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OSD Technical policy relating to extreme weather hazards

Rev 3 August 2005

Contact:

M Birkinshaw - OSD4.2
HSE
Priestly House
Priestly Road
Basingstoke
RG24 9NW

Document development - QA sheet:

Purpose of issue Rev Date of issue Technical author Policy contributions Technical editor
Draft for comment 0 24-Jan-03 V Forbes OD5.2 
M Birkinshaw OD5.2
M Birkinshaw OD5.2
 Wayne Jones OD5.1 
Robert White OD5.3
V Forbes OD5.2
Draft including comments 1 14-Feb-03 V Forbes OD5.2 
M Birkinshaw OD5.2
M Birkinshaw OD5.2 
Wayne Jones OD5.1 
Robert White OD5.3
V Forbes OD5.2
Draft including further comments 2 2-April-03 V Forbes OD5.2 
M Birkinshaw OD5.2
M Birkinshaw OD5.2 
Wayne Jones OD5.1 
Robert White OD5.3
V Forbes OD5.2
Finalisation including UKOOA Comments 3 2 Aug 05 V Forbes OD 5.2
M Birkinshaw OD 5.2
M Birkinshaw OD 5.2
Wayne Jones OD 5.2
Robert White OD 5.3
V Forbes OD 5.2

Technical policy

Dutyholders must demonstrate that structures have a low probability of catastrophic failure when subjected to extreme environmental actions.

The emphasis of this Technical Policy is that primary control for extreme weather is inherent safety, achieved by the ability of structures to withstand extreme loads through adequate design with suitable safety factors (e.g. to 100-year return period), and a subsequent check to a longer return period (e.g. to 10,000-year return period). The preferred 'inherently safe' situation for fixed and jack up installations is one where an air gap is provided for the 10,000-year return period - indeed, this will be expected as the minimum standard for all new installations. Ongoing operational safety shall be demonstrated by inherent safety in design, in conjunction with an appropriate integrity management system.

Impact of this policy

There should be no adverse impact on new installations designed to the new standard. The standard set by this policy is compatible with the International Standards Series for Offshore Structures.

The number of manned installations at risk of not meeting this standard depends on;

and is not known. Some estimates put it as high as 30% of manned installations.

The impact on existing installations of failing to meet the standard is thus determined on a case by case basis and the range of possible remedial measures are also installation specific.

Nature of threat

Extreme weather can cause the installation substructure or equipment located below the first deck to fail either through inappropriate original design (loading or resistance) or setting of the design first deck elevation such that waves impact on the deck causing stepwise increase of loading to the substructure and damage to equipment.

On normally unattended installations (NUI) the risk is not to life but to asset and safety critical equipment that is vulnerable.

Risk level

Good quality data and processes are not available to accurately estimate the risk level with sufficient confidence. However recent advances in reliability and advanced analysis put the probability of catastrophic failure in the region of 10-2 to 10-5 depending on structure type, standard designed to and whether wave impact on deck is adequately accounted for. The methods to achieve this standard have been encapsulated in the ISO Series of Standards for Offshore Structures which, together with the Regional Annexes, are considered to be good practice.

The criteria by which this is estimated are, in themselves, uncertain. However, refinements in estimation methods have seen significant reductions in criteria without having methods for dealing with the large uncertainties.

Accident history and foreseeability

There have been failures of fixed structures and jack ups in the Gulf of Mexico (GoM) (each followed by a more onerous loading requirement). The GoM has a more predictable storm regime and a policy of evacuation prior to major storm. Some installations in the North Sea have seen deck impacts. There have been no recorded fatalities for these events on the UKCS; nor indeed in the GoM due to the precautionary evacuation strategy adopted for hurricanes there.

The failure history of large ocean going vessels is a significant cause of concern in the shipping industry. This is of relevance to the offshore sector in relationship to FPSO's.

The foreseeability of severe weather is self evident for offshore installations. The reasonableness is reflected through the level of integrity set for the hazard.

Worldwide practice

The level of integrity has traditionally been set through prescribed Regulation in the North Sea, by an industry standard (API) for the USA, and Class Rules. These are currently being harmonised through ISO to a set of appropriate criteria, similar to those in place in Norway (NPD Regulations and guidance, NORSOK etc.) except for jack-ups. This Technical Policy is based on the ISO criteria.

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Background

The historical situation for the extreme weather hazard is dependent on the type of structure.

Fixed structures

Fixed structures have traditionally been designed for 50 to 100 year return events, combined with factors of safety included from codes of practice. It has also been important to ensure that structures can survive events with longer return periods so that risks to people are as low as reasonably practicable.

Historically, the air gap (the gap between the highest crest elevation for the design return period and the underside of the lowest deck level) has been set around 1.5m. This figure is not risk-based and will be insufficient to ensure that wave crests, associated with waves larger than the design, do not strike deck structures (particularly for structures in deeper water).

Jack ups

Jack ups have been designed to similar practices as those for fixed structures and classed by Classification Societies. For a specific site it is usual to perform a site-specific assessment and traditionally these assessments have followed various methodologies.

Floating structures

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. Thus for monohull structures such as FPSO's and FSU's, the Rules implicitly include load effects for which the derivation is not clear. Such effects may not be site-specific. For semi-submersibles, being newer types of structure and closely linked with the offshore industry, the relevant Rules tend to appear more open and rigorous.

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Legal requirements

Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR), Regulation 4 and 5(1)(a) and (e). Offshore Installations (Safety Case) Regulations 1992 (SCR), Regulation 8(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.

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Examples of evidence

The Technical Policy for Extreme Weather, in accordance with DCR Reg 5(1), will be achieved by the demonstration that the structure will retain, for an appropriate probability of the event, sufficient integrity to enable action to be taken to safeguard the health and safety of persons on or near the installation. HSE interprets the probability of such an event to be a value of Pf< 10-4. This is achieved by designing, with associated factors of safety, to a certain return period (e.g. 100 years) in conjunction with a check at a longer return period (e.g. 10,000 years). This check takes different forms ranging from use of an airgap control to use of sophisticated analysis techniques (including pushover analyses) to demonstrate appropriate integrity. Claimed failure probabilities of less than 10-4 should require further justification using, for example, structural reliability analysis.

The use of quantified risk assessment or reliability analysis is a comparatively new technique, and can be used in exceptional circumstances where the above are not met. The use of generic failure rate data should be used with caution, and only where it is fully relevant to the specific application in terms of geographical factors, environmental factors and structural form.

Fixed structures

In general, maximum environmental actions cause maximum responses. The adequacy of structures may be demonstrated using various approaches, including:

Jack ups

For extreme events, maximum environmental actions cause maximum responses. In the case of a Jack-Up, which can set the deck level on a site-specific basis, a primary aspect of the demonstration is the provision, wherever reasonably practicable, of sufficient airgap to accommodate the 10-4 crest elevation (referred to as the 10,000-year air gap). In addition the adequacy of the structure should be demonstrated based on the following hierarchy: compliance with an industry or company site-specific assessment standard, which implicitly provides a minimum probability of failure, Pf, of less than 10-4. HSE considers compliance with the SNAME T & R Research Bulletin 5-5A 'Guideline and Recommended Practice for Site Specific Assessment of Mobile Jack-Up Units' revision 2 January 2002 combined with an air gap above the design wave appropriate to accommodate the 10-4 crest elevation to be capable of achieving such a minimum. Alternative methods can be used provided they have been appropriately calibrated to the 10-4 criteria.

  1. Where it is not reasonably practicable to achieve a 10,000-year airgap, extrapolation of results of the specific site assessment accounting for actual elevation and including the associated wave-in-deck loads based upon a return period of the order of 10,000 years. Alternatively the use of a site-specific non-linear analysis (also termed 'pushover') to demonstrate that partial failures do not threaten the overall integrity of the structure nor of those areas where people may be sheltering.
  2. Precautionary evacuation of normally manned installations in advance of severe environmental events. As this approach is not based on the inherent safety of the structure, it should be examined very closely to assess the uncertainty in weather forecasts and the risks due to evacuation in poor weather conditions. It is not normally considered to be a viable demonstration to the previous two options, for Jack-Up structures. Given the inherent difficulties with precautionary evacuation, due to the uncertain behaviour of jack-ups in extremely severe wave loading conditions, particularly if waves contact with the deck, it is unlikely to be an acceptable alternative to meeting criterion (a) above.

Floating structures

Peak responses tend to occur when the wave lengths are close to relevant dimensions of the structure and so the highest waves for certain periods will be more critical than the highest waves overall. There may be increased opportunity (when compared to other structural types) to actively manage integrity during severe weather, e.g. by control of the ballast and/or cargo distribution so as to minimise stresses. The following matters will need to be addressed:

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Relevant publications

Pertinent technical issues

This policy covers the following elements:

Updated 2012-12-13