Wednesday, 4 May 2011


Floating Offshore Structures.

     Floating structures have various degrees of compliancy. Neutrally buoyant structures, such as semi-submersibles, Spars and Drillships are dynamically unrestrained and are allowed to have six degrees of freedom (heave, surge, sway, pitch, roll and yaw). Positively buoyant structures. such as the Tension Leg Platforms (TLPs) and Tethered Buoyant Towers (TBTs) or Buoyant Leg Structures (BLS) are tethered to the seabed and are heave-restrained. All of these structures with global compliancy are structurally rigid. Compliancy is achieved with the mooring system. The sizing of floating structures is dominated by considerations of buoyancy and stability. Topside weight for these structures is more critical than it is for a bottom-founded structure. Semi-submersibles and ship-shaped hulls rely on waterplane area for stability. The centre of gravity is typically above the centre of buoyancy. The Spar platform is designed so that its centre of gravity is lower than its centre of buoyancy, hence it is intrinsically stable. Positively buoyant structures depend on a combination of waterplane area and tether stiffness to achieve stability.

     Floating structures are typically constructed from stiffened plate panels, which make up a displacement body. This method of construction involves different processes than those used in tubular construction for bottom-founded structures. Neutrally buoyant floating structure motions can be accurately determined as a single six-degrees of freedom system subjected to excitation forces. Positively buoyant floating structures in deep water will have restraining systems with substantial mass, and the restraining systems are subjected to excitation forces as well. The motions of the platform are coupled with the dynamics of the mooring system. The coupling of motions between the platform, risers and mooring systems becomes increasingly more important as water depth increases.

Floating Platform Types.

     The floating structures may be grouped as Neutrally Buoyant and Positively Buoyant. The neutrally buoyant structures include Spars, Semi-submersible MODUS and FPSs, Ship-shaped FPSOs and Drillships. Examples of positively buoyant structures are TLPs, TLWPs and Buoyant Towers. Floating platform functions may be grouped by their use as mobile drilling-type or production type. The number of units in these categories installed worldwide is shown in table below as of 2002.

Floating systems as of 2002 [Offshore, 2002]

     There is little standardisation of floater units. Shell offshore and their partners achieved significant cost savings when they designed multiple TLPs following similar design practices (Le. Auger, Ram-Powell, Mars. Ursa and Brutus). Kerr-McGee achieved some saving by designing the Nansen and Boomvang Spars identically. However, for the most part, each deepwater field has been developed with a “fit for purpose” design.

Production Units (FPSO and FPS)

     Most floating production units are neutrally buoyant structures (which allows six-degrees of freedom) which are intended to cost-effectively produce and export oil and gas. Since these structures have appreciable motions, the wells are typically subsea-completed and connected to the floating unit with flexible risers that are either a composite material or a rigid steel with flexible configuration (i.e. Compliant Vertical Access Risers). While the production unit can be provided with a drilling unit, typically the wells are pre-drilled with a MODU and the production unit brought in to carry only a workover drilling system.

     The FPSO generally refers to ship-shaped structures with several different mooring systems. Early FPSOs in shallow waters and in mild environment had spread mooring systems. As more FPSOs were designed and constructed or converted (from a tanker) for deepwater and harsh environments, new more effective mooring systems were developed including internal and external turrets. Some turrets were also designed to be disconnectable so that the FPSO could be moved to a protective environment in the event of a hurricane or typhoon.

     The use of FPS in offshore oil and gas development is proliferating around the world. FPS technology has been in commercial use since the early 1970s when Hamilton Bros. utilized a converted MODU to produce from the Argyll Field in the UK sector of the North Sea. However, Petrobras gets the credit for widespread application of the FPS concept beginning in the late 1970s. The combination of depressed oil prices and advances in subsea production technology made the FPS concept more attractive. Another important reason for its popularity was that Petrobras had the insight on the cost and schedule advantages of MODU conversions and arranged the MODU lease, charter contracts to ensure the ownership transfer of the MODUs to Petrobras at the end of their contracts (typically a two- or three-year contract). FPS technology has become an effective solution for both the marginal and the deepwater field development. Although the advantage of converting semi-submersibles and other mobile offshore drilling units (MODUs) into FPS existed in the 1980s, with the surplus of such MODUs most FPSs put into service in the 1990s were based on newly constructed semi-submersible and Spar units. These structures have the advantages of versatility, mobility (in re-location, adverse weather or politics), relative low cost and self-containment.

     Among the nations that are involved in the development and installation of FPS, Brazil has aggressively pushed into deepwater frontier. They first set the goal to produce from 1000 m depth and established a multi-faceted research and development programme to achieve this objective. Once this objective was achieved, they raised the bar and established a new goal of producing from 2000 m water depth. To achieve this target, Petrobras has created Procap 2000, Program for Technological Capability for Deepwater Production, to develop deep and ultra-deep waters of Campos Basin.

     Floating Production System units were also installed in the US in the Gulf of Mexico. Unfortunately, the first three units to be installed (Placid Oil’s Green Canyon Block 29, Enserch’s Garden Banks Block 387/388, Tatham Oil’s Ewing Bank Block 958,959) were less than successful due to poor reservoir conditions. The Gulf of Mexico has seen discoveries of more than 50 oil and gas fields with recoverable reserves of more than 40 million BOE in water depths greater than 1968 ft (600 m). It is likely that most of these fields will be developed utilising FPS and perhaps FPSO systems.

              Offshore Structure Analysis, Inc. Plainfield, Illinois, USA

Friday, 29 April 2011


Two Phase Flow.
  • All of the discussion so far has been confined to single phase flow i. e. flow of either gas or liquid.
  • There are situations in which are economically advantageous to transport both a gas and a liquid in a single pipeline simultaneously.
  • This is called 2-phase flow.
  • An example 2-phase flow involves certain off-shore oil operations,where it is extremely expensive to separate the liquid and gas phases in deep water.
  • As the technology advances two-phase flow is likely to become common place in situations such as this.
  • Two-phase flows may occur over a wide range of liquid to gas ratios and manifest a variety of flow patterns.
two phase flow patterns

  • Two phase flow technologies is much more complex than single-phase flow. However research suggest that the design of such systemsis possible.
  • Experimental studies have revealed that2-phase-flow shows larger drops in pressure compared to single-phase flows.
  • Two components of pressure drop are suggested in literature for 2-phase flows :
  1. The pressure drop due to friction which increases with increase in gas flow rate. This is the only component in horizontal lines.
  2. The pressure drop due to the head of liquid in inclined lines.Practically all of this pressure drop occurs in the uphill section of the pipeline.
Iran-Turkmenistan pipeline
source: Prof. Dr. O. Cahit ERALP

Wednesday, 27 April 2011

offshore engineering and some history of oil wells.

written by: amir aziz

      Offshore Engineering applies ocean engineering studies to the engineering of devices within the benthic environment, such as Marine Foundations, Mooring Systems, Ocean Mining, Offshore Disposal, Offshore Pipelines and Cables, Offshore Structures, Submersible Vehicles, Submarine Habitats and Salvage Operations.
        According to the Wikipedia, there were also the installation of structures and pipelines in a marine environment for the production and transmission of oil and gas that known as offshore construction. According to the source, Construction in the offshore environment is a dangerous activity and where possible the construction is modular in nature with the individual modules being assembled onshore and using a crane vessel to lift the modules into place. Oil platforms are key fixed installations from which drilling and production activity is carried out. Drilling rigs are either floating vessels for deeper water or jack-up designs which are a barge with liftable legs. Both of these types of vessel are constructed in marine yards but are often involved during the construction phase to pre-drill some production wells. Specialist floating hotel vessels known as flotels are used to accommodate workers during the construction and hook-up phases. So, that is the reason why did a person have to be trained before becoming specialist in order to constructing the platform construction.
       Other key factors in offshore construction are the weather window which defines periods of relatively light weather during which continuous construction or other offshore activity can take place. Safety is another key construction parameter, the main hazard obviously being a fall into the sea from which speedy recovery in cold waters is essential.
       In constructing the pipeline for the transmission of the oil and gas, the main types of vessels used for pipe laying are the "Derrick Barge (DB)", the "Pipelay Barge (LB)" and the "Derrick/Lay Barge (DLB)" combination. Diving Bells in offshore construction are mainly used in water depths greater that 120ft, less than that, the divers use a metal basket driven from an "A" frame from the deck. The basket is lowered to the water level, then the divers enter the water from it to a maximum of 120ft. Bells can go to 1500ft, but are normally used at 400 to 800ft.
       In searching the information about oil platform, I found that offshore platform, often referred to as an oil platform or an oil rig, is a large structure used to house workers and machinery needed to drill wells in the ocean bed, extract oil and/or natural gas, process the produced fluids, and ship or pipe them to shore. Depending on the circumstances, the platform may be fixed to the ocean floor, may consist of an artificial island, or may float.
       Most offshore platforms are located on the continental shelf, though with advances in technology and increasing crude oil prices, drilling and production in deeper waters has become both feasible and economically viable. A typical platform may have around thirty wellheads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 5 miles (8 kilometres) from the platform. Remote subsea wells may also be connected to a platform by flow lines and by umbilical connections; these subsea solutions may consist of single wells or of a manifold centre for multiple wells.
       In searching the information about the offshore engineering, I also found some information about the history of oil wells. Around 1891 the first submerged oil wells were drilled from platforms built on piles in the fresh waters of the Grand Lake St. Marys (a.k.a. Mercer County Reservoir) in Ohio. The wide but shallow man made reservoir was built from 1837 to 1845 to provide water to the Miami and Erie Canal.

  • Around 1896 the first submerged oil wells in salt water were drilled in the portion of the Summerland field extending under the Santa Barbara Channel in California. The wells were drilled from piers extending from land out into the channel.
  • Other notable early submerged drilling activities occurred on the Canadian side of Lake Erie in the 1900’s and Caddo Lake in Louisiana in the 1910’s. Shortly thereafter wells were drilled in tidal zones along the Texas and Louisiana gulf coast. The Goose Creek field near Baytown, Texas is one such example. In the 1920’s drilling activities occurred from concrete platforms in Venezuela’s Lake Maracaibo. 

  • The oldest subsea well recorded in Infield’s offshore database is the Bibi Eibat well which came on stream in 1923 in Azerbaijan. The technique used here involved using landfill to raise shallow portions of the Caspian Sea.
  • In the early 1930s the Texas Co., later Texaco (now Chevron) developed the first mobile steel barges for drilling in the brackish coastal areas of the gulf.
  • In 1937 Pure Oil Co. (now Chevron) and its partner Superior Oil Co. (now ExxonMobil) used a fix platform to develop a field 1 mile offshore of Calcasieu Parish, Louisiana in 14ft of water.
  • In 1946, Magnolia Petroleum (now ExxonMobil) drilled at a site 18 miles off the coast, erecting a platform in 18 ft of water off St. Mary Parish, Louisiana.
  • In early 1947 Superior Oil erected a drilling/production platform in 20 ft of water some 18 miles off Vermilion Parish, La. But it was Kerr-McGee Oil Industries (now Anadarko Petroleum), as operator for partners Phillips Petroleum (ConocoPhillips) and Stanolind Oil & Gas (BP) that completed its historic Ship Shoal Block 32 well in October 1947, months before Superior actually drilled a discovery from their Vermilion platform farther offshore. In any case, that made Kerr-McGee’s well the first oil discovery drilled out of sight of land.
  • The Thames Sea Forts of World War II are considered the direct predecessors of modern offshore platforms. Having been pre-constructed in a very short time, they were then floated to their location and placed on the shallow bottom of the Thames estuary. In 1938, the Superior Oil company constructed the first offshore oil platform off the Gulf Coast of Louisiana. However, the first producing platform out of sight of land was installed by the Kerr-Mc Gee Company in 1947. This is the date generally accepted as the start of the offshore oil era.

       Offshore & Dredging Engineers make structures such as fixed and floating platforms for the oil and gas industry. They also design undersea pipelines and other underwater equipment for this sector. An important feature is the design of dredging equipment for land reclamation, maintenance and the recovery of embedded minerals in deep-sea locations. Another application of offshore engineering is the design of offshore wind farms. Offshore & Dredging Engineers are the people who design facilities for the 70% of the earth’s surface area that is not land.

single anchor leg mooring (SALM)


-A mooring facility dedicated to the offshore petroleum discharge system. Once installed, it permits a tanker to remain on station and pump in much higher sea states than is possible with a spread moor.
·         A SALM system consists of the following main components
·         Buoy Body
·         Universal joint
·         Chain with chain swivel or tubular column
·         Flexible pipe
·         Base (ballasted and/ or piled)
-The Single Anchor Leg Mooring or SALM: Prevents collision damage to the swivels by placing them underwater and below the keel level of the tanker. Any damage should then only affect the simple surface buoy and be relatively cheap to repair. The underwater swivels do however have maintenance disadvantages. To prevent the flexible loading pipe clashing with the mooring chains the catenary is replaced by a single, nominally vertical, tensioned chain mooring leg. In shallow water the fluid swivels are on the base. In deep water the fluid swivels could be attached part way up the mooring leg. This would ease maintenance of the swivels and the flexible pipes from the swivels to the tanker.
 The buoy body provides buoyancy and place to accommodate the other components.
The floating buoy is anchored to the seabed by one single anchor leg, connected to a base type anchor point (ballasted and/ or piled). The buoy can be attached to the base by either one single chain or by a chain or tubular column, this can be seen in the pictures below.
The connection between the buoy and the vessel, and between the buoy and the base, can be established in various ways. These are described below.
 Fluids flow either through a flexible pipe from the base on the seabed directly to the ship or flow through the base and the tubular column, via a swivel to the ship. The submarine hoses are long enough to adapt to all the motions of the buoy. The fluids are transferred between the buoy and the FPSO through one or more floating hoses. The flow through the buoy goes via a swivel, which allows flow between the geostatic parts and the rotating parts of the buoy.
Auxiliary components can be things such as navigational aids for maritime visibility, fenders to protect the buoy, boat landing, power provisions, etc.
 The SALS system (Single Anchor Leg Storage) is a specially designed principle for offloading purposes. The system consists of the following components: a slim riser, which is connected to a mooring base below, and a rigid mooring yoke through a universal joint above. In the rigid mooring yoke, a buoyancy tank is incorporated, which keeps the riser under pretension by way of its positive buoyancy. The rigid mooring yoke is connected to the storage tanker by a rigid arm.
The cargo line is constructed either of pipes and jumper hoses to bridge the universal joints or of pipes and swivels without hoses.
The fluid swivel is mounted on a platform, which is an integral part of the mooring yoke. This means that the swivel is not a stressed member. Besides, as the swivel remains almost permanently out of water, there results a smaller and simpler construction requiring less maintenance.
Because of the special anchoring system, the behaviour of the SALS is almost completely independent of the water depth.
The buoy can be attached to the base by either one single chain or by a tubular column.

SALM system, with a single chain.

SALM system, with a tubular column.
There are two types of buoy bodies:  
Turntable buoys
 The body that provides the buoyancy is geostatically fixed with the mooring lines. The upper part can rotate freely, allowing the FPSO to weathervane.
 Turret buoys
 The turret is geostatically fixed with mooring lines. The body that provides the buoyancy can rotate around the turret and thus allows the FPSO to weathervane.

The FPSO can be moored to the buoy in two ways:
These are polyester or nylon mooring lines. Usually, a chain section is attached to both sides of the elastic section, to prevent wear and abrasion at the fairleads. The elastic part of the hawsers damp the loads and tanker movements.

A rigid arm is used to connect the FPSO to the buoy. This takes away the risk of the FPSO bumping into the buoy. However, this system can only be used for permanently moored FPSO’s and is more costly than a hawser arrangement.

:All weather functionality.

Number of risers

:Only one.
:Applicable in a great range of water depths.

Construction/ Installation
:Construction and installation are relatively fast and cheap.
:Cost-effective system.

:The past has shown that the system is very reliable.


Tuesday, 26 April 2011