Spiral Pipe for Offshore Application

In certain parts of the world it is highly relevant to use spiral welded pipes for offshore applications. This is driven by cost, project characteristics and the desire to manufacture the pipe close to where it is to be used. Spiral welded line pipe has been used extensively for onshore applications, however there has been some reluctance to specify spiral welded line pipe for offshore applications. A joint industry project is beeing carried out together with coil manufacturers, pipe manufacturers, installation contractors and operators to review the status regarding offshore applications for spiral welded pipes and identify the most critical technology gaps using a technology qualification process. Detailed suggestions as to how the gaps can be met have been made. An update on efforts to close these gaps is ongoing. The challenges for spiral welded line pipe include design, metallurgical and quality control issues. The design issues include fracture arrest, collapse and displacement controlled loading conditions which are all highlighted in DNVs standard for submarine pipelines (DNV OS F101). The design issues regarding load controlled displacement are mainly due to limited experience with spiral welded line pipe subjected to large strains. For running fracture the limited experience with spiral welded pipe for offshore applications is an issue.There are 5 new spiral welded pipe mills in United States so availability has improved. The review includes an assessment of typical pipe material test results and whether properties required for offshore applications can reasonably be expected.

Introduction
Det Norske Veritas (U.S.A.), Inc. (DNV) and MCSKenny are carrying out a joint industry project (JIP) to investigate the suitability of spiral welded pipe for offshore applications. It appears that the industry has a general understanding that the performance of spiral welded (SAWH) pipes is different to Submerge Arc Welded (SAWL)/ High Frequency Welded(HFI)/ Electric Resistance Welded ERW linepipe when exposed to the same loading conditions, and that currently existing design standards for offshore applications may not be applicable. An important issue is to establish how the spiral wound linepipe can be produced consistently to a high level of quality, and what is required by the design standard for spiral welded pipe to be fit for purpose for offshore use. Some of the main areas of concern regarding the quality of spiral wound linepipe will be discussed. The aim is to assess whether SAWH linepipe can be considered equivalent to SAWL and HFI/ERW linepipe. The use of spiral welded linepipe (SAWH) for pipelines has generally been the most popular manufacturing choice of linepipe for onshore low pressure pipelines, pipelines transporting water, ship borne piping, or very shallow water, low pressure pipelines (≤ 500 ft).

Recently there has been more interest in the use of spiral wound linepipe, due to the following reasons:
• There are five new SAWH pipe mills in America with “state-of-the-art” technology.
• SAWH linepipe is a cost-effective solution compared to the other manufacturing processes.
• Generally, the chemical compositions, mechanical properties and dimensional tolerances are assumed to be
comparable to SAWL pipe.
• SAWH linepipe can be manufactured in 80 ft lengths with diameters from 20 to more than 100-inch OD and wall
thicknesses ranging from approximately 9 to 25 mm.
• Some SAWH pipe mills have coating capabilities for 80 ft pipe lengths (FBE and 3-layer coating systems). 80 ft
pipe lengths could mean less fabrication costs for the installation contractors.

Source: Spiral Wound Linepipe for Offshore Applications

G. Heiberg, Det Norske Veritas, A. Eltaher, MCSKenny, P. Sharma, Det Norske Veritas, P. Jukes, MCSKenny, M. Viteri Det Norske Veritas

On-Bottom Stability of Offshore Pipeline

One important aspect in designing an offshore pipeline system is its stability for being underwater, on the seabed for a life time service (operation). The analysis of keeping the pipeline system remained on the seabed is known as On-Bottom Stability. There are few methods to maintain pipeline at the seabed, such as pipe burial, trenching, as well as building a rock berm, and thicken the concrete coating. On-bottom stability consists of vertical stability and dynamic lateral stability.

Vertical Stability

Total pipe weight is the weight of the pipe alloy steel material, anti-corrosion coating, and field joint coating. A cross-section of an offshore pipeline can be seen through the image below:

In order to avoid floatation in water, the submerged weight of the pipeline shall meet the following criteria:

where:

ΥW = safety factor

b = pipe buoyancy per unit length : ρw • g • ∏ • D2 / 4

ρw = mass density of water

g = acceleration of gravity

D = pipe outer diameter (including all coating)

ws = pipe submerged weight per unit length

sg = pipe specific density : (ws+b)/b

If a sufficiently low probability of negative buoyancy is not documented, the safety factor ΥW = 1.1 can be applied.

Dynamic Lateral Stability

The objective of a dynamic lateral stability analysis is to calculate the lateral displacement of a pipeline subjected to hydrodynamic loads from a given combination of waves and current during a design sea state. On-bottom stability is a highly non-linear phenomenon with a large degree of stick/slip response. This is particularly important to keep in mind for large values of current to wave ratios and large wave periods, and more so for stiff clay and rock than for soft clay and sand where the build up of penetration and passive resistance is more pronounced.

1. Current Condition

The steady current flow at the pipe level may have components from:

• Tidal current
• Wind-induced current
• Storm surge induced current
• Density driven current

2. Short Term Wave Condition

The wave induced oscillatory flow condition at the pipe level may be calculated using numerical or analytical wave theories. The wave theory shall be capable of describing the conditions at the pipe location, including effects due to shallow water, if applicable. The short-term, stationary, irregular sea states may be described by a wave spectrum Shh(ω) i.e. the power spectral density function of the sea surface elevation. Wave spectra may be given in table form, as measured spectra, or in an analytical form.

For instance, the JONSWAP spectrum, the spectral density function reads:

3. Forces Affecting Pipeline On-Bottom Stability

• Hydrodynamic force, consists of drag force and inertia force (can be calculated using morrison formula), as well as lift force. Lift force is a vertical hydrodynamic force. This would happen with the concentration of streamline on the pipe.
• Soil friction force, is a horizontal force influenced by friction coefficient between pipe and seabed. Value of the friction coefficient depends on the seabed soil characteristics. For example, friction coefficient for clay-soil is 0.2 and friction coefficient for sandy-soil is 0.6.

Source: https://nonerieska.wordpress.com/2013/01/30/on-bottom-stability-of-offshore-pipeline/ January 2015

Offshore Pipeline Corrosion Prevention

Corrosion can be defined as the destruction or deterioration of a material because of reaction with its environment. Corrosion is a natural occurance and inevitable. Especially in seawater environment, corrosion is a threat for carbon steel pipe (offshore pipeline). Corrosion will damage pipeline and leads to pipe leak in which will be dangerous for the circumstances surround. Petroleum industry spends a million dollars per day to protect its pipelines. And so, there is urgency to protect and prevent pipeline from corrosion.

There are several methods that can be used to prevent and decrease the rate of corrosion on offshore pipeline. These methods are:

Material Selection

This method is just simply selecting the best and appropriate alloy carbon steel to a particular environment. For instance, the use of nickel-based alloy steel allows pipeline to withstand seawater environment without putting additional sacrificial anodes or impressed current, yet it’s far more expensive than having ordinary carbon steel with cathodic protected.

Use of Inhibitor

Sometimes corrosion in offshore pipeline attacked from inside (compounds brought by the fluid inside pipe e.g. sulphate). This can be helped by adding inhibitor. Inhibitor is a substance that when added in small concentrations to an environment, decreases the corrosion rate, such as chromate and nitrate.

Cathodic Protection

Cathodic protection is achieved by supplying electrons to the metal structure to be protected. Basically, cathodic protection has the pipeline become cathode, instead of anode, that way it won’t be corroded. There are two ways to cathodically protect a stucture. Firstly, Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP).

1. Impressed Current Cathodic Protection (ICCP)

ICCP supplies electron by flowing electrical current from a power supply. This method is suitable for large structures regarding cost.

For pipelines, anodes are arranged in groundbeds either distributed or in a deep vertical holes depending on several design and field condition factors including current distribution requirements.

2. Sacrificial Anode Cathodic Protection (SACP)

This method is also known as Galvanic Coupling. In the usual application, a galvanic anode, a piece of a more electrochemically “active” metal, is attached to the vulnerable metal surface where it is exposed to the corrosive liquid. Galvanic anodes are designed and selected to have a more “active” voltage (more negative electrochemical potential) than the metal of the target structure.

Coating

Relatively thin coatings of metallic and inorganic materials can provide a satisfactory barrier between metal and its environment. The chief function of such coatings is to provide an effective barrier.

Coating can be in the form of, for example, cladding. Cladding involves a surface layer of sheet metal put on by rolling two sheets of metal together. For instance, a nickel and a steel sheet are hot-rolled together to produce a composite sheet with, say, 1/8 inch of nickel and 1 inch of steel. This way the steel are protected with its environment since nickel is layered on the surface. Moreover, in the application for offshore pipeline, high density polyethylene (HDPE) and polypropylene layer can be coated on pipe bare surface. Both HDPE and polypropylene coating have low water permeation which will improve isolation of the pipe from seawater surrounds. Coating for pipeline is illustrated as below:

Source: https://nonerieska.wordpress.com/2013/01/30/offshore-pipeline-corrosion-protection-and-prevention/ January 2015

Flexible Pipe

In the advanced development of technology, oil and gas industry explores further from shore and deeper to the basin. It is inevitable that the deeper it goes, more pressure it receives. This become a concern in planning a safe pipeline system. For that reason, a new kind of offshore pipeline is invented, the flexible pipe.

A flexible pipe is made up of several different layers. The main components are leak-proof thermoplastic barrier and corrosion-resistant steel wires. The helically steel wires give the structure its high pressure-resistant and excellent bending characteristic, so it provides flexibility.

A figure of typical flexible pipe.

Main Components

Figure below identifies the main components of flexible pipe cross section:

1. Interlocked stainless steel carcass
2. Internal pressure sheath, made from nylon, poly vinylidene flouride (PVDF) and high density polyethylene (HDPE)
3. Zeta spiral (pressure armour), made from rolled carbon steel
4. Tensile armour (double cross wound armours), made from flat rectangular wires
5. Outer thermoplastic sheath, made from non-metallic materials

Main Characteristics

• Flexible. Makes it possible to spool the pipe on a reel or in a carousel for efficient and quick transportation and installation.
• Easy to install. Since the flexible pipe is in continuous form, laying speed will be much faster than laying ordinary rigid carbon steel pipeline.
• Modularity. The independent layers of a flexible structure enable it to be tailored to the precise needs of a specific development.
• Corrosion resistant. Since the steel wires are not in direct contact with the conveyed fluid, they do not require the same corrosion resistance as steel pipe.
• High pressure resistant. Flexible pipes resist all fluid pressures encountered in the most complex subsea application. Besides, the modularity of the flexible pipe manufacturing process enables to adjust pipe thickness, shape, and number of steel wire layers to satisfy a specific requirement.

The image above shows the physical form of flexible pipe.

Source: https://nonerieska.wordpress.com/2013/01/31/offshore-flexible-pipe/ January 2015

Upheaval Buckling of Offshore Pipeline

When production starts through a pipeline, internal temperature and pressure will rise. The temperature increase will lead to thermal expansion of the steel. A pipeline will be restrained variously along the routing due to soil friction, and the temperature rise will result in axial compressive forces in the pipe. As a response to the longitudinal compressive force interacting with local curvature of the pipe, global buckling may occur.

A pipeline can buckle downwards in a free span, sideways on the seabed or upwards for buried pipelines. Vertical buckling of a pipeline is called upheaval buckling, and the direction of the buckle is upwards because this is the way of least resistance. If a vertical buckle leads the pipe into exposure on the seabed, this is a severe problem. An expensive and time consuming operation is needed to re cover the pipe at this location. If the buckle damages the pipeline, this part must be replaced before re covering takes place.

Figure describes upheaval buckling on buried pipe.

For upheaval buckling to occur, the pipeline must first have an initial imperfection. Imperfections are typically due to the pipeline being laid over a boulder or due to irregularities in the seabed profile.

Figure below illustrates a sequence of events which initiates buckling in a buried pipeline:

The pipeline is laid across an uneven seabed (a) and later trenched and buried (b). The trenching and burial operations modify the profile of the foundation on which the pipe is resting, so that it is not precisely the same as the original profile. Trenching may smooth the profile overbends, but may also introduce additional imperfections, if, for instance, a lump of bottom soil falls under the pipe.

The occurrence of an upheaval buckle is highly depending on the smoothness of the seabed profile. According to the DNV-RP-F110 (Global Buckling of Submarine Pipelines), it gives criteria to avoid upheaval buckling from occurring by designing sufficient cover providing enough resistance for pipelines to remain in place. Therefore, upheaval buckling is considered as an ultimate limit state (ULS) in the RP.

Source:

https://nonerieska.wordpress.com/2013/02/01/upheaval-buckling-of-offshore-pipelines/ January 2015

Pipeline Free Span Fatigue Analysis

Construction of unburied pipeline is the most common method in offshore pipeline system. Unburied pipeline should be designed appropriately due to the bathymetry condition. And it is inevitable founding the existence of free span. Free spanning in offshore pipelines mainly occurs as a consequence of uneven seabed and local scouring due to flow turbulence. An illustration of free span is showed by the figure below:

According to Fredso and Sumer (1997), resonance is the main problem for offshore pipelines laid on the free spanning. Resonance happens when the environment’s frequency becomes equal to the pipe natural frequency. Resonance may lead to develop more fatigue on pipelines. In order to reduce the risk caused by free spanning, a maximum allowable length of free span should be determined. Span length is described with the following image:

An allowable length of free span can be calculated by the following formula (DNV 1998 & ABS 2001) :

in which E = modulus of elasticity; I = bending moment of inertia pipeline; C = coefficient of seabed condition; Vr = reduced velocity (Fredso and Sumer, 1997).

Vr defined as:

where U = streamwise flow velocity; D = outer diameter of pipe; me = effective mass (including structural mass, mass of content and added mass); fn = natural frequency of the pipe free span.

Natural frequency of free span pipe defined as:

In practice, the use of these formula for estimation of maximum free span length is not very applicable since there is difficulties in determining the exact seabed conditions.Therefore, different approaches usually adopted. One of the method is modal analysis.

Modal Analysis

Natural frequency of pipelines can be obtained using the Euler-Bernoulli beam equation which is defined as (Xu et al, 1999 and Bai, 2000):

with y = in-line displacement of pipe; x = position along the pipe span; t = time; C = total damping ratio; T = axial force of pipe (positive under tension); and F(t,u,y) = total external forces.

External forces and damping ratio only influence the resonance amplitude, so it can be ignored and the pipe free vibration equation is expressed in the following equation:

There are several codes that can be used as reference containing free spanning on offshore pipeline, like DnV RP F105 (Pipeline Free Spanning) and API RP 11 11, 1999.

PREVENTION

In order to prevent crack due to free spanning, supports can be made to reduce the stress on the free span area. These supports include sand-filling or mini structure. A mini structure is shown in figure below:

Source:

https://nonerieska.wordpress.com/2013/01/31/free-span-fatigue-analysis/ January 2015

Cathodic Protection for Offshore Pipelines

Deepwater designs and manufactures retrofit anode systems to extend the life of aging offshore pipelines.

Deepwater provides cathodic protection for new and aging offshore pipelines all over the world. In addition to standard bracelet anodes, we produce retrofit systems that replace or supplement anodes on mature pipelines needing a boost in cathodic protection levels or to fully replace systems needing life extension. With the majority of the world’s offshore infrastructure beginning to reach the end of its original design life, it is more important than ever to address corrosion control and asset integrity for offshore pipelines.

Our N.A.C.E.-certified team of corrosion engineers and cathodic protection designers have engineered anode systems for offshore pipelines at all depths and in all types of conditions. Our team has the expertise to provide complete project management from design to installation, whether for a new build or for extending the life of an aging onshore or offshore asset. With a variety of proprietary systems and a long, proven track record of dependability and cost savings, Deepwater is way ahead in knowledge and experience.

For new pipelines – For new pipeline projects, we provide stockaluminum anode bracelets made at one of our certified ISO 9000 anode foundries. All foundry facilities are located only a few miles from where the design team and engineers manage each project, ensuring that strict QA/QC oversight is observed for all anode materials used in our systems.

For aging pipelines – For mature or damaged pipelines or lines in need of supplemental cathodic protection, we have two anode systems: RetroSled for standard bottom conditions, and SmartMat for difficult conditions, pipeline crossings and other special circumstances.

The RetroSled system is a standard rigid (or expanding) aluminum anode sled, installed on the sea floor next to the pipe and attached via diver or ROV with the RetroClamp. The SmartMat is a concrete mattress with integral aluminum or zinc anodes that simultaneously stabilizes, provides separation between lines and delivers cathodic protection current through a RetroClamp connection.

For hot spots – To focus a small amount of cathodic protection on a localized problem area (or “holiday”), the RetroClamp CP operates like a normal RetroClamp, except it has a small amount of anode material electrically attached to the body of the clamp, providing a small diver-or-ROV-installable clamp-on anode.

Source: http://www.stoprust.com/pipelinecp.htm January 2015