Patent Publication Number: US-2020277028-A1

Title: Offshore wellhead platform

Description:
The present invention relates to an offshore wellhead platform for use in the oil and gas industry, and in particular to an offshore wellhead platform that receives hydrocarbon fluids from at least one well and carries out processing of the received hydrocarbon fluids to produce processed or part processed hydrocarbon fluids for storage and/or transport to another installation. 
     Offshore platforms used in the oil and gas industry should be arranged in a manner that satisfies the required function of the platform and also that ideally allows for efficiency both in terms of the operation of the platform as well as also efficient manufacture and assembly. An offshore platform must include equipment specific to the platform&#39;s purpose, for example equipment for handling and processing hydrocarbons, and it must also include means for accessing the platform, which in known platforms may include a heli-deck and a landing area for a vessel. The platform must be able to be operated and maintained by remotely controlled systems and/or by personnel on the platform. Conventional platforms hence typically also include control and monitoring systems such as sensors and CCTV as well as facilities for personnel, such as lighting, living areas and so on. As well as holding all of the equipment and ancillary features required for operation the platform must also allow for access for maintenance and be arranged to permit loading and unloading of materials such as consumables and new equipment. Such materials may be delivered by helicopter or by a service vessel, with the delivery method varying dependent on the nature of the materials and their weight. Thus, known platforms typically also include a crane for lifting heavy items to and from a service vessel as well as a laydown area for receiving such items. There may be a gangway such as a so-called Walk to Work (W2W) systems for transfer personnel and loading or unloading of smaller/lighter items. 
     The structure of the platform has two main parts. A supporting sub-structure that extends from below the surface of the sea to above the surface of the sea, and a topside structure on top of the supporting structure. A commonly used supporting structure is a jacket, which has several columns and a framework connecting these columns. The sub-structure can be a floating structure or it may have a foundation at the sea bed. In the latter case the columns are usually held by pilings that are driven into the sea bed. The topside structure generally consists of a number of decks that hold the equipment required by the platforms. The decks are linked by a supporting framework and by stairways. 
     Most offshore platforms include at least a spider deck, which is the lowermost deck and interconnects between the topside and the jacket; a cellar deck, which usually includes a laydown area and can be the location for heavier equipment like transformers and compressors; and a weather deck, which is the uppermost deck and holds the crane as well as providing space for helicopter access. If the offshore platform is a production platform then there may also be a separate deck for the emergency shutdown valve (the ESDV deck) and a process deck for holding process equipment. In this document process equipment is defined as equipment for processing the hydrocarbon fluids to produce processed or part processed hydrocarbon fluids. This typically includes equipment directly involved in the separation, removal and/or transformation processes carried out on hydrocarbon fluids received from the well which are employed to obtain hydrocarbons suitable for use, sale, storage and/or transport. Other decks may also be present, with the number of decks and the size of the platform varying depending on the required function of the platform. 
     Viewed from a first aspect, the present invention provides an unmanned offshore wellhead platform for use in the oil and gas industry, the platform comprising: riser hang-off equipment for connection to at least one riser for flow of hydrocarbon fluids from at least one well; and process equipment for processing the hydrocarbon fluids to produce processed or part processed hydrocarbon fluids for storage and/or transport to another installation, wherein all of the process equipment is on a single process deck of the platform. 
     With this arrangement the platform is an unmanned platform and has essentially one main deck. Advantageously the platform may be a single-deck platform, comprising the single process deck and no further deck(s), aside from optionally a weather protection deck and/or a lower access level for maintenance as described below. All of the process equipment (i.e. equipment as defined above, such as equipment that is directly involved in the separation, removal and/or transformation processes carried out on hydrocarbon fluids received from the well that are employed to obtain hydrocarbons suitable for use, sale, storage and/or transport) on the platform is located on the single process deck and thus may all be placed in essentially the same plane. This is in clear contrast to many known arrangements where multiple decks are used as mentioned above. Reducing the number of decks simplifies the construction of the platform and saves on costs and material usage. Placing all of the process equipment on a single process deck further simplifies the platform arrangement and may allow for more straightforward automation of the operation of the platform. These simplifications have a synergy with the additional proposed feature that the platform is unmanned (i.e. that it generally operates with no personnel present as discussed further below) since having a simpler platform reduces the need for maintenance operations and having a single process deck for all the process equipment can facilitate more straightforward automation of maintenance. For example, to move materials such as spare parts or consumables around a single process deck then a single remotely controlled handling system may be provided to move items horizontally around the single process deck and this will only need to operate in a restricted vertical extent, since a floor level for all of the process equipment may generally be in a single plane. 
     In example embodiments the single process deck is the main deck of the platform and there are no other decks for equipment relating to the processing or handling of hydrocarbon fluids. For example, there may be no other decks aside from one or more decks provided for the purpose of facilitating weather protection, materials handling and/or access to the single process deck. 
     The processing equipment may include equipment for processing or part processing the hydrocarbon fluids, such as equipment for water handling and separation for re-injection, hydrocarbon separation, and/or gas reinjection equipment such as via ESP. The platform may comprise ancillary equipment required for operation of the wellhead platform, and some or all of this ancillary equipment may be located on the single process deck along with the process equipment. For example, the platform may include an electrical cabinet and/or a hydraulic cabinet for holding an electrical and/or a hydraulic control system for the wellhead platform, and this cabinet is advantageously located on the single process deck. Example embodiments use an electrical system rather than a hydraulic system in order to allow for minimal maintenance and reduce the need for personnel to be present at the unmanned platform. 
     The single process deck may be arranged to allow personnel to access the process equipment for maintenance purposes. However, since the platform is an unmanned platform then it is not intended for personnel to be present for normal operation. To allow for maintenance operations the single process deck may have a walkway for enabling personnel to access the process equipment and optionally other equipment on the single process deck. This walkway may also form an evacuation route for personnel to leave the platform single process deck in the event of an emergency such as a fire. An added advantage of the single process deck is that evacuation time is reduced and this allows for further enhancements to the design of the platform, such as in relation to fire protection as discussed below. 
     The single process deck may be arranged with the riser hang-off equipment at the centre. This may also involve placing the riser hang-off equipment at the centre of the platform, for example at the centre of both the single process deck and at the centre of a jacket that supports the deck. In this way the clearance around the riser hang-off equipment is maximised, and fluid pathways such as risers extending to the platform from a subsea location can also be sited with maximum clearance from the structural of the platform and/or the jacket, such as columns that support the single process deck. The riser hang-off equipment may be arranged to couple to multiple risers and all of the riser hang-offs may be grouped together centrally in the single process deck. The riser hang-off equipment may comprise riser hang-offs as well as associated structures and connections, such as a manifold for the hydrocarbon fluids. 
     Processing equipment may be located at a non-central location on the single process deck and may be at an outer part of the deck spaced apart from the centre of the deck. This can have advantages in relation to access to lift the processing equipment as discussed below. Other potentially heavy items such as the optional electrical or hydraulic cabinet may also be located at a non-central location on the single process deck and may be at an outer part of the deck spaced apart from the centre of the deck. 
     The single process deck may include one or more materials handling device(s) for movement of materials around the single process deck. The materials handling device(s) may be arranged for movement of equipment about the plane of the single process deck. For example, this may be a crane such as a gantry crane. 
     The platform may comprise a weather deck as mentioned above. The weather deck may have the primary purpose of shielding the single process deck from the weather. Optionally, the weather deck may support a crane for lifting heavy items to and from the single process deck. For example, a slewing jib crane may be used. The weather deck and the single process deck may be arranged such that there is access to certain equipment on the single process deck from above, for example by using the crane on the weather deck for lifting said equipment off the single process deck. This might include heavier equipment that is advantageously located at an outer location of the single process deck and spaced apart from the centre, such as the process equipment as set out above. In one example, the weather deck does not fully cover the single process deck and instead the single process deck may extend horizontally outward below the weather deck at the locations of said equipment to be lifted. In this case a crane on the weather deck, or potentially a crane on a service vessel, can lift the equipment vertically from the single process deck since the weather deck will not obstruct the lifting operation. 
     There may be a first crane on the single process deck for moving equipment in the plane of the single process deck and a second crane on the weather deck for lifting equipment vertically from the single process deck. In that case the platform may advantageously include a laydown area on the single process deck that is accessible to both the first crane and the second crane. 
     The platform may include an access level beneath the single process deck allowing for access for maintenance. For example, the access level may permit access to lower parts of the riser hang-off equipment and or to utilities passing beneath the floor of the deck for maintenance and inspection. The access level can hold essential, non-process equipment, such as Emergency Shutdown Valves (ESVs) and also provide a riser pull-in level for use when the platform is first commissioned and risers are connected. It is noted that platforms of the type described herein may include various valve arrangements for flow control and/or safety equipment that handles the hydrocarbons but does not process them, i.e. does not perform any transformation on the hydrocarbons. These valves and other similar safety components are not process equipment as defined herein. 
     In some examples the platform includes a single process deck holding all of the process equipment on the platform, a weather deck located above the single process deck and an access deck located below the single process deck. This platform may include no further decks or floor levels. 
     The proposed unmanned wellhead platform with a single process deck has a restricted size and extent compared to multi-deck platforms and this means that the evacuation of personnel from the platform can be achieved within a relatively short time. This is another advantage of a single process deck arrangement. It will be appreciated that in this instance the personnel on the platform are temporarily present, for example for maintenance operations that cannot be performed remotely, since the platform is an unmanned platform. In general the platform would only be operated with personnel on board when a vessel such as a service vessel was also present and thus in this case the evacuation time required is the time needed for personnel to exit the platform and board the vessel, as well as for the vessel to be piloted to a safe distance. In alternative arrangements the platform may be used in a development including multiple platforms connected via bridges and therefore the evacuation time may include personnel exiting the platform via a bridge. Since there is only a single process deck then the maximum evacuation time will not need to include any significant allowance for traversing stairways or crossing multiple decks. Instead it may simply require any personnel on board to exit the single process deck (or the access level, where present) to a stairway or ladder that allows them to board the vessel (or cross a bridge). The evacuation time can therefore be very short. 
     The ability to reduce the evacuation time allows for other advantages to be obtained and for other simplifications to be made to the platform. For example, it can be possible to evacuate the platform quickly enough to avoid escalation even when there is no emergency depressurisation mechanism, such as a hot flare, and therefore the platform may have no emergency depressurisation mechanism. Alternatively, or in addition, in can be possible to evacuate the platform sufficiently quickly to avoid the need for any active fire protection (AFP) systems such that the platform may have only passive fire protection (PEP) systems. Moreover, the amount of passive fire protection that is required can be minimised. The fire protection of the platform may be designed such that the platform includes only passive fire protection and such that the passive fire protection is included only to the extent required to allow for evacuation, with the fire then being allow to escalate after the required evacuation time has passed. 
     It goes counter to established practice to allow for a fire to escalate and potentially destroy valuable equipment and thus it is not obvious to omit emergency depressurisation and to provide only a minimum of passive fire protection. The inventors have realised that the potential cost of permitted escalation of a fire (when evacuation is completed) is outweighed by the benefits to simplification of the platform since this allows for reduced cost and complexity when manufacturing and assembling the platform as well as reduced cost of operating the platform. In particular, an emergency depressurisation mechanism and active fire protection may require regular maintenance and inspection, which requires personnel to be present, so a platform without such features requires lesser maintenance and fewer visits from personnel. Passive fire protection may also require inspection and/or maintenance and so further advantages are realised when this is reduced. Allowing for minimised maintenance means the platform can operate for longer periods without personnel present and this contributes to the gains in efficiency and reductions in costs that arise through using an unmanned platform. 
     The platform may be arranged to have evacuation time that is at most 15 minutes. This puts some limitations on the size of the platform and on the accessibility and length of the evacuation route(s). The platform may be arranged to have a maximum evacuation time of 10 minutes or below, optionally about 7 minutes or below. In example embodiments the evacuation time may be as low as 4 minutes or below. Reducing the maximum evacuation time by restricting the size of the platform can be done by reducing the size of the single process deck, arranging the deck for direct access to exit toward the escape route to the vessel (or bridge) and so on. Those skilled in the art will appreciate that the variables relating to the maximum evacuation time can be controlled during design of the structure and layout of the platform, especially when there is a focus on minimising the amount of equipment that is present. 
     Having a restriction on evacuation time sets a limit on the size of the platform when one considers the possible speed of movement of personnel during evacuation. The platform dimensions and layout, and in particular the dimension and layout of the single process deck, may be determined with reference to such considerations. The deck may have a maximum length and/or width of less than 30 m, optionally less than 25 m and in some examples less than 20 m. For example the deck may be a square or rectangle with both length and width of less than 25 m or optionally less than 20 m. 
     The platform is an unmanned platform and hence it is a platform that has no permanent personnel and may only be occupied for particular operations such as maintenance and/or installation of equipment. The unmanned platform may be a platform where no personnel are required to be present for the platform to carry out its normal function, for example day-to-day functions relating to handling of oil and/or gas products at the platform. There are added advantages to making an unmanned platform as compact as possible, and thus there is a synergy between the proposed single process deck and the fact that it is an unmanned platform. 
     An unmanned platform may be a platform with no provision of facilities for personnel to stay on the platform, for example there may be no shelters for personnel, no toilet facilities, no drinking water and/or no personnel operated communications equipment. The unmanned platform may also include no heli-deck and/or no lifeboat, and advantageously may be accessed in normal use solely by a gangway to a vessel or a bridge to another platform, for example via a Walk to Work (W2W) system as discussed below. 
     An unmanned platform may alternatively or additionally be defined based on the relative amount of time that personnel are needed to be present on the platform during operation. This relative amount of time may be defined as maintenance hours needed per annum, for example, and an unmanned platform may be a platform requiring fewer than 10,000 maintenance hours per year, optionally fewer than 5000 maintenance hours per year, perhaps fewer than 3000 maintenance hours per year. There is of course a clear inter-relationship between reducing the maintenance hours needed and the minimisation of fire protection, amongst other things. The current platform has been developed as a part of a general philosophy of minimising the amount of, and complexity of, the equipment on the unmanned platform, thereby allowing for the smallest and most cost effective platform for a given capability in terms of providing a function in the oil and gas installation. 
     A further synergy arises due to the realisation by the inventors that an unmanned platform can be operated on the basis that whenever personnel are present on the unmanned platform then there should always be a way for direct access and egress by the personnel via a gangway or a bridge. This can lead to reductions in the evacuation time and thus aid in meeting the restrictions on the size of the platform. 
     In relation to the absence of emergency depressurisation mentioned above, the platform may have no mechanism for emergency depressurisation of a hydrocarbon inventory in the event of a fire, and the platform may be arranged to permit a fire to escalate by combustion of the hydrocarbon inventory after time is allowed for evacuation of any personnel that happen to be present. 
     The absence of depressurisation such as a flare can reduce the size and complexity of the platform, and whilst the lack of depressurisation generates an added risk in the event of escalation of a fire it has been unexpectedly found that the capability for reduced size and consequently reduced evacuation time means that the risk to personnel can be avoided. Thus, counter-intuitively, the absence of depressurisation does not result in an increase in risk, provided it is accompanied by a suitable restriction on the platform size, which can easily be achieved with the proposed single process deck. The restriction on the size is aided by the absence of a mechanism for emergency depressurisation, which typically requires a large amount of space and thus increases the possible maximum evacuation time. In addition, contrary to conventional platforms, the hydrocarbon inventory is allowed to burn if the fire is large enough to escalate to the hydrocarbon inventory, for example by rupture of the pressurised piping, and equipment on the platform can be treated as sacrificial in that situation. 
     In some cases the platform may have no depressurisation mechanism of any type, although it may sometimes be useful to allow for a cold vent system for use in maintenance. It will be appreciated by those skilled in this field that there can be a capability for a slow speed depressurisation for use in maintenance (for example over several minutes or hours), whilst also having no ability for emergency depressurisation, which should occur at high speed with emission of large amounts of hydrocarbons in a short space of time, within seconds for example. There may be no flare, in particular there may be no hot flare and optionally no cold flare. For example there may be no large bore cold vent. In other arrangements a cold flare may be present but there may be no hot flare. The exact set-up may depend on regulatory requirements and on the nature of the equipment on the platform, which determines the size of the hydrocarbon inventory and the risks in the event of a fire. 
     The platform may be arranged so that the equipment and piping are left at an operating pressure in the event of a fire. The piping on the platform may be isolated from wells that are located subsea or at a separate structure and/or from pipelines having large inventories of oil or gas. For example, isolation valves may be present at appropriate locations, with these isolation valves being arranged to isolate the hydrocarbon inventory of the platform in the event of a fire. Thus, the equipment and piping may not brought down to atmospheric pressure in the event of a fire, but instead an operating pressure is left in the system. The pressure may change as a result of operation of other equipment such as the isolation valves and/or a drain tank or similar. 
     In the event of a fire the time to escalation without emergency depressurisation will generally be decreased compared to a similar platform with depressurisation. The operating pressure is not released, which means that the pipe stress will remain high or increase while the material ultimate tensile strength will decrease as it heats in the fire. Rupture will therefore occur sooner and at a higher pressure, causing the fire to escalate sooner than would be the case for a depressurised system. However, with the proposed arrangement this is quicker time to rupture can be acceptable. Due to the short evacuation time resulting from the restricted size of the platform via the use of a single process deck, then if maintenance personnel happen to be present they can still evacuate to a safe distance when the escalation of the fire happens. For certain pipes and/or equipment passive fire protection may be required to extend the time before escalation and allow evacuation, but as explained below the amount of passive fire protection can be minimised. 
     The layout and size of the platform may be based on determining a maximum permitted evacuation time based on an estimate of the expected time for escalation of the fire, and then using this time to determine what size of platform can be permitted, which may be in combination with the absence of emergency depressurisation and/or in combination with the absence of active fire protection. This can be done based on identifying the longest safe evacuation time based on the expected time to escalation of the fire, and ensuring that all evacuation routes can be used within that evacuation time. The layout and/or size of the platform may be arranged in order to reduce the evacuation time if required. Passive fire protection may be included in order to increase the maximum permitted evacuation time, for example by adding optimised fire protection as described below. 
     The evacuation time for a given route can be calculated based on assessing the nature of each part of the evacuation route, allocating a time required for a person to traverse each part of the evacuation route, and summing the times. For example, an evacuation route may require personnel to cross one or more deck(s), ascend or descend one or more flights of stair(s), and cross a gangway or bridge. In the case of evacuation via a vessel then the evacuation route may include boarding a vessel, detaching the vessel from the platform and piloting the vessel away from the platform to a safe distance. The time required for a person to traverse each part of a route may be based on the length/distance for the route and on a set speed for different types of route. Preferably the speed is based on evacuation of an injured person. Optionally the speed may be based on favourable weather conditions. In the case of an unmanned platform (as discussed below) personnel would not board the platform during adverse weather and therefore it may not be necessary for the speed during evacuation to take account of adverse weather. The speeds can be based on past experience and/or empirical calculations for speed of movement of a person. 
     The evacuation time may take into account the time required for all personnel on the platform to exit the platform. Multiple personnel may wish to use the same evacuation route, or the same part of a route, at the same time. For example, there may be a queue to board a vessel. The determination of the maximum evacuation time may be done on basis of a maximum number of people on the platform and may include taking account of the time required for this number of people to all complete certain stages of the evacuation route, for example using a ladder, boarding a vessel and so on. The method may include the platform having a maximum limit on the number of personnel present. For example the platform may always have no more than 20 people present at any one time, optionally no more than 15 people, and in some cases no more than 10 people. There may be a maximum limit on the number of people permitted to be present in order to thereby control the evacuation time. 
     In addition to the time required to move from a location on the platform to escape the platform and/or get to a safe distance from the platform via an evacuation route the method may also include adding a time allowance for personnel to evaluate and understand the situation before a decision to escape the platform is made. As the platform is very limited in size and therefore complex thought should not be required to determine the best evacuation route then this time may be set at just a few seconds, for example as 15 seconds or less, or as 10 seconds or less. A further time allowance may be added for personnel to evaluate and address injuries to other personnel before evacuating along with the injured personnel. The maximum evacuation time may include these types of thinking time as well as the time needed to pass along the evacuation route. 
     The assessment of evacuation time may include using a speed for a person crossing a deck, for example a speed in the range of 0.3 to 0.7 m/s for an injured person being evacuated across a flat deck, optionally a speed in the range 0.4 to 0.6 m/s, for example a speed of 0.5 m/s. The same speed may be used for an injured person crossing a flat gangway or bridge. An adjusted speed may be used in the event that the evacuation route includes an inclined walkway such as an inclined gangway. The assessment of evacuation time may include using a speed for an injured person evacuating via ascending or descending stairs, for example a speed in the range of 0.1 to 0.3 m/s for stairs of standard size, for example a speed of 0.2 m/s. The assessment of evacuation time may include using a speed for an injured person evacuating via ascending or descending ladders, for example a speed in the range of 0.05 to 0.2 m/s, such as a speed of 0.1 m/s. Stairs of standard size may be defined as stairs with a maximum pitch of stairs not to exceed 38° and step height in the range of 12-22 cm. The assessment of evacuation time may allow a set time for particular actions during the evacuation, such as opening a barrier, boarding a vessel, detaching the vessel from the platform and so on, and these times may be determined based on past experience and/or testing. Where a vessel is involved then the assessment of evacuation time may include using a speed and/or a set time for piloting the vessel to a safe distance. Since the platform is unmanned then this speed and/or time could be determined based on favourable (or non-severe) weather conditions on basis that the platform is only accessed by personnel in favourable weather, or at least not in severe conditions. 
     The platform may include optimised passive fire protection that is provided to equipment and/or piping on the platform in order to prevent escalation of the fire that would create a risk to personnel on the evacuation route(s) during the determined evacuation time, but that may permit escalation of the fire after the evacuation time has passed. 
     This allows for the amount of fire protection to be optimised such that it can be implemented at a minimum level based on the determined maximum evacuation time. A small and compact single-deck platform can hence be developed with a minimal amount of fire protection. Of course, a safe platform could be easily provided with extra fire protection compared to the proposed optimised fire protection, but the inventors have realised that significant gains in efficiency are possible by the use of the proposed optimisation. Advantageously, the passive fire protection may be provided only to the extent required to prevent escalation of the fire that would create a risk to personnel on the evacuation route(s) during the determined evacuation time. Thus, there may be no further passive fire protection on the platform. Preferably there is no active fire protection at all. By minimising the amount of fire protection then the maintenance required for the fire protection can be minimised, and the space needed on the platform can also be kept to a minimum. As well as this, the installation costs are reduced. The inventors have taken the non-obvious step of providing fire protection that is optimised based on evacuation and would effectively allow for the equipment on the platform to be sacrificed in the rare event of a fire, since provided the platform is kept safe for evacuation then further escalation may not be restricted by the fire protection. 
     An unmanned platform can easily satisfy the requirement for a gangway or a bridge for use in evacuation since such a platform may either be interconnected with another platform, with personnel escaping via a bridge for example, or it may only have personnel present when the vessel that provided transport for the personnel is also present and provides a part of the evacuation route(s). Thus, the method may involve evacuation route(s) making use of a so-called “Walk to Work (W2W)” system for example using a gangway from a service vessel. 
     In the case where the method involves the use of a bridge to another platform, then the other platform may typically be associated with the same oil and gas installation and it may be the same type of platform or a different type of platform. For example, the unmanned wellhead platform to be evacuated may be connected by a bridge to another wellhead platform or to a production platform. 
     The length of the bridge may be set in order to provide a safe distance for evacuation, although it is envisaged that other factors will require a bridge to be of sufficient length, and probably longer than required. For example, the distance between platforms may need to be above a certain minimum based on allowing safe navigation of vessels. The length of the bridge may be about 50 m or above, optionally about 75 m or above. 
     The evacuation route(s) may include different routes from different locations on the platform to an escape point via the gangway or bridge. The platform may have just one gangway or bridge that is hence common to all evacuation route(s). In the case of a vessel connecting to the platform via a gangway then the evacuation route may include personnel boarding the vessel and moving away from the platform to a safe distance by using the vessel. In the case of a bridge, for example to another platform, then the evacuation route may include traversing some or all of the bridge to get to a safe distance. In determining the evacuation route(s) the method may include considering all possible locations for personnel on the platform, and the route(s) that these personnel may use to escape via the gangway or bridge. Identifying the evacuation routes can include taking account of the routes required for traversing decks, climbing and/or descending stairs, climbing and/or descending ladders, descending escape chutes and/or moving around obstructions. Obstructions might include equipment permanently on the platform with a location or a possible location that could block some routes, for example a crane that might obstruct a preferred evacuation route in some positions. Obstructions might also include temporary objects, such as objects being loaded onto or removed from the platform during installation or maintenance. Identifying the evacuation routes may also include taking account of routes that may not be available in the case of evacuation of injured personnel. The method may include identifying multiple possible evacuation routes for the different locations for personnel on the platform. 
     A maximum evacuation time can be determined based on the steps discussed above and this may then be used in assessing the risk and determining the required passive fire protection for optimized passive fire protection. Assessing the risk to personnel using the evacuation route(s) in accordance with the determined maximum evacuation time in the event of a fire can include determining the likelihood of escalation that would affect the evacuation route(s) within the evacuation time. This may include taking account of the expected progression of evacuation of personnel along the evacuation route(s). For example, an increase in the level of danger at start of the evacuation route may be permitted once sufficient time has elapsed for personnel to have moved away from the immediate area. The platform may include passive fire protection that is provided for equipment and/or piping on the platform in order to prevent escalation of the fire that would create a risk to personnel on the evacuation route(s) during the determined evacuation time. This may include passive fire protection provided to the extent required to remove the risk to personnel on the evacuation route(s) during evacuation, and optionally the passive fire protection may only be provided to such an extent. By way of example, if there is a risk of escalation within the maximum evacuation time due to rupture of certain pipework in the vicinity of an escape route, or liable to affect an escape route then passive fire protection may be provided to restrict the increase in temperature of the pipework during a fire and/or to increase the strength of the pipework to make it more resistant to rupturing. Alternatively or additionally, if there is a risk of escalation within the maximum evacuation time due to hydrocarbons present in certain equipment in the vicinity of an escape route, or liable to affect an escape route then passive fire protection may be provided to restrict the increase in temperature of the equipment during a fire and/or to protect the equipment from to make it more resistant to ignition of the hydrocarbons and/or explosion of the equipment. Such equipment may include compressors, scrubbers, coolers, metering devices, valves and so on. 
     Another factor in prior art fire protection is avoidance of risk to the structural stability of the platform. This can provide benefits for the proposed platform as well, although it will be appreciated that the decision could alternatively be taken to sacrifice the platform entirely in some cases, for absolute minimum fire protection despite a risk to the platform structure. In the case of a relatively compact platform even with the optional absence of depressurisation it is typically found that with appropriate isolation and hence containment of the hydrocarbon inventory, then the hydrocarbon inventory may be made small enough to permit it to burn out before any risk to the structural stability of the platform, whilst avoiding the need to add any further fire protection. Thus, in some examples, by including isolation of the hydrocarbon inventory then the platform can include optimised fire protection both for the protection of evacuating personnel and for the protection of the platform structure. 
    
    
     
       Certain embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying drawings in which: 
         FIG. 1  is a view of a 3D model of an example unmanned wellhead platform with a single deck for process equipment; and 
         FIG. 2  shows the process equipment deck of the example platform with the weather deck omitted so that the layout of single process deck can be seen. 
     
    
    
     The proposed unmanned wellhead platform can be used in a field development comprising one or more wellhead platforms along with associated processing platforms, which may advantageously also be unmanned in the same way as the wellhead platform. In some examples the proposed unmanned wellhead platform is implemented as a part of an unmanned field development similar to that described in GB 1615681.2, GB 1615683.8, GB 1615686.1 or GB 1615687.9 and the unmanned wellhead platform described herein may replace one or more of the wellhead platforms described in those applications. 
     The platform is shown in in  FIGS. 1 and 2  with an example layout. The example offshore wellhead platform  10  is an unmanned platform and hence personnel are not permanently present. The platform  10  also omits features that would be required for permanent personnel, such as a heli-deck, accommodation and toilet facilities and so on. A single process deck  12  is the main deck of the platform  10 . This single process deck  12  holds riser hang-off equipment  13  at a central part of the deck  12  and it also holds all of the process equipment  14  for the platform  12 . Thus, there is no process equipment  14  on the platform  10  aside from the process equipment  14  on the single process deck  12 . The process equipment  14  may include water removal equipment, separators and so on. In most cases the process equipment  14  will only carry out partial processing of hydrocarbons received at the platform  10  via risers attached to the riser hang-off equipment  13 , for example processing to remove water for reinjection and/or other processing to allow for more efficient transport of hydrocarbons to other installations for further processing. The riser hang-off equipment  13  may include connections to multiple risers as well as associated manifolds and the like. 
     In addition to the process equipment  14  the single process deck  12  also holds ancillary equipment needed for operation of the unmanned wellhead platform  10 , such as an electrical cabinet  16  that holds an electrical control system and other electrical sub-systems for the unmanned wellhead platform  10 . The single process deck  12  further includes a walkway  15  for use by personnel when they are present, i.e. for access to the equipment  14 ,  16  for maintenance or inspection and for use in evacuating the platform  10  when needed. 
     It will be appreciated that the layout of the equipment on the single process deck  12  could be varied compared to that shown whilst still obtaining the advantages of having all of the process equipment on a single floor. 
     The platform  10  further includes a weather deck  20  above parts of the single process deck  12  and an access level  24  below the single process deck  12 . The weather deck  20  protects the single process deck  12  from weather and the access level  24  allows for personnel access below the single process deck  12  for maintenance and the like. The access level  24  can hold ESVs and also provide a riser pull-in level for use when the platform is first commissioned and risers are connected. 
     Handling of materials such as component parts and the like is enabled via a gantry crane  18  for moving items around the plane of the single process deck  12  and a jib crane  22  on the weather deck  20  for lifting items off the process deck  12 . Local handling for each item may involve the use of permanently installed pad eyes and monorails and/or temporary equipment in addition to the two cranes. The single process deck  12  is designed for internal horizontal transport handling from laydown areas to and from the location where the items are needed. The weather deck  20  does not extend across the full extent of the single process deck  12  in order to allow for vertical access to heavier equipment such as the process equipment  14  and the electrical cabinet  16 . Thus, the jib crane  22  can have access to vertically lift such equipment from the single process deck  12 . 
     Since all the main equipment, including all of the process equipment  14 , is on the single process deck  12  and hence is in a single plane then the proposed platform  10  allows for greater automation and the use of robotics to augment the materials handling solutions that are used. Thus, the platform  10  may be arranged for remote and/or automated operation of the gantry crane  18  and/or the jib crane  22 , as well as optionally including further automated systems for materials handling and/or for monitoring or maintaining the platform equipment, amongst other things. There are advantages to be gained from minimising the need for active human intervention and allowing for maximised unmanned operation of the platform. 
     The platform  10  will allow for various evacuation routes from differing locations using the walkway  15  about the single process deck  12  and the stairways/ladders shown in the Figures. The evacuation routes need to be established with the slowest evacuations being used as the basis for a maximum evacuation time, which can then be used in determining what fire protection should be included in some examples, i.e. to allow for optimised (minimised) fire protection. The platform  10  is provided with passive fire protection (PFP) in order to ensure that a fire will not escalate until after any personnel that are on the platform have been safely evacuated. The platform  10  has no hot flare and in this example there is no mechanism of any sort for emergency depressurisation. It should be noted that the absence of a flare can increase the risk of a dangerous escalation of a fire, since there is no depressurisation. However, the absence of the flare contributes to allowing for the size of the platform  10  to be reduced and the evacuation time to be minimised. Moreover since the platform  10  is an unmanned platform then personnel will only be present with a connection via a bridge (not shown in this example) or a gangway to a service vessel being present as well, which means that the evacuation process can be very quick. It is estimated that personnel can escape to the stair tower within 1 minute after the initial incident, and a conservative assumption is that personnel will be on the service vessel within 10 minutes. 
     The evacuation time and/or the length of the route is assessed for various evacuation routes, or at least for the longest routes, in order to identify the evacuation route with the longest evacuation time. The evacuation time is calculated based on assessing the nature of each part of the evacuation route, allocating a time required for a person to traverse each part of the evacuation route, and summing the times. The time required for a person to traverse each part of a route is based on the length/distance for the route and on a set speed for different types of route. Preferably the speed is based on evacuation of an injured person. Optionally the speed may be based on favourable weather conditions. In the case of an unmanned platform personnel would not board the platform during adverse weather and therefore it may not be necessary for the speed during evacuation to take account of adverse weather. The speeds can be based on past experience and/or empirical calculations for speed of movement of a person. 
     By way of example, the speed of movement may be set as follows: 
     Evacuation of uninjured person: 1.0 m/s for corridors (flat decks), 0.6 m/s for stairs and 0.3 m/s for ladders. 
     Evacuation of injured person: 0.5 m/s for corridors, 0.2 m/s for stairs and 0.3 m/s for ladders. 
     The example platform above is about 20 m by 20 m. The longest evacuation route is determined to be from the far corner of the access level  24  or the far corner of the single process deck  12  to the stairway for access to a service vessel. Conservatively, the distance around the outer periphery across the deck is used. The escape route is hence as follows: walk around deck (about 40 m) and then walk via stairs to service vessel (about 30 m, partially stepped). 
     Using the speeds set out above, the evacuation time for non-injured and injured personnel can then be found. For a non-injured person the timings are: walk around deck—40 s, walk via stairs to service vessel—50 s (using the slower stair speed for the whole distance to get a conservative time), with a total time of 90 s. For evacuating an injured person the timings are: walk diagonal across deck—80 s, walk via stairs to service vessel—150 s, with a total time of 230 s. 
     The evacuation time is used in assessing the risk and determining the required passive fire protection. Passive fire protection is provided to equipment and/or piping on the platform in order to prevent escalation of the fire that would create a risk to personnel on the evacuation route(s) during the determined evacuation time. For minimum fire protection this includes providing passive fire protection only to the extent required to remove the risk to personnel on the evacuation route(s) during evacuation. Thus, if there is a risk of escalation within the maximum evacuation time due to rupture of certain pipework in the vicinity of an escape route, or liable to affect an escape route then passive fire protection is provided to restrict the increase in temperature of the pipework during a fire and/or to increase the strength of the pipework to make it more resistant to rupturing. Alternatively or additionally, if there is a risk of escalation within the maximum evacuation time due to hydrocarbons present in certain equipment in the vicinity of an escape route, or liable to affect an escape route then passive fire protection is provided to restrict the increase in temperature of the equipment during a fire and/or to protect the equipment from to make it more resistant to ignition of the hydrocarbons and/or explosion of the equipment. Such equipment may include compressors, scrubbers, coolers, metering devices, valves and so on.