Strength in monitoring

Structural integrity monitoring should be a lifecycle tool and could make jackup moves safer. Meg Chesshyre reports from Oceanology International.

Jackups. Photo by staff.

A call for further development of structural monitoring systems for offshore use was made by a member of the UK Health and Safety Executive’s Energy Division at the Oceanology International conference in London this spring.

In a joint paper, presented by Alexander Stacey on behalf of himself and John Sharp of Cranfield University, Stacey said structural monitoring techniques complemented periodic techniques as tools for integrity assurance and that their use provides clear benefits in the maintenance and integrity of offshore structures. But, he said there is wide range of techniques that vary in terms of maturity and applicability, and that further development is required if detection and continuous monitoring of defects is to be achieved.

Stacey says that structural monitoring systems have been used on a number of offshore installations, but they are only being used on an as-required basis, and have not been adopted by the offshore industry in routine inspection. He added: “There is little guidance on the application of structural monitoring techniques.” It’s an industry not immune to structural failure. Stacey gave examples such as the short-lived Sea Gem rig in 1965, which was the first occurrence of fatigue offshore, resulting in 13 deaths. Next, there was the catastrophic failure of the Alexander Kielland semisubmersible drilling rig in 1980, which occurred in Norwegian waters, resulting in 123 deaths.

Stacey says the offshore lifecycle could be divided as follows:

1970 - 1990: The design, construction and installation phase for UK Continental Shelf (UKCS), with codes and standards developed (API, D.En/ HSE Guidance Notes);

1980 - 2000: The operational phase, certification in the UK (until 1998), the safety case regime implemented from 1995, following Piper Alpha; in the US, reassessment for post-hurricane Andrew damage; the development of international ISO offshore standards;

2000: Life extension, decommissioning and re-use, aging installations institutions in the North Sea and the need to review post design life. More than 160 UKCS North Sea installations, over 60%, are now over 25 years old.

Lifecycle adoption

Stacey says there is a “bathtub curve,” starting with early life failures, followed by a steady failure rate, then the wear out phase, pointing to a need to focus on structural integrity management throughout the lifecycle. This requires good data on current condition, and an understanding of degradation processes and structural response. The use of best technology/latest assessment- methods (e.g. system strength) is required to determine safety margins, as well as the development of new techniques, such as online monitoring.

There have been lot of technological developments over the past decade. Some structural integrity monitoring techniques are suitable for localized operation, e.g. acoustic emissions and fatigue gauge, and some could provide global platform information, such as natural frequency monitoring, but are limited in capability to detect smaller defects.

Stacey points to the SIMoNET program – www.simonet.org.uk – a joint venture between industry and government organizations, managed by University College London and Cranfield University. It provides a forum for the exchange of information on structural monitoring techniques.

Photo from unknown photographer/Norwegian Petroleum Museum.

Measurement equals management

“If you can measure it, you can manage it,” commented Karthik Subramanian senior engineer, ABS London, in a presentation on jackup motions monitoring for safety and operational efficiency.

Key performance indicators (KPIs) could be of use in monitoring rig moves. The KP lagging indicators, which show output results, are easy to measure, but hard to improve or influence. The KP leading indicators, which measure changes in input variables, are hard to measure, but easier to influence, he said. In terms of rig moves, the lagging indicators are the incident rate, and the leading indicators are measuring the hull motions and the sea states.

The hull motions can be captured with accelerometers mounted on the hull. The signals are sent to the onboard data acquisition system, then to signal processing onshore. The sea states can be measured using laser wave sensors mounted on hand rails. The wave elevation signals are then sent to the data acquisition system.

ABS is interested in jackup monitoring because most of the jackup fleet is classed under ABS. “We need our clients to operate safely,” Subramanian said. “Basically, we want to bridge the gap between the sensor technology and rig move operations.”

Almost 70% of the accidents for jackup units happen during a rig move, he said. The industry has developed a number of monitoring committees. The International Association of Drilling Contractors (IADC) jackup committee was formed 20 years ago. The International Organization for Standardization (ISO) formed Panel 54 working on the installation and retrieval process for jackup rigs. The underwriting community, the Society of Naval and Marine Engineers and ISO have both been developing standards for jackup units for site specific assessments and rig design for over 20 years.

Guidance due

ABS has prepared draft guidance notes on “Hull motions and monitoring systems for jackup units,” which it intends to publish around June, once necessary approvals are in place. It will include performance specifications, recommendations, and methodology for data analysis, jackup modeling and analysis. Subramanian said that the guide will help sensor technology developers understand how they can bring their technology into the jackup unit, and how the jackup rig moves and operations can be made safer.

Real-time monitoring of marine conditions, vessel response, and structural integrity are key to managing operational risk and safety for offshore assets, particularly as operators move into deeper, more remote areas, said Robert J. Barker, business development – global sales and marketing, BMT Scientific Marine Services.

An integrated marine monitoring system provides both offshore decision support and the means for conducting long-term integrity assessment over the design life of the facility. It adds value and saves cost. Such systems have been successfully integrated into platform safety management programs. Real-time data is being used successfully to support operational risk and reliability programs. Archived data from marine monitoring systems is key for performing forensic and engineering analysis, he said.