Introduction to Offshore Structures-PART-2

Types of Off shore Oil Platforms

Floating production system:

These platforms can take the form of either floating semisubmersible platforms or drill ships.

The basic idea behind their design is that, once the well has been drilled, much of the production equipment can be mounted on the seafloor and the petroleum pumped to the surface facilities through flexible risers.

Meanwhile, the platform or ship stays in position with anchors or a dynamic positioning system. This approach allows oil companies to reach depths of up to 6,000 feet (1,829 meters).

The floating production system (FPS) is anchored in place and can be dynamically positioned using rotating trusters. Connected to wellheads on the ocean floor, this system can be used in water depths up to 6,000 feet.


Tension leg platform:

This platform is essentially a king-sized version of the Sea Star platform, except the tension legs extend from the ocean floor to the platform itself.

It experiences more horizontal motion and a certain degree of vertical motion, but it allows oil companies to drill at depths of up to 7,000 feet (2,134 meters), well over a mile (1.6 kilometers) beneath the waves.

TLPs and SPAR platforms consist of floating structures held in place by vertical tendons connected to the sea floor. TLPs and SPAR platforms account for 39 percent of announced of deepwater projects.


Subsea system:

This approach takes the idea of mounting the wellhead on the sea floor and applies it to even greater depths -- 7,000 feet (2,133 meters) or more.

Once the well has been drilled by a surface platform, the automated systems transfer the oil and natural gas to production facilities by either risers or undersea pipelines.

Subsea systems (SS), connected to nearby platforms, can operate at great depths. However, the drilling and completion cost penalties of subsea systems make these arrangements less preferable than floating structures.


Spar platform:

Finally, if you absolutely need to drill a hole at a depth of 10,000 feet (3,048 meters), then the spar platform is the oil rig for you. With this design, the drilling platform sits atop a giant, hollow cylindrical hull. The other end of the cylinder descends around 700 feet (213 meters) into the ocean depths. While the cylinder stops far above the ocean floor, its weight stabilizes the platform. A network of taunt cables and lines trail out from the cylinder to secure it to the ocean floor in what is called a lateral catenary system. The drill string descends down through the length of the cylinder's interior and down to the ocean floor.


Deep-sea waters reach nearly freezing temperatures, contain pressures great enough to crack iron casings and are subject to rough, deep-sea currents.

Engineers have to design equipment that can stand up to the pressure, while also preventing boiling oil from hot, underground depths from cooling to a solid form and rupturing pipes when it emerges into the chilly ocean environment. While antifreeze has played an important part in preventing this thus far, more advanced methods are under development











In offshore petroleum production, steel risers (tubing) are used to bring oil from the seabed to floating production platforms. In deepwater applications (3,000 to 5,000 feet), heavy steel risers require expensive tensioning and buoyancy systems. For ultradeep operations in water depths of 5,000 to 13,000 feet, the size and weight of conventional steel risers becomes cost prohibitive, inhibiting the exploitation of substantial offshore domestic oil reserves (Silverman 1999).

The use of lighter-weight composite production risers (CPRs) will reduce platform weight together with floatation and tensioning requirements and overcome one of the greatest limiting factors in offshore oil production, that is, increasing riser loads that floating platforms have to support in increasing water depths. However, the cost of CPRs has been significantly higher than steel risers because prior manufacturing techniques have not been optimized and production levels have not achieved economies of scale.

Umbilicals in Off shore

An umbilical is a group of hydraulic fuel lines and/or electrical cables reinforced with metal wires designed for subsea applications

Major Failures – Storage Tank

Alexander Kielland Oil Rig Platform Disaster

On March 27, 1980, Alexandra L Kielland Platform, a semi-submersible oil drilling platform located at North Sea capsized during a storm. The platform supported by five columns standing on five 22 meter diameter pontoons. The five 8.5 diameter columns on the pontoons were interconnected by a network of horizontal bracings. The cracked bracing made five other bracing break off due to overload. And subsequently the platform became unbalanced and capsized. 123 killed

Fatigue crack had propagated from the double fillet near the hydrophone mounted to one of the horizontal bracing. Some cracks related to lamellar tearing were found in the heat affected zone (HAZ) of the weld around the hydrophone..

A circular hole was introduced in D6 bracing & a pipe (hydrophone) was mounted into the circular hole by welding. The hydrophone was 325 mm in dia. with a 26 mm wall thickness and was welded using a double fillet weld with a weld throat of 6 mm.

Failure caused by leakage of boiling and expanding vapour explosion

25 mm thick Plate ASTM 516 Gr 70 steel Rolled and welded using SAW process without PWHT

Six inches crack noticed at HAZ

Mechanism of cracking Hydrogen Induced cracking

Major Failures – Storage Tank

Codes & Certifying Bodies

Structural design has to comply with specific offshore structural codes. The worldwide leading structural code is the API-RP2A. The recently issued Lloyds rules are also important.

The major certification bodies are: Det norske Veritas (DnV) Lloyds Register of Shipping (LRS) American Bureau of Shipping (ABS) Bureau Veritas (BV) Germanischer Lloyd (GL)

Insurance companies covering transport and installation require the structures to be reviewed by warranty surveyors before acceptance. The warranty surveyors apply standards, if available, on a confidential basis.

Types of loads

Maximum design forces for steel jacket platform are as follow:

Vertical load: around 50 MN Horizontal load: around 5 MN Overturning moment: around 10 GN.m

It is not advisable to impose any further lateral loading on the offshore installation as this may affect the overall strength of the platform which could create a potential safety hazard

Steel jacket type platform on a pile foundation is by far the most common kind of offshore structure

"substructure" or "jacket" is fabricated from steel welded pipes and is pinned to the sea floor with steel piles, which are driven through piles guidesPiles are thick steel pipes of 1 to 2 metres dia. and can penetrate as much as 100 m into the sea bed. The jacket can weigh up to 20,000 tonnes. To ensure that the installation will last for the required service life, maintenance must be carried out including the cathodic protection used to prevent corrosion.

Types of loads

Introduction to Offshore Structures-PART-2

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steel risers

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water depths

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