1. The following are bare minimum design documentation requirements. Supplier shall discuss with Purchaser and agree on full extent of documentation requirements.
2. For jacket, pilings, and deck, structural drawings shall be prepared with sufficient details for fabrication of all items included in the design. All accepted-for-construction drawings for U.S.
developments shall require the seal and signature of a licensed professional engineer
3. Upon completion of the design, as a minimum, the following reports shall be submitted as a final record unless Purchaser agrees otherwise:
a. Up-to-date design basis.
b. General descriptions of the structures, weight, and CG report, sketches showing allowable loads on the platform, and other unique features.
c. For each analysis performed, a description of the computer model with plots and sketches, description of the analyses performed, summary of loads, and unity check reports.
d. Hand justification and other special analyses performed.
e. Detailed AFC drawings.
f. Design documentation organized such that a third-party reviewer/verifier can reference the documents easily for all necessary data and information.
g. MMS permit package for all U.S. installations.
APPENDIX A
ASSESSMENT OF WIND-INDUCED VORTEX SHEDDING
Step 1 Adjust the extreme one-minute wind speed using the wind variation with elevation law: Vy = (y/H)1/nVH
where
By = wind velocity at height y
VH = wind velocity at reference height H y = elevation of member at its centroid n = 7 for the 1 minute sustained wind
Step 2 Compute the natural frequency (f) of the member in air:
f = (22.4
*
(E*
l/m)0.5) / (2*
*
LL2) whereI = moment of inertia of member (in4) E = modulus of elasticity of steel (lb/in2) m = mass per unit length (slugs/in) LL = joint-to-joint member length (in)
Note: Member is assumed fixed at both ends.
Step 3 Compute the critical velocity for the member:
vcritical = 4.7
*
f*
dwhere
vcritical = critical velocity (in/sec) f = natural frequency of member (sec-1) d = outside diameter of member (in)
Step 4 Adjust the critical velocity to account for member relative orientation with respect to the wind direction.
Step 5 If a member critical velocity is less than its corresponding design wind speed, the following vibration suppression methods may be considered:
• Change member size/span to increase the critical speed over the design speed.
• Use cables to shift the member frequency and thus its critical speed.
Other methods of vibration suppression shall not be used unless specifically accepted by Purchaser in writing.
APPENDIX B
DESIGN GUIDELINE FOR VESSEL IMPACTS
B.1 Introduction
The objective of this appendix is to define generic design criteria to design fixed steel offshore structures for accidental vessel impact. The criteria are a compilation of acceptable industry practices and should be combined with regional specific criteria for the platforms to be assessed.
B.1.1 Background
During the operational life of a fixed offshore platform, it may be accidentally impacted by a vessel, which can be a major hazard. Due consideration shall be given to the design of these structures to provide robustness against such events by designing them to survive the initial vessel impact and the post impact damage.
B.2 Reference Documents
The following documents form a part of this design basis. Unless otherwise specified herein, use the latest edition.
B.2.1 Codes and Standards
Several worldwide offshore codes and standards offer guidance on vessel impact for new and existing platforms (summarized below). Code requirements and additional descriptions of vessel impact analysis approaches and guidance can be found in these documents.
American Petroleum Industry (API)
API RP 2A-WSD Section 18, Fire, Blast, and Accidental Loading Det Norske Veritas (DNV)
DNV-RP-C204 Design Against Accidental Loads International Organization for Standardization (ISO) ISO 19902 Clause 10, Accidental Situations Standard Norge (NORSOK)
NORSOK N-004 Design of Steel Structures, Annex A, Design against accidental actions United Kingdom Health and Safety Executive (HSE) Guidance Notes
HSE Guidance Notes Section 15, Loads B.2.2 Publications
UK Health Safety Executive, Loads, OTR 13/2001, 2002 DNV Technical Note, Impact Loads from Boats, TNA202, 1981 Veritec, Design Against Accidental Loads, Report No. 88-3127, 1988
Health and Safety Executive, Technical Policy Relating to Structural Response to Ship Impact, December 2006
B.3 Definitions
Design impact event—Design impact event represents an event selected from multiple impact scenarios that require specific design considerations. Design impact events are primarily based on accident scenarios involving vessels that are expected to operate in the vicinity of the platform.
Vessel impact scenarios shall be developed by a risk assessment process, involving a multi-discipline team of experienced engineers. The most likely impact scenarios are the broadside impact of one of the legs of the platform and the bow/stern impact of one of the braces in the splash zone. Practices that account for accidental scenarios are provided in Section 18 of API RP 2A.
For the purpose of a rigorous impact analysis, design impact events shall be established
representing bow, stern, and broadside impacts on exposed platform elements. Vessel orientation and velocity shall further define the impact event. Operational restrictions on vessel approach sectors may limit the exposure to impacts in some areas of the structure.
Design impact events shall consider two energy levels of vessel impacts, i.e. accidental vessel impact, representing a rare condition with a high energy level, and operational vessel impact, representing a frequent condition with a low energy level.
Accidental vessel impact—Accidental vessel impact represents an ultimate condition based on the vessel drifting out of control in the worst seastate where it may operate close to the platform.
For accidental vessel impact, the impact loads should be resisted or impact energy should be absorbed without complete loss of the structural integrity.
Operational vessel impact—Operational vessel impact represents a serviceability condition based on the type of vessel that would routinely approach alongside the platform with a velocity representing normal maneuvering of the vessel approaching, leaving, or standing alongside the platform. For operational vessel impacts, a vessel speed of 0.5 m/s (1/0 knot) is commonly used.
(The metric system of units, which is the predominant system used in the documents listed in the reference section, is used in this appendix. In some cases, the U.S. units are also provided.) During operational vessel impacts, the impact energy shall be absorbed by localized denting of brace or leg and elastic deformation of the structure only. The structure should only suffer minor damage without impairing the functionality of the platform.
Impact zone—The impact zone is defined by the portion of a platform vulnerable to impact by an attendant vessel. The impact zone is a function of the vessel freeboard, tidal range, and operating sea states. The following conditions shall be considered in determining the range of possible impact zones:
1. Vessel maximum and minimum draft 2. Mean low and high water spring tides
3. Operating sea states when the vessel may be in use 4. Associated surge with the operating wave height 5. Platform settlement
6. Water depth tolerance
7. Vessel geometry for bow, stern, and broadside impacts
The greatest frequency of impact shall be near the mean stillwater level. All exposed elements at risk in the impact zone shall be assessed for vessel impact during normal operations.
Attendant vessels—It is not practical or economical to design a platform for a major collision, hence the structure shall be designed to absorb the impact energy from vessels regularly visiting the platform, i.e., the supply vessels. These vessels vary in size from 2,000 to 5,000 tonnes. The vessel size in a specific region shall be confirmed prior to assessment. By way of example, for the northern North Sea, a vessel can be 5,000 tonnes, whereas in the southern North Sea a mass of around 2,500 tonnes is more normal. For Gulf of Mexico structures in mild environments and reasonably close to their base of supply, a 1,000 tonne vessel represents a typical 55 m to 60 m (180 to 200 ft.) supply vessel. For deeper and more remote locations in the Gulf of Mexico, the vessel size may be different.
Note In this document, the term tonne means metric ton, which is approximately 2,205 pounds or about 1.1 short tons.
The attendant vessel details shall include vessel velocity, displacement, added mass, flexibility, maximum and minimum draft, and vessel shape.
Accident scenario—Accidents result from the occurrence of a series of one or more events that combine to cause an undesirable and unplanned outcome. Such a series of events constitutes an accident scenario. The events may result from mechanical fault or human and organizational error.
Ductility—Ductility is a generic term that characterizes the ability of a component or system to deform without experiencing collapse due to brittle fracture or buckling. A ductile component or system may experience some diminishing strength as it deforms and still be considered ductile.
Linear analysis—Linear analysis assumes that all components and system respond linearly to loading.
Non-linear analysis—Non-linear analysis takes into consideration the non-linear effects of individual component behavior, including non-linear material behavior as well as the non-linear deflection of the structural components and system.
Residual strength—When a component is damaged or is removed from a structural system, the system capacity is lower than in the undamaged condition. The system capacity of the damaged structure is referred to as the “residual strength.”
B.4 Mechanics of Vessel Impact B.4.1 General
A vessel impact is characterized by a rapid dissipation of kinetic energy by the impacted structure and the vessel as strain energy. In some instances, the vessel will strike a glancing blow and a portion of the impact energy will remain as kinetic energy following the impact.
B.4.2 Vessel Impact Absorption Mechanism
During an impact between an attendant vessel and a steel structure, a number of mechanisms are available to absorb the strain energy:
1. Local denting of the impacted member
2. Platform structural deformation, including local bending of the impacted member and platform global deformation
3. Vessel local indentation
Local Denting of Impacted Member. Under lateral impact, circular tubular sections are susceptible to localized denting. This energy absorption can be determined either from load-deformation curves or by detailed modeling of the impacted member.
The contribution to energy dissipation from local denting is normally of significance for jacket legs only. For braces in typical jackets, the denting energy dissipation is small compared to the total impact energy and may be neglected.
Platform Structural Deformation. Apart from local denting of the impacted member, energy shall be absorbed by elastic and plastic deformation of the impacted member, the platform, and foundation. This energy shall be calculated using the area under the platform load-displacement curve at the point of impact obtained from the ship impact analysis.
In general, resistance to vessel impact is dependent upon the interaction of member denting and member bending. Platform global deformation may be conservatively ignored. For platforms of a compliant nature, it may be advantageous to include the effects of global deformation.
Vessel Indentation. The deformation of the vessel can be a significant energy absorption component when vessel impacts on jacket leg. Energy absorption by local deformation of the vessel may be based on the force-indentation curves provided in DNV RP C204 if no specific data is available. It is noted that these curves were developed based on a North Sea supply vessel with a displacement of 5,000 tonnes.
For vessel impacts on jacket braces, it is typically assumed that all energy is dissipated by braces.
B.4.3 Design Impact Energy
Several offshore codes offer guidance on the determination of design impact energy for accidental vessel impact. However, they have different approaches.
The default vessel impact energy recommended in API RP 2A is based on the attendant vessel size and a minimum vessel speed of 0.5 m/sec. Guidance is given in the
commentary section of APR RP 2A, C18.9.2, “Vessel Collision.” This approach is tailored to the Gulf of Mexico (GOM) environment and operating practices.
Norwegian codes specify a vessel size of 5,000 tonnes displacement drifting at 2.0 m/s yielding a kinetic energy of 14 MJ (10,300 ft-k) for broadside impact and 11 MJ
(8,100 ft-k) for bow or stern impact. These design kinetic energies are to be shared by the platform and the vessel.
The UK HSE Guidance Notes define the accidental broadside impact energy of a
5,000 tonne vessel traveling at 2 m/s as 14 MJ, which is the same as the Norwegian code requirement. Based on studies of observed platform damage from actual vessel impacts, however, HSE has modified the theoretical impact energy from an accidental vessel impact to account for known deficiencies in the theoretical method. HSE requires that the platform’s contribution to energy dissipation should be minimum 4 MJ. This is different from the Norwegian code requirement where the share of energy is not prescribed, but depends on the relative stiffness of the vessel and platform.
The recent ISO 19902 standard reinforces the HSE approach but highlights the need to establish accidental design conditions taking account of known site-specific vessel operations.
The UK HSE approach is adopted in this appendix to derive the design impact energy for an accidental vessel impact. If acceptable to Purchaser, another relevant design code may be used.
B.5 Calculation of Vessel Impact Loads
Vessel impact loads are typically characterized in terms of impact energy. The total kinetic energy involved in a vessel impact can be calculated using Equation 1 below.
(Eq. 1) where
E = kinetic energy of the vessel (KJ) m = vessel mass (tonnes)
a = added mass factor
= 1.4 for broadside impact
= 1.1 for bow or stern impact v = vessel velocity (m/s)
The key factors in determining the vessel kinetic energy are mass and velocity.
B.5.1 Calculation of Vessel Impact Loads—Accidental Vessel Impact Design Energy
The velocity at which a drifting vessel may impact a facility depends on the actual sea state in which the impact occurs. The vessel drifting velocity is related to the expected environmental conditions under which the vessels will be operating. In lieu of measured, site-specific velocities, Equation 2 below may be used.
(Eq. 2) where
v = vessel drifting velocity (m/s)
Hs = maximum permissible significant wave height for vessel operations near the platform (m)
The deficiencies with this approach are as follows:
1. The added mass factor is dependent on impact duration and is not straightforward to estimate.
2. The vessel is unlikely to come to a complete stop (in sway, yaw, and roll) and hence not all the energy will go into the impact.
3. The platform will not see all the energy; some will be absorbed by the vessel itself.
2
2 1amv E
Hs
v 2
1
In recognition of these deficiencies, the UK HSE carried out studies of observed platform damage to determine the amount of energy actually absorbed by the platform. On the basis of the study results, the HSE Guidance Notes define the accidental broadside impact energy of a 5,000 tonne vessel traveling at 2 m/s as 14 MJ, but only require 4MJ to be absorbed by the jacket structure without collapse. The vessel velocity of 2 m/s represents a vessel drifting out of control in a sea state with significant wave height of approximately
4 m.
On this basis, it is possible to define, in simple terms, a design impact energy seen by the platform structure only as calculated in Equation 3 below.
(Eq. 3) where
KE = design impact energy to be absorbed by the platform structure only (MJ).
Using Equation 3, a broadside impact of a 5,000 tonne vessel operating in a seastate with a significant wave height of approximately 4 m produces a design impact energy of 4 MJ, which shall be absorbed by structure alone.
This formula takes no account explicitly of current velocity and may therefore be seen as appropriate for non-tidal or open water operational conditions where diurnal velocities are low. Where current velocities are significant (typically in near-shore and estuarine areas), it is proposed that the formula in Equation 4 be used.
(Eq. 4) where
Uc = operational current velocity (m/s)
Note that the constant 3500 in the equations above is only applicable if the specified metric units are used.
The operational current velocity shall be set according to the circumstances, and be consistent with the operational sea state, Hs. It shall include only a small wind-induced component, comprising mainly tidal effects. It may be argued that the wind-induced component is already included in the wave-induced velocity computation (Hs/2), and thus only tidal current should be added.
The vessel mass should be the mass for the size of the supply vessel expected to service the platform. The velocity should be the drifting velocity that would be reached by that vessel in the maximum operating storm condition. The vessel size and the maximum operating storm condition could be determined from a site-specific risk assessment.
Design engineers shall attempt to obtain the vessel size and the maximum operating storm condition from the project team. Once those are established, the vessel drifting velocity can be determined.
Table B-1 provides various vessel sizes and design impact energy values (both with and without considering current) for six different geographical regions including Gulf of Mexico (GOM), northern North Sea (NNS), southern North Sea (SNS), offshore east coast Trinidad, offshore northern Angola, and shallow water offshore Nigeria. For preliminary engineering, or in areas where a risk assessment is not carried out, these values can be used to estimate design impact energy prior to obtaining operations input for site-specific analysis.
Note that the values in Table B-1 represent the impact energy criteria required to be dissipated by the structure alone. These values may not be conservative and should be used with caution. The lowest criteria of the Gulf of Mexico reflect the smaller vessel sizes and the lower operational sea states. The most onerous criterion is in the northern North Sea where vessel sizes are larger and operating sea states are more severe.
The minimum impact energies with the current velocity ignored are also presented in Table B-1. For the northern North Sea, a minimum impact energy of 4 MJ is computed when current is ignored. The more conservative approach is that the current velocity is taken into account.
B.5.2 Calculation of Vessel Impact Loads—Operational Vessel Impact Design Energy
For operational vessel impacts, i.e., planned vessel berthings at boat landings and barge bumpers, a vessel speed of 0.5 m/s is commonly used. Table B-2 provides various design energy values for operational vessel impact at 0.5 m/s for six different geographical regions including Gulf of Mexico (GOM), northern North Sea (NNS), southern North Sea (SNS), offshore east coast Trinidad, offshore northern Angola, and shallow water offshore Nigeria. For preliminary engineering, or in areas where a risk assessment is not carried out, these values can be used to estimate design impact energy prior to obtaining operations input for site-specific analysis.
Note that the values in Table B-2 represent the impact energy criteria required to be dissipated by the structure alone. These values may not be conservative and should be used with caution.
B.5.3 Impact Load Application
The width of contact area during impact is in theory equal to the height of the vertical, plane section of the ship side that is assumed to be in contact with the tubular member. For large widths, and depending on the relative rigidity of the cross section and the ship side, it may be unrealistic to assume that the tube is subjected to flattening over the entire contact area. In lieu of more accurate calculations, it is proposed that the width of contact area be taken equal to the diameter of the hit cross section.
In the global analysis of the impacted member and the structure, the impact load is often modeled as a concentrated load applied at the point of impact. This is a reasonable
In the global analysis of the impacted member and the structure, the impact load is often modeled as a concentrated load applied at the point of impact. This is a reasonable