Understanding your asset is vital before making the big decision. Clarus Subsea’s Vinayak Patil and John MacDonald explain the key challenges of subsea integrity management in the Gulf of Mexico.
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Figure 1: Loop current frontal eddies (Coastal Marine Institute (LSU) – MMS Contract 1435-01-99-CA-30951-85247) |
Operators in the Gulf of Mexico (GoM) must remain as proactive in the approach to subsea integrity as they have been in pioneering deepwater. Many of the early deepwater facilities are nearing the end of their design life, bringing to light the decision of what is next. However, making confident decisions in terms of life extension or decommissioning requires a thorough understanding of the asset history. For example, events such as high eddy current speeds are often not the priority during operations but may become a driver of life extension. Also while pushing designs for deepwater requires bespoke systems and flexible regulations, there are few prescriptive requirements to guide best practice. These leave operators and subsea integrity management (IM) contractors to determine the best practices for their asset IM.
Loop currents
Loop currents are a unique feature to the Gulf of Mexico and have recently exceeded 4.3 knots (Eddy Lazarus), which approaches a 100-year return period event in accordance with the new metocean guidelines (API-RP-2MET). An image depicting the typical movement of loop currents and associated eddies is shown in Figure 1. Combined with these loop currents is the largest concentration of deepwater steel catenary risers (SCRs), which are subject to vortex induced vibrations (VIV) that can reduce the riser fatigue life and increase the potential for fatigue failure.
State-of-the-art subsea IM programs can take a stepwise approach to managing accumulated fatigue due to high currents and VIV: 1) design, 2) monitor and 3) response measurement. The first, design, is beyond the scope of this article, but what is relevant is that design predicts a baseline fatigue life in response to current profiles (including loop currents).
As a leading indicator of fatigue performance, fatigue can be analyzed indirectly through accumulation of measured current speeds. Response limits are derived based upon the design analysis predictions and measured values. The measured current speed compared to design becomes a cheap and easy key performance indicator (KPI) of both short- and long-term riser fatigue accumulation. The objective is to signal an alarm well before the design life is compromised and to allow time to respond economically with retrofit strakes or vessel repositioning.
Where measured data or life extension targets show the design to be un-conservative, riser response, including strain and motion, can be directly monitored by installing subsea data loggers on the SCR. By measuring the SCR response for a period of time, some conservatisms and assumptions can be replaced in the design models and a new fatigue life estimated. This is particularly valuable if the monitored data includes extreme events, such as a hurricane or the recent high loop currents. However, this is a two-way street particularly for older designs where newer methods and metocean data may actually show the original analysis to be un-conservative, but a mature IM program in a frontier region such as the deepwater GoM will look for opportunities to reduce long-term risk by increasing confidence in the system. Preventing incidents has proven to be significantly cheaper than responding to them.
Bureau of Safety and Environmental Enforcement regulations for integrity management
US Bureau of Safety and Environmental Enforcement (BSEE) regulations require operators to manage integrity of the outer continental shelf (OCS) facilities in accordance with the safety and environmental management systems (SEMS) rule (30 CFR 250, Subpart S). The SEMS rule, based on the API-RP-75, is not prescriptive in terms of subsea IM. This allows operators latitude in development of their IM programs, leaving a gap in what is commonly accepted as “best practice.” The SEMS program is required to be audited within two years of the initial implementation and at least once every three years thereafter. The benefit of an open specification is that it allows the operator to set their own IM plans and priorities based on risks relevant to their system. There are many serial number one designed equipment in use in the GoM. The challenge is passing the required audit with little direction from the auditor.
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Figure 2: Subsea integrity management elements |
Clarus has supported audits on behalf of operators and can confirm that the expectation is to identify and determine, in writing, an IM plan and then to implement/execute phases to resolve elements addressed within said plan. Subsea IM plans typically address the six elements shown in Figure 2 as a minimum, but Clarus has identified three key components that are adopted in more mature and efficient programs.
DNV GL has released recommended practices in the past few years covering both riser and subsea equipment integrity management. Both have good recommendations and considerations to get the most of an IM program. In the end, an efficient and effective IM program will be more valuable to the operators and will increase the likelihood of passing the BSEE audits, as a small side benefit.
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Figure 3: GoM vessels age |
Age of deepwater structures
The earliest deepwater structures in the GoM are approaching the end of their design service life, which is typically 20 years as shown in Figure 3. Approximately 2% of the total assets in the GoM are beyond their original design life, and approximately 15% have been in operation for more than 15 years and are approaching their design service life. As the service life approaches, operators have to decide between decommissioning, life extension or divestment. Not only is this a complex decision, but it is one that many GoM engineering teams are facing for the first time in their career. New subsea tiebacks to existing platforms offer an economical option for the operators in comparison to the installation of new structures, especially in the current financial environment. However, subsea tieback of new developments to the existing flowline-riser systems that are nearing their service life requires a life extension assessment.
Confident decisions regarding life extension or decommissioning of aged structures can take a few or more years. A comprehensive IM program, incorporating some of the elements already discussed from the design phase through the operations phase, can provide a clear “health history” of the aged structure as a first step. The most commonly accepted steps of a life extension assessment include:
Life extension is a new activity for the GoM industry, and guidance on life extension assessment is developing such as ABS’ draft life extension methodology for floating production installations. Starting early with engineering efforts and communication with regulators is recommended and gives the best possibility for life extension over forcing decommissioning or divestment.
Conclusion
The GoM faces many unique challenges including those related to long term IM of subsea systems. Program development and execution is a key to proactively manage these challenges. It enables operators to stay in compliance with regulations and provides visibility to future opportunities with the asset. An IM program does not have to be expensive. Using the same efficient approach used to make this market a success can ensure that the GoM systems and the people operating within those units are safe. But, in order to do so, IM programs need to be written down and reviewed on a regular basis to address the unique issues faced by each asset.
Vinayak Patil is an integrity specialist at Clarus Subsea Integrity in Houston. He holds bachelors and master’s degree in chemical engineering and has accumulated seven years’ experience in subsea integrity management, corrosion management and chemical process engineering. His project experience includes working with major operators in the Gulf of Mexico to provide integrity management support and solutions.
John MacDonald is currently vice president at Clarus Subsea Integrity in Houston, a recent spin off of sister company 2H Offshore. He holds a BS in ocean engineering from Texas A&M University and is both a Chartered Engineer and a certified project management professional. He has accumulated 14 years’ engineering experience in riser analysis, naval architecture, verification, acceptance testing and integrity management for 2H Offshore and Bath Iron works.
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