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Health and safety issues associated with changes from dry gas to wet gas operations

SPC/TECH/OSD/18

Version No:
3
OG Status:
Fully Open
Author Section:
OSD 3.1
Issue Date:
21 November 2011
Review Date
01 November 2012
To
All Inspectors in OSD

Purpose

This document has been produced for the guidance of inspectors and highlights health and safety issues to consider when assessing installations that have or plan to change from dry gas to wet operations.

Introduction

Operators of offshore gas installations are adopting a number of strategies to reduce costs. These include reducing permanent engineering and operations staff, employing more contract staff, changing the installation mode of operation from attended to normally unattended (NUI) with remote control and monitoring of operations.

Reduced reservoir pressure from the ageing fields is also driving this trend towards minimum facilities installations.

The option that is favoured by a number of operators is to simplify the plant on the offshore installations by reducing the amount of gas processing that is done offshore. This involves removal/isolation of the gas drying equipment, such as glycol contactor and associated glycol recovery plant, and allowing the "wet" gas to flow into the pipelines. Extent of change depends upon the current process and existing layout of the installation. Main emphasis by the operators has been to use as much as possible of the existing hardware simplifying the process in this way means less equipment maintenance is required which in turn reduces the number of maintenance personnel on the installation. The reduction in staffing levels and simplification of the process also means a significant reduction in risk and is clearly an obvious safety benefit.

However a number of recent incidents involving release of production fluids on the installations that have changed from dry gas to wet gas operations have raised some concerns. This SPC, for the benefit of the HSE Inspectors, outlines key safety issues, which installation operators need to address when considering a change in the mode of operation. It is a result of consultations with Shell, Amoco, British Gas, UMIST, MATSU, HID and OSD colleagues.

Legislative requirement

Under Reg 14 of the Offshore Installations (Safety Case) Regulation 2005, a duty holder is required to revise a current safety case when appropriate.

Regulation 22 of the Pipeline safety Regulations 1996 requires the operator of pipeline to notify the Executive if there is any change in fluid composition or type.

The change over to wet gas operations is considered significant enough to require the operator to revise the safety case and instigate appropriate action to comply with the pipeline safety regulations.

Key safety issues

Corrosion management

A change from dry gas to wet gas operation processing is expected to change the corrosion rate as well as corrosion pattern in topsides equipment to a limited extent but more significantly in the pipeline. Presence of CO2 in the wet gas stream will also increase the general corrosion rate. H2S in wet environment can cause rapid development of sulphide stress corrosion cracking in susceptible materials.

As part of the change over to wet gas production the operator should have reassessed the corrosion risks, identified the high risk items and implemented corrosion control measures. Systems known to be particularly at risk include process system sections running wet gas and pipelines. The favoured control measure tends to be the use of corrosion inhibitors (note this is not effective in dealing with H2S induced cracking problems). Additional corrosion monitoring using corrosion coupons or probes may be required in order to evaluate the effectiveness of the inhibitor.

Pipeline in particular will require special attention.

The operator should have carried out the following as a minimum:

Pipeline integrity management

Change to wet gas operation will introduce additional hazards which the operator needs to consider when developing the pipeline integrity management plan. Issues to be addressed include:

Process system and operating parameters

Change in the process fluid composition could result in a significant change in the operational characteristics of the process system, which in turn will markedly influence the operating parameters. The change to wet gas operation should be treated as a major modification requiring a complete reappraisal of the process design, assessment of the adequacy of the plant to operate under the new process conditions and operating parameters i.e. liquid levels in vessels, in particular interface between water and condensate. There should be clear indication of where additional liquids can upset the process balance and this should be reflected in a reappraisal of capacities, protection settings and operating procedures.

There may also be an impact upon the blowdown sequence as liquids are harder to get rid of. This means a re appraisal of blowdown times etc. Other protection systems such as HIPPS (valve closure times) may be affected by increased liquid inventory within the pipework.

Failure to follow such an approach will lead to problems such as hydrate formation and blocking of fuel lines, liquid carry over to flare or vent system, salt deposition etc. Issues to be considered include:

Redundant equipment

The redundant gas drying equipment would need to be made safe and mothballed or removed. Hazards associated with such activity also need to be assessed and the risks managed.

Valve specification

Greater water content could increase corrosion of valve seating material requiring more frequent inspection or replacing with a more resistant material. Presence of liquid may also affect the closure times, which would need to be considered when developing the emergency shutdown logic/philosophy. Issues to be considered include:

Hydrocarbon release

Increased liquid content may lead to a change in the dispersion characteristics of the hydrocarbon release, which in turn will affect the estimation of explosion overpressure calculations, and fire characteristics.

Structural changes

Structural assessment should be carried out in order to check the effect of any addition or reduction of hardware or any other operating loads as a result of this change.

Drains handling capability

Issues to be considered include:

Maintenance regime

Installations, which were designed for all dry gas operation, may require a higher degree of scrutiny of material specifications. Pipework and instrumentation may not be as robust against corrosion attack in a wet environment and this should be reflected in revised maintenance frequencies.

Installation attendance pattern

Change in mode of operation from attended to normally unattended, will have a significant effect on the maintenance of equipment particularly where a number of trips have to be made in order to complete a task. For example, in a study carried out for a typical southern North Sea gas platform it was estimated that three eight hour visits by nine persons per week would be required in order to carry out the maintenance work.

This is based on the assumption that all the tools would be available to carry out the particular task, that the weather would be favourable, that the fastenings would be easy to remove and that sufficient aids will be on hand to assist in lifting, manipulating and moving of heavy parts. Until more experience has been gained in managing this activity, including the very detailed planning, it is recommended that safety implication of the change in attendance pattern is assessed before the change is implemented.

By demanning the beneficial effect of operator intervention to control an unsafe situation is also removed.

Installation flare/blowdown philosophy

Increased water content in the gas could lead to formation of hydrates during blowdown and possibly block the pipes and valves. Pipework and knock out drum capacity need to be assessed for adequacy to cope with increased fluid. The change in the gas composition could lead to a significant change in the combustion characteristics and affect the flare tip's ability to burn the product efficiently and safely. The effect of drop in temperature on the material performance during blowdown should be reassessed.

Training and consultation

Change in the mode of operation would require retraining of the operatives. This could be accomplished by posting the operations staff to work with the commissioning and testing contractor during both the construction and commissioning phases. Although views of the operations staff were sought by some operators in designing the modifications, they were not always implemented. This is an area where improvement is needed. Use of computer-based training also appears to be popular and has been found to be very effective by some operators.

Health issues

Hazardous substances

The amounts of methanol, monoethylene glycol and corrosion inhibitors are expected to increase, following the change in operational mode. The operator will need to reassess the risks, taking into account the increased inventory on the installation and more frequent replenishment from the supply boats. Additional control measures, training of operatives, modifications of management and work procedures and changes to the requirements for personal protective equipment could result from the change in operational mode.

As part of the initial survey of the existing equipment carried out to gather information for designing the modifications, hazardous materials, including asbestos, should be identified so that appropriate precautions can be implemented during the execution of the modifications.

Physical hazards

The presence of low specific activity (LSA) scale may increase over a period of time, following the conversion to wet gas processing. The operator will have to establish the likelihood of this occurring by analysing the product and carrying out laboratory tests. If the possibility is established then the organisational structure may need changing to reflect the additional responsibilities concerning the management of all activities that could involve contact with LSA scale. Control of exposure to the radiation is vital, so strict working and monitoring procedures would have to be followed. Extensive PPE will have to be used, including wet suits, breathing apparatus, rubber boots and gloves. In order to prevent contamination of other parts of the installation, strict decontamination procedures would be required. Safe disposal of the LSA scale will also have to be managed.

Ergonomic hazards

As part of the modification of the plant, manual handling of various items is expected. To reduce the possibility of this causing harm to the workers, these activities would have to be planned and appropriate equipment provided to assist with handling the items. Training of operatives to recognise the harm that can be done through unsafe handling and how to carry out these tasks safely will also help. Sufficient time to complete the task without hurrying will ensure that short-cuts are not taken and thereby reduce the possibility of causing damage.

Psychosocial hazards

The change in the installation operational mode could lead to an increased feeling of insecurity and helplessness if the operator changes the staffing arrangements at the same time. By involving the workforce in discussions concerning the change and addressing their fears honestly and fairly, some of the feeling of insecurity could be deflated. It would also help if their workloads are manageable and they are adequately trained to meet the demands of the job.

Management of change

Issues that the operator needs to consider in planning and implementing the change over to wet gas operation include:

Effect on on-shore plant

Main purpose of the change is to decrease offshore operational cost, by shifting some of the processing to on-shore base. However, this in turn can affect the risk on the on-shore operation. Therefore it is very important for OSD to maintain close liaison with HSE colleagues responsible for regulating the on-shore operations.

Conclusions

In the drive to reduce costs a number of operators have either changed or plan to change over to wet gas operations. Recent incidents involving hydrocarbon releases suggest that the change has caused problems with significant safety implications. In this SPC a number of the safety and health issues (highlighted in bold) have been identified that the operator should address in order to reduce the possibility of similar incidents occurring and to generally improve the installation safety.

Reduction in the inventory and maintenance requirements as a result of removing the drying equipment will no doubt reduce the risks on the installations. However the transference of the gas drying system to the on-shore base is likely to lead to an increase in risk.

Further information

Further information can be obtained from: Head of Section OSD3.1 (Process Integrity), Redgrave Court, Bootle.

Updated 2012-05-01