Pipeline Ending Manifold (PLEM)

Subsea manifold is a flow-routing subsea hardware (subsea flow router) that connects between subsea trees and flowlines. It is used to optimize the subsea layout arrangement and reduce the quantity of risers connected to the platform. If connected to dual flowlines, the manifold can typically accommodate pigging and have the capability of routing production from a particular tree to a particular flowline.

Pipeline End Manifold (PLEM)

It a simpler version of a cluster manifold generally designed to direct fluids for only one or two subsea Christmas trees. A PLEM generally connects directly to a subsea flow line without the use of a pipeline end termination (PLET).

Manifold Compenents 

A manifold is typically composed of the following major components:

• Pipework and valves – contains and controls the production and injection fluids.
• Structure framework – protects and supports the pipework and valves.
• Subsea connection equipment – allows subsea tie-in of multiple pieces of equipment. Types include vertical, horizontal and stab-and-hinge-over connections.
• Foundation – interface between the manifold structure and seabed.
• Controls Equipment – allows the remote control of any hydraulically actuated subsea manifold valves and the monitoring of production and injection fluids. Control pods may be either internal or external to the manifold.

Valves

Valves on the manifold are essential for directing and controlling the flows. They can be either manual or hydraulically actuated. Sometimes chemical injection valves are placed on the manifold as well.

• Branch valves are generally slab type gate valves (similar to tree valves). Their sizes are based on the production/injection tree size.
• Flowline header valves are also gate type, but ball valves have been used previously. Their sizes are based on the flowline size.
• Materials are chosen for compatibility with production and injection fluids. Most of time, it is CRA-clad.
• Double barrier philosophy generally used against production fluids.
  -Two valves in series
  -One valve and one pressure cap
  -Primary seal is generally a metal-to-metal seal

Pipework

A wide range of pipework configurations is possible. Each header connects to an individual flowline. the pipework sizing is based on the tree piping size and the flowline diameters. The main circuit is designed to accommodate pigging operations. The material of construction needs to be compatible with production and injection fluids.

• Test headers can be incorporated to test individual or groups of trees
• Test headers can be a second or even third header isolated in the manifold
• Insulation may be required for unscheduled or emergency shutdowns

Control System

Control system for the manifolds is the same as the control system for the trees. Multiple options for the control system have been used in the manifold design

• No controls on the manifold. The manifold is controlled by tree subsea control modules (SCMs).
• SCMs on the manifold.
• Manifold with control system distribution units with flying leads going to trees.

Framework Structure

The framework is a welded structure to provide support for the pipework and valves and contain the foundation interface structure. The pipework is allowed to float inside the framework within limits and it is not rigidly attached to the frame. The frame can also be used for lifting and landing of the jumper tie-in tools.

Foundation

• Mud mats – a simple foundation resting directly on the seabed, generally with a short skirt around the perimeter to resist lateral loads.
• Piles – long cylindrical structures embedded into the soil intended to hold a subsea structure above the seabed. Foundations may utilize one or more individual piles.
• Intermediate Structures – an intermediate structure can be used to interface a subsea manifold with a pile foundation to reduce weight of the manifold structure or to ease retrieval of the manifold. Intermediate structures can be either retrievable or permanent structures.

Tie-ins to wells and flowlines

The tie-in hubs placed on the outer edge of the manifold, which are used to tie-in jumpers that bring in fluid from the production wells and export fluid into the flowlines (production manifold). The tie-in sizing is based on the tree piping size and the flowline diameters. and the loads applied from the flowlines

Insulations

Generally gas manifolds are not insulated and oil manifolds are insulated. For oil production, insulation is necessary to allow adequate cool-down time to treat or remove trapped production water. Gas production is generally treated continuously with chemicals to prevent hydrates.

Deployment method

The following vessels are typically used for manifold deployment:

• Drill Rig: through moon pool or keel-hauled on drill string
• Heavy Lift vessels (Derrick Barges): through moon pool or over side
• Work-class vessels: over side on crane or winch

The following equipments are typically required:

• Manifold hydraulic installation tool
• Sling sets, either wire rope or synthetic fiber

Applicable API Specs

• API Spec 17P – Templates and Manifolds
• API Spec 17D – Specifications for subsea wellhead and Christmas tree equipments
• API Spec 17A – Recommended practice for design and operation of subsea production systems
• API Spec 17H, ISO 13628-8 – ROV Interfaces

Source : http://www.piping-engineering.com/pipe-flanges-types-systems.html

Pipeline Upheaval Buckling

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.

Source : http://brage.bibsys.no/uis/bitstream/URN:NBN:no-bibsys_brage_25027/1/Ommundsen,%20Marius%20Loen.pdf

http://www.engr.mun.ca/~spkenny/Courses/Undergraduate/ENGI8673/Reading_List/1990_Palmer_Upheaval_Buckling.pdf

Pipe in Pipe

Pipe in pipe systems allow a range of advanced and highly efficient insulation materials to be used to achieve Overall Heat Transfer Coefficients less than 1 W/m2 K. These systems are important components of subsea developments where untreated well fluids may have to be transported large distances and wax and hydrate problems have to be managed. However, as a result of this efficient insulation, thermal expansion challenges are increased and techniques such as probabilistic analysis, upheaval buckling design, snake lay or cooling spools employed to mitigate high expansion loads. 
CHALLENGES 

• High Temperature Well Fluids 

High temperature well fluids increase the overall levels of energy in the system increasing the risk of uncontrolled buckling of the flowlines and increasing project costs through the use of pipeline anchors, rockdump or trenching, more exotic materials and the concern that the predicted design temperature is outside the range of applicability of existing design codes. 

• Wax/Asphaltene/Hydrate formation within product stream 

In heavy oils or multiphase pipelines excessive cooling of the product during transportation can result in drop out of high molecular weight waxes and asphaltenes. Increasing the operation increasing the operational pigging requirement of the system, increasing OPEX. In wet gas systems hydrate formation during startup or blow-down conditions can block pipelines. 

• Long distance tie-backs 

Long tie back from a subsea facility to the host platform increases the length of time that the production fluid is subjected to heat loss through the pipeline wall, increasing the risk of flow assurance issues, particularly in the event of shutdown condition and extended residence time of thefluid in the pipeline. 

• Low OHTC value requirement (<1 W/m2K) 

For long tie backs or production fluids with high critical temperatures the Overall heat transfer Coefficient requirements of the system can be onerous. Requiring conventional wet insulation thickness in excess of practical thicknesses for application or installation. 
SOLUTIONS 

• Use of Cooling Spools 

There can be significant cost benefit from cooling the product stream from very high temperature wells to minimise the thermal expansion forces and then maintaining this lower temperature for through efficient pipeline insulation reducing the volume of post lay rock dump or trenching required or enabling more conventional materials and analysis techniques to be employed. 

• Pipe In Pipe 

Installing a second pipeline around the product pipeline isolates the carrier pipeline from the seawater surrounding it and creates a dry chamber around the pipeline that can be engineered to accommodate a range of advanced insulation techniques. 

• Materials 

Typically pipeline insulation must be able to withstand the stresses imposed as a result of the installation methods and also be strong enough to withstand constant external pressure and function effectively when submerged and saturated. The dry, load-free environment within the annulus of the pipe allows non-typical insulation materials with much lower thermal conductivities to be applied subsea than has historically been possible e.g. rock-wool systems etc. 

Source : http://www.jpkenny.com/SiteCollectionDocuments/PIPE%20IN%20PIPE%20REV%2001.pdf

Vortex Induced Vibration on Pipeline

Before discussing about VIV, we may take a look at flow around circular cylinder. Flow around cylinder usually characterized by Reynold number (based on the diameter of the circular member).

Reynold number is defined as:

where v = mean velocity of the object relative to the fluid; L = characteristic linear dimension; μ = dynamic viscosity of the fluid; ν = kinematic viscosity; ρ = density of the fluid.

At very low Reynold numbers, the streamlines of the resulting flow is perfectly symmetric as explained in potential theory. However, with the increasing Reynold number, the streamline becomes assymetric.

When a body is immersed in a fluid and is in relative motion, the drag is defined as the component of the resultant force working on the body, in the direction of the relative motion. Drag = pressure drag + skin friction drag.

Flow past a circular cylinder:

Reynold number < 0.5; therefore inertia effect can be ignored and pressure recovery is nearly complete.

Reynold number ranged between 5 – 40; in this case, separation of boundary layers occurs.

With the increasing Reynold number, there is tendency of eddies elongation which then begin to oscillate until Reynold number of 90, depending on free stream turbulence level. The streams break away from the cylinder.

With further additional of Reynold number, the laminar streamline transforms into turbulence.

Then, as Reynold number getting bigger, separation of boundary layer begin to existence.

Then, vortex begins to appear and takes place in special flow velocities (according to the size and shape of the cylindrical body). In this flow, vortices are created at the back of the body and detach periodically from either side of the body.

VORTEX-INDUCED VIBRATION (VIV)

Vortex-Induced Vibration, abbreviated as VIV, are motions induced on bodies facing an external flow by periodical irregularities on the flow. A simple example of VIV is an underwater cylinder, offshore pipelines.

VIV happens when the vortices are not formed symmetrically around the body (with respect to its mid plane), different lift forces develop on each side of the body, and leading to motion transverse to the flow. This motion changes the nature of the vortex formation leading to a limited motion amplitude.

There are two types of VIV, self-excited oscillations and forced oscillation.

1. Self-excited oscillations

This type of VIV occurs naturally. For instance, when the vortex-shedding frequency and the natural frequency are approximately the same. This is the real VIV, vortex-induced vibration.

2. Forced oscillations

This VIV occurs at velocities and amplitudes which are preset and can be controled indepedently of fluid velocity. This is not the “real” VIV, this is vibration-induced vortices.

In order to prevent VIV phenomenon, some offshore structures are design with strakes to suppress VIV. Strakes can be seen in the following figure:

Sources : http://www.leb.eei.uni-erlangen.de/winterakademie/2007/report/content/course01/pdf/0117.pdf

http://en.wikipedia.org/wiki/Vortex-induced_vibration

http://web.mit.edu/towtank/www/viv.html

http://en.wikipedia.org/wiki/Reynolds_number

Pipeline Hot Tap Installation

Pipeline hot tap is a connection made to an existing pipeline without the interruption of emptying that section of pipe. This means that a pipe can continue to be in operation whilst maintenance or modifications are being done to it. The process is also used to drain off pressurised casing fluids.

Typical connections consist:

• Tapping fittings like Weldolet®, Reinforced Branch or Split Tee. Split Tees often to be used as branch and main pipe has the same diameters.
• Isolation valve like Gate or Ball valve.
• Hot tapping machine which includes the cutter, and housing.

Mechanical fittings may be used for making hot taps on pipelines and mains provided they are designed for the operating pressure of the pipeline or main, and are suitable for the purpose.

• Design: ASME B31.1, B31.3, ASME B31.4 & B31.8, ASME Sec. VIII Div.1 & 2
• Fabrication: ASME Sec. VIII Div.1
• Welding: ASME Sec. IX
• NDT: ASME Sec. V

There are many reasons to made a Hot Tap. While is preferred to install nozzles during a turnaround, installing a nozzle with equipment in operation is sometimes advantageous, especially if it averts a costly shut down.

Remarks before made a Hot Tap

• A hot tap shall not be considered a routine procedure, but shall be used only when there is no practical alternative.
• Hot Taps shall be installed by trained and experienced crews.
• It should be noted that hot tapping of sour gas lines presents special health and metallurgical concerns and shall be done only to written operating company approved plans.
• For each hottap shall be ensured that the pipe that is drilled or sawed has sufficient wall thickness, which can be measured with ultrasonic thickness gauges. The existing pipe wall thickness (actual) needs to be at least equal to the required thickness for pressure plus a reasonable thickness allowance for welding. If the actual thickness is barely more than that required for pressure, then loss of containment at the weld pool is a risk.
• Welding on in-service pipelines requires weld procedure development and qualification, as well as a highly trained workforce to ensure integrity of welds when pipelines are operating at full pressure and under full flow conditions.

Hot Tap setup

For a hot tap, there are three key components necessary to safely drill into a pipe; the fitting, the valve, and the hot tap machine. The fitting is attached to the pipe, mostly by welding.
In many cases, the fitting is a Weldolet® where a flange is welded, or a split tee with a flanged outlet (see image above).

Onto this fitting, a valve is attached, and the hot tap machine is attached to the valve. For hot taps, new bolts, gaskets and a new valve should always be used when that components will become part of the permanent facilities and equipment.

The fitting/valve combination, is attached to the pipe, and is normally pressure tested. The pressure test is very important, so as to make sure that there are no structural problems with the fitting, and so that there are no leaks in the welds.

The hot tap cutter, is a specialized type of hole saw, with a pilot bit in the middle, mounted inside of a hot tap adapter housing.

The hot tap cutter is attached to a cutter holder, with the pilot bit, and is attached to the working end of the hot tap machine, so that it fits into the inside of the tapping adapter.

The tapping adapter will contain the pressure of the pipe system, while the pipe is being cut, it houses the cutter, and cutter holder, and bolts to the valve.

Hot Tap operation

                       Split Tee 

The Hot Tap is made in one continuous process, the machine is started, and the cut continues, until the cutter passes through the pipe wall, resulting in the removal of a section of pipe, known as the "coupon".

The coupon is normally retained on one or more u-wires, which are attached to the pilot bit. Once the cutter has cut through the pipe, the hot tap machine is stopped, the cutter is retracted into the hot tap adapter, and the valve is closed.

Pressure is bled off from the inside of the Tapping Adapter, so that the hot tap machine can be removed from the line. The machine is removed from the line, and the new service is established.

Hot tapping is also the first procedure in line stopping (isolation of a piping system to provide a shut off where none exist) , where a hole saw is used to make an opening in the pipe, so a line plugging head can be inserted, as can be seen from the figure above. Physically hot tap is figured as below:

Sources : http://en.wikipedia.org/wiki/Hot_tapping

http://en.wikipedia.org/wiki/Line_stopping

http://www.wermac.org/specials/hottap.html

Offshore Pipeline Corrossion 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:

Sources :

http://en.wikipedia.org/wiki/Cathodic_protection

http://met-engineering.blogspot.com

http://www.tenaris.com/en/Products/OffshoreLinePipe/Coating/ExternalAnticorrosion.aspx

Offshore Pipeline Route Selection

Planning and designing are the first things to do before installing offshore pipeline system. Before the pipeline design is made, a series of surveys is done to determine the best plan (the safest, the cheapest, the most environmental friendly) for the pipeline project. One aspect of planning that impacts a pipeline project from beginning to end, is it’s route selection.

In the planning stage, there is one key component that all pipeline projects have in common-how the initial routing of the pipeline will affect the eventual interface of all activities required for the project.

Pipeline route selection or routing is choosing the best route or path for the pipeline system to be outstretched toward the existing bathymetry and platform location. There are some criteria to be considered in routing, such as:

• The safest
• The shortest
• The easiest to install
• The minimum cost

THE SAFEST

Safety first. That would probably be the most crucial thing in the whole process of producing the pipeline system. The safest route should be considered due to its minimum risk and impact for human and for the existing environmental surround. Moreover, existing landscape should be taken into account. Troughs, volcanoes, scarps, faults, and other extreme landscape (including geohazard) should be avoided in selecting the pipeline route.

THE SHORTEST

The shortest means the most efficient and effective route. Minimum material will be needed, minimum pressure loss, as well as minimizing installation risk. Please note that longer pipeline will be more susceptible to pressure loss. If long pipeline route shall be installed, then putting some additional compressor along will be necessary.

THE EASIEST TO INSTALL

As told before, installing offshore pipeline system is not an easy task. Therefore we should manage the simplest way to install them offshore.

THE MINIMUM COST

If the three criteria mentioned before is obeyed, then it’s possible to have the minimum cost all the way.

Source : http://www.offshore-mag.com/articles/print/volume-71/issue-1/flowlines-__pipelines/survey-assesses-geohazards-for-record-subsea-pipeline.html

http://www.mustangeng.com/AboutMustang/Publications/Publications/Pipeline%20Route%20Selection%20rev%2012-7-11.pdf