|Purpose of Issue||Rev||Date of Issue||Technical
|Policy Contributions||Technical Editor|
|Issued||A||December 06||M Birkinshaw||M Birkinshaw||M Birkinshaw|
Duty holders should be able to demonstrate that fixed and jack up structures and their equipment are sufficiently robust when subjected to the impact of attendant vessels.
Duty holders should be able to demonstrate that floating structures are sufficiently robust when subjected to the impact of attendant vessels or shuttle tanker collisions.
The impact of a ship on an installation can cause the installation substructure to be impaired to an inappropriate integrity by damaging critical structural members and in some cases to fail catastrophically.
Equipment can also be damaged or affected by collision such as:
Further consequences from the above damage can contribute to:
Factors affecting the risk include geographical location, age and structural form of the installation, and position and angle of impact on the structure. The impact load required of the installation is determined through existing best practice. The actual quantified risk level of structural failure due to ships impact is difficult to assess.
There have been failures of fixed structures, jack ups and FPSOs in the North Sea from ship impact. There have been no recorded fatalities for these events. The reasonable foreseeability of ship impact is self evident for offshore installations.
The ships collision and impact scenario is seen as an accidental limit state in the design and assessment of offshore installations. Historically worldwide practice has varied, but has sought a generic ‘robustness’ for all installations irrespective of their specific type and location. This robustness was originally set by the Fourth Edition Guidance (Ref 11, now lapsed) and more recently has been set using the output of a site-specific risk assessment following the practice defined in ISO 19902 (Ref 20).
There is no adverse impact on new installations where structures are engineered to withstand impact. There is minimum impact on existing installations as most were brought up to the generic impact requirements of the earlier guidance produced by HSE (Ref 11). Where the assumptions of Ref 11 have changed (e.g. change of vessel size, speed or generic configuration) a new assessment may be required and further control measures considered.
Fixed structures have traditionally been designed for ship impact loads by an assessment of the impact of attendant vessels. Shuttle tanker collisions have traditionally not been considered. Jack ups have been designed to similar practices as those for fixed structures and classed by Classification Societies.
Since 1988 it has been standard practice to design for an impact energy of 14 MJ for sideways impact, and 11 MJ for bow or stern impact. These values are based on attendant/support vessels of 5000 tonne displacement moving at 2 m/s. The added mass is assumed to be 0.1 for bow or stern contact and 0.4 for side impact. The impact energy is in the form of kinetic energy of the moving vessel together with its added mass. For smaller or larger vessels, the modified impact energy E can be calculated.
The recent ISO 19902 Standard (Ref 20) reinforces the above approach but highlights the need to establish accidental design conditions taking account of known site specific vessel operations.
On impact, the vessel and platform will absorb part of the energy as it deforms elastically and plastically (and in fracture if there is such failure). The rest will remain as kinetic energy when the vessel and part of the fixed platform move together. At the end of the event the energy spent in elastic deformation will be converted to kinetic as the vessel rebounds. The rebound is largely due to the global bending of the fixed platform.
Plastic deformation and fracture can affect structural integrity. They can also affect the integrity of the equipment supported by the affected members or located behind them. As the platform bends and then rebounds, the accelerations at deck level can be significant and equipment can be affected.
Floating structures have traditionally been designed using Classification Rules. These Rules have been developed over many decades for trading ships, and include a number of empirical or experience-based requirements. The impacts of attendant vessels and shuttle tanker collisions have been considered.
Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR), Regulation 5(1)(a) and (e).
Offshore Installations (Safety Case) Regulations 2005 (SCR) Regulation 12(1)(d).
The evidence should demonstrate the integrity of the Safety Critical Elements (SCEs) of the installation to withstand an appropriate impact energy level. This includes both direct impact loading and any subsequent acceleration loads. Providing the duty holder is to operate within the defined criteria, HSE considers that Section 7 Accidental Loads of ‘OTO 013/2001 LOADS’ provides criteria suitable for ‘good industry practice’ given non violation of underlying principles of vessel size and approach speed. For such cases a site specific analysis should be carried out.
The sub-structure may incur permanent damage due to impact loads however it should, so far as reasonably practicable, continue to ‘retain sufficient integrity to enable action to be taken to safeguard the health and safety of persons on or near it’ (DCR regulation 5(1)(e)). This can be demonstrated if the installation is shown to be capable of withstanding a suitable subsequent environmental event without collapse. Additional SCEs may exist in the direct impact zone eg risers, conductors, fire water caissons. Evidence should also be provided that, so far as reasonably practicable, their integrity is ensured through suitable location or protection.
Some existing installations may not be able to withstand the impact energy from a 2m/s collision, and/or change of vessel size. In such cases the ALARP principle should be applied to decide whether any modifications or further detailed investigations are justified or not. To apply the ALARP principle, it is necessary to carry out a study to determine the required modifications or further detailed investigations and their cost. This will allow a comparison of cost against the benefits (or the potential cost of inaction), leading to an appropriate decision.
For new designs as well as existing installations, if equipment is likely to be affected, a consequence analysis will form part of the study. If the consequences based on what is considered to be a conservative estimate of acceleration and equipment vulnerability are unacceptable. Modifications or further detailed investigations required to assess the accelerations and equipment vulnerability more accurately.
The references given above for Fixed Structures provide information useful in evaluating the damage to semi-submersibles and TLPs. The columns are the areas most vulnerable to collision with attendant vessels. These columns generally have higher energy absorption capacity than jackets or the legs of jack-ups. However structural damage can also affect marine integrity by allowing water ingress and/or causing damage to pipes essential for marine operations. Additionally, collision with bracings should not be discounted.
Analysis of recent collision incidents between shuttle tankers and FPSOs indicate that impact energy of 40 MJ is foreseeable.
There are several commercial ship/installation collision models that can be used to calculate the frequency of a passing vessel colliding with an installation. Those currently available and the organisations that developed them include:
In part, the collision models use data contained in the shipping traffic database to predict the frequency of a ship/installation collision. It is important that the model uses traffic data that is accurate for the existing or proposed location of the installation under consideration.
In general the models calculate collision frequency from:
Validation of the COLLIDE model predicted 3.77 powered collisions up to 1995 and 0.69 drifting vessel collisions. In the same period there were three actual powered collisions and no drifting collisions. The parameters used by some of these models have been modified to fit the model’s predictions to historical incident data.
The following text applies to the impact of attendant vessels with fixed platforms (such as jackets and jack-ups) and compliant installations such as articulated columns, semi-submersibles and TLPs. They do not apply to shuttle tanker collisions. Energies to be used in FPSO/shuttle tanker collisions are discussed in section 3.3.
Since 1988, it has been standard practice to design for an impact energy of 14 MJ for sideways impact and11 MJ for bow or stern impact (refs 3, 4, 11, 12, 15).
These values are based on attendant/support vessels of 5000 tonne displacement moving at 2 m/s. The added mass is assumed to be 0.1 for bow or stern contact and 0.4 for side impact. The Impact Energy is in the form of kinetic energy of the moving vessel together with its added mass. For smaller or larger vessels, the modified impact energy E in MJ can be calculated using the following equation:
E = 0.5 (M+a) v 2
E = The Modified Impact Energy in J M = the displacement in kg,
a = added mass of vessel (0.1M for bow or stern impact & 0.4 for side impact) and
v = velocity of impact in m/s
Note: For new designs the NPD regulations (Ref. 14) require that M and v to be assumed to be not less than 5000 tonne and 2m/s respectively.
Impact velocity of 2m/s is based on probable velocity achieved during erroneous operation or drifting in significant wave height of 4m.
On impact, the vessel and platform will absorb part of the energy as they deform elastically and plastically (and in fracture if there is such failure). The rest will remain as kinetic energy when the vessel and part of the fixed platform move together. At the end of the event the energy spent in elastic deformation will be converted to kinetic as the vessel rebounds. The rebound is largely due to the global bending of fixed platform. In the case of floating units, rebound is less significant.
Plastic deformation and fracture can affect structural integrity - and buoyancy/stability in the case of floating units. They can also affect the integrity of the equipment supported by the affected members or located behind them. As the platform bends and then rebounds, the accelerations at deck level can be significant and equipment can be affected. The following paragraphs briefly consider the effects of vessel impact on structural integrity, equipment and buoyancy/stability.
Assessment of structural damage due to impact is normally calculated using the force indentation curves developed by DNV and others. They are presented in References 4 and 5. A more recent set of curves is presented in Reference 16. These references also provide guidance in using the curves. They were developed for an impact of a 5000 tonne vessel with infinitely stiff vertical cylinders of 1.5 and 10m diameters. Even though the applicability of these curves is limited, they are still being used widely. Unless it is certain that the use of these curves and the analysis method used provides conservative results, it is prudent to ensure that the energy absorbed by the installation is not less than 4 MJ (ref 11).
Detailed methods for calculating damage to leg or braces of jacket structures can be found in References 4, 16 and 19. The methods described in these references can also be applied to jack-up units. Techniques specifically developed for jack-ups can be found in References 5 and 6.
The information given in References 4, 5 and 16 are also useful in evaluating the damage to semi-submersibles and TLPs. The columns are the areas most vulnerable to collision with attendant vessels. These columns generally have higher energy absorption capacity than jackets or the legs of jack-ups. However structural damage can also affect marine integrity by allowing water ingress and/or causing damage to pipes essential for marine operations. Additionally, collision with bracings should not be discounted.
Analysis of recent collision incidents between shuttle tankers and FPSOs indicate that impact energy of 40 MJ is foreseeable. The curves presented in References 4, 5 and 16 cannot be used in designing or checking for collision damage.
In order to assess the potential consequences of vessel impact, deck accelerations, the vulnerability and tolerance of various equipment to accelerations, deflections and the relative displacements or rotation of their supports should be known.
Investigation of accelerations due to vessel impact on three jacket structures is reported in Reference 10. The accelerations varied between 0.1 to 0.9 g depending on the structure and location of impact on the deck. There are no validated, simplified methods to calculate deck accelerations. Finite Element models developed for strength analysis are often used. Some such models may not be suitable for dynamic loads. Failure to consider the softening effects of impacted area could also produce unreliable results.
Accelerations significantly greater than 0.1g can cause problems for equipment. There are no suitable guidelines available to assess the vulnerability of offshore equipment to such accelerations. A 'walk down' study (Reference 9) can identify equipment, which is obviously vulnerable to lateral loads or accelerations. It cannot identify all potential problems. In the cases below, ALARP principles should be applied to decide whether any modifications or further detailed investigations are justified or not.