Synthetic Mud Ropes for Offshore Mooring Applications Field History and Testing Data

Transcription

1 Synthetic Mud Ropes for Offshore Mooring Applications Field History and Testing Data Jason D. Pasternak Delmar Systems, Inc Wilcrest Dr., Suite 225 Houston, TX USA John Shelton Delmar Systems, Inc Wilcrest Dr., Suite 225 Houston, TX USA Justin Gilmore Samson Rope 2090 Thornton St. Ferndale, WA USA Abstract With the continuing development of synthetic ropes and real-world experience gained through their use in offshore mooring operations, it is evident that previous reservations about the use of synthetics in soils are no longer warranted. This is proven through the extensive documentation of field service history, coupled with rope and fiber testing of a jointly developed High Modulus Polyethylene (HMPE) mud rope, which is presented to industry in this paper. The mud rope examined in this paper is of 8x3 open-braid HMPE construction. This open-braid construction facilitates visual inspections of each rope, allowing for significant time savings during expensive offshore operations. Each of the subropes within the main rope has its own separate filter barriers and jackets. These individually jacketed subropes allow for more residual strength in the rope should a breach in the jacket occur and cause premature fiber wear and internal abrasion. As the ropes are comprised of lighter HMPE material, they are easier to handle offshore on the deck of an Anchor Handling Vessel (AHV), reducing the likelihood of accidents and injuries. This low density also allows the ropes minimal interference in the freefall trajectory of the gravity-installed anchor to which they are directly connected. The ropes have initial lengths of ~335-ft, and due to their direct connection to the gravity-installed anchor, much of the rope is under cyclic tension under the mudline. Given the excellent fatigue and abrasion resistance of HMPE, in addition to all of the aforementioned benefits, it is an excellent choice for ropes that are under continual use under harsh conditions. The service life of this rope spans from its first installation into Gulf of Mexico soils on 3/12/08 to the final retrieval date of 1/20/10 (~679 days). The actual total time in the mud is documented to be ~544 days. During that service life, the rope was placed on two semi-submersible MODUs (Mobile Offshore Drilling Units) and in six different mooring system configurations. In late August of 2008, the rope was connected to a MODU during hurricane Gustav, experiencing storm loads calculated to be in excess of ~1,050-kips (~1,050,000-lbs). The rope was inspected and measured to determine if any permanent damage was done to it. As the rope did not experience loading near its MBL (Minimum Break Load) of 800 metric tones (~1764- kips), the rope was deemed safe for further use. After minor jacket repairs, the rope was placed back into service, where it remained until January of After being retired from service, the rope underwent several destructive tests. Testing was performed on the rope fibers to confirm fiber creep, static stiffness, and post-service MBL (Minimum Break Load). A similar testing scope was performed on the rope to examine overall rope creep, rope static stiffness, and post-service MBL. This paper documents the detailed records of an HMPE mud rope throughout its service life, along with the rope and fiber testing techniques and results that were used to verify its integrity after the rope was retired. I. BACKGROUND High Modulus PolyEthylene (HMPE) fibers, with their very high strength to weight ratios, are utilized more and more frequently in heavy industrial and offshore applications. These fibers, when incorporated into large ropes, produce ropes that are orders of magnitude stronger than wire ropes of similar weight. The benefits of this lighter rope material are seen in several applications where heavier weights or larger rope diameters are less desirable. For example, HMPE ropes increase payload capacities during deepwater heavy lift and deployment operations. Another key benefit over other rope fibers includes an increased abrasion resistance. Offshore deepwater mooring is an industry that benefits greatly from the increasing utilization of HMPE ropes. When used as a major mooring component, they allow greater variable deck loads on Semisubmersibles through mooring weight reductions as well as lower current drag loading on mooring lines due to smaller rope diameters. The OMNI-Max anchor is a relatively new development within the deepwater mooring industry. It allows for rapid installation and removal of high holding capacity anchors with significant out-of-plane loading capabilities. An integral part of the rigging design that allows for rapid deployment and recovery is the HMPE mooring mud rope (the orange and blue rope depicted on the left of Figure 1). For the use of the rope documented in this paper, the OMNI-Max mud rope, a jacketed open-braid HMPE rope with load bearing fibers consisting of Dyneema SK 75 was chosen. The light weight (near-neutral buoyancy), high abrasion resistance, and excellent fatigue life properties were all factors 1

2 in the rope selection. The light weight prevents free-fall trajectory issues with the OMNI-Max gravity-installed anchor, while facilitating rope handling and inspection. The resistance to abrasion is necessary for the numerous deployment and recovery operations, due to rope interactions with vessel decks and stern rollers, as well as during soil impact and drag below the mudline. The long fatigue life allows for a longer service life of the rope, making it a more cost-effective solution than some other options. II. SERVICE RECORDS The service life of the mud rope documented in this paper spans from 3/12/08 to 1/20/10 (approximately 679 days) with a total in-place time in the mud of ~544 days. During that service life, the rope was placed on two Mobile Offshore Drilling Units (MODUs), six different mooring system configurations and locations, and experienced hurricane Gustav. The following is a brief summary of the service record of the rope. The first deployment was on a taut mooring system for Diamond Offshore s Ocean Valiant semi-submersible MODU in an area of complex seafloor bathymetry within a block in East Breaks on March 12, The mooring configuration for these lines had a high anchor uplift angle, partially due to the downward slope of the seafloor and partially due to the necessity of the taut mooring line to clear subsea infrastructure. The OMNI-Max anchor was chosen for these preset lines due to the highly sloped and fault-ridden seafloor, which made using drag anchors near impossible and suction pile anchors undesirable. As a method of internal validation, the client requested that the mooring lines containing OMNI-Max anchors be proof loaded as high as possible, which resulted in a load of ~611-kips (~232 metric tons) at the mooring mud rope and anchor. When the rope was recovered on July 27, 2008 (~137 days later), it was inspected and found to be in good condition, without visible defects (Figure 2). During the second deployment, the mud rope experienced both its longest continuous time in soil and its highest loading condition prior to post-retirement MBL testing. The anchor and mud rope were preset in Mississippi Canyon on August 5, 2008 for the Transocean Sedco Forex (TSF) Amirante. While on location and connected, the Amirante was hit by Hurricane Gustav, which applied a maximum estimated load of ~1,096 kips (~497 metric tons) to the mud rope and anchor. The rope was recovered on January 12, 2009 (~159 days after installation) with a visible tear in one subrope jacket. As the damage did not extend to the load bearing fibers, the rope was repaired and made ready for redeployment. The third mud rope installation was the shortest continuous time in soil during service and lowest loading condition of the rope. The anchor and mud rope were preset in Mississippi Canyon on May 13, 2009 for the TSF Amirante. The anchor was recovered ~29 days later on June 10, During this deployment, no significant anchor or mud rope tension was experienced, as the anchor was not proof loaded or connected to the MODU. The rope was deployed and recovered in good condition. The fourth installation of the mud rope was on June 10, 2009 in Mississippi Canyon. The rope was preset and connected to the TSF Amirante. During the ~110 days of deployment, the largest tension experienced by the rope was during proof-loading (~339 kips at the mud rope). The rope was recovered without incident and in good condition on September 28, The fifth mud rope deployment was on October 3, The rope was preset and connected to the TSF Amirante in East Breaks. During the ~34 days of deployment, the largest tension experienced by the rope was during proof-loading (~347 kips at the mud rope). The rope was inspected in the water with a Remotely Operated Vehicle (ROV) on November 6, 2009 (Figure 3) and wet towed to the sixth installation location. The sixth and final deployment of the mud rope was on November 6, The rope was preset and connected to the TSF Amirante in East Breaks. During the ~75 days of deployment, the largest tension experienced by the rope was during proof-loading (~361 kips at the mud rope). During recovery operations on January 20, 2010, several subropes came loose from the braided eyes, requiring the rope to be sent back to shore for splicing. The overall length of the rope was initially measured (before service) at a reference tension (based on standard Cordage Institute recommendations of 200*d 2 ) to be 350-ft long [4]. After service was discontinued, the rope was measured at the same reference tension to be ~357-ft long, a measured permanent elongation of 2%. Delmar internal specifications for rope retirement due to creep allows a measurement of up to 6% elongation in the mud rope, which is significantly below API recommendations of 10% [1]. III. BREAK TEST SAMPLES Two HMPE rope sections, each of ~63-ft length from pin to pin (Figure 4), were created from each end of a single 350-ft OMNI-Max mud rope (Figure 5). Each rope consisted of the same construction (Figure 6) with a 10-ft long center rope section, 6-ft long soft eyes, and ~20.5-ft long splices. One section was taken from the anchor side of the mud rope, which contained a repaired cover of a section of subrope, while the other was taken from the mud mat side. IV. BREAK TEST PROCEDURES This section of the paper documents the Delmar-proposed break test procedures that were followed to determine the Minimum Break Loads (MBLs) of each rope sample. The test ropes were soaked in water for a duration of 22 to 26-hours prior to testing, placed expeditiously within the testing apparatus, and kept moist during the tests (by soaker hoses). During rope soaking, water temperature was maintained between 15 C and 25 C (59 F and 77 F). All testing recorded load and extension in adequate sampling intervals such that an accurate representation was captured without fitting a curve to the data points. Testing was not interrupted in the middle of test sets. Time was recorded throughout the test so cycle rates could be verified. Tolerances 2

3 on the loads applied to the test ropes were maintained within 2% MBL. The proposed break test of each rope was specified to consist of a 10-cycle bedding-in stage, immediately followed by a slow pull to break (Figure 7). Each bedding-in cycle was defined as a loading from 1% MBL to 30% MBL, and relaxing back to 1% MBL at a rate of ~10% MBL / min. The break test was defined as a linearly increasing load from 1% MBL to break at a rate of 1% MBL per minute. Given the slow rate of loading, a static stiffness approximation for the rope can be obtained from the MBL data. V. BREAK TEST RESULTS Due to the manual nature of the testing apparatus, the proposed testing procedure in Figure 7 was followed as closely as possible, with constant manual adjustments, but the actual loading / unloading rates are largely scattered from the desired rates (Figure 8). For Figures 8-11, some of the erroneous readings from the raw data collected were removed to better portray usable results. Figure 9 depicts the corresponding extension versus time for the two rope sections. Figure 11 depicts a very close correlation between the examined approximate static stiffness of each rope test section. As the percent of elongation (depicted in Figures 9 and 10), which is used to calculate static stiffness (Figure 11), is based upon the pin to pin sample length, not a measured test section of mid-span rope, the static stiffness results obtained from this test should be taken as upper-bound approximations of actual rope sections provided by Samson to Delmar. Each test rope section broke in excess of the original specified rope MBL of 1,763.7-kips (800-mt). The sample taken from the mud mat side of the mud rope broke at ~2,086.7-kips (~118% of new MBL), and the sample taken from the anchor side broke at ~1,824.9-kips (~104% of new MBL). The high variance seen in the break tensions from the specified MBL is likely due to the rope jacket repair preventing proper load sharing in the rope splice, causing the lower break strength. VI. YARN TESTING Yarn samples were removed from several sections within the mud rope, including the rope sections used in the break tests, for comparison with new yarns. No abnormalities were noted in the sample yarns, with all test results yielding acceptable strength and elongation results (Figure 12). VII. FILTER CLOTH INSPECTION The rope jacket was removed from the rope in several places, including the rope sections used in the break tests, to allow inspection of the filter cloth. No particle ingress was noted on either end of the rope, with very slight staining of the filter cloth and adjacent fibers observed. Figure 13 depicts the filter cloth and fibers contained within it. VIII. CREEP TESTING Creep testing was performed by Samson on yarns from the test rope by loading the yarns to 40% of their average breaking strength at a temperature of 50 C (122 F). This higher temperature increases the creep rate, allowing for a shorter duration test. The test yielded creep rates of 1% elongation per 13.4 hours for new yarns and 1% elongation per 16.7 hours for the used yarns, a 20% reduction in creep life for the used yarns. Due to the 50% rope efficiency (rope strength versus the strength of the sum of the sub ropes), this translates to a loss of creep life in the rope of ~10%, reducing the original DSM estimated new rope creep life of 14 years by ~1.4 years (12.6-year creep life remaining). Due to the increased temperature used during testing, this estimated 10% reduction in creep life examined in the rope is considered conservative. IX. CONCLUSIONS Testing results show that no abnormal reduction in break strength or creep life was examined within the mud rope due to described service conditions, including no observed soil ingress. According to all of the examined evidence, the difference in break tensions of the two samples was observed most likely due to a rope jacket repair located within the eye splice, which may have caused improper load-sharing amongst the sub ropes. However, this reduced capacity was still above catalog MBL ratings for a new rope. ACKNOWLEDGMENT The authors would like to thank Dillon Shuler and Robert Garrity of Delmar for their contributions to this project. REFERENCES [1] API RP 2I, In-Service Inspection of Mooring Hardware for Floating Structures, 3 rd Edition, April [2] API RP 2SK, Design and Analysis of Station Keeping Systems for Floating Structures, 3 rd Edition, October [3] API RP 2SM, Recommended Practice for Design, Manufacture, Installation, and Maintenance of Synthetic Fiber Ropes for Offshore Mooring, 1 st Edition, March [4] CI , Cordage Institute Test Methods for Fiber Rope, 2 nd Edition, May [5] Del Inspection Report rev , Samson Inspection Report, September

4 Figure 1: OMNI-Max Anchor Assembly Ready to Deploy Figure 3: Typical Screen Shot Taken from ROV Rope Inspection Video (11/06/09) Figure 2: Typical Screen Shot Taken from on Deck Rope Inspection Video (7/29/08) Figure 4: Screen Shot Taken from Rope Testing Bed ANCHOR END UPPER END EXISTING SPLICE 116' for sling 116' for sling 47' (approx repair location) Figure 5: OMNI-Max Mud Rope Details [5] 4

5 UPPER END 63'-0" 6'-0" 20'-6" 10'-0" Figure 6: Typical Test Section Details [5] Tension (%MBL) Time (Minutes) Figure 7: Proposed Break Test Procedure Load [% MBL] Mud Mat Side Mud Rope (Load vs. Time) Anchor Side Time [sec] Figure 8: Actual Break Test (Loading) 5

6 Mud Rope (Extension vs. Time) Mud Mat Side Anchor Side Elongation [%] Time [sec] Figure 9: Actual Break Test (Extension) Load [%MBL] Mud Mat Side Mud Rope (Load vs. Extension) Anchor Side Elongation [%] Figure10: Actual Break Test (Load vs. Extension) 6

7 28 Mud Rope (Static Stiffness) Mud Mat Side Anchor Side Stiffness [xmbl] Load [%M BL] Figure 11: Actual Break Test (Approximate Static Stiffness vs. Load) Figure 12: Yarn Test Results [5] 7

8 Figure 13: Filter Cloth and Subrope Yarns [5] 8