What Is A Jack-Up Platform Or Barge?
A Jack-Up Platform is a floating barge that has movable legs attached to the hull. These legs can be retracted and extended vertically, meaning that once it makes contact with the sea bed, the platform begins to move upwards and out of the water.
The hull on which these legs are built is water-tight, buoyant, and similar to a ship. It has ballast tanks, living spaces (if required), a bridge for operators to work from, and machinery for lowering and raising the legs. Jack Up refers to how the legs are jacked up or down.
The first thing to note is that Jack Up platforms are primarily used in shallow and intermediate water depths. They perform well in depths up to around 120 meters.
At deeper depths, they face issues in structural compliance (discussed later). Jack Ups can be used for drilling and installation operations, and our mobile units as compared to other gravity-based structures.
They can be either towed like other barges or have propulsion units at the aft that move the structure under its own power. The self-propelled variants are less common due to the large power that such units consume.
They are also stable since they have widely-spaced supporting legs. The number of these legs depends on the type and functionality of the barge but is usually 3, 4, 6, or 8.
The buoyancy of the hull means that the structure can lift objects out of the water, such as turbines, smaller vessels, or offshore drilling platforms.
Jack ups are one of the most common types of offshore platforms, with over 700 barges in service as of 2019. The first jack-up used in India was procured by ONGC and built-in Hiroshima, Japan.
The tallest jack-up rig to date is the 214-meter-tall Noble Lloyd (the height refers to the height of the platform legs).
The Origin And Development Of Jack Up Barges
The concept for jack-up platforms stemmed from the need for a vessel that could install offshore structures easily. The offshore oil and gas industry was still in the early stages of setting up in the mid-1900s (from around 1940 onwards).
Once explorations were conducted into the feasibility of setting up underwater wellheads, the problem was installing an O&G platform.
Colonel Leon B. DeLong of the United States Army was a part of the Army Corps of Engineers, who worked with temporary military platforms for troops to use.
Post World War 2, he began working on an idea to use jack-up platforms in Greenland for army operations. He refined this idea and designed the DeLong Platform.
Once he left the army in 1949, he started his own organization called the DeLong Engineering and Construction Company. There, the idea of using jack-up platforms for the O&G industry was brought up and DeLong’s team began working on it.
In 1950, he began working on a contract for Magnolia Petroleum (a famous and reputed petroleum company of the early 20th century) to design and build a 6-leg jack-up barge.
In 1952, DeLong began collaborating with McDermott International Inc (an American engineering and construction company currently operating out of Panama). They designed the DeLong-McDermott No. 1 for another major O&G company- Humble Oil (which is known today as Exxon).
The No. 1 was a MODU i.e., it was mobile and could be towed. The design also featured an innovative feature- spud cans, designed to stabilize and improve sea bed gripping.
Over time, the DeLong-McDermott company was taken over by The Offshore Company (known today as Transocean Ltd. based out of Vernier, Switzerland).
However, their idea was used by other companies to revolutionize the offshore industry and increase the number of platforms that could be set up. Zapata Offshore in collaboration with the LeTourneau Company worked on the machinery that operated the legs and looked at reducing the structural weight.
Today, jack-up barges are the most common type of offshore platform and can be found across the world’s oceans- from the Gulf of Mexico to the Pacific Rim.
How Do Jack Up Barges And Standalone Platforms Function?
A jack-up barge is comprised of 4 main parts:
1. The buoyant and water-tight hull on which machinery and deck space is located,
2. The legs that are used to jack the platform vertically,
3. The machinery that is used to move the legs (located on the hull), and
4. The bottommost region of the legs, known as the footing. Spud cans are a common type of footing used in modern jack-up platforms.
The hull is an enclosed structure made of steel, capable of withstanding hydrodynamic loads while floating and stationary. The machinery is stored on the hull and it is usually divided into 3 decks.
The uppermost deck is where the living quarters and helideck are. It is also used as a storage site for heavy machinery and essential items. The intermediate deck is known as the equipment deck, where the drilling rig equipment is housed. It also includes the machinery that is used to move the legs vertically.
Lastly, the lowermost deck is used to store grout/drill mud (in the case of drilling rigs) and store the drilled oil and gas. Over its lifetime, the hull can assume 2 main modes of operations:
1. Floating – During transit and when the legs do not touch the sea bed i.e., fully dependent on buoyancy for supporting the hull weight.
2. Standing – When the legs are anchored in the sea bed with the footing i.e., the weight is supported either solely by the leg reaction force or partially with buoyancy on the hull (if it is in contact with the water).
The jack-up legs are either solid or truss and provide the unit with its unique name. Solid legs are more expensive but are resistant to heavy wave forces.
They are not very common due to the high cost associated with them. Truss legs, on the other hand, make use of legs that are triangulated structural members and provide support in both tension and compression.
They are cheaper to build and can be used in most wave conditions. The advantage with truss members is that the wave resistance is significantly reduced, making the structure transparent to sub-surface hydrodynamic forces.
The type of machinery used to retract and extend the legs vary depending on the platform, but the most commonly used systems are:
1. The mechanical rack and pinion mechanism, and,
2. The hydraulic ram pin-in-hole mechanism.
The rack and pinion system makes use of a gear mechanism that works by rotating a pinion about a rack positioned on the leg. The pinion (on the hull) is part of the hull and climbs up and down the leg using motors that power it.
The hydraulic system is also very common and makes use of a series of rams that can slot into holes along the platform leg. It works by sequentially moving the ram into the hole, hydraulically moving the hull, and then locking it into place. There are 4 rams- 2 upper pins and 2 lower pins, separated by a vertical piston.
In case the deck is being lowered into the water, the ram pins first lock into their holes. Then, the upper pins release, the pistons contract, and then the upper pins move downwards.
Then, they lock into place, the lower pins release, the pistons extend, and then the lower pins again lock into place. This way, the mechanism retracts and extends the legs (the process is in reverse in the case of raising the hull).
Lastly, the footing is used to secure the base of the legs to the sea bed. The most common footing design is the spud can. This is a large flat disk that is slightly inclined upwards on the top surface, closer to the axis. It is used to improve the leg grip and prevent the footing from sinking into soft soil.
Now that we have discussed the various parts, let us look at the operation of installing the jack-up platform.
There are 6 main steps:
1. The jack-up barge is towed to the installation site, with minimal machinery and equipment on board. Any loose items on deck are secured to prevent any accidents during the subsequent operations.
2. The legs are then lowered into the water until it reaches the sea bed.
3. Then, the leg-extending machinery pushes the legs further into the sea bed, while the hull floats.
4. A step known as preloading is then undertaken, whereby the hull is ballasted to full capacity.
5. The hull is then de-ballasted, and it begins to climb the legs, away from the surface.
6. Finally, the hull exits the water and maintains a sufficient air gap. Lastly, any moorings are removed, and the jack-up barge is allowed to face environmental loads without any external protection.
Most of these steps are self-explanatory. However, preloading is an often confusing operation because it seems unnecessary.
When the legs are already in the sea bed, what is the need to ballast the hull? The reason is that the substratum is often unstable and can shift once environmental loads are allowed to act.
To allow the spud cans to properly grip the soil and the legs to sink into the sea bed, the downward load on them is increased by ballasting the hull. This increases the weight acting on the spud cans and secures them in place.
Preloading is a safety procedure since it ensures that the soil can take the hull weight without slipping. In case of foundation failure at this stage, the hull will only sink below the surface, while there will be no fatalities.
The jack-up can be repeatedly preloaded to ensure that it is secured in a stable soil region. During preloading, the hull is brought out of the water to a height of around 1.5 meters. Since there is an abrupt weight transfer from buoyancy to the leg reactions, there is a chance of sudden impact loads.
For this reason, failure simulations should be undertaken to check safety at impact conditions. To prevent the chance of failure, geotechnical investigations are carried out to determine the suitability of a site for setting up the jack-up platform.
Once these steps are completed, the equipment and machinery required for drilling are installed on the different decks. When it is time to shift to a new location, the spud cans are loosened by pumping water through nozzles (to dislodge it from the soil or clay), the legs retracted, and the hull immersed in the water.
An interesting thing to note is that during the floating transit mode, the legs can be used to handle wind and wave loading. By lowering the legs further into the water (without touching the sea bed), the vessel becomes less responsive to wave loads and more stable. This is particularly useful when used in conjunction with ballasting.
How Do Jack Up-And-Fixed Drilling Platforms Work?
Other than working as a standalone platform or barge, jack-ups can also be used to drill oil from the sea bed in more complex operations. Such a jack-up platform is composed of 2 main parts:
1. The fixed unit, and
2. The jack-up unit
These 2 units are located adjacent to each other and work in tandem. They are connected by structural members to provide added stability. Since the jack-up is one of the most stable structures, it can be used for motion-sensitive tasks such as drilling.
The first step in the installation is for the fixed unit to be towed to the site. The fixed unit can be of different types depending on the drilling requirement and is generally a jacket structure. It has limited machinery and houses storage facilities for drilled oil and gas.
Then, the jack-up platform is brought to the site, moored at a fixed distance from the fixed unit, and then jacked up. The jacking height is such that it is nearly level with the deck of the fixed unit.
The main structural consideration for this is that there should be a suitable air gap of around 1.5 – 3 meters between the water’s surface and the underside of both structures.
Once the steps from the previous sections are followed, the platform is ready for the final process. Long cantilever beams are placed between both units and fixed to the decks.
While welded joints are occasionally used, riveted and bolted structures are more common since it is easier to install and dissipates stress better than welded joints.
The cantilever beams are checked for safety and stability, and to ensure that they can be used to cross between both platforms. They often include winch harnesses that can be used to haul goods and material across the gap.
There are also dedicated beams for pipelines that ferry oil and gas from the jack-up platform to the storage facilities on the fixed unit. It may use a system of interlocked beams to add strength to the structure.
Since the jack-up platform is stable, it is used for motion-sensitive tasks such as drilling. It also hosts structures such as the drilling rig, derrick cranes, and helideck.
On the other hand, storage is on the fixed platform. Living quarters are often on the jack-up since they should ideally be located away from the storage depots. The positioning of the fixed unit depends on the expected direction of the wind.
The flare stack which is used to vent fumes is positioned on the fixed unit in such a way that it is leeward i.e., downstream from the wind. This prevents poisonous gases and fumes from fouling the living quarters.
The Types Of Jack Up Barges
Since jack-up barges are ideal for different purposes, they have been modified to suit the varied needs of different industries. In this section, we look at the 4 common types of jack-up barges.
1. Mobile Offshore Drilling Units (MODUs)
A MODU is a type of drilling platform that is not unique to jack-ups. It refers to a class of floating platforms that can be moved across different locations. Unlike older variants of oil rigs that were cemented or firmly connected to the sea bed, MODUs can be detached from supports and towed elsewhere. By fitting drilling equipment on the deck and using a series of connectors and equipment to link it with a wellhead on the sea bed, the jack-up can be converted to an oil drilling platform. The primary modifications to a conventional jack-up are the onboard storage facilities (for the drilled oil) and structures that are required by drilling equipment (such as the Christmas Tree and Blow Out Preventer).
2. Turbine Installation Vessel (TIV)
Offshore wind turbines are commonly found in areas such as the North Sea (near Nordic countries). When several turbines are set up nearby, it is known as a wind farm. These farms are capable of providing a significant amount of power to the neighbouring countries. Their upkeep is an important factor that must be considered before installation. The main challenge in the North Sea wind farms is the high wave heights, strong winds, and violent currents. Even at other locations, repair and maintenance crafts could face problems while attempting to approach the turbine. The stability of the installing and repair vessel is of paramount importance in this field.
By using jack-up barges, installation and repair can be easily achieved since it presents a stable platform. They work just like conventional jack-ups- their legs are lowered, preloaded, and then the platform elevated off the water’s surface. Once this is complete, a series of piles are driven into the seabed for the turbine’s base to be laid. Then, the column with the hub and blades at the top is lowered from the side of the jack-up until it connects with the base. Once the installation is complete, the barge can simply lower itself, lift its legs off, and sail to the next location.
A famous jack-up TIV is the MPI Resolution (aka Mayflower Resolution) that uses a system of 6 legs and hydraulic machinery to install multiple wind turbines. She can elevate to a height of 3 – 46 meters and was the first jack-up TIV.
3. Modified Heavy Lift Vessel (m-HLV)
Heavy Lift Vessels (HLVs) are ships and floating barges capable of lifting heavy structures from the surface of the sea or elevated support columns. For instance, the top offshore platform builders are situated in South Korea, Japan, and China.
However, the platforms are often destined for locations such as the North Sea, the Gulf of Mexico, and the African coastline. Other than semi-submersibles, the vast majority of platforms cannot be floated or towed to their installation site and require another vessel to lift and transport them. This is where HLVs come in.
Jack-up barges can be modified to function as HLVs. A traditional HLV works by ballasting to below the structure and then de-ballasting until it can lift the structure off the surface. However, jack-ups work by positioning the platform under the structure, preloading, elevating till it can lift the structure out, loosening the sea bed supports, and then retracting the legs. It can only be used in regions with shallow water due to the restrictions on leg lengths.
The m-HLV is heavily reinforced about the midship region to prevent sagging and structure failure. The main challenge during the lifting operation is that the platform should not raise too high above the surface, since it requires buoyancy to support the weight of the structure once the legs are retracted. Detailed calculations on the optimal draft of the jack-up are calculated before the operation to minimize the chance of failure.
4. Regularized Barges
Lastly, the jack-up platform can be used as a regular barge for transporting goods. They do not perform any specialized functions like the jack-ups mentioned above but can be used as modified barges that transport 2 main types of cargo- large and unwieldy (such as turbines), and smaller vessels (for service, repair, or deployment). Their legs are generally unused when transporting goods, but the benefit is that when not transporting goods, they can double up as a standard jack-up.
Considerations While Designing, Building, And Using Jack Up Barges/Platforms
Jack-up barges, like any floating vessel, require certain design, structural, and safety considerations to be kept in mind while designing and building them. These are as follows:
1. During preloading, impact load stability and strength must be monitored to prevent point overloading and subsequent failure.
2. Spud cans should be tested in shallow conditions to confirm that the components, such as the nozzles, are working.
3. If the installation site is soft or clayey, the spud cans can be joined together to form a continuous mat-like structure, known as a mud mat.
4. The foundation must be checked to ensure that it can support the hull and machinery weight. Along with preloading, punch through must also be check to ensure that the structure is resting on a firm and hard surface. This reduces the chance of damage during storms or other adverse weather conditions.
5. Uplift forces and spud can break out forces that will act on footings that have been in place for a long period. They can cause structural damage to the footing, legs, and even the hull. The maximum break-out forces occur in clay, and when the spud can is raised by about 0.1 to 02 times its diameter. Cavitation of the lower surface also begins to occur. To prevent this, water jetting through nozzles, prior excavation of soil, cyclic loading, or a combination of these methods can be used.
6. High-strength steel is subject to embrittlement when the oxygen content is low (as is the case underground) and when hydrogen gas is formed due to the cathodic protection system used to prevent rusting. Sufficient care must be taken to inspect the legs for embrittlement or signs of damage.
7. The hull must be located at least 1.5 – 3 meters above the surface, to prevent waves from stripping away the weld joints. While this may seem like a minor problem, it is prevalent in the North Sea region.
8. Safety studies and analysis should be undertaken using the probabilistic method and must account for normally unexpected environmental forces based on 100 years.
9. At very deep-water depths, the spud cans act as hinges, meaning that they become very compliant with environmental forces. They can no longer remain stiff, and the structure is in danger of being loosened from the sea bed. To prevent this, the operating depth is kept at 120 meters. In the case of large platforms that can function at depths of over 200 meters, they use multiple legs that use larger spud cans with more surface area.
For joint drilling platforms that make use of both fixed and jack-up platforms, there are other considerations to be kept in mind. These are as follows:
1. The height of the jack-up should be fixed since if there is a mismatch between the jack-up and fixed units, the cantilever cross beams will come under extraordinary stress and eventually buckle or fail. This could have catastrophic consequences, including the complete structure breaking down. Oil may leak into the ocean, and the lives of staff and crew onboard would be put at risk.
2. The cross beams should be secured after much consideration and planning. This is because people use the beams to cross over, and drilled products also pass through them. In case of an accident, it could lead to a loss of life and property. Extensive tests and simulations are carried out to ensure the cantilever beams are safe for use.
In this article, we have seen how instrumental jack-up barges are to various industries such as the offshore oil and gas sector and the offshore energy sector. They allow large and complex machinery to be installed simply and efficiently. However, safety is an important aspect while working offshore, and this must be kept in mind at all times.
You might also like to read:
- Life On An Oil Rig – Do You Know What It Takes?
- Types Of Mobile Offshore Drilling Units (MODU)
- Offshore Well Drilling: A General Overview
- Understanding Offshore Lifting Operations And Engineering Analysis
- Different Types Of Offshore Oil and Gas Production Structures
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