Design of FPSO systems for re-use, decommissioning

Multi-field use requires many up-front considerations

The following article is a summary of a Deep Offshore Technology conference (DOT '97) paper entitled "Design and Evaluation of Floating Production Storage Offload (FPSO) Systems for Decommissioning and Reuse," authored by M. Kawase of Modec, H. B. Skeels, of FMC, and P. A. Stemmler, of FMC Sofec.
The full-length paper is available from the authors.
With the popularity of purpose built floating production, storage, and offloading (FPSO) vessels, the industry needs a standardized classification process to group these vessels into handy categories that would help owners place them on other fields when their purpose-built job is complete. Also, helpful would be the design of these vessels with the prospect that one day modifications would be necessary to market the vessel.

Though the process facilities of these vessels are generally custom made for a specific application, other major components including pressure vessels, piping, and equipment that can be used on similar fields in future applications.

If initially designed with an eye towards an extended life, and the potential for expansion, the equipment could more easily be converted and moved to another field with similar fluid characteristics. FPSOs lend themselves readily to such conversions and movements because of their ship shape.

This minimizes the need for offshore work, since the vessels can quickly travel to conversion yards under their own power and then travel to the new field. By reducing offshore work, costs are also lowered.

Vessel categories

Although topsides are generally custom-built, FPSOs can be divided between those with conventional production systems and those with enhanced production systems:

  • Conventional systems: This type of vessel contains a production manifold, several stages of two or three phase separators (including a test separator), a water treatment system, gas flare/disposal system, a heat exchanger, crude transfer pumps, and a control system.
  • Enhanced systems: This unit features a gas compression system for gas lift, gas injection, and gas export, water injection system, desalters, NGL recovery systems, in addition to the conventional production system.
In general, enhanced production systems have a large processing capacity. This means they will have more chances to be used again with fewer modifications. To ease the redesign process for FPSOs, the following recommendations should be considered in the initial design:

  • Provide space for future expansion of additional equipment
  • Provide additional space in pipe racks and cable racks
  • Provide spare I/O ports for control panels
  • Provide spare branches on piping for future connections
  • Operator's policy and preference for operation, maintenance and safety.
Engineering documents of the equipment should be checked for compatibility with the new production fluid composition and design conditions: manifolds, pressure vessels, valves, piping, pumps, heat exchangers, water treatment facility, flare system, gas compression system, and E&I system.

This is followed by a thorough inspection survey of existing equipment and infrastructure. Once the existing and new requirements are investigated for suitability, a revised PFD and P&ID is developed along with the appropriate modification plan.

The FPSO vessel is categorized by storage capacity. Vessel categories follow classical groups for categorizing trading tankers. The small vessel category includes tankers with a deadweight range less than 90,000 tons. Crude oil storage capacity is less than 700,000 bbl.

The medium vessel category includes mostly Suez Max tankers. Deadweight range is from 100,000 dwt to 150,000 dwt. Crude oil storage capacity is between 700,000 bbl to 1 million bbl.

The large vessel category includes VLCC and ULCC size tankers. Deadweight range is from 160,000 dwt to 400,000-dwt. Crude oil storage capacity is more than 1 million bbl. In almost every case, a large vessel has more versatility than a small vessel.

The following requirements are key factors to judge re-usability of an FPSO vessel for a new field:

  • Operation duration requirement at the new field
  • Crude storage capacity requirements
  • Double hull requirement.
If an FPSO is judged to be reusable for the next assignment, the following factors are studied and the conversion plan will be developed accordingly:

  • Life extension and repair
  • Conversion of offloading system
  • Conversion of tank heating
  • Conversion of safety facility
  • Regulations
  • Site environment.

FPSO mooring

FPSO tanker turret facilities can vary tremendously in type, size, and function. Turret facilities may be the external or internal type, and may be either permanent or disconnectable.

External turrets are typically cantilevered off the bow or stern of a vessel. Internal turrets penetrate the body of the vessel and may be located near the bow, near the stern, or near amidships. In general, these turrets perform the same basic functions although additional functions may be served.

Turret design standardization further facilitates re-use in that plans for additional, or future risers may be well thought out avoiding the complications due to structure or piping interference in a future modification.

Swivel and piping/manifold requirements are usually too field specific to employ much planning for reuse. However space-control planning, similar to topsides space planning, will aid in future modifications. Standardization of swivel mounting interfaces will aid swivel stack changes.

Turret re-utilization

Given that the most basic capabilities of a turret are governed by load capacity and riser space availability, turrets may be categorized accordingly:

  • Load capacity: Turret load capacity may be categorized as either low load capacity or high load capacity for simplicity. Low load capacity may be considered when the total maximum resultant load due to off-vessel moorings plus risers is less than approximately 1,200 tons.
  • Riser capacity: Turret categories based on riser capacity are: few risers, less than 10; moderate number of risers, between 10 and 25; and many risers, greater than 25.
Realizing that the turret forms a plan of the flow path, a review of the new field key parameters to assess turret system suitability will include most of the key parameters.

The turret system piping, manifold, swivels, and safety features must be evaluated for compatibility with the new field requirements following a thorough inspection to determine the existing condition.

Evaluation of the swivels, piping, manifold, and other components must consider materials, corrosivity, pressure, size, and pigging requirements.

Other factors involved in the process of decommissioning and re-use include the following:

  • Decommissioning: Prior to disconnection of the mooring and riser systems, the machinery related to anchor leg or riser installation/deinstallation is reconditioned.
  • Refurbishment, conversion: Turret system refurbishment is best carried out at a shipyard. Drydocking may be required depending on the modification plan.
  • FPSO mooring: Anchor leg patterns and component sizes, grades and lengths are generally specific to a certain site.
However, with proper consideration of corrosion, wear, fatigue and handling, modern anchor chain and wire components may be considered for reuse. FPSO mooring systems may be categorized as all-chain system, combination system, and polyester system.

The following are key parameters for judging the reusability of the off-vessel mooring components:

  • Field life: Is residual life adequate for new field life?
  • Strength: Is size and grade of materials adequate?
  • Design temperature: Is the vessel and component material suitable?
  • Installation equipment: Can it be properly handled?

Anchoring components

Anchor chain may have been used many times, although this is seldom the case for an FPSO permanent mooring system. However, inspection programs to re-certify anchor chain exist and should be adopted or modified to meet the new field's functional requirements.

Careful planning for the recovery of the anchor legs must allow for handling operations, which will not damage the components. Generally, these procedures may be a reversal of the original installation procedures. Any wire rope to be considered for reuse must be very carefully handled especially when handling the spelter ends to avoid overbending of the wire rope.

Risers

Typically risers are custom designed for a specific site. The design of the structure and the configuration is a complicated function of fluid service, site, water depth, environment, and flowing vessel response characteristics.

However, careful design in the choice of riser configuration, size and interface between vessel and the wellheads can allow the re-utilization of the FPSO without major modifications at the riser interface.

In the initial design, it is important to properly design the structure and configuration to enhance the potential for re-use. Configurations like the lazy wave and free hanging cantenary risers are more readily adaptable to re-utilization as the riser forms part of the flowline after touchdown; thus the length of the riser is not too dependent on water depth.

The following are key parameters for judging the reusability of the riser system:

  • Service life: Is the residual life adequate for the new field life?
  • Type of service: Is the product type similar, sweet or sour?
  • Design temperatures and pressures: Is the riser structure suitable?
  • Site particulars: water depth and metaocean environment?
  • Riser configuration: Does the riser configuration allow for use at another site?
  • Installation equipment: Can it be properly handled?
A thorough inspection plan must be carried out on the recovered riser. This includes the end fittings, external sheath wear, external buoyancy modules, and possible NDE inspection of the inner carcass. Re-certification of the riser by the riser manufacturer or the certifying authority may also be required.

A detailed inspection plan with specific reject/accept criteria must be developed with the riser manufacturer and/or classification society for the new field. A detailed examination of the external sheath and the end fittings will be required. Buoyancy modules will need to be removed, refurbished, and replaced during re-installation.

Subsea systems

Because subsea systems are a packaged system, they incorporate intricate machined hardware sized for the flow rates, well shut-in pressures, installation vessel interfaces, and intra-field connections.

Suitability of subsea hardware for decommissioning and re-use is primarily a function of valve size, material class, and its remaining useful design life. Another obstacle to re-use may be the tree's flowloop configuration and the number of valves as it passes from one field's requirements to another.

Subsea trees can be broken down into four major types and two sub-categories: water depth and trim. Water depth is the primary category for describing the type and complexity of a subsea system:

  • Mudline: <120 meters. mudline systems are simple designs, with stacked valve configurations, and rely heavily on human intervention to connect equipment together and play a key role in subsequent interventions and decommissioning.
  • Diver assist: <200 meters. diver assist systems are rudimentary, but a little more complex and larger in size because they are usually placed on top of subsea, floating drilling, wellheads.
  • Diverless: 200-800 meters. at around 200 meters, the economics of diver intervention verses remote operation give way to our third group of subsea trees, diverless systems. these trees, as their name implies uses rot's and rov exclusively to perform all installation and intervention tasks.
  • Guidelineless: >100 meters, but usually >800 meters. This last group of subsea trees is the guidelineless tree. These trees are also diverless, but they do not require the four guidepost/guideline guidance from surface to the subsea well site, as the others.
Mudline trees are, by definition, smaller, minimal wall thickness design systems. Their designs are more susceptible to metal loss and pitting corrosion because of the minimal wall thickness unless CRA's are used. They also are limited in production capacity because of smaller bore configurations.

The other three subsea tree categories are typically designed with composite valve blocks, offering considerably more metal which is available for metal loss without sacrificing pressure containment or pressure control for longer design lives at the lower trim levels.

Diverless trees are probably the most versatile since remote intervention can be accomplished from many different types of vessels in a wide variety of water depths.

Other equipment

There are three other subsea equipment areas that should be mentioned which could affect successful reuse: wellheads, foundations, control systems, and flowline connection techniques.

Mudline wellhead systems are designed with below mudline abandonment in mind. They unscrew the landing string from the casing hanger joints inside the well, and leave the rest behind. Subsea, floating drilling, wellheads have the high pressure and low pressure housings recovered during abandonment work.

Reworked wellheads and housings are cost effective for drilling programs where a single wellhead may be used for three or more successive drilling programs over a short period of time. Many subsea completions specify metal sealing equipment. If so, seal bores in a wellhead only need to suffer minor corrosion or mechanical damage to prohibit its reuse.

Mudline trees can fit on most mudline wellheads since the completion interface usually consists of casing joints threaded together between the mudline wellhead's landing sub and the tree completion hardware. Therefore reused mudline trees can be used on a number of different mudline wellhead systems so long as the casing program is the same.

Subsea wellheads are a different matter. Subsea wellheads have an external profile in one of two categories, clamp hub, or mandrel profile. The subsea tree has a special hydraulic connector that fits over and locks onto one of these external profiles.

Subsea foundations are designed to structurally support subsea equipment on the seafloor for the life of the field. Several systems are in use today, including drilled or hammer founded piles, flat mudmats and skirted mudmats, auger bit anchors, caisson silos, and various suction anchor designs.

Subsea control systems are a wild card when considering decommissioning and re-use. There are three basic types of subsea control systems: direct hydraulic, piloted hydraulic, and various forms of electro-hydraulic controls.

Flowline and umbilical decommissioning and reuse should address a fundamental question at the beginning of an FPSO project, "Can, or should, the intra-field flowlines be abandoned in place, or should they be recovered for economic and/or regulatory reasons?"

Removing a diver installed spool piece between the end of the flowline and the tree is the simplest method to decouple and recover the two subsea components separately. If an operator wants to re-use existing equipment, he either:

  1. Builds in all the "what ifs," paying a higher price for it at the front end of the project;
  2. Pays for an overhaul of the tree to change its configuration, trading off newbuild costs for refurbishment costs;
  3. Works hard to operate within the limits of what is given him and work around potential equipment limitations.

Subsea decommissioning

Subsea hardware decommissioning is accomplished during well abandonment. After the well is controlled and killed, cement is usually circulated into the well through prepared perforations in the tubing string.

Once the well is dead, the flowlines and associated subsea hardware, such as subsea manifolds, flowline connection hardware, and other components are circulated with water. By pumping from the FPSO to the tree and drilling vessel or from the drilling vessel to the FPSO, all equipment is purged of hydrocarbons and allowed to flood to hydrostatic pressure.

Then, flowline and control umbilical connections are disconnected and the subsea tree is recovered. Subsea wellheads are then cut approximately 3 meters below the seafloor and pulled up as a single salvaged unit; wellhead, guide bases, casing hangers, packoffs, and casing joint stubs. All remaining subsea hardware is either removed from the seafloor, or abandoned in place, depending on local regulation requirements, water depth, proximity to shipping lanes or anchorages, and abandonment costs.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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