Subsea System Engineering and Operations Consulting

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 …

 

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…Brian is very knowledgeable on subsea systems including horizontal and vertical trees, completion risers and subsea test trees. I would recommend Brian highly for any work in this area….
Steve PierceDeepwater Well Intervention AdvisorChevron North America E&P