Pipeline Thermal Insulattion and Pipe in Pipe (PIP) as a Solution

Pipeline thermal insulation is a prevention due to temperature difference between pipeline outer surface and inner part of the pipeline. Pipeline thermal insulation is considered as important since its contribution to corrosion as well as wax and hydrate problems in offshore pipeline system.

1. Condensation That Lead to Corrosion                                                                                       Pipes operating at relatively low temperature (with hot fluid flows inside, e.g. offshore pipeline) increases the potential for existing water vapour to condense on pipe surface. And this moisture may lead to corrosion on the pipeline surface.

2. Wax and Hydrate Formation

In oil and gas industry, excessive cooling of the product during transportation can result in drop out of high molecular weight waxes and asphalt. This happens due to the working temperature of the pipeline (deep in the depth, above the seabed where the temperature is relatively low). In wet gas systems, hydrate formation can block pipelines (flow).

Offshore Pipeline Corrosion Protection and 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).

Pipeline Ending Manifold (PLEM)/PLET

Pipeline end terminators (PLET)/pipeline end manifold (PLEM), and inline structures (ILS) are subsea structures designed to attach the pipeline end and then lowered to the seabed in the desired orientation. The PLET/PLEM is located at the end of a subsea pipeline, while the inline structure is located in the middle of pipeline.

The design and installation ofe PLET/ILS include first-end, middle, and second-end options. The components of them may include from a single hub with manual isolation valve, to two or three hubs with ROV actuated valves, chemical injection, pig launching capabilities and more. The foundation of PLET/ILS may be a mudmat, or a single suction pile. A rigid or flexible jumper is utilized to tie-in the PLET/ILS to the other subsea structures, eg. Tree, manifold, or other PLET/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).

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Horizontal Directional Drilling

Horizontal directional drilling (HDD) was pioneered in the United States in the early 1970s by an innovative road boring contractor who successfully completed a 183 m (600 ft) river crossing using a modified rod pushing tool with no steering capability (DCCA 1994). By integrating existing technology from the oil well drilling industry and modern surveying and steering techniques, today's directional drilling methods have become the preferred approach for installing utility lines, ranging from large-size pipeline river crossings to small-diameter cable conduits.

The HDD industry is divided into three major sectors--large-diameter HDD (maxi-HDD), medium-diameter HDD (midi-HDD), and small-diameter HDD (mini-HDD, also called guided boring)--according to their typical application areas. Although there is no significant difference in the operation mechanisms among these systems, the different application ranges often require corresponding modification to the system configuration and capacities, mode of spoil removal, and directional control methods to achieve optimal cost-efficiency. 

Directional drilling methods utilize steerable soil drilling systems to install both small- and large-diameter lines. In most cases, HDD is a two-stage process. Stage 1 involves drilling a pilot hole approximately 25 to 125 mm (1 to 5 in) in diameter along the proposed design centerline. In stage 2, the pilot hole is enlarged to the desired diameter to accommodate the pipeline. The pilot hole is drilled with a surface-launched rig with an inclined carriage, typically adjusted at an angle of 8 to 18 degrees with the ground for entrance and 8 to 12 degrees for exit angle (Miller the Driller 2002). The preferred minimum radius in feet for steel pipe is typically 100 times the diameter of pipe in inch. For plastic pipe, the multiplication factor is 40, i.e., 40 times of diameter of pipe in inch.

Most systems adopt either fluid-assisted drilling or a high pressure fluid jetting method to create or enlarge the bore hole. In a few instances, some mini-HDD systems utilize dry bore systems (with compressed air) in hard, dry soils and calcified or soft rock formations.

Pipeline Free Span Mitigation

During pipeline routing evaluation, consideration has to be given to the shortest pipeline length, environment conservation, and smooth sea bottom to avoid excessive free spanning of the pipeline. If the free span cannot be avoided due to rough sea bottom topography, the excessive free span length must be corrected.

Free spanning causes problems in both static and dynamic aspects. If the free span length is too long, the pipe will be over-stressed by the weight of the pipe plus its contents. The drag force due to near-bottom current also contributes to the static load. To mitigate the static span problem, mid-span supports, such as mechanical legs or sand-cement bags/mattresses, can be used.

Free spans are also subject to dynamic motions induced by current, which is referred to as a vortex induced vibration (VIV). The vibration starts when the vortex shedding frequency is close to the natural frequency of the pipe span. As the pipe natural frequency is increased, by reducing the span length, the VIV will be diminished and eliminated. Adding VIV suppression devices, such as strakes or hydrofoils, can also prevent the pipe from vibrating under certain conditions. The VIV is an issue even in the deepwater field since there exists severe near-bottom loop currents.

To prevent static and dynamic spanning problems, a number of offshore pipeline spanning mitigation methods in table below have been identified.

Pipeline On Bottom Stability

On bottom stability analysis is performed to ensure the stability of the pipeline when exposed to wave and current forces and other internal or external loads (e.g. buckling loads in curved pipe sections). The requirement to the pipeline is that no lateral movements at all are accepted, or alternatively that certain limited movements that do not cause interference with adjacent objects or over stressing of the pipe are allowed.

Hydrodynamic stability is generally obtained by increasing the submerged weight of the pipe by concrete coating. There are other ways such as increasing the steel wall thickness, placing concrete blankets or bitumen mattresses across the pipeline, anchoring or covering it with gravel or rock. Alternatively, the hydrodynamic forces may be reduced by placing the pipeline in a trench on the seabed, prior or subsequent to installation. The natural backfilling of a pipeline depends on the environmental conditions and the seabed sediment at the location.

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Pipeline Route Selection

Oil and gas pipeline routes are pivotal pieces of information upon which pipeline engineering depends. The route will define the pipeline size, terrain, soils, and engineering analysis requirements. Engineering assessment based upon agreed alignment selection criteria is an important part of a linear project. To be able to reach the best construction line and optimise its components, the phases – namely corridor, route, alignment, and construction line selection — should be studied in the given order.

Selecting the optimum route does not end with geotechnical challenges, as it also requires interactive coordination between the owner, the engineer, the regulator, the landowners, the construction contractor and a multitude of other project stakeholders and interested parties.

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