Offshore oil facilities come to the forefront of our thinking these days only when a catastrophic accident occurs aboard a platform or other facility. That’s because they are generally out to sea and out of sight. It’s difficult to comprehend the varied and complex piping systems that allow a rig or a drill ship to operate successfully and safely, but they’re an integral part of what’s happening every day. These piping systems contain a multitude of valves built to multiple specifications to handle a variety of flow control challenges. They also control various system loops and contain pressure relief devices.
The arterial heart for oil production is the oil or gas recovery piping system. Although not always on the platform, many production systems also use “Christmas trees” and piping systems that operate in inhospitable depths of 10,000 feet or more. This production equipment is built to a myriad of exacting American Petroleum Institute (API) standards and are referenced in several API Recommended Practices.
The drill platform or drill ship contains all of the normal trappings of an onshore oil well drilling and gathering system. However, the components, including the valves, also must have the capability to withstand the harsh corrosive salt air and water environment in which they operate. Besides having this capability, weight and size are often key valve selection criteria, depending on the application.
The crude or natural gas piping systems found on production platforms and ships are considered part of the “upstream” and “midstream” categories of the industry. [To review: Upstream is all the equipment (piping and valve-wise) from down in the drill hole to the choke on the wellhead. The choke is a regulating valve and usually the last flow control element on the Christmas tree). Midstream is everything from the choke to the refinery fence, while downstream is everything inside the fence and throughout the refinery.]
All of the wellhead valve designs are covered in upstream documents such as API 6A, Petroleum and Natural Gas Industries–Drilling and Production Equipment–Wellhead and Christmas Tree Equipment. On most large platforms, additional processes are applied to the raw fluid coming from the wellhead. These include separating water from the hydrocarbons and separating gas and natural gas liquids from the fluid stream. These post-Christmas-tree piping systems are generally built to ASME B31.3 piping codes with the valves designed in accordance with API valve specifications such as API 594, API 600, API 602, API 608 and API 609. Some of these systems may also contain API 6D gate, ball and check valves. Since any pipeline on the platform or drill ship is internal to the facility, the strict requirements in API 6D valves for pipelines do not need to be followed.
Although multiple valve types are used in these piping systems, the type most frequently chosen is the ball valve. The lightweight and relatively small footprint of this type of valve for isolation service helps greatly in the space-starved confines compared to a type such as gate valves. Ball valves also help with the weight-conscious environs of the offshore structure. An important consideration in these cases is on-site repairability because the facilities are separated from on-shore repair shops by many miles of water.
The already smaller ball valve designs were reduced even further in the early 1990s with the advent of compact-style valves. Before these compact design ball valves, the top-entry API design ball valve was a popular choice for offshore isolation service.
The compact valves are designed and built by reducing end-to-end dimensions, providing OEM-specific end connections, changing construction detail parameters and circumventing design standards such as API 608 and API 6D. The design and stress calculations of each valve are performed by the valve manufacturer. Since they do not comply with API or the American Society of Mechanical Engineers (ASME) standards, the purchaser is responsible for accepting the manufacturer’s valve design and supporting design data as suitable for the service conditions.
These valves have found acceptance in the offshore field because of their reduced size, weight and design modularity. Because offshore production facilities are populated by many modular manifold systems, the design of the compact valve works well for these types of piping systems. On a floating drilling ship, where every pound of weight requires a matching pound of buoyancy, the valves have received very broad acceptance. The compact design principles are also used to build ball valves with two distinct ball and seat sets encased in one body.
In addition to ball valves, the compact design philosophy has been used for other types such as check valves. They are built with a variety of end connections, including butt-weld ends, ANSI flange adapters, threaded union and hub-type ends.
In addition to the ball valves in isolation service, more and more butterfly valves are used. These include both the lower-pressure elastomer or rubber-sealed types as well as the high-performance triple offset designs. One of the drawbacks of high-performance butterfly valves is that they can be more difficult to repair on-site than ball valves.
Valve materials for offshore applications have to take into consideration the harsh, salt-laden environment, as well as corrosion issues encountered in hydrocarbons. To counter the ambient corrosive effects, heavy exterior coatings are applied to the piping systems. Austenitic stainless steels such as 316 and 316L are used in these systems since they are resistant to the saline atmosphere. They also can help resist fluid corrosion situations such as sour gas.
Plain carbon steels are still popular as well. One of the most common combinations of ball valve materials is WCB or A105 steel with either stainless-steel trim or electroless nickel plating on the valve trim. Higher strength materials such as 17-4 PH are used when additional component strength is needed. Since many ball valves are at the top end of the feasibility size range for floating ball valves, grease fittings are sometimes supplied. This is to allow for infusion of a lubricant to reduce the friction and coincidental higher opening torque.
Oftentimes the hydrocarbons may contain levels of hydrogen sulfide (H2S), also called sour gas. In high-enough concentrations, this fluid can be both highly corrosive and deadly. Protection of lives and property in these situations requires H2S-resistant materials as specified in the NACE International specification MR0175/ ISO 15156-1, Petroleum and Natural Gas Industries–Materials for use in H2S Containing Oil and Gas Environments in Production.
A key component of gas and oil processing system piping is the emergency shutdown system (ESD). This component is the means by which the oil and gas processing system is made safe in the event of an emergency. The ESD employs a variety of key emergency shutdown valves to stop flows through the various lines. These valves are fire-safe and are often controlled by spring-loaded actuators.
FPSO AND FLNG SHIPS
One of the trends today in offshore production is the use of Floating Production, Storage and Offloading (FPSO) ships. These vessels are frequently converted from tankers or purpose-built from scratch. An FPSO looks like a small refinery sitting on the deck of a ship, oftentimes containing a flare system rising high above the deck. The production piping systems on an FPSO are called topsides. Since the FPSO takes over many of the operations that might be handled on a rig structure, such as oil, gas and water separation, the same material requirements apply to the piping and valves onboard those FPSOs.
The topside oil and gas processing equipment is designed and constructed in accordance with API and ASME offshore and refinery standards. However, the standards must take into account the movement of the vessel caused by ocean conditions. Additional loadings from wave action can be considerable and have a serious effect on the fatigue life of the piping systems. Since many of the systems are built in modular subassemblies, compact and lightweight valves are often chosen.
The cold-blooded second cousin to the FPSO is the Floating Natural Gas (FLNG) ship. The FLNG is a complete floating liquefaction unit built into a ship. The unit converts natural gas into Liquid Natural Gas (LNG) which is then offloaded to LNG tankers for delivery to other locations and gasification. FLNGs are heavily laden with piping and valves, and have the additional caveat of low-temperature piping and valve requirements as the liquefaction process operates at temperatures down to –260°F (–162°C). Because of this, extended bonnet butterfly, ball, gate and globe cryogenic valves are often used.
UTILITY PIPING SYSTEMS
Offshore platforms and ships have an infrastructural piping system just like large onshore plants. These systems are critical to the day-to-day operation of the facility as well as its safety. The valves in these systems usually do not fall under API pipeline, production or process valve standards. In United States waters, they are covered by U. S. Coast Guard (USCG) or American Bureau of Shipping (ABS) regulations. These piping systems include any type of ballast-controlling piping, especially where there are direct outlets to the sea.
Fire protection piping systems are also critical in this case, and they are specialized for the offshore environment. The firefighting system on an offshore platform begins with a series of high-pressure pumps drawing seawater to fight a potential fire. The pumps spread water throughout the vessel or structure through a lattice of pipes and deluge nozzles. Popular materials for these systems are copper/nickel alloy (see Table 1).
While weight is an important consideration on any floating offshore facility, linear valves such as gates and globes are still used in many of these utility applications. Since space is also often at a premium on drill ships and FPSOs, the non-rising stem design is often employed for gate valves.
ABS divides shipboard piping systems into three classes—I, II & III, with class I the most critical. The piping code chart generally relates more critical classifications to valves in service as the operating temperature and pressure increase.
ABS has a process whereby it approves or validates piping components, including valves. The first step in the process is called the “unit certification.” The unit certification is for a particular valve or component used in a specific installation on a vessel. It is typically a one-time approval. An ABS inspector usually witnesses parts of the manufacturing process and then approves the component’s use for a particular project.
A more generic type of approval is the “product design assessment” (PDA). This is a generic approval that provides all parties with a “pre-approved” starting point that helps to reduce backlogs and expedite final approvals. The approval is given when an engineer or surveyor accepts the assessed item for a specific user and installation.
A “manufacturing assessment” (MA) is issued for each manufacturing facility. To earn an MA, a valid PDA is required along with satisfactory demonstration that the product can be consistently manufactured according to that PDA. If a product such as a valve has both a valid MA and PDA, it can be considered for ABS Type Approval. Having type-approved valves is very desirable for the manufacturer.
COAST GUARD AND THE CFR
Important U.S. government regulations for shipboard piping and valves are found in the Code of Federal Regulations (CFR), Title 46, Part 56. The primary valve regulations are located in Subpart 56.20 “Valves.” The USCG is the regulatory entity for these documents.
Resilient-seated valves that fall under USCG CFR regulations are required to pass stringent test procedures. These are similar to the fire testing requirements of API 607 and API 6FA, in that the valve, without its resilient seating components, must still provide partial sealing in accordance with specific allowable leakage parameters. These parameters are based on whether the valves are installed on board the ship in vital systems or positive shutoff service.
The critical piping systems on offshore facilities and ships have to be robust enough to handle the harsh conditions encountered from the North Sea to the South Atlantic. The need for reliable high-quality valves in these systems is paramount.
Although the offshore industry may be running at idle for a while until the price of oil creeps back up, the future of much of our energy production lies below the sea floor. An armada of safe and productive offshore ships and facilities is vital to retrieve this black gold.
Types of Offshore Facilities
Many types of offshore oil and gas production structures exist. The choice of which the production company uses is based on economics, which are in turn based on water depth, projected field lifespan, and whether the equipment must be reused, to name a few. All of these structures are loaded with piping and valves. The FPSOs and FLNG ships are especially valve laden.
Fixed Platform—The fixed platform is a solid design with fixed length or limited adjustment height control. The fixed structure is limited in depth of water (DOW) to what can be built in the shipyard and transported to the proposed drill site via ship or barge. Fixed platforms are not nearly as popular as they once were because of the facility’s one-use status (lack of mobility) and the expense of decommissioning the structure should it need to be abandoned.
Jackup—Jackups are the most common type of offshore drilling rig. They are usually comprised of a floating barge and a main drilling structure with legs that are jacked down to the ocean floor, causing the drilling structure to rise above the waves. Because they make solid contact with the bottom, their useable DOW is limited.
Semisubmersible—The operating deck of a semisubmersible sits high above the waves on submerged pontoons. The drill structure is held in position by a system of anchors. These anchors are sometimes augmented by powered systems to help maintain position over the drill target. Semisubmersibles are used around the world in water depths up to 10,000 feet.
Drill Ship—Drill ships are basically oil rigs supporting equipment built into a ship. The drill ships are self-powered and completely independent. Like the semisubmersibles, drill ships are used in water depths of up to about 10,000 feet. They are able to stay in position through use of sophisticated GPS systems. Because they are mobile, they can move out of harm’s way to avoid severe weather events or relocate to a new field when required.
FPSO—FPSOs handle many of the operations contained on a drilling platform, except for the drilling. These activities include oil, gas and water separation and preparing gas and water for injection. The FPSO also adds storage of about 500,000 barrels of oil. This on-site storage capability negates the need for long offshore product pipelines because the oil in the FPSO is offloaded into transit tankers as needed.
FLNG—An FLNG ship contains all of the necessary processing equipment to convert natural gas to liquid natural gas (LNG) and then offload the LNG to LNG tankers. The on-site capability eliminates the need for expensive undersea natural gas pipelines that would be laid from the well to the onshore liquefaction plant.