The loading and unloading sequence is a time consuming event, as the feed lines have limited capacity. To cope with this limitation, often more than one loading arm are connected at the same time in order to double or triple the capacity.
Existing jetties which are to be upgraded in terms of handling ships with larger capacities, or to handle different kinds of fluids in addition to the ones already handled, would normally need to be provided with additional marine loading arms.
Addition of loading arms on an old jetty often requires a re-arrangement of the jetty. Sometimes the re-arrangement includes an extension of the jetty in order to accommodate the additionally required marine loading arms. Extension of jetties usually is a time consuming and costly event.
Adding more lines without the need for major adjustment of the jetty arrangement would be a cost saving and more simple solution.
Recently marine loading arms were introduced with two fully separated fluid lines on a shared base plate, who can be operated independently from each other.
This loading arm type has been launched with the purpose to double the capacity on the same base plate as a single-line marine loading arm.
Design of marine loading arms
Loading and unloading of tankers principally needs articulated equipment to follow a ship’s movements in the horizontal plane were there are requirements to deal with drift and sway, and in vertical plane because of changing water and draft level suggests either loading hoses or loading arms. Loading arms offer the possibility of including safety accessories and remotely operated quick couplers to make life easier to operators.
Marine loading arms consist of a piping assembly with two moveable pipes. The flexibility is achieved by swivelling joints, which are the critical component of loading arms. When marine loading arms became common some 50 years ago, the swivel joints were much less reliable than today and as a result of that a supporting structure taking over the loads of the piping was absolutely required. Nowadays, swivel joints strength have drastically improved, which eliminates the need for a support frame for almost every application. A self supporting loading arm is requires less steel and less components and is therefore significantly more efficient with regard to the old generation marine loading arms where the liquid carrying line is supported by a separate frame.
Because of the high weight of the steel piping, both moveable pipes must be counterweight balanced. A well-thought out design would optimise the design in such a way, that only one rotating counter weight is needed for both moving pipes. Moreover a rigid connection between the counterweight and outboard pipe instead of a cable and wheel system further increases the efficiency (Refer to figure 1).
Design of symmetric marine loading arms
Conventional marine loading arms have permanent overturning moment on the jetty due to the fact that first moveable pipe (called the inboard) is mounted aside the fixed part, called stand post or riser. This is unavoidable since the swivel joints connect the moveable pipes via elbow pipes and the bend radius of two elbows creates the off-set between the riser and the inboard.
A symmetric design however equally divides the mechanical forces on the swivel joints and within the loading arm structures itself significantly extending service life. In addition, the symmetry reduces the overturning moment (OTM) on the jetty to almost zero. This makes them especially suited for extraordinary applications like oversized dimensions and mounting on moveable carts and floating jetties (refer to figure 2).
The symmetry is achieved by applying two inboard, each of the with the same offset towards the riser.
Possibilities of symmetric marine loading arms
A symmetric design clears the way towards a number of very interesting possibilities with regard to different executions, if not only the inboard is doubled, but the riser and outboard as well. It requires very careful design, but the additional riser, inboard and outboard can be configured as a completely separate second liquid line, fully part of the self-supporting construction of the symmetric marine loading arm.
The additional line provides the following applications:
1. Integration of large diameter and high pressure vapour return lines.
In an increasing number of cases, local authorities require a vapour recovery system in order to minimise the escape of VOC’s to the atmosphere.
Symmetric marine loading arms provide the possibility to not only accommodate a so-called piggy-back mounted vapour line, where the liquid line takes all the weight from the vapour arm, but also to fully integrate the vapour line in the structure, meaning that there are basically no limitations regarding the diameter of the vapour line. The only remaining limitation is the design pressure due to the connecting hose.
This is a significant improvement, since conventional marine loading arm would only accommodate a piggy-back mounted vapour line up to a certain maximum depending on the carrying structure, which will soon end up to be huge and massive.
2. Integration of a second fully rigid line for a different product
Basically the same configuration of the marine loading arm with integrated vapour line can be applied for a second, fully rigid liquid line. In stead of the connecting hose, a rigid connection can be made for both of the liquid carrying lines.
Two liquid lines in one structure significantly saves costs when compared to two single-line marine loading arms, moreover the jetty does not need to be extended or even reinforced, since the overturning moment still remains zero because the symmetric design is maintained.
This configuration is suited to connect one liquid line at the same time.
3. Integration of a second fully rigid line to double capacity
The limitation of the previously described execution being connection of only one liquid line at the same time is a result of both lines being part of the same construction.
In case the aim is to double the capacity by connecting two liquid lines at the same time, an entirely new concept must be developed. Symmetry needs to be maintained while simultaneous connection of two liquid arms would require full independent movement of each liquid line.
Both requirements have been met with the recently introduced so-called twin arm. This concept consists of two independently controlled product lines mounted on one base plate. The liquid lines are mirror shaped and the still existing symmetry still ensures a virtually zero overturning moment on the jetty. In fact they combine two full-option marine loading arms in one assembling only, sharing the same jetty floor space initially assigned for one loading arm only.
Simultaneous connection of two liquid lines from the same marine loading arm assembly candouble the capacity and it provides an enhanced feature, which enables newly designed jetties to be considerably smaller and existing jetties to be upgraded without major jetty adjustments. This is a very important cost-saving feature.
Loading efficiency can be significantly increased by applying multi-line marine loading arms, integrated vapor lines including the ones with large diameters. High-reliable symmetric MLA’s were already available for self-supporting applications. Recently a multi-line marine loading arm has been launched with fully independent product lines for simultaneous connection of both liquid lines.
This feature doubles the capacity of the assigned jetty floor space for loading arms and it means a significant contribution to improving jetty efficiency.