Ship collision and falling objects

The design of concrete structures for ship collisions and impact from falling objects will mainly imply the design of walls and shells for various kinds of punching effects. The probability of occurrence of collision loads exceeding certain limits determines whether the design is performed according to the ultimate or accidental limit state criteria.

By ship collisions, the magnitude and distribution of a static equivalent load is mainly determined by the speed, mass and deformability of the colliding vessel itself. Impact mechanics and impact loads are dealt with in Chapter 3, Section 3.6.

As a rule the ordinary design formula for the resistance to local concentrated static loads (punching shear capacity) are applied. Shear reinforcement is often required in structures exposed to ship collisions between specified levels above and below the operational mean water level. In the case of collision with large ships, the global resistance of the structure will often be decisive. Falling objects are usually regarded as stiff bodies with a specified kinetic energy hitting the concrete structure at a concentrated contact surface. The impact of falling objects with high velocity and small cross section may penetrate into the concrete and eventually lead to direct perforation of the concrete shell structure.

The exposed upper domes of a Condeep structure or the pontoons of a floating concrete structure are often protected against moderate impacts from falling objects by a layer of low- strength lightweight aggregate concrete. In this case the load may be determined by the

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dynamic crushing strength of the protective concrete, and the penetration depth by equating the impact energy and the internal work. The resistance of the main structure is related to the static punching shear resistance with increased material strengths due to the high strain rates. The existing rules may not be fully applicable for extremely concentrated loads.

5.3.6 Fatigue

Fatigue of the longitudinal or shear reinforcement due to the large number of load cycles from the wave action may be decisive for certain parts of concrete platform structures. Fatigue of concrete in compression is seldom decisive.

Design rules concerning fatigue of reinforced and prestressed concrete are constantly developing. Recommended references are CEB-FIP Model Code (CEB-FIP Model Code 1990) and Norwegian Standard (NS3473, 1992). Safety against fatigue failure may be differentiated dependent on the basis of failure consequences and inspection accessibility. An example of safety differentiation is found in the regulations by the Norwegian Petroleum Directorate (NPD, 1992), where the number of cycles during the assumed service life is multiplied by a specified fatigue factor.

5.3.7 Prestressing

Several advantages can be achieved by the prestressing of structures. Prestressing may be necessary in certain parts of the structure to comply with specified requirements regarding water-tightness or limitation of crack widths to avoid corrosion of the reinforcement. Prestressing is also applied to decrease the stress range in the ordinary reinforcement in structures subjected to fatigue-load cycles. Prestressed structures are to a larger extent performing in the un-cracked state, with larger stiffness and better conformity between the linear analysis and the design. The use of high strength prestressed reinforcement substituting a larger amount of ordinary reinforcement will decrease the weight of the structure, which will be advantageous in highly weight-sensitive floating structures.

On the other hand, prestressing of areas of the structure, which will be subjected to large compression stresses by reversal of the direction of the load, might give undesirable additional compressive stresses.

The degree of prestressing of offshore structures is often presented as the percentage of the load effect of the characteristic wave action which is counteracted by the prestressing without tensile stresses in the critical section.

The effect of prestressing is usually taken into account by a basic load case in the global analysis. The time dependent loss of prestress is taken into account by determination of an approximate single loss factor for the actual part of the structure.

The needed prestressing force depends on the load effects. The optimal choice of degree of prestressing is discussed and decided in each single case. The necessary prestressing is decided mainly by the requirements regarding durability and water-tightness.

By the checking of maximum allowable crack widths to ensure durability without corrosion, it is usually accepted to calculate the load effects (normal force and moments) of waves

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occurring 100 times during the service life of the structure. These load effects will be about 75% of the effect of the characteristic wave with 100-year return period.

The usual water-tightness criterion for important buoyancy compartments is that resulting membrane tension should be avoided in service state. An important question is for which wave load (return period) this requirement should be satisfied.

If the same load level as for the durability requirement is applied, there is a risk that structures designed for water-tightness may experience cracking through the full sections due to seldom loads, but still with a high probability of occurrence during the service life of the structure. Cracks through the section represent a weakness zone with potential water leakage also after the closure of the crack.

Prestressing capable of resisting the axial forces due to the full characteristic (100-year) wave action without cracks through the section is recommended for structures where the tightness of the structure during the full service life is emphasized. It seems, however, reasonable to take into account a safe portion in the tensile strength of the concrete when designing for such rare wave loads.

The requirements in NS3473 regarding the maximum allowable stresses in the prestressing reinforcement may also influence the effectiveness of the prestressing.

In the section of NS3473 concerning the Serviceability Limit State it is stated: The stresses in the prestressed reinforcement shall for no combination of actions exceed 0.8 f_y, alternatively 0.8 f₀₂.

During prestressing, however, stresses up to 0.85 f_y, alternatively 0.85 f₀₂, may be permitted provided it is documented that this does not harm the steel, and if the prestressing force is measured directly by accurate equipment.

The background for these requirements is mainly to prevent excessive loss of prestress in the service state. This may occur if the steel is stressed significantly above the proportionality limit or exposed to such high sustained stresses that the relaxation of the steel increases considerably. Because of the gradual decrease of the stresses in the prestressing steel with time due to the general prestressing losses, the above requirements will usually be decisive during prestressing. However, the stress in the prestressing steel may exceed the initial prestress in special cases with especially high service loads. This may be the case if the load effects of the full characteristic 100-year wave are applied in the Serviceability Limit State. The required limitation of the service stresses may then be decisive for the prestressing steel demand and the possible choice of the prestressing level.

When designing for such seldom loads, it seems acceptable to allow stresses up to 0.85 f_y, alternatively 0.85 f₀₂ also in service states of short duration, provided that inelastic strain in the prestressing steel is compensated for by initial prestressing to the same level.

The passive anchors of the prestressing cables may be placed internally or at the surface of the structure. The active prestressing anchor must be accessible from the outside and may be placed in recesses or outside ribs. The use of recesses is the most practical method in slip-form construction. The anchor recesses will disturb to some extent the flow of forces and the general reinforcement in the structure. The recesses are to be well distributed to avoid continuous weakness zones in the structure. The recesses are grouted when the prestressing is finished, but the compression strength of the grouted section is still somewhat reduced. The reduced compressive and tensile strength loss is to be compensated by additional reinforcement if necessary.

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It is important to calculate the tensile forces due to the deviation of the general forces around anchor zones and where the prestressing cables themselves change direction, and provide the necessary reinforcement. Walls and shells are to be equipped with both transversal and extra longitudinal reinforcement in the anchorage zones.

5.4 Reinforcement