Oil and gas offshore installations or units are located at sea to reach oil wells thousands of meters below the seabed. With an average capability to house a crew of 50-100 man and women, the units are build in various shapes and sizes. They offer a true technological workplace!
Resembling the field of oil and offshore, you can find yourself working in units employed for Exploration or Production. Below we provide you with a detailed description regarding the types of vessels/rigs on where you can expect to be working, each with their own individual characteristics.
The one factor which governs more the Exploration side than the Production side of the industry is mobility; be moved from one location to another. In exploration fields you will normally find yourself working on a Mobile Offshore Drilling Unit or MODU for short.
MODU configurations comprise Jack-ups, Semi-submersibles and Drill Ships. The task of these vessels is to locate oil in various water depths.
Jack-up Drilling Rigs.
A jack-up is similar to a fixed platform while in its drilling mode and will drill in relatively shallow waters (400 feet- cm) seeing as it stands on the seabed. Therefore, the depth of water in which each Jack-up can operate in depends on the length of its legs.
The rig is towed to its drilling location with several towboats or, when distances are long, the unit will be placed on heavy-lift vessels and transported to the location. With the use of GPS the vessel is manoeuvred into its “Final Position” or the position that the Oil Production Company hiring the rig wants it to drill at. Once in position on the wellsite, the vessel’s 3 or 4 legs are lowered into the seabed so that the rig can ’Jack’ itself from the surface of the water. This practice shall raise the hull approximately 60 feet out of the water, creating the “Air Gap” (distance from the water line to the hull).
The structure of the jack-up can be designed to allow drilling through slots in the hull or it can enable a series of wells to be drilled by extending the cantilever. A Slot-type jackup will be positioned to drill additional development wells or to work over existing wells. In the second case, the rig is able to extend the drilling package (drill floor) on a part of the rig referred to as the cantilever, in some cases extending over 75 feet from the rig. This practice can move the drill floor and provide the conditions for the jackup to perform drilling operations over existing production platforms. The rig moves close besides a platform, extends the drilling package over the top of the “template “on the seabed and drills a sequence of wells.
The jack-up’s deck provides space for drilling equipment, supplies and living quarters.
The jack-up is considerably stable given that waves pass underneath the hull.
Semi-submersible Drilling Rigs
Unlike the jack-up rigs, a Semi-submersible does not rest on the sea floor but instead stands on giant pontoons and hollow columns, and is held in position by a series of anchors which are laid out in a pattern approximately one mile away from the vessel. Such a system permits the vessel to drill in much deeper waters than any Jack-up, reaching 600-7500 ft (feets).
A semi submersible will sail or be towed by means of several vessels to reach the location of the well, with the unit floating. At the site, its pontoons and columns will be pumped with seawater for the rig to partially submerge until the calculated depth (deballasting), giving it his name semi-submersible. The construction is maintained above the well by either Dynamic Positioning Systems (DP) and GPS signalisations that employ propellers to regulate wind and wave pressures, by using anchoring (mooring) systems wherein the vessel is connected to the seabed using cables, or by a combination of both.
Due to the fact that most part of the hulk is put below the water’s surface (wave transparency), the semi is referred to as a stable platform useful for drilling in rough waters since currents have only a slight influence on its stability. It has been a well suited rig for the industry to handle large water depths of up to 9140 meters and high formation pressures.
Drill ships cover all seagoing vessels that have been adapted with drilling mechanism to carry out drilling operations in waters as deep as 10,000 feet. Besides the normal equipment found on a large marine ship, they are recognized by a drilling platform and derrick stationed in the middle of the deck. Wells will be drilled through an opening in the platform of the boat that reaches the surface of the water (“Moon Pool”).
Unlike the previous two MODUs, their construction and engineering is comparable to that of a marine vessel where faster mobility is possible and where tugboats are not required for transportation.
Eventhough using similar dynamic position systems and mooring as the semi-submersible in maintaining stability, the vessels differentiate as these are much more subjected for movements in rough water.
The Jack-up drilling rigs will likely be the most common type of vessel you will be working on, in particular as these vessels drill a majority of offshore wells around the world. Nevertheless, as the search for oil is never ending and most shallow waters around the globe have been, or are being explored at large, the trend is moving to explore deeper waters with every year. Advances in the drilling/production technology have also made reaching deeper waters more feasible and economically viable. Meaning that Semi-submersibles and drill rigs will be playing a major part in the industry for the next 10 to 15 years.
The character of the Production side of the industry calls for constructions that remain permanently on location for serving very long term projects for pumping up oil from wells. These offshore installations are known as Offshore Production Platforms or Fixed Platforms. Find below their characteristics.
Offshore Production Platforms
Build on concrete legs, a Fixed Rig is anchored directly to the ocean floor and kept in place due to the major weight of the construction itself. Such installations can only be positioned as far as the leg construction permits, which makes them impractical to be used in very deep water. Usually they are limited to water depths not greater than 1.500 feet.
The rig is towed on a large barge by tugboats to the location. A stoollike construction will be placed before fastening the jacket to the seabed with piles, which is done with the help of large cranes. This practice facilitates the installation of the jacket which is located on the system with the use of remotely operated vehicles (ROV).
Once the middle piece (jacket) is secured with legs that stretch out until the seabed, will the deck and additional modules be placed. The dimensions of the modules will vary significantly based on the requirements to complete the project (e.g. quantity of fluid to be managed, number of personnel, future productions calculated). A 200 feet by 200 feet dimension per deck level will cover four modules that include space for drilling, production and crew operations (e.g. accommodations), and crew quarters.
A system of connected pipes (pipeline), previously laid down by a pipe laying vessel, will collect the hydrocarbons and, depending on the distance, transport it to a destination at the coast. Manpower is contracted to monitor these lines for pressure change and maintenance requirements (leakages). ROV are employed to inspect the conditions through video-shots.
The secure position of the platform generates only a slight exposure to movements created by waves and wind.
Helicopters and supply boats are used to transport workers, equipment, and suppliers from shore to the rig.
As indicated by the name, a supply vessels works to assist the Semi-submersible rigs by supplying handling duties such as transporting merchandise and personnel to and from the rig or providing any other necessary support. In many occasions these vessels have a fire fighting role during emergencies offshore.
These vessels are manned by crews who carry the trival task of looking out for the safety of the rig and its personnel. They are constantly ‘Standing By’ watching over the rig’s safety, equipped with a highly trained crew who are ready to meet the challenge head on when an emergency does occur.
Flotels are “rigs” that serve as a hotel for extra manpower needs of a production platform in the summer months while upgrading work is being carried out to the platform.
Floating Production Storage and Offsload (FPSO’s).
The FPSO is a ship that is connected to a nearby platform or oil well on the seabed to store hydrocarbons. It pumps the oil or gas into storage tanks until another vessel arrives to offload the oil and take it back to an oil refinery onshore, or until pipelines are ready to transport the substance to onshore.