Fabrication
A flexible dolphin has two main elements: the single pile or multiple piles that are to be installed in the harbour bottom, and the dolphin head or topside. The dolphin head mainly consists of a platform equipped with a bollard or quick release hook and an energy-absorbing fender system or berthing beam connecting multiple piles. Most flexible dolphins are made from either steel or wood, for instance azobé (Lophira alata) or basralocus (Dicorynia guianensis).
Steel dolphins
Although steel is used for several parts of flexible dolphins – such as bollards, structural parts of fender systems, handrails and decks – the largest amount of steel can be found in the tubular piles. Therefore, the focus of this section is on steel tubular piles, which are considered to be flexible monopiles.
Larger diameter monopiles are made up of separate pile sections, which are typically multiples of 3 m. The designer must evaluate the production capacity of different suppliers to find the optimal weight of each dolphin for each factory and get the optimal economic offer with respect to the number of butt welds between pile sections, in order to compare each manufacturer's prices. Steel plates are rolled into the correct diameter and seamed with a vertical weld. The individual sections are interconnected with a horizontal splice. In this way it is also possible to vary the wall thickness over the various pile sections. Vertical welds typically have a staggered orientation over the pile length to avoid having a point where four welds come together. All welds in the fabrication yard are made automatically. This reduces the risk of weld imperfections and benefits the quality control of the structure. The allowable section length, diameter, steel quality and wall thickness depend on the type of mill available in the steel fabrication yard.
The dolphin head or topside can be made in situ and/or be prefabricated; the decision is up to the contractor executing the work. Prefabricated topsides are typically used in order to reduce the construction time by reducing the amount of work done above water and to benefit the quality control of the structure, as it can be inspected onshore. Topsides can be prefabricated at a steel fabrication yard. For ease of welding, the steel quality of topsides (with e.g. a small platform and bollard connections on top) is lower than the quality of the dolphin pile itself. It is also possible to apply concrete in the dolphin head. This is mainly done when a dolphin will be subjected to high berthing impact loads or high mooring loads on quick release hooks. Concrete can then be used to distribute the high berthing and mooring loads, allowing the contractor to install a relatively light steel head and use the concrete that will be installed later to take the loads.
Material properties
Tubular piles used for flexible dolphins are either produced according to the European standard EN 10219-1 [7.1] for cold-formed tubes or the American standard API 5L PSL 1 or 2 for line pipes. Tubes produced according to the API 5L [7.3] generally have to meet more strict production tolerances and properties, as the requirements of the offshore industry are usually higher than those for foundation purposes and flexible dolphins.
Nevertheless, it is common practice in the design of dolphins to use the higher API steel grades, such as X65 and X70. This is for multiple reasons:
- Higher API steel grades have no European equivalent in EN 10219. Therefore, X65 and X70 steel grades are applied to realize a more economical design in which additional cost prices for higher steel grades are lower compared to overall cost savings.
- The availability of higher API steel grades is greater compared to equivalent steel grades of EN 10219, leading to lower base material prices and relatively quick delivery lead times.
- As line pipes are usually welded on site to very tight tolerances, the chemical compositions of API grades are known to guarantee good weldability, also for higher steel grades.
While 'X' grades according to API 5L PSL 1 have no restrictions with regard to the carbon equivalent and Charpy impact, additional requirements are often demanded by project owners. A maximum carbon equivalent of 0.43%, consistent with API5L-PSL 2, and a Charpy V notch of 27J at 0 degrees Celsius are usually requested.
Designers should be aware that clients sometimes impose very strict rules concerning maximum material quality, not always with the use as a flexible dolphin in mind. These rules often lead to limitations on the base material and production method, for example only longitudinally welded tubes may be used. The client must therefore be made aware of these limitations and the possible consequences, like inefficient and uneconomical designs.
Geometric tolerances
The production of steel profiles is a difficult process. In both EN 10219-2 [7.2] and API 5L [7.3], tolerances are given for all dimensional parameters. For dolphins, the production tolerances of EN 10219-2 are usually respected. It is therefore important that these tolerances are taken into account in the design calculations for the stability calculations. The Eurocodes allow the nominal values to be used for structural design calculations. However, for the stability calculations in accordance with EN1993-1-6 the fabrication tolerance quality class A, B or C (of Annex A of the EN 10219-2) must be taken into account.
Special attention must be paid to tolerances on straightness, thickness and out-of-roundness. During the rolling process, wall thickness tends to increase due to wear-out of the rolls. As the rolls form a large part of the capital investment costs in a production process, pricing is directly linked to the lifetime expectancy and residual value of these rolls. Therefore the entire wall thickness tolerance range, including the minimum tolerance, is utilized in the production process of the base material for a tube. This variation in wall thicknesses has to be taken in to account in the design of the dolphin. Sometimes the designers has to limit the permitted wall thickness tolerances, if stricter tolerances than EN 10219 (±10%) are necessary. An additional request to the suppliers can be to comply the base material to EN 10029 (2010) Class A, which reduces the permissible negative wall thickness variation. This has to be defined by the designers in either the drawings or contract between the client and the supplier.
The out-of-roundness tolerance according to EN 10219-2 is 2%. Annex A of EN 10219-2 gives a guideline for additional tolerances for out-of-roundness when tubes are used as bearing piles and/or primary elements of combined wall systems. These tolerances are usually relevant for diameters D > 900 mm and D/t > 100. Three fabrication classes (A, B and C) are defined with decreasing tolerances (0.7%, 1%, 1.5%). While these fabrication classes and tolerances are used in the design calculation models according to EN 1993-1-6 [7.4] and CUR 211 [7.5], they are difficult to achieve in practice in production. Especially for higher D/t ratios, the out-of-roundness tolerance given by production class C is often the highest attainable.
Production methods tubular piles
Cold-formed tubes are produced from either hot-rolled coils or hot-rolled plates. As a result, there are two main production methods: longitudinally welded and spirally welded. Longitudinally welded tubes are rolled from hot-rolled plates. Either the width or the length of a plate determines the circumference of a tube. Larger diameters are possible, but then two or even more longitudinal welds are needed. The tube sections are limited in length, depending on the size of the plates, and vary from 3 m to 18 m. Longer tubes can be fabricated by butt welding sections together.
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Figure 7-1 Process of spirally welded pile fabrication.
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Figure 7-2 Fabrication mill of spirally welded piles.
Spirally welded tubes are rolled from hot-rolled coils in a continuous production process. In theory, the length can be infinite, but it is limited by the size of a production facility and the lifting capacity. Production of spirally welded tubes is possible for diameters ranging from 0.6 m to 3 m, with a thickness depending on the availability of coils. Hot-rolled coils are limited to a thickness of 25.4 mm (1 inch), but developments regarding increasing this thickness are ongoing. Due to the continuous production method and lower base price of coils, the unit price of spirally welded tubes is usually lower than their longitudinally welded counterpart. Neither the design standards (EN 1993 series) nor the standards for cold-formed tubes (EN 10219 and API 5L) distinguish differences between the two production methods. There is no technical reasons to prefer one type of produced tube over the other.
It is highly recommended to verify the standard sizes for spirally welded tubes. However, the production possibilities are not limited to these standard sizes as the diameter can be set accurately to the mm.
Surplus tubes
In order to reduce the unit prices and delivery lead times, tube suppliers keep large stocks of tubes in standard inch sizes. These tubes are mostly new, surplus tubes from the oil and gas industry, and are either spirally or longitudinally welded. While these surplus tubes are economically attractive, the certifying documents are often not available. In such cases, supplier and client must agree upon what is needed for their project and alternative testing regimes can be set up to guarantee both the required mechanical and chemical properties and the fabrication tolerances. First of all, lots need to be defined on which an agreed test regime will take place. While identification used to be done using the Brinell hardness test (Equotipping), more and more lots are now defined based on the chemical composition measured using handheld XRF analysers. The subsequent testing procedures are usually more strict (e.g. with regards to test frequency and NDT on welds) than given in the standard, as the base material and the welding procedures used to produce the tubes are unknown.
New developments
AMLoCor
AMLoCor is a new low-corrosion steel grade. The main advantage of AMLoCor is a significant reduction of the corrosion rates in both the low-water zone and the permanent immersion zone. In situ test specimens have proven that the loss of thickness of AMLoCor is reduced by a factor of 3 for the permanent immersion zone and a factor of 5 for the low-water zone. AMLoCor also provides better protection against ALWC (accelerated low-water corrosion), which is often related to biological activity intensifying the degradation of steel in the low-water zone. For more information, see the Arcelor AMLoCor documentation.
AMLoCor is a new low-corrosion steel grade. The main advantage of AMLoCor is a significant reduction of the corrosion rates in both the low-water zone and the permanent immersion zone. In situ test specimens have proven that the loss of thickness of AMLoCor is reduced by a factor of 3 for the permanent immersion zone and a factor of 5 for the low-water zone. AMLoCor also provides better protection against ALWC (accelerated low-water corrosion), which is often related to biological activity intensifying the degradation of steel in the low-water zone. For more information, see the Arcelor AMLoCor documentation.
Regarding the mechanical properties, AMLoCor can be considered equivalent to a 'standard' carbon steel used in the foundation industry. Hence, it can be designed and installed based on standard design procedures and guidelines valid for steel piles, for instance according to EN 1997, EN 1993, EN 12063[7.7], EAU 2012 [7.6], etc. In the design, a combination of additional protection methods may be considered for zones where the steel is less effective, for example coatings or concrete sleeves.
X80
In the foundation industry, X70 is already standard. In the meantime, there is a tendency towards further optimizing tube sizes and using even higher steel grades. X80 has already been produced and used for flexible dolphins, and some designs even go up to S690 (or X100).
In the foundation industry, X70 is already standard. In the meantime, there is a tendency towards further optimizing tube sizes and using even higher steel grades. X80 has already been produced and used for flexible dolphins, and some designs even go up to S690 (or X100).
30 mm thick coils
In the last 10 years, the thickness range for Hot-rolled coils increased from 25.4mm (1 inch) to 30 mm in S355 and to 28 mm in steel grades upto X70
In the last 10 years, the thickness range for Hot-rolled coils increased from 25.4mm (1 inch) to 30 mm in S355 and to 28 mm in steel grades upto X70
Wooden dolphins
In the past, many inland berths and locks were protected by wooden dolphins, but most of these have now been replaced by steel tubular piles. However, for recreational vessels and in cases where a traditional appearance is required, wood can be a sustainable alternative. Especially when wood is produced in a sustainable forest, it has a much smaller impact on the environment than steel. Wood can also be used as a fender panel installed on a flexible dolphin. In that case, the use of wood and steel is optimal, because the brunt of the wear and tear is taken by wooden panels and the structural capacity is delivered by the steel substructure.
When using wood, be aware that:
Horizontal surfaces should have a slight slope so that water runs off. Water weakens surfaces over time.
Horizontal surfaces should have a slight slope so that water runs off. Water weakens surfaces over time.
Holes drilled in the wood for connection to the behind laying steel structure by means of bolts, in most cases, must be drilled from the backside of the wood, connecting side, to ensure that holes fit the behind laying structure, because holes tend to get an offset during drilling which results in not fitting of the bolts, if drilled from the frontside, because of relatively small tolerances on the boldhole in the steel side.More information can be found in CUR 213 'Hout in de GWW sector'[7.8].