GB2489279A

GB2489279A - Protective cover and fluid environment for subsea equipment

Info

Publication number: GB2489279A
Application number: GB1104959.0A
Authority: GB
Inventors: Neil Douglas
Original assignee: Viper Subsea Technology Ltd
Current assignee: Viper Subsea Technology Ltd
Priority date: 2011-03-24
Filing date: 2011-03-24
Publication date: 2012-09-26
Also published as: GB201104959D0

Abstract

A method and apparatus for protecting subsea equipment such as a subsea distribution unit 1 on the seabed 5 from electrical short circuit or corrosion caused by seawater or calcareous or coral growth is provided by a fluid tight impermeable habitat over the equipment, and displacing at least some of the seawater from the habitat by a buoyant fluid such as an electrically insulating or low electrical conductivity gas or liquid or dielectric oil less dense than seawater so that at least part of the equipment such as electrical connectors (2) of cables 4 is no longer in contact with seawater. Fluid levels in the habitat may be monitored by a float or sensor and maintained to a level. The habitat may be a rigid structure or comprise a flexible impermeable membrane.

Description

I

METHOD AND APPARATUS FOR THE PROTECTION OF SUBSEA EQUIPMENT

This invention relates to a method and apparatus for the protection of subsea equipment from deleterious effects of exposure to seawater.

Seawater is an electrolyte and is therefore electrically conductive, and it is therefore necessary to protect electrical circuits which are located in sea water against water ingress, which has the effect of creating electrical short circuits between electrical conductors and between the electrical conductors and earth (ground potential). In F addition, seawater may degrade materials in contact with it, for instance by corrosion.

In the subsea fluid extraction industry, equipment is usually located on the seabed to control the flow of hydrocarbons from subsea wells. This equipment is usually connected to a facility on the surface or on land via a subsea umbilical, through which electrical power, hydraulic power and communication signals are routed, for operation and control of the subsea well complex A subsea distribution unit (SDU) is typically used to provide connection from the subsea umbilical to electrical connections which are provided on the SDU to which further electrical equipment may be connected The functionality of the SDU may be incorporated into an umbilical termination assembly Great effort is required to design the packaging of electrical equipment intended for sub sea use in such a way that it can operate reliably while submerged for extended penods of time Factors which affect the long term reliability of electrical equipment in a subsea environment include the quality of the design, build and test of the equipment, and the long term performance of the materials exposed to sea-water and electrical stresses In addition, mechanical damage to electrical components can occur dunng installation and maintenance of the subsea system Subsea electrical failures predominantly result from water ingress to a connector enclosure, or cable Such electncal failures typically manifest themselves as earth leakage problems due to the fact that seawater provides an electncally conductive path to ground Electncally insulating barriers can fail which may result in a subsea electrical fault. Evidence for long term degradation mechanisms is provided by failures which have taken place after many years of fault-free operation.

In such cases it can often be difficult to diagnose which component has failed or where water is ingressing. Fault finding under such circumstances typically involves replacement of equipment, often having to replace several pieces of equipment sequentially, until the fault is eliminated. This can be a very costly exercise and can also result in creating new subsea failures.

When electrical equipment fails, the consequential costs can be very high, not just in the fault diagnosis and equipment replacement, but also due to the potential loss of oil and gas production resulting from the loss of control and monitoring in the absence of electrical power or communications. Therefore, very high availability and reliability is required from electrical distribution equipment used in the subsea hydrocarbon extraction industry A continuous working life of 25 years is typically called for The design and packaging of electncal equipment for reliable operation is therefore particularly relevant to the subsea hydrocarbon extraction industry It is known for subsea electrical distribution equipment to be built with two or more independent barriers between each electncal conductor and the sea water Examples of techniques by which this may be achieved are insulated wires installed within pressure-balanced oil filled hoses, electrical connectors with two independent oil chambers around each contact, and rubber moulded connectors housed within an oil-filled enclosure Due to the cnticahty of subsea electrical distnbution equipment extensive effort is spent on design, manufacturing, assembly, testing and installation methodologies in order to maximise the probability of fault free operation Despite these efforts due to the high number of subsea electncal components in any one subsea installation (for example relating to an oil field) the probability of a subsea electrical failure within the life time of the subsea installation is high Vanous technologies are known for monitonng and isolating elements of subsea electrical equipment in the event of a failure While such techniques may help to minimise the impact of failures in subsea electrical equipment when they take place, they do not provide for improved reliability of the subsea electrical equipment.

In addition to the problems identified above, a further problem of calcareous or coral growth exists, with some waters exhibiting high rates of said growth. This growth prevents proper functioning of the equipment by interfering with mechanical operation.

For example, electrical and/or fluid connectors may be prevented from being disconnected without prior removal of the growth.

According to a first aspect of the present invention there is provided a method of protecting subsea equipment from seawater by providing an impermeable habitat over the equipment, and displacing at least some of the seawater from the habitat by a buoyant fluid so that at least part of the equipment is no longer in contact with seawater.

F

According to a second aspect of the present invention there is provided an impermeable habitat for protecting subsea equipment from seawater comprising an impermeable container which is arranged to be installed over the equipment, and from which at least some of the seawater may be displaced by a buoyant fluid so that at least part of the equipment is no longer in contact with seawater, the buoyant fluid being contained within the container.

The subsea equipment may comprise electrical equipment, and the buo9ant fluid may be electrically insulating or of low electrical conductivity. The buoyant fluid may comprise a liquid.

The habitat may be arranged so that it is suitable for installation after the installation of the subsea equipment The habitat may provide protection dunng the normal operation of the equipment The habitat may be arranged such that when the habitat is installed an opening is provided through which connections may be made to the equipment The habitat may comprise a monitoring, means to monitor the level of buoyant fluid in the habitatS The monitoring means may comprise a float which is more dense than the buoyant fluid, and less dense than the seawater. The monitoring means may comprise an electrical sensor which is either a capacitance sensor or a resistance sensor.

The habitat may comprise a replenishment means which may maintain the level of buoyant fluid in the habitat within acceptable limits.

The habitat may be provided with lifting attachments The habitat may be provided with a means for securing it in place, directly or indirectly, to the seabed.

The habitat may be provided with a fluid connector which communicates with the interior of the habitat.

The habitat may comprise a flexible membrane. The habitat may comprise a rigid container.

The habitat may be arranged to be substantially conformal to the shape of the subsea equipment to be protected.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic of a typical subsea distribution unit installed without a protective habitat, Figure 2 is a schematic of an embodiment of the present invention installed over the SDU of Figure 1 Figure 1 shows a typical subsea distribution unit installed without the benefit of the invention The SDLJ 1 is installed on a flat surface 5, typically another subsea structure or the seabed A series of bulkhead electncal connectors 2 will be installed on the SDU 1 Connected to the bulkhead connectors 2 are a plurality of corresponding free connectors 3, which are themselves connected to electrical cables 4. These connectors 2, 3 and cables 4, distribute the electrical conductors from the SDLJ 1 to remote consumers. The electrical connectors 2, 3 are all exposed to surrounding seawater in this conventional arrangement of the SDU and so are subject to the risks outlined herein before.

Figure 2 shows the SDU of Figure 1 with a habitat 6 in accordance with an embodiment of the present invention protecting it from the seawater environment. The habitat 6 is arranged such that it is suitable for deployment over the SDU 1, and in use is deployed over the SDU 1. The habitat as illustrated in Figure 2 takes the form of an inverted container which is fluid tight so that buoyant fluid may be contained therein. The habitat as illustrated is rigid, and is substantially frusto-pyramidal in shape with a rectangular open base and a rectangular closed upper surface. The upper surface of the container F is slightly larger in plan than the SDU, and the integral sidewalls of the container flare down towards the flat surface 5, ending in a ledge feature 9. The habitat 6 as illustrated is rigid, and may comprise, for example, glass reinforced plastic (GRP). The habitat 6 may be constructed from steel or a non-metallic material such as GRP. The habitat 6 a material which is impermeable to the buoyant fluid which it contains in Lifting attachments 7 are provided whereby the habitat may be lifted using a lift line or similar from a vessel on the surface. The habitat 6 may be guided into position by Remote Operated Vehicle (ROV), diver or separate mechanical guidance arrangement.

The habitat 6 is arranged such that its shape substantially conforms to that of the SDU 1 and may be of bespoke design to the needs of a particular installation Other equipment positioned near the SDU 1 may for example impose further restrictions on the shape of the habitat 6 Once installed the habitat 6 may be secured by providing ballast to ensure that the habitat is held in position by the combined weight of the habitat and ballast A ledge feature 9 is provided on which ballast may be readily located Alternatively the habitat F. 6 may be locked in position using latches, or secured to the seabed or subsea structure P by other conventional means It will be appreciated that the addition of buoyant fluid (i e fluid which is less dense than seawater) to the interior of the habitat 6 may result in F the habitat having positive buoyancy in the absence of ballast F Buoyant fluid is added to the habitat 6 so that the interior contains the buoyant fluid after installation. The buoyant fluid may comprise a gas or a liquid. The buoyant fluid preferably comprises an electrical insulator and inhibits calcareous growth. The buoyant fluid is compatible with the materials comprising the SDU 1, and is preferably selected to reduce the rate at which materials comprising the electrical components of the SDU I degrade, or to entirely prevent their degradation. The buoyant fluid may conveniently comprise a dielectric oil. The buoyant fluid is preferably non-hygroscopic and remains an insulator in contact with seawater. The buoyant fluid is preferably environmentally friendly, and creates a minimum of environmental problems when released into the seawater environment.

Conveniently, the buoyant fluid may be added to the habitat 6 after it has been secured over the equipment to be protected. The buoyant fluid may be added through a fluid connector which communicates with the interior of the habitat 6, or may be released below the habitat and allowed to rise into the habitat. The addition of the buoyant fluid displaces the seawater from the interior of the habitat. Sufficient buoyant fluid is provided to ensure that the connectors 2, 3 are bathed in the fluid rather than seawater.

The interface between the fluid and the seawater is denoted by the line 10 in Figure 2.

As an alternative to a substantially rigid structure, the habitat 6 may comprise a flexible impermeable membrane which may be deployed over the SDU 1 Such an arrangement would be of reduced weight and would be easier for a diver to deploy The flexibility of such an arrangement provides advantages in that the shape of the habitat 6 may be adapted to the circumstances of deployment more readily Such a flexible membrane may also be fed through small gaps between adjacent items of subsea equipment in a way that may not be possible with a rigid habitat The flexible membrane may be tailored such that it forms a specific shape when filled with buoyant fluid, for example to conform with the SDLJ. A flexible membrane may furiher be P cheaper to manufacture than a rigid habitat.

An opening B is provided in the habitat 6 at low level to allow the cables 4 to exit from F the habitat 6 while maintaining the fluid interface 10 between the seawater and buoyant fluid such that the electrical connectors 2 3 of the SDU are fufly immersed in buoyant fluid and are therefore not in contact with seawater The SDU I may be elevated so that the fluid boundary 10 may more readily be maintained at a level that ensures that all the elements of the SDU 1 that are vulnerable to failure modes arising from contact with seawater are immersed in the buoyant fluid. Alternatively, the SOL) 1 may be designed to cooperate with the habitat 6 such that the vulnerable elements of the SDU 1 are immersed in buoyant fluid in operation with the habitat 6 deployed.

Level monitoring means (not shown) may be provided on the habitat 6 to monitor the level of the fluid interface 10 within the habitat. A number of techniques may be used to achieve this. For example a capacitance probe may be used to monitor the level of the interface 10 between the conducting seawater and the buoyant fluid. Alternatively a pair of electrical contacts mounted at a specific level may be used to determine that the interface 10 is above or below a set level by monitoring the resistance of the fluid between the two contacts. Another option is to use a float with a density greater than the buoyant fluid and less than seawater, such that it is supported substantially at the level of the interface 10. Such a float may be used to operate an electrical circuit, or mechanically coupled, for example to a mechanical valve or switch. A visual indication of the interface level 10 may be provided on the exterior of the habitat 6 to a diver or F ROV.

Electrical conductivity monitoring means may be provided to monitor the electncal conductivity of the buoyant fluid to ensure that it has not degraded over time Information relating to the position of the fluid interface and the electrical conductivity of the buoyant fluid may be reported back to the surface via a conventional subsea communications network The monitoring sensors may communicate wirelessly through the wall of the habitat Once in place the habitat 6 may prevent or restrict ROV or diver access to the SDU I F and therefore prevent certain maintenance operations disconnection of electrical connectors for example The habitat may therefore be arranged to be conveniently removable or provided with opening windows to allow access for maintenance When such access is required the buoyant fluid may escape and be displaced by seawater

B

A means may be provided to replenish the buoyant fluid. Such replenishment may be required in the event that buoyant fluid has escaped, or where the electrical conductivity of the fluid has increased to an unacceptable level. Replenishment may take place automatically from a local reservoir of buoyant fluid via a locally controlled valve, based on the information provided by the level monitoring means and/or electrical conductivity monitoring means.

The habitat may remain in place during the normal operation of the equipment and may remain in place for the entire service life of the equipment, only being removed as dictated by maintenance requirements.

It will be appreciated that immersing electrical components in a benign, electrically insulating fluid in this way will improve their reliability and consequently the reliability of the protected subsea equipment It will be further appreciated that the use of the invention may have the effect of providing a remedy to existing faults that are related to conduction through seawater It will further be appreciated that the invention prevents or inhibits calcareous growth on subsea equipment Although the example shown relates to an SDU it will be appreciated that the invention may equally be applied to provide protection for non-electrical subsea equipment For example, the reliability of fluid connectors which are subject to failure modes arising from calcareous growth may be improved by the use of the invention Whilst this invention has been described in relation to a subsea oil and gas production F system it will be appreciated that the concept can be applied to many other applications that utilise electrical connectors glands or penetrators in the subsea environment

Claims

CLAIMS: 1. A method of protecting subsea equipment from seawater by providing an impermeable habitat over the equipment, and displacing at least some of the seawater from the habitat by a buoyant fluid so that at least part of the equipment is no longer in contact with seawater.
2. The method of claim 1 wherein the subsea equipment comprises electrical equipment and the buoyant fluid is electrically insulating or of low electrical conductivity.
3. The method of any preceding claim wherein the habitat is installed after the installation of the subsea equipment.
4. The method of any preceding claim wherein the habitat provides protection during the normal operation of the equipment.
5. The method of any preceding claim wherein the habitat is arranged such that when the habitat is installed, an opening is provided through which connections may be made to the equipment.
6. The method of any preceding claim wherein a monitoring means is provided to monitor the level of buoyant fluid in the habitat.

7 The method of any preceding claim wherein a replenishment means is provided which maintains the level of buoyant fluid in the habitat within acceptable limits S The method of any preceding claim in which the fluid compnses a liquid 9 The method of claim 6 in which the monitonng means comprises a float which is more dense than the buoyant fluid and less dense than the seawater The method of claim 6 in which the monitoring means comprises an electncal sensor which is either a capacitance sensor or a resistance sensor 11. The method of any preceding claim in which the habitat is provided with lifting attachments.12. The method of any preceding claim in which the habitat is provided with a means for securing it in place, directly or indirectly, upon or to the seabed.13. The method of any preceding claim in which the habitat is provided with a fluid connector which communicates with the interior of the habitat.14. The method of any preceding claim in which the habitat comprises a flexible membrane.15. The method of any of claims Ito 13 in which the habitat comprises a rigid F: container.16. An impermeable habitat for protecting subsea equipment from seawater comprising an impermeable container which is arranged to be installed over the equipment, and from which at least some of the seawater may be displaced by a buoyant fluid so that at least part of the equipment is no longer in contact with seawater, the buoyant fluid being contained within the container.17 The habitat of claim 16 wherein the habitat is arranged so that it is suitable for installation alter the installation of the subsea equipment 18 The habitat of either claim 16 or 17 wherein the habitat is arranged to be substantially conformal to the shape of the subsea equipment to be protected.19 The habitat of any of claims 16 to 18 wherein the habitat is arranged such that when the habitat is installed an opening is provided through which connections may be made to the equipment 20. The habitat of any of claims 16 to 19 comprising a monitoring means to monitor the level of buoyant fluid in the habitat 21. The habitat of any of claims 16 to 20 comprising a replenishment means which maintains the level of buoyant fluid in the habitat within acceptable limits.22. The habitat of claim 20 in which the monitoring means comprises a float which is more dense that the buoyant fluid, and less dense that the seawater.23. The habitat of claim 20 in which the monitoring means comprises an electrical sensor which is either a capacitance sensor or a resistance sensor. F! 24. The habitat of any of claims 16 to 23 in which the habitat comprises lifting attachments.25. The habitat of any of claims 16 to 24 in which the habitat comprises means for securing it in place, directly or indirectly, upon or to the seabed.26. The habitat of any of claims 16 to 25 in which the habitat comprises fluid connectors which communicate with the interior of the habitat.27. The habitat of any of claims 16 to 26 in which the habitat comprises a flexible membrane.28 The habitat of any of claims 16 to 26 in which the habitat compnses a ngid container.