Seismic surveys are carried out extensively in the North Sea to search for potential reserves of oil and gas in the subsea rock formations. Large seismic survey vessels are used to tow steamers suspended below the surface which carry hydrophones. Sound waves are transmitted from the vessel using compressed air guns which travel down through the seabed and reflect back from the different layers of rock. These relfected sound waves are received by the hydrophones located along the seismic streamers which when processed gives a three dimensional picture of the substrata.
Click on photo left to see an animation on seismic surveys.
Seismic survey vessels usually tow several streamers behind them (from 1 to 12) and of varying lengths up to 5000 metres. They show shapes and lights for a vessel restricted in its ability to manouevre. The streamers (cables) are spread by diverters, similar to a type of mid-water trawl door, and can extend to over 500 metres in width. The end of each streamer is marked by a tail buoy carrying radar reflector and flashing lights.
Seismic survey vessels tow at a speed of 4 - 5 knots and need to be on a straight line whilst surveying and are usually accompanied by a "Chase Boat" who will assist in notifying other vessels of the seimic operation. Survey areas vary greatly in size and may cover extensive areas of the sea surface. Please co-operate with seismic survey vessels during your own operations.
Bigfoot 1, DP 3 Derrick Pipelay Barge, heavylift 400 mT offshore mast crane, 60 mT pipe handling crane, classified under ABS register, LOA (excluding stinger) 135.00 m, 6x2200kW Rolls Royce Azimuth Thrusters, 8 point Mooring Winches, KONGSBERG DPS Class 3, KONGSBERG HiPAP 500 system, 270 mT pipelaying capacity, 6×50 mT Pipe Repair Davits, A&R Winch capacity of 300MT, three sections-90m Stinger, helideck–Sikorsky S-61N & S-92 with refueling system, 150HP ROV, accommodation 240 POB, Built in 2010 in GSP Shipyard, upgraded in 2011.
Relevant missions: pipelaying, heavy lift works.
The barge is capable of installing 6” to 60” diameter pipelines.
where they are welded together photo right.
Photo left the length continues over the stern of the barge and is layed onto the seabed with the barge moving in a continuous movement.
After the pipe arrives on the seabed the sand is back filled over it.
Click photo above to see an animation of a laybarge.
This link show an animation of one of many types of trenching machines used to bury the piplines
As stated on admiralty charts "Mariners are advised not to anchor or trawl in the vicinity of pipelines". The note goes on to say that damage to oil or gas pipelines could cause your vessel to lose buoyancy through escaping gas. Seabed conditions such as sand waves can create spans on pipelines which fishing gear can become snagged under and put your vessel in severe danger.
Pipelines are used to transport oil and gas from wells to the shore and come in a variety of sizes from 4 inches (100mm) up to 48 inches (1200mm) in diameter. They are mainly constructed of steel and some may have extra protective coatings of concrete.
Pipelines can be trenched into the seabed using a plough and then backfilled with the seabed spoil from the trench. This method is typical for smaller diameter pipelines, where as larger diameter pipelines are simply laid on the seabed. Smaller diameter pipelines are vulnerable to damage from heavy trawl doors, beam trawls or clump weights and there is a risk of serious environmental impacts if a pipeline is damaged.
Short sections of pipelines connecting subsea wells and manifolds are often protected by placing tunnels over the top of them. These protective structures, as seen below, are constructed of steel or reinforced concrete and several pipelines can be protected within the one structure.
Where two pipelines have to cross over each other there is a need to provide some protection to the pipeline crossing. There are several ways which this can be achieved. Protection is mostly provided at crossings by laying small pieces of rock to form a bridge over the bottom pipeline which in turn is covered by rock. This method is undertaken using a rock placement vessel, as seen in the picture to the right.
These specialist vessels are capable of laying the rock in exactly the correct location, direct from the surface. Stone mattresses may also be used in conjunction with rock placement, as seen in the image above right.
When pipelines are installed, great care is taken to ensure they are as safe as possible to other seabed users. However, due to an uneven seabed, tidal currents or scouring, some pipelines may develop free spans.
A free span on a pipeline is where the seabed sediments have been eroded, or scoured away and the pipeline is no longer supported on the seabed.
The gap under the pipeline may be sufficient to catch fishing gear as in the image right.
The recovery of oil and gas from the subsea environment involves the use of many seabed structures. These seabed structures present snagging dangers to the fishing community. Many subsea structures are protected by safety zones usually of 500 meters though others do not have the benefit of safety zones and therefore, it is vital that their positions are known.
Here we will look at the most commonly found seabed structures:
Production subsea wellheads
Subsea Production Wellheads
When a well is drilled the structure placed on the seabed is called a wellhead. There may be a single wellhead, though often there may be several units grouped together to form a block. Attached to the top of the wellhead are the control units called subsea trees.The wellhead assembly is usually contained within a supporting structure called a template, with many also having a protection structure covering all the subsea equipment, as shown in the image left.
A single wellhead can be 3-4 metres square and up to 5 metres high. When several are grouped together within a template they become very large structures on the seabed.
The protection structures covering wellheads and their associated equipment are extremely large structures, which although offering some protection to / from fishing activities, they should still be treated with extreme caution.
Below you can see a protection frame seen on the back deck of a support vessel prior to being installed on the seabed. From this picture you can see the scale of these structures by the size of the crewman.
After initial drilling, a well may be temporarily suspended if an operator intends to carry out further operations at a later date. In this event a guide base is left on the seabed to facilitate re-entry if required image right. The structure left in place resembles an inverted table with legs approximately 4 metres high. A central pipe about 20 inches in diameter normally projects about 4 - 5 metres from the seabed.
Suspended wellheads do not have any safety zones attached to their location and are not usually marked however some may have buoys attached. These structures are therfore extremely vulnerable to fishing operations and should be avoided at all times.
These structures are attached to the top of subsea wells to control the flow of oil / gas to or from a well. When attached to a subsea well the combined structures can extend to 7 metres above the seabed in height.
They are made up of pipework and valves within a supporting steel framework and present a substantial hazard to towed fishing gear. When several subsea trees and associated wells are used as a group they are placed on a structure called a template.
A subsea tree and manifold are enclosed within this protective structure. This is typical of many subsea structures on the UK Continental Shelf. The size of this structure, approximately 9 metres tall. The associated pipeline connections and other seabed infrastructure connected to such structures emphasise the need for protective safety zones around all seabed structures.
A subsea template is a large steel structure which is used as a base for various subsea structures such as wells and subsea trees and manifolds. The image below shows a number of wellheads on a subsea template:
The wellheads can be seen with their guidebases (4 x white rods) pertruding from each corner, which are used to locate other equipment, i.e. trees.The size of a template is dependant on the number of structures attached to it. Many will have protective structures covering them, as does the template pictured right. This helps prevent damage from fishing activities and also improves fishing safety by reducing the likelihood of nets becoming snagged on the equipment.
Photo right shows a subsea template supporting wellheads and a protective structure.
A subsea manifold is a large metal peice of equipment, made up of pipes and valves and designed to transfer oil / gas from wellheads into a pipeline. Manifolds are usually mounted on a template and often have a protective structure covering them - as with the image below right.
Manifolds vary greatly in size and shape, though can be huge structures reaching heights of 30 metres.
Most subsea templates / manifolds will be protected by a 500 metre safety zone centered on one position. However, other equipment may also be clustered within the same area, justifying the need to have a large safety zone.
The image right shows how associated seabed structures may extend more than 100 metres from the central location of the safety zone. This demonstrates just how important it is that a safety zone is recognised by all seabed users.
Click on the photo right for an animation of how this technology on the seabed works.
Diagram left shows the Chevon's Frade field in the Northern Campos Basin about 121 kms (75 miles) off Rio de Janeiro, Brasil.
The field is situated in waters with a depth of approximately 1,128 meters (3,700 feet) . With both heavy oil API of 19 to 22 degrees and gas.
Frade has recoverable reserves are estimated at 200 to 300 MMbo.
Bear in mind that the sea bed is not flat like it is shown and everything has been placed there remotely.
Here is a diagram showing the under water structures of various types of rigs and platforms. The structure will depend on the water depth.
Dependent on the geographic location and the continenal shelf will effect the water depth and distance from the coast where these different types of platforms will be used.
Below is a description of all of these and more rigs or platforms in the diagram as well as some other types whichare common in some parts of the world.
Jack-up platforms (or jack-ups), as the name suggests, are platforms that can be jacked up above the sea using legs that are lowered, much like jacks. They usually have 3 or 5 legs. These platforms are typically used in water depths up to 400 feet (120 m), although some designs can go to 550 ft (170 m) depth. They are designed to move from place to place, and then anchor themselves by deploying the legs to the ocean bottom using a rack and pinion gear system on each leg.
I used to work as a diver on these rigs in the southern North Sea our main job was to check the amount of sand around each of the legs. Because they were in shallow water the currents were faster and washed the sand awa. This could make the platform lean over, and make it impossible to jack the legs down. If the case gets serious the leg may buckle and cause the rig to collaps.
Click on photo to see an animation of a jackup rig.
These platforms are built on concrete and/or steel legs anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are designed for very long term use . Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed.
Concrete caisson structures, pioneered by the Condeep concept, often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore (Norwegian fjords and Scottish firths are popular because they are sheltered and deep enough) and then floated to their final position where they are sunk to the seabed. Fixed platforms are economically feasible for installation in water depths up to about 1,700 ft (520 m).
The first oil platforms in the North Sea were of this type in many old fields they were drilled main platform, theywere called satelite wells as they were drilled around the main and the oil or gas is pumped ashore. With the advent of directional drilling satelite platforms were no longer needed as all the wells were drilled from one place in various directions around the platform.
Click on photo to see an Dunbar in the North Sea during a storm.
These platforms have hulls (columns and pontoons) to allow the structure to float, but of weight sufficient to keep the structure upright. Semi-submersible platforms can be moved from place to place either under their own power or under tow. They can be ballasted up or down by altering the amount of flooding in buoyancy tanks; they are generally anchored by combinations of chain, wire rope and/or polyester rope during drilling or production operations, though they can also be kept in place by the use of dynamic positioning. Semi-submersibles can be used in water depths from 200 to 10,000 feet (60 to 3,000 m).
Click on photo to see a film of the Scarabeo 9 semisubmersible rig.
A drillship is a vessel that has been fitted with appuratus for drilling. They are often used for exploratory drilling of new oil or gas wells in deep water but can also be used for scientific drilling (mud sampling or techtonic plate surveys).
Early versions were built on a modified tanker hull, but purpose-built designs are used today. Most drillships are fitted with a dynamic positioning system to maintain position over the well. They can drill in water depths up to 12,000 ft (3,700 m).
Click on photo to see an excellent animation of a Maersk drillship.
TLPs are floating platforms tethered to the seabed in a manner that eliminates most vertical movement of the structure.
They are used in water depths up to about 6,000 feet (2,000 m). The "conventional" TLP is a 4-column design as on the left, which looks similar to a semisubmersible. They are relatively low cost, used in water depths between 600 and 4,300 feet (180 and 1,300 m). Mini TLPs can also be used as utility, satellite or early production platforms for larger deepwater discoveries.
Click on photo to see various TLP platforms.
Spars are moored to the seabed like TLPs, but whereas a TLP has vertical tension tethers, a spar has more conventional mooring lines. Spars have to-date been designed in three configurations: the "conventional" one-piece cylindrical hull, the "truss spar" where the midsection is composed of truss elements connecting the upper buoyant hull (called a hard tank) with the bottom soft tank containing permanent ballast, and the "cell spar" which is built from multiple vertical cylinders. The spar has more inherent stability than a TLP since it has a large counterweight at the bottom and does not depend on the mooring to hold it upright. It also has the ability, by adjusting the mooring line tensions (using chain-jacks attached to the mooring lines), to move horizontally and to position itself over wells at some distance from the main platform location. The first production spar was Kerr-McGee's Neptune, anchored in 1,930 ft (590 m) in the Gulf of Mexico; however, spars (such as Brent Spar) were previously used as FSOs.
ENI's Devil's Tower photo left located in 5,610 ft (1,710 m) of water, in the Gulf of Mexico, was the world's deepest spar until 2010. The world's deepest platform is currently the Perdido spar in the Gulf of Mexico, floating in 2,438 meters of water. It is operated by Royal Dutch Shell and was built at a cost of $3 billion.
Click on photo left to see the Shell Perdido Spar platform.
Gravity Based Structures
A GBS is a support structure held in place by gravity. A common application for a GBS is an offshore oil platform. These structures are often constructed in fjords since their protected area and sufficient depth are very desirable for construction. A GBS intended for use as an offshore oil platform is constructed of steel reinforced concrete, often with tanks or cells which can be used to control the buoyancy of the finished GBS photo left.
When completed, a GBS is towed to its intended location and sunk. The platform structure which a GBS supports is called the topsides photo right.
Gravity-based structures are also widely used for offshore wind power plants. By the end of 2010, 14 of the world's offshore wind farms were supported by Gravity-based structures. The GBS are suited for water depths up to 20 meters. The deepest registered offshore wind farm with Gravity-based structures is Thornton Bank 1, Belgium, with a depth up to 27.5 meters. However, as offshore wind power plants are growing in size and moving towards deeper waters, the GBS is not considered competitive in comparison with other support structures.
Click on photo left to see the Shell Troll "A" GBS platform.Floating Production Storage Offloading vessel
The main types of floating production systems are FPSO (floating production, storage, and offloading system). FPSOs consist of large monohull structures, generally ( not always) shipshaped, equipped with processing facilities. Many are built using tanker hulls.These platforms are moored to a location for extended periods, and do not actually drill for oil or gas. Some variants of these applications, called FSO (floating storage and offloading system) or FSU (floating storage unit), are used exclusively for storage purposes, and host very little process equipment. In most cases shuttle tankers are used to remove the oil and take it to shore. This may be done through a floating hose or a buoy.
Click on photo left to see an animation of the fundermentals of an FPSO.
Floating Drilling Production Storage and Offloading
Murphy West Africa Ltd, Societe Nationale Des Petroles du Congo, and PA Resources AB will put the world’s first FDPSO into operation on the Republic of Congo’s Azurite field
The field development program for Azurite consists of a spread-moored floating, drilling, production, storage, and offloading (FDPSO) vessel tied to a subsea manifold. The manifold has 10 slots of which six for oil and gas production and four for water injection are connected to the FDPSO by three flexible high-pressure risers. Two of the risers are production risers, and one is for water injection. Ten enhanced vertical deepwater trees, are tied in to the subsea manifold by flexible well jumpers. A multiphase pump will provide artificial lift for the field. Petroserv Dynamic Producer working in the Guará field off the state of Rio de Janeiro, Brasil where it is known as a FPWSO I guess W is for Well.
(Image courtesy of William Jacob Management Inc.)
In many cases it is easier and cheaper to have a loading off-loading buoy rather than build a terminal. Below are descriptions of some of the different ones that are used. Found at http://www.trends.risoe.dk
There are various types of export facilities, ranging from single point off take systems linked by pipeline to the offshore production installation. to complex floating production process and storage systems. Some systems are subsurface and others surface based systems.
The first type of export facility was the catenery anchor leg mooring (CALM) where the hydrocarbons are transferred by long, floating hose to the amidships manifold of the off take tanker. This system needs reasonably good weather and favourable environmental conditions and is not suited to harsh environments or exposed locations, particularly the deep water extremities of the continental shelf.
The concept of offshore exporting via shuttle tanker has now become accepted as a permanent way of life of field solutions for production areas worldwide. Increasingly the concept is being used for waters in the even harsher environment of the Atlantic Frontier.
To confuse the issue just a bit photo right is the drilling rig Sevan Brasil
Surface Single Point Systems
There are various types of surface single point mooring loading systems, including an articulated loading platform (ALP) and single point mooring (SBM). A common feature of single point systems is that their upper sections are above the surface and that they have a single terminal offloading point around which the off take tanker can normally weathervane. The loading hose and where relevant, the mooring hawser are connected to the bow section of the off take tanker.
Articulated Loading Arm (ALP)
An articulated loading platform, normally consists of a column that is attached to a gravity base structure on the seabed by a universal joint assembly that allows it to articulate around the x-y axis. A rotating head weighing in the region of 350 tonnes, sits on the column at a sufficient height above the sea surface to avoid contact with the 100 year wave. Risers are routed partly inside the column and through a swivel joint to the end of a loading boom that is part of the loading head. From there, normally, a 20” flexible hose in the region of 80 or 90 metres long connects with the loading manifold.
Single Point Mooring (SPM)
Typically a single point mooring, export system consists of a surface buoy attached by chains to piled anchors on the seabed, as few as six, possibly up to 12 in number. The hydrocarbon flow line from the production installation is laid on the seabed and is connected to a seabed manifold, from where a flexible riser carries the hydrocarbons to the surface buoy and from there to the off take tanker. The surface buoy is invariably fitted with a turntable through which the hawser and flexible export pipeline are routed to the off take tanker. There are a number of variations of the BBM type system, such as the CALM (Catenery anchor leg mooring system) and the SALM (single anchor leg mooring).
There are various types of submerged systems, including OLS (offshore loading system), STL (submerged turret loading), TCMS (tripod catenery mooring system), SAP (single anchor production) and SAL (single anchor loading). The OLS was the first type of submerged system and replaced some of the earlier
Offshore Loading Systems (OLS)
The offshore loading system, consists of a seabed template that is connected to the production installation by a hydrocarbon pipeline. A mod water riser buoy is secured to the template by vertical chain or wire and is also connected by flexible hydrocarbon carrying hose. There is sufficient depth clearance above the level of the riser buoy to allow deep draft vessels to overrun it. There is generally a swivel arrangement on the buoy turntable that allows unobstructed freedom for the attached off take tanker to weathervane. The loading hose is connected to the off take tanker at its bow section in a manner similar to the arrangements for a surface single point loading systems. There is no hawser in the OLS and tanker positioning is by DP.
Submerged Turret Loading (STL)
The STL system, is a refinement of the OLS and is more robust. The STL system is different in a number of ways particularly that the subsurface buoy is secured to the seabed by a number of anchor chains. In the STL system there is no flexible hydrocarbon riser from the buoy to the tanker, the subsurface buoy being the highest point in the system. The subsurface buoy is designed to fit into a specially configured STL compartment in the hull of the tanker, normally located in way of one of the forward centre cargo tanks. The STL compartment houses the HP swivel around which the tanker can rotate. The subsea mooring system associated with the STL system is not solely for use with off take tanker operations, but can also be used as means of positioning and transferring hydrocarbons to FPSOs and FPSUs. Tankers used with this system need not be DP, the system being capable of operation with conventional propelled and controlled tankers. However, they must be fitted with STL compartment and HP swivel.
Tripod Catenary Mooring System (TCMS)
The TCMS, is ideally suited to extended well test off takes where the production/test installation, normally a mobile drilling rig, is located approximately 1 to 2km distant. Although first developed for extended well tests (EWT) of up to 90 days duration, the system is now considered technically capable of carrying out the early production phase in marginal fields and is also considered to have the potential to be a life of field solution.
The mooring system comprises a three legged anchoring system, where the legs of the chain or wire join art a single node point. A chafe chain assembly then rises to the bow of the tanker, where it is connected to a standard bow stopper arrangement. The mooring arrangement can be deployed by an anchor handling vessel.
Once connected to the mooring system and the loading hose, the moored tanker can be used as either a shuttle tanker or as a storage facility. In storage mode the tanker would remain moored on location and export crude oil via a shuttle tanker operating in tandem. In order to export in this way, it is necessary for the tanker to undergo modifications, in particular having an appropriate discharge system fitted, e.g. stern discharge system (SDS).
Single Anchor Production (SAP) & Single anchor loading (SAL)
Both SAP and SAL, are similar in concept to the TCMS, being suitable for extended well tests (EWTs), early production phases (EPPs) and also for supporting offshore loading.
Surface Production and Storage Systems
The two principle systems are floating storage unit (FSU) systems and floating production storage and offloading (FPSO) systems. Typically both involve the use of ship shaped vessels secured to the seabed by a number of different mooring systems, such as STL. In both cases the FSU and FPSO are able to weather vane, at some locations without restriction, but at others with only a limited degree of freedom. The normal means of export is by stern loading to an off take tanker. The generic term for this is tandem loading. The off take tanker can be either DP controlled or a conventional tanker.
Floating Storage Units (FSU)
FSUs are either converted tankers or custom built vessels. There are a number of different ways of securing the unit to the seabed, such as by an STL system where the securing point is normally in a specially constructed compartment in the hull of the unit or by a yoke type surface connection located directly on the bow of the unit. Hydrocarbons are pumped by subsea pipeline to the FSU from the production installation some distance away, normally in the region of 1.5 to 2km. The FSU loads directly into its cargo tanks and when nearing completion an off take tanker arrives to carry out a tandem offload using the stern discharge system (SDS). There are no process facilities on board the FSU, only storage. Storage capacities vary and can be as much as 600, 000 barrels of oil.
Floating Production Storage and Offloading (FPSO)
Many of the features associated with the FSU are shared by the FPSO, the principle differences being that there are process facilities on board the FPSO unit, consisting of crude oil separators, gas compression plant, flaring, venting systems and chemical injection modules. Normally, FPSOs are ship shaped and may be custom built installation or a converted tanker. Method of securing the FPSO to the seabed are invariably by a submerged mooring system, by STL or by a variation of the concept. In addition, export of the hydrocarbons is normally by SDS to an off take tanker in a tandem arrangement.
The off take tanker has considerable freedom to manoeuvre on its approach to all of the sub-sea systems referred to above. Unless there are other surface installations in the vicinity there are generally no surface obstructions and thereby no restrictions on the tanker’s direction of approach. The presence of obstructions in the vicinity of the location normally result in the establishment of exclusion zones, into which tankers are not allowed. This is particularly relevant to tanker approaches. In all cases i.e. OLS, STL or TCMS there is generally a surface messenger line that is attached to the loading hose or subsurface buoy and it is therefore necessary for the tanker to know the location of the messenger before starting its approach. Typically, a tanker support vessel is used in the approach stages to locate and then pass up the end of the messenger to the tanker.
Generally, off take tankers can moor up and maintain production at subsurface systems in more adverse environmental conditions that is considered acceptable for surface based off take systems.