OFFSHORE PLATFORMS

 Offshore structuring has become a prominent field of marine exploration, where, man has started trying his manipulation by building mesmerizing structures along and inside the oceans. These structures are susceptible to corrosion, damage caused by dropped objects, and contacts from vessels.

The technology needed to design and build deep-ocean compliant structures, such as tension leg platforms & floaters, continues to evolve to meet technical and economic needs for deepwater development. This rapid evolution in technology needs to be independently verified to ensure continued safety of operations and protection of the environment. These structures are very helpful in manufacturing marsh gases, deep oil deposits and petroleum. This paper presentation introduces you to various offshore platforms and their analysis.

 

 The first oil offshore platform in the world is the Oil Rocks (Neft Daşları), built near Baku in Soviet Union. Building on the platform began in 1949, with Soviet Tankers transporting Oil from the first Well to Baku in 1951.The Oil Rocks lies 45–50 km (about 25 nautical miles) offshore on the Caspian Sea. The most unique feature of the Oil Rocks is that it is actually a functional city with a population of about 5000. The Oil Rocks is a city on the sea, with over 200 km of streets built on piles of dirt and landfill. Most of the inhabitants work on shifts; a week on Oil Rocks followed by a week on the shore.

Offshore platforms are used for exploration of Oil and Gas from under Seabed and processing. The First Offshore platform was installed in 1947 off the coast of Louisiana in 6M depth of water. Today there are over 7,000 offshore platforms around the world in water depths up to 1,850M.

Specifications of platforms:

         Platform size depends on facilities to be installed on top side eg. Oil rig, living quarters, Helipad etc.

Classification of water depths: 350 M- Shallow water, 1500 M - Deep water, 1500 M- Ultra deep water. US Mineral Management Service (MMS) classifies water depths greater than 1,300 ft as deepwater, as and greater than 5,000 ft as ultra-deepwater.

Types of platforms:

Platforms are broadly divided as fixed and floating type platforms.

FIXED PLATFORMS:

1. Fixed Platforms: 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, by virtue of their immobility, designed for very long term use (for instance the Hibernia platform). 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. Fixed platforms are economically feasible for installation in water depths up to about 1,700 feet (520 m).

2. Compliant Towers, consist of narrow, flexible towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 1,500 and 3,000 feet (450 and 900 m).

3. Semi-submersible Platforms having legs of sufficient buoyancy to cause the structure to float, but of weight sufficient to keep the structure upright. Semi-submersible rigs can be moved from place to place; and can be ballasted up or down by altering the amount of flooding in buoyancy tanks; they are generally anchored by cable anchors during drilling operations, though they can also be kept in place by the use of dynamic positioning. Semi-submersible can be used in depths from 600 to 6,000 feet (180 to 1,800 m).

4. Jack-up Platforms, as the name suggests, are platforms that can be jacked up above the sea, by dint of legs than can be lowered like jacks. These platforms, used in relatively low depths, are designed to move from place to place, and then anchor themselves by deploying the jack-like legs.

5. Drillships, a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory drilling of new oil or gas wells in deep water but can also be used for scientific drilling. It is often built on a modified tanker hull and outfitted with a dynamic positioning system to maintain its position over the well. 

FLOATING PLATFORMS:

Floating production systems are large ships equipped with processing facilities and moored to a location for a long period. The main types of floating production systems are FPSO (floating production, storage, and offloading system), FSO (floating storage and offloading system), and FSU (floating storage unit). These ships do not actually drill for oil or gas.

1. Tension-leg Platforms, consist of floating rigs tethered to the seabed in a manner that eliminates most vertical movement of the structure. TLPS are used in water depths up to about 6,000 feet (2,000 m). The "conventional" TLP is a 4-column design which looks similar to a semisubmersible. They are relatively low cost, used in water depths between 600 and 3,500 feet (200 and 1,100 m). Mini TLPs can also be used as utility, satellite or early production platforms for larger deepwater discoveries.

2. Spar Platforms, moored to the seabed like the TLP, but whereas the TLP has vertical tension tethers the Spar has more conventional mooring lines. Spars have 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 with the bottom soft tank containing permanent ballast, and the "cell spar" which is built from multiple vertical cylinders. The Spar may be more economical to build for small and medium sized rigs than the TLP, and 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 use of chain-jacks attached to the mooring lines, to move horizontally over the oil field.

3. Normally unmanned installations are small platforms, consisting of little more than a well bay, helipad and emergency shelter. They are designed for operate remotely under normal operations, only to be visited occasionally for routine maintenance or well work.

4.FPSO: A Floating Production, Storage and Offloading vessel is a type of floating tank system used by the offshore oil and gas industry and designed to take all of the oil or gas produced from a nearby platform (s), process it, and store it until the oil or gas can be offloaded onto waiting tankers, or sent through a pipeline. A FSO is a similar system, but without the possibility to do any processing of the oil or gas. Oil has been produced from offshore locations since the 1950s. Originally, all oil platforms sat on the seabed, but as exploration moved to deeper waters and more distant locations in the 1970s, floating production systems came to be used.

Oil produced from offshore production platforms can be transported to the mainland either by pipeline or by tanker. When a tanker solution is chosen, it is necessary to accumulate oil in some form of tank such that an oil tanker is not continuously occupied while sufficient oil is produced to fill the tanker. Often the solution is a decommissioned oil tanker which has been stripped down and equipped with facilities to be connected to a mooring buoy. Oil is accumulated in the FPSO until there is sufficient amount to fill a transport tanker, at which point the transport tanker connects to the stern of the floating storage unit and offloads the oil.

PARTS OF A PLATFORM: There are 3 main parts.

1. Topside        2. Moorings and anchors              3. Risers.

TOPSIDE:

 Topside is tailored to achieve weight and space saving. It is a flat opening which Incorporates process and utility equipment. namely Drilling Rig, Injection Compressors, Gas Compressors, Gas Turbine Generators, Piping, HVAC, Instrumentation, Accommodation for operating personnel, Crane for equipment handling, Helipad.

 MOORINGS & ANCHORS:

 These are used to tie platform in place. Anchors are usually made of steel in order to keep the structure intact. The other materials used are Steel chains, steel wire rope. If Catenary shape due to heavy weight is implemented, then the total Length of rope is more as the structural weight is high. Even synthetic fiber ropes are used. If Taut shape due to substantial less weight than steel ropes, less rope length is required and is Corrosion free

RISER:

 Risers are various supports provided as link pipes. Pipes used for production, drilling, and export of Oil and Gas from Seabed. Riser system is a key component for offshore drilling or floating production projects. The cost and technical challenges of the riser system increase significantly with water depth. Design of riser system depends on filed layout, vessel interfaces, fluid properties and environmental condition. Risers Remains in tension due to self weight. Profiles are designed to reduce load on topside.

Types of risers

    -Rigid

    -Flexible - Allows vessel motion due to wave loading and compensates       heave motion. Simple Catenary risers: Flexible pipe is freely suspended between surface vessel and the seabed.

INSTALATION OF PLATFORMS:

                 Since Platforms are huge structures, large cranes and computerized facilitators are required to function the machines.

BARGE LOADOUT:

Various methods are deployed based on availability of resources and size of structure. Barge Crane is the most efficient machine to unload and load the risers and topsides accurately. Flat over - Top side is installed on jackets. Ballasting of barge includes Smaller jackets can be installed by lifting them off barge using a floating vessel with cranes. Large 400’ x 100’ deck barges capable of carrying up to 12,000 tons are available as platforms or bases.

PLATFORM FOUNDATIONS:

The loads generated by environmental conditions plus by onboard equipment must be resisted by the piles at the seabed and below. The soil investigation is vital to the design of any offshore structure. Geotech report is developed by doing soil borings at the desired location, and performing in-situ and laboratory tests. Pile penetrations depend on platform size and loads, and soil characteristics, but normally range from 30 meters to about 100 meters.

Corrosion protection:

The usual form of corrosion protection of the underwater part of the jacket as well as the upper part of the piles in soil is by cathodic protection using sacrificial anodes. A sacrificial anode consists of a zinc/aluminum bar cast about a steel tube and welded on to the structures. Typically approximately 5% of the jacket weight is applied as anodes. The steelwork in the splash zone is usually protected by a sacrificial wall thickness of 12 mm to the members.

STRUCTURAL DESIGN:    

Loads Design:  

Offshore structure is designed for following types of loads:

–        Permanent (dead) loads.

–        Operating (live) loads.

–        Environmental loads

                                           Wind load

                                           Wave load

                                           Earthquake load

–        Construction - installation loads.

–        Accidental loads.

The design of offshore structures is dominated by environmental loads, especially wave load.

1. Permanent Loads:

Weight of the structure in air, including the weight of ballast. Weights of equipment, and associated structures permanently mounted on the platform. Hydrostatic forces on the members below the waterline. These forces include buoyancy and hydrostatic pressures.

 2. Operating (Live) Loads: Operating loads include the weight of all non-permanent equipment or material, as well as forces generated during operation of equipment. The weight of drilling, production facilities, living quarters, furniture, life support systems, heliport, consumable supplies, liquids, etc.Forces generated during operations, e.g. drilling, vessel mooring, helicopter landing, and crane operations. Following Live load values are recommended in BS6235: Crew quarters and passage ways: 3.2 KN/m2. Working areas: 8, 5 KN/m2

3. Wind Loads: Wind load act on portion of platform above the water level as well as on any equipment, housing, derrick, etc.For combination with wave loads, codes recommend the more unfavorable of the following two loadings: 1 minute sustained wind speeds combined with extreme waves. 3 second for gusts.

              When, the ratio of height to the least horizontal dimension of structure is greater than 5, then API-RP2A requires the dynamic effects of the wind to be taken into account and the flow induced cyclic wind loads due to vortex shedding must be investigated.

 4. Wave load:  The wave loading of an offshore structure is usually the most important of all environmental loadings. The forces on the structure are caused by the motion of the water due to the waves Determination of wave forces requires the solution of,

a) Sea state using an idealization of the wave surface profile and the wave kinematics by wave theory.

b) Computation of the wave forces on individual members and on the total structure, from the fluid motion.

Design wave concept is used, where a regular wave of given height and period is defined and the forces due to this wave are calculated using a high-order wave theory. Usually the maximum wave with a return period of 100 years is chosen. No dynamic behavior of the structure is considered. This static analysis is appropriate when the dominant wave periods are well above the period of the structure. This is the case of extreme storm waves acting on shallow water structures.

•         Wave theories

Wave theories describe the kinematics of waves of water. They serve to calculate the particle velocities and accelerations and the dynamic pressure as functions of the surface elevation of the waves. The waves are assumed to be long-crested, i.e. they can be described by a two-dimensional flow field, and are characterized by the parameters: wave height (H), period (T) and water depth (d).

Wave forces on structural members

Structures exposed to waves experience forces much higher than wind loadings. The forces result from the dynamic pressure and the water particle motions. Two different cases can be distinguished:

Large volume bodies, termed hydrodynamic compact structures, influence the wave field by diffraction and reflection. The forces on these bodies have to be determined by calculations based on diffraction theory.

Slender, hydro-dynamically transparent structures have no significant influence on the wave field. The forces can be calculated in a straight-forward manner with Morison's equation. The steel jackets of offshore structures can usually be regarded as hydro-dynamically transparent

As a rule, Morison's equation may be applied when D/L < 0.2, where D is the member diameter and L is the wave length.

- Morison's equation expresses the wave force as the sum of,

–         An inertia force proportional to the particle acceleration

–         A non-linear drag force proportional to the square of the particle velocity.

 Earthquake load:  Offshore structures are designed for two levels of earthquake intensity. Strength level. Earthquake, defined as having a "reasonable likelihood of not being exceeded during the platform's life" (mean recurrence interval ~ 200 - 500 years), the structure is designed to respond elastically. Earthquake, defined as close to the "maximum credible earthquake" at the site, the structure is designed for inelastic response and to have adequate reserve strength to avoid collapse.

5. Ice and Snow Loads: Ice is a primary problem for marine structures in the arctic and sub-arctic zones. Ice formation and expansion can generate large pressures that give rise to horizontal as well as vertical forces. In addition, large blocks of ice driven by current, winds and waves with speeds up to 0.5 to 1.0 m/s, may hit the structure and produce impact loads.

6. Temperature Load: Temperature gradients produce thermal stresses. To cater such stresses, extreme values of sea and air temperatures which are likely to occur during the life of the structure shall be estimated. In addition to the environmental sources, accidental release of cryogenic material can result in temperature increase, which must be taken into account as accidental loads. The temperature of the oil and gas produced must also be considered.

7. Marine Growth: Marine growth is accumulated on submerged members. Its main effect is to increase the wave forces on the members by increasing exposed areas and drag coefficient due to higher surface roughness. It is accounted for in design through appropriate increases in the diameters and masses of the submerged members.

8. Installation Load: These are temporary loads and arise during fabrication and installation of the platform or its components. During fabrication, erection lifts of various structural components generate lifting forces, while in the installation phase forces are generated during platform load out, transportation to the site, launching and upending, as well as during lifts related to installation.

All members and connections of a lifted component must be designed for the forces resulting from static equilibrium of the lifted weight and the sling tensions.

Load out forces are generated when the jacket is loaded from the fabrication yard onto the barge. Depends on friction co-efficient

9. Accidental Load: According to the DNV rules, accidental loads are loads, which may occur as a result of accident or exceptional circumstances.  Examples of accidental loads are collision with vessels, fire or explosion, dropped objects, and unintended flooding of buoyancy tanks. Special measures are normally taken to reduce the risk from accidental loads.

Load Combinations:

The load combinations depend upon the design method used, i.e. whether limit state or allowable stress design is employed. The load combinations recommended for use with allowable stress procedures are: Normal operations

            Dead loads plus operating environmental loads plus maximum live loads.   Dead loads plus operating environmental loads plus minimum live loads.

Extreme operation: Dead loads plus extreme environmental loads plus maximum live loads. Dead loads plus extreme environmental loads plus minimum live loads

Environmental loads should be combined in a manner consistent with their joint probability of occurrence. Earthquake loads are to be imposed as a separate environmental load, i.e., not to be combined with waves, wind, etc.

Analysis and modeling;

         The analytical models used in offshore engineering are similar to other types of on shore steel structures. The same model is used throughout the analysis except supports locations. Stick models are used extensively for tubular structures (jackets, bridges, flare booms) and lattice trusses (modules, decks).Each member is normally rigidly fixed at its ends to other elements in the model. In addition to its geometrical and material properties, each member is characterized by hydrodynamic coefficients, e.g. relating to drag, inertia, and marine growth, to allow wave forces to be automatically generated.

Structural analysis: Integrated decks and hulls of floating platforms involving large bulkheads are described by plate elements. Deck shall be able to resist crane’s maximum overturning moments coupled with corresponding maximum thrust loads for at least 8 positions of the crane boom around a full 360° path. The structural analysis will be a static linear analysis of the structure above the seabed combined with a static non-linear analysis of the soil with the piles. Transportation and installation of the structure may require additional analyses detailed fatigue analysis should be performed to assess cumulative fatigue damage the offshore platform designs normally use pipe or wide flange beams for all primary structural members.

Acceptance Criteria: The verification of an element consists of comparing its characteristic resistance(s) to a design force or stress. It includes:

-a strength check, where the characteristic resistance is related to the yield strength of the element,

-a stability check for elements in compression related to the buckling limit of the element.

-An element is checked at typical sections (at least both ends and mid span) against resistance and buckling.

-Tubular joints are checked against punching. These checks may indicate the need for local reinforcement of the chord using larger thickness or internal ring-stiffeners.

-Elements should also be verified against fatigue, corrosion, temperature or durability wherever relevant

Offshore Standards (OS):

          Provides technical requirements and acceptance criteria for general application by the offshore industry eg.DNV-OS-C101

Recommended Practices (RP): Provides proven technology and sound engineering practice as well as guidance for the higher level publications eg. API-RP-WSD

BS 6235: Code of practice for fixed offshore structures.

–        British Standards Institution 1982.

 

What it is

An increasing portion of oil and gas production is coming from offshore fields. Advanced technologies now permit production from deepwater fields and from marginal fields. In parallel, platform technologies are evolving from fixed platforms suited for shallow waters, to semi-submersible platforms (TLP, SPAR) and to Floating Production Units (FPU, FPSO). The latter reduce project lead-time, have higher performance flexibility, and may be moved from a depleted field to a new field, thus greatly reducing investments. They are the most common solution for deepwater and marginal fields.

Offshore platforms are equipped with the machinery needed to extract oil and natural gas but have some critical challenges that should not be underestimated. Thanks to its many years of worldwide experience, GE Oil & Gas can provide optimum technical solutions and the project management experience needed to maximize production while helping customers to meet or accelerate their "First Oil" date.

How it works

Offshore production platforms/FPSO collect the hydrocarbons produced under the seabed by means of specially designed flow-lines and risers. The platform also contains the necessary monitoring & control equipment, and gear for furnishing electric and/or hydraulic power to the subsea equipment installed at the various field wells.
Power generation, compression and pumping equipment are generally installed on the platform. This machinery is used to collect the hydrocarbons and convey them to onshore receiving facilities, or for the re-injection of associated gas back into the well to enhance production.