Strength Calculations

(iii) Loads due to motion of the vessel on which the gangway is mounted (maximum transit/survival accelerations), ref. 4.1.6.

(b) Additional considerations:

(i) Increased abrasion on part of the gangway system. The hydraulic luffing cylinders are a typical example of parts that may be exposed to increased abrasion. During the gangway's operating condition, the hydraulic cylinders are usually exposed to less than 2  10⁵ load cycles. If the hydraulic cylinders are part of the system supporting the bridge in transport condition, they are exposed to additionally 10⁸ load cycles due to ship movement. Even if the loading in transport condition is smaller than those in working condition, the transport condition may, due to the large amount of cycles (500 times more cycles than that for working condition) be of significance when considering the expected life duration of the cylinders.

(ii) The design check of a gangway does not cover investigations whether the gangway interferes with other equipment onboard the ship. For example, if the bridge points along the ships longitudinal axis, the transverse displacement of the bridge tip in a storm may be significant. The ship buyer/owner is to, ensure that the gangway does not interfere other equipment, not only for working condition, but also for transport condition.

(iii) Calculation of natural-frequencies and eigenmodes is normally not covered. The natural period of the bridge is quite different when the bridge rests in a cradle compared to when it is supported by hoisting wire and/or luffing cylinders. If, for instance, the ship movement has the same period as a natural period for the bridge, quite a dynamic amplification of the displacements in the bridge may occur.

Additional securing systems for the bridge may be required if the in-service experience of the gangway shows that large vibrations may occur under transport condition.

4.3 Strength Calculations

4.3.1 General

(a) It is to be shown that structures and components have the required safety against the following types of failure:

(i) Excessive yielding (see 4.3.2) (ii) Buckling (see 4.3.3)

(iii) Fatigue fracture (see 4.3.4).

(b) The safety is to be evaluated for the load combinations defined in 4.2. For each of these cases and for each member or cross section to be checked, the most unfavorable position and direction of the forces are to be considered.

(c) The strength calculations are to be based on accepted principles of structural strength and strength of materials. When applicable, plastic analysis may be used. If elastic methods are not suitable to verify sa fety, for instance due to pre-stressing, plastic analysis may be required.

(d) The verification of safety may be based on the permissible stresses method (work stress design, WSD) or the limit state method (load and resistance factor design, LRFD). With the factors given in the Guidelines there will be only a formal difference between the two methods. If LRDF method is used, LRFD load factors in Table 4-1 are to be applied.

CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH 4.3 Strength Calculations

Table 4-1 LRFD load factors

Load categories

G Q E D

(a) 1.3 1.3 0.85 1.0

(b) 1.05 1.05 1.3 1.0

G = permanent load (self-weight of the structure and all installed equipment, vertical and horizontal loads due to operational motions)

Q = variable functional load (live load)

E = environmental load (loads due to climatic effects, loads due to motion of the vessel on which the gangway is mounted)

D = deformation load

(e) For steel structures, the capacity check is normally to be based on CR Rules and Standards or alternatively other internationally recognized standards (e.g. EN 1993-1).

(f) For aluminium structures, the capacity checks are normally to be based on internationally recognized standards (e.g. EN 1999-1).

(g) For structures with nonlinear behavior, however, significant differenc es may occur. In such cases the limit state method is to be used, or the safety factor is to refer to load and not to stresses.

4.3.2 Checking with respect to excessive yielding

(a) General

With reference to method of analysis and method of verification of safety given in Table 4-2, σy is the guaranteed minimum yield strength (or 0.2% proof stress). If σy is higher than 0.8 times the ultimate strength σu, it is to be used 0.8  σu instead of σy.

When using elastic analysis for cases of combined stresses, the permissible stresses (or the required safety factors) given in Table 4-2 refer to the equivalent stress according to von Mises. Local peak stresses in areas with pronounced geometrical changes may be accepted by case by case evaluation.

Joints are not to be weaker than the minimum required strength of the members to be connected. For riveted joints, bolted joints, friction-grip joints, and welded joints the design is to be based on an internationally recognized standard.

(b) Aluminium

In the case of welded connections, the respective mechanical properties in the welded condition are to be assumed. If these values are not available, the corresponding values in the soft condition are to be assumed.

For aluminium structures, the safety factors in Table 4-2 are to be multiplied with an additional safety factor, SFAL = 1.05.

CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH 4.3 Strength Calculations

Table 4-2

Criteria for the Checking with Respect to Excessive Yielding

Method of verification Load Case I Load Case II Load Case III

Safety factor

Elastic analysis 1.50 1.33 1.10

Plastic (ult. str.) analysis 1.69 1.51 1.25

Permissible stresses Elastic analysis σy /1.50 σy /1.33 σy /1.10

4.3.3 Checking with respect to buckling

(a) The guiding principle is that the safety against buckling is to be the same as the required safety against the yield limit load being exceeded. This principle indicates that the factors given in the second line of Table 4-2 above is to represent the normal requirement. However, other values may be required or allowed, for instance due to uncertainty in the determination of the critical stresses (or load) or due to the post -buckling behavior. Required factors are given for various types of buckling in Table 4-3 below.

Table 4-3

Safety Factors for the Checking with Respect to Buckling

Type of buckling Load Case I Load Case II Load Case III

Elastic buckling 1.86 1.66 1.38

Elastic-plastic buckling 1.69 1.51 1.25

(b) The safety factors given in Table 4-2 above are based on the assumption that the critical stresses (or loads) are determined by recognized methods, taking possible effects of geometrical imperfections and initial stresses into account. Elastic buckling in Table 4-3 above means that elastic buckling stress does not exceed the yield strength.

(c) Calculation methods and corresponding required safety factors as specified by other internationally recognized standards for structural design may also be used.

4.3.4 Checking with respect to fatigue

(a) Checking with respect to fatigue is to be based on an internationally recognized standards applicable for structures intended to be used offshore.

(b) The fatigue assessment is to be performed on the gangway structure considering the cumulative damage effects of both the operational (including deployment/retrieval) and transit/parked cases and is to consider (but not limit to) the following gangway specifics:

(i) Operation time: not less than 20 years.

(ii) Translation and/or rotation cycles in the directions/around X, Y and Z axis (e.g. telescoping, luffing, slewing, etc.).

(iii) Loads due to motion of the vessel on which the gangway is mounted.

(iv) Wind load may usually be excluded.

(v) Type I only: On and off-load cycles at full Live load (LL): not less than 6/day (e.g. 3 working shifts).

The on and off-load cycles are to be agreed with the Society and be noted in the certificate.

Note:

1. On-load: gangway subject to full LL

CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH 4.3 Strength Calculations

2. Off-load: gangway completely unloaded (LL=0) (vi) Deployment/retrieval cycles/day.

(vii) The design fatigue factor (DFF) is not to be less than 2.

(viii) The load combinations for the fatigue assessment can be based on the load combinations defined in Table 4-4 and Table 4-5 below (as applicable).

(ix) Stress acceptance levels according to the fatigue standard used.

(c) The stress range spectrum is to be defined by the designer considering the above minimum limitations.

(d) Different fatigue design parameters are to be agreed with the Society on a case by case basis.

(e) For gangway pedestal below the slewing ring, in addition to the above defined conditions, the introduction of relative stress in the pedestal caused by global deformation of the asset is also to be evaluated, if relevant.

Table 4-4

Type I Gangway - Load Combinations

LC 1a LC 1b LC 2a LC 2b LC 3 LC 4

(1) SW includes gangway self-weight and all installed equipment.

(2) Dynamic factor (DFZ / DFY) due to vertical/horizontal loads due to operational motions.

(3) Stresses in the gangway structure above the slewing bearing (e.g. in the bridge, etc.) may be accepted up to Load Case III allowable stresses, if extreme vessel accelerations (i.e. probability level 10^-8) are used. If vessel accelerations with higher probability level (i.e. 10^-4) are used, then stress levels are to comply with Load Case II safety requirements.

(4) Other relevant load cases and/or combinations are to be agreed with the Society.

(5) MOA – maximum operational accelerations, MTA – maximum transit/parked accelerations.

CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH 4.3 Strength Calculations

Table 4-5

Type II Gangway - Load Combinations

LC 1a LC 1b LC 1c LC 2a LC 2b LC 3 LC 4

Centrifugal force 100%, as applicable

Green sea loads 100%, as

(1) SW includes gangway self-weight and all installed equipment.

(2) Dynamic factor (DFZ / DFY) due to vertical/horizontal loads due to operational motions.

(3) Stresses in the gangway structure above the slewing bearing (e.g. in the bridge, etc.) may be accepted up to Load Case III allowable stresses, if extreme vessel accelerations (i.e. probability level 10^-8) are used. If vessel accelerations with higher probability level (i.e. 10^-4) are used, then stress levels are to comply with Load Case II safety requirements.

(4) Gangway in uplift position (cantilever), load applied at the free end (tip).

(5) Other relevant load cases and/or combinations are to be agreed with the Society.

(6) Fully motion compensated gangways are to have special consideration; Not all load combinations and stress acceptance levels in the table are directly applicable.

(7) MOA – maximum operational accelerations, MTA – maximum transit/parked accelerations.

4.3.5 Design and strength of particular components

(a) General

The design and strength of particular components, such as slewing bearings, flanges, pedestals and pedestal adapters are to be based on the Rules for Cargo Gear or other recognized standards accepted by the Society.

(b) Wheel rolling on rail/structure

Calculation of stress is to be done according to applicable internationally recognized standards accepted by the Society (e.g. EN 13001-3-1 Annex C.4, EN 1993-6).

Alternatively, Finite Elenent calculations are to be provided.

CHAPTER 4 STRUCTURAL DESIGN AND STRENGTH