Flooding risk to machinery spaces of floating offshore installations: Guidelines on inspection of ship side valves; flood detection and control; inspection and training

Offshore Information Sheet No. 4/2011

(Issued May 2011)

Contents

Introduction

This sheet gives guidance on what is required by the duty holder to demonstrate that:

  • The ship side valves are suitable, suitably maintained and in good repair.
  • They have arrangements that deal with flooding incidents.

This information sheet replaces information sheet 8/2007 which is now withdrawn.

This sheet is of particular relevance to long term moored installations such as FPSOs, floating storage units (FSU) and floating production units (FPU).

Background

HSE are aware of instances of flooding of the machinery spaces of floating installations and receive, on average, two reports of flooding incidents per year.

Shipside valves ensure the watertight integrity of the hull and internal pipe work from the sea. Operation of the valves allows maintenance on pipe work and equipment. They are used to stop water ingress into compartments after a pipe or equipment failure. Hence, the valves are required to provide a watertight barrier when closed.

It is a legal requirement that the ship side valves are suitable, maintained in an efficient state, in efficient working order and in good repair.

On normal trading vessels, the vessel will be dry docked and the ship side valves will be inspected to comply with Classification Society rules. This inspection will confirm the condition of the valve and that it will seal when closed. For installations such as mobile offshore drilling units, flotels or well intervention vessels, this inspection can be carried out by dry docking or repair periods in port however, for permanently moored installations, these inspection activities become more involved.

Effect of flooding

The effect of flooding will be dependant on the volume of the flooded compartment and the equipment contained in the flooded compartment, if not protected in an effective manner. The consequence of uncontrolled flooding varies from minor, through to loss of control of stability and possible overstressing of the hull structure.

Dependant on the design of the installation this could result in the loss of fire pumps, cooling water supplies and power generation. These failures could lead to loss of production and down staffing of the installation.

Assessment

On a floating offshore installation there will be a large number of ship side valves of varying sizes, from 12mm to 600mm or larger. The larger the diameter of the connection, the greater the consequence of a failure. The duty holder should have completed an assessment of the flooding risk and put in place appropriate measures.

The assessment should identify which valves are critical based on the consequence of a failure of the associated pipe work or attached equipment. Once the critical valves have been identified the duty holder should have suitable arrangements in place to maintain the ship side valves and deal with flooding scenarios.

Hull integrity

The ship side valve integrity can be considered as two distinct areas, the hull aperture and connection to the ship side valve and the ships side valve its self. If the ship side valve and its connection to the hull are well maintained, closing the valve will prevent water ingress into the hull.

The integrity of the connection between the hull and the ship side valve can be confirmed using non destructive testing. The construction of the sea chest and pipe connections are easily inspected by this method. The integrity of the bolting connections should also be confirmed. The inspection regime should ensure that hull and pipe work inspections in the vicinity of ship side valves are done at suitable intervals.

Ship side valve integrity

The purpose of the ship side valve is to be able to provide a watertight seal, to prevent water ingress. The duty holder has to be able to demonstrate that the valve provides an adequate seal and is in good repair. This can be done by a combination of close visual inspection and in-situ testing.

Ship side valve visual inspection

To allow for visual inspection the valve will have to be removed and inspected. This will require the valve to be isolated from the system and the sea. To isolate the valve from the system will require closure of the system to the valve. To isolate the valve from the sea a blank will have to be placed on the hull opening by ROV or diver. Once the isolation is proven the valve can be removed for inspection. This type of inspection allows the internals of the valve to be inspected, as well as the sealing face of the valve. The valve can then be pressure tested to prove it provides a water tight seal.

Testing

It is possible to prove that a valve provides a watertight seal without removing the valve. To demonstrate that the valve provides an adequate seal, the valve should be closed and the down stream side of the valve vented to prove that the leakage past the valve is at an acceptable value. Ideally the valve should have zero leakage; however, in practice, a certain amount of leakage past the valve may be acceptable. The duty holder should define acceptable leakage rates and include these in the work instruction for ship side valve testing.

In certain circumstances this test will routinely be done when suction strainers are cleaned. With other connections, such as overboard discharges; this cannot be easily achieved, as there is normally no arrangement in place to achieve this. It is possible to split the flanges downstream of the valve; however this can damage the joint and if the valve passes, water will ingress into the compartment; hence this method is not recommended. If in-situ testing is to be carried out pipe work modification may be required to allow easy testing of the valve. This involves fitting a drain/vent line inboard of the ship side valve and a further isolating valve in the line. With this arrangement, both valves are closed and the drain line opened to assess the leakage rate.

When a valve has passed the leak rate test, it proves that it provides an adequate watertight seal. It does not however prove that the valve is in good internal condition. A visual inspection of the valve internals is required to ensure it is in suitable condition and not suffering from conditions likely to cause failure such as damage or internal corrosion.

With larger valves it is possible to inspect by using a small ROV, in essence swimming the ROV down the pipeline to inspect the valve internals. To facilitate this modification to the pipe work and suction strainer lids may be required. For smaller valves it may be possible to inspect the valve internals by endoscope; however it may be more practical and cost effective to remove these valves for inspection.

The duty holder should have a suitable plan in place for the inspection of all ship side valves. The inspection interval should be risk based, taking into account the consequence of the valve failing, the valve materials and the sea water environment. As with any risk based inspection programme, the inspection intervals should be reviewed periodically and take into account any failures or identified deterioration.

Flood detection

The machinery space, compartments and void spaces below the water line or other spaces that could be liable to flooding should be fitted with water ingress detection; bilge alarms. If the compartment holds critical equipment, or flooding of it would have an effect on the installation stability, then two independent bilge alarms should be fitted. In large compartments this can be achieved by fitting multiple bilge alarms each with a separate alarm channel. In smaller sized compartments it may not be practicable to have multiple bilge alarms at the same level; in addition to a "normal" bilge alarm a high level alarm should be fitted to the space.

The flood detection system should be periodically tested. The testing should include full end to end testing, proving that a build up of water will generate a bilge alarm.

In addition to bilge alarms compartments, particularly machinery spaces that are un staffed for any length of time, can be fitted with cameras, to allow the control room operator (CRO) to quickly assess the situation in the event of a bilge alarm. In semi submersibles that use lifts to access to the pontoons, cameras can provide confirmation that the operator is not being lowered into danger. In the case of some FPSOs where a large diameter pipe work is used to supply process plant with cooling water, the size of which often exceeds the "normal" engine room pipe work, hence the leakage rate could be large, then a camera can provide early information to the CRO to remotely close system valves, reducing the extent of a flooding incident.

The provision of cameras does not reduce the need to investigate and deal with the cause of the activation of a bilge alarm. The camera should only be considered as an aid to the CRO in assessing the speed of response required to investigate the bilge alarm.

Remote operation

In the event of a flooding incident it is important that the ship side valves can be closed to reduce the size of the incident. If the water ingress is large it may be the case that local access to the ship side valves is not possible, hence shipside valves should be fitted with remote means of operation. This can be extended spindles, pneumatic, electric or hydraulic operation.

For semi submersibles the remote operation of theses valves should be accessible from a space above the final damage water plane after damage.

For mono hull vessel the remote operation should be accessible from a place that does not expose an operator to danger when closing the valve. In considering the sighting of extended spindles the location of these should consider the speed of operator response and the foreseeable flooding rates.

Instruction and training

The duty holder should have in place suitable instructions for personnel to deal with foreseeable flooding events. These instructions should cover what the control room operator does on receipt of a bilge alarm, what the operator does to minimise the water ingress and remove the water from the compartment. The instructions should cover each of the pump devices/systems the operator can use to dewater the compartment and provide guidance on the appropriate one to use.

As well as providing suitable instruction, the duty holder should have provided suitable training. This can range from on the job training covering operation of bilge pump, or emergency bilge suction to damage control drills as part of the emergency response exercises.

Personnel involved in dealing with flooding incidents should be able to locate the equipment used in dealing with flooding incidents and explain/demonstrate their operation to an inspector.

Summary of duty holder actions

  1. Duty holders should assess the flooding risk and the criticality of ship side valves.
  2. Duty holder should define the inspection and testing requirements for ship side valves.
  3. Duty holders should assess arrangements for flood detection in machinery spaces and their procedures in response, including the remote operation of ship side valves.
  4. Duty holders should define the testing requirements of the flooding detection systems.
  5. Duty holders should provide suitable instruction and training to key personnel required to respond to a flooding event.
  6. Duty holders should consider damage control drills as part of their emergency response exercises.

The main relevant legal requirements are:

References

Research Report RR387 - Stability HSE Books 2005

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.

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Updated 2021-02-12