OFFSHORE STRUCTURES.

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.

MOSES SSIP TLP

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.

source :  HANDBOOK OF OFFSHORE ENGINEERING

              SUBRATA K. CHAKRABARTI

              Offshore Structure Analysis, Inc. Plainfield, Illinois, USA