70-years Marvel of Producing Oil & Gas from Ocean

The oil & gas industry has marched into ocean for 70 years (1954-2024). It has been an amazing marvel where human being has ventured to deep waters in oceans to harvest oil and gas hidden underneath the ocean floor. This article reviews the timeline of the offshore oil & gas developments and highlights the important moments when technological advancements were made.

First Floating Drilling in 1950s

It started with Gulf of Mexico, when the oil production has been depleted onshore in Texas and Louisiana. Engineers created the very first mobile offshore drilling unit, Mr. Charlie, which started drilling in the shallow coast in 1954. It was a submersible barge built specifically to float on its lower hull for transportation to location. It had to run a sequence of flooding the hull stern down in order to rest on the bottom to begin drilling operations in 40-ft water. However, strictly speaking, it was not "floating" drilling. The very first "floating" drilling vessel to use subsea well control was the Western Explorer (figure 1) owned by Standard Oil of California (now Chevron), which spudded its first well in 1955 in the Santa Barbara Channel. Mooring lines are vaguely visible in front of the bow in the picture. The spudding from a floating vessel may be considered as the first milestone in the history of floating drilling.

(Figure 1) It was the first drillship in history compared with a modern drilling semi
(Courtesy of the Laborde Family and Transocean)

With Mr. Charlie (bottom-founded) and Western Explorer (floating) as the first offshore drilling units, another concept showed up in the form of a "jack up." This type of unit floated to the location on a hull with multiple legs sticking out under the hull. Once on location, the legs were hydraulically jacked down to the ocean bottom, and then the hull was jacked up out of the water. With this approach, a stable platform was available from which to drill. The first jackup built for drilling in 1956 was for 80-ft water depth. In the 1990s, enhanced jackups were designed and built, which could drill in slightly deeper depths greater than 400 ft.

Oil companies saw the need to have a more "motion-free" floating drilling platform in the deeper and stormier waters of the GOM. They noticed that submersibles like the Mr. Charlie had much smaller motions afloat compared with monohulls. They had the idea of putting a mooring system on a submersible, thus converting it to a semi-submersible (or semi) that floats. Thus, in 1961, the submersible Bluewater I was converted to a semi. Then came the Ocean Driller, the first semi built from the keel up. The Ocean Driller went to work for Texaco (now Chevron) in 1963. The unit was designed for approximately 300 ft of water depth.

Great Evolutions in Drilling Vessels over 1960-1990

In the 1960s, owners of drilling barges employed mooring systems consisting of wire ropes connected to six or eight anchors. Using anchors became a common practice for station-keeping. During the decade, these mooring systems allowed floating units to drill in deeper water than jack-ups could. The deeper water imposed challenges on mooring arrangements. Large vessel offset due to weather could deflect the riser underneath and possibly lead to riser failure. Therefore, mooring needs to be carefully designed. Analysis needs to be performed to allow drilling risers to stay connected to the blow-out preventer (BOP) on seafloor. Mooring became an important engineering discipline. Since then, drilling semis have gone through generations of designs in the following decades. In the early 1970s, second-generation semi was built with more sophisticated mooring and subsea equipment. The design generally was for 600-ft water depth or deeper. In the mid 1980s, a number of third-generation semis were designed and built that could moor and operate in greater than 3,000 ft of water depth. The displacement of these units went from approximately 18,000 tons in the 1970s to more than 40,000 tons in the 1980s. In the late 1990s, the fifth-generation units, became even larger at a displacement greater than 50,000 tons. A fifth-generation drilling semi is shown in the figure.

(Figure 2) The author working in an offshore oil field to install polyester mooring ropes

Technologies Advanced Deepwater Exploration in 1990s

Initially, floating drilling was conducted in shallow water of less than 100 feet, and then it was gradually moved to slightly deeper waters. Up to the 1980s, drilling vessels were moored in water depths no more than a few hundred feet. During that period, wire ropes and chain were the components used on every mooring job. Technologies in mooring have improved significantly since then. Now, in deeper waters, drilling can be conducted by vessels employing dynamic positioning (DP) systems, that use computer-controlled, motor-driven propellers, called "thrusters," to counter the wind, waves, and current loads.

(Figure 3) Excessive platform offset could cause the drilling riser to bend (Courtesy of Stress Engineering); author inspecting a riser joint in a factory

On the other hand, moored drilling and production vessels can also operate in deeper water with the advances of mooring technologies. One significant breakthrough for deepwater mooring is the technological advancement of synthetic fiber rope mooring. Polyester and other fiber ropes were studied for deepwater moorings in the early 1990s. The studies showed that polyester rope has desirable weight and stiffness characteristics for use as mooring lines. The first use of polyester ropes in a permanent mooring system was tried successfully by Petrobras in the mid-1990s. Today, polyester mooring has become the most commonly used mooring system for deepwater floating production around the world.

In addition to polyester moorings, other technological developments have enabled the industry's migration to deep water over the last fifty years. For example, anchors have been improved as well. In early days of floating drilling, small conventional drag anchors were used exclusively. Today, high-efficiency drag anchors are available for drilling operations. For floating production units, more powerful anchors that can take high vertical loads such as suction piles are widely deployed. These technological advancements enabled the offshore oil & gas industry to venture into deeper water and a harsher environment.

Oil and gas have been produced from offshore locations since the late 1940s. Bottom-founded structures such as fixed platforms and compliant towers were initially used, which are limited to water depth of about 1000 -1800 ft. As exploration and production moved to deeper waters and more distant locations, four types of floating production systems came to play. All four types, spar, semisubmersible, TLP, and FPSO, have experienced significant growth over the years. Particularly, FPSOs have the much larger number of units installed in the world than the other three types. Floating production moved into deepwater at 6000 - 8000 ft of depth during the 2000s. Massive production platforms were built for large oil fields in deepwater; it was a deepwater boom.

Deepwater Drilling and Production Boom in 2000s

Offshore drilling operations are conducted by mobile offshore drilling units, which can be categorized as at least three types: jack-up barge, drilling semi, drillship. Jack-up barges use their vertical legs to stand on the seabed and raise their hulls above the sea surface, so they may not be considered as one of the vessel types for "floating" drilling. However, they can only work in shallow water areas due to the limited length of their legs. The other two types use either a mooring system or a DP (Dynamic Position) system to provide station-keeping. They can go to deepwater areas. The operation to drill an exploratory well is normally of short duration, lasting 30 to 90 days. Therefore, there is a need for the vessel to move periodically from one well site to another. The frequent moves require the mooring system to be designed for easy retrieving and re-deploying.

(Figure 4) Lessons learned from mooring incidents in offshore oil fields around the world

Most floating drilling vessels are of semisubmersible type which are designed with good stability and seakeeping characteristics. A semi-submersible retains most of its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a drillship. The drilling operation is conducted through a drilling riser, which is connected to a Blow Out Preventer on the seabed. In recent years, drilling semis used in ultra-deep water (for example, 8000 ft) have become very large, and they are equipped with advanced DP systems.

Drillship is another type of drilling vessels which can operate in deepwater. They can be spread moored or dynamically positioned, and have been used in floating drilling operations with a long history. Because of the relatively poor motion characteristics of a ship shaped vessel, moored drillships are seldom used today. In recent years, drillships used in deepwater and ultra-deepwater become very large. They are equipped with advanced DP systems. These DP systems maintain the position of a drillship within a small circle.

Offshore Oil Production Matured in 2010s

Fixed platforms were initially used to produce oil and gas in water depth up to about 1200 feet. As exploration moved to deeper waters, floating production systems were deployed. There are mainly four types of floating production platforms: spar, semi, TLP, and FPSO. Many have been built and installed in the past 30 years. Experience has been accumulated and lessons have been learned since the deepwater boom. Many failures and near-misses, such as mooring incidents, have been collected and corrected. Floating production platforms have become a mature technology.

(Figure 5) Spar, Semisubmersible, and FPSO
(Courtesy of Technip, Aker Solutions, Bluewater)

Spar is a type of floating production platform typically used in deeper waters. The deep draft design of spars makes them less affected by waves, and thus the better motion characteristics allow for both dry tree and subsea (wet tree) production. A classic spar consists of a large-diameter, single vertical cylinder supporting a high deck. The cylinder is ballasted at the bottom by a chamber, thus improving stability. Spars are permanently anchored to the seabed by way of a spread mooring system.

A semi-submersible platform, simply phrased as semi, is a specialized platform designed for offshore drilling, production, accommodation, or a combination of these functions. When oil wells are drilled and completed by drilling vessels, production semis are towed to the field and hooked up with their permanent mooring systems. Semi-submersibles offered stable and cost-effective platforms. As the offshore oil and gas development moved into deeper water, the use of semisubmersible platforms became increasingly popular because of its spacious deck area to accommodate large topside equipment and the ease of topside-hull integration at quayside.

The concept of tension-leg platform (TLP) is a vertically moored floating platform that is suitable for water depths between 1000 and 5000 ft. The platform is permanently moored by tendons. The tendons effectively eliminate most vertical motion of the platform. This allows the platform to have the production wellheads on deck, connected directly to the subsea wells by rigid risers. Their installation however requires precision and careful handling.

A floating production storage and offloading (FPSO) system is a ship-shaped platform used for producing and storing crude oil. An FPSO vessel is designed to receive hydrocarbons produced from oil wells. It has equipment on deck to process the hydrocarbons. It also stores the oil until it can be offloaded onto a shuttle tanker or, less frequently, transported through an export pipeline. FPSOs have been widely chosen, significantly more than the other types. At one time, there were close to 300 vessels deployed worldwide. Produced oil can be transported to the mainland either by shuttle tanker or export pipeline.

Closing Remarks
Offshore oil and gas industry seems to be slowing down in recent years. It is partly because the sudden rising of shale oil in the Permian basin in West Texas and New Mexico. The innovations of hydraulic fracking and horizontal drilling became a game changer for onshore production. The shale oil puts United States back on the No. 1 spot of oil producing countries. Besides that, corporate consolidation and conservative spending plans of oil companies also contribute to the slowdown of offshore oil developments. Nonetheless, the achievement in the past 70 years made by the offshore oil industry is remarkable. Nearly 30% of crude oil is produced from the oceans. There are over 300 massive oil platforms or vessels floating around the globe, not to mention thousands of fixed jackets standing on the ocean floor. It is truly a marvel created by human being, thanks to engineers of various disciplines including civil, marine, mechanical, petroleum, geotechnical, and others.