Types of Natural Gas

Natural gas is  fossil fuel (form from decays of organic matter) that mostly consists of methane (CH4), which is extracted by drilling wells into underground reservoir. The natural gas is divided into:
  • Conventional;
  • Unconventional.
Conventional Gas
The conventional gas reservoirs are largely consist of porous sandstone formations capped by impermeable rock, where the natural resources are trapped. It is easier to access and extract compared to unconventional.

The resources that can be extracted from conventional petroleum reserves include crude oil, condensate and natural gas. The products that can be refined include liquefied petroleum gas, fuel oils, petrol, diesel, kerosene, asphalt base and others.

The resources are extracted by drilling down through the ‘cap’ rock and allowing petroleum to flow up the well through the well pressure.

Unconventional Gas
The unconventional gas is produced by complex geological systems which significantly limit the production rate. Previously, it was uneconomical due to difficulties in extraction and low production rates, however, recent advances in the oil and gas technology has made it feasible.

The unconventional gas is divided into:
  • Shale gas;
  • Tight gas; or
  • Coal seam gas.
All the gas resource types are shown in figure below:

  1. http://www.parliament.vic.gov.au/publications/research-papers/8927-unconventional-gas-coal-seam-gas-shale-gas-and-tight-gas
  2. http://www.nt.gov.au/d/Minerals_Energy/?header=What%20is%20the%20difference%20between%20Conventional%20and%20Unconventional%20Gas?
  3. https://www.ehp.qld.gov.au/management/non-mining/conventional.html

Linear Wave Theory

Linear wave theory is also known as:
  • Airy theory;
  • Sinusoidal wave theory;
  •  Small amplitude wave theory.
  • It has only single sinusoidal component;
  • It is the simplest wave theory obtained by taking the wave height, H to me much smaller than both the wave length, λ and water depth, d. 
  • Wave crest height, AC = Wave trough height, AT and is denoted the wave amplitude, A, hence H=2A.
  • Linear wave theory is the first approximation to the wave motion. In order to achieve theoretical results closer to the reality requires to solve the equations to higher perturbations (non-linear system).

Wave Related Defination

Significant wave height, HS
The average height (trough to crest) of the highest one-third waves in the indicated time period, also denoted H1/3.

Peak Period, TP
It is the inverse of the frequency at which a wave energy spectrum has its maximum value.

Zero-up-crossing period, TZ
It is the average time interval between two successive up-crossing of the mean sea level.

Keulegan-Carpenter number, KC
  • V = flow velocity; 
  • T = period;
  • D = pipeline diameter.

Keulegan-Carpenter number also known as Period Number. It generally describes two things:
  1. Relative importance of drag force over inertia force (Small KC number, inertia dominates; large KC number, the drag force is important);
  2. Wave ellipse size relative to the pipeline diameter (Small KC number, current dominated; large KC number, the wave is current-like)


Wave can be classified into:
  1. Regular wave - a periodic wave with a single period.
  2. Random wave (spectrum) - a superposition of a number of regular linear waves of differing heights and periods.
Regular Wave
There are two types of regular wave:

  • Linear wave theory (Airy) - symmetric (Crest height = Trough height);
  • Non-linear wave theories (Stoke's 5th order theory, Dean's stream function theory etc.) - asymmetric (Crest height > Trough height).
Random Wave
Random wave also known as spectrum. A real sea state is best described by a random wave model, since the ocean waves are irregular and random in shape, height, length and speed of propagation. Examples of random wave are JOHNSWAP, ISSC etc.


A J-tube is a conduit that has the shape of the letter “J” that is attached to the platform. J-tube consists of the following components:
  • Vertical Tube
  • Bottom Bend
  • Bell Mouth
It is a common practice in the offshore industry to use J-tube as a method of installation of SMALL diameter flowline, electrical cable or pipeline bundle as connecting a small diameter pipeline to the topside facilities through a small diameter riser is not very convenient in terms of supports. To overcome difficulties of installation and protection, small diameter pipelines / cables are installed by pulling-in through J-tube.

The J-tube pull-in design can be found in the following link:

  1. A. C. Walker & P. Davis, A design basis for the J-tube method of riser installation, Journal of Energy Resources Techonology, ASME, Sept 1983.
  2. Full scale pull-in loads for multiple flowline bundles in North Sea's Troll Field, Oil & Gas Journal, Sept 1985.
  3. http://www.marktool.com/tool-rubber/J-Tubes

Subsea Insulation Valve (SSIV)

Subsea/ Sub-surface Insulation Valve is often placed in offshore pipeline to give an emergency stop point in the event of line rupture or on fire.


Monolithic Insulating Joints (MIJ)

Monolithic Insulating Joints are used to electrically isolate pipeline segments. The joints are used for the sectioning of main pipelines (gas, oil or water) and service lines and ensure cathodic protection and electrical safety in pipelines.