Subsea Design ( Conceptual, Pre-FEED, FEED, Detailed Design )

Subsea design services are applicable to Pre-FEED, FEED and Detailed Design phases of the project.  The extent of the study deliverables depend on the Client's expectation and project's EPCI schedule as a whole. 
A Pre-FEED study aims to find a general solution or development option without dealing with details. However, the main challenges and concerns need to be addressed as well.

A FEED study is supposed to address all uncertainties and suggest a robust solution. In fact track projects there will be very limited possibility to meet project milestones if procurement is postponed till the end of the detailed design.  Therefore, FEED needs to be mature enough to ensure no major change occurs in the next phases of the project as the procurement for long lead items need commence by the end of the FEED.

Typical FEED deliverables include:

  • Flow assurance
  • Geotechnical and geophysical surveys;
  • Field Layout (pipeline routes and control system distribution)
  • Control system philosophy
  • Riser system solution
  • Weight and dimensions and foundation solution for subsea structures
  • General arrangement Drawings
  • Specifications and datasheets for long Lead Items
  • EPCI cost and schedule estimates

A detailed design studies is supposed to address all engineering issues in details.  Construction and installation will be finalised based on the detailed design reports and drawings.  Therefore, detailed design is supposed to provide a very extensive coverage on all aspect of the project.

Field Development

Selection of an appropriate scenario for developing an offshore field is the most critical decision to make. 

Experience has shown that for an isolated marginal field the best development scenario is a tie-back to an adjacent host.  However, for a large field a stand-alone solution is more preferred.

For cluster marginal fields mainly a phase development is of interest especially for emerging operators.  This is to minimize capital exposure.
There are several ways to developing an offshore field.  To select the optimum scenario, we develops life-time economic model for all possible scenarios and extracts techno-economic indicator. 

Z-Subsea develop life-time economic model through the following steps:

  • EPIC schedule estimate
  • Cost estimate
  • Income estimate
  • Cash flow generation
  • Extracting economic indicators such as NPV, IRR and UTC.

In addition to the economic consideration, other issues such as emerging technologies and concept maturity need to be assessed.

More detailed discussions such as optimum pipeline system is addressed under this service as well. Scenarios such as single pipe, piggybacked pipe, pipe-in-pipe and bundle are dominating pipeline system solutions.  Through a techno-economic assessment the optimum solution can be extracted.

Field Layout and Route selection

To perform a field layout study, basic information such as (a) location of drilling centres, (b) location of host facility and (c) geotechnical and geophysical survey results need to be available. The outcome of the study is the field architecture.


GEotechnical and geophysical surveys

First step of performing a geotechnical and geophysical survey is to develop the scope and specification of the survey.  The scope of survey needs to cover subjects such as survey datum, bathometric charts, debris, seabed features, interrupted conditions, soil description, soil properties, plastic and liquid limits, void ration, in situ testing, sampling (grabs, cores) and shears strength tests.


Manifolds and structures locations

Manifolds need to be as close as possible to drilling locations. There is no X-mas tree mounted on traditional manifolds.  The basic function of a manifold is to comingle wells fluids.  Several wells (branches) can get connected to the production heading. 

Subsea Isolation Valves (SSIV) Structures need to be placed as close as possible to the host platform.  This is to minimise the inventory between SSIV and platform's topside in case of a hydrocarbon release.

Templates are the structures which act as a base for drilling, X-mass trees in which case manifolds are mounted on templates.  Location of template depends on subsurface and reservoir studies.


Pipeline routes

Pipeline outing needs to be assessed based on geotechnical and geophysical surveys.  The route should avoid seabed features such as boulders and pock marks, anchoring zones, seabed slopes and etc. 

Minimum clearness between pipeline and other infrastructures need to be maintained as well.


Pipeline system (single, piggybacked, PIP or bundle)

A single pipe solution is the first choice however, due to flow assurance or buckling requirement, pipe-in-pipe pipelines may be used. 

If a relatively large diameter pipe and a small diameter pipe need to be installed in parallel, a piggybacked solution can be utilised.  This is to minimise installation and seabed intervention cost. 

For more than two pipelines with the same start and end point a bundle solution may be used.  Umbilical cores can be placed in the bundle and if so, there is no need to install an umbilical.


Controls ( Umbilicals, Subsea Distribution Units and Flying Leads)

Umbilicals are used to connect topside control system to Subsea Distribution Units (SDUs) and SDUs to manifolds, templates, x-trees and etc.

SDUs are used to distribute hydraulic fluids, power supply and communications between topside control system and Subsea Control Modules (SCM) mounted on manifolds, templates and X-mass trees.  An SDU can be mounted on a subsea structure or alternatively can have its own dedicated support and protection structure.

Flying leads are used to connect umbilical terminations to SCMs, SCMs to valves and instruments and etc.

Subsea Pipelines



Z-Subsea provide most competent team of experts to deliver a robust and economically viable design to meet the project requirements.

The pipeline and riser design process can be based on PD8010, DNV, ASME and other industry recognised standards. The initial calculations of subsea pipelines wall thickness at FEED stage is based on:

  • The hoop stress in the pipe due to differential pressure
  • Pipeline collapse due to external pressure
  • Buckle propagation due to initiation of a buckle

However, for High Pressure-High Temperature (HPHT) pipelines, consideration should be given to:

  • High compressive axial force which could result in near yield stresses. Means of reducing axial stress should be sought at early stages
  • Start-up/shut-down cycles for HPHT pipelines will result in considerable fatigue in the girth welds which requires awareness at early stage of the project. 


Depending on the linepipe dimensions, a thin or thick wall theory should be used to assess the stresses in the pipe.

There are also other factors in selecting wall thickness for a pipe. For HPHT systems, there are usually HIPPS systems used to protect pipeline from over pressure. Depending on the HIPPS valve closure time, the pipeline will have a fortified zone.  Also, if the protection system upstream, and isolation systems downstream the pipeline fail to react on demand, there is possibility of pipeline over pressurisation which requires probabilistic design against burst or "loss of pressure containment". Other factors like accidental loads and seismic activities could affect the initial sizing of a pipeline. These accidental or exceptional load cases should be looked at using reliability and probability of failure analysis.


On-bottom stability

The vertical and lateral stability of pipelines during installation, testing, and operation depends on the hydrodynamic forces imposed on the pipeline, submerged weight of pipeline, and the type of the seabed soil. There are several approaches for calculation of hydrodynamic force based on the water depth and wave characteristics.

To ensure on-bottom stability of pipelines, simplified methods as well as finite element modelling may be emplyed. Nonlinear FE modelling in time domain in this case requires accurate soil behavior and hydrodynamics and can result in more economical design.


Risers and Umbilicals

Z-Subsea offers a very wide range of expertise in design of risers for shallow and deep water. A wide range of engineering software such as Orcaflex, Flexcom, and Sheer7 are used to obtain the design forces of the risers and estimate the effect of vortex induced vibration condition of the riser and fatigue life. The various type of risers covered in this category are:

  • Single rigid risers
  • Pipe-in-Pipe Risers
  • Caisson risers
  • Retrofit risers
  • J-tubes
  • Flexible riser
  • Umbilicals and cables
  • Steel Catenary Risers (SCR)
  • Drilling and work over risers

For installation of pipelines and risers, based on the RAO of the installation vessel and given sea state and seab
ed condition, various software packages are used for S-Lay, J-Lay, reeled pipes, and abandonment and recovery for shallow and deep water.

The installation parameters, including lay tension, bending stress/strain, departure angle and pipeline/stinger tip separation can be optimised.

Subsea Structures and Pipeworks

The design of subsea structures and foundation for operational conditions, dropped objects, trawl board impact, hydrodynamic forces, and seismic excitation is fully conducted for structures including:

  • SSIV
  • Manifold
  • Foundation (Mud mat, Pile, Suction pile)

Materials And Coating Selection

As part of successful oil and gas project execution, appropriate materials in various forms such as pipelines, piping system, pressure vessels, subsea structure, etc. should be selected, procured, fabricated and installed. Fit for purpose of the selected materials in terms of the operational condition, resistivity against wear, fatigue, corrosion and protecting requirements (coating and CP systems, etc.) are amongst other Z-Subsea in-depth materials/corrosion offering to the industry. At Z-Subsea, our materials Engineering group has the necessary capability and expertise to deliver the above services starting from high level materials screening/selection during the conceptual phase, provision of procurement specifications especially for the long lead components (Corrosion Resistance Alloys, Clad/lined materials, etc.), support during the operation until the end of the design life and beyond (asset life extension) with aiming at reducing the total life cycle cost. 

Z-Subsea materials group could also assist our client by provision of support on procurement specifications, Component fabrication, Vendor assessment and qualification.

Corrosion Protection Philosophy and CP Design

Corrosion mechanisms modelling as part of the materials selection study or Integrity Engineering services and Fitness for Service (FFS) assessments.

Corrosion protection services such as suggestion of the inhibition strategy, preparation of CP design philosophies, reviewing and upgrading existing CP procedures, performing CP surveys (Offshore/onshore) and coating selection and survey. Stages of CP selection are:

  • CP selection
  • CP interference
  • CP and anode weight calculation

Subsea Controls

Subsea control systems as a whole comprise following components:

  • Electrical Power Unit (EPU): To provide electrical power for functions required at SCM and MCM;
  • Hydraulic Power Unit (HPU): To provide hydraulic power for functions required at valves (via SCM and MCM);
  • Topside Umbilical Termination Unit (TUTU).  This unit is located between HPU and umbilical hang-off assembly.  The function of TUTU is to distribute hydraylic to umbilical cores.  It also includes ESD valves to shutdown hydraulic power if required
  • Subsea Distribution Unit (SDU): The function of this unit is to distribute hydraulics, electrical power and communications to different SCMs and MCMs
  • Umbilical Termination Assembly (UTA): this is a means of separating and terminating umbilical's cores and cables at subsea structures.   This unit provides a proper mechanical connection between umbilical and subsea structure
  • Subsea Control Module (SCM): SCM is located on X-mas tree and its function is to receive signals from topside and operate X-mas tree's valves.  In addition, it sends reading of pressure and temperature transmitters and multi phase flow meters to topside
  • Manifold Control Module (MCM): MCM is located on manifold and its function is to receive signals from topside and operate manifold tree's valves.  In addition, it sends reading of pressure and temperature transmitters and multi phase flow meters to topside.

Component Design

Subsea pipelines always require the design of mechanical components such as:

  • Flanges
  • Bulkheads
  • Tee-pieces
  • Y-Pieces

Theses components are traditionally designed based on stress linearization and stress categorisation. However, with increased computation powers of desktop computers, economical and robust design of components is possible using nonlinear finite element method and LRFD design criteria.

The advanced methods of components design utilise “Design By Analysis” (DBA) approach based on Load and Resistance Factors Design (LRFD) codes such as BS EN-13445, PD5500, or ASME VIII.

Based on DBA and employing LRFD, the load factors (>1.0) are applied to a component with resistance factors (<1.0) applied to the material strength. The criteria of design are usually based on limiting the strain in the component or check the structural stability or solution convergence.

Cost and Schedule Estimate

Cost and schedule estimate are among the main deliverables of a Pre-FEED and FEED study. A project will not be sanctioned unless a clear understanding of capital expenditure and the schedule for completion of the project is obtained and submitted to the management.  Cost and schedule estimates can be performed in different levels of accuracy.  These levels are mainly dictated by the client.  Typical value for a Pr-FEED cost study is +/- 40%. For a FEED study it needs to be narrowed down to +/- 20%.

The schedule needs to cover all engineering, procurement, installation, pre-commissioning and commissioning activities.

Subsea Process

The following are among the subsea process capabilities:

  • Subsea Process drawing development e.g. P&ID, PFD, C&E diagrams
  • Design pressure & temperature philosophy
  • Choke valve sizing
  • Start-up and Shutdown operating guidelines
  • Hydrate remediation strategy
  • HIPPS system operational test procedures
  • HAZID & HAZOP, LOPA participation
  • Equipment duty specification/datasheet
  • Technical evaluation of vendor proposals

Flow Assurance

The following are among the flow assurance capabilities:

  • Fluid property calculations using PVTSIM
  • Steady State analysis using PIPESIM (including pipeline sizing optimisation, pressure and temperature profiles, turndown, erosion velocity ratio, slugging etc.)
  • Hydrate and Wax management strategy
  • Insulation selection (U-value optimisation)
  • Liquid management and slug catcher optimisation
  • Transient analysis using OLGA (including Pipeline cool-down, restarts, ramp-up, packing, blowdown analysis)
  • HIPPS response time evaluation
  • Pigging simulation and optimisation