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OSD Technical policy relating to seismic events hazard

Revision record

Purpose of Issue Rev Date of Issue Technical
Author
Policy Contributions Technical Editor
Issued A December 06 V Karthigeyan M Birkinshaw Wayne Jones

Technical policy

Duty holders should be able to demonstrate that structures have a low probability of catastrophic failure when subjected to earthquakes, and that supports of both safety critical plant and equipment are sufficiently robust to withstand the accelerations, displacements and relative deflections caused.

The emphasis of this Technical Policy is that primary control of the seismic hazard is inherent safety, achieved by the ability of structures and equipment supports to withstand seismic forces and vibrations through adequate design with suitable safety factors (e.g. to 200 year return period) and a subsequent check to a longer return period (e.g. to 10,000 year return period). Equipment safety is provided by appropriate specification and attention to vulnerability of supports. The implication of acceleration, displacement and deflection for the integrity of safety critical elements also need to be considered but this is beyond the scope of this Technical Policy, which is limited to structural aspects.

Impact of policy

There is no adverse impact on new installations. There is some impact on existing installations concerned with the requirement to explicitly consider the hazard. This may lead to some modifications which are to be determined following the ALARP principle.

Nature of threat

The UKCS is considered as an area of moderate seismic activity and structures should be appropriately designed for seismic loads to reduce the likelihood of damage to the structure, its equipment and systems during these seismic events.

Risk level

Technology is not available to accurately estimate the risk level. However, the risk level can be controlled reference to accepted Standards in which integrity for earthquakes covers two levels:

Accident history and forseeability

Though the UKCS is considered as an area of moderate seismic activity, seismic events of a severity sufficient to cause structural and equipment damage have occurred, and statistical analysis indicates the likelihood of more severe events. They are therefore considered to be reasonably foreseeable.

Worldwide practice

Practice varies worldwide. In areas of the world with earthquake activity it is usual to apply an analysis following the practice of API RP 2A. In the Norwegian sector the seismic provisions in NPD regulations and guidance for offshore structures have been followed for more than 10 years. These have been adopted as NORSOK standards containing seismic provisions (in 1999). Spectra amplification factors presented in NPD standards are widely use in the UK.

The draft ISO Standard for fixed offshore structures contains detailed provisions for seismic design. The ISO draft for jack-ups in preparation will also contain these provisions. Applicability of ISO standards to UKCS structures have not been tested.

Background

Fixed installations and jack-ups

Fixed structures, designed for seismic forces, have traditionally applied a strength and ductility level approach. The historical situation re design for earthquakes on the UKCS is mixed. The amount of specific analysis carried out is small. Where analysis has been carried out to check the vulnerability of the jacket, some duty holders have conducted a crude seismic analysis to obtain the base shear due to seismic loads and compared that with the base shear for storm load. This method of assessment for the seismic loads neglects the fact that the loads are different in nature and the pattern of shear force distribution between the two cases is different Some duty holders also claim that the forces generated in the transportation condition assure that the equipment can withstand seismic loads. This claim neglects the fact that the frequencies of load application are different and some equipment can have sea fastenings during transportation and be filled with liquids during operation. It also neglects the fact that vibration-sensing devices will not be working during transportation.

For jack-ups it is usual to perform a site-specific assessment. Where the installation is to be located in a seismically active zone, this will include a seismic evaluation. The seismic assessment would follow a similar approach to that of fixed structures.

Floating structures

It has generally not been practice to consider the effects of earthquakes on anchor systems nor on hull strength. NPD regulations require that the low frequency waves be considered for a 10-4 seismic event.

Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR), Regulation 4 and 5(1)(a) and (e).
Offshore Installations (Safety Case) Regulations 2005 (SCR) Regulation 12(1)(d).

Remedial actions

Before considering remedial actions it is important to ensure that the current level of conformance with this policy is fully understood. The most appropriate updates of criteria and methods should be used to ascertain the level of conformance. Installations failing to meet the standard set by this policy are to be assessed in line with ISO 13822 Assessment of Existing Structures and remedial actions implemented in accordance with Annex H of that Standard.

Examples of evidence

It is the expectation that the Technical Policy for seismic events will be achieved by the demonstration that the installation can withstand the ‘reasonably foreseeable’ event. This is interpreted to be as defined in existing standards and be:

  1. without any significant damage, acceleration, displacement or relative displacement at the 5x10-3 earthquake; and
  2. without major collapse or other significant disruption to safety critical equipment at the 10-4 earthquake level.

Since earthquakes occur without warning, precautionary evacuation is not an option therefore, duty holders should ensure that their installations have sufficient ‘inherent safety’. Aspects for consideration are: base excitation; liquefaction of subsoil; seabed stability; tsunamis; waterborne shock waves (see Appendix for further technical detail on these aspects).

Installations and equipment supports

The demonstration of appropriate control may take various forms:

Relevant publications

Appendix of pertinent technical issues

a) Base excitation

The effects of base excitation need to be covered in adequate detail and the following paragraphs provide additional details. The adequacy of the installation to withstand base excitation may be demonstrated by:

(i) The structure withstanding an earthquake which will have a reasonably low likelihood of exceedance during its lifetime with little or no damage and can maintain its integrity without major collapse which may cause large loss of life during a rare, intense earthquake with a very low probability of exceedance during its lifetime.

(ii) The levels of vibrations, displacements and relative deflections at all locations are low enough in order that the safety critical equipment supported by the structure is not affected adversely. Alternatively the safety critical equipment can be demonstrated to be robust enough to withstand the support movements expected.

b) Liquefaction of subsoil

Liquefaction needs to be considered if the soil is silty.

c) Seabed stability

Consideration of seabed stability may be required dependant of the slope of seabed or under-laying layers and their properties in the vicinity of the installation.

d) Tsunamis

Tsunamis are not expected to be a problem for fixed installations in UKCS.

e) Waterborne shock waves

Waterborne shock waves need only be considered if the hull is susceptible to high frequency loads.

NOTE: Fault movement and Mud volcanoes So far there is no evidence that fault movements and mud volcanoes may be a problem in UKCS.

f) Structural strength of installations

This sub-section applies to non-compliant structures, including jacket structures; gravity based structures (GBS), and some compliant structures such as TLPs and articulated towers, which are not compliant in all modes (or directions). The following provisions apply to such installations, which can be affected in the non-compliant direction or mode.

SLE assessment is required in cases where the effective acceleration at mud level, taking into account the effects of soil in the Southern North Sea and Northern North Sea for the 200-year event, is generally more than 0.05g. Recent evidence suggest that Central North Sea may be seismically ‘more active’ than predicted in OTO 2002/005 and SLE assessment will also be expected in this region, based on nominal bedrock acceleration of 0.05g.

DLE assessment can be expected for many of the larger installations and those with un-braced frames in the Southern and Northern North Sea. In using the provisions of ISO/CD 19901-2, it should be noted that the target annual probability of failure for all installations in the UKCS is 1x10E-4 and this is more onerous than the exposure level for L1 specified in ISO/CD 19901-2 hence evaluation of SLE would be required for all installations in UKCS.

Adequate dynamic analysis needs to be carried out to assess seismic loads on the structure. Analysis model should be prepared to suit the seismic load. If spectral analysis is used, an adequate number of modes should be considered. This may rule out some models used for wave loads and simple use of single degree freedom oscillators. API RP2A and ISO/CD 19901-2 provide the basic guidelines for the informed analyst. Since vertical accelerations are considered as important as horizontal components, the model should cater for this by incorporating axial effects in jacket legs and flexure in cantilevers and inclined members such as flare towers.

g) Equipment

The possible effects of structural vibration on safety-critical plant and equipment should be considered. For the informed analyst, both ISO/CD 19901-2 and API RP2A provide some principles or guidelines to be adopted in calculating the vibration levels at various locations and taking into account their properties.

Safety critical equipment should comply with the requirements of Clause 11.7 of ISO/CD 19901-2. The following may be used to substantiate compliance:

Where consequence analysis or quantitative risk assessment (QRA) is used, the magnitude of structural vibrations relevant to the return period should first be determined followed by the effect of the calculated acceleration on the equipment or its support. QRA or consequence analysis should consider the fact that the same seismic event may have simultaneously affected many of the equipment barriers being considered. In calculating the Individual Risk (IR) or Potential Loss of Life (PLL), the ‘rare’ event (1x10E-4) does not necessarily represent the worst case, which may arise as a result of a less severe but more frequent event.

Claims that generic failure rate data used in QRA implicitly take account of earthquakes will not be accepted unless evidence is shown that the failure rate data used takes into account failures of similar equipment during earthquakes.

A qualitative assessment of plant and equipment vulnerability, possibly involving a ‘walk-down’ study on the installation is a poor substitute to the considered assessment of accelerations and their consequences in a systematic manner. However in the absence of such assessment, ‘walk-down’ may be considered as acceptable as an interim measure and will, in any case, be a useful supplement to engineering analysis.

h) Jack-ups

Requirements for fixed structures are also valid for jack-ups. The following additional items are worth noting:

  1. Simpler models may be used in dynamic analysis, provided the following are taken into account:
    • The effect of possible eccentric deck mass on torsional vibration;
    •  Higher bending modes of the leg;
    •  Vertical vibration due to axial compression of leg; and
    •  Possible P-Delta effects.  
  2. The effects of higher relative displacements should be considered on itself, risers, conductors, other connections and other structures, particularly when a jack up is working alongside another installation.
  3. Effect of possible variation of spud can fixity on the dynamic characteristics and consequently on the seismic loads should be considered.

It is recognized that it would be excessive to perform a new assessment for every site; therefore, an approach may be adopted whereby conservative values are used in a generic seismic assessment, which would cover all possible rig locations. Since they move from location to location, the fact that they may be in one location for a limited time cannot be used as an excuse for not considering seismic effects.

i) Floating structures

The effects of ground acceleration, liquefaction of subsoil and seabed stability on the anchor structure are considerations in assessing the effects of earthquakes.

Waterborne shock waves should be considered if the hull could not have been designed to be robust or vulnerable.

This information sheet contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.

Updated 2013-08-08