Subsea System Engineering and Operations Consulting

Global Riser Analysis – “Creating a Bridge between Riser Analyst and Operational Consultant”

Key Messages of Blog:

  • Providing Operational input (**)  to support the analyst create the GRA (Global Riser Analysis)  model is essential and can result in lower cost , and improved quality of results.
  • System boundary conditions between riser /landing string need to be defined early to ensure GRA analysis is accuracy
  • Understanding Subsea Equipment functional and structural capacities factor into GRA and adjustments to operability ranges for various uncontrolled events like weather , sea state and currents.
  • End user should have a quality program to ensure that GRA model accurately depicts the “system” which includes the MODU (Mobile Offshore Drilling Unit), Riser / landing string, and compensation systems.
  • Assisting the riser analyst understand the significant aspects of the GRA model and ensure that boundary conditions and constraints are identified working as a team will deliver the best overall result.

Open water deployed Completion and Work-over Riser systems (CWOR)  and Specialized Landing strings with SSTT (Subsea Test Trees) run inside Marine Drilling riser systems are two modes to establish bore access to a subsea production tree.   Deployment is either from a MODU or MSV (Multi-Service Vessel) with motion compensation hoisting systems to decouple vessel heave from the riser or landing string.

Riser analyst are often contracted to provide a global riser or landing string analysis which defines allowable operational envelopes for the completion and intervention system.  A typical challenge is many of the riser analyst do not have full understanding of the offshore boundary conditions and constraints and need technical / operational input from operator / consultant (eg DeepMar).

Creating a global model of the subsea equipment, wellhead , Subsea tree and riser  with appropriate stiffness factors at each transition is vital to ensure screening results are estimated accurately.   Additionally, building an interface model with the vessel compensation system to ensure stiffness of compensation during heave, stroke limits , and offset limits are established to ensure riser does not contact MODU /MSV hull all are contributing factors in the GRA analysis.

Typically the end user (eg Operator) has to provide oversight to ensure appropriate boundary conditions  (e.g.  accurate compensation stiffness factors, soil stiffness factors, specific to the well)   are key factors in the analysis process.   Offering practical and operational guidance early in the build of the GRA model can be invaluable and result in an overall lower cost solution. Training and mentoring analyst to improve operational boundary condition understanding  is essential to creating an analytical tool with value.

** DeepMar has provided riser / landing string Design and Operational Assurance service on behalf of major operators to improve safety and operability.

Subsea Riser Operability – What is it and Why is it important ?

What is an Operability Plan ?

An Operability plan is a operational document bridging output results from global riser analysis and/or testing phase of the various system components to deliver a set of “operability curves” – establishing boundary limits for a specific vessel, riser system, dynamic metocean environment and associated well system.  Understanding of such structural and functional limits are especially critical to manage risk given such  safety functions are barriers impacting well control or well containment.

Operational curves should illustrate limits for various weather conditions related to floating vessel offset, riser tension, Internal pressure, Hydraulic response times (eg latency time to disconnect and appropriate closure sequences -ESD/EQD).  The plan should include  weak point analysis , leakage /functional and structural limits to ensure field operations personnel can manage surface offsets to maintain adequate safety margin of equipment for planned and unplanned DP events.

The Intervention Engineer is typically challenged to consolidate this complex data set and create a functional operational plan.

The following are selected areas such as :

  • Loading conditions for the riser system to evaluate both planned and unplanned conditions
  • Vessel Response to normal, extreme and accidental Weather/ current limits ,
  • Vessel response  conditions when the vessel is connected/disconnected to subsea well assets,
  • System control logic, shut in sequence  and critical function latency time to compare to vessel drift rate to manage risks while connected at extreme and accident offsets.
  • Process should ensure all the assumptions, analysis results compiled are simplified for field personnel to manage (eg Watch circle),  Competency Training of key operational personnel.

Depending on type of MODU (Mobile Offshore Drilling Unit), when a DP (dynamic positioned)  MODU is connected to a subsea asset, the Global operability analysis and plan shall include an unplanned DP drive off / drift off event as contingency to normal operations.  Many reasons may cause a DP event to occur  and such reasons are outside the  scope of this blog.

Note: Links below offer graphic “Example” illustrations to be used for reference only.

Reference Link : DP Operations Guidance

General Description – Subsea Riser System

The System functions as the primary fluid conduit and provides structural attachment between the well and surface vessel comprising of  many components and sealing elements    Such include:  vessel /MODU, riser or landing string system, control system which are integrated and managed by field operations personnel through a comprehensive Operability Plan across uncontrolled weather events.  Creation of the operability plan is one of the most important tasks needed to ensure “The System” can be deployed and provides the functional connection within design and operational limits per the given environment.  The safety components require both structural and functional limits to ensure well bore isolation is achieve at extreme and accidental loads.

Selected Subsea well  intervention operations  require attaching a tubular conduit between a dynamic vessel (eg MODU) and the seafloor or subsea well.  Connecting between the surface floating vessel (MODU) and the seafloor can occur in multiple ways.  Deployment in open water , it may occur using a completion work over riser (CWOR), or through a Marine Drilling riser using a dedicated Landing string as part of a Subsea Test Tree system.  Reference Link – Subsea Riser Connected to MODU.  The illustration highlights the impact of change to surface nominal position when connected to subsea asset.   Surface position of the MODU or MSV (Multi-Service vessel ) can be  caused by loss of Dynamic Position signal, loss of power resulting in limited thruster power to hold position for a given weather event, or limits of riser and disconnect functionality.

NOTE: For Reference an Engineer may reference DNV – RP – H101 for more information to consider when evaluating DP system risks.

Output from Analysis – Station keeping

Global systems analysis is generated within the engineering phase using specific riser component testing data, MODU data and environmental inputs to evaluate selected operational load conditions.  The intervention engineer should fully understand such systems analysis,  performance limits, and  failure modes to ensure appropriate watch circle envelope is generated and communicate safe operational constraints.  Reference Illustration of Watch circle Graphic

The operational plan must consider a loss of station keeping condition as part or the design and global analysis program for a given riser system, while maintaining functional and structural integrity  and establishing critical barriers resulting in control of the well.   Key elements of an operability plan includes a safe shut in – disconnect sequence / well control strategy, while managing offset and riser working stress conditions.   Watch circle envelopes graphically illustrate boundaries which require action to be taken as offset may increase from nominal well center surface position.    The typical watchcircle uses (GREEN – SAFE), (YELLOW – Take Appropriate Action), (Red – Disconnect) concentric circles to aid field management as to when to take appropriate action at the appropriate time.  Additionally, using drift rate prediction curves, the envelope should highlight safe shut in periods if loss of power or other unforeseen failure events may result in uncontrolled surface offset.

Other selected factors which impact envelopes are:

  • MODU Drift rate (caused in the event of failure of power or thruster system for various weather conditions),
  • Riser Internal Pressure and Tension Conditions,
  • Current loading which may result in lowering fatigue life of riser system
  • Current loading which may impact control functionality and reliability
  • Knowledge of Integration of hardware and control system.  The output of various analysis both structural and hydraulic response limits , static and dynamic  loading conditions, control system response time predictions are input elements to the Watch circle Envelope.
  • Pre -Engineering: Global Analysis  engineering combined with test qualification data  used to define functional (eg Leakage)  and structural limits (eg Failure)  of various system component in the primary load path.

Comprehensive Elements of Plan

Various load conditions should be pre-defined which typically are  changes in pressure, tension, bending caused by offset of the vessel, or operational changes.  Conducting analysis to simulate the various system limitations across a range of loading conditions is critical.  This complexity both on the design of the riser & control systems response times evaluated within  vessel drift limits  is vital to achieving desired operational limits within the prescribed safety envelope.   Changes to vessel infrastructure , global  regions , various loading conditions impact offset conditions and limits and directional shape.

Creating a comprehensive Operational  plan, should be an iterative process both with Engineering and Operations to ensure proper training and understanding of the limits of the analysis are conveyed to the field.   The operational plan is complex and shall  incorporate  a range of environmental conditions, equipment performance limits based on load cases, control response times, vessel response motion characteristics all built into simplified procedure resulting in the formation of a safe operability plan with clear performance limitations of each component in the primary load path.

How can DeepMar Consulting Assist ?

DeepMar Consulting has extensive experience in systems engineering and operational planning to many clients, which conduct riser based operations in offshore environments.  This blog is intended to offer a subsea engineer the basics and  illustrate selected Operability elements to create safe and executable  offshore  plans, while offering useful links to reference which aid in improved operational safety , reliability and efficiency.    To manage such complexity requires in depth front end planning, multi-team buy-in (eg Engineering and Operations)  and continuous attention to understanding the performance boundaries of equipment to control risks across an array of conditions.

We welcome and look forward to your comments on the blog …


Developing an “Actively Managed” Subsea Asset Intervention Strategy

Written by: Brian Saucier : President, DeepMar Consulting


This blog shares  key learning’s for engineering and operational teams to consider when building a successful subsea asset production & intervention strategy.   Early identification of design opportunities to enhance safety, reliability and efficiency for subsea field optimization is essential.   This concept is demonstrated in the simple INFLUENCE DIAGRAM (ref Figure 1 )  .   Often many design opportunities are missed which could, implemented result in improved Life of Project  value and reduction of risks.  During project sanction, increased awareness of future intervention & abandonment operations should be incorporated and considered within the design basis.   Information within the blog could be a reference tool to both intervention and asset managers who are continuously monitoring many of the parameters in order to achieve maximum ROI within a dynamic oil and gas price environment.


Development of oil and gas fields employing subsea technology results in a higher cost of intervention as opposed to surface well or offshore platform well location.  Subsea projects carry an economic liability both in management of production optimization, using high cost assets to intervene, and future Plug and Abandonment liability which can erode overall project ROI if not adequately managed.

Creating and maintaining a “Active Managed” subsea intervention strategy is essential to manage field economics, technology changes and production contingencies. Subsea assets must have a flexible Intervention strategic plan to forecast “life of field” intervention cost and ensure specialized equipment and technology gaps for future intervention requirements are met. With the lower price environment, the need to focus on how to manage existing assets and to reap optimized returns (eg added value) while maintaining lower cost and risk offers tremendous opportunity.


  • Subsea Interventions are inevitable, and should be fully planned and incorporated within every phase of the Subsea development design phase.  Projects should adequately simulate via probability modelling, a range of dynamic parameters, intervention types and be an integral service component to the subsea asset team
  • Information gained from continuous surveillance and influence changing parameters should be seamlessly integrated between intervention engineering and production asset team.
  • Given the limited published industry data related to subsea component reliability, the need to create “live” database resource files to track regional component failure rates and trends
  • Front end design decisions should integrate subsea intervention requirements into subsea production equipment design to enable production optimization and lower risks when performing future intervention operations.  Intervention failure rates, frequency and types of intervention operations shall be integral to the decision as to whether a Horizontal or Vertical tree is selected.
  • Lower cost intervention vessels verse typical MODUs, offer a suite of intervention operations which can be performed and should be considered and be a factor in the economic model.
  • Setting realistic expectations on intervention frequency, types of operations and risk in a dynamic oil and gas price environment should be continuously monitored and managed.
  • Forecasting specialized tooling and identification of intervention assets to capitalize on market price cycles should be constantly monitored to reduce cycle time for intervention readiness.   Consider investment in intervention readiness when price is low to be able to reduce intervention cycle time when market shifts to higher price.
  • Improve wellbore design by engagement with intervention specialists to evaluate intervention tool compatibility and risks on the front end to avoid identification of an enhancement during the intervention planning engineering – which is too late.
  • Enhance subsea assets to allow intervention access points and offer opportunities which can be explored using lower cost intervention vessels.
  • Develop Plug and Abandonment strategies during field life to capitalize on lower price market cycles and development of core lower cost enabling technology to be realized.
  • Improve competency and training to ensure tools, equipment and processes are well defined and functional and structural performance limits defined.
  • Build appropriate simulation models to cover the range of “realistic uncertainty” within the life of field development plan.


  • During the economic sanction and evaluation of the intervention strategy, the team should focus on creating “realistic simulations” which can be processed within deterministic and probability models simulating a range of uncertainty in both positive and negative directions.  Selected dynamic parameters which should be included in Intervention Strategy include:
o   Deviations in oil and gas price environment
o   Technology and/or Process improvement enablers
o   Improved Reliability thru enhanced design assurance and Root cause analysis
o   Data Acquisition systems to manage maintenance and lowering of downtime risks
o   Execute Verification and Validation process
o   Identification of “all” Liabilities – PA, Well Control / Well containment
o   Equipment (over-supply or under supply conditions)
o   Leverage Contractor strategies and share risks (where possible)
o   Regional vessel /MODU market conditions
o   Knowledge and application of operations to meet regional regulatory requirements


  • Incorporate Intervention input within the design of lower and upper completion.   Focused on tubing isolation barriers, casing & tubing selection, auxiliary component selection, bore size compatibility with specialized fishing and logging tools, future re-completion or repair activities, etc.  Often opportunities are missed during the initial design phase resulting in greater completion risks, higher cost and/ or potential loss of well.


  • Maintaining production and intervention readiness metrics, acceptance standards and KPI’s to ensure all teams are aligned and focused on optimization and opportunity recognition. Such include established alignment of intervention operations sequences to ensure well shut in times are optimized with multiple trip, cost saving solutions and operational methods.


  • Equipment verification and validation plan to improve reliability, integrity and safety. Enhance component performance through rigorous verification and validation especially on tools which are rental and limited performance history is known.  Details to enhance the overall reliability of components and processes to ensure that operational conditions do not expose equipment to subsequent failures are:
o   Established Load case definition and performance application
o   Testing and qualification simulation to be aligned with operational conditions
o   Standardized interfaces and methods with continuous improvement on learning


  • Partnerships and contracting strategy to lower cost, improve efficiency and enhance cycle time readiness.    Partners become familiar with E&P operators risk profile and methods by which work in conducted.   This knowledge improves over time and leveraging aligned expectations and bridging of safety cultures aid in overall learning curve efficiency enhancements.


  • Identification of transitional points within the field development process such as (field architecture, well intervention flexibility incorporation into completion, subsea tree selection) are a few of the key focus items. Ability to influence or “refine” the design diminishes over time, while the cost of change and schedule increases exponentially over time.   Successful project teams recognize the point of inflection where teams work towards brainstorming optimized solutions yet transition to converge to decision and build appropriate focus and execution strategy to manage risks and deliver the project.   Realization of this ‘inflection point” is a key transition towards maturing the concept with additional engineering evaluation.  (Figure 1)

Figure 1: Typical Influence Diagram


Closing Comment  

Management of subsea assets involves the work and coordination of many multi-disciplined teams.  Coordination of the teams and communication via a “dynamic” Intervention strategy for the field which keeps focus and awareness of the opportunities is one of the keys to success.  Several of the recommendations offered in this paper were a result of many years of experience and observations noted.  DeepMar can assist your team in various aspects both strategic and tactical to deliver a managed plan resulting in a low cost more optimized solution.

Brian Saucier – DeepMar Consulting

Brian is an independent consultant with over 33 years of industry experience specializing in deep water subsea project design, installation and life of field intervention. He has consulted for several of the major E&P companies in various project roles.  Brian is a member of ASME and has authored and co-authored several industry papers in the field of subsea engineering and patent holder.  Brian can be reached at