SHORE END CABLE PROTECTION CASE STUDY－ SUBMARINE CLIFFS
XiaoLong Zhang, Stuart Wright Email: [email protected] Huawei Marine Networks Co., Limited. No. W3C West Sec. Binhai Fin. Ave, No51 3rd Street, Tianjin, China Abstract: A recent project involved the planning and installation of a submarine cable between isolated oceanic islands of volcanic origin. One of many project challenges was the complexity of the cable landings. In the approach to the landing sites, the cable had to ascend vertical submarine cliffs with heights in excess of 20 m – a zone influenced by strong swell and corrosive seawater. During the planning phase, with directional drilling and rock cutting considered impractical, a decision was made to pin the cable encased in articulated pipe to the cliff face. The difficulties in achieving this, and the subsequent impact of a storm with magnitude in excess of The Perfect Storm, are discussed. 1. INTRODUCTION
In a submarine cable project, the shore end is defined as the section of cable laid in shallow waters approaching land. Commonly, it indicates the area from about 13 m water depth to beach manhole – typically a high risk area for submarine cable systems because of the strong hydrodynamic forces in this zone. Multiple protection methods exist to help protect these shore end cables. These include the following.
� Cable burial;
� Upgraded cable armor;
� Encasement within metal alloy or polymer piping;
� Stabilization clamps, pins and bags;
� Horizontal directional drilling;
� Support/guide frames.
Depending on the shore end site conditions, these protection methods are adopted singularly or in combination.
Typically, cable route engineers look for benign gently sloping beaches covered with sediment as cable landing sites. However, this is not always possible. Sometimes, a cable landing with more challenging conditions has to be selected. This is often the case when a cable lands on isolated cliff lined oceanic islands. The special environment conditions result in a challenge when it comes to protecting cables. This case study will examine planning considerations on a recently delivered cable project to land two shore ends to the same landing site on such a cliff lined island. 2. SUBMARINE CLIFF
ENVIRONMENT
2.1 Landing Area Situation
The cable landing lies on an isolated island surrounded by cliffs composed of hard volcanic rocks. The selected landing site lies in a small rocky bay (see Picture 1), where a steep access road extends to the sea (see Picture 2).
The coastline is very rough, formed from lava flows entering the sea, then pounded by strong marine hydrodynamic forces. Some isolated rocks expose offshore (see Picture 1).
Picture 1. The small rocky bay for cable
landing
Picture 2. Steep concrete road extending
to small rocky bay
Picture 3. Slipway into sea
Picture 4. Step marks the top of
submarine cliff
Approximately twenty-five metres beyond where the access road enters the sea, a step marking the top of a submerged cliff can be clearly observed (see Picture 3 and Picture 4). 2.2 Submarine Cliff Feature
In order to investigate the submarine cliff in the shore end section, a full diver survey was performed from the beach manhole (located on the access road, see Picture 1) to about 22.0 m water depth (far from the cliff foot). A total of 5 survey lines were executed including 3 lines along the cliff face and 2 lines along the lateral cliff face (see Figure 1).
Figure 1. Diving survey lines planned
The survey results indicate that the cliff can be divided into 3 parts: cliff top, cliff
face and cliff bottom. Cliff Top. The cliff top can be divided into the land part and the submerged part. The land part is about 20.0 m in length from the beach manhole to waterline (see Picture 1) along a concrete roadway. The submerged part is about 25.0 m in length where a natural or manmade gully (about 2.0 m wide) extends from the waterline to the top of the submarine cliff at about 2.2 m water depth(see Picture 3). The submerged part lies within the surf zone where storm waves impact, rolling rubble up and down the gully. The designed end of cable ducting lies at about 1.0 m water depth, within the shallow gulley (see Figure 1). Cliff Face. Composed of lavas, the cliff face has a vertical drop of 17.0~18.0 m from the cliff top to the cliff foot. At the top of the cliff, the fall is almost vertical. The middle and lower slopes are slightly less extreme and further reduced by rock
Figure 2. Profile of cliff face along the
diving survey lines
Picture 5. Gullies on the cliff face
terraces (see Figure 2).Some discontinuous natural gullies incise the cliff face; generally vertical in nature (see Picture 5). Hammer tests carried out during diving survey indicated that the rock was solid, with little weathering (see Picture 6).
Picture 6. Hammer tests on the cliff face Cliff Bottom. At the cliff bottom, the seabed was found to be relatively flat, with the exposed rock intermittently covered with boulders, sand or gravel (see Picture 7).
Picture 7. Cliff bottom conditions
Lateral Traverse. The survey results along the lateral cliffs showed that a route that traversed down the cliff would encounter similar rough conditions to reach the base of the cliff. 3. CABLE PRTECTION SCHEME
3.1 Cable Type Design
Based on the nature of the submarine cliff, a heavy double armored cable was proposed and applied in the shore end
section to mitigate abrasion risk. In the more corrosive seawaters sometimes experienced in volcanically active areas, the traditional polypropylene cable serving was replaced by an external polyethylene sheath. 3.2 Cable Protection Analysis
HDD and Rock Cutting:
HDD is considered to be an effective protection method to avoid adverse nearshore conditions. Unfortunately, HDD is also very expensive, especially when drilling hard lavas in remote locations. Additionally, the narrow landing site (see Picture 1) and the steep drop of the terrain (see Picture 2) complicate HDD operations. HDD was therefore considered impractical for this project. A similar conclusion was made with regard to significant rock cutting. Beyond the practicalities, large scale rock cutting of hard volcanic rock is expensive and time-consuming. When combined with the high energy marine environment, such operations can also constitute a significant safety risk for divers. Pinned Casings:
If shore end risks could not be limited through avoidance via HDD or trenching, then the cable would have to be protected by other means. In such high energy environments, traditionally this would mean articulated pipe made from cast metal alloy as opposed to polymer based casings which were considered less robust. Such piping would be pinned to the seabed to avoid movement induced by wave action. Stabilization:
Where the cable was suspended, such areas would have to be supported and stabilized using stabilization bags. Corrosion Protection:
In the highly corrosive seawaters sometimes associated with volcanically active areas, the articulated pipe selected was of high grade, designed to resist corrosion. To reduce the risk of fixing failure, non-ferrous polymer bolts would be used to secure the pipes to the rocks. 3.3 Cable Route on Cliff
To provide a level of diversity between the two cable landings, it was decided that one link would opt for a direct ascent of the submarine cliff to the landing site, while the other would descend the lateral cliff to traverse gradually towards the seabed. Direct Ascent:
Based on the diving survey data, the cliff face was found to be rugged with many crests and natural gullies. To avoid or reduce suspension on the cliff face, the optimal route tried to utilize the natural gullies. At the top of the cliff - because the nominal bend radius of the articulated pipe was limited to 2.0 m - even though a natural gully was chosen as route, localised
rock cutting was necessary to avoid stressing the articulated pipe as it passed over the cliff top (see Figure 3). At the foot of the cliff, again due to the bend radius of the articulated pipe, a long suspension would be created. The practical method to mitigate the area of risk was to pillow the suspension using stabilisation bags (see Figure 3). To avoid diver rock cutting, support structures at the cliff top and bottom were considered. These included bend restrictors and prefabricated cable tracks (see Picture 8). However, difficulties in securely fixing such apparatus so that they could withstand the hydrodynamic forces expected were considered impractical, and as such discounted.
Picture 8. Prefabricated cable tracks
Lateral Descent:
The survey results indicated that a route selected on the lateral cliff provided a contoured descent down the cliff face (see Figure 3). On this route, some natural support points could be used to secure the cable to the cliff. However, articulated pipe on the lateral slopes would only be supported by clamps pinned to the rock face, with cable between in constrained suspension. 3.4 Articulated Pipe & Clamps design
To combat the expected corrosion mechanisms on this project, high grade articulated pipe was selected. It was proposed that the pipe would extend at
least 10.0 m from the base of the submarine cliff to approximately 22.0 m water depth. Depending on circumstances, articulated pipe is typically clamped and pinned approximately every 2.0 m in the surf zone, every 5.0 m in the shallows water (less than 5.0 m), and every 10.0 m until the end of the articulated pipe. Considering local conditions for this project, it was decided to securely fix the articulated pipe to the cliff face at least every 1.0 m (or wherever there was an opportunity) from the cliff top to the cliff bottom, then every 5.0 m from the cliff foot to the end of the articulated pipe. To ensure that the articulated pipe was secure on the cable, a decision was made that all sections of the articulated pipe should be bolted. 3.5 Stabilization Bag Design
The stabilization bags used at the base of the cliff were filled with grout. The bags had dimensions: 0.55 m × 1.00 m, weight: 30 kg ~ 35 kg, composition: 34% concrete, 33% gravel and 33% sand. To remove the cliff base suspension, it was estimated that approximately 180 grout bags would be required. A number of pins with length 0.5 m were driven through the grout bags to help prevent the bags from being dislodged. 4. CABLE PROTECTION
INSTALLATION
In October 2013, the cables were successfully landed; articulated pipe clamped and bolted; stabilization bags installed (see Picture 9). After installation was finished, testing was executed to assess clamp effectiveness.
� Random test to the bolts of clamps using a wrench torque (see Picture 10),
� Firmness of clamps using lift bags with approximately 400 kg lifting force at different depth (see Picture 11),
� Hand shaking to make sure that the clamps was securely tightened.
Picture 10. Clamps installed and bolts
tested
Picture 11. Lift bags test
At the end of the operations, a video survey was performed to demonstrate the cable system was secure. 5. FAULTS BY THE WINTER
STORM
During late December 2013 and early January 2014, an unusually deep polar low developed over the North Atlantic Ocean. One of these polar lows, Winter Storm “Hercules”, was one of the deepest polar low systems observed in the last 60 years, and more ferocious than the 1991 Perfect Storm. The system approached the case study shore end around the 5th January. The storm had maximum wind speed 55 to 60 knots; producing up to ~11 m significant wave heights (see Figure 4). The winter storm seriously impacted the cable system shore end.
Figure 4. Significant wave height (solid line) and mean wave directions (open circles) for during the Winter Storm “Hercules” (ECMWF Date Server)
After the Winter Storm, a video survey was carried out. The survey results showed that a total 23 clamps on the cliff top and cliff face (approximately one in three) were broken (see Picture 12), all 180 grout bags anchored together with pins vanished (see Picture 13), and rubble falling from the cliff top and face buried about 8 m of cable at the cliff base (see Picture 14). Most damage was experienced on the
direct descent route, although some clamps were also broken on the lateral route. However, fortunately, both cables retained their integrity and the articulated pipe was undamaged.
Picture 12. Broken clamps on cliff top
and cliff face Picture
Picture 13. Approximate 10.0 m cable suspension following disappearance of
stabilizing grout bags
Picture 14. Cable overlaid by gravels
and rubbles
6. CABLE PROTECTION IMPROVEMENT
In order to improve the security of both cable legs, it was decided that both cables should be laid on the lateral cliff, with the direct route abandoned. To achieve this, the following remedial operations were undertaken. 6.1 Clearing & Release
During remedial operations, debris was first cleared from the cable lines, and clamps released so as to free the cable for re-route. 6.2 Cable Route Adjustment
Although the clamps were severely damaged during the storm, the adjusted route on the cliff top and upside of the cliff face were retained within the natural trench. It was considered that this area would provide the more secure conditions than the surrounding exposed surfaces. At the cliff foot, as the suspended cable could not be adequately supported by grout bags, the cable was pinned to the adjacent cliff face, descending obliquely to the seabed (see Picture 15). The sketch map of adjusted route is as the red line in Figure 5.
In order to obtain the suitable support points for the adjusted cable route along the lateral cliff, minor rock cutting operation in two locations were performed. These reduced ridges and allowed more secure cable clamping (see Picture 16).
Picture 16. Rock cutting operation on
the lateral cliff 6.4 Clamps Reinstallation and testing
Clamps were then reinstalled along the adjusted cable route to provide stability where required. Extended clamps were used to provide stability to the small suspensions along the adjusted cable route (see Picture 17).
Picture 17. Extended clamps and used in
this case to avoid suspension Tests were performed to examine the security of the installed clamps at different water depths using a larger lift bag with lifting force approximately 1000 kg. Although not a dynamic test, the conclusion was that the clamps installed along the adjusted cable route were secure as could be achieved. 7. CONCLUSION
The Case Study introduces the protection experiences of a cable system crossing an unavoidable submarine cliff in the approach to a cable landing. Although route diversity prompted the use of a direct decent of submarine cliffs in a shore end area, a less diverse option to route a cable gradually down a cliff wall is currently ensuring system security. The study shows that the power of storms should not be underestimated. Albeit an extreme event, a single storm at the study
site severely compromised initial efforts to secure the cable system, even to water depths in excess of 20.0 m. Only the strength of the cable and the quality of the cable protection ensured the continued operation of the cable. Although an indirect approach to the landing, the current landing configuration traversing the lateral cliff face will perhaps prove to be the best solution for the study cable system. Huawei Marine Networks continue to monitor system integrity. 8. REFERENCES [1] ‘Naoki Maekawa, Mikinori Niino, Yutaka Kiuchi’, ‘Cable Protection Methodology at Surf Zone in Maldives’, ‘Suboptic 2013’. [2] ‘Ryan Wopschall, Klaus Michels’, ‘Cable Protection Methods and Applications in an Arctic Environment’, ‘Suboptic 2013’. [3] ‘Jorn Jespersen’, ‘Protection of Greenland connect landfall in Arctic Water’, ‘Suboptic 2013’.
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