Locating Ship-to-Ship (STS) Transfer Operations via Multi-Criteria Decision Analysis (MCDA): A Case Study

Abstract

Nowadays Ship-to-Ship (STS) transfer has become common practice. However, it remains a complex and difficult procedure, with risk assessment essential for both vessels and location selection. A variety of risk assessment techniques are thus commonly applied in order to evaluate the factors affecting STS transfers. This paper proposes a novel approach to the risk assessment of different locations for STS transfer operations, using the ELECTRE methodology borrowed from the Multi Criteria Decision Aid (MCDA) discipline. The proposed MCDA methodology is properly developed and thoroughly analysed with the aim of supporting operators in choosing the best alternative STS location. To this end, a case study is presented with which to testify the effectiveness and verify the strength of the suggested approach. In particular, four different locations within the Mediterranean Sea, which represent the set of alternative actions, are evaluated according to four different groups of criteria, with a view to selecting the most appropriate location at which to conduct the transfer operations. Different operational, economic, environmental and safety-security criteria regarding each location are assessed and evaluated by a team of three experts designated by the stakeholders (decision makers) of a shipping company. In addition, a robustness analysis is performed in order to control the stability of the model results. The objective is to develop an MCDA model with which to select the most appropriate location according to its operational eligibility.

Appendices

Appendix 1: Analytical Description of the ELECTRE I Methodology

In MCDA modeling, a set of actions A = a ₁, a ₂, …, a _m is evaluated based on a consistent family of criteria F = g ₁, g ₂, …, g _n under the assumptions that

$$ \displaystyle\begin{array}{rcl} g_{j}(a)> g_{j}(b)& \Leftrightarrow a\mbox{ is preferred to }b&{}\end{array}$$

(8.1)

$$\displaystyle\begin{array}{rcl} g_{j}(a) = g_{j}(b)& \Leftrightarrow a\mbox{ is indifferent to }b&{}\end{array}$$

(8.2)

To implement the ELECTRE I methodology for the aforementioned problem, three different types of data must be addressed:

• Weights of the criteria p ₁, p ₂, …, p _n : These refer to the relative importance of each criterion (see the recent survey [29]) and can be calculated via direct or indirect methods. In direct assessment, users assign a value to each criterion, with these values then normalized accordingly. A more effective and complex indirect approach involves the use of pairwise comparisons between the selected criteria, applying goal-programming methodologies. The sum of the weights in both cases is equal to 1.

  $$\displaystyle{ \sum _{i=1}^{n}p_{ i} = 1 }$$

  (8.3)

• Concordance threshold s: An absolute number selected by the analyst which can range from 0.5 to 1, the concordance threshold is applied in order to determine the pairs subject to discordance evaluation, and thus examine the outranking relation for the actions to which the pair members refer.

• Veto thresholds v ₁, v ₂, …, v _n : Veto thresholds are employed to control large differences between the values of specific criteria for the corresponding actions.

In particular, for a pair of actions or alternatives (a, b) the outranking relation is determined via the equation

$$\displaystyle{ a\,S\,b \Leftrightarrow (a,b)\mbox{ satisfy the concordance and veto conditions} }$$

(8.4)

Assuming that the criteria weights have been calculated and the concordance and veto thresholds determined, the implementation of the ELECTRE I methodology comprises the following general steps:

Step 1. Concordance validation::

To validate an outranking relation between a pair of actions (a, b), the concordance indicator is introduced via the function C(a, b) as follows:

$$\displaystyle\begin{array}{rcl} C(a,b): A \times A \rightarrow [0,1]& & {}\end{array}$$

(8.5)

$$\displaystyle\begin{array}{rcl} C(a,b) =\sum _{i^{{\ast}}}p_{i},\quad \mbox{ for }i^{{\ast}}\in \{ i\vert g_{ i}(a) \geq g_{i}(b)\}& & {}\end{array}$$

(8.6)

Relations (8.5) and (8.6) show that C(a, b) is the aggregation of the criteria weights where action a is preferred or the user is apathetic regarding action b. Due to Eq. (8.3), the aggregation of the weights cannot exceed a value of 1. The pair (a, b) satisfies the concordance condition when

$$\displaystyle{ C(a,b) \geq s,\quad \mbox{ where }s\mbox{ is the concordance threshold} }$$

(8.7)

Step 2. Discordance validation::

A pair of actions (a, b) satisfies the discordance condition

$$\displaystyle{ g_{j{\ast}}(b) - g_{j{\ast}}(a) \leq v_{j{\ast}},\quad \forall j^{{\ast}}\in \{ i\vert g_{ i}(a) \geq g_{i}(b)\} }$$

(8.8)

Indicator j ^∗ belongs to the consistent family of the criteria for which action b is preferred over action a, and v _j∗ is the veto threshold for criterion j ^∗. In case that action b has a difference in values that exceeds the veto threshold of a particular criterion, the latter criterion opposes the veto regarding the outranking relation of action a over action b.

In summary, the outranking relation employed in the ELECTRE I methodology is determined as follows:

$$\displaystyle{ aSb \Leftrightarrow \left \{\begin{array}{@{}l@{\quad }l@{}} C(a,b) =\sum _{i_{{\ast}}\in \{i\vert g_{i}(a)\geq g_{i}(b)\}}p_{i} \geq s,\quad 0.5 \leq s \leq 1 -\min _{j\in F}p_{j} \quad \\ \qquad \qquad \qquad \qquad \qquad \qquad \mbox{ (concordance validation)} \quad \\ g_{j}(b) - g_{j}(a) \leq v_{j},\quad \forall j \in F\mbox{ (discordance validation)}\quad \end{array} \right. }$$

(8.9)

The discordance validation is conducted under the condition that the concordance validation has a positive result. Following the outranking relation determination, results are presented in the form of the kernel of the outranking graph, after the elimination of any circuits.

Step 3. Kernel construction::

We can define the kernel of the outranking relation as a set of actions Π, which is itself a subset of a set of actions A, for which the following attributes must be satisfied:

$$\displaystyle\begin{array}{rcl} (1)\,\,\forall b \in A-\varPi,\,\exists a \in \varPi: aSb\mbox{ (external condition)}& & {}\end{array}$$

(8.10)

$$\displaystyle\begin{array}{rcl} (2)\,\,\forall a_{1} \in \varPi \mbox{ and }a_{2} \in \varPi,\,\,a_{1}/ Sa_{2}\mbox{ and }a_{2}/ Sa_{1}\mbox{ (internal condition)}& & {}\end{array}$$

(8.11)

By definition, the kernel consists of the most important actions in set A that the decision maker must take into account before making a final decision. Therefore, the efforts of the analyst should be concentrated on the minimization of kernel-included actions. The optimization of kernel actions can be achieved via sensitivity or robustness analysis, both of which provide the ability to modify the variables p, s and v accordingly.

Fig. 8.7

Left: topological map of Malta. Right: statistical data retrieved from the Poseidon Live Access Server (LAS) regarding significant wave height for the Malta STS area for the year 2014

Appendix 2: Short Introduction to the Proposed STS Transfer Locations

Malta: The island of Malta is strategically located (35∘50’N 14∘50’E) at the center of the Mediterranean Sea and is the common point of sea lanes linking Europe, North Africa and the Middle East (Fig. 8.7). Malta also offers a comprehensive suite of inspection, testing and surveying facilities for disparate cargoes ranging from heavy to light. Many STS providers are available to assist with transfers. However, the transfer area itself is not adequately sheltered and environmental conditions may have an adverse effect on operations. The area on Hurd Bank has been used for many years to conduct such operations and thus the local authorities are familiar with procedures. STS operations can be undertaken both within and outside territorial waters. The average height of waves is approximately 1.07 m, with maximum wave height reaching 4.68 m. Malta is classified by the World Lightering Organisation as a level one response area.

Cyprus: (34∘20’ N 33∘20’ E) Located in Limassol, the Cypriot site (Fig. 8.8) is one of the most important and strategic STS areas in the north eastern Mediterranean. As operations are conducted in international waters there are no port fees. Approximately 200 miles from the Suez Canal, Cyprus provides clients with the option to both deliver to eastern Mediterranean markets and to transship larger parcels of cargo east, avoiding the inherent delays facing larger vessels transiting the Turkish Straits. STS transfers can be conducted either stationary or underway. Average wave height is 0.8 m and maximum wave height 2.9 m.

Fig. 8.8

Left: topological map of Cyprus. Right: statistical data retrieved from the Poseidon Live Access Server (LAS) regarding significant wave height for the Cyprus STS area for the year 2014

Fig. 8.9

Left: topological map of the Strait of Gibraltar. The areas shown in light represent protected landscape/seascape areas according to Natura2000. Right: statistical data retrieved from the Poseidon Live Access Server (LAS) regarding significant wave height for the Gibraltar STS area for the year 2014

Fig. 8.10

Left: topological map of Crete. Right: statistical data retrieved from the Poseidon Live Access Server (LAS) regarding significant wave height for the Crete STS area for the year 2014

Gibraltar: (36∘20’ N 5∘40’ W (−5.4)) The STS transfer area is located close to the Strait (Fig. 8.9). Strict regulations must be followed regarding emissions, with zero impact to the environment. Additional fees must also be paid to ensure permission from the Gibraltar port authorities for the transfer of crude oil and/or other by-products. Operations can be conducted at anchorage only. Average wave height is 0.42 m and maximum wave height 2.43 m.

Crete: (34∘50’ N 24∘50’ E) The area of Kaloi Limenes (Fig. 8.10) in southern Crete has recently been designated by local authorities as an STS transfer operation location. As a result, local staff have limited experience and no records of previous STS operations exist. Average wave height is 0.90 m and maximum wave height 5.57 m (Fig. 8.11).

Fig. 8.11

General arrangement of the vessel used in the practical example

Appendix 3: Main Characteristics of the Vessel Used in the Practical Example

Table 14