Free Span Mitigation Methods

The DnV-RP-F105 (Free Span Pipelines, 2006) and DnV’s FatFree Program can be used to check for the maximum allowable free span length. If the actual free span length exceeds the maximum allowable free span length, the free span should be corrected using one of the mitigation methods below (also see Figure 12.3):
• Alteration of seabed (cut-off high seabed spots by plough or trencher)
• Concrete mattress or sand-cement bags
• Mechanical support
• Strakes or fairings

Figure 12.3 Examples of Free Span Mitigation Methods

(a) Mechanical support

(b) Strakes

(c) Fairings

Source:

Lee, Jaeyoung. 2007. Introduction to Offshore Pipelines and Risers.

Pipeline Fress Span Analysis

Pipeline free spans could exist at irregular seabed terrain or fault areas. The best way is to avoid free spans but if not avoidable, it is necessary to check if the anticipated free span length is acceptable for static and dynamic loads. The static loads include dead weight of the pipe and waves & current induced hydrodynamic load. Figure 12.1 shows one example of static pipe stress near free span areas. The dynamic loads come from

vortex induced vibration (VIV, see Figure 12.2) and fatigue damage.

Source:

Lee, Jaeyoung. 2007. Introduction to Offshore Pipelines and Risers.

CRA (Corrosion Resistant Alloy) Pipe

Depending on alloy contents, CRA pipe can be broken into follows:
• Stainless steel: 316L, 625 (Inconel), 825, 904L, etc.
• Chrome based alloy: 13 Cr, Duplex (22 Cr), Super Duplex (25 Cr), etc.
• Nickel based alloy : 36 Ni (Invar) for cryogenic application such as LNG (liquefied natural gas) transportation (-160oC)
• Titanium: Light weight (56% of steel), high strength (up to 200 ksi tensile), high corrosion resistance, low elastic modulus, and low thermal expansion, but high cost (~10 times of steel). Good for high fatigue areas such as riser touchdown region, stress joint, etc.
• Aluminum: Light weight (1/3 of steel), low elastic modulus (1/3 of steel), high corrosion resistance, but low strength (only up to 90 ksi tensile). Applications can include casing, air can, and risers.
Some key properties of each material are introduced in Table 7.2.1.

Depending on sour contents in the fluid, different chrome based alloy pipe should be selected as shown in Table 7.2.2.

Source:

Lee, Jaeyoung. 2007. Introduction to Offshore Pipelines and Risers. 

Pipeline Welding

• SMAW or Stick Welding

Shielded Metal Arc Welding (SMAW) is frequently referred to as stick welding. The flux covering the electrode melts during welding and this forms the gas and slag to shield the arc and molten weld pool. The slag must be chipped off the weld bead after welding.

• GMAW or MIG Welding

Gas metal arc welding (GMAW) uses an arc between a consumable constant filler metal electrode and the weld pool. Shielding is provided by an externally supplied shielding gas. This method is also known as metal inert gas (MIG) welding or metal active gas (MAG, i.e. carbon dioxide or oxygen) welding. GMAW consists of a DC arc burning between a thin bare metal wire electrode and the work piece. The arc and weld area are encased in a protective gas shield. The wire electrode is fed from a spool, through a welding torch which is connected to the positive terminal. The technique is easy to use and fast (high productivity) and there is no need for slag-cleaning since no flux is used. The MAG process is suitable for steel, low-alloy, and high-alloy based materials. The MIG process, on the other hand, is used for aluminum and copper materials.

• GTAW or TIG Welding

Gas tungsten arc welding (GTAW) is more commonly known as tungsten inert gas (TIG) welding. It is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The electrode used in  TAW is made of tungsten, because tungsten has the highest melting temperature among metals. As a result, the electrode is not consumed during welding, though some erosion (called burn-off) may occur. GTAW is most commonly used to weld thin sections of stainless steel and light metals such as aluminum, magnesium, and copper alloys. The process is known for creating stronger and higher quality welds than SMAW and GMAW. However, GTAW is comparatively more complex and difficult to master. It is also significantly slower than most other welding techniques.

Source:

Lee, Jaeyoung. 2007. Introduction to Offshore Pipelines and Risers. 

Underwater Welding

Underwater welding is performed while the welder is submerged, often at elevated barometric pressures.  This introduces a variety of challenges that require specialized skills and training that are taught at CDA Technical Institute (formerly Commercial Diving Academy).  Because of the adverse conditions and inherent dangers associated with underwater welding (also known as wet welding) divers must be trained to an exceptionally rigorous standard with highly specialized instruction.

Wet Welding

Welding underwater can be acheived by two methods: wet welding & dry welding. Wet welding entails the diver to perform the weld directly in the water. It involves using a specially designed welding rod, and employs a similar process used in ordinary welding. Here are advantages to wet welding:

• Cheap and fast
• high tensile strength
• ease of access to weld spot
• no habitat
• no construction

Dry Welding / Hyperbaric Welding

Another method of welding underwater is hyperbaric welding or dry welding. Hyperbaric welding is the process by which a chamber is sealed around the structure that is to be welded. It is then filled with a gas (typically mixture of helium and oxygen, or argon), which then forces the water outside of the hyperbaric sphere. This allows for a dry environment in which to perform the weld. Here are some advantages to dry welding:

• welder / diver safety
• higher weld quality
• surface monitoring
• non-destructive testing

Source:

https://www.commercialdivingacademy.com/underwater-welding.cms

Flange Connection

Flanges General

A flange is a method of connecting pipes, valves, pumps and other equipment to form a piping system. It also provides easy access for cleaning, inspection or modification. Flanges are usually welded or screwed. Flanged joints are made by bolting together two flanges with a gasket between them to provide a seal.

Materials for Flanges

Pipe flanges are manufactured in all the different materials like stainless steel, cast iron, aluminium, brass, bronze, plastic etc. but the most used material is forged carbon steel and have machined surfaces.

In addition, flanges, like fittings and pipes, for specific purposes sometimes internally equipped with layers of materials of a completely different quality as the flanges themselves, which are "lined flanges".

The material of a flange, is basically set during the choice of the pipe, in most cases, a flange is of the same material as the pipe.

All flanges, discussed on this website fall under the ASME en ASTM standards, unless otherwise indicated. ASME B16.5 describes dimensions, dimensional tolerances etc. and ASTM the different material qualities.

Dimensions of Flanges

Each flange according to ASME B16.5 has a number of standard dimensions. If a draftsman in Japan or a work preparer in Canada or a pipefitter in Australia is speaking about a Welding Neck flange NPS 6, Class 150, Schedule 40 ASME B16.5, then it goes over the flange which in the image here below is shown.

If the flange is ordered, the supplier want to know the material quality. For example ASTM A105 is a forged carbon steel flange, while A182 is a forged stainless steel flange.

So, in a correct order to a supplier two standards must be specified:

Bolted Flange connections

A bolted flange connection is a complex combination of many factors (Flange, Bolts, Process, Temperature, Pressure, Medium). All these various elements are interrelated and depend upon one another to achieve a successful result.
The reliability of the flanged joint depends critically upon competent control of the joint making process.

The industry has recognized the critical nature of installation and assembly for several years.
In Europe, the emphasis has been on ensuring that joint making is undertaken by trained and validated technicians and this has led to the publication of a European Technical standard: TS EN 1591 Part 4 entitled "Flanges and their joints. Design rules for gasketed circular flange connections. Qualification of personnel competency in the assembly of bolted joints fitted to equipment subject to the Pressure Equipment Directive (PED)".

The standard provides a methodology for the training and assessment of technicians involved in the making and breaking of flange joints and can be viewed as being analogous to the training required for welders involved with pressure vessel work. Its publication demonstrates the importance placed upon the competent control of joint making process in ensuring leak-free performance from the flange.

The gasket is but one of many reasons a bolted flange joint connection can leak.
Even when all the complex inter-related components of a bolted joint flange connection work in perfect harmony, the single most important factor leading to success or failure of that bolted flange connection will be attention given to properinstallation and assembly procedures by the person installing the gasket. If done properly, the assembly will remain leak-free for the target life expectancy.

Sumber:

http://www.wermac.org/flanges/flanges_pipe-connections_pipe-flanges.html

Pigging Procedure

(Sumber: http://www.tedberrycompany.com/wpdemo/wp-content/uploads/2013/10/piggingLaunch_and_Retrieval_Methods-1.jpg)







Source:
Guo, Boyun. (2005) Offshore Pipeline. Gulf Professional, U.S.A