Different Types of Offshore Platforms used worldwide

 On reading this article, one should be able to:

ü  Explain terminologies for different types offshore structures

ü Identify and differentiate between the various types of offshore structure based on their operation and characteristics.

✔ You can also learn about Offshore Structures in Bangladesh

Offshore Structures

1. Introduction

It is important to note that the development of offshore platforms depends on various factors, namely

      (i)            structural geometry with a stable configuration;

    (ii)            ease of fabrication, installation, and decommission;

   (iii)            low capital expenditure (CAPEX);

   (iv)            early start of production; and

    (v)            high return on investment through increased and uninterrupted production.

Offshore plants are unique in many ways. The most important fact is that the newly developed platforms do not have the similar existing ones to compare and understand the design/construction complexities. It is therefore imperative to examine the basics of planning and then select the most suitable structural configuration of the platform.

2. Offshore Platforms: Structural action and form improvements

Energy is the driving force for the progress of civilization. Industrial advancement was first powered by coal and then by oil and gas. Oil and gas are essential commodities in world trade. Oil exploration, which initially started onshore, has now moved to deep waters because of the paucity of resources in shallow waters.

To date, there are more than 20,000 offshore platforms of various kinds installed around the world. Geologists and geophysicists search for potential oil reserves beneath the ocean floor, and engineers take the responsibility of transporting the oil from the offshore site to the shore location.

There are five main areas of operation from exploration to transportation of oil, namely

(i)  exploration,

(ii) exploration drilling,

(iii) development drilling,

(iv) production operations, and

(v) transportation

Ever since the first offshore structure was constructed, more advanced design technologies have emerged to build larger platforms specifically for deeper waters. Each design is unique to the particular site.

A precise classification of offshore platforms is difficult because of the large variety of parameters involved, such as

• functional aspects,
• geometric form,
• construction and
• installation methods, and so on.

However, platforms are broadly classified based on geometric configurations. Offshore installations are constructed for various purposes, namely

      (i)            exploratory and production drilling,

    (ii)            water or gas injection into the reservoir,

   (iii)            processing oil and gas,

   (iv)            cleaning the by-products of the produced oil for disposal into sea, and

    (v)            accommodation facilities.

They are not classified on the basis of their functional use but on the basis of their geometric (structural) form. Because the platforms are designed for greater water depths, their structural form changes significantly; the same form cannot be used at different water depths. This means that the geometric evolution of the platform has to be adapted to accommodate the environmental loads at the chosen water depth.

The present trend is to design and install offshore platforms in regions that are inaccessible (difficult to reach) and where the use of existing technologies is difficult. The structural form of every platform is largely derived on the basis of structural innovation rather than on the basis of functional advantage. Revisiting existing platforms constructed around the world will impart a decent level of knowledge to offshore engineers.

Offshore platforms are classified either as

• bottom-supported or
• floating.

Bottom-supported platforms can be further classified as

• fixed or
• compliant structures;

Compliant means moving (mobility).

Floating structures are classified as

• neutrally buoyant (e.g., semisubmersibles, FPSO, and monocolumn spars) and
• positively buoyant (e.g., tension leg platforms).

It is important to note that buoyancy plays a very important role in floating offshore structures, because classifications are made on this basis. Table 1.1 shows the list of jacket platforms constructed worldwide.

Fixed platforms are known as template structures, and they consist of the following:

ü  A jacket or a welded space frame designed to facilitate pile driving and also acting as a lateral bracing for the piles

ü  Piles permanently anchored to the seabed to resist the lateral and vertical loads that are transferred from the platform

ü  A superstructure consisting of the deck to support operational activities

Offshore platforms fall into three major categories, namely

1. fixed platforms,
2. compliant platforms, and
3. floating platforms.

1. Different types of fixed platforms are

      (i)            jacket platforms and

    (ii)            gravity platforms.

2. Different types of compliant platforms are

      (i)            guyed towers,

    (ii)            articulated towers, and

   (iii)            tension leg platforms.

3. Different types of floating platforms are:

      (i)            semisubmersibles,

    (ii)            floating production units (FPUs),

   (iii)            floating storage and offloading (FSO) systems,

   (iv)            floating production storage and offloading (FPSO) systems, and

    (v)            spars.

Jacket Platforms

Fixed offshore structures are stiffer and tend to attract more forces. Jacket platforms are steel structures and are especially suited for soft-soil regions, for example, clayey soil. They are usually insensitive to the lateral loads by virtue of their rigidity and bottom fixity.

Their salient advantages are

1. they support large deck loads;
2. They have the possibility of being constructed in sections and transported;
3. they are suitable for large-field and long-term production (can support a large number of wells); and
4. the piles used for their foundations result in good stability.

We can think of these factors as advantageous for a production platform in a remote location in the sea, but they come with a few undesirable characteristics, namely

1. they fail instantaneously without sufficient warning;
2. they are not suitable for cyclic loading; and
3. they make the installation process time-consuming and expensive, which delays the start of production.

A few other disadvantages are:

1. their cost increases exponentially with increase in water depth;
2. they have high initial and maintenance costs;
3. they are not reusable; and
4. the steel structural members are subject to corrosion causing material degradation during service life. Because of their massive geometry, they are also expensive.
5. Economic considerations limit the development of fixed (rigid) platforms to water depths of about 500 m. Jacket structures are also known as template structures because the legs act as a template for driving the piles.
6. A few of the important terms associated with this platform are topside or surface facilities, jacket, pipeline, and support services. Topside or surface facilities are the part of the platform that contains the drilling module, the production module, and the crew quarters.
7. Typically the dimensions of the topsides could be 66 m × 66 m per deck level, with four decks with an overall height of 33 m.

Gravity Platforms

In addition to steel jackets, concrete has also been widely used to build some offshore structures. These structures are called gravity platforms or gravity-based structures (GBS). A gravity platform relies on the weight of the structure to resist the encountered loads instead of pilings.

In regions where driving piles become difficult, structural forms are designed to rely on their own weight to resist the environmental loads. These structures have foundation elements that contribute significantly to the required weight and spread over a large area of the seafloor to prevent failure due to overturning moments caused by lateral loads.  Gravity platforms are capable of supporting large topside loads during tow out, which minimizes the hook-up work during installation.

Additional large storage spaces for hydrocarbons add to the advantages, which include

      (i)            construction onshore for transport;

    (ii)            towing to the site of installation;

   (iii)            quick installation by flooding; and

   (iv)            use of traditional methods and labor for installation.

These platforms are also known to be responsible for seabed scouring due to large foundations and for causing severe environmental impact.

Gravity platforms have serious limitations, namely

(i) unsuitability for sites with poor soil conditions that would lead to significant settlement of the foundation;
(ii) long construction periods delaying the start of production; and
(iii) natural frequencies falling within the range of significant power of the input wave spectrum.

Gravity structures are constructed with reinforced concrete and consist of a large cellular base surrounding several unbraced columns that extend upward from the base to support the deck and equipment above the water surface.

Gravity platforms consist of production risers as well as oil supply and discharge lines contained in one of the columns; the corresponding piping system for exchange of water is installed in another column, and drilling takes place through the third column.

This particular type is referred to as a CONDEEP (concrete deep water) structure and was first designed and constructed in Norway.

The base of the platform is constructed in dry dock after which it is floated and moored in a deep harbor. The construction is then completed by slip forming the large towers in a continuous operation until they are topped off (finished). The structure is then ballasted and a steel prefabricated deck is floated over the structure and attached to its top. The construction of gravity platforms obviously requires deep harbors and deep tow-out channels.

The floatation chambers are used as storage tanks, and platform stability is ensured by skirts. Steel gravity platforms exist off Nigeria, where the presence of rock close to the seafloor rules out the possibility of using piles to fix structures to the seabed.

 

Figure 1.8 shows the steel gravity platform located in the Maureen Field, United Kingdom, 2001.

The platform is a steel gravity base structure with a weight of 112,000 tons and a height of 241 m, and has steel skirts for penetration into the seabed.

Compliant Platforms

To overcome the negative factors of gravity platforms, we can design a structural form, which will attract fewer forces and remain flexible to withstand cyclic forces. The structural action and the form are modified based on the “mistakes” learned from fixed-type platforms.

This is a special kind of reverse engineering, which makes offshore platforms unique. It leads to a continuous improvement from one platform to the next. Hence FEED (front end engineering design) is on a constant update as new structural forms are being tried for oil and gas exploration in deep and ultra-deep waters.

Fixed-type offshore structures became increasingly expensive and difficult to install in greater water depths. Hence a modified design concept evolved for  structures in water depths beyond 500 m. A compliant tower is similar to that of a traditional platform and extends from the surface to the sea bottom but is transparent to waves.

A compliant tower is designed to flex (bend or curved) with the forces of waves, wind, and current. The classification of compliant structures includes those structures that extend to the ocean bottom and are anchored directly to the seafloor by piles or guidelines.

Guyed towers, articulated towers, and tension leg platforms fall into this category. The structural action of compliant platforms is significantly different from that of fixed ones because they resist lateral loads not by their weight but by their relative movement.

In fact, instead of resisting the lateral loads, the structural geometry enables the platform to move in line with the wave forces.

To facilitate the production operation, they are position-restrained by cables/tethers or guy wires. By attaching the wires to the compliant tower, the majority of the lateral loads will be counteracted (cancelled) by the

ü  horizontal component of tension in the cables;

ü  the vertical component adds to the weight and improves stability.

Guyed Towers

A schematic view of a guyed tower is shown in Figure 1.9. A guyed tower is a  slender structure made up of truss members which rests on the ocean floor and is held in place by a symmetric array of catenary guy lines.

The foundation of the tower uses a spud-can arrangement, which is similar to an inverted cone placed under suction. The structural action of the guyed tower makes its innovation more interesting, which is one of the successful form improvements in offshore structural design. The upper part of the guy wire is a lead cable, which acts as a stiff spring in moderate seas. The lower portion is a heavy chain, which is attached with clump weights.

Under normal operating conditions, the weights will remain at the bottom, and the tower-deck motion will be nearly insignificant.

However, during a severe storm, the weights on the storm ward side will lift off the bottom, softening the guying system and permitting the tower and guying system to absorb the large wave loads.

Because the guy lines are attached to the tower below the mean water level, close to the center of applied environmental forces, large overturning moments will not be transmitted through the structure to the base. This feature has allowed the towers to be designed with a constant square cross section along their length, reducing the structural steel weight compared with that of conventional platforms.

The advantages of guyed towers are:

      (i)            lower cost (than steel jacket);

    (ii)            good stability because guy lines and clump weights increase the restoring force; and

   (iii)            possible reuse.

The few disadvantages are

      (i)            high maintenance costs;

    (ii)            applicability to small fields only;

   (iii)            exponential cost increases with increased water depth; and

   (iv)            difficulties in mooring. These factors promoted further innovation in platform geometry resulting in articulated towers.

Articulated Towers

One of the earliest compliant structures, the articulated tower started in relatively shallow waters and slowly moved into deep water.

A schematic view of an  articulated tower is shown in Figure 1.10. An articulated tower is an upright tower that is hinged at its base with a universal hinge, which enables free rotation about the base. When there was a need to improve the structural form from fixed to compliant, researchers thought of both modes of compliancy, namely (i) rotational and (ii) translational.

The capability of large translational motion could make the platform free from being position restrained, and hence rotational compliancy was attempted.

In such geometric forms, it is important to note that the design deliberately introduces a single point failure, which is the universal joint. The tower is ballasted near the universal joint and has a large buoyancy tank at the free surface to provide a large restoring force (moment).

The tower extends above the free surface and accommodates a deck and a fluid swivel. In deeper water, it is often advantageous to introduce double articulation, the second being at mid-depth. Provision of more articulation reduces the bending moment along the tower.

Fatigue is an important criterion for this type of system design because the universal joints are likely to fail under fatigue loads. The advantages of articulated towers are:

      (i)            low cost;

    (ii)            large restoring moments due to high center of buoyancy; and

   (iii)            protection of the risers by towers.

The few disadvantages are:

(i) suitability for shallow

water only because the tower shows greater oscillations for increased water depth;

(ii) inoperability in bad weather;

(iii) limitation to small fields; and

(iv)  universal joint fatigue leading to a single point failure.

In both the above-mentioned structural forms of compliant towers, it can be seen that the structure (tower) extends through the water depth, making it expensive for deep water.

Therefore, successive structural forms are intuited (work out or understand) toward the basic concept of not extending the tower to the full water depth, but only to retain it near the free surface level as far as possible. In such kinds of structural geometry, it is inevitable that the platform weight is dominant.

To improve the installing features and decommissioning procedures, the geometry attempts to be buoyancy dominant instead of weight dominant (buoyancy force exceeds the weight by manifold).

While this enables easy fabrication and installation, it also demands skilled labor and high expertise for installation and commissioning of such platforms. The evolved structural Geometry is tension leg platforms.

Tension Leg Platforms

A tension leg platform (TLP) is a vertically moored compliant platform.

A  schematic diagram of a TLP is given in Figure 1.11. Taut mooring lines, called tendons or tethers, vertically moor the floating platform with its excess buoyancy. The structure is vertically restrained while it is compliant in the horizontal direction, permitting surge, sway, and yaw motions. The structural action results in low vertical force in rough seas, which is the key design factor. Substantial pretension or claim is required to prevent the tendons falling slack even in the deepest trough, which is achieved by increasing the free floating draft.

Typical natural periods of the TLP are kept away from the range of wave excitation periods and typically are achieved through proper design for TLP resonance periods of 132 s (surge/sway) and 92 s (yaw) as well as 3.1 s (heave) and 3.5 s (pitch/roll).

The main challenge for the TLP designer is to keep the natural periods in heave and pitch below the range of significant wave energy, which is achieved by an improved structural form. Tension leg platform technology preserves many of the operational advantages of a fixed platform while reducing the cost of production in water depths of up to about 1500 m. Its production and maintenance operations are similar to those of fixed platforms.

TLPs are weight sensitive but have limitations in accommodating heavy payloads. Usually a TLP is fabricated and towed to an offshore well site where the tendons are already installed on a prepared seabed.  Then the TLP is ballasted down so that the tendons may be attached to the TLP at its four corners. The mode of transportation of the TLP allows the deck to be fixed TLP at dockside before the hull is taken offshore.

Under lateral loads like earthquakes, TLPs have shown a favorable response, even though the compliancy in the design is offered in displacement degrees of freedom.

The advantages of TLPs are

      (i)            mobility and reusability;

    (ii)            stability, because the platform has minimal vertical motion;

   (iii)            low-cost increase with increase in water depth;

   (iv)            deep water capability; and

    (v)            low maintenance cost.

The few disadvantages are

      (i)            high initial cost;

    (ii)            high subsea cost;

   (iii)            fatigue of tension legs;

   (iv)            difficult maintenance of subsea systems; and

    (v)            little or no storage.

Spar Platforms

A spar platform consists of a deep-draft floating caisson (a large watertight chamber), which is a hollow,  cylindrical structure similar to a very large buoy. The major components of a spar platform are the hull, moorings, topsides, and risers.

Figure 1.17 shows the details of a spar platform. The distinguishing feature of a spar is its deep-draft hull, which produces very favorable motion characteristics.

Some salient features of spars are:

1. water-depth capability ranging up to 3000 m;
2. full drilling and production capabilities;
3. direct, vertical access production risers (surface production trees);
4. surface blowout preventer for drilling and workover operations;
5. steel catenary risers (import and export);
6. inherent stability because the center of buoyancy is located above the center of gravity;
7. favorable motion compared with other floating systems;
8. traditional construction (steel or concrete hull);
9. cost insensitive to water depth;
10. potential oil storage;
11. relocatable over a wide range of water depths; and
12. conventional drilling and process components can be used.

The advantages of spar platforms are:

1. low heave and pitch motion compared with other platforms
2. use of dry trees (i.e., on surface);
3. ease of fabrication;
4. unconditional stability because the center of gravity is always lower than the center of buoyancy, resulting in a positive GM; and
5. derives no stability from the mooring system and hence does not list or capsize even when completely disconnected from its mooring system.

The few disadvantages are

1. difficulty of installation because the hull and the topsides can only be combined offshore after the spar hull is upended;
2. little storage capacity, which brings along the necessity of a pipeline or an additional FSO; and
3. lack of no drilling facilities.

Exploratory Platforms

Deepwater offshore platforms are expected to grow from 3% of the total in 2003 to 10% in 2020. Platforms for exploratory drilling have also been improved with advanced structural configurations.

A platform configuration with a semisubmersible is shown in Figure 1.18, and the details of the single buoy mooring (SBM) are shown in Figure 1.19.

The most commonly used exploration platforms are jack-up platforms,  semisubmersibles, and drill ships (Figure 1.20).

The jack-up platform

 is used down to a water depth of 90 m whereas semisubmersibles are used down to a depth of 180 m; drill ships operate at greater depths, as deep as 2000 m.

The jack-up platform consists of three legs with a deck supporting the helideck, drilling mast (Figure 1.21), and so on.

The operation of jack-up platform—drilling a well—is shown in Figure 1.22. The floating deck is used to tow the jack-up platform, with the three legs above the deck, as shown in Figure 1.23.

Jack-up platforms are most vulnerable when afloat and under tow during severe weather, whereas they are susceptible to hurricane/cyclone damage while elevated. The punch-through of jack up rigs is due to sudden failure of the soils while preloading during the operational phase.

The buoyancy for a semisubmersible is provided by pontoons, as shown in Figure 1.24, which is kept well below the water surface to reduce the wave action.

The semisubmersible is kept in position by mooring lines, as shown in Figure 1.25. Both dry and wet tows can be used for transportation.

The hull is constructed by normal marine and shipyard fabrication methods. In the classic or full cylinder hull form, the whole structure is divided into upper,  middle, and lower sections.

The upper section is compartmentalized around a flooded center well, which houses different types of risers, namely production riser, drilling riser, and export/import riser. This also provides the required buoyancy while the middle section is configured for oil storage. The bottom section, called the keel, is also compartmentalized to provide buoyancy during transport and to contain any field-installed, fixed ballast.

The platform position is restrained using a lateral catenary system of 6–20 lines or taut mooring system made possible due to the small motion.

Mooring lines are anchored to the seafloor with a driven or suction pile.

The footprint created by the mooring system can reach out to a large diameter (typically for a half-mile or more) from the hull to the anchor piles, which can be significantly reduced by taut mooring lines.

This is essentially required to protect the platform from the impact of vessels deployed for oil transportation.

The topside configuration follows typical fixed platform design practices. The larger top sides consist of drilling, production, processing, and quarters facilities and could also accommodate remote wells/fields being tied back to the spar for processing.

The decks can accommodate a full drilling rig (3000 hp) or a workover rig (600–1000 hp) plus full production equipment. Total operating deck load—which includes facilities, contained fluids, deck structural and support steel, drilling/workover rig, and work over variable loads—can be about 6600 tons or more.

Floating, production, storage, and offloading systems (FPSO)

are typically  converted or newly built tankers, which are designed for production and storage of hydrocarbons.

Offloading indicates transfer of produced hydrocarbons from an  offshore facility into shuttle tankers or barges for transport to terminals or deep water ports.

An FPSO relies on subsea technology for the production of hydrocarbons, and would typically involve pipeline export of produced gas with a shuttle tanker (offloading) transport of produced liquids. FPSOs are usually ship-shaped structures and are relatively insensitive to water depth.

To date, nearly all FPSOs have been installed in water depths less than 1000 m. One recent example was Petrobras’ installation of the Marlim Sul Field FPSO in approximately 4700 ft of water located in the Brazilian Campos Basin.

Figure 1.26 shows the Marlim Sul Field FPSO deployed by Petrobras.

The FPSOs currently in operation cover a wide range of environmental conditions, vessel sizes, production rates, operating water depths, and number of  risers.

The mooring systems of FPSOs

are classified as “permanent mooring” or “turret mooring.”

The majority of FPSOs deployed worldwide are permanently moored, that is, the FPSOs with their moorings and riser systems are capable of withstanding extreme storms in the field location.

On the other hand, disconnectable FPSOs have recently attracted more attention. A disconnectable FPSO is typically turret moored. A disconnectable turret is designed for FPSOs to be able to disconnect to avoid certain extreme environments.

The lower part of the turret (lower turret buoy) which connects the mooring lines and risers can be disconnected to allow the FPSO to sail away prior to the approach of a storm.

After disconnection, the lower turret with anchoring legs and riser terminations sinks to 50–100 m below the water surface and thus clears off the wave zone.

When the extreme weather passes, the FPSO can come back and pick up the submerged mooring/riser buoy and reconnect to it. However, this operation of disconnecting and reconnecting takes considerable time. The primary function of the turret is to allow the FPSO to weather-vane without disrupting the transfer of production fluid between the FPSO and subsea wells.

Turret types may be external or internal. FPSOs offer many attractive features, namely

      (i)            environmental hazard is minimized because the platform with crude storage can be disconnected and go to a sheltered area in the event of severe storm;

    (ii)            the lower turret with moorings and riser connections goes below the wave zone and is not significantly impacted by the storm;

   (iii)            the mooring system and its associated turret components can be of smaller size because it only needs to be designed against less severe storms when the FPSO stays  connected; and

   (iv)            greater safeguards against platform damage and liability to the operator in the event of a storm.

Advantages are

      (i)            low cost;

    (ii)            mobility and reus ability;

   (iii)            reduced lead time;

   (iv)            quick disconnecting capability which can be useful in iceberg-prone areas;

    (v)            little infrastructure required; and

   (vi)            turret  mooring system enables FPS (converted ship type) to head into the wind/waves reducing their effect.

The few disadvantages are

      (i)            limited to small fields;

    (ii)            low deck load capacity;

   (iii)            damage to risers due to motion;

   (iv)            poor stability in rough seas; and

    (v)            little oil storage capabilities.

Deepwater platforms

have processing equipment to  facilitate production equipment.

Storage of oil is the main concern in offshore operations. Once the whole drilling operation is complete, the drilling platform becomes a well-protector and a storage platform.

Mostly, a tanker ship is anchored next to the treatment platform and serves as oil storage. Personnel employed in the offshore platforms are generally transported by boats or helicopters; hence all platforms must be provided with helipads, mooring bits, bumpers, cranes, stairs, and so on, for use with workboats and crew boats.

Crew transported by helicopter are generally spared seasickness on arrival and remain prepared for work.

Apart from considerable time savings and reduction in cost, helicopter reliability and capability in bad weather is much better than that of boats.

Offshore platforms should be generally designed with a large helicopter landing area to handle loading and unloading operations easily and quickly.

Basic dimensions of the helipad on the platform deck depends on the overall length of the helicopter; usually a circular pad of diameter equal to the overall length of the helicopter is considered for the design.

The heliport landing area must be large enough to handle loading and unloading comfortably.

Provision is generally made on the deck to enable a slant take-off of helicopters. A typical offshore helicopter is some 24 m × 24 m to 49 m × 49 m in size.

The offshore deck should be designed for a helicopter load in one of two ways. Either it is designed for a concentrated load up to 75% of the gross weight of the largest helicopter or for an impact factor twice the weight of the largest helicopter.

Wind indicators are provided for the heliport to indicate the true wind direction, along with the landing boundaries marked in yellow.

Helipads are preferably located at one corner of the platform to facilitate easy landing and takeoff. It preferable to locate the power units, flare stack, and drilling units/equipment far away from the location of helipad to reduce possible risk during helicopter operations.