Attention to environment key to managing mooring integrity

How three floating production system (FPS) operators faced deep-sea mooring line challenges.

By Jim Macklin and Kent Longridge, Intermoor, Inc. March 28, 2016

Permanent mooring systems typically have a design life span of 20 to 30 years. Floating production system (FPS) operators are quite often faced with mooring system integrity issues, including mooring line failure. When we contemplate the value of mooring integrity management (MIM) we first have to ask; how have we structured the MIM program? How frequently should we assess the mooring system’s reliability? Are we collecting the necessary data with in-service inspection plans (ISIP) to make informed and pragmatic decisions as part of the structural integrity management (SIM) to safely hold the floating production unit on station? Have we developed the relevant data or are we "shot-gunning" our data collection to acquire anything that is convenient or everything we can?

Maritime history shows that mooring systems fail in a wide range of water depths, regions, and environmental conditions. The consequences of mooring failure can range from asset damage and production interruption to environmental issues and even personnel loss. When it comes to mooring line failure, there are several elements to watch for, such as basic wear-and-tear, corrosion and integral production defects.

Among these elements are:

Wear can occur when the mooring line rubs on adjacent line components at connecting links, fairleaders, bending shoes, etc.

Fatigue damage can be due to crack initiation and propagation from repetitive axial and bending stress.

Abrasion from the chain coming in contact with the seabed; sediments are abrasive and friction can erode the chain.

Corrosion and rust can result from chemical reactions between the material and the surrounding environment.

Growth of marine life, such as algae and barnacles on fiber ropes, can further contribute to the need to replace the mooring line to avoid failure.

Damage to chain, wire rope or polyester rope can happen during installation or inspection operations, or from dropped objects.

Impure materials, improper heat treatment, improper or non-compliant assembly, or poor coating and/or lubrication are all reasons that can lead to a weaker material; and eventually to failure.

Excessive tension on mooring lines can lead to damage during severe storms and cyclonic events.

Thus, it is prudent for operators to define and implement MIM programs that are tailored to their floating production assets (i.e., based on their vessels’ mooring system types, composition, age and condition) and include the following at a minimum:

  • Regular inspection and evaluation of a mooring system’s in-place condition to identify weakened or damaged on and off-vessel mooring equipment.
  • Identification and execution of routine maintenance operations to reduce the potential for failure (e.g., winch or chain jack maintenance, fairlead lubrication, and regular adjustment of mooring line payout to prevent long-term stress concentrations and wear on the short sections of line that bend through fairleaders, etc.).
  • Qualitative and/or quantitative estimates of a mooring system’s probability of failure and remaining life expectancy that are based on the mooring system’s present condition, line tension measurements, "wear and tear" at fairleaders, corrosion and abrasion rates, damaged components, soil erosion from trenching and scour around anchors, and the operator’s knowledge of previous real world operating and survival conditions and anticipated future events.
  • A mooring repair plan that: a) identifies capable spot market installation vessels, mooring equipment replacement suppliers, their lead times, and viable staging and mobilization docks, b) defines installation aid fabricators and suppliers, and c) develops preliminary offshore operations procedures to repair compromised line components before or after a failure occurs with little or no production loss from down time.
  • Procurement, fabrication, storage, and maintenance of spare mooring components and installation aids.

Mooring integrity practices consist of bottom chain replacement, mid-water chain, top chain, and other mooring components for numerous floating production storage and offloading (FPSOs); some with as little as 10 years in service.

For the following three case studies, replacement operations were performed in West African and other locations in water depths ranging from 50 to 4,000 ft. (15 to 1,200 m). This repair work was necessary due to some of the aforementioned issues.

Case No. 1: Chain replacement

The FPSO was installed in 2005 in 3,100 ft. of water using eight mooring lines made from chain and polyester. Upon the discovery of that one of the mooring legs had lost its catenary shape, the offshore installation manager (OIM) first assumed general repairs might be in order. After investigation, it was decided to replace certain sections of chain in each mooring leg.

Four vessels were utilized for the repair: two tug boats for position and heading control while each leg was disconnected from the FPSO, and two more anchor handling tugs (AHT) for chain replacement. The first of the AHTs with heave compensated crane and ROV support cut and removed the condemned chain link, leaving an open link in a chain table. The second AHT, with ROV support, lowered the replacement chain with the H-link connector to the chain table, connecting it into the existing chain on the seabed, then paying out the new replacement chain linked on the deck into the polyester that had been recovered to the vessel storage drum.

The polyester was the paid out to the FPSO, being careful to maintain tension, preventing it from being laid on the seabed and then reattaching it to the top chain. The bottom chain for all eight mooring lines were replaced in this manner, restoring the mooring system’s structural integrity. 

Case No. 2: Low depth, high tension

InterMoor was contracted by an FPSO operator to replace a top chain at a depth of 49 ft. The FPSO in this case was spread moored (a group of mooring lines distributed over the bow and stern of the vessel to anchors on the seafloor.) on 10 wire-chain mooring lines. The chain at the stopper had been weakened by friction caused by chafing at high tensions that were triggered by the mooring system’s reaction to the vessel’s motions in shallow waters (see Figure 1).

Because the FPSO deck and topsides arrangements were not originally designed to accommodate chain replacement operations, it was difficult to maneuver the FPSO’s old and new chain and successfully transfer it to and from the anchor handler vessel (AHV) in the field. Through detailed and complex rigging, long sections of chain were transferred from the FPSO’s crane-lay-down-area to the foredeck. This had to be accomplished while adhering to safety standards set by the client, InterMoor’s engineers and installation crews prior to the start of the work.

It also was necessary to separate two of the FPSO’s original mooring lines, which had previously been crossed and hooked up by a different contractor to the wrong fairleads during the FPSO’s original installation campaign (see Figure 2). After careful inspection, the crossing pattern of the mooring lines was defined and the mooring lines were uncrossed while new top chains were installed. 

Case #3: Life extension

The third example encompasses more than just repair and maintenance. The ultimate goal was to extend the life of the mooring system within class standards and maintain approval for the upgraded design. The original contract covered replacing old mooring chains and wire ropes for eight of the 12 mooring lines along with the installation of two drag anchors on an FPSO that was already in service with years of production still possible.

The engineering work began in October 2014 and the installation was completed in March 2015. As the work progressed, the existing components of the mooring lines were thoroughly inspected as they were being disconnected to determine which portions required replacement with new chain and connectors. During the operation, the scope of work was also extended to replace an additional compromised mooring line.

Documentation was vital to the installation to ensure the client and regulatory agencies had accurate, as-built information for the upgraded mooring system. The project was finalized with all documentation delivered in April 2015. 

Mooring integrity management in practice

Each of the above three projects demonstrates a different aspect of MIM in practice:

  • The sudden and unexpected failure that occurs without symptomatic indications and requires investigation, inspection, and repair.
  • The observed deterioration of components, which can be measured and evaluated prior to replacement, and subsequently needs to be replaced or repaired.
  • The continued inspection of subsea components over the facility’s life cycle, the collection and recording of the component condition in-situ, and the needs evaluation for replacement of prolonged service beyond the original design life.

The first example involving bottom chain illustrates a manufacturing problem. The early failures in mooring systems that have been observed can be a result of a manufacturing defects not caught during production. Because these products are used in more extreme conditions, inspection and testing programs require heightened consideration and diligence. It can be an issue for newer steel mills to quench and temper steel to achieve specific properties. These processes need to be monitored to assure the quality programs are rigidly enforced.

The second example, the replacement of top chain was more predictable, as observation was possible on a daily basis. There are practices to manage this type of wear such as mooring line adjustments to reposition chain links at or near the chain stopper for example. However, this may also mean that the chain jacks and related equipment must be maintained to handle the chain at each stopper. While maintenance can result in additional operating expense, doing so makes it is possible to delay or avoid replacement costs.

In the third and most interesting case, the operator had inspection records and updated data from previous interventions. He was also informed and aware of the condition of the mooring system. That data enabled the operator to plan and budget for the necessary refurbishment and upgrades. Mid-water wire was replaced with chain to provide a robust design, while connectors that were commonly used at the time of the original installation were exchanged for more reliable ones.

United States Coast Guard (USCG) memorandums issued on Jan. 19, 2016, provide guidance for structural integrity management, which is applicable to the hulls of FPSs and includes mooring integrity evaluations, and for the data necessary for submission and review, as it relates to floating facilities life extension. These memoranda provide a more definitive guidance in targeting the critical areas for inspection and increase the efficiency of the USCG in reviewing the criteria for compliance.

MIM is practiced in a variety of ways. Methods range from the combination of rigorous quality control measures and predictive in-service assessments with planned and preemptive actions, to unexpected problems and emergency repairs. As the world’s offshore production fleet ages and as additional offshore energy reserves come online, it is apparent that quality control and rigorous evaluations are beneficial, while older members of the fleet can directly benefit from mooring repair plans. It is also helpful to know what regulatory agencies will require and recommend.

Because mooring systems are robustly designed, at times having double redundancy, we often see them installed and then ignored. MIM programs define the care, maintenance and actions needed to prevent and respond to mooring failures. In all cases, mooring design experts with offshore installation knowledge and experience are essential contributors to MIM in practice.

Jim Macklin is the VP of Projects & Engineering, and Kent Longridge is the principal mooring engineer at InterMoor Inc.

Original content can be found at Control Engineering.