Thursday 18th Aug, 2022

Using jackets for offshore berth construction

Richard Morgan, Director of Aspec Engineering, and Daniel Squires, Director of Rendel Limited, examine design considerations for offshore marine bulk loading terminals, including techniques used previously for open sea tanker terminals.


The expansion of Australia’s minerals exports has created the need for suitable export facilities. Sizes of dry bulk material carriers are increasing with the largest class up to 400,000 dead weight tonnes (DWT) potentially placing further demands on export facilities.

Suitable sites in natural protected deep-water harbours are the ideal locations for export facilities but are scarce and are not always politically acceptable. With vessels of such deep drafts as modern bulk carriers, the cost of creating artificial harbours by conventional means involving dredging and breakwater construction can be prohibitively expensive. Because of this, a feasible option can be to load large vessels in the open sea. Suitable design of the terminal and lower berth occupancies to compensate for loss of time due to bad weather are usually necessary for open sea conditions.

Design of terminals for open sea conditions is a challenging exercise due to factors such as:

a) Large height of structure above the seabed required to accommodate vessel draft, tidal range, movement of vessel in waves, vessel keel clearance and requirements for clearance of deck above extreme waves including allowance for climate change

b) Large berthing and mooring forces

c) Large environmental loads due to waves, wind and earthquake

d) Possibility of difficult and uncertain foundation conditions

e) Difficulty of construction over water

Jacket structures can be used as an alternative to freestanding piles for the wharf and jetty substructure for offshore berths. An advantage of this form of construction is that the jacket modules are fabricated off site and the amount of site work can be dramatically reduced. Piles are typically driven through the legs of the jacket framework. This form of construction is particularly suitable for construction in deeper water.

Design considerations

Ship size

One of the first criteria which needs to be established is the maximum size of ship likely to use the terminal. There is a trend that larger vessels result in a more economic freight rate. This trend is more pronounced on longer voyages. However, to produce an efficient bulk transport operation, the selection of the ship size must be considered as part of the total system, which includes the materials handling and marine facilities at both the loading and discharge ports, and the availability of other cargoes that may be handled as part loads or backloads.

The maximum size of ship will allow design draft, berth pocket size and outreach of loading equipment to be determined. Sizes of smaller ships also need to be considered in the design of berthing, mooring and loading facilities.

Figure 1 shows the distribution of ship sizes over 65,000 DWT in the world fleet as of 2017. There are a significant number of large ships over 200,000 DWT which require deep water berths. In terms of maximum size, typical design values for length, beam and draft for a very large 400,000 DWT Chinamax bulk carrier are 360 metres, 65 metres and 24 metres respectively.

Figure 1: Ship Sizes >65,000 DWT

Depth of water required

The depth of water required below low tide is determined by the maximum vessel draft and the required under keel clearance. Allowance also needs to be made for movements of the ship due to waves while moored and while approaching and leaving the berth. Prediction of these movements is a complex subject which is preferably handling by dynamic computer simulation.

Environmental effects

Wind, waves, tide, current and earthquakes are important considerations in the siting, orientation and layout of a terminal as well as in consideration of the resultant loadings in the detailed structural design. The tidal range and operational wave heights will determine the level at which such items as shiploaders, mooring hooks, walkways etc must be placed and hence the deck level. It is desirable that the deck level be above the highest water level due to high tide, surge and wave conditions occurring in combination. Allowance also needs to be made for sea level rise due to climate change.

Wave loadings on offshore structures can be very high. Effects due to drag and inertia must be considered. For jackets this is generally done by determining water particle velocities using a suitable wave theory and applying the Morrison equation to determine induced forces.

Wave uplift factors on a submerged deck can be of very high magnitude. These forces should be considered in design for structures where the deck level cannot be placed above a combination of extreme wave, surge and tide level, particularly in cyclonic areas. This is usually the case for dolphin decks which are usually at a lower level than the main wharf.

Wind effects are important, particularly regarding the forces acting on a ship at berth during operational conditions and extreme wind loads on superstructure elements such as shiploaders, and berth superstructure.

In certain areas in Australia and overseas, earthquake loadings on the structure can be significant and must be considered in the design.

Berthing and mooring forces

Berthing and mooring forces are generally the critical loads for design of berthing and mooring dolphins. In offshore terminals dolphins have different functional requirements to the deck and supporting elements for shiploaders, conveyors and roadways and are often configured as separate structures.

Mooring dolphins are usually isolated structures supporting quick release mooring hooks with access provided by catwalks. Ships of the sizes considered require four mooring dolphins for each berth. Each dolphin must be designed for line pulls of up to 4000 kilonewtons from several lines.

Berthing dolphins have rubber fendering systems on their faces but must be designed to take overloads in the event of an abnormal or extreme berthing situation. This is normally done by allowing plastic deformation in the structure to occur for extreme overloads.

Integration of the berthing dolphin function with the jackets supporting the wharf deck by mounting marine fenders and mooring hooks on the jackets can be carried out. In this case an overload mechanism should be incorporated into the design so that plastic deformation can occur in the upper part of the jacket structure so underwater repairs are not required.

Berth configuration

The type of shiploader and overall combination of shiploader and berth structure greatly influences the berth configuration.

The length of the berth is determined by the largest ship expected and to a lesser extent by the type of shiploader selected. In the case of a long travelling shiploader shown in Figure 2, the travel length (and berth length) should be at least equal to the distance between extreme hatches on the largest ship using the berth to avoid the need to move the ship along the berth (termed warping).

Figure 2:  Long travelling shiploader
Figure 2: Long travelling shiploader

A variation of the long travelling shiploader is the long travelling slewing shiploader as shown in Figure 3. This has the advantage that it requires a shorter wharf rail length as the shiploaders can slew to cover the end bow and stern hatches. The slewing shiploader also allows for efficient use of wharf space as the conveyor is positioned between the rails. This type of shiploader can also allow ships to be loaded on each side of the, allowing for layout efficiencies. There are other types of shiploaders. However, the long travelling type is most suitable for offshore berths using jacket construction.

Figure 3: Long travelling slewing shiploader with telescoping boom

Where there are a range of ship sizes from small to large, a long travelling slewing shiploader may be fitted with a telescoping boom to give a greater range of coverage than with a fixed boom. This is shown on Figure 3.

Jacket structures

A jacket is a braced framework which can sit directly on the seabed or be suspended above the seabed on temporary piles (spuds). Piles are typically driven through the leg of the jacket and the annulus between the pile and the jacket leg grouted, welded or swaged to provide the connection.

In some cases, the berthing dolphin can be made integral with the wharf jacket to reduce the amount of piling required on site. A rubber diaphragm closure is often used to prevent water entering the jacket leg and improve buoyancy during jacket placement. The rubber seal is broken when the pile is installed. Above the diaphragm closure is a grout seal which also acts as a wiper to prevent soil entering the annulus between the pile and jacket leg. In some cases, additional inflatable seals are used.

There are primary grout inlets at the base of the jacket leg. Depending upon the height of the jacket, additional inlets may be placed up the pile to allow grouting in stages. The grout inlets are connected by pipes to the surface.

An advantage of the jacket structure is that the jacket modules are fabricated off site, reducing the amount of site work required. Jacket structures generally have a smaller number of heavily loaded piles than freestanding pile structures. However, in order to achieve the high axial capacities required, it is often necessary to construct drilled sockets or bells at the base of the piles or to use groups of skirt piles. The jackets also provide lateral restraint to the piles against buckling.

Analysis and design

Several software systems are available for structural analysis, wave loading, code checking, and fatigue analysis for efficient jacket structure design. The software typically has wave modules, structural analysis modules, member and joint code check module, and fatigue modules

Jackets are fabricated from steel tubes. The start of the jacket fabrication process is the cutting of square and profiled tube ends including the weld preparation. This is preferably done with a computer-controlled flame cutting machine. The process is reliable and reduces errors in fabrication.

After individual elements are cut and end elements profiled, members are assembled, and tack welded. Following inspection for straightness, circumferential joints are welded. Semi-automatic methods such as submerged arc are often used. Where possible seamless pipe should be used for jacket bracing members up to 610 millimetres in diameter.

Fatigue critical joints should be fabricated separately as a joint can with attached stubs to allow welding of profiled brace-chord connections from both sides. Following fabrication of individual leg and bracing members, side panels to the jackets are assembled and welded in the yard.

These panels are then transported to the assembly area and lifted upright into saddles supporting the legs on the ground and secured by guy ropes at the top. Horizontal and diagonal bracing members are welded into position, scaffolding being required for access to the top level.

After fabrication jackets are lifted or skidded onto a barge or heavy lift ship and secured for shipment. Analysis and design of loading configuration, sea fastenings and transportation loads are also carried out.

Jacket installation

Jacket construction on site requires heavy floating cranes or a heavy lift ship for unloading and positioning jackets. Jackets can be placed on the seabed in a storage area and the legs flooded to provide stability. In some cases, anchoring is also necessary for stability in rough weather. At the appropriate time, jackets are positioned in their final position on the line of the berth. Flooded legs may be de-ballasted with compressed air to reduce the effective to assist in lifting to the required position.

Once the jacket position is set, top closure plates on the jacket legs are removed ready for piling. Guide frames may be required to support the piles during driving. The ability to move the pile inside the jacket leg during the placing of superstructures to tops of piles allows for some dimensional inaccuracies. It also allows the jacket to be levelled to the correct elevation if required.

Guides are positioned in the jacket legs to provide a gap between the pile and inside of the jacket leg for grouting and to prevent fouling with the seals at the base of the jacket leg. The top of the guide is tapered to allow the pile to slide over the protrusion.

Jacket piles are usually heavily loaded. Different pile types can be considered depending upon foundation conditions, such as driven piles, grouted insert piles or belled piles.

In the case of grouted inert piles and belled piled, rock drilling will be required. Drilling to the required depth is carried out first then the belling operation is carried out with a special belling tool. Spoil is carried to the surface in the drilling mud (bentonite) which is filtered and recirculated.

Shear connectors are welded inside the pile during fabrication to allow for force transfer between the pile and the drilled foundation. The base of the pile may also have a thickened pile shoe for hard driving into rock.

Following construction of the drilled socket at the base of the pile, the gap between the inside of the jacket leg and the pile is grouted to provide a structural connection. Grout is injected at the base of the jacket leg displacing water within the leg towards the surface. Pumping of grout is continued until the grout escaping from the top of the jacket legs is of the correct consistency.

Grout commonly used is a mixture of cement and sea water with additives. It has a colloidal consistency before setting and expands on setting. To increase workability a plasticiser may be added.

Grout should have the following properties:

  • Adequate fluidity for at least 1 hour from time of mixing at ambient temperature
  • Minimum water/cement ratio compatible with workability
  • Unrestrained expansion not more than 10 per cent and not less than one per cent
  • Settlement of sediment from the grout not exceeding two per cent at three hours by volume
  • Strength of set grout not less than 10 megapascals at 24 hours, and 30 megapascals at seven days

Superstructure

Different superstructure components are constructed for different functions. The philosophy is to prefabricate as much as possible of the superstructure and deck off site including both steel and concrete items. The rail girders for supporting the superstructure are integrated with cross girders supporting the wharf deck so that large modules can be lifted into position with offshore cranes or a heavy lift ship, minimising offshore work.

Pile stubs are welded to the soffit of the superstructures and decks at the fabrication works. In some cases, this is done on site as actual dimensions are required to be confirmed. Stubs are welded to the piles in the field with complete penetration welds.

Conclusion

Installations providing for mooring of large vessels in deep water produce conditions which can be beyond the practical capacity of conventional freestanding pile construction. A system using prefabricated steel jackets provides an alternative, developed and proven for offshore oil drilling platforms.

Jackets can be used on dry bulk terminals for export of mineral ores as the trend towards larger ship sizes continues. This type of construction requires heavy and specialist plant. The method is well suited to projects some distance offshore where economy of scale can be realised.

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