With offshore developments moving into deeper water coupled with more severe operating conditions, subsea production is facing more challenges. The industry needs more innovative technologies for subsea production to assure safety, integrity, reliability, efficiency and enhanced recovery rates.

DNV GL sets the standard for subsea equipment and components, assisting the industry in evaluating the risks and then applying technology qualification, analysis and verification/certification processes by means of endorsed industry standards, specifications and recommended practices (RPs).

Technology development updates

HP/HT and API 17TR8: As an attempt to provide the industry some design guidance for subsea equipment used in HP/HT environments, the API 17TR8 technical report intends to serve as a reference to establish design methodologies, material functional specifications and design validation requirements.

The proposed HP/HT design method recommended in the report is to follow a design-by-analysis through finite element analysis methodologies either by linear-elastic or elastic-plastic analysis techniques, depending on the HP/HT scenario. The pressure rating and temperature class are treated as separate design considerations. The pressure rating is primarily proposed to drive the decision-making process for design method. Once pressure rating is determined, temperature effects as they affect changes in material properties are brought into the design decision-making process.

However, the combined HP/HT effect on design and materials properties seems to be the main issue as suggested in the technical report. Separately, high pressure with low temperature or low pressure with high temperature has proven to be successfully developed. The combined HP/HT effect can create far worse conditions for stress corrosion cracking, corrosion fatigue and stress relaxation of seals. Design verification and validation tests become more complex for combined HP/HT conditions.

HP/HT and 30 CFR 250: While the release of API 17TR8 is the first of several editions serving as a design guideline for subsea equipment, forward thinking of how the guideline can be applied to current 30 C.F.R. §250.807 regulations should be considered—namely, examination of HP/HT equipment design verification process; examination of HP/HT equipment design validation, functional testing and procedures; and an explanation of why the analysis, processes and procedures ensure that the HP/HT equipment is fit for service.

In all cases, the regulators should be engaged at the earliest opportunity to apprise them of the qualification programs and the proposed design methodology.

Material toughness: In recent years, a significant number of documented field failures took place, almost all due to brittle fracture of parts made from low-alloy steels, stainless steels and even highly corrosion-resistant Ni-base alloys.

The failure root causes all point to low materials toughness due to improper materials processing, environmental embrittlement and drawback in design. In response to this trend, materials toughness is becoming an important materials property and a challenging issue to tackle. This is apparent in the API 17TR8 technical report for HP/HT equipment and revision of the API 6A 718 standard to add a series of Ni-base alloys other than only alloy 718.

Current subsea drilling and production systems have become a mix of complex structures with all sorts of materials and coatings being used (metals, elastomers and ceramics) and with all kinds of geometries, interfaces and joints having been designed and manufactured. Surrounding these complex materials structures are usually corrosive fluids with unwanted physical and chemical reactions occurring all the time.

To mitigate the subsea risks, reduce manufacturing quality assurance/quality control and operational cost and ensure equipment lifetime integrity, a design and materials verification methodology centering on maximizing and maintaining materials toughness—not material strength—needs to be created and applied for HP/HT subsea equipment design verification and even technical qualification processes.

Mooring integrity management: Recently Ma, et al. (OTC-24025) reported on mooring failures occurring during the last decade. They reported on a total of 21 mooring incidents, including eight major incidents. The major incidents included multiple line failure, loss of station keeping, emergency production shutdown and damage to risers with associated hydrocarbon leak.

The authors concluded that the annual probability of mooring system (i.e. multiple lines) failure is on the order of 3 x 10-3, which is raising concern among industry experts. A number of failure mechanisms were identified, including some considered relatively novel, such as out-of-plane bending (OPB), chain hockling/twisting/knotting, flawed flash butt welds, low fracture toughness and pitting corrosion. A number of mooring lines failed or were found to be understrength because improper weld repairs were performed on defects. In the majority of instances, mooring line failures occurred at an interface or discontinuity.

There is a need for better wear and corrosion degradations models; a better understanding of OPB-induced fatigue; data for large-diameter, higher-strength chain grades; better, more reliable inspection and monitoring tools; and more effective integrity management systems that can work across a corporation’s business unit boundaries.

DNV GL efforts

With project experience in the most challenging subsea environments, DNV GL continues to work with the industry on subsea production challenges.

Subsea equipment and documentation standardization: DNV GL has established initiatives for both the standardization of subsea equipment and subsea documentation.

Developing subsea systems is costly and time-consuming, due partly to fields frequently having tailor-made solutions with extensive qualification programs. Adding to the cost are the special installation tools required, often on specialized vessels. By standardizing tie-in technology and module sizes, it will be easier to combine different types of technology to adapt to project needs.

Similarly, subsea documentation is another costly and time-consuming requirement for projects. This project aims to present the minimum set of documentation requirements for all major subsea components. It will improve industry practices by helping both customers and suppliers manage requirements, providing the correct/complete information for the right equipment and user and increasing predictability for all parties involved.

New subsea report on technology, safety and future trends: DNV GL recently issued the report “Subsea—Technology Developments, Incidents and Future Trends” on behalf of the Norwegian Petroleum Safety Authority. The report aims to raise awareness and share knowledge within the industry on safety, cooperation, degradation mechanisms, failure modes, monitoring, integrity management and incident-related information.

New RP on subsea integrity management: Following the successful completion of the Subsea Integrity Management joint industry project, DNV GL has now published the corresponding RP. The RP outlines a step-by-step guide for how to assess and manage integrity of subsea facilities and aims to provide a reliable point of reference for both the industry and authorities, helping to raise the standard within the subsea integrity management community.

Editor’s note: Additional information on projects and documents described can be viewed or downloaded for free on the DNV GL website at dnvgl.com.