8. Generic design basis

06.04.2016
This section provides an overview of the design data, which shall be clarified before issuing a tender for the design or the construction of an unmanned wellhead platform, UWHP.  

Most operators have their own standard list of content for a specific design basis, however, the content will more or less cover the following main items:

  • System design
  • General design data
  • General design regulations and standards
  • Topside disciplines
  • Structures
  • Battery limit and interface.

More detailed information about a set of generic design basis data for unmanned platforms can be found in the sub-report, ref. /10/.

 

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8.1 System design

This section includes information about the overall set-up of the UWHP, the connecting pipelines and the host facility, i.e. an overview of the entire architecture.

The general project description can in most cases also be used in the Scope of Work for a consultant and or contractor, who is assigned to design and or construct the platform.

 

8.2 General design data

This section includes information about the general design data to be used as basis for most of the disciplines. The content will e.g. be information about the type of platform, the number of People on Board (POB), water depth, design life, helideck and or boat landing, number of well slots, producers and injectors, pigging requirements etc.

 

8.3 General design regulations and standards

This section includes information about definition, of which regulations, norms, codes and standards as well as relevant European Directives and internal company technical standards and procedures are to be used as basis for the design. Furthermore, a definition of which 3rd party verification is required, e.g. “Certificate of Compliance” for the platform part and a similar approval from a recognised Marine Warranty Surveyor for the weighting prior to load out.

 

8.4 Topside disciplines

This section includes information about the different topside disciplines i.e.:

  • Health, Safety and Environment (HSE) / Technical Safety
  • Process
  • Piping and layout
  • Mechanical and material handling
  • Material technology
  • Electrical
  • Instrumentation
  • Telecom
  • Operation
  • Maintenance (including well service requirements and strategy)
  • Weight control
  • Structure.

as well as necessary information about the following:

  • Reservoir (fluid composition, pressure and temperature)
  • Drilling (wellbay area, well spacing, elevations)
  • Pipeline (interfaces, dimensions, pressure and temperature)
  • Marine operation (load and sail out, installation).

References are made to attachment 6 for further details.

 

8.5 Battery limit and interfaces

This section includes information about the battery limit and the interfaces to e.g. the pipelines, risers, drilling rig and well intervention, well growth and well spacing etc.

 

8.6 Special cost considerations

The selection of the relevant type of unmanned platform depends on many factors. The main selection criteria for example depend on the field size, the depletion philosophy, the number of wells, the water depth, the operation and maintenance strategy, the well intervention requirements, the distance to the host, the required utilities (umbilical versus installation on the platform) and the access requirements (helideck versus boat landing).

In section 8.7 and 8.8, the requirement for an umbilical and methods of access are discussed.

 

8.7 Umbilical versus installations at the platform

The most cost-effective solution for the unmanned wellhead platform may be to supply the electrical power, chemicals for injection, communication and hydraulic power from a nearby host via an umbilical. The installation of an umbilical will eliminate the need to install power generation, chemical tote tanks and pumps etc. on the unmanned wellhead platform.

However, the installation of a standard umbilical with, e.g. 10 kV electrical power, 24 fibre-optical communication cables and 3 tubes for chemicals as well as the pulling head, hang-off, flanges and J-tubes has a certain cost. While the alternative, the installation of, e.g. a power generation module providing 2 times 75 kVA incl. a bunker station, diesel oil storage as well as a hydraulic power pack, chemical tote tanks and line of sight communication at the platformmost likely has a lower cost.

By including the operation and maintenance costs in the equation, the umbilical solution will, however, most likely be the most cost-effective solution, see Figure 8-1.

 

Figure 8-1: Costs of umbilical solutions

Figure 8-1: Costs of umbilical solutions

 

The new trend is to use an umbilical to supply all the utilities from a host platform, where the operation and maintenance activities are more cost-effective than on an unmanned platform. Furthermore, this solution is the most safe approach, as the visits and the maintenance work on the unmanned platform are limited.

 

8.8 Helideck versus boat landing

The most cost-effective platform type is the one without a helideck, as the helideck requires some sort of accommodation/emergency shelter, and the present legislation in Norway also requires that fire water pump(s) (a NORSOK requirement) are installed on platforms with helidecks.

The first unmanned platform built in Denmark was without a helideck (Maersk Oil), however, later some unmanned platforms were built with a helideck (DONG E&P) in order to obtain a higher availability.

The new trend is to use a telescopic gangway (walk-to-work system), e.g. from Ampelmann or similar, in order to avoid the boat landing and the access limitations due to wave heights.

 

Figure 8-2: Examples of gangway access methods

Figure 8-2: Examples of gangway access methods