In addition to the below stated loads, other relevant loads are to be considered, as applicable.
4.1.2 Principal loads
(a) The loads due to dead weight of the components: self-weight of the structure and all installed equipment;
(b) the loads due to live load.
(c) In addition, the following loads are to be considered, as applicable:
(i) Loads due to self-weight of:
(1) personnel waiting area (see 5.6)
(2) access to the gangway and/or waiting area (see 5.6) (3) driver’s cabin.
(ii) Loads due to live loads on:
personnel waiting area (see 5.6).
4.1.3 Vertical loads due to operational motions
Vertical refers to the coordinate system of the gangway (Z axis direction).
(a) Inertia forces due to acceleration or deceleration of vertical motions
Forces are to be determined on the basis of the maximum possible acceleration with the given machinery, and on the basis of the maximum possible deceleration with the given brakes. Typically, forces of this type occur by starting and stopping of luffing motions (e.g. during deployment/retrieval of the gangway).
The inertia forces are to be taken into account by multiplying the self-weight of the gangway by a “dynamic factor - DFZ” (see para. 4.3.4/Table 4-4 and Table 4-5 Note 2).
The dynamic factor is to be calculated by the designer based on the stiffness of the gangway taking into account all elements from gangway tip to pedestal; however, it is not to be less than 1.10.
For the dynamic case (LC 2b), these are to be added to the vertical vessel acceleration.
4.1.4 Horizontal loads due to operational motions
CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH 4.1 Design Loads
Horizontal refers to the coordinate system of the gangway (Y axis direction). It is assumed that horizontal is so defined that it corresponds to physical horizontal in the ideal position with zero “heel” and “trim” of the vessel/unit on which the gangway is mounted.
It is to be noted that these horizontal forces act in addition to possible simultaneously acting horizontal components of the principal loads, see 4.1.2.
(a) Inertia forces due to acceleration or deceleration of horizontal motions
(i) Forces are to be determined on the basis of the maximum possible acceleration with the given machinery, and on the basis of the maximum possible deceleration with the given brakes. Typically, forces of this type occur by starting and stopping of slewing motions. The inertia due to angular acceleration/deceleration of rotating machinery components is to be taken into account when this effect is significant.
(ii) The lateral force to be applied at the gangway (bridge) center of gravity (CoG) is to be calculated based on the below formula:
FH = (SW/100) (2.5 + 0.1 r n) ≥ 5% SW (kg)
where:
FH = lateral force
SW = gangway self-weight (kg)
r = radius/distance from revolving axis to gangway (bridge) CoG (m) n = revolutions per minute
(i) The centrifugal/radial force may be determined on the basis of maximum angular velocity and radius to the considered mass and is to be calculated based on the below formula:
CF = (SW/1000) (n2 r) (kg)
(ii) For the dynamic case (LC 2b), these are to be added to the relevant horizontal vessel acceleration (longitudinal/transverse).
4.1.5 Loads due to climatic effects
(a) Wind load
(i) Generally, the wind loads on the gangway is to be calculated according to the Rules for Steel Ships, Part III, Chapter 13. Other internationally recognized standards may be used for a more complex approach.
(ii) The below wind speed values are considered speed at 10 m above ground (or sea level). The wind speed/pressure is to be modified accordingly for the gangway location with the variation of height.
The design wind velocity and pressure are to be based on one-minute mean gust wind speed at the gangway location.
(1) For the operational case, the design wind speed is not to be less than 20 m/s. The gangway is to be parked when the one-minute mean gust wind speed exceeds this value.
CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH 4.1 Design Loads
(2) For the deployment/retrieval case, the design wind speed is not to be less than the “Operational design wind speed” – recommended value: 36 m/s.
(3) For the transit/survival/parked case, the design wind speed is not to be less than 44 m/s.
(iii) For gangways intended to be installed and/or operated on offshore installations compliant with the MODU Code, the gangway design wind speeds (for operational, deployment/retrieval and transit/survival cases) are to be in accordance with MODU Code Chapter 3 requirements (i.e. 51.5 m/s transit/survival/parked wind speed).
(iv) For gangways that are to be installed on vessels intended to “maintain station” or “wait on weather”, the gangway design wind speed for the parked/transit case is to be correlated with the maximum wind speed that the supporting vessel is designed to operate in (e.g. when the wind speed is expected to be higher than 44 m/s or 51.5 m/s).
(b) Vortex induced oscillations
Consideration is to be given to loads from vortex shedding on individual elements due to wind, current and waves. Vortex induced vibrations of frames are also to be considered. The material and structural damping of individual elements in welded steel structures are not to be set higher than 0.15% of critical damping.
The problem of wind induced VIV (vortex induced vibrations) of members in space frame offshore structures is to be treated as an on-off type. Either the member will experience vibrations and then there is a fatigue problem or it will not experience vibrations and then there is no danger of fatigue cracks.
Such members are therefore to be designed according to an avoidance criterion that will ascertain that the structure will not vibrate.
(c) Sea pressure loads (green sea loads)
These loads will vary according to vessel type and the actual location of the gangway on vessel; in general environmental loads on MOUs will be less than those on ships.
Sea pressure loads are to be calculated according to CSR-H Pt.1 Ch.4 Sec.5[1].
4.1.6 Loads due to motion of the vessel on which the gangway is mounted
(a) Vessel motions are dependent on the vessel/MOU on which the gangway will be installed, a s well as on the specific location of the gangway on the supporting vessel.
(b) The vessel/MOU accelerations for the parked/transit/survival case is to be based on the extreme values given in the governing code for the supporting vessel/MOU.
(c) The vessel accelerations for the operational and deployment/retrieval cases are to be stated by the designer.
(d) The inertia forces caused by the vessel motions are to be combined according to relevant rules/calculations for the vessel/MOU considered. Alternatively, combinations of the maximum values may be used:
(i) vertical force alone
(ii) vertical and transverse force (iii) vertical and longitudinal force
(iv) vertical, transverse and longitudinal force.
(e) Typical values for the calculated accelerations may, for a circa 180 m ship with 60,000 tonnes displacement and the gangway near the bow/aft, be:
(i) Combined vertical acceleration: aV = 1.0·g (ii) Combined transverse acceleration: aT = 0.7·g (iii) Combined longitudinal acceleration: a = 0.3·g
CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH