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Optimising offshore combined operations manning

18 August 2017

The pressures faced by operators to maintain and enhance oil and gas producing installations can require significant scopes of work to be carried out.  Limited bed space often requires mobile accommodation units such as flotels or mobile offshore drilling units (MODUs) to accommodate additional workers.  In this article, Paul Murray of DNV GL Risk Advisory takes a look at the issues raised.

When two or more installations are joined by a gangway link, there is a need to define a suitable plan for emergency response should a major accident hazard occur on either installation. This plan would include all aspects from emergency mustering to abandoning the installation and would be referenced in a ComOps notification to the regulator. Given the need to optimise production in the current economic climate, additional complexity occurs when undertaking ComOps without a platform being shut down. 

Manning levels

Excluding any bed space limitations, the upper manning limit on a stand-alone installation is often dictated by the total capacity of lifeboats or Totally Enclosed Motor Propelled Survival Craft (TEMPSC) adjacent to its temporary refuge (TR). For installations that are composed of multiple jackets, manning limitations are also often placed on jackets away from the TR due to the possibility of bridges to the TR being impaired in a major accident event.

During ComOps, there is often a need to increase manning levels significantly above the normal levels, which will generally require evacuation to the accommodation unit in an emergency. Taking benefit for the gangway in emergency response does however need to be justified and this is likely to be case specific.

Decision aspects

Manning limitations during ComOps are defined by a number of criteria, including:

•  Overall bed space (and welfare) which sets an upper bound on the number of personnel that can be accommodated

•  Escape, Evacuation and Rescue (EER) provisions to evacuate safely in an emergency

•  Control of work, i.e. capacity to safely manage a number of coincident work streams

•  Risk tolerability to individuals and the larger than normal number of workers exposed to platform hazards (i.e. societal risk).

Escape, evacuation and rescue

A hierarchy of emergency evacuation and escape options is available on offshore platforms:

•  Helicopters, which are capacity constrained and may not be available to rapidly evacuate

•  Fixed bridge to an unaffected jacket where personnel can wait-out an event, though this may not be possible all personnel on board (POB)

•  TEMSPCs, whose capacity often defines the maximum manning and whose use has an associated risk

•  Liferafts, which are far less protected than TEMPSC and credit may often not be taken for them in defining manning limitations.

In the UK, the Prevention of Fire Explosion and Emergency Response (PFEER) regulations and its approved code of practice state, unless shown otherwise by appropriate risk assessment, that the TEMPSC capacity should be 150% of the POB with 100% readily accessible from the TR. Often a risk assessment may conclude that an optimal TEMPSC philosophy may be to include one spare TEMPSC at the TR, which aligns with the approach in NORSOK. This is particularly relevant for installations such as flotels, with a high POB, where applying the 150% rule would result in a significant number of TEMPSCs.

In almost all cases, the total TEMPSC capacity will likely be greater than the number of beds on the installation. Operators often take advantage of this during walk/fly to work campaigns where personnel are transported onto the installation during on-shift time and are accommodated elsewhere during off-shift time.

During ComOps, the additional accommodation is permanently linked to the installation by a gangway that also acts as an additional means of evacuation from an installation to a place of safety (i.e. the flotel or MODU) with its own lifeboat provision.

The key part of a ComOps EER strategy is whether to take credit for the gangway and so relax the normal manning constraints for the operational facility. In setting manning limits, consideration should be taken for:

•  The reliability/availability of an evacuation method in events for which it is required

•  Diversity/redundancy in the form of back-up evacuation means. For example. evacuation to another jacket. In this case, the location of the alternative evacuation means in relation to the accommodation unit may be critical.

Gangway considerations

If it is not justified to take credit for the gangway during evacuation scenarios (i.e. the likelihood of it being impaired in the hazardous event requiring evacuation is high), then the manning restrictions cannot increase above the limited capacity of the operational facility.

Normally, the gangway should connect to a relatively safe location on the production installation. However, just having a low frequency of impairment is not sufficient. Crucially, if the gangway is to offer an alternative to additional TEMPSC provision on the operating facility then it must be available for most of the events that would drive evacuation by TEMPSC. This is therefore not a straightforward risk assessment as the gangway needs to provide similar availability to TEMPSC in the conditions where a TEMPSC would be required.

There are additional considerations for a flotel using dynamic positioning, as there are circumstances where the gangway will need to lift, particularly in the case of poor weather. Weather forecasting will provide warning, allowing the production facility to be downmanned in a controlled manner. However, some contingency needs to be in place for cases where the gangway is unexpectedly unavailable.

Relaxing the 150% criterion

Normal operations manning restrictions recognise potential failure mechanisms in TEMPSCs and so ensure redundancy usually either by provision of 150% capacity, or one spare TEMPSC justified by a risk assessment.

The presence of a gangway potentially negates the need for the primary evacuation means to be by TEMPSC – they still provide the redundancy, or secondary evacuation method, should the gangway be unavailable. Due to this, the manning level may be increased up to the full TEMPSC capacity on the platform.

Should the gangway availability be guaranteed in all foreseeable evacuation events, for example, due to its location, or protection offered in its design, then reliance on TEMPSCs is removed and the manning restriction implied by EER provision could be lifted.

Multiple jackets

Given the difficulty in guaranteeing gangway availability in all scenarios, the total TEMPSC capacity on a single platform can only be exceeded after careful analysis of the hazards and the overall EER strategy. However, in the case of an installation with multiple jackets or multiple accommodation units, the fixed bridges or gangways may offer a considerable amount of redundancy if they have diverse landing points. This additional redundancy, again negating the need to use TEMPSCs, may provide justification for a further manning increase.

Platform status

During ComOps maintenance and construction campaigns, it is often necessary to shutdown and depressurise sections of process plant and so a range of operating statuses may apply such as:

•  Producing - Platform ‘live’ as in normal operations, with hydrocarbon systems pressurised

•  Topsides depressurised - All topsides systems isolated and depressurised. However, wells, risers and pipelines may remain pressurised

•  Fully depressurised - As above, but with wells isolated at the downhole safety valve and annuli depressurised. All risers isolated and depressurised up to the subsea isolation valve, or interfield pipelines fully depressurised

When a platform operating status is changed from the norm, hazards are reduced or eliminated, such that relaxing the manning constraints further may be justified as the likelihood of gangway impairment in an evacuation event may be significantly reduced or removed entirely. In some cases, a fully hydrocarbon free installation may have no manning constraints from an EER perspective if, other than hazards that occur with no warning and are not necessarily designed for, such as sudden and severe ship impact, there are no remaining major accident hazards that could impair the gangway and result in insufficient evacuation means on the host platform.

Liferafts

While liferafts are considered the secondary means of evacuation on normally unmanned installations without TEMSPC, their use on platforms with TEMPSC is normally only as a tertiary means of escape. The intention is that they are only used in rare cases of an emergency and multiple TEMSPC failure, or the unlikely scenario of persons being trapped in one part of a platform unable to reach the TR or a TEMPSC. However, under highly controlled conditions and appropriate risk assessment, it may be appropriate to take some credit for them. Likely restrictions on their use are that:

•  There is another evacuation route (most likely a bridge to a flotel, or other jacket) with a remote possibility of being unavailable or impaired

•  It is only possible in calm weather with the ERRV in close standby

•  The numbers of persons involved is limited.

While the above is rightly an onerous set of conditions, it may temporarily allow for additional manning on, for example, a bridge linked platform during ComOps.

Summary

The pressure to expedite work requires man-hours to be maximised during ComOps. However, the need to manage the safety implications of having a large number of personnel on board, particularly when an installation is producing, may place a limit of the maximum manning. DNV GL has assisted several operators in optimising their ComOps manning strategy and, as each operation is different, the chosen manning strategy needs rigorous assessment to demonstrate that risks are As Low As Reasonably Practicable (ALARP).

About the author

Paul Murray is a Chartered Mathematician and Principal Consultant with DNV GL Risk Advisory. He has more than ten years’ experience in undertaking various Technical Safety studies for offshore installations in the North Sea and worldwide. Based in Aberdeen, Paul works closely with operators and engineering contractors to provide consulting services for Offshore Installations at various stages in their lifecycle - from concept design through to decommissioning. His main areas of focus are in risk based decision making, ALARP demonstration, Quantitative Risk Assessment (QRA), and consequence modelling.


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