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-- Edited by admin on Thursday 15th of March 2012 11:24:35 PM

nkossa II - mooring sistem

Jeremy Beckman
Editor
Europe

Bouygues Offshore's multi-function barge is one of several deepwater production/drilling concepts originating from France for the Gulf of Guinea.

• Paris-based Prosernat supplied this separation package for Enterprise's Pierce FPSO in the UK North Sea. ProserNat is working with other French contractors to combine its technology into deepwater subsea separation systems. [44,486 bytes]
• Field development layout for Kuito Phase 1a - Coflexip Stena Offshore is responsible for much of the subsea equipment/installation. [31,229 bytes]
• Offshore-related work is beginning to stream into the new Lobito yard in Angola, under construction by a joint venture involving Sonangol and ETPM. [26,386 bytes]

Low oil prices are impacting activity in the French oil and gas sector, as elsewhere. "The general feeling is uncertainty about the future," says Victor Vachier, executive-president of national industry association GEP. "If this situation is going to last, if you believe in the forecasts - they are usually wrong, but you must take them into account - it becomes imperative to re-engineer your activities."

France's two major oil companies have reacted in different ways. Total became bigger, especially downstream, through a merger with PetroFina. Elf has resisted so far the idea of an alliance, but has sought to cut costs, partly through restructuring of its operations in Po Pau.

Elf has claimed it can afford to stay independent. Although its net income has decreased, it has not suffered as much as other oil majors such as Shell. Total is in a similar situation, according to Philippe de Panafieu of French petroleum research body CEPM. "Both claim to be getting good returns, even at $10/bbl. But French contractors and service companies are suffering more, because the E&P budgets of these and other oil companies worldwide have decreased substantially.

"Upstream specialists in geophysics, like CGG, and drilling services (Geoservices), will be more affected by these cuts - CGG, for instance, cancelled an option for a second newbuild 3D seimic vessel from Chantiers de l'Atlantique. But others engaged more in deep offshore engineering have good order books, such as Bouygues Offshore, Doris Engineering, and ETPM."

A high percentage of those orders relate to the Elf-operated Girassol development in Angolan Block 17. Vachier feels that the contractors are still too reliant on Elf and Total, and ought to expand their activities more with other companies, they have the capabilities for it. On the other hand, these two are at present among the more lucrative patrons.

While others are cutting back on new field work, Elf has started up production this year from the $250 million Maharaja-Lela gas-condensate project offshore Brunei. With Agip, Elf has signed a contract to re-develop the Dorood oilfield, lying mainly offshore Kharg Island in the Persian Gulf, in order to boost recovery from the field to 1.5 billion bbl. Investment costs here will be just under $1 billion.

In Nigeria, it is part of a consortium that is increasing throughput at the Bonny Island LNG terminal from 7.4 to 11 bcm annually. And in deepwater Mississippi Canyon 305 block in the Gulf of Mexico, where Elf is operator, it has just announced a major discovery on the Aconcagua prospect in 2,155 meters of water.

Total also reported recently a significant find in Mississippi Canyon block 942, where it is a joint shareholder with operator Shell. The well, drilled to 5,500 m T/D in waters 1,200 m deep, encountered two oil-bearing horizons with total thickness of 60 m.

The merger with PetroFina brings Total further acreage in the Gulf of Mexico, as well as strong development prospects in the UK North Sea and offshore Vietnam. Also offshore Iran, Total operates stage two and three of the South Pars gasfield development, which involves commissioning of two new platforms. Field production, anticipated in summer 2001, could yield up to 2 bcf/d of gas plus associated condensate.

With other European and Asian oil companies negotiating Iranian buy-back contracts, GEP is responding by planning a technical mission to Iran to introduce French contractors/service groups to NIOC representatives.

The fact that American companies are barred from competing in Iran and Iraq clearly benefits the French, Vachier says. On the other hand, French contractors have made little progress themselves in the Gulf of Mexico, despite a fanfare over numerous ventures with companies in Houston. North America constitutes no more than 5% of French oil industry turnover, Vachier claims.

There are exceptions. Doris is still active in the PetroCanada-led Terra Nova development offshore Newfoundland. Off California, Coflexip Stena Offshore installed Exxon's Harmony Heritage pipeline last fall, the first application for the CSO modular J-Lay spread. CSO Constructor was deployed for the work in 300 m of water (although the technique is designed for use in up to 1,500 m of water). The 12-in. line was initiated by a J-tube pull and terminated with a steel catenary riser.

Although North Sea work has tailed off (for everyone, not just the French), CSO continues to pick up subsea assignments. Recently it devised a new riser configuration for a shallow water installation on Kerr-McGee's Janice Field, where two mid-water arches were specified to support risers and umbilicals connecting the subsea installation to the Janice `A' platform.

The water depth, 79 m, was below the 90 m minimum normal for such arches. In shallow waters, there is a risk of flowline/riser clash, plus swell impact, but arches had to be installed on Janice for cost purposes. CSO's solution involved use of a riser bend limiter and hold-back clamp tethered to a gravity base which maintains the riser and umbilical configuration, at the same time securing it against wave actions from a 100-year storm.

Elsewhere, ETPM has become more active offshore Brazil, following the dissolution of its alliance with McDermott. For instance, it is bidding the EPCI contract for the umbilical riser flowline for Petrobras' Barracuda Phase 1 development. This is based on an FPSO plus a Spar or TLP linked to subsea wellheads, in waters 800 m deep. Up to 60 risers and flowines, 30 producer wells and 15 water injectors are planned. Production is due to start in 2001, at 200,000 b/d.

CGG is reportedly performing Brazil's first aeromagnetic surveys, over the Santos and Espiritu Santo basins, covering blocks to be issued for tender in the country's first open concession bidding round this June. CSO remains strong in Brazil, although its manufacturing venture Brasflex anticipates a temporary dip in flexible pipe sales.

In the Black Sea, Bouygues Offshore and Bouygues have just been awarded a $360 million contract by the Caspian Pipeline Consortium to build a new marine terminal near Novorossisk to handle oil piped overland from the Caspian.

Investments

But West Africa remains the most prospective source of offshore income. "French contractors are very much focused on West African markets," says de Panafieu, "trying to make breakthroughs on problems such as J-Lay for deepwater pipelines. CSO's Kitty, ETPM's DLB Polaris and Saibos' FDS are all examples of new or converted vessels in DP mode designed for this task, deploying laser-based, quadruple joint welding to accelerate the pace of laying. All this, however, still has to be qualified or accepted."

The renegotiations over the Girassol FPSO topsides construction affects the French contractors concerned in commercial terms - particularly the appointed yard in Fos, southern France which looks like losing the work to a lower-cost facility in the Far East. But this scenario is inevitable in a low price oil environment, says de Panafieu, emphasising that the South Pars platform contracts also went elsewhere (to a yard in Abu Dhabi). Welders outside Europe cost up to five times less, he points out.

A bigger achievement than profitability on Girassol may be the successful installation of the 1.3 km long riser towers. This is the responsibility of the Alto Mar Girassol team, including ETPM for fabrication and Doris for engineering. "There will be 140 bundles to tow 250 km from the beach at Soyo, Angola to the field," de Panafieu says, "in the form of carrier pipes. And 277 connections will be needed, all by ROV. To do that at 1,350 m water depth is a big challenge."

Tank tests in Toulon, and up ending trials in a deep lake in the south of France (involving specially developed instrumentation) produced quantitative data approximating to what had been calculated, according to Doris' Michel Deguen. A bottom tow trial of 200 m long bundle over 500 km in Angola was successful also, says de Panafieu, "in that they didn't lose the bundle, and positioned it well.

"But to install an actual riser tower, 1,300 m long, of composite assembly, housing four production bundles, plus service lines - all assembled in a syntactic foam for buoyancy and insulation; and then when it's on site, upend it from the horizontal to the vertical, the foam must not break loose and flow to the surface - all this is complicated. The incentive for this to succeed is enormous - the prototype must not fail.

"Another problem is that Girassol's oil is viscous, paraffinic and not very hot. The temperature between the wellhead and FPSO must be strictly maintained - no more than one degree C per km may be lost. And a further challenge is to inject gas at the foot of the riser to lift the column. However, they must be careful as flow must be continuous without too many slugs forming.

"Institut Francais du Petrole is engaged in software simulation in this regard for polyphasic flow calculations. Also in Solaize near Lyon, IFP's experimental loop is reproducing characteristics of Girassol's oil gas and water to watch for the danger of hydrate or paraffin deposits."

Vachier adds that the mooring system for Girassol, which includes use of suction anchors and polyester ropes to anchor the associated loading buoy, is another technical breakthrough.

Nigeria

Beyond Girassol, French contractors are monitoring another deepwater Elf-operated project off Angola and also Elf's shallow water Amenam project offshore Nigeria. Tenders may have been issued for a fixed platform and storage vessel, producing up to 150,000 b/d. TPG, Bouygues Offshore, Doris, and ETPM are thought to be bidding, with ETPM's package including use of its Smart Leg installation technique.

Also off Nigeria, ETPM is in the frame for Shell's shallow water Ea project, bidding an EPCI contract for four wellhead platforms which would be built at its Warri yard.

Another new Elf project is Tchibeli offshore Congo. Here ETPM has the contract to lay an electric cable and water injection line, plus a 25 km insulated production line between the Tchibeli Field and its host platform, the Nkossa barge. DLB Polaris will employ S-Lay for the installation. The production line features pipe-in-pipe technology for the insulation, created by Interpipe. The latter, based in Louveciennes, Paris, also developed a pipe-in-pipe system for Shell Expro's HT/HP ETAP project in the UK North Sea.

CSO is part of the consortium working on the Chevron-led Kuito Phase 1a facilities, another deepwater Angolan development. CSO is supplying flexible flowlines and risers and umbilicals, plus two gas lift distribution manifolds and one subsea control distribution manifold.

France's CEPM is also evaluating numerous other research projects for 3,000 m water depths. One is for a new drilling riser, such as Sedco Forex or Pride/Foramer. Cameron in Beziers is working on a BOP for drilling in these depths.

The problem with drilling at these greater depths, says de Panafieu, "is that the transmission time between the surface and sea bottom is too long if you use a hydraulic pulse. While waiting 2-3 minutes for that, you can have a blow-out. So you must use an alternative technique that works faster, such as acoustic links. You must also diminish the weight of the riser - perhaps develop a concentric riser to cut mud volumes."

Other new R&D projects being undertaken by French contractors include:

• a deepwater wellhead and drilling tension raft platform
• synthetic ropes for West African mooring systems
• launching and uprighting of long skirt piles from a barge
• deepwater pipeline tee installations
• H2S migration through polymer coating in flexible pipes (IFP and CSO).

Offshore concrete structure

Offshore concrete structures have been in use successfully for about 30 years. They serve the same purpose as their steel counterparts in the oil and gas production and storage. The first concrete oil platform has been installed in the North Sea in the Ekofisk field in 1973 by Phillips Petroleum. Since then 47 major concrete offshore structures have been built, whereby 21 of the 47 concrete substructures have been designed (concept and detail designs ) by Dr. techn. Olav Olsen.

Introduction

Concrete offshore structures are mostly used in the petroleum industry as drilling, extraction or storage units for crude oil or natural gas. Those large structures house machinery and equipment needed to drill and/or extract oil and gas. But concrete structures are not only limited to applications within the oil and gas industry. Several conceptual studies have shown recently, that concrete support structures for offshore wind turbines are very competitive compared to common steel structures, especially for larger water depths.

Depending on the circumstances, platforms may be attached to the ocean floor, consist of an artificial island, or be floating. Generally, offshore concrete structures are classified into fixed and floating structures. Fixed structures are mostly built as concrete gravity based structures (CGS, also termed as caisson type), where the loads bear down directly on the uppermost layers as soil pressure. The caisson provides buoyancy during construction and towing and acts also as a foundation structure in the operation phase. Furthermore, the caisson could be used as storage volume for oil or other liquids.

Floating units will be held in position by anchored wires or chains in a spread mooring pattern. Because of the low stiffness in those systems, the natural frequency is low and the structure can move in all six degrees of freedom. Floating units serve as productions units, storage and offloading units (FSO) or for crude oil or as terminals for liquefied natural gas (LNG). A more recent development is concrete sub-sea structures.

Concrete offshore structures show an excellent performance. They are highly durable, constructed of almost maintenance-free material, suitable for harsh and/or arctic environment (like ice and seismic regions), can carry heavy topsides, often offer storage capacities, are suitable for soft grounds ad are very economical for water depths larger than 150m. Most gravity-type platforms need no additional fixing because of their large foundation dimensions and extremely high weight.

Fixed structures

Since the 1970s, several fixed concrete platform designs have been developed. Most of the designs have in common a base caisson (normally for storage of oil) and shafts penetrating the water surface to carry the topside. In the shafts normally utility systems for offloading, drilling, draw down and ballast are put up.

Concrete offshore platforms of the gravity-base type are almost always constructed in their vertical attitude. This allows the inshore installation of deck girders and equipment and the later transport of the whole structure to the installation site.

The most common concrete designs are:

• Condeep (with one, two, three or four columns)
• ANDOC (with four columns)
• Sea Tank (with two or four columns)
• C G Doris
• Arup Concrete Gravity Substructure (CGS)

Condeep Type

Condeep refers to a make of gravity base structure for oil platforms developed and fabricated by Norwegian Contractors in Norway. A Condeep usually consists of a base of concrete oil storage tanks from which one, three or four concrete shafts rise. The original Condeep always rests on the sea floor, and the shafts rise to about 30m above the sea level. The platform deck itself is not a part of the construction. The Condeep Platforms Brent B (1975) and Brent D (1976) were designed for a water depth of 142m in the Brent oilfield operated by Shell. Their main mass is represented by the storage tank (ca. 100m diameter and 56m high, consisting of 19 cylindrical compartments with 20m diameter). Three of the cells are extended into shafts tapering off at the surface and carrying a steel deck. The tanks serve as storage of crude oil in the operation phase. During the installation these tanks have been used as ballast compartment. Among the largest Condeep type platform are the Troll A platform and the Gullfaks C. Troll A was built within four years and deployed in 1995 to produce gas from the Troll oil field which is operated by Norske Shell. A detailed overview about Condeep platforms is given in a separate article.

Concrete Gravity Base Structures (CGBS) is a further development of the first-generation Condeep drilling/production platforms installed in the North Sea between the late 1970s and mid '90s. The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a float-over mating method. Current or most recent projects are:

• Sakhalin-II platforms (Molikpaq (Piltun-Astokhskoye A; PA-A) platform, Piltun-Astokhskoye B (PA-B) platform and Lunskoye (LUN-A) platform)
• Malampaya
• Wandoo

C G DORIS Type

The first concrete gravity platform in the North Sea was a C G Doris platform, the Ekofisk Tank, in Norwegian waters. The structure has a shape not unlike a marine sea island and is surrounded by a perforated breakwater wall (Jarlan patent). The original proposal of the French group C G DORIS (Compagnie General pour les Developments Operationelles des Richesses Sous-Marines) for a prestressed post-tensioned concrete "island" structure was adopted on cost and operational grounds. DORIS was general contractor responsible for the structural design: the concrete design was prepared and supervised on behalf of DORIS by Europe-Etudes. Further example for the C G DORIS designs are the Frigg platforms, the Ninian Platform and the Schwedeneck platforms. The design typically consists of a large volume caisson based on the sea floor merging into a monolithic structure , which is offering the base for the deck. The single main leg is surrounded by an outer breaker wall perforated with so called Jarlan holes. This wall is intended to break up waves, thus reducing their forces.

McAlpine/Sea Tank

This desigin is quite similar to the Condeep type.

ANDOC Type

To achieve its goal and extract oil within five years after discovering the Brent reservoir Shell divided up the construction of four offshore platforms. Redpath Dorman Long at Methil in Fife, Scotland getting Brent A, the two concrete Condeeps B and D were to be built in Norway by Norwegian Contractors (NC) of Stavanger, and C (also concrete) was to be built by McAlpine at Ardyne Point on the Clyde (which is known as the ANDOC design). The ANDOC design can be considered as the British construction industry's attempt to compete with Norway in this sector. McAlpine constructed three concrete platforms for the North Sea oil industry at Ardyne Point. The ANDOC type is very similar to the Sea Tank design, but the four concrete legs terminate and steel legs take over to support the deck.

Arup Concrete Gravity Substructure (CGS)

The Arup dry-build Concrete Gravity Substructure (CGS) concept was originally developed by Arup in 1989 for Hamilton Brothers' Ravenspurn North. The Arup CGS are designed to be simple to install, and are fully removable. Simplicity and repetition of concrete structural elements, low reinforcement and pre-stress densities as well as the use of normal density concrete lead to economical construction costs. Typical for the Arup CGS is the inclined installation technique. This technique helps to maximise economy and provide a robust offshore emplacement methodology. Further projects have been the Malampaya project in the Philippines and the Wandoo Full Field Development on the North West Shelf of Western Australia.

Floating structures

Since concrete is quite resistant to corrosion from salt water and keeps maintenance costs low, floating concrete structures have become increasingly attractive to the oil and gas industry in the last two decades. Temporary floating structures such as the Condeep platforms float during construction but are towed out and finally ballasted until they sit on the sea floor. Permanant floating concrete structures have various uses including the discovery of oil and gas deposits, in oil and gas production, as storage and offloading units and in heavy lifting systems.

Common designs for floating concrete structures are the barge or ship design, the platform design (semi-submersible, TLP) as well as the floating terminals e.g. for LNG.

Floating production, storage, and offloading systems (FPSOS) receive crude oil from deep-water wells and store it in their hull tanks until the crude is transferred into tank ships or transport barges. In addition to FPSO’s, there have been a number of ship-shaped Floating Storage and Offloading (FSO) systems (vessels with no production processing equipment) used in these same areas to support oil and gas developments. An FSO is typically used as a storage unit in remote locations far from pipelines or other infrastructures.

Semi-Submersible

Semi-submersible marine structures are typically only movable by towing. Semi-submersible platforms have the principal characteristic of remaining in a substantially stable position, presenting small movements when they experience environmental forces such as the wind, waves and currents. Semi-Submersible platforms have pontoons and columns, typically two parallel spaced apart pontoons with buoyant columns upstanding from those pontoons to support a deck. Some of the semi-submersible vessels only have a single caisson, or column, usually denoted as a buoy while others utilize three or more columns extended upwardly from buoyant pontoons. For activities which require a stable offshore platform, the vessel is then ballasted down so that the pontoons are submerged, and only the buoyant columns pierce the water surface - thus giving the vessel a substantial buoyancy with a small water-plane area. The only concrete semi-submersible in existence is Troll B.

Tension Leg Platform (TLP)

A TLP is a buoyant platform, which is held in place by a mooring system. TLP mooring is different to conventional chained or wire mooring systems. The platform is held in place with large steel tendons fastened to the sea floor. Those tendons are held in tension by the buoyancy of the hull. Statoil's Heidrun TLP is the only one with a concrete hull, all other TLPs have steel hulls.

Barge/Ship Design

FPSO or FSO systems are typically barge/ship-shaped and store crude oil in tanks located in the hull of the vessel. Their turret structures are designed to anchor the vessel, allow “weather vaning” of the units to accommodate environmental conditions, permit the constant flow of oil and production fluids from vessel to undersea field, all while being a structure capable of quick disconnect in the event of emergency.

The first barge of prestressed concrete has been designed in the early 1970s as an LPG (liquefied petroleum gas) storage barge in the Ardjuna Field (Indonesia). This barge is built of reinforced and prestressed concrete containing cylindrical tanks each having a cross-section perpendicular to its longitudinal axes that comprises a preferably circular curved portion corresponding to the bottom.

Surface Floating Concepts:

The axes of ocean colonization / floating real estate building lots on the water / Plate Seastead - Plate Floating Element for Ocean Colonization / Catamaran Concrete Floating Elements - Base for Ocean Living / Floating Concrete Breakwater Marina / Ocean colonization how to get there / Ramform ship island as ocean base mobile stable scaleable / small honeycomb floating concrete structures in cartagena / Seabreaks for dampening colossal ocean waves / Ocean colonization technology / Ocean colonization company / Oustanding floating concrete structures / ocean colonization general considerations / Interesting projects for ocean colonization / Aquaculture, business, trade, mininig, energy, salvage, making money afloat /

Submerged Concepts:

The captain nemo float out - seasteading / Sub movement finished - Submarine Yacht / Is submarine living space expensive? / concrete pressure vessel / Concrete submarine project / submarine yacht / concrete submarine yacht supporter club / Submerged living space bubble concept basics / Exotic Submerged Bubble Hotel / sea orbiter / Current Turbine Concrete Hull /

The big five business fields of ocean colonization: | oceanic transport | oceanic energy | oceanic real estate | deep sea mining | oceanic aquaculture |

The four fundamental quests of ocean colonization : | The quest for interference freedom | The quest for mobility | The quest for oceanic resources | The quest for space on the planet |

Concepts: / Lens shell pictures overview / / Ramform floating home pictures / / c-shell floating home pictures / / Floating concrete building methods / / shell cluster pictures / / investor proposal list /

Vachier feels that the contractors are still too reliant on Elf and Total, and ought to expand their activities more with other companies, they have the capabilities for it.

 Dry Rot

The Nkossa field development will consist of two fixed steel drilling platforms, the concrete production barge and two storage/loading vessels.

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