Rig-based mooring inspection saves millions

Inspections are critical for safety, but moving floating systems off station to undergo shore-based mooring inspection can be very expensive. A new approach that allows for inspections carried out from the rig while offshore has cost-savings implications for the entire moored semisubmersible sector because it reduces the need for costly and time-consuming relocations.

The desire to find a better solution spurred the alliance among a Houston-based driller, a Canadian offshore services company, a Houston-based mooring company, and a Norwegian certifying authority. The result of their efforts is a pioneering service that surveys mooring components onboard moored drilling rigs. By working closely with the client and the class society, Welaptega has been able to develop and introduce a risk-based inspection strategy robust enough to satisfy class while maintaining optimum performance for the client.

A new approach
The inaugural inspection of the rig-based inspection system was conducted in the summer of 2009 on a mobile offshore drilling unit (MODU) offshore Spain during a rig move. The inspection employed Welaptega’s Chain Measurement System (CMS), an optical caliper that has a proven record with floating production systems. The successful application of the CMS under water convinced the company it could be adapted for use on the topsides, given that a dry inspection on deck would be simpler in many ways than one carried out subsea. Normally the CMS is deployed by a remotely operated vehicle (ROV) and is used for inspecting mooring chains on floating production systems. The CMS generally is combined with Welaptega’s rope measurement system (RMS), an ROV-deployed frame equipped with a hydraulic sled and geometrically fixed cameras.

For in-air inspection, where the moorings are retrieved, inspections include missing or loose studs; bent links; and excessive corrosion, including surface pitting, leading to sharp gouges and surface cracks. Welaptega personnel applied the CMS to survey chain and the RMS to survey rope moorings, gathering data for the Special Periodic Survey recertification of the moorings by DNV.

Welaptega’s equipment and operators, along with chain inspectors from InterMoor (Europe), were deployed to the drilling rig during mooring recovery and relocation to another well. Measurements were conducted on 5% of chain links on each of the eight MODU mooring legs during two rig moves. The program produced sufficient information to receive a recertification of the mooring system by DNV. The application of this inspection technique saved the driller an estimated US $10 million or more in maintenance costs on one rig alone.

An offshore crew prepares to carry out an inspection. (Images courtesy of Welaptega Marine Ltd.)

Rig-based model trumps traditional approaches
Rig-based inspection is an improvement over traditional mooring inspection methods for a number of reasons. The most obvious are cost, time, and convenience. Inspecting chains, wire, and connectors on deck rather than in dry dock cuts maintenance costs and reduces downtime.

The topside inspection model also maximizes downtime efficiency when rigs have to be docked for large-scale maintenance operations. Because moorings have already been inspected, valuable time is available for other more pressing maintenance issues. Inspecting mooring systems in situ also reduces the risks associated with moving mooring lines dockside, including potential damage to the rig (caused by chain drop, unspooling of wire, etc.), and more importantly, the risk of injury to personnel.

Topside inspections also meet an important emerging industry need to conduct comprehensive mooring inspections following severe storm conditions. In the wake of recent hurricanes, reporting rules have changed. Now facilities in the US Gulf of Mexico are required to prove they are operational after a hurricane. The onboard inspection technique enables quick post-hurricane inspections, which means MODUs can be back online more quickly. There is potential for this inspection method to be used to survey mooring systems before a rig is deployed.

It is advantageous for operators to know what condition mooring chains are in before a unit is deployed. Dry land inspection can provide intelligence that can be used to determine which mooring components to deploy for a particular job. Another advantage of the CMS is speed. The optical nature of the CMS allows for fast data acquisition. In one case, an entire line was fully inspected and measured in 80 minutes during a rig move.

Each inspection is planned depending on the operator, mooring spread, the type of rig, and the class requirement for the MODU. Where traditional offshore inspection has required the mooring lines to be laid out across the deck or dock, Welaptega’s approach allows both chain and ropes to be inspected during the standard retrieval/deployment process without the requirement of deck space for layout. For surveying purposes, measurements are taken of the intergrip diameter on mooring chains, including transverse diameter at the crown if the intergrip diameter is near discard. Surveys also measure link elongation and look for fatigue cracking at the crowns and shoulders of individual links. Wire and fiber rope surveys measure the cross-sectional profile along the full length of the rope.

The CMS, an optical caliper, is deployed by an ROV. The successful application of the CMS under water convinced the company it could be adapted for use on the topsides.

Video cameras provide a full 360º inspection of broken strands; excessive corrosion; and gross deformations, including kinking, crushing, core protrusion, etc. In situ inspection allows chains to be measured under tension, producing more accurate results for link elongation. Wire ropes also are measured under tension, either while installed or during pull-in.

Taking it subsea
Ideally, MODU mooring inspection takes place on deck, but if this is not possible, the same techniques can be used subsea using ROV-deployed chain or rope measuring systems. Much in-water inspection is limited to components in the catenary rather than in grounded or buried sections of the mooring system. In-water inspection can be made more comprehensive by using an anchor-handling vessel to lift grounded/buried components. An inspection can be a hybrid of in-water and rig-based options, depending on the situation.

For instance, if the MODU can pay the moorings partially in or out while on station, the full inspection can be accomplished in part by ROV-deployed equipment and in part from the rig deck, depending upon vessel circumstances, contractual allowances, and drilling schedules. Welaptega’s approach to inspection largely removes human influence from the point of inspection, ensuring consistency of results among sites, projects, and teams, and because the inspections are largely video-based, a permanent record is created that can be archived and reviewed after inspection.

Video data is standardized and is made available on a permanent basis. Rapid retrieval and easy availability enhances decision-making processes on the adequacy of components and inspection methodologies. The visual data also can be made available to class societies or other technical authorities. Welaptega is the only company that can certify mooring systems in situ for the floating production market with approvals from Lloyd’s Register and DNV and validated by the International Association of Classification Societies. Approval by both ABS and Bureau Veritas is under way.