Industry Information
Deepwater Definition
In the 2002 World Petroleum Congress (WPC), water depth less than 400m is recognized as conventional water depth, 400-1500m as deepwater and above 1500m as ultra-deepwater.
Offshore Engineering Challenges
The offshore industry requires continued development of new technologies in order to produce oil in regions, which are inaccessible to exploit with the existing technologies. Sometimes the cost of production with existing know-how makes it unattractive. With the depletion of onshore and offshore shallow water reserves, the exploration and production of oil in deep water has become a challenge to the offshore industry. Offshore exploration and production of minerals is advancing into deeper waters at a fast pace. New oil/gas fields are being discovered in ultra-deep water. Many of these fields are small and their economic development is a challenge to the offshore engineer. These have initiated the development for new structures and concepts.
Jacket Platform
The jacket or template, structures are still the common offshore structures used for drilling and production. Some structures contain enlarged legs, which are suitable for self-buoyancy during its installation at the site. Fixed jacket structures consist of tubular members interconnected to form a hress-dimensional space frame. These structures usually have four to eight legs battered to achieve stability against toppling in waves. Main piles , which are tubulat. Are usually carried with the jackets and friven through the jacket legs into the seafloor. The term jacket structure has evolved from the concept of providing an enclosure(“jacket”)for the wrll conductuors. Thers platforms generally support a superstructure having 2 or 3 decks with drilling and production equip,ent, and workover rigs. The use of these platforms has generally been limited to a water depth of about 500~600 ft (150~180m) in the harsh North Sea enviorment( typical design wave of 100 ft /30,). In the more intermediate Gulf of Mexico environment (typical design wave of 75 ft/23m) many jackets have been installed in deeper water.
TLP Platform
A Tension-leg platform or Extended Tension Leg Platform (ETLP) is a vertically moored floating structure normally used for the offshore production of oil or gas, and is particularly suited for water depths gre ater than 300 metres (about 1000 ft).
The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This makes for a cheaper well completion and gives better control over the production from the oil or gas reservoir.
The first Tension Leg Platform was built for Conoco's Hutton field in the North Sea in the early 1980s. The hull was built in the dry-dock at Highland Fabricator's Nigg yard in the north of Scotland, with the deck section built nearby at McDermott's yard at Ardersier. The two parts were mated in the Moray Firth in 1984.
Spar Platform
A spar is a deep-draft floating caisson, which is a hollow cylindrical structure similar to a very large buoy. Its four major systems are hull, moorings, topsides, and risers. The spar relies on a traditional mooring system (that is, anchor-spread mooring) to maintain its position. About 90 percent of the structure is underwater. Historically, spars were used as marker buoys, for gathering oceanographic data, and for oil storage. The spar desig n is now being used for drilling, production, or both. The distinguishing feature of a spar is its deep-draft hull, which produces very favorable motion characteristics compared to other floating concepts. Low motions and a protected center well also provide an excellent configuration for deepwater operations. Water depth capability has been stated by industry as ranging up to 10,000 ft.
The first Spars were based on the Classic design. This evolved into the Truss Spar by replacing the lower section of the caisson hull with a truss. The Truss Spar is divided into three distinct sections. The cylindrical upper section, called the “hard tank,” provides most of the in-place buoyancy for the Spar. The middle truss section supports the heave plates and provides separation between the keel tank and hard tank. The keel tank, also known as the “soft tank,” contains the fixed ballast and acts as a natural hang-off location for export pipelines and flowlines since the environmental influences from waves and currents and associated responses are less pronounced there than nearer the water line.
Compliant Tower
Compliant towers are similar to fixed platforms in that they have a steel tubular jacket that is used to support the surface facilities. Unlike fixed platforms, compliant towers yield to the water and wind movements in a manner similar to floating structures. Like fixed platforms, they are secured to the seafloor with piles. The jacket of a compliant tower has smaller dimensions than those of a fixed platform and may consist of two or more sections. It can also have buoyant sections in the upper jacket with mooring lines from jacket to seafloor (guyed-tower designs) or a combination of the two. The water depth at the intended location dictates platform height. Once the lower jacket is secured to the seafloor, it acts as a base (compliant tower) for the upper jacket and surface facilities. Large barge-mounted cranes position and secure the jacket and install the surface facility modules. These differences allow the use of compliant towers in water depths ranging up to 3,000 ft. This range is generally considered to be beyond the economic limit for fixed jacket-type platforms.
The portion of the tower that contains the drilling, production, and crew quarter modules is the surface facility. Individually, size is dictated by the dimensions needed to handle production, drilling operations, and crew accommodations. The surface facilities are smaller by design on compliant towers than on fixed platforms because of the decreased jacket dimensions that support them.
The supporting structure, for a compliant tower; it may consist of a lower and upper section. Typically, the tower’s jacket is composed of four leg tubulars that can range from 3 to 7 ft in diameter and are welded together with pipe braces to form a space-frame-like structure. The lower jacket is secured to the seafloor by weight and with 2- to 6-ft piles that penetrate hundreds of feet beneath the mudline. Both the lower and upper jacket dimensions can range up to 300 feet on a side. The water depth the structure will reside in dictates the height of the jacket.
A series of buoyant tanks (up to 12) located in the upper part of the jacket places the members in tension, reducing the foundation loads of the structure. The tanks can range up to 20 ft in diameter and up to 120 ft in length. The amount of buoyancy is computer controlled, keeping the appropriate tension in the structure members during wind and wave movements. This buoyant system can also be incorporated into some member designs, minimizing the size and placement of the tanks.
Floating Production, Storage and Offloading (FPSO)
Floating production, storage, and offloading systems receive crude oil from deepwater wells and store it in their hull tanks until the crude can be pumped into shuttle tankers or oceangoing barges for transport to shore. Use of the FPSO’s has the potential to improve industry’s capabilities of developing oil and gas reserves on the Gulf of Mexico Outer Continental Shelf (OCS) in waters so deep that they either challenge or exceed existing deepwater production techniques and transportation systems.
In addition to FPSO’s, there have been a number of ship-shaped Floating Storage and Offloading (FSO) systems (vessels with no production processing equipment) used in these same areas to support oil and gas developments. A Floating Storage and Offloading (FSO) unit can be considered to be a subset of FPSO’s. The FSO system lacks the oil and gas production processing capabilities of the FPSO. An FSO is typically used as a storage unit for production processed from other platforms that are remote from infrastructure and lack an oil pipeline to transport the oil to the refinery.
FPSOs generally are an amalgam of marine and petroleum functions, and therefore, present many specialized challenges for those involved in their creation. Their turret structures are designed to anchor the vessel, allow “weather vaning” of the units to accommodate environmental conditions, permit the constant flow of oil and production fluids from vessel to undersea field, all while being a structure capable of quick disconnect in the event of emergency.
An FPSO system is an offshore production facility that is typically ship-shaped and stores crude oil in tanks located in the hull of the vessel. The crude oil is periodically offloaded to shuttle tankers or ocean-going barges for transport to shore. FPSO’s may be used as production facilities to develop marginal oil fields or fields in deepwater areas remote from the existing OCS pipeline infrastructure. FPSO’s have been used to develop offshore fields around the world since the late 1970’s. They have been used predominately in the North Sea, Brazil, Southeast Asian/South China Seas, the Mediterranean Sea, Australia, and off the West Coast of Africa. As of 2004 there were 70 FPSO’s in operation or under construction worldwide.
